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This book gathers the proceedings of the 11th workshop on Direct and Large Eddy Simulation (DLES), which was held in Pisa, Italy in May 2017. The event focused on modern techniques for simulating turbulent flows based on the partial or full resolution of the instantaneous turbulent flow structures, as Direct Numerical Simulation (DNS), Large-Eddy Simulation (LES) or hybrid models based on a combination of LES and RANS approaches. In light of the growing capacities of modern computers, these approaches have been gaining more and more interest over the years and will undoubtedly be developed and applied further. The workshop offered a unique opportunity to establish a state-of-the-art of DNS, LES and related techniques for the computation and modeling of turbulent and transitional flows and to discuss about recent advances and applications.

This volume contains most of the contributed papers, which were submitted and further reviewed for publication. They cover advances in computational techniques, SGS modeling, boundary conditions, post-processing and data analysis, and applications in several fields, namely multiphase and reactive flows, convection and heat transfer, compressible flows, aerodynamics of airfoils and wings, bluff-body and separated flows, internal flows and wall turbulence and other complex flows.

### Adaptive Direct Numerical Simulation with Spatially-Anisotropic Wavelet-Based Refinement

In the wavelet-based adaptive multi-resolution approach to the numerical simulation of turbulent flows, the separation between resolved energetic structures and unresolved flow motions is achieved through the application of a wavelet thresholding filter. For very small threshold values, the effect of residual motions upon the resolved flow dynamics can be completely neglected, which leads to the adaptive Wavelet-based Direct Numerical Simulation (W-DNS) approach.

G. De Stefano, E. Brown-Dymkoski, O. V. Vasilyev

### Towards Adaptive Mesh Refinement for the Spectral Element Solver Nek5000

When performing computational fluid dynamics (CFD) simulations of complex flows, the a priori knowledge of the flow physics and the location of the dominant flow features are usually unknown. For this reason, the development of adaptive remeshing techniques is crucial for large-scale computational problems. Some work has been made recently to provide Nek5000 with adaptive mesh refinement (AMR) capabilities in order to facilitate the generation of the grid and push forward the limit in terms of problem size and complexity [10].

N. Offermans, A. Peplinski, O. Marin, P. F. Fischer, P. Schlatter

### Discrete Conservation of Helicity in Numerical Simulations of Incompressible Turbulent Flows

Helicity is the scalar product between velocity and vorticity and, just like energy, its integral is an inviscid invariant of the 3D incompressible Navier–Stokes equations, $$\frac{\partial {u}_{i}}{\partial t} + \mathscr {N}_{i}({u}) = -\frac{\partial {p}}{\partial x_{i}} + \frac{1}{\text {Re}}\frac{\partial ^2 {u}_{i}}{\partial x_{j}\partial x_{j}}, \quad \quad \frac{\partial {u}_{i}}{\partial x_{i}}=0 \;,$$ where $$\mathscr {N}_{i}(u)$$ is the non-linear convective term and Re is the Reynolds number.

D. Vallefuoco, F. Capuano, G. Coppola

### A Massively Parallel, Direction-Splitting Solver for DNS in Complex Geometries

The Direct Numerical Simulation of turbulent flows (DNS) has proved itself, over the years, an extremely valuable tool to investigate the fundamental properties of turbulence, often rivalling experiments by virtue of its accuracy and of the insight it offers to the investigator (Moin and Mahesh, Ann Rev Fluid Mech 30:539–578, 1998) [15].

F. Auteri, M. D. de Tullio, J.-L. Guermond, D. Montagnani, P. D. Konghar

### An Analysis of Time-Integration Errors in Large-Eddy Simulation of Incompressible Turbulent Flows

There is widespread theoretical and numerical evidence that Large-Eddy Simulation (LES) of turbulent flows has to be performed using high-order accurate numerical schemes. Much of the research has so far focused on the spatial discretization, coming to the conclusion that: (1) higher-order methods ( $${\ge }2$$ ) are preferred, to ensure that the magnitude of the truncation error does not overwhelm the subgrid-scale model contribution, and (2) non-dissipative (centered) schemes should be employed, so that the energy cascade mechanism is not artificially contaminated (Capuano et al, J Comput Phys 295:209–229, 2015), [2], (Capuano et al, Progress in turbulence VI. Springer, Berlin, 2016), [3].

F. Capuano, E. M. De Angelis, G. Coppola, L. de Luca

### Evaluation of the Spectral Element Dynamic Model for LES on Unstructured, Deformed Meshes

Discontinuous finite element methods (DFEM) such as the discontinuous Galerkin (DG) (Cockburn et al, Discontinuous Galerkin methods: theory, computation, and applications. Springer, Berlin, 2000), [1] or the spectral difference (SD) (Kopriva and Kolias, J Comput Phys 125(1):244–261, 1996), [7], (Liu et al, J Comput Phys 216(2):780–801, 2006), [9], (Wang et al, J Sci Comput 32(1):45–71, 2007), [21] methods show a strong potential for the direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent flows on realistic geometries. These methods are characterized by a rather peculiar mix of features, such as their high-orders of accuracy, the ability to handle unstructured meshes, curved boundary elements and the compactness of the stencil, which allows for optimal parallelism. The extremely low level of numerical dissipation which can be achieved when high-orders are selected, and the consequent significant increase in resolving power, make DFEM particularly well suited for LES.

G. Lodato, J. B. Chapelier

### A Discontinuous Galerkin Variational Multiscale Approach to LES of Turbulent Flows

In recent work (Chapelier et al, Comput Method Appl Mech Eng 307:275–299, 2016), [1] we have developed a variational multiscale simulation (VMS) approach based on a modal discontinuous Galerkin (DG) method. The separation of scales is achieved in each element via projection onto the discontinuous modal space. In (Chapelier et al, Comput Method Appl Mech Eng 307:275–299, 2016), [1], the DG-VMS technique was applied to the Taylor–Green vortex (TGV) flow at $$Re=3\,000$$ demonstrating the potential of this approach to perform LES.

M. de la Llave Plata, E. Lamballais, V. Couaillier

### Implicit LES Approaches via Discontinuous Galerkin Methods at Very Large Reynolds

We consider the suitability of implicit large-eddy simulation (iLES) approaches via discontinuous Galerkin (DG) schemes. These are model-free eddy-resolving approaches which solve the governing equations in unfiltered form and rely on numerical stabilization techniques to account for the missing scales. In DG, upwind dissipation from the Riemann solver provides the baseline mechanism for regularization. DG-based iLES approaches are currently under rapid dissemination due to their success in predicting complex transitional and turbulent flows at moderate Reynolds numbers (Uranga et al, Int J Numer Meth Eng 87(1–5):232–261, 2011, [1], Gassner and Beck, Theor Comput Fluid Dyn 27(3–4):221–237, 2013, [2], Beck et al, Int J Numer Methods Fluids 76(8):522–548, 2014, [3], Wiart et al Int J Numer Methods Fluids 78:335–354, 2015, [4]). However, at higher Reynolds number, accuracy and stability issues can arise due the highly under-resolved character of the computations and the suppression of stabilizing viscous effects.

R. C. Moura, J. Peiró, S. J. Sherwin

### Implicit LES of a Turbulent Channel Flow with High-Order Discontinuous Galerkin and Finite Volume Discretization

Owing to the permanently growing computational resources and the known predictive deficiencies of unsteady Reynolds averaged Navier-Stokes simulations, scale-resolving simulations become affordable methods to further study the unsteady phenomena of complex flows. Especially high-order spatial discretizations, such as the discontinuous Galerkin method, seem to be well suited for these simulations due to their superior dispersion and dissipation properties in comparison to their low-order counterparts. In this paper, we investigate the implicit large eddy simulations of a fully developed turbulent channel flow with an high-order discontinuous Galerkin method and a second-order accurate finite volume method. Statistical quantities, obtained with varying approximation orders but by using the same number of degrees of freedom, are compared to reference DNS data.

M. Bergmann, C. Morsbach, M. Franke

### An Implicit Discontinuous Galerkin Method with Reduced Memory Footprint for the Simulation of Turbulent Flows

In recent years the increasing availability of High Performance Computing (HPC) resources strongly promoted Large Eddy Simulation (LES) as a viable approach to the simulation of those moderate Reynolds flow conditions where Reynolds-averaged Navier–Stokes (RANS) formulation fails, e.g. massively separated flows. In particular, the practice of an implicit LES (ILES) based on the Discontinuous Galerkin (DG) method showed to be very promising due to the favourable dispersion and dissipation properties (Bassi et al., Eur J Mech B-Fluid Part 2 55:367–379, 2016, [1]). The high potential of DG approximations for the under-resolved simulation of turbulent flows has already been demonstrated in literature and research on this topic is growing fast (Chapelier, et al., Comput Fluids 95:210–226, 2014, [2], de Wiart et al., Int J Numer Methods Fluids 78:335–354, 2015, [6]). However, how to integrate in the most efficient way the semidiscrete set of NS equations exploiting at best such large computational facilities is an active research topic.

A. Crivellini, M. Franciolini, A. Nigro

### On the Development of an Implicit High-Order Discontinuous Galerkin Solver for a Hybrid RANS-LES Model

Recent years have seen an ever-increasing interest in turbulence models able to go beyond the limited predictive capability of the Reynolds-averaged Navier–Stokes (RANS) formulation. In the range of moderate Reynolds numbers, availability of large HPC resources now allows to employ Large Eddy Simulation (LES) also in complex flow applications. In this context, the practice of an implicit LES (ILES) based on the Discontinuous Galerkin (DG) method showed to be very promising due to the good dispersion and dissipation properties of DG methods. However, to date, characteristic Reynolds numbers of many industrial applications are too large for a fully resolved LES. For these applications the use of a hybrid RANS-LES model or a wall modelled LES approach seems mandatory. In hybrid RANS-LES models the RANS equations are active close to solid walls, where LES would be prohibitively costly, while LES is used in regions of separated flow where larger eddies can be resolved.

F. Bassi, L. Botti, A. Colombo, A. Ghidoni, F. Massa, G. Noventa

### Assessment of High-Order Discontinuous Galerkin Methods for LES of Transonic Flows

This paper concerns implicit large eddy simulation (ILES) of turbulent flows of industrial interest using high order discontinuous Galerkin method (DGM). DGM has a high potential for industrial applications using ILES. As dissipation is only active on very small scale features, the method mimics a subgrid scale model, while its high accuracy ensures that large scale dynamics are not contaminated by dispersive/dissipative errors. Previously DGM/ILES has been assessed on many low Mach number canonical test cases (e.g. Carton et al. Numer Methods Fluids, 78:335–354, (2015), [3]). This paper recapitulates recent validation on transonic benchmarks (Hillewaert et al. Proceedings of CTR summer program, pp. 363–372, Stanford University, (2016), [6]) and proceeds to the application on the LS89 cascade, a well-known turbomachinery benchmark.

J. S. Cagnone, Z. Zeren, A. Châtel, M. Rasquin, K. Hillewaert, L. Bricteux

### Efficient Pressure-Correction Method for Interfacial Tracking Appropriate for the Immersed Boundary Method

Solving the Navier–Stokes equations to simulate incompressible two-fluid flows with interfaces, is still a developing scientific field. One of the main challenges, is the reduction of the computational cost which is still significantly higher when compared to single-fluid problems. This is mainly due to the variable coefficients Poisson equation arising from the projection method to impose mass conservation.

C. Frantzis, D. G. E. Grigoriadis

### On the Eddy Viscosity Associated with the Subgrid Stresses

Thanks to its simplicity and robustness, the models based on the eddy viscosity concept represent the most common procedure to introduce the effect of the unresolved scales in the equations of motion for the Large Eddy Simulation (LES) approach. Indeed, the subgrid scale (sgs) viscosity approach allows from an energetic point of view to respect the dissipative nature of turbulence.

A. Cimarelli, A. Abbà, M. Germano

### Implicit/Explicit Spectral Viscosity and Large-Scale SGS Effects

In order to investigate the scale-selective influence of SGS on the large scale dynamics, DNS and LES are performed for the Taylor-Green vortex problem. An a priori analysis confirms the interest of the hyperviscous feature at small scale as used in implicit LES, SVV and VMS. However, the assumption of zero SGS dissipation at very large scales is found unrealistic for the high Reynolds number and coarse LES mesh considered. A posteriori analysis shows that SGS modelling based on the assumption of an inviscid cascade leads to a bottleneck effect on the kinetic energy spectrum with a significant underprediction of the total SGS dissipation. The simple addition of a constant eddy viscosity, even targeted to be optimal in terms of SGS dissipation, is unable to give realistic results. To allow accurate predictions by LES, a specific closure that incorporates both the hyperviscous feature (i.e. regularisation) and the expected SGS dissipation at large scales has to be developed.

E. Lamballais, T. Dairay, S. Laizet, J. C. Vassilicos

### Realizable Dynamic Large Eddy Simulation

A very attractive feature of large eddy simulation (LES) is the possibility to apply the dynamic subgrid scale model calculation developed by Germano et al. Phys. Fluids A 3:1760–1765, 1991, [1]. This is a method for the calculation of model parameters as functions of time and space as the simulation progresses. It avoids empirical treatment of model parameters such as damping or wall modeling near the wall boundaries. On the other hand, dynamic LES models usually suffer from instabilities. The mechanism of instability of dynamic sub-grid scale (SGS) models has not yet been fully clarified. Several methods are in use for the stabilization of dynamic SGS models. The most popular methods are clipping of model parameters and their space averaging in homogeneous directions. These stabilization techniques are often difficult or even impossible to apply. In real flows, there are no homogeneous directions in space. It is also difficult to find appropriate clipping values for dynamic LES parameters, which can depend on the type of flow, Reynolds number and grid resolution.

R. Mokhtarpoor, S. Heinz, M. K. Stoellinger

### The Dynamic Smagorinsky Model in Pseudo-Spectral LES of Decaying Homogeneous Isotropic Turbulence at Very High

We consider the large-eddy simulation (LES) of turbulent flows, in the classical view where no regular explicit filtering is added to the truncation/projection due to the LES grid. The truncation of the complete field $$u_i$$ (experimental or from direct numerical simulation, DNS) to the much coarser LES grid corresponds to the incomplete LES field and is noted $$\overline{u}_i$$ . Assuming perfect numerics, the “effective subgrid-scales (SGS) stress” is then obtained as : i.e., the product of LES quantities minus the product of complete quantities, and further truncated to the LES grid. The divergence of that stress (i.e., the “effective SGS force”) represents the effect of the removed scales on the LES scales. As there is no information beyond the LES grid cutoff, the SGS stress (or the SGS force) can only be modeled.

O. Thiry, G. Winckelmans, M. Duponcheel

### Nonlinear Subgrid-Scale Models for Large-Eddy Simulation of Rotating Turbulent Flows

We aim to design subgrid-scale models for large-eddy simulation of rotating turbulent flows. Rotating turbulent flows form a challenging test case for eddy viscosity models due to the presence of the conservative Coriolis force. We therefore propose a new subgrid-scale model that, in addition to a dissipative eddy viscosity term, contains a nondissipative nonlinear model term that can capture transport processes, such as those due to rotation. We show that the addition of this nonlinear model term leads to improved predictions of the Reynolds stress anisotropy in large-eddy simulations of a spanwise-rotating plane-channel flow, while maintaining the prediction of the mean velocity profile that is obtained when only using an eddy viscosity model.

M. H. Silvis, R. Verstappen

### A New Subgrid Characteristic Length for LES

Large-eddy simulation (LES) equations result from applying a spatial commutative filter, with filter length $$\varDelta$$ , to the Navier–Stokes equations \begin{aligned} \partial _t \overline{\varvec{u}} + \left( \overline{\varvec{u}} \cdot \nabla \right) \overline{\varvec{u}} = \nu \nabla ^2\overline{\varvec{u}} - \nabla \overline{p} - \nabla \cdot \tau ( \overline{\varvec{u}} ) , \quad \nabla \cdot \overline{\varvec{u}} = 0, \end{aligned} where $$\overline{\varvec{u}}$$ is the filtered velocity and $$\tau (\overline{\varvec{u}})$$ is the subgrid stress (SGS) tensor and aims to approximate the effect of the under-resolved scales, i.e. $$\tau (\overline{\varvec{u}} ) \approx \overline{\varvec{u}\otimes \varvec{u}} - \overline{\varvec{u}} \otimes \overline{\varvec{u}}$$ . Most of the difficulties in LES are associated with the presence of walls where SGS activity tends to vanish. Therefore, apart from many other relevant properties, LES models should properly capture this feature [1].

F. X. Trias, A. Gorobets, A. Oliva

### On the Richardson Extrapolation of the Reynolds Stress with the Systematic Grid and Model Variation Method

A new operational Richardson extrapolation has been proposed to reconstruct the Reynolds stresses in LES. The method is based on three LES simulations as suggested in the Systematic Grid and Model Variation approach, and two new terms appear in the formalism. For a turbulent planar jet these two terms are small but reasonable and the extrapolation of the resolved shear stress works qualitatively well. The method is flexible and can be combined with different strategies. First results are promising, but more experience is needed with different flow configurations and higher Reynolds number.

M. Klein, G. Scovazzi, M. Germano

### Spatial Filtering for Reduced Order Modeling

Spatial filtering has been central in the development of large eddy simulation reduced order models (LES-ROMs) (Wang et al. in Comput. Meth. Appl. Mech. Eng. 237–240:10–26, 2012, [9], Xie et al. in Data-driven filtered reduced order modeling of fluid flows, 2018, [11], Xie et al. in Comput. Methods Appl. Mech. Eng. 313:512–534, 2017, [12]) and regularized reduced order models (Reg-ROMs) (Iliescu et al. in Int. J. Numer. Anal. Mod. 2017, [4], Sabetghadam and Jafarpour in Appl. Math. Comput. 218:6012–6026, 2012, [7], Wells et al. in Int. J. Numer. Meth. Fluids 84:598–615, 2017, [10]) for efficient and relatively accurate numerical simulation of convection-dominated fluid flows. In this paper, we perform a numerical investigation of spatial filtering. To this end, we consider one of the simplest Reg-ROMs, the Leray ROM (L-ROM) (Iliescu et al. in Int. J. Numer. Anal. Mod. 2017, [4], Sabetghadam and Jafarpour in Appl. Math. Comput. 218:6012–6026, 2012, [7], Wells et al. in Int. J. Numer. Meth. Fluids 84:598–615, 2017, [10]), which uses ROM spatial filtering to smooth the flow variables and decrease the amount of energy aliased to the lower index ROM basis functions. We also propose a new form of ROM differential filter (Sabetghadam and Jafarpour in Appl. Math. Comput. 218:6012–6026, 2012, [7], Wells et al. in Int. J. Numer. Meth. Fluids 84:598–615, 2017, [10]) and use it as a spatial filter for the L-ROM. We investigate the performance of this new form of ROM differential filter in the numerical simulation of a flow past a circular cylinder at a Reynolds number $$Re=760$$ .

L. C. Berselli, D. Wells, X. Xie, T. Iliescu

### A RANS Assisted LES Approach

In the particular approach herein proposed, RANS and LES are combined in order to achieve a detailed description of turbulent flows without incurring in infeasible computational effort. This model is based on the hybrid filter approach proposed by Germano [5]. One of the most interesting point of this approach, is that the equations obtained by filtering Navier–Stokes equations, already include terms which are able to represent the interactions between RANS and LES regions. Therefore no further artificial terms are needed to allow the appropriate energy and momentum transfer between RANS and LES. This approach has already been tested [8, 10] showing promising results.

A. Abbà, M. Germano, M. Nini

### Analysis of a Synthetic Turbulence Generation Method for Periodic Configurations

With increasingly available computational resources, scale-resolving simulations are beginning to become affordable for industrially relevant flows in turbomachinery. While full Large Eddy Simulations (LES) may still be out of reach, the combination of Reynolds-Averaged Navier-Stokes (RANS) and LES methods is a promising approach. Both LES and zonal RANS-LES methods require the prescription of resolved velocity fluctuations at the inflow or the RANS-LES interfaces. To save computational resources, the flow in a turbomachinery configuration is usually assumed to be periodic in the spanwise or blade-to-blade directions. Synthetic turbulence generators based on Fourier reconstruction of the fluctuating velocity field using random wave number vectors are, by definition, not periodic in a given direction. This leads to a violation of continuity at periodic boundaries of the computational domain and can, in turn, result in abnormal turbulence statistics downstream. Motivated by this deficiency, a simple correction, which restores periodicity, is proposed and its performance with respect to statistical properties of turbulence such as Reynolds stresses, two-point correlations and energy spectra is evaluated.

C. Morsbach, M. Franke

### The Effect of Lossy Data Compression in Computational Fluid Dynamics Applications: Resilience and Data Postprocessing

The field of computational fluid dynamics (CFD) is data intensive, particularly for high-fidelity simulations. Direct and large-eddy simulations (DNS and LES), which are framed in this high-fidelity regime, require to capture a wide range of flow scales, a fact that leads to a high number of degrees of freedom. Besides the computational bottleneck, brought by the size of the problem, a slightly overlooked issue is the manipulation of the data. High amounts of disk space and also the slow speed of I/O (input/output) impose limitations on large-scale simulations. Typically the computational requirements for proper resolution of the flow structures are far higher than those of post-processing. To mitigate such shortcomings we employ a lossy data compression procedure, and track the reduction that occurs for various levels of truncation of the data set.

E. Otero, R. Vinuesa, P. Schlatter, O. Marin, A. Siegel, E. Laure

### Augmented Prediction of Turbulent Flows via Sequential Estimators

Among the numerous research aspects in the analysis of complex flow configurations of industrial interest, the accurate prediction of turbulent flows is one of the ultimate open challenges. Investigation via classical tools, such as experiments and numerical simulation, is difficult because of fundamental drawbacks which can not be completely excluded. Experiments provide a local description of flow dynamics via measurements sampled by sensors. A complete reconstruction of the flow behavior in the whole physical domain is problematic because of the non-linear, strongly inertial behavior of turbulence. While reduced-order models, such as POD (Lumley, Stochastic tools in turbulence. Academic Press, New York, 1970, [4]), have been extensively used for this purpose, they usually provide an incomplete reconstruction of turbulent flows for the aforementioned reasons.

M. Meldi, A. Poux

### High Performance CFD/DEM Approach in Complex Geometries on Unstructured Meshes

Over the years, attention has been drawn to Computational Fluid Dynamics (CFD) as a necessary tool to get a better understanding of Fluidized Bed Reactors (FBR) dynamics and to avoid time-consuming and costly experiments on pilot-scale reactors. In industry, FBR typically consists of ten up to a few tens meter high cylinders also containing more complex parts and gathering several hundred billion particles (see Van Der Hoef et al, Adv Chem Eng J 31:65–149, 2006, [8] for a complete review on FBR modeling). In such complex geometries, unstructured meshes are easier to build and locally refine, but often shunned because of the extra coding effort required. Moreover, the employ of thousands of processors is often essential to ensure sufficient memory and performance of the code.

Y. Dufresne, G. Lartigue, V. Moureau, E. Masi, O. Simonin

### Direct Numerical Simulation of Spherical Bubbles in a Downward Turbulent Channel Flow

Downward bubbly flows occur when the direction of the fluid velocity is opposite to the direction of the rise velocity of the bubbles. This type of flow represents an important element of several industrial processes such as drilling (Rehm et al, The why and basic principles of managed well-bore pressure, 2008, [8]) and reactions in chemical plants (Kunii et al, Bubbling fluidized beds, 2013, [4]). Many differences arise between upward and downward flows and one basic, yet dominant issue is the transfer of energy between the fluid and the bubbles.

C. Santarelli, J. Fröhlich

### DNS of Thermocapillary Migration of Deformable Droplets

A nonuniform distribution of temperature field on a fluid-fluid interface leads to surface tension gradients, which induce shear stresses that produce the motion of a drop in the direction of the temperature gradient. This phenomenon is known as thermocapillary flow or Marangoni migration. In addition to its importance from a fundamental point of view, thermocapillary flows play an important role in micro gravity environments (Subramanian and Balasubramaniam, The motion of bubbles and drops in reduced gravity. Cambridge University Press, Cambridge, 2001, [1]) and micro-devices (Darhuber and Troian, Annu Rev Fluid Mech 37:425–455, 2005, [2]).

N. Balcázar, O. Antepara, J. Rigola, A. Oliva

### The Motion of Settling Particles in Isotropic Turbulence: Filtering Impact and Kinematic Simulations as Subfilter Model

Turbulent two-phase flows with small particles are quite common in environmental and industrial contexts. The dispersed phase is involved in a range of phenomena, including preferential concentration, collisions/agglomeration, and wall deposition. In computations of practical flow cases, beyond relatively low Reynolds numbers and simple geometries, DNS reveals to be overly expensive, even in the point-particle approximation with the one-way momentum coupling. Therefore, LES has gained more and more interest over the years. When feasible, the LES becomes particularly well suited for situations where the solution of instantaneous eddy structures is crucial for the prediction of the particulate phase. The subgrid scales (SGS) may have an impact on the motion of particles, especially those of lower inertia.

J. Pozorski, B. Rosa

### Evaporation Dynamics in Dilute Turbulent Jet Sprays

Evaporation of dispersed droplets within a turbulent flow is of crucial importance in several applications (Jenny et al, Prog Energy Combust Sci 38(6):846–887, 2012, [1]). A typical example consists in designing innovative internal combustion engines, capable to increase combustion efficiency and reduce pollutants emission levels. These goals are directly related to the accurate control of the vaporization process which, in turns, affects the mixing homogeneity. In particular, turbulent sprays are complex multiphase flows in which liquid evaporating droplets are dispersed within a turbulent gaseous phase. The evaporation process occurs via mass, momentum and energy exchanges between the two phases causing the spray dynamics to be a challenging modeling task due to the presence of unsteady, multi-scale and multiphase processes.

F. Dalla Barba, F. Picano

### A Novel Turbulent Inflow Data Generation Method and its Application to the Simulation of Primary Breakup

Direct numerical and large eddy simulation of inhomogeneous flows are very sensitive to turbulent inflow boundary conditions. This contribution presents a new turbulent inflow data generation method based on an auxiliary simulation of forced turbulence. It combines the flexibility of synthetic turbulence generation methods with the accuracy of precursor simulations. The approach allows full control over turbulence properties and provides divergence-free boundary fields, featuring realistic Navier-Stokes dynamics.

S. Ketterl, M. Klein

### Studying Transient Jet Flames by High-Resolution LES Using Premixed Flamelet Chemistry

A transient piloted turbulent non-premixed methane jet flame approaching its blow-off limit is numerically studied by high-resolution Large-Eddy Simulations (LES). In the statistically steady jet phase, the high turbulence intensity leads to local flame extinction and re-ignition events. During the transient phase, the pulsation leads to a global flame extinction soon after the blow-off velocity is reached. The flame then re-ignites when the strain is relaxed. To model turbulent combustion with a minimum set of equations in order to reduce the computational effort, a tabulated detailed chemistry approach is tested.

E. Inanc, F. Proch, A. M. Kempf

### Identification of Combustion Trajectories Using t-Distributed Stochastic Neighbor Embedding (t-SNE)

With increasing computational power, direct numerical and large eddy simulation (DNS and LES) of reacting flows with complex chemistry are becoming common, e.g. Yoo et al (Proc Combust Inst, 34(2):2985–2993, 2013, [1]), Duwig and Iudiciani (Fuel 123:256–273, 2014, [2]), Fooladgar et al (Comput Fluids 146:42–50, 2017, [3]). The resulting data which may occupy hundreds of gigabytes of storage, consists of millions to billions of points each of which is described by tens to hundreds of chemical species. To explore and analyze this large, high-dimensional data, conventional visualization techniques such as scatter plots, histograms and pairs plots are limited. Human visual perception is well tuned to identify patterns and trends in graphs with one or a few data variables at a time, calling for new automated identification tools.

### Impact of Scalar Dissipation Rate on Turbulent Spray Combustion Investigated by DNS

Spray combustion includes a lot of physical processes that occur simultaneously, most prominently injection, atomization, dispersion, evaporation, and combustion. Therefore, it is not sufficient to rely only on experimental techniques for understanding this problem. As a complementary source of information, highly accurate numerical models can be used to perform such investigations. Using high-performance computers (HPC), even parametric studies become possible.

A. Abdelsamie, D. Thévenin

### Modeling of Convective and Conductive Conjugate Heat Transfer in a Kerosene/Air Spray Flame Used for Aeronautical Fire Resistance Tests

Airworthiness standards require a fire resistance demonstration for aircraft or helicopter engines to obtain a type certificate. This demonstration relies on tests performed with prototype engines in the late stages of the development. In these tests, a propane or a kerosene standardized flame with imposed burnt gas temperature and heat flux is placed next to the engine casing during a given time. The aim of this work is to provide a better characterization of a kerosene/air certification burner in order to reach a better understanding of the thermal environment during fire tests.

L. Boulet, P. Bénard, G. Lartigue, V. Moureau, S. Didorally

### Towards the Direct Numerical Simulation of a Simplified Pressurized Thermal Shock

The integrity assessment of a Reactor Pressure Vessel (RPV) is considered to be an important issue for lifetime extension of nuclear reactors. A severe transient that can threaten the integrity of the RPV is the existence of a Pressurized Thermal Shock (PTS) during a Loss-of-Coolant Accident (LOCA) (Shams et al, Nucl Eng Des 300:282–296, 2016, [1]). A PTS consists of a rapid cooling of the RPV wall under pressurized conditions that may induce the criticality of existing or postulated defects inside the vessel wall. The most severe PTS event has been identified by Emergency Core Cooling (ECC) injection during a LOCA. The injected cold water mixes with hot water present in the cold leg, and flows towards the downcomer, causing further thermal mixing and, therefore, large temperature gradients. This sudden change in temperature may induce high stresses in the RPV wall, leading to the propagation of flaws inside the vessel wall, especially in the embrittled region adjacent to the core. A proper knowledge of these loads is important for the RPV remnant lifetime assessment.

A. Shams, E. M. J. Komen

### Study of the Flow Around a Heated Cylinder in Mixed Convection Regime

Overhead Lines (OHL) are a key component to transmit and distribute electrical energy. The aero-thermal design of OHL is generally carried out considering the cable as a perfect heated cylinder immersed in a uniform cross flow (IEEE standard for calculating the current-temperature relationship of bare overhead conductors, 2013, [1]). Moreover, The design also assumes that only two regimes can occur, depending on the Reynolds number (based on the line diameter and the incoming velocity): (a) natural convection for low velocities, e.g. below a wind speed of 0.6 m/s which roughly corresponds to a Reynolds number of 850; (b) forced convection at higher Reynolds numbers. In both of these cases analytical correlations exist to compute the Nusselt number.

S. Rolfo, K. Kopsidas, S. A. Rahman, C. Moulinec, D. R. Emerson

### Direct Numerical Simulation of Convective Turbulent Channel Flow of Fluid Mixtures

Fluid mixtures play an important role in a large variety of applications ranging from atmospheric flows to intracellular transport mechanisms. At the same time they present a significant challenge with regards to modeling and computational costs. In this work, the feasibility of treating a mixture of two fluids with different fluid properties by considering one species as the carrier fluid and treating the effect of an added trace species as a perturbative modification is explored. Towards this end, the concentration of the trace constituent is introduced as an active scalar within the flow and used as an expansion parameter analogously to the temperature in the Boussinesq approximation. For the example of a mixture of water vapor and dry air, the effects due to the added buoyancy are investigated using direct numerical simulations (DNS) of turbulent channel flow. The limitations concerning the validity of the strict Boussinesq approximation are determined, and an extended formulation is employed to explore higher order effects which become significant towards and outside of the limits of this range.

P. Bahavar, C. Wagner

### Momentum and Buoyancy Repartition in Turbulent Mixed Convection

Turbulent mixed convection is a common phenomenon in applications such as heat exchangers and climatisation in transport vehicles and buildings and was often studied in the past. Metais and Eckert (J Heat Transf 86(2):295–296, 1964, [6]) experimentally analyse the heat transport in a vertical pipe heated from the outside.

T. Wetzel, C. Wagner

### Buoyancy-Driven Flow Inside An Asymmetrically Heated Cavity

Buoyancy-driven flows inside enclosures are in the center of problems related to heat transfer because they can provide a significant insight into the physical mechanisms of heat transfer. Typical examples of such flows include Rayleigh–Bénard convection, differentially heated cavities and partially divided enclosures. In the present study, the buoyancy-driven flow inside an asymmetrically heated closed cavity is investigated and proposed as a benchmark case for future studies to assess the accuracy of simulations and to help in the validation of coarsened turbulence models. Additionally, from an application perspective such a configuration is highly relevant, e.g., in view of its similarity with passive solar systems such as ventilated building facades (Puangsombuta et al, J Fluid Mech 42(6):2218–2226, 2007, [1]) and Trombe walls (Zamora and Kaiser, Heat Mass Transf 45(11):1393–1407, 2009, [2]).

A. D. Demou, D. G. E. Grigoriadis, B. J. Geurts

### LES of Natural Convection in a Closed Cavity

Natural convection cavity flows are of interest due to their occurrence in a range of engineering situations, such as in gas and steam turbines during shutdown. These natural convection flows can affect the operation of gas and steam turbines so reliable prediction of these flows is desirable. Accurate prediction of the wall heat transfer is of particular importance to engineers.

A. Pilkington, B. Rosic

### Polynomial Adaptivity in LES: Application to Compressibility Effects Investigation on Bluff Bodies

The use of a Discontinuous Galerkin framework to perform a compressible Large Eddy Simulation is an effective and accurate way to simulate complex turbulent flows. However, since the turbulent scales size is not known a priori, assumptions during the grid generation must be made. With the aim of reducing the influence of such assumptions, lowering the computational effort required to perform the LES and moving towards the adaptive LES postulated by Pope, New J Phys, 6(1):35, 2004, [5], we introduced a polynomial adaptive framework in Tugnoli, J Comput Phys, 349:33–58, 2017, [7]. In the present work we employ the aforementioned procedure to assess the effects of a varying Mach number in a flow around a square section cylinder, a configuration of interest for example for flame holders in combustors. Since most of the reference data on this kind of flows is obtained by incompressible computations (e.g. Rodi et al, J Fluids Eng, 119(2):248–262, 1997, [6]) and experiments (e.g. Lyn et al, J Fluid Mech, 304:285–319, 1995, [4]) the aim is to assess what are the effects of slightly higher Mach numbers in such flows, and what is the error involved into comparing compressible simulations results with incompressible reference data.

M. Tugnoli, A. Abbà

### Direct Numerical Simulation of Compressible Flows Around Spherical Bodies Using the Immersed Boundary Method

The three-dimensional flow around a sphere is one of the most classical subjects of investigation for fundamental analysis of external aerodynamics. In fact this flow configuration, which is described by a very simple geometrical shape, exhibits the potential for complex multi-physics analysis. Some aspects that can be investigated include turbulence, acoustics and heat transfer, and this test case is particularly favorable for the analysis of coupled problems. In addition, the emergence of a number of different regimes is observed for moderate Reynolds number, which are extremely sensitive to the Mach number Ma investigated. Furthermore, multiple physical systems can be modeled by multi-spherical bodies in motion involving complex interactions. Owing to this large number of aspects which are relevant for industrial applications, this case represents an important benchmark for validation of new numerical/modeling strategies.

H. Riahi, E. Constant, J. Favier, P. Meliga, E. Serre, M. Meldi, E. Goncalves

### Large Eddy Simulation of Highly Compressible Jets with Tripped Boundary Layers

In high-speed aircraft, supersonic jets used for propulsion can lead to very intense aerodynamically generated acoustic noise. Thus, there is a need to study the aerodynamic and aeroacoustic properties of highly compressible jets. In previous studies (Gojon et al, Temperature effects on the aerodynamic and acoustic fields of a rectangular supersonic jet, 2017, [1], Gojon et al, On the response of a rectangular supersonic jet to a near-field located parallel flat plate, 2017, [2]), several simulations of supersonic jets have been conducted. Unfortunately, the turbulence intensity at the nozzle exit was dependent on the internal geometry of the nozzle and could not be tuned. This is a pity given that, as shown experimentally (Zaman, AIAA J, 50(8):1784–1795, 2012, [3]) and numerically (Bogey et al, J Fluid Mech, 701:352–385, 2012, [4], Brés et al, Nozzle wall modeling in unstructured large eddy simulations for hot supersonic jet predictions, 2013, [5]) for subsonic and supersonic jets, the boundary layer state of the jet affects the jet flow and noise.

R. Gojon, C. Bogey, M. Mihaescu

### Analysis of Dense Gas Effects in Compressible Turbulent Channel Flows

In this work we investigate the influence of dense gas effects on compressible wall-bounded turbulence. Turbulent flows of dense gases represent a research field of great importance for a wide range of applications in engineering. Dense gases are single-phase fluids with a molecular complexity such that the fundamental derivative of gas dynamics (Thompson, Phys Fluids, 14:1843–1849, 1979, [1]) $$\varGamma := 1 + \frac{\rho }{c} \left. \frac{\partial c}{\partial \rho }\right| _s$$ (where $$\rho$$ is the density, p the pressure, s the entropy, and c the sound speed), which measures the rate of change of the sound speed in isentropic transformations, is less than one in a range of thermodynamic conditions close to the saturation curve. In such conditions, the speed of sound increases in isentropic expansions and decreases in isentropic compressions, unlike the case of perfect gases. For dense gases, the perfect gas model is no longer valid, and more complex equations of state must be used to account for their peculiar thermodynamic behavior. Moreover, in the dense gas regime, the dynamic viscosity $$\mu$$ and the thermal conductivity $$\lambda$$ depend on temperature and pressure through complex relationships. Similarly, the approximation of nearly constant Prandtl number Pr= $$\mu c_p/\lambda$$ is no longer valid. Numerical simulations of turbulent dense gas flows of engineering interest are based on the (Reynolds-Averaged Navier–Stokes) RANS equations, which need to be supplemented by a model for the Reynolds stress tensor and turbulent heat flux. The accuracy of RANS models for dense-gas flows has not been properly assessed up to date, due to the lack of both experimental and numerical reference data.

L. Sciacovelli, P. Cinnella, X. Gloerfelt

### Effect of Inflow Turbulence on LES of an Airfoil Flow with Laminar Separation Bubble

The turbulence intensity of the incoming flow can have a strong impact on the arising flow field around bodies especially when transition to turbulence plays an important role. A classical example is the flow past airfoils at moderate Reynolds numbers (e.g., for micro air vehicles), where laminar separation bubbles are often observed in the boundary layer on the suction side. For such cases experimental investigations carried out in different wind or water tunnels typically show strong variations of the separation, transition and reattachment locations, which to a great extent is caused by different levels of the turbulence intensity of the oncoming flow. Beside these deviations observed due to the natural turbulence level of the facility used, the effect of inflow turbulence becomes even more significant when the turbulence intensity is artificially increased for example by active or passive grids in order to mimic an atmospheric boundary layer. During the last years an increasing interest to simulate these flow phenomena is observed. Eddy-resolving simulations such as LES or hybrid LES-URANS are in principle the right choice for this challenging task. However, the inflow turbulence has to be prescribed in such a manner that it reaches the region of interest. Recently, a source term formulation (Schmidt and Breuer, Comput Fluids, 146:1–22, 2017, [9]) based on a synthetic turbulence inflow generator (STIG) was suggested. It allows to superimpose turbulent fluctuations in finer resolved flow regions, where the damping of small structures due to an inadequate grid resolution is negligible. This technique is applied here to investigate the flow past a SD7003 airfoil at Re $$_c$$ = 60,000 and an angle of attack $$\alpha = 4^{\circ }$$ for a wide range of turbulence intensities of the oncoming flow ( $$0 \le TI \le 11.2\%$$ ). The results of the reference case without inflow turbulence are compared with the predictions with increasing turbulence intensities.

M. Breuer, S. Schmidt

### Flow Around Thick Airfoils at Very High Reynolds Number. Stall and Dynamic Stall Applications

With the increase of the power and rotor diameter of modern wind turbines, blade loads must be predicted with high confidence in order to optimize accurately the complex blade internal structure. Unsteady aerodynamic loadings such as dynamic stall are the main challenges for state-of-the-art numerical tools (Leishman, Challenges in modeling the unsteady aerodynamics of wind turbines, 2002, [5]). Dynamic stall can appear on horizontal-axis wind turbines (HAWT) in several operating conditions: misalignment with the wind direction, free-stream turbulence, fast pitch maneuvers... Wind tunnel experiments and RANS or URANS simulations are the state-of-the-art tools to obtain estimations of aerodynamic forces, specifically in stall and dynamic stall cases. The present work aims at getting a better insight into the dynamics of the flow around thick wind turbines airfoils thanks to Large-Eddy Simulation (LES), which resolves a broader range of turbulent scales. These thick airfoils operate at very high Reynolds number because of the dimensions of the rotor. In order to perform LES with realistic CPU time, a Wall-Modeled LES (WMLES) strategy is considered. Several simulations are carried out at Reynolds number of $$1.6\cdot 10^6$$ on the FFA-W3-241 profile, a $$24.1\%$$ relative thickness profile. Attached flow is first investigated, then detached flow in steady and oscillating conditions are studied. The impact of spanwise length is considered, in particular for stalled cases.

F. Barnaud , P. Bénard, G. Lartigue, V. Moureau, P. Deglaire

### On the Resolution of Mean Skin Friction by Hybrid RANS/LES Simulations at High Reynolds Numbers

This study addresses the following question: how much do the RANS model and the LES-resolved fluctuations contribute to mean skin friction when a Wall-Modelled LES is performed by means of a hybrid RANS/LES strategy (RANS near-wall modelling, LES outer layer resolution)? The paper relies on both Wall-Resolved and Wall-Modelled LES obtained by means of the Zonal Detached Eddy Simulation technique, together with very high Reynolds number predictions provided by RANS boundary layer simulations. A scale decomposition is performed by applying a spectral analysis to a recent physical decomposition of mean skin friction involving the production term of turbulent kinetic energy. The results suggest that half of mean skin friction may be resolved by some hybrid RANS/LES methods used as WMLES at very high Reynolds numbers.

N. Renard, S. Deck

### DNS of Separated Low-Re Flow Around a Cambered Aerofoil

Recent improvements in manufacturing and control technology have led to the introduction of a new class of micro and even nano sized air vehicles (Micro/Nano UAVs) operating in flight conditions characterized by a medium-to-low Reynolds number and by background flow unsteadiness that may lead to critical aerodynamic conditions such as massive separation, dynamic stall and aerodynamic hysteresis. Although the effect of low Reynolds number regimes and its impact on boundary layer separation and flow unsteadiness have been the subject of many previous studies, fully stalled conditions did not receive much attention in the past. Using Direct Numerical Simulation, the present contribution addresses the spatio-temporal characterisation of the flow around a cambered aerofoil in deeply stalled conditions. A joint analysis of the flow structure and of the velocity time signal sampled in different locations allows to highlight the presence of a number of unsteady features that govern the overall behaviour of the flow field.

M. F. Shahab, M. Omidyeganeh, A. Pinelli

### High Reynolds Number Airfoil: From Wall-Resolved to Wall-Modeled LES

Wall-Modeled Large-Eddy Simulation (WMLES) alleviates the near-wall grid requirement by employing a wall-model to reconstruct the wall shear-stress. In this way, WMLES simultaneously reduces the computational cost associated with Wall-Resolved LES (WRLES) and opens the door towards higher Reynolds numbers (Piomelli, Wall-modeled large-eddy simulations: present status and prospects. Springer, Netherlands, 2010, [1], Larsson et al, Mech Eng Rev, 3, 2016, [2].

A. Frère, K. Hillewaert, P. Chatelain, G. Winckelmans

### Robust Feedback Control of Two and Three Dimensional Flow Separation Around a NACA0012 Profile Using Plasma Actuators

Closed-loop flow control is aimed at altering a natural flow state into a more desirable state, which is chosen depending on control objectives. The control input is usually an electric signal, which has to be converted to a physical quantity by means of an actuator. A new and original technology using non-thermal surface plasmas has witnessed a significant growth in interest in recent years, as they: have no moving parts; exhibit an extremely fast time-response; are characterised by low mass and low input power. These surface dielectric barrier discharge (DBD) actuators are used to accelerate the near-wall flow, thus modifying the velocity profile within the boundary layer. In this paper, we focus on the robust feedback control of the flow separation using plasma actuators. Our objective is to solve the problem of directly controlling the unsteady flow separation using real-time velocity measurements, which are available in realistic applications. We propose this flow separation problem as a practical application of the new theoretical results in Marino and Tomei, Automatica, 60(8):2213–2218, 2015, [4]. The aim of this paper is to show how, despite the high complexity of the system, a simple robust output regulator is sufficient to effectively suppress the flow separation along an aerofoil, using two actuator/sensor pairs. Accurate two-dimensional (laminar flow) and three-dimensional (turbulent flow) numerical simulations of incompressible flows on a NACA0012 at Reynolds $$Re=20{,}000$$ are performed in order to illustrate the effectiveness of the proposed approach. In the two-dimensional case a robust, fast flow reattachment is achieved, along with both stabilisation and increase/reduction of the lift/drag, respectively. The control system shows good dynamic performances, as the angle of attack is varied. For the three-dimensional test, a Large Eddy Simulation (LES) approach has been chosen for the modelling of the turbulence dynamics, whereas the eddy viscosity is calculated according to the well established classical Smagorinsky model.

R. Broglia, D. Durante, L. Pasquale

### Performance Analysis of a Heaving Wing Using DNS and LES

The weight and structure of civil transport aircraft are mainly dictated by the loads they experience at the limit of the design envelope (e.g. turbulence, gusts and manoeuvres). During the design process aircraft loading is often predicted using simplified models. However, under extreme operating conditions aircraft experience erratic unsteady loads that are not well understood, and hence difficult to predict using state of the art reduced models. Achieving a more profound understanding of the flow structures dictating the aerodynamic loads under extreme conditions will be crucial for the design of efficient aircraft. An example of such conditions occurs when a wing starts heaving or encounters a gust near its stall angle (e.g. during landing); unsteady flows and transient effects, including flow instabilities and vortex shedding, take place, making the aerodynamic loads highly unpredictable (see for example Von Ellenrieder et al, J Fluid Mech, 490:129–138, 2003, [1]). These complex physical mechanisms can only be fully captured by experiments and accurate numerical simulations, both of which are currently expensive to be used during the design stages, but can provide useful insight. Direct numerical simulations (DNS), being free from simplified modelling assumptions, have the advantage of including all the relevant flow physics. On the other hand, the large eddy simulation (LES) technique can be used to simulate realistic flow conditions, but its accuracy in predicting complex flow phenomena including flow instability and vortex shedding is not clear. This work focuses on assessing the performance of OpenFoam’s LES solver for the prediction of the aerodynamic loads acting on a heaving wing at incidence through comparisons with DNS and experimental results.

N. De Tullio, Z. Xie, J. Chalke, N. D. Sandham

### A Numerical Study of Low-Aspect-Ratio Flapping-Wings in Forward Flight

Unsteady aerodynamics at low Reynolds number is receiving an increasing attention from the scientific community due to the recent development of Flapping Micro Air Vehicles (FMAV). These vehicles generate thrust and lift by flapping their wings, like insects or small birds do. The specific unsteady aerodynamic mechanisms involved in the generation of forces in flapping flight have been studied by several authors, as reviewed in [6]. However, our understanding of flapping flight is still limited.

A. Gonzalo, G. Arranz, M. Moriche, O. Flores, M. García-Villalba

### The Influence of the Reynolds Number on the Auto-Rotation of Samaras

As samara seeds fall from trees, they enter into auto-rotation. In this way they maximize dispersal distances (Green, Am J Bot 169:1218–1224, 1998, [2]). In this study, we present direct numerical simulations of the autorotation of a model samara at Reynolds numbers, Re, somewhat smaller than those observed in nature. The aim is to detect the presence of a LEV, if it exists, and to understand how the kinematics of the seed are related to the vortical structures of the flow.

G. Arranz, M. Moriche, M. Uhlmann, O. Flores, M. García-Villalba

### A Priori Analysis and Benchmarking of the Flow Around a Rectangular Cylinder

The flow around bluff bodies is recognized to be a rich topic due to its huge number of applications in natural and engineering sciences. Of particular interest is the case of blunt bodies where a reattachment of the separated boundary layer before the definitive separation in the wake occurs. One of the main feature of this type of flows is the combined presence of small scales due to the occurrence of self-sustained turbulent motions and large scales due to classical vortex shedding. The complete understanding of these multiple interacting phenomena would help for a correct prediction and control of relevant features for engineering applications such as wind loads on buildings and vehicles, vibrations and acoustic insulation, heat transfer efficiency and entrainment. Archetypal of these kind of flows is the flow around a rectangular cylinder. Many studies have been carried out in the past. The general aim is the understanding of the main mechanisms behind the two unstediness of the flow, the shedding of vortices at the leading-edge shear layer and the low-frequency flapping mode of the separation bubble, see e.g Cherry et al (J Fluid Mech, 144:13–46, 1984, [1]), Kiya and Sasaki (J Fluid Mech, 154:463–491, 1985[2]), Nakamura et al (J Fluid Mech, 222:437–447, 1991, citeNakamura).

A. Cimarelli, A. Leonforte, D. Angeli

### Benchmark on the Aerodynamics of a 5:1 Rectangular Cylinder: Further Experimental and LES Results

The flow around a rectangular cylinder, having chord-to-depth ratio equal to 5, has been the object of a benchmark (BARC) launched in 2008 ( http://www.aniv-iawe.org/barc/ ). The BARC configuration is of practical interest, e.g. in civil engineering, and, in spite of the simple geometry, the related flow dynamics and topology is complex. Indeed, the high-Reynolds-number flow around such a stationary rectangular cylinder is turbulent with flow separation from the upstream corners and unsteady reattachment on the cylinder side. Furthermore, a vortex shedding also occurs from the rear corners and interferes with the leading-edge vortices, according to the mechanism of impinging shear-layer instability (Nakamura et al, J Fluid Mech, 222:437–447, 1991, [1]).

C. Mannini, A. Mariotti, L. Siconolfi, M. V. Salvetti

### Large-Eddy Simulation of a Sheared Air-Water Flow Around a Cylinder

The objective of the present work is to study wake dynamics behind a surface-mounted circular cylinder immersed between two highly stratified fluids in the crossflow, by performing LES. The focus is to evaluate the degree of interference and interaction caused by the presence of obstacles and of the air-water interface over incoming turbulent boundary layers, while keeping the interface flat.

S. López Castaño, V. Armenio

### Scaling Laws in the Axisymmetric Wake of a Sphere

An axisymmetric turbulent wake is often assumed to be self-similar when the streamwise location x is sufficiently far from the wake generator. Thus, profiles of the single-point statistics have the local wake width, $$\delta (x)$$ , and the centerline streamwise mean defect velocity, $$U_0 (x)$$ , as the characteristic length and velocity scales. Under self-similarity, the evolution of the scaling parameters, $$U_0$$ and $$\delta$$ , is described by power laws: $$U_0/U_\infty \sim x^m$$ and $$\delta /D \sim x^n$$ where D is a characteristic length scale of the body.

K. Chongsiripinyo, A. Pal, S. Sarkar

### Hybrid Versus Pure-LES Models Comparison for Subcritical Cylinder Flows

In Computational Fluid Dynamics applications, there is a need for turbulence models which deliver good predictions for flows involving both laminar and turbulent boundary layers, without knowing in advance the regions where turbulence occurs, and then without changing their parameters according to such an a priori knowledge of the flow characteristics. In this work, which extends the study (Itam et al., 6th Symposium on Hybrid RANS-LES Methods, Strasbourg, France, pp. 26–28, 2016, [7]), we are interested in the assessment of hybrid models for the computation of subcritical flows with laminar boundary layers and in the improvement of the wake behavior prediction in a hybrid RANS/LES model (Itam et al, Application of a hybrid variational multiscale model to massively separated flows. 3AF, Toulouse, France, 2015, [6], Itam et al, 6th Symposium on Hybrid RANS-LES Methods, Strasbourg, France, pp. 26–28, 2016, [7], Moussaed et al, J Fluids Struct 47:114123, 2014, [10]). The performances of a DDES model are compared with a dynamic variational multi-scale (DVMS) large eddy simulation model.

E. Itam, S. Wornom, B. Koobus, A. Dervieux

### Modeling of Wind Gusts for Large-Eddy Simulations Related to Fluid-Structure Interactions

The dimensioning of lightweight structures under wind loads strongly depends on realistic flow conditions. Two different setups have to be distinguished. For a long-term analysis such as dynamic fatigue, temporally and spatially correlated velocity distributions are required as inflow conditions for a large-eddy simulation (LES) to mimic a realistic physical setup (Wood et al, Flow Turbul Combust 97(1):79–119, 2016, [9]).

G. De Nayer, M. Breuer, P. Perali, K. Grollmann

### Dissipation in Front of a Wall-Mounted Bluff Body

The dissipation rate is required to assess the Kolmogorov scales, the smallest scales of motion in turbulent flow. A priori knowledge about these scales is needed to design both experiments and numerical simulations. However, an explicit evaluation of the dissipation is difficult. Direct Numerical Simulation (DNS) without turbulence model is still rare for practical flow problems.

W. Schanderl, M. Manhart

### Dynamic Unified RANS-LES Simulations of Periodic Hill Flow

The hybrid RANS-LES methodology intends to combine the most favorable aspects of Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) to take advantage of both the computational efficiency of RANS and ability of LES to resolve instantaneous large scale flow structures. In this paper we present a new hybrid RANS-LES model which benefits from two important properties.

R. Mokhtarpoor, S. Heinz, M. K. Stoellinger

### DNS of Separated Flow: Scale-by-Scale Analysis

The Direct Numerical Simulation (DNS) of a turbulent channel containing a bump is carried out to study the turbulence dynamics in the separated region. Our study is intended to fill the gap among the idealised conditions, e.g. plane channel flows typically addressed in literature and the actual flow geometries where the effects of wall curvature and the presence of bluff bodies immediately generate a substantial separated flow region in the bulk of the flow and produce turbulent wakes behind the bluff body. Such configuration allows retaining, with the minimum level of complexity, all the basic features of separated wall-bounded flows such as the presence of the recirculating region that acts as the main energy source for the turbulent velocity fluctuations in the bulk of the flow and in the wake behind the bump.

J.-P. Mollicone, F. Battista, P. Gualtieri, C. M. Casciola

### Investigation of Turbulent Flow Over Two Wall-Mounted Cubes Using LBM

Large eddy simulation (LES) of a turbulent flow over two cubic prisms is performed based on the Lattice Boltzmann Method (LBM). Two wall-mounted cubic prisms are arranged in tandem on the bottom wall of a fully developed turbulent channel flow for a Reynolds number of ReH $$=$$ 4000 (based on the centerline velocity, Uc, and the prism height, H). One major objective of the present study is to investigate the typical flow patterns around the cubic prisms and vortex structures in the wake region. Although the flow geometry is symmetric, the wake interaction introduces asymmetric behavior into the instantaneous flow.

M. Teng, D. J. Bergstrom

### Drag Reduction of Boat-Tailed Bluff Bodies Through Transverse Grooves

The present work describes a strategy for the aerodynamic drag reduction of elongated axisymmetric bluff bodies, which can be viewed as simplified models of road vehicles. One well-known method to reduce the drag of this type of body is a geometrical modification denoted as boat-tailing, which consists in a gradual reduction of the body cross-section before a sharp-edged base (Maull et al, J R Aeronaut Soc 71, 854–858, 1967, [1], Mair, Aeronaut Q 20, 307–320, 1969, [2], Wong and Mair, J Wind Eng Ind Aerodyn, 12, 229–235, 1983, [3]). We combine herein boat-tailing with properly contoured transverse grooves to further delay boundary-layer separation and to reduce drag.

A. Mariotti, G. Buresti, M. V. Salvetti

### Flow Over a Realistic Car Model: WMLES Assessment and Turbulent Structures

Most CFD research on automotive external aerodynamics has been carried out using very simplified models such as the Ahmed car (Aljure, Lehmkuhl, Rodríguez, Oliva, Comput Fluids, 96:122–135, (2014), [1]). To reduce the gap between the production cars and models used for academic purposes, the DrivAer car model was introduced in 2012. References (Heft, Indinger, Adams, SAE Tech Pap, No. 2012-01-0168, (2012), [2], Strangfeld, Wieser, Schmidt, Woszidlo, Nateri, Paschereit, SAE Tech Pap, No. 2013-01-1251, (2013), [3]) performed experimental observations on this geometry. Their results suggested that the Reynolds number dependency of the force coefficients decreased as Re increased. Two values have been reported for Re number independence, $$Re=2\times 10^6$$ (Strangfeld, Wieser, Schmidt, Woszidlo, Nateri, Paschereit, SAE Tech Pap, No. 2013-01-1251, (2013), [3]) and $$Re=4.87\times 10^6$$ (Heft, Indinger, Adams, SAE Tech Pap, No. 2012-01-0168, (2012), [2]).

D. E. Aljure, J. Calafell, A. Báez, A. Oliva

### Numerical Study of the Flow Around Ahmed Bodies with Hybrid Turbulence Models

The aim of this work is to explore the efficiency of different improved Reynolds-Averaged Navier-Stokes (RANS) and hybrid LES/RANS approaches to study the external aerodynamics related to ground vehicles. These computational techniques should be able to build a bridge between accuracy and robustness in order to compute complex high Reynolds number bluff-body flows like ground vehicle flows. Bluff body flows are characterized by separated regions, containing wide spectra of turbulent scales. These regions, especially in the wake behind the body, are responsible for the main part of the drag forces. An accurate computation of these areas is a difficult task.

F. Delassaux, I. Mortazavi, V. Herbert, C. Ribes

### Large Eddy Simulation of a Compressor Blade Passage Operating at Low Reynolds Number

This paper presents the preliminary results from the numerical study of an axial compressor operating at low maximum Reynolds number of $$5\times 10^4$$ , flow coefficient $$\phi = 0.32$$ and head coefficient of $$\psi =0.28$$ . The overall goal is to use high fidelity simulations to accurately calculate the acoustic sources responsible for noise generation in the compressor. However, this is a challenging task and the first step is to gain confidence in the accuracy of the LES with respect to mesh resolution and sub-grid scale modelling. The objective of this study is therefore to assess the effect of mesh resolution and subgrid scale models on the flow field. The main points of interest are to accurately resolve the unsteady wall pressure spectrum at the blade trailing edge, as well as turbulent length scales and intensities in the wake. These results can be used as direct inputs to analytical noise propagation theories to predict the generated noise in the far-field.

O. Wilsby, S. Rolfo, A. Agarwal, P. Harley, C. Moulinec

### Eddy Resolving Simulations of Intake Under Crosswinds

Modern aircraft engine designs with higher bypass ratio and lower fan pressure ratio offer significant fuel burn benefits of upto 25%. To compensate for the additional increase in the drag and weight, relatively shorter intakes and nacelles are employed. However, shorter intakes suffer under off-design conditions of high incidence and crosswinds. The flow experiences severe acceleration around the intake lip, relaminarization, flow separation and transition to turbulence. Using eddy resolving simulations, the current paper aims to capture the flow over the intake lip under crosswinds and further explore effects of the Reynolds number.

N. R. Vadlamani, P. G. Tucker

### On Stability and Transition in Bent Pipes

This work is concerned with the investigation of the instability and transition to turbulence of the viscous, incompressible flow inside curved pipes. For the first time, the impact of the curvature is analysed over the whole parameter space, presenting new results for both the steady flow and the instabilities encountered by this flow.

J. Canton, R. Örlü, P. Schlatter

### Scaling of High-Order Statistics in Turbulent Pipe Flow

Direct numerical simulations of turbulent pipe flow involving friction Reynolds numbers of $$Re_\tau$$ =180,360,720,1500 were carried out and investigated in terms of high-order statistics. A logarithmic dependency on the Reynolds number was found for the streamwise Reynolds stress where $$Re_\tau \ge 360$$ , the streamwise skewness and the wall-normal flatness for $$Re_\tau \ge 360$$ . The scaling failure of the latter quantities is related to large-scale outer flow motions that become important at high Reynolds number flow and penetrate into the near-wall region. For the lowest Reynolds number $$Re_\tau =180$$ the streamwise Reynolds stress peak and the wall-normal flatness at the wall exhibited discrepancies to values obtained from channel flow simulations, which can be explained by the different flow geometry interacting with the wall structures that are of large size compared with the geometry at such low Re.

C. Bauer, C. Wagner

### Turbulent-Drag Reduction by Oblique Wavy Wall Undulations

Reducing the turbulent skin-friction drag over civilian aircraft is a potentially high-reward target, as this drag component accounts for about half of the total drag in cruise conditions. Thus, even modest reductions convert into material savings, resulting in significant cuts in costs. Active-control techniques can be remarkably effective at suppressing turbulence and drag, but pose major engineering challenges in terms of actuation, efficient operation, reliability and maintainability. In contrast, passive techniques based on riblets are easier to implement, but face important durability and maintenance limitations related to the extremely small spacing of the grooves. The alternative passive-control method that is the subject of the present paper was first proposed in Chernyshenko (Drag reduction by a solid wavy wall emulating spanwise oscillations. Part 1. [physics.flu-dyn]( arXiv:1304.4638 ), (2013), [1]). The key characteristic of the method is that it involves wavy surface undulations directed obliquely to the mean flow and having wave lengths two orders of magnitude larger than riblets, and would thus be much more practical to manufacture and maintain.

S. Ghebali, S. I. Chernyshenko, M. A. Leschziner

### Estimation of the Roughness Function in Turbulent Flows Using the Slope of the Roughness

In the last decades, important efforts have been made to better understand the effects of surface roughness on the mean flow. These studies have been performed investigating turbulent channel flows, turbulent boundary layers or pipe flows. The most evident effect of the roughness is the increase of the overall resistance, corresponding to a decrease of the mean streamwise velocity profile in the logarithmic region. This reduction is known as roughness function $$\varDelta U^+$$ (the symbol $$^+$$ represents quantities made non dimensional using the friction velocity $$u_{\tau }$$ , or the viscous length scale $$\nu /u_{\tau }$$ ).

M. De Marchis, B. Milici, E. Napoli

### Large-Eddy Simulation of Reactive Plume Dispersion Over Hypothetical Urban Areas

Air pollution poses major threat to premature mortality (Lelieveld, Evans, Fnais, Giannadaki, Pozzer, Nature, 525:367–371, (2015), [8]) but its levels over $$80\%$$ of cities are unhealthy (WHO: WHOs urban ambient air pollution database - update 2016. In: World Health Organization (2016). www.who.int/phe , Cited 1 July 2017, [15]). Although large-scale computational fluid dynamics (CFD) models are the common research solutions to detailed air quality studies (Russell, Annu Rev Energy Environ, 22:537–588, (1997), [12]), analytical models offer quick screening tools that are surrogates for prohibitively expensive sensitivity tests in practice (Moonen, Allegrini, Environ Model Softw, 72:77–91, (2015), [9]). The Gaussian models, which are analytical tools developed based on open terrain and chemically inert pollutants, have been well received in the industry for decades (Roberts, Proc Roy Soc Lond A, 104:640–654, (1923), [11]). Their results for chemically reactive pollutants over urban areas must be interpreted cautiously.

C. H. Liu, Z. Wu, Y. K. Ho

### Large-Eddy Simulation of an Open Channel Flow with Submerged Rigid Vegetation

Flexible slender structures embedded in fluid flows are everywhere. Hairy surfaces are abundant in nature and perform multiple functions: e.g. thermal regulation, water harvesting, flow sensing, transport of species. While a great deal is known about the interplay between individual fibres and fluid flows, considerably less work has been done on flows interacting with ensembles of fibres, such as fur, hair, feathers or vegetation canopies. The particular case of vegetative canopies in river flows represents a paradigmatic example of the wide range of possible interactions between flows and ensembles of slender elements and how nature or human intervention can exploit this interplay to either mitigate disruptive events or to enhance the efficiency of a number of transport processes highlighting the importance of canopy-flow interactions on societal and environmental issues and the advantages that can be achieved by properly managing their reciprocal action.

A. Monti, M. Omidyeganeh, A. Pinelli

### Detached Eddy Simulations of the Flow Around the Japan Bulk Carrier (JBC)

The accurate prediction of the velocity field in the wake of a ship is a challenge of crucial importance since it affects the optimal design of the propeller rotating in a non-uniform velocity field. This non-uniformity causes temporal variations of the propeller thrust and torque and, consequently, possible vibrations and fatigue of the propulsive system.

E. Guilmineau, G. B. Deng, P. Queutey, M. Visonneau, J. Wackers

### Large Eddy Simulation of a Tornado Flow Around a Train

A tornado is a destructive rotating column of air extending from a cloud to the ground. It may kill people and damage property. Also there are possibilities of derailment of trains. Some train-turnover accidents are suspected to have been caused by tornadoes in Japan. For instance, a train of the JR Uestu line was overturned by a gust in 2005. According to the accident analysis, a tornado or downburst probably generated the gust (Aircraft and railway accidents investigation commission: railway accident analysis report RA2008 -4 (2008), (in Japanese), [1]). In 2006, a train of the JR Nippo line was overturned by a tornado (Aircraft and railway accidents investigation commission: railway accident analysis report RA2008 -6 (2008), (in Japanese), [2]).

K. Obara, S. Krajnovic, G. Minelli, B. Basara, N. Okura, M. Suzuki

### Large Eddy Simulation of a Wind Farm Experiment

The growing interest in renewable resources [1] is encouraging research on wind turbines. The present work is focused on the turbine wake because it plays a key role in the power production of the entire wind farm. The wake is strongly turbulent and it persists more than fifteen rotor diameters (D) downstream (Chamorro, PortéAgel, Boundary Layer Meteorol, 132(1):129–149, (2009), [2]), while the distance between two “in line” turbines is much less than this length. Therefore, the downstream turbines are impinged by a velocity that is completely different from the undisturbed one and the turbine operates off-design. A good representation of the evolution and decay of the turbine wake is needed to accurately predict the turbine performance.

B. Rocchio, U. Ciri, M. V. Salvetti, S. Leonardi

### On Direct Aeroacoustics Calculations of the Vocal Tract

Voice production and the verbal expression through speech are crucial components of human communication. The human voice is not just conveying information directly through words, but also indirectly as paralinguistic information such as the speaker’s emotional state through tonality (Zhang, J Acoust Soc Am, 140(4):2614–2635, (2016), [7]). As such, voice is generated through a two-part process: First, a source signal is produced by the vocal folds that are pulsating the lung pressure and volumetric flow rate in a particular frequency through periodic opening and closing. Second, the vocal tract causes an attenuation or amplification of this source signal at certain frequencies depending on its specific shape. The voice generation process can therefore be described by a source-filter model with the vocal folds acting as the source and the vocal tract as an acoustic filter (Titze, Alipour, The myoelastic aerodynamic theory of phonation. National Center for Voice and Speech, (2006), [6]). Thus, we are able to produce different vowels and sounds as we manipulate the vocal tract during phonation.

L. Schickhofer, A. Dahlkild, M. Mihaescu