New Results in Numerical and Experimental Fluid Mechanics XIV

The effects of the wing skin distortion on the boundary layer of a highly flexible wing are analyzed in a wind tunnel experiment using infrared thermography measurements. Considerable differences in the boundary layer flow are observed when comparing the sections of the wing near the ribs, where the design shape of the wing is preserved, and in between the ribs. At the spanwise locations between the ribs, the sectional wing shape distorts and triggers boundary layer transition close to the leading edge. The differences between the design behavior of the wing and the experimental results of the boundary layer analysis demonstrate the need for considering the skin deformation and its effects on the boundary layer flow when designing highly flexible wings.

In this paper, the effect of a statically deflected spoiler on the transonic flutter boundary is investigated by means of numerical airfoil models. For this purpose, a spoiler geometry is added to the NACA64A010 airfoil and a wing cross-section of the NASA’s Common Research Model. The deflection angle and the hinge-line position of the spoiler are varied. Unsteady aerodynamic loads are obtained with a linearized CFD method in the frequency domain. The flutter results show an increase in flutter velocity in the transonic dip region due to the spoiler deflection, which seems to be connected to a change of the aerodynamic loading of the airfoil. However, deterioration in the flutter stability boundary can be found for some specific spoiler deflection angles, especially at high Mach numbers. A careful use of the spoiler and a dedicated scheduling scheme dependent on the Mach number are therefore suggested if the spoiler effect is exploited as part of future flutter suppression technologies.

The DLR FR8-LAB is a self-contained swap-body container, equipped with an on-board power supply, data acquisition system and remote-access communication. The oncoming flow is derived from the surface pressure measurements on the FR8-LAB container’s front/rear/side surfaces and a calibration developed in a 1:15 scaled closed-return wind-tunnel experiment. The wind-tunnel model of the container was statically yawed over a range of −90:90°, mapping the relationship between changing surface-pressure and yaw angles. This relationship is used to infer the oncoming flow characteristics – until now, uncharacterized – that the FR8-LAB experiences in real-world operation, through application of the calibration to the measured transient surface-pressure. Oncoming flow conditions can be used to discover and characterize the real-world operating conditions at specific locations where aerodynamics is important, as well as input into aerodynamic/multi-body models to assess the risk of overturning, and modelled in simulations/experiments. Measurements on operational freight trains - validated against ultrasonic anemometers - have demonstrated that the FR8-LAB can be used to not only determine peak pressure loads and their resulting forces and moments, but also as a multi-hole dynamic-pressure probe to infer the oncoming flow conditions that freight trains experience.

The approach and results of a study on the optimization of airfoils for wind turbines through geometrical modification of the leading edge are presented. The objective is to produce a constant lift-coefficient after the airfoil stalls and to remain at this constant lift for as long as possible. An experimental and numerical methodology that yields optimized geometries that influence lift this certain manner has been devised. The procedure is performed by experimentally testing a base airfoil (TEG2618) for a wide range of $$\alpha $$ α at Re = $$8 \times 10^{5}$$ 8 × 10 5 ( $$u_{\infty } = 38\,\mathrm{m\,s}^{-1}$$ u ∞ = 38 m s - 1 ) and using the produced experimental data as a reference for validating computational fluid dynamic (CFD) models. A software tool is developed to automatically carry out analyses of all predefined geometrical variations between $$\alpha = -10$$ α = - 10 and $$20^{\circ }$$ 20 ∘ . The leading edge variants yielding the best results in the CFD analyses according to the criteria are selected, 3D printed and experimentally analyzed. The results demonstrate that the maximum lift coefficient at stalling can be maintained for at least another $$\varDelta \alpha $$ Δ α of $$6^{\circ }$$ 6 ∘ with reasonable loss of the total maximum lift compared to the unmodified base airfoil.

The present paper deals with a wall shear stress sensor that is an improved modification of a conventional sublayer fence probe. Using a novel probe geometry - a so-called star probe - the measurement technique is capable of detemining the shear stress or the velocity close to the wall in magnitude and direction instantaneously at high temporal resolution. Hence, the novel probe enables significant gain in sensor capabilities compared to the conventional probe configuration. However, the directional resolution becomes more and more biased and damped with increasing frequency.

The flow around a train in a tunnel was experimentally investigated at the Tunnel Simulation Facility of the DLR in Göttingen. A 1:25 scaled moving train model equipped with flow sensors passed through a 10 m long model tunnel. Parallel to the experiments simulations based on a quasi-1D formulation of the flow were performed. It could be shown that with appropriate adjustment of empirical parameters, the simulation can reproduce the measured pressure and velocity. All deviations between the measured and calculated data can be plausibly explained.

For a high-speed train operating in the desert, a certain proportion of sand may be lifted up from the ground in the vicinity of the underfloor caused by the turbulent slipstream. The train under investigation here has a number of lattice doors (side flaps) near the ground where air is sucked in with a certain amount of volume flow. To experimentally address the situation in the desert, a test rig was designed and constructed which establishes a flow on the surface of the side flap in full scale, whereas in addition the flow can be enriched with a constant, predetermined mass fraction of sand. The setup finally allows for comparison of different kind of filters and sealing attached to the side flap. Here, the design and construction of the experimental rig is presented.

Wind tunnel tests were performed to measure the loads on a train model under disturbed incoming flow conditions. For this purpose, a wing system with movable flaps was used to generate harmonic fluctuations in the flow upstream of the train model. The model was mounted above a moving belt to generate a more realistic underbody flow and the loads were measured using an internal balance. The tests show that the mean drag and the lateral force response can be analyzed although vibrations in the setup influence the measured force signals. Evidence was found that the drag coefficient increases when incoming flow disturbances are present. Phase-averaging was used to filter out the part of the lateral force fluctuation related to the incoming flow disturbance. The resulting lateral force response depends strongly on the frequency of the incoming flow disturbances.

The unsteady forces acting on a sphere laterally mounted in a wind tunnel with transient sinusoidal inflow are investigated. The study shows that the low-frequency dynamic is characterized by a spontaneous switching between two partially stable states, also known as bistability. Additionally, oscillatory forces exist, which, on the one hand, are induced by the sinusoidal transient inflow and, on the other hand, originate from natural vortex shedding. By separating the bistability and the oscillatory forces using proper orthogonal decomposition, a low-order model based on the Duffing equation is developed both for the bistability and the oscillatory multi-scale induced forces. The bistability is represented by a chaotic bistable Duffing equation, whose parameters are determined by manual adjustment. In contrast, the parameters of the oscillatory multi-scale forcing are determined by expanding the forcing of the Duffing equation to three terms and preserving the phase angles between the three characteristic frequencies, the flapping frequency, its multiples and the natural vortex shedding.

An experimental investigation was carried out to assess the effectiveness of non-circular air-jet vortex generators (AJVGs) in controlling shock-induced separations. Single rows of spanwise-inclined AJVGs with circular, rectangular, and four triangular orifice shapes were tested in an incoming supersonic crossflow at Mach 2.52 and $$Re_\theta = 8225$$ R e θ = 8225 . All cases were characterised by the same hydraulic diameter for comparability. Surface oil-flow visualisations and PIV in a wall-parallel plane are used for flow diagnostics. The results reveal an improved performance by all non-circular AJVGs in controlling the shock-induced separation, in comparison to the traditional circular AJVG. The improved effectiveness is expected to be a result of increased turbulent mixing and favourable jet/jet interactions between neighbouring jets in the array.

In recent years, advancements in the machine learning and applied mathematics community have led to the development of Physics-Informed Neural Networks (PINNs) which implement a set of governing partial differential equations (PDEs) into the framework of a classical Neural Network. Through these equations it is possible to discover latent physical quantities from an incomplete dataset such as pressure and density fields from velocity measurements. While the potential has been shown in several studies with synthetic data, there are very few publications with experimental data and even fewer for compressible, high-speed applications. In this contribution, a PIV dataset of a turbulent shockwave-boundary layer interaction at Ma = 2 is used as a base to test the PINNs ability to work with experimental data. Multiple cases are constructed to enable comparisons with analytical results and show the influence of different domain sizes and boundary conditions. The results are able do match the theoretical predictions and appear to be quite robust towards measurement noise. This suggests that PINNs can be a powerful tool to extract additional information from a given set of experimental data and future improvements in the computational framework will only increase their versatility.

In this study, different surrogate models for flow field prediction are applied to a size-limited experimental dataset of a 2D car wake. The dataset consists of DrivAer model wakes influenced by different rear wing positions. The objective is to identify the most robust and accurate method for small training datasets in order to exploit the time-reducing benefits of surrogate models in the experimental field. Three methods (random forest regression, Gaussian process regression and feedforward neural network (FNN) with different activation functions) trained with different dataset sizes are compared. It is shown that Gaussian process regression is the most accurate but most time-consuming method. Interestingly, the random forest regression predicts the flow fields as good as the FNN, but its training time is two orders of magnitude faster. In terms of FNN, rectified linear unit 6 (ReLU6) stood out from the other activation functions as it achieved the best wake predictions with only five training samples.

A validation of a polyatomic model for a kinetic Fokker-Planck solver based on a master equation ansatz is presented. The reference data are polynomial fits of experimentally measured heat capacity. An analytical solution based on the master equation ansatz is derived and the convergence of the Fokker-Planck solution towards it is shown. Deviations between the analytical and the reference solution are pointed out and explained.

In the present work, we study the sensitivity of expanding hypersonic air flows in the HEG facility with respect to the thermochemical relaxation rates and reservoir conditions, along with the impact of uncertainties in the rates and reservoir conditions on uncertainties of the free-stream parameters. A simplified 1-D formulation of the flow equations is utilized to allow for simultaneous variation of a large number (10) of uncertain parameters. The simulation results show large uncertainties in the vibrational temperature of oxygen and molar fractions of oxygen-containing species, mainly due to uncertainties in the reservoir parameters and data on thermochemical relaxation rates of molecular oxygen.

Ensuring reliability in the measurement and interpretation of plasma diagnostics is essential for characterizing thruster concepts in the growing field of electric propulsion. Langmuir probes are often used to determine the properties of low temperature plasmas. However, not much research is found on Langmuir probe measurements on electron cyclotron resonance thrusters with magnetic nozzle, especially no detailed studies on variable probe orientations with respect to magnetic field lines. The question arises which differences can be shown in the electron energy distribution regarding the probes orientation and in what way it is possible to interpret probe alignment consequences on determined plasma characteristics. We perform our study as follows: We measure in two orientations, parallel and orthogonal to magnetic field lines, at different operating states of the thruster (variable power and volume flow settings). The Langmuir probe data shows clearly a dependency on the probes orientation. We use the measured ion energy distribution of the retarding potential analyzer as a comparison tool to the determined electron energy distribution of the single Langmuir probe measurements. Due to the determined anisotropic non-Maxwellian distribution mainly visible in parallel orientation we conclude a parallel orientation of Langmuir probes with respect to magnetic field lines is preferable for ECRT with MN.

The hydrogen combustion process within the propulsion unit of wind tunnel scramjet models produces NO and H2O. A precise determination of the concentration of both species applying laser absorption spectroscopy is necessary to validate numerical simulations. A novel approach to obtain absorption spectra of both species in the infrared region within μ-second range is based on the beating analysis of two combined frequency combs generated with quantum cascade lasers. This technique allows multi-species measurements of absorption spectra. The concentrations of species can be inferred from characteristic absorption features. The generated frequency combs are within the 1740 ± 20 cm−1 wavenumber region which includes strong NO and H2O spectral lines, thus enabling an identification of these species. The method offers a current-tuning based sweeping measurement mode for an increased spectral resolution. An extended calibration of all possible measurement modes under different conditions is reported.

The concept of aerotow for gliders with a tow-drone instead of a tow-plane is presented based on the historical development of aerotow and compared to the current state of the art for towing gliders using the measurements from the thrust flap = “Schubklappen-Antrieb” project SKA.

Load alleviation technologies are a promising approach to increase the fuel burn efficiency. Ideally, they should be taken into account already in the early design phase, where the geometry shape is defined. Implementing the capability for a correlated evaluation requires a multi-disciplinary simulation approach. The conceptual aircraft design framework within this paper includes coupled and physics-based models of the disciplines aerodynamics, structures and flight dynamics. The software ASWING from the Massachusetts Institute of Technology is included and provides an unsteady lifting-line calculation combined with non-linear Euler beam theory. The wing shape is optimized within a nine-dimensional design space via a surrogate model approach. The objective function is a multi-point mission block fuel. The difference between an optimized aircraft design without and with maneuver load alleviation shows a potential of 5.3% combined block fuel savings. The effects of gust load alleviation on loads and the overall aircraft design are discussed.

This work presents the virtual design activities performed within the Virtual Product House (VPH) start-up project. In this project a multi-disciplinary process for virtual design, manufacturing and testing is developed. The VPH acts as integration plateau where the assessment capabilities of multiple partners from research, industry and certification authority are combined and applied to use cases.The multi-disciplinary analysis setup enabled the investigation and improvement of aircraft designs and allows the impact assessment of modifications on a existing configurations.As starting point, three disciplines are considered: aerodynamics, structural and system design. This paper presents results for selected modifications on trailing edge moveable devices of a representative research aircraft, including variations of the applied structural constituent and design, actuation system as well as number of flap tracks.

Future highly efficient long-haul aircraft take advantage of increased cruise performance due to wings of higher aspect ratio and geared turbofan engines. Further improvements can be achieved by using the control surfaces to adapt the wing shape to different flight conditions. The objective of the present work is to optimize the control surface deflections of a long-haul aircraft to reduce block fuel and thus the CO2 emissions.An optimization of the deflection angles of six control surfaces along the trailing edge of the high aspect ratio wing has been performed. In this optimization process, a flow solver based on the Reynolds-averaged Navier-Stokes equation is used to determine the cruise performance considering the static aeroelastic deformations for three different flight conditions. Depending on the flight mission a fuel burn reduction between $${0.5}{\%}$$ 0.5 % and $${1.7}{\%}$$ 1.7 % is predicted.

Solving multidisciplinary problems by coupled black-box solvers using splitting methods, such as the nonlinear block-Gauß-Seidel method, has severe limitations on convergence. We implemented a MPI-parallel full Newton-Krylov method involving the next-generation CFD solver CODA, combining the HPC platform FlowSimulator with the open-source multidisciplinary analysis and optimization Python framework OpenMDAO. The framework implementation uses the Python entry points from CODA for its residual vector, its Jacobian and its linear solution routine as matrix-free operators. For the ONERA M6 wing ( $$1.3{\cdot }10^{6}$$ 1.3 · 10 6 DoF) with torsional spring attachment, we recovered the quadratic convergence of Newton’s method and showed the reduction in computational work compared to the baseline method. The implementation is capable of handling the computationally large NASA DPW5-CRM half-plane ( $$144{\cdot }10^{6}$$ 144 · 10 6 DoF) distributed across a HPC cluster.

This contribution presents the implementation and results of a Gaussian Process regression based surrogate model for prediction of the aerodynamic drag coefficient $$C_D$$ C D of a transonic transport aircraft wing. The wing contains a hybrid laminar flow control system, alongside variable camber integration via an Adaptive Dropped Hinge Flap. The latter enters the envisaged input parameter space of the surrogate through specification of its deflection angle, alongside variations in cruise lift coefficient and altitude. The model builds upon high-fidelity computational fluid dynamics (CFD) results and aims at incorporating them into an overall aircraft design workflow. The predicted drag coefficients agree well when compared to CFD validation data, showing the model being suitable for the present application case.

The discretization of the incompressible Navier-Stokes equations leads to a saddle-point problem in which the discrete equations for the pressure and momentum are coupled. Traditionally, this coupling is handled by segregated methods in which separate equations for momentum and pressure are solved with the constraint of obtaining a divergence-free velocity field. In the fully coupled approach, a single block system of equations for the momentum and pressure is formulated and solved simultaneously. Furthermore, the solution of such a coupled system has been shown to improve robustness and convergence properties. In the present work, a coupled approach for the solution of the incompressible Navier-Stokes and RANS equations have been developed in the finite volume framework of the CFD software CODA (CFD for ONERA, DLR and AIRBUS). The implementation is verified and validated with various test cases.

Field Inversion and Machine Learning is an active field of research in Computational Fluid Dynamics (CFD). This approach can be leveraged to obtain a closed-form correction for a given turbulence model to improve the predictions. The fundamental approach is to insert a parameter into the system of RANS equations and determine it in a way such that, for example, a given pressure dissipation is better approximated compared to the one obtained with the original set of equations. The goal of this article is twofold. Numerical arguments are presented that these kinds of problems can be severely ill-posed. Second, instead of introducing a field parameter into a given turbulence model, an approach is presented to directly reconstruct the eddy viscosity field along with an example of an RAE2822 airfoil in transonic conditions. A regularized Gauss-Newton method is used for a realization. Finally, an outlook is presented in which way this approach can be used to possibly modify or improve turbulence models such that not only one, but a larger number of test cases are considered.

2nd order finite-volume discretization schemes generally offer a good compromise between computational effort and accuracy. For a given mesh, the accuracy of an approximate solution often depends significantly on the discretization applied to the underlying grid. Therefore, the discretization has to be adapted to the chosen grid metric, including the types of elements present in the mesh. To keep the number of degrees of freedom acceptable, in regions of the grid, where steep gradients need to be resolved, quadrilateral, hexahedral or prismatic elements with an extreme cell-stretching are required. Hence, one is now forced to develop unstructured discretizations that reliably produce accurate solutions for mixed grids with elements of high anisotropies. In this work we review and demonstrate, in which way these anisotropies can be meaningfully incorporated into different, commonly used discretization schemes and how the schemes can be extended for unstructured grids in order to achieve good quality solutions.

Various numerical applications in the context of multidisciplinary high-fidelity aircraft design, e.g. fluid-structure interaction or shape optimization, require the consideration of changes in the geometry of the aircraft shape. Such geometry changes can be realized using mesh deformations, and we demonstrate such a volume mesh deformation method based on elasticity analogy in conjunction with line-implicit solvers. Line-implicit solvers have already been successfully used in CFD simulations with strongly anisotropic cells in a viscous boundary layer, and are now applied to mesh deformation problems of such meshes. In this approach, the solution of a block-tridiagonal subsystem of the Jacobian matrix is computed exactly using the Thomas algorithm. Here, the selection of the tridiagonal part – representing the lines – is of crucial importance for the speed of convergence. A new algorithm for line identification is presented and compared with existing ones. Also, a comparison and combination of these methods with multigrid methods is performed and the implications for industrially relevant test cases are demonstrated.

The capabilities of Reynolds stress model compared to eddy-viscosity RANS turbulence models and scale-resolving simulation methods for the flows characterized by separation from smooth surfaces and subsequent vortex formation are assessed. To this end, the flow over a delta wing and a diamond wing at the incidence angle of $$13^\circ $$ 13 ∘ and $$12^\circ $$ 12 ∘ , respectively, are investigated. Predictions obtained for the aforementioned configurations by different approaches are compared to each other and to experiments. In this paper, improvements and detriments observed in Reynolds stress model predictions compared to RANS predictions and to scale-resolving simulations are presented.

The DLR New Generation Train Cargo concept (NGT-Cargo) aims to increase the rail share of the European freight traffic market. Computational Fluid Dynamics (CFD) is a useful tool in analyzing high-speed operations which are an important part of this concept. The development of aerodynamic forces under unsteady on-flows should be analyzed within the context of industrial standards, for example guidelines are provided for CFD assessments at cross-wind conditions using the RANS (Reynolds-averaged Navier-Stokes) equations. A major challenge arises because the multi-scale nature of these flows are characterized by a large range of energetically significant flow scales. Wind-tunnel measurements, focused on the development of the aerodynamic forces acting on a train, are being currently performed on a 1:25 scaled NGT model in the cross-wind facility Seitenwindversuchsanlage Göttingen (SWG) located at the DLR Göttingen. This paper presents preliminary work assessing the ability of CFD to reproduce both the observed on-flow conditions as well as the aerodynamic drag of the vehicle.

The present work focuses on the performance analysis of the DG-SEM implementation of the CFD solver CODA. The turbulent Taylor-Green vortex is employed as a simple testcase for scaling behavior, while for a more detailed node-level performance analysis more granular kernel benchmarks are used. Bottlenecks in the implementation are highlighted and possible solutions proposed.

The unsteady flow phenomena of a delta wing configuration with multiple leading edges are investigated by means of high-resolution numerical simulation and comparisons with experimental data. The goal is to establish a validated numerical baseline for further research on potential mode alternations due to topological instabilities of the complex vortical flow, which have been observed on the DLR-F23 combat aircraft wind tunnel model during previous experiments. In order to draw conclusions from the underlying flow physics, it needs to be shown first, that coherent and plausible flow field data can be calculated by numerical simulations. Hence, a challenging test case at an angle of attack of $$\alpha =21^\circ $$ α = 21 ∘ and a transonic Mach number of 0.85 is chosen. Comparisons between experimental and numerical data show, that the hybrid-RANS-LES approach reproduces the flow to a satisfying degree.

In this work, we establish a relation between the generation of broadband sound for the inertial subrange in turbulent flows and the turbulence cascade mechanism. A dimensional analysis of the perturbed convective wave equation (PCWE) source term is used to obtain the time scale of the aeroacoustic source term and its relation to the turbulent length scales. Based on these findings, a new method to estimate the grid cut-off frequency ( $$f_{GCO}$$ f GCO ) is proposed. The frequency $$f_{GCO}$$ f GCO determines the maximum frequency up to which the turbulent structures are fully resolved by the grid of the flow simulation. For hybrid aeroacoustic simulations, this maximum resolvable frequency of the flow simulation also determines the maximum frequency content of the aeroacoustic sources and therefore the highest frequency that can be resolved in the subsequent acoustic simulation of broadband sound. The ability of the new method to estimate the $$f_{GCO}$$ f GCO is tested against two common methods frequently used in literature. The test case of a forward-facing step is chosen which is a common aeroacoustic benchmark case and known to generate broadband sound. The new method shows promising results when the estimated grid cut-off frequency is compared against the estimations from the two reference methods.

In this paper a method is presented, which allows to simulate the traffic noise exposure of people living in an urban area. This is in contrast to classical noise maps, where the noise exposure of locations is calculated. A temporally resolved noise mapping is used in combination with an agent model, which simulates the location of persons in an urban area. Noise exposure is then assigned to agents based on their specific locations throughout the day, and averaged over time. The question is answered what difference in noise exposure the method shows in comparison to classical noise-maps. The method was successfully applied for a part of Berlin. It is shown that the average noise to which the agents are exposed during the day is increased due to their motion. This is not always due to the fact that people spend more time in places with high noise exposure. Rather, the usual consideration of the noise exposure in a log scale is responsible for this. This always results in a dominance of the noisier locations in the daily mean of the considered agents. It is to be discussed whether this shift towards higher noise exposure corresponds to the actual subjective perception of persons.

A distributed propeller configuration was investigated by using CFD flow field data and evaluating the acoustics in the far field with the Ffowcs Williams-Hawkings approach. The noise sources were localized by implementing two permeable surfaces within the CFD simulation as input for the acoustic formulation. It can be shown that solely obtaining the propeller noise is not sufficient. Since three propellers are present, the propeller-wing interaction noise has a great impact on the overall noise emission. To reduce the noise, the propellers were vertically shifted, which led to a reduction in propeller-wing interaction noise and an overall noise reduction of 2.4 dB in sound power level. Finally, an extensive parameter study was carried out, showed that propeller distance/phase and direction of rotation, had only minor influence on the overall noise emission.

Porous bleed systems are widely used to mitigate the shock-induced boundary layer separation, e.g., in supersonic air intakes. However, the complex geometry makes simulations expensive and motivates the application of suitable models. Existing models are based on applying a continuous blowing/suction boundary condition (continuous porosity) along the porous plate, leading to an overestimation of the boundary layer thinning. The new method using distributed suction by a local porosity is applied on a Mach 1.6 supersonic flow and improves the prediction of the effect on the boundary layer significantly. In contrast to the existing methods, the wall shear stress is not overestimated, and the boundary layer profiles downstream of the bleed region are better reproduced. Moreover, all expansion fans and barrier shocks caused by the bleed holes are captured.

For a simplified but effective chamber concept of boundary layer suction for hybrid laminar flow control, different velocity distributions inside suction chambers under consideration of pressure gradients are simulated. The flow inside these chambers is calculated on 2D and 3D grids to predict suction velocity as well as the wall pressure distribution at different suction pressures. The classical concept of a microporous metal with a substructure of stringers and single chambers for adapted inner pressure will be investigated, as well as modern chamberless applications with micro etched metal foils at varying porosity for each surface part, stabilized by metal fabric below. By the latter approach, extensively long suction regions result in non-negligible pressure gradients inside the porous sheet and the pressure distribution on the outside wall, which is also taken into account. These effects require an additional pressure gradient boundary condition, which was added and successfully evaluated by comparison with experimental data, allowing the future prediction of suction distribution for an optimal transition suppression.

We describe the aerodynamic design of an HLFC system for the wing of a large transport aircraft and compare it to the HLFC system installed in the VTP of the DLR A320 ATRA aircraft. For the VTP we used the ALTTA-design with a laser-drilled microperforated titanium sheet with only one microperforation and small suction chambers beneath it. For the wing we designed a system with only one large plenum and controlled the suction with the help of variable microperforation. Some specific challenges of an HLFC design on a wing in contrast to a VTP application are highlighted here.

Direct numerical simulations are used to gain insight into the effect of pitch and skew angle variations of vortex generator jets. The skew is investigated for a range from 30 $$^{\circ }$$ ∘ to 150 $$^{\circ }$$ ∘ for a pitch of 30 $$^{\circ }$$ ∘ and 45 $$^{\circ }$$ ∘ . The baseline consists of a laminar wall shear flow at a low Reynolds number of 165 based on the displacement thickness and freestream velocity. The resulting compound flows have been classified by integral values such as the friction coefficient and the momentum gain coefficient. The dependency on the pitch is small except at a small region close to the jet exit. Transition to turbulence occurs for skew angles above 90 $$^{\circ }$$ ∘ . The jet trajectories are extracted from the data and a polynomial model that characterizes the trajectories as a function of skew and pitch is proposed.

Counter-rotating cylindrical roughness pairs are used to control the Tollmien-Schlichting instability-induced laminar-turbulent transition. Direct numerical simulations are performed to study this active flow-control method’s potential for delaying an H-type transition, which is indicated by a linear stability analysis by the current authors [15]. A complete stabilization of the TS-instability-induced transition in the investigated integration domain is obtained. A Fourier analysis of the streamwise evolution of instabilities reveals that both fundamental and subharmonic components of the controlled TS waves are attenuated. The present work thus proposes and investigates an active technique using wall-normal finite-length cylinders, a widely used structure, to control boundary layer transition.

This article presents a novel approach to include turbulent wedges in local correlation-based $$\gamma $$ γ transition models. The turbulent wedge is modeled by increasing the intermittency at the wedge apex. The wedge develops downstream without further interference in the transition model behavior. The method is demonstrated for an experimental high Reynolds number test case on the NASA CRM-NLF configuration. Wedges are successfully created, but the wedge angles are too large compared to the experimental data. Grid spacing, initial disturbance size, and scaling of the diffusion term only have a minor effect on the wedge angles.

The forward-swept NLF-ECOWING-FSW model is designed to facilitate extensive natural laminar flow (NLF) at a high Mach number of 0.78. A wing-fuselage half-model with a half-span of 1.25 m was tested in the European Transonic Windtunnel (ETW). A first comparison of the experimental data with CFD simulations is presented in this paper. The objective is to understand the steady aerodynamics of the NLF-ECOWING-FSW configuration. This work aims at building the foundation for future unsteady investigations of this model.

Turbulent concentric coaxial pipe flows are numerically investigated as canonical problem addressing spanwise curvature effects on heat and momentum transfer that are encountered in various engineering applications. It is demonstrated that the wall-adapting local eddy-viscosity (WALE) model within a large-eddy simulation (LES) framework, without model parameter recalibration, has limited predictive capabilities as signalized by poor representation of wall curvature effects and notable grid dependence. The identified lack in the modeling of radial transport processes is therefore addressed here by utilizing a stochastic one-dimensional turbulence (ODT) model. A standalone ODT formulation for cylindrical geometry is used in order to assess to which extent the predictability can be expected to improve by utilizing an advanced wall-modeling strategy.

In this publication, an open source numerical flow solver for Large Eddy Simulations (LES) optimized for graphics processing units (GPUs) is analyzed and presented. Its applicability is demonstrated with a series of test cases calculated with different LES models and the results are analysed. In addition, the performance and effectiveness of this approach is examined, the parallelization of the solver is explained and finally the overall system is discussed.

This paper presents analyses of the flow topology around and behind a roughness element in a laminar flat plate boundary layer. Hydrogen-bubble and dye visualization performed in a laminar water channel give insights into primary structures, such as the horseshoe vortex, hairpin vortex and recirculation zone. Moreover, two secondary structures are presented: the $$\varLambda _1$$ Λ 1 vortex, known from classical laminar-turbulent transition, and a novel vortex, here named $$\varLambda _2$$ Λ 2 vortex. The $$\varLambda _2$$ Λ 2 vortex is elongated and orientated like the $$\varLambda _1$$ Λ 1 vortex parallel to the wall. However, it lies near to the wall and is believed to be initiated by the $$\varLambda _1$$ Λ 1 vortex. Hot-film measurements reveal that the fluctuation amplitude of the $$\varLambda _1$$ Λ 1 vortex increases up to the highest value downstream of the roughness element. This indicates that the $$\varLambda _1$$ Λ 1 vortex contributes most to laminar-turbulent breakdown.

A correction of the turbulent diffusion terms inside a Reynolds-stress model for mean-streamline curvature is presented. The correction is based on work by Zeman and is reformulated in this work to be applicable in general CFD codes. For this purpose, a new and grid-point local gradient Richardson number is derived which is used within the correction term in order to characterize mean-streamline curvature. These quantities are verified for a channel with U-turn and delta wing vortex. The final correction term was finally applied to these test cases: The overall effect of the correction term on the local Reynolds stresses depends the relative effect of diffusion on the turbulent kinetic energy balance. In this sense, no effect on the solution of the mean flow field was observed for the channel test case, whereas lower values of turbulent kinetic energy appear for the delta wing vortex.

Local stability theory (LST) combined with an $$e^N$$ e N method is a well-suited approach for predicting transition. However, simpler or surrogate LST models are often used in an automated transition prediction framework for robustness and efficiency reasons. There are, however, few surrogate-based stability methods for the instabilities encountered in three-dimensional compressible flows. To address this problem, this paper proposes a radial-basis-function-based surrogate model capable of reproducing the instability characteristics of two-dimensional Tollmien-Schlichting waves and stationary cross-flow instabilities. The basic flows necessary to set up the model stem from compressible local Falkner-Skan-Cook similarity solutions. The suitability of the proposed method is demonstrated by N-factor computations for three ATTAS flight test points at transonic conditions.

In the low speed wind tunnel Braunschweig (DNW-NWB), three measurement campaigns with sucked boundary layer for laminarisation have taken place. Their aim was to improve effectiveness and efficiency of HLFC systems on the path to industrial applications. The present article gives an overview over the modular wind tunnel model, the conducted measurement series and the obtained results. Multiple suction surfaces were used. Conventional approaches with laser drilled cylindrical perforations are compared with recently developed suction surfaces, using a fine etched metal foil and a supporting mesh structure. Also two layouts of suction slots in contrast to the cylindrical perforations are considered. All investigated surfaces turned out to be suitable for transition delay. Differences of the effectiveness between the suction panels could be found depending on the suction rate. Moreover, numerical investigations are presented, allowing a deeper insight into aerodynamical processes of unstable boundary layer modes. The focus was on the deviation of experimentally measured transition lines and numerical predictions by an $$e^N$$ e N method with regard to effectiveness of suction and predictability.

A conventional differential, near-wall Reynolds stress model (RSM) and its eddy-resolving version, sensitized appropriately to account for the turbulence unsteadiness (termed as Improved Instability- Sensitized RSM model - IIS-RSM), are applied within the unsteady Reynolds-Averaged Navier Stokes (RANS) computational framework to simulate the annular flow formed by two concentrically arranged cylinders. Two characteristic situations, in which the rotation of the inner cylinder by a constant angular velocity and that of the outer cylinder were considered individually, were studied in a range of rotational intensities. The corresponding Reynolds numbers based on the hydraulic diameter and axial bulk velocity of amount to 8900 and 12500 for the inner and outer cylinder rotations, respectively. The scale-supplying equation governing the inverse turbulent time scale relies on the ’homogeneous dissipation’ rate ( $$\omega _h=\varepsilon ^h/k$$ ω h = ε h / k ). The eddy-resolving capability of the IIS-RSM is enabled by a selective enhancement of the turbulence production by introducing an additional production term in the length-scale determining transport equation, in accordance with the Scale-Adaptive Simulation (SAS) strategy. The results for the cases with inner cylinder rotation obtained by both model versions show substantial mutual agreement and agree well with the reference LES data. The flow driven by the outer cylinder rotation at higher rotational rates is correctly predicted by the IIS-RSM model, in contrast to the conventional RANS approach resulting in premature relaminarization.

Besides coughing and sneezing, breathing is the most frequent particle emission event of aerosol droplets carrying the SARS-COV-2 virus or viruses of other airborne diseases. Direct Numerical Simulations (DNS) of ‘jet-like’ emissions of particle clouds through the mouth caused by coughing and breathing are performed in a cuboidal simplified room to study the spreading of respiratory droplets with different momentum and size. Contrary to coughing, we found that no droplet follows a ballistic trajectory after a breathing event since all the droplets are trapped in the humid puff of air. The detailed analysis and the comparison of the predictions obtained for respiratory droplets emitted by single breathing and coughing events are further discussed. Despite the major difference between the maximum exhalation speeds reached during coughing and breathing, the horizontal propagation distance differs by less than 30%. Additionally, a comparison of the results of the present DNS neglecting aerosol evaporation and considering buoyancy forces with the results of an earlier DNS study from the literature taking evaporation into account but neglecting buoyancy, revealed that buoyancy damps the horizontal propagation of the humid puff and enhances the upward motion.

Three-dimensional Reynolds-Averaged Navier-Stokes simulations are performed to calculate the wind field of a full-size urban district of 1 km $$^2$$ 2 around the campus of Technical University of Berlin with the inlet wind direction as a parameter. Two-dimensional snapshots of the simulation data set are used for model reduction by proper orthogonal decomposition (POD) to reduce the complexity of the system. The POD modes are then used to estimate a high resolution wind field from sparse velocity sensor measurements by using the Gappy POD as a data reconstruction method. The sensor measurements are taken from a simulation test case that is not included in the snapshot basis. The sensor placement problem that needs to be solved to find effective sensor locations is investigated using two different methods. The performance of the data reconstruction is then assessed by calculating the least-squares reconstruction error. An application of the estimated urban wind field is demonstrated by finding a wind-based minimum-energy path from a start to a final location for an operation of unmanned aerial vehicles. The wind-based path is compared to the shortest path for a reduced order model taking only 5 sensor measurements. An overall mean energy reduction of 5.5 $$\%$$ % was calculated by the full-order model and of 3.9 $$\%$$ % by the reduced order model. This suggests that trajectory planning may be performed on inexpensive reduced-order models of the urban wind field.

We present results from novel field, lab and computer studies, that pave the way towards non-invasive classification of localised surface defects on running wind turbine rotors using infrared thermography (IRT). In particular, we first parametrise the problem from a fluid dynamical point of view using the roughness Reynolds number ( $$R\!e_{k}$$ R e k ) and demonstrate how the parameter regime relevant for modern wind turbines translate to parameter values that are currently feasible in typical wind tunnel and computer experiments. Second, we discuss preparatory wind tunnel and field measurements, that demonstrate a promising degree of sensitivity of the recorded IRT data w.r.t. the key control parameter ( $$R\!e_{k}$$ R e k ), which is a minimum requirement for the proposed classification technique to work. Third, we introduce and validate a local domain ansatz for future computer experiments, that enables well-resolved Navier–Stokes simulations for the target parameter regime at reasonable computational costs.

A low-cost thermal human manikin with seven zones has been developed for comfort assessment in passenger cabins. The total costs for tor the manikin amount to less than 1, 000 € and it allows for temperature measurements in the individual zones, where the small differences in the resistance depending on the temperature are recorded. Based on this dependence both, the surface temperature and the equivalent temperature, can be determined on a zonal basis. The latter allows for a comfort assessment of the individual zones in accordance with common standards.

We report the results of numerical simulations of aerosol formation and dispersion in the aircraft cabin. The simulation was performed in a full-scale model of the Do728 aircraft cabin using the open-source software OpenFOAM. Aerosol cloud formation was studied during normal human breathing, with particles modeled as a composition of solid nuclei (1% of total mass) and a solution of NaCl (1%) and water (98%). The time of transition to the equilibrium state as a function of particle size, as well as the spatial position and size distribution of the formed aerosol cloud were investigated. Further, dispersion of aerosol particles in the cabin was simulated and compared for two ventilation configurations: Mixing Ventilation (MV) and Cabin Displacement Ventilation (CDV). The comparison of particle residence time in air, the fraction of particles deposited, and the concentration of particles in the breathing area revealed certain advantages of the CDV configuration.

Surgical Smoke is generated during the cauterization of tissue with high-frequency (HF) devices and consists of 95% water vapor and 5% cellular debris. When the coagulation tweezers, which are supplied with HF voltage by the HF device, touch tissue, the electric circuit is closed, and smoke is generated by the heat. In-vivo investigations are performed during tracheotomies where surgical smoke is produced during coagulation of tissue. Furthermore, in-vitro parametric studies to investigate the particle number and size distribution and the spatial distribution of surgical smoke with laser light sheet technique are conducted. With higher power of the HF device, the particles generated are larger in size and the total number of particles generated is also higher. Adding artificial saliva to the tissue shows even higher particle counts. The study by laser light sheet also confirms this. The resulting characteristic size distribution, which may include viruses and bacterial components, confirms considering the risk arising from surgical smoke. Furthermore, the experiments will provide the database for further numerical investigations.

Investigations of complex patient-specific flow in the nasopharynx requires high resolution numerical calculations validated by reliable experiments. When building the validation base and the benchmark of computational fluid dynamics, an experimental setup of the nasal airways was developed. The applied optical measurement technique of tomo-PIV supplies information on the governing flow field in three dimensions. This paper presents tomo-PIV measurements of the highly complex patient-specific geometry of the human trachea. A computer-tomographic scan of a person’s head builds the basis of the experimental silicone model of the nasal airways. An optimised approach for precise refractive index matching avoids optical distortions even in highly complex non-free-of-sight 3D geometries. A linear-motor-driven pump generates breathing scenarios, based on measured breathing cycles. Adjusting of the CCD cameras’ double-frame-rate PIV- $$\Delta $$ Δ t enables the detailed analysis of flow structures during different cycle phases. Merging regions of interest enables high spatial resolution acquisition of the flow field.

A method for the measurement of position and size of sessile water droplets is presented. Droplets originate from condensation on a plane vertical surface in a vented cuboidal cavity with a mixed convective flow with humid air as the working fluid. Condensation is observed through a transparent cooling device coated with polyvinyl chloride with an average contact angle of 80.0(3) $$^\circ $$ ∘ . The implemented detection algorithm is based on the circle Hough Transform together with sophisticated pre- and post-processing steps, which are detailed in this work. Validation experiments yield a detection of over 97% of the area covered by droplets by detecting a minimal radius of $${13.8}\,\upmu \textrm{m}$$ 13.8 μ m . Additionally, first experimental results of droplet size distributions are presented.

The global COVID-19 outbreak in 2020 has made understanding pathogen-laden aerosol transport and the associated transmission routes more relevant than ever. To determine how aerosol particles generated by continuous breathing accumulate in confined spaces, the particle concentrations in a small room resembling a train entrance are investigated. The room is ventilated and equipped with two heated manikins, one of which is continuously exhaling aerosol through the mouth for 30 min. For this setup we conducted local particle measurements in the center plane and a RANS simulation including the prediction of the transient particle transport. It is shown that the particle concentration increases logarithmically and attains a nearly steady state. The resulting local particle concentrations normalized to the source concentrations are subsequently compared. We find good agreement with the experiment in the exhalation zone of the breathing manikin and larger differences for the sensor positions beneath the ventilation inlet.

Innovative helicopter rotor blades with a combined forward-backward double-sweep at the outer part of the blade enable a reduction in noise emission and enhance the overall performance of a rotor. In this context, the influence of the aeroelastic behaviour in connection with the dynamic stall phenomenon is of great importance. It is accompanied by large aerodynamic load peaks, primarily seen in the lift and the pitching moment, impacting the structural integrity of the blades and adjacent control components. Double-swept model rotor blades were developed and investigated experimentally at the German Aerospace Center (DLR) in Göttingen regarding the dynamic stall behaviour in a four-bladed rotor configuration at the Rotor Test Facility Göttingen. Due to an axial inflow to the rotor disc a sinusoidal variation in pitch angle is introduced to trigger the dynamic stall phenomenon once per revolution. The numerical study simulates the conducted experiments utilizing two different blade modelling approaches: elastic and rigid bodies. The corresponding computations are carried out with the use of computational fluid dynamics (CFD) and a multibody system (MBS). With the inclusion of blade elasticity both domains are connected together by using a strong aeroelastic coupling scheme. Three test cases with a rotor speed of 23.6 Hz will be presented comprising two test cases with fixed collective pitch angle and one with a superposed cyclic variation in pitch angle in order to introduce the dynamic stall phenomenon. Finally, a comparison is carried out with experimental data including the measured rotor thrust as well as the displacements at the blade tips. They show good agreement with the numerical results in both of the considered fields comprising the structural behaviour of the blades as well as the surrounding fluid flow.

Experimental investigations are performed on a helicopter wind tunnel model for three rotor heads with different blade stub lengths. This investigation is intended to judge the influence of the blade stub spanwise dimension on the drag, lift, and especially pitching moment characteristics. Aerodynamic forces, moments, and the wake flow fields are analyzed. As expected, the longest blade stubs provide the highest drag and lift coefficient along the angle of attack polar, while both shorter blade stubs show a similar force course. The flow field of the largest rotor head exhibits pronounced blade tip vortices, which influence the pitching moment polar at high angles of attack. The medium and long stubs show a similar flow field, while the hubcap vortex dominates the smallest blade stub configuration.

In preparation for a future wind tunnel test of a rotor with twist-actuated blades (STAR II), numerical predictions of this test have been carried out by the contributing partners. In this paper, the simulated results of the vortex-induced stall operating condition, synonymous with a highly loaded flight condition, are presented. The current conclusion is that a noticeable spread of the results is given when searching for the maximum attainable thrust, depending on the onset of stall once perceived by the individual simulation methodology. Some general trends among the simulations could still be identified with respect to the actuation settings that reduce vibrations and the required power. Generally, most CFD-based results allowed to capture this physical phenomenon, but carefully tuned low-fidelity aerodynamic tools also managed to capture the same trends at a fraction of the computational cost.

The current numerical investigations analyze the influence of the vertical stabilizer on the aerodynamics and aerodynamic stability, especially roll stability, of a multi delta wing configuration. Planform studies established to enhance the rolling moment behavior by means of strakes and wing leading edge slats are taken as a reference for the current vertical tail plane studies. The aim is to identify the differences between the configuration with and without vertical tails. In particular the influence of the vortical flow topology and interaction with the vertical tail planes on the aerodynamic behavior should be identified. This should lead to design recommendations for the current fighter design approaches within the DLR project Diabolo.

Vortices and vortex breakdown flow structures over a multi-delta wing configuration are often analyzed by applying flow field invariant analysis techniques from critical point theory and flow topology research. In particular the three invariants of the velocity gradient tensor are used to classify and analyze flow fields, mainly for the assignment of vortical flow regions and assessment of their stability. In this work, the extended optimal triple tensor decomposition is applied to the vortical flow field delivering new tensor fields which allow to recalculate the rate of strain and rotation tensors in an associated basic reference frame. These new tensor fields contain separated distinct flow properties. After retransformation into the original coordinate system, flow field invariant analysis techniques applied to these newly derived tensor fields provide further insights into the dynamics of vortical flows over delta wings.

In this study, the performance of side jet control for two different altitudes (10 km/30 km) was numerically investigated and compared with the effectiveness of secondary injection thrust vector control for a supersonic missile at M = 6.2 and 7.0. This comparison also considered the behavior of the transient force signal caused by the side jet thrust increasing with time, resulting in a time delay between reaching the maximum side jet thrust and reaching the maximum control force. It was shown that, in contrast to thrust vector control, the performance of side jet control decreases with increasing altitude as a result of a less effective shock wave boundary layer interaction. With respect to this interaction, it was found that the boundary layer state has a great influence on this interaction and affects the induced force to a certain extent.

The design of realistic shock control bumps requires multiple disciplines. In this paper, several shapes of static shock control bumps are investigated numerically and evaluated under structural considerations due to constraints imposed by a realistic morphing spoiler structure.The bumps are optimized to reduce mainly wave drag of two-dimensional airfoils at a given lift coefficient. The shapes are not only evaluated by their drag reduction potential but also by their structural feasibility. The aerodynamic optimization is done with surrogate based modeling with up to five parameters. It is shown that up to $$30\%$$ 30 % of drag reduction is reached. The penalty of only allowing structurally feasible bump shapes is within an acceptable range of only few percentage points.

Within this work, 2D numerical simulations are used to assess the potential of variable camber (VC) to increase the effectiveness in terms of buffet onset delay of shock control bumps on the airfoil of a HLFC wing. Since the bumps are restricted to the spoiler, positioning of the bump relative to the shock is not ideal, which limits the potential in shifting the transonic buffet boundary to higher lift coefficients. VC is used to move the shock towards the leading edge. This increases the capabilities of shock control bumps in moving buffet onset to higher lift coefficients values over a wide range of bump crest positions and bump heights. In addition, VC enables the usage of a structurally designed bump on a morphing spoiler, which is optimized for wave drag reduction without VC and can be used for buffet onset delay with VC applied.

This work describes parameter studies of shock control bumps focused on delay of buffet onset. A two-dimensional wing section simulation setup was derived, consistently with the 3D flow about the mid-wing of a swept wing aircraft equipped with a hybrid laminar flow control suction system. Two bump shapes respecting constraints of the existing spoiler were defined and analyzed for their ability to delay the onset of transonic buffet. Both types of geometry were found to be able to delay the occurrence of buffet at the given cruise conditions, with both cases requiring bumps whose crest is positioned far downstream. Both smooth curvature devices and structurally realizable bumps showed potential benefits, moving buffet onset to higher lift coefficients $$c_L$$ c L .