New Results in Numerical and Experimental Fluid Mechanics XV

Inhaltsverzeichnis

1. Frontmatter

2. Aerodynamics of Ground Vehicles

   1. Frontmatter

   2. Wind Tunnel Calibration Methodology for Measuring Aerodynamic Loads on Operational High-Speed Trains

      Alexander Buhr, James R. Bell, Lars Siegel, Arne Henning, Martin Köppel, Matthias Härter, Daniela Lauer, Mathilde Laporte, Robert Winkler-Höhn

      Abstract

      A reduced-scale wind-tunnel experimental methodology is presented that provides calibration and insight for measurements on an operational high-speed train; the Deutsche Bahn advanced TrainLab. A 5-hole dynamic pressure probe mounted at the train’s front is used to determine the incoming flow magnitude and direction. A 1:10 scale wind-tunnel experiment was performed to determine probe corrections to remove inflow disturbances induced by the train head. A 1:25 scale wind-tunnel experiment was performed to determine drag, side-force and lift polars for the first carriage. The corrected probe measurements and force calibration provide quasi-steady force estimations under real-world operation. Furthermore, the surface pressure was measured around the circumference at three positions along the length of the first carriage as well as on the coupler cover of the models for additional insight into the causal flow physics and potential for surface-pressure measurements on operational high-speed trains in the future.

   3. Experimental Investigation of the Flow Field of a Notchback and Estate Back Full-Scale DrivAer Model with Ground Simulation

      Lisa Knaus, Johannes Haff, Christoph Lietmeyer, Keith A. Weinman, Uwe Fey, Klaus Ehrenfried, Claus Wagner

      Abstract

      Computational Fluid Dynamics (CFD) methods are becoming increasingly important in the modern automotive design process, where the main objective is to predict the effect of geometry changes on drag and lift. To ensure the accuracy of the results, the methods and setups must be validated, using reliable experimental data from different geometries under identical boundary conditions. The DrivAer model allows the effects of these different geometric modifications to be analyzed with reduced complexity compared to a production car. Two geometry variants, the notchback and the estate back, were investigated in a full-scale wind tunnel including ground simulation. As the flow field of the notchback model has proven to be particularly difficult to reproduce in CFD, the experimental data presented focuses on this area. Additional investigations with the estate rear end configuration show the presence of a pronounced upwash from the underbody, resulting in an upward distortion of the wake of the vehicle. A strong dependence of the rear lift and drag on the diffuser angle is observed. For the present study, validation data were collected using different rakes, equipped with pitot-probes mounted directly on the model and on the traverse system of the wind tunnel.

   4. Calibration of CFD Methods for Drag Prediction Under Unsteady On-Flow in the DLR SWG Wind Tunnel

      Keith A. Weinman, Tobias S. Müller, Uwe Fey, Klaus Ehrenfried

      Abstract

      DLR NGT-Cargo [1] is a logistics concept using rail as the central mode of freight transport. This concept requires the development of competitive transport systems with respect to operational and associated ecological costs. Within this framework an accurate assessment of the aerodynamic drag under realistic vehicle operating conditions is essential. Air flows about trains are characterised by a large range of energetically significant flow scales which challenge accurate numerical simulation. However, CFD methods for certification of trains are now accepted with some restrictions by the transport industry. However at the present time there are no acknowledged international standards concerned with CFD drag prediction under unsteady on-flow conditions for rail vehicles. It is therefore useful to develop sufficiently accurate CFD models to assist in the study of these vehicles, particularly with respect to operational cost and safety requirements. This paper extends previous work assessing the ability of CFD to reproduce the aerodynamic drag of a wind tunnel train model under unsteady on-flow conditions.

   5. Aerodynamic Flow Phenomena at Wheel Rotation Units in Modern 1:1 Vehicle Wind Tunnels

      Michael Willmann, Alexander Wäschle, Peter Dannhäuser, Bettina Frohnapfel

      Abstract

      Modern vehicle wind tunnels, like the Aeroacoustic Wind Tunnel (AAWT) from Mercedes-Benz AG, are a fundamental tool in car aerodynamics development. The AAWT features a boundary layer conditioning system and a five-belt system consisting of one center belt and four wheel rotation units (WRUs) for road simulation. The WRUs are connected to the underfloor balance, which leads to design-related gaps to the stationary floor. The wheels are usually positioned in the center of the WRU belt. Previous measurements have shown the existence of a vehicle-dependent connection of the spacing between the tire sidewall and the edge of the belt on the measured drag coefficient. This study investigates the influence of the flow phenomena in the close area around the WRUs on the vehicle aerodynamics of a sedan. The flow was primarily analyzed by using numerical simulations. For validation purposes, experimental measurements were conducted. This involves the static surface pressure around the WRUs and the velocity field in specific planes. The results show that the low-pressure area above the longitudinal gaps, caused by the flow around the wheels, leads to a pressure gradient within the gap between the balance room and the plenum. The static pressure in the balance room always corresponds to the undisturbed plenum pressure. For this reason, a pressure-driven secondary flow occurs through the gaps. In case there is an interference between the gap leakage and the rotating wheel, the loss area of the tire wake increases. The altered wheel flow affects the rear, which results in local pressure differences. In particular, reducing the track width of the WRUs is critical, as the gap leakage directly blows onto the tire.

3. Aeroelasticity and Structural Dynamics

   1. Frontmatter

   2. Experimental and Numerical Investigation of a Generic Aeroelastic Delta Wing

      Konstantin Bantscheff, Christian Breitsamter

      Abstract

      Predicting structural loads on elastic delta wing structures exhibiting complex flow phenomena is crucial for the aircraft design process. In that regard, wind tunnel experiments are essential for validating numerical simulations. To ensure comparability concerning full-scale applications, the consideration of elastically scaled, deformable wings plays an important role. This work investigates such a design for a 3D-printed half-model delta wing, called Model53e, featuring a leading-edge slat and adjustable trailing-edge flaps. The model is studied for a Reynolds number of \(Re=1.0 \times 10^6\) and high angles of attack. For the experimental setup, unsteady deformation analyses are performed using a photogrammetry setup. Additionally, a comparison with acceleration sensors is conducted to assess the capabilities of photogrammetric measurements. The experimental findings are used to validate aeroelastic simulations. The results show that the mean deformation of the wing significantly influences the surface pressure distribution, while the structural vibration magnitude is small and thus negligible for simulations.

   3. FSTraceInterface: Towards Coupling TRACE with Structure Solvers

      Christian Berthold, Ramandeep Jain, Immo Huismann

      Abstract

      The increasing complexity of turbomachinery blade designs, driven by advanced materials and geometries, necessitates more accurate simulations to ensure engine safety and to increase efficiency. Traditional one-way coupled methods used in industry are becoming insufficient for capturing complex phenomena such as flutter and nonlinear blade vibrations. To address this, a new coupling interface, FSTraceInterface, is being developed to integrate the turbomachinery CFD solver TRACE [8] with the Flow Simulator Data Manager (FSDM) library, enabling robust Fluid-Structure Interaction (FSI) simulations. This integration already supports Conjugate Heat Transfer (CHT) simulations and offers the flexibility to couple TRACE with various structural solvers already present in FSDM. The FSTraceInterface interface facilitates the exchange of data between non-matching fluid and structural meshes. The coupling methodology follows an iterative Gauss-Seidel process, ensuring the consistent exchange of temperature and heat flux between the fluid and structural domains. Verification and validation of the tool are demonstrated through CHT analysis of a plate with finite thickness, and conjugate heat transfer simulations of a turbine blade with internal cooling.

4. Technical Flows

   1. Frontmatter

   2. Numerical Simulations of the Sloshing Behavior in Aircraft Hydrogen Tanks for Different Flight Maneuvers

      Andreas Goerttler

      Abstract

      Sloshing is the dynamic oscillation of liquid within a tank caused by external accelerations. This study quantifies loads in the form of forces exerted by the liquid on the inside of the tank. It focuses on a geometry that represents a realistic version of a hydrogen tank that could be used in the future in a short to medium range aircraft. Five filling levels, corresponding to a third to two thirds full tank, are examined. The reference tank geometry is also configured with two different baffle options in order to investigate the influence of the baffles as anti-sloshing device on the forces and oscillations using the three different versions. Furthermore, it is investigated how the longitudinal forces generated by the sloshing fuel differ depending on the fill level and the load case. It was observed that baffles significantly reduce forces, the anti-sloshing devices lead to an increase in free oscillation frequency, and excitations near the tank’s natural frequency are critical to sloshing behavior in aircraft tanks.

   3. Measuring the Deflection of an Exhalation Jet by an Air Curtain Using PIV

      Andreas Kohl, Daniel Schmeling, Claus Wagner

      Abstract

      Understanding the (particle) transport processes through the planar turbulent air curtain jet – a ventilation concept designed to reduce the spread of airborne particles and thus possible airborne infections – is important prior to its implementation in a future passenger cabin. In the present study, two-dimensional Particle Image Velocimetry is used to investigate the deflection of an exhalation jet by an air curtain as well as the deflection of the air curtain itself as an indicator of a possible breakthrough of the exhalation jet. The strength of the two opposing jets is varied by changing the air curtain momentum flux to achieve four different momentum flux ratios \(\gamma \) between 0.17 \(\le \gamma \le \) 6.88. Strong deflections of the exhalation jet were observed for \(\gamma = 3.47\) and \(\gamma =\) 6.88, while less deflection and a (temporal) breakthrough of the air curtain were obtained for \(\gamma = 0.65\) and \(\gamma =\) 0.17.

   4. Investigating Slip-Velocity Boundary Conditions in Turbulent Thermal Convection Using a Lattice Boltzmann Method

      Sai Ravi Gupta Polasanapalli, Marten Klein, Heiko Schmidt

      Abstract

      The present study investigates the impact of various surface boundary conditions on turbulent Rayleigh–Bénard convection within a cubic cavity configuration. Simulations are conducted with a characteristic-based off-lattice Boltzmann method (LBM) solver at a Rayleigh number of \(Ra=2\times 10^6\) and a Prandtl number of \(Pr=4.38\) (water) using a direct numerical simulation (DNS) approach. The current study considers different boundary conditions such as no-slip, free-slip, and Navier-slip conditions on the walls with variations in slip length and wall-slip anisotropy. Results are evaluated through mean isotherms, streamlines, root-mean-square fluctuations, and Nusselt number. The results obtained demonstrate that the selection of wall-boundary conditions has a significant influence on the flow organization within the cavity and on the heat transfer across it.

   5. Confluence of Wall Shear Stress and its Relation to Vorticity Surface Flux

      Markus Rütten

      Abstract

      Methods stemming from Vortex Dynamics have the potential to support the aircraft design and optimization process. In particular, the analytical description of wall bounded vorticity flux allows to identify promising quantities which could be manipulated in order to reduce overall drag and increase lift. It also reveals the connection between surface forces acting on an aircraft and its geometrical shape contour, but also the pressure and viscous flow related fluxes. By investigating the sources of wall-shear stress and using elements of the boundary vorticity flux relations, an analytical extension could be derived allowing to identify the flow physical relations to wall-shear flow pattern, expressed by the quantity confluence, and to flow separation criteria. These derived quantities are visualized on the surface of a DLR wide body aircraft configuration.

   6. Respiratory Particle Transport Predictions in a Ventilated Generic Room: Comparison of URANS and RANS Simulations with Experiments

      Florian Webner, Andreas Kohl, Daniel Schmeling, Claus Wagner

      Abstract

      The SARS-CoV-2 pandemic highlighted the need to understand aerosol transport and associated disease transmission, and motivated many numerical flow studies using different numerical approaches to predict Lagrangian particle transport for infection risk modelling, with varying degrees of accuracy and computational cost. To evaluate the trade-off between these different flow simulation approaches, we compare particle concentration predictions based on solutions of the steady and unsteady Reynolds-averaged Navier-Stokes (RANS) equations with experimental data. A ventilated generic train entry segment is chosen because it is easy to set up for experiments and numerical flow simulations. Two heated dummies are placed in this ventilated space, one of which continuously exhales aerosol. The RANS approach predicts significant particle accumulations that are not observed in either the experiments or the URANS simulations. However, the averaged absolute deviation from the experimental data is reduced by a factor of 2.4 when URANS simulations are performed, albeit at an eightfold increase in computational cost.

5. Bio- and Microfluids

   1. Frontmatter

   2. High-Resolution Flow Investigations in Membrane Lungs for Understanding Shear-Induced Blood Clot Formation

      Michael Kranz, Daniel Pointner, Maria Stella Wagner, Matthias Lubnow, Karla Lehle, Lars Krenkel

      Abstract

      Complex blood flow phenomena in membrane lungs (MLs) play a crucial role in intra-device clot formation and the occurrence of thromboembolic events. At present, however, the local flow conditions within an ML are not yet sufficiently known. The aim was to gain a deeper understanding of local flow regimes inside MLs by performing highly resolved computational fluid dynamics (CFD) of generic and native fiber mat bundles. Straight cylinders with a diameter of 380 \(\upmu \)m in parallel arrangement were the foundation of the generic model. For validation, a method for reconstructing a native geometry from a microcomputed tomography (\(\upmu \)CT) scan was established, with both models used for CFD. While the generic model showed a symmetrical flow regime without indicating any pathological flow, the native model did show an irregular fiber arrangement and no symmetrical flow regime. In conclusion, the fiber arrangement significantly affects the local flow regimes inside MLs.

   3. Towards Experimental Validation of Models of Shear-Induced Aerosol Generation in the Human Respiratory System

      Johanna Michel, Lars Krenkel

      Abstract

      Numerical modeling is a valuable tool to research shear-induced aerosol generation inside the human respiratory system. While the volume of fluid method and Eulerian wall film models have been used to predict the stripping of particles from the mucus film, sufficient validation data is lacking. Here, we present an experimental method to create such validation data. A film of mucus mimetic hydrogel with an initial thickness of 1 mm covering the floor of a rectangular channel (75.5 mm \(\times \) 25.5 mm \(\times \) 3 mm) was exposed to an airflow with a flow rate of 9.5 and 21.6  L/min. The number of created particles and the emergence of waves on the mucus surface were measured. Shear-induced aerosol generation was triggered successfully and caused an increase of mean particle flow. Different wave profiles were observed at varying film depths.

6. Rotorcraft

   1. Frontmatter

   2. Evaluation of Wind Tunnel Test Data from a Helicopter Model with Novel Fuselage Geometry and Additional Passive Lift Devices

      Erik Brehl, Oliver Schneider

      Abstract

      In the framework of the DLR guiding concept 4 “The Rescue Helicopter 2030” a new medium size wind tunnel model was developed. Two wind tunnel test campaigns were conducted where a number of new ideas were tested like a lower drag optimized fuselage and the option of side wings. The basic model setup without rotor blades was tested first in 2020, where the aerodynamic properties of two different fuselages were measured. In 2024 the model was completed with a spinning rotor and further instrumentation. A sophisticated Data Acquisition System was developed for measuring data from the rotating domain. A completely new set of rotor blades with instrumentation was manufactured in-house. Two wings were designed and manufactured, which could be mounted optionally at the main model structure. This paper evaluates the measured data from sensors in the rotating and non-rotating frame.

   3. Comprehensive Code Modeling Impact on Maximum Thrust, and Beyond, of an Isolated Rotor in Hover: Application of a Free-Wake Method

      Berend G. van der Wall

      Abstract

      A comprehensive rotorcraft simulation code is used to predict the rotor maximum thrust limit (due to stall limits) in hover and the thrust and power characteristics when the collective control angle is further increased. The aerodynamic modeling factors that may significantly affect the results are: steady vs. unsteady aerodynamics, steady vs. dynamic stall, blade tip losses, curvature flow, yaw angle, inflow model, and blade-vortex interaction.

   4. Propeller-Rotor Interaction in Helicopter Air-To-Air Refueling

      Berend G. van der Wall

      Abstract

      During helicopter air-to-air refueling the rotor of the helicopter might enter the slipstream of the tanker aircraft's propeller. Based on blade element momentum theory, the impact of this jet on rotor blade aerodynamics is formulated. Rotor controls required to re-trim are solved analytically and verified by numerical solution of the problem. The collective and cyclic controls needed for disturbance rejection are computed for a typical air-to-air refueling scenario. A propeller wake affecting the retreating side of the rotor requires much more pilot controls to retrim than an impingement on the advancing side. Variations of rotor angle of attack, propeller radius, propeller thrust and rotor thrust are performed.