Silvia Reuß

Scale-resolving simulations of wall-bounded flows with an unstructured compressible flow solver

The fully-developed channel flow at \(Re_\tau \approx 395\) is used to validate scale-resolving simulations with the unstructured compressible DLR-TAU code. In a sensitivity study based on wall-resolved LES a low-dissipative spatial scheme is derived, which allows to predict the channel flow in fair agreement with DNS. Then the scheme is used in Improved Delayed DES computations in order to assess TAU’s capabilities for wall-modelled LES. As pointed out in a grid study, a tangential resolution of about \(\varDelta x^+ \approx 40\), \(\varDelta z^+ \approx 20\) is required to obtain acceptable mean-flow results. Besides, the combination of IDDES with a vorticity-dependent subgrid filter width is shown to yield consistent results, and the effect of the underlying RANS approach up to Reynolds-stress modelling is analysed.

Hybrid RANS/LES simulations of a three-element airfoil

In this paper the Spalart-Allmaras based Delayed Detached Eddy Simulation (DDES [1]) and Improved Delayed Detached Eddy Simulation (IDDES [2]) are used to simulate the flow about an industrially relevant airfoil-configuration with deployed high-lift devices. Here, the potential advantage of the computationally very challenging hybrid approaches over pure RANS simulations in the case of incipient separation is investigated.

Scale-Resolving Simulations with a Low-Dissipation Low-Dispersion Second-Order Scheme for Unstructured Flow Solvers

A new low-dissipation low-dispersion second-order scheme is applied to scale-resolving flow simulations using compressible and incompressible unstructured finite volume solvers. In wall-resolved and wall-modeled large-eddy simulations of the plane channel flow, the new scheme yields substantial improvements compared to the more dissipative/dispersive standard central scheme over a considerable range of Reynolds numbers. For general hybrid Reynolds-averaged Navier–Stokes/large-eddy simulations, a numerical blending approach is derived that uses a local sensor function to switch between the new scheme in the large-eddy simulation branch and the standard scheme in inviscid flow regions. After determining a suitable sensor formulation, the hybrid numerical scheme is applied to simulate a backward-facing step flow, for which satisfactory results and a reduced grid sensitivity are obtained. To demonstrate its potential in relevant aeronautical flows, the new scheme is successfully applied to hybrid Reynolds-ave...

Simulation of Wing and Nacelle Stall

Numerical stall simulations are challenging in terms of physical models involved, overall computation effort, and the needed efforts for validation. The present paper describes coordinated, fundamental research into new simulation methodologies and their validation for wing and nacelle stall that also include the effects of atmospheric gusts. The research is carried out by the DFG funded Research Unit FOR 1066, which is composed of German Universities and the German Aerospace Center, DLR. The Research Unit investigates advanced models of turbulence, advanced physics-based gust models, and new numerical approaches for gust simulation. These modeling and computational activities are supplemented by new validation experiments, that aim at providing stall data on wings and engine nacelles with well defined, generic distortions of the onset flow.

Simulation of Wing Stall

Simulation capabilities for low-speed aircraft stall prediction are important for determining the limits of safe aircraft operations during design processes. The simulations are extremely demanding in terms of physical models involved, overall computation effort, and the needed efforts for validation. The present paper describes coordinated, fundamental research into new simulation methodologies for wing stall that also include the effects of atmospheric gusts. The research is carried out by the DFG funded Research Unit FOR 1066 composed of German Universities and the German Aerospace Center, DLR. The research Unit investigates advanced models of turbulence, advanced physics-based gust models, and new numerical approaches for gust simulation. These modeling and computational activities are supplemented by an unique validation experiment, that aims at providing stall data on a high-lift wing with well defined, generic distortions of the onset flow.

Assessment of Local LES-Resolution Sensors for Hybrid RANS/LES Simulations

Different sensors that provide a measure for the resolution of the LES content in hybrid RANS/LES computations are proposed and investigated. In an a-priori test on isotropic turbulence a suitable sensor is identified. Based on that sensor an automatic local mesh refinement is performed for an IDDES of the flow over a backward facing step. The results obtained on locally adapted grids are compared to results on globally refined grids. It is shown, that the proposed sensors can detect underresolved LES regions and that the local mesh refinement can help to reduce resolution errors caused by a too coarse grid spacing.

Hybrid RANS/LES Study of the Development of an Airfoil-Generated Vortex

We consider the numerical simulation of a generic lateral vortex, that is generated by a rapidly deflected NACA0021 airfoil. We use the SST k-\(\omega \) URANS approach and a hybrid RANS-LES method based on the SST model, where a zonal RANS-LES interface is located little downstream of the airfoil trailing edge. The results are assessed by reference with the experimental data. The mean flow behavior and the induced angle of attack of the vortex can be predicted satisfactorily by the URANS approach and the improvement using hybrid RANS/LES is small. The hybrid RANS/LES results exhibit a delayed shear layer instability of the wake, and the arising 2D roller type vortices lead to a significant overprediction of the turbulent shear stress. We then apply a stochastic forcing in the region of the airfoil trailing edge. This leads to a fast generation of turbulent content in the wake and clearly improves the predictions for the turbulent stresses.

Computations of Separated Flows with a Hybrid RANS/LES Approach

In an effort to accurately compute nacelle stall processes, the research unit FOR1066 (Simulation of Wing and Nacelle Stall) has been working extensively on the development of advanced simulation methods. Due to the high dependency of the separation aerodynamics on the turbulent structures developed within the boundary layer, embedded LES methods appear promising to reliably compute such processes. Nevertheless, these approaches are characterized by exhibiting a very long “grey area“ (also known as adaptation distance) that may lead to the degradation of the whole solution. To shorten this adaptation distance, an advanced synthetic turbulence generator is implemented that forces the development of resolved turbulence at the inlet of the LES domain. The implementation is accessed for a zero pressure gradient flat plate, the HGR-01 airfoil, and a subsonic flow-through nacelle case. The numerical results are validated against experimental data and compared with numerical results without applying synthetic turbulence forcing. Results show that the implementation considerably reduces the required adaptation distance enhancing the overall solution. However, the success of the computation also depends on the solver numerical settings and the grid resolution.

Investigation of the Resolution Requirements for a Hybrid RANS/LES Simulation of a Multi-element Airfoil

This work is dedicated to the investigation of the resolution requirements for hybrid RANS/LES simulations for aerodynamic flows at high-lift. First, results of a local DDES for a limited section of a highlift wing with deployed slats and flaps of a full-aircraft configuration are presented. Based on the resolution of this simulation the computational effort for a hybrid RANS/LES simulation of the complete high-lift wing is estimated. Since this estimate results in prohibitively high costs, the focus is shifted to the scale-resolving simulation of a quasi two-dimensional segment of a three-element wing for the further investigations. Three approaches, a zonal DDES, a global DDES and an IDDES, are presented and the last one is evaluated with respect to the resolution of the boundary layers as well as the free shear layers.