Dieter Schwamborn

A grid and flow adaptive wall-function method for RANS turbulence modelling

This paper presents a grid and flow adaptive wall-function method for RANS turbulence modelling with emphasis on aerodynamic flows. A near-wall grid adaptation technique ensures a locally appropriate resolution depending on both the near-wall flow physics to be captured and the range of validity of the wall-function model. The near-wall RANS solutions of the Spalart-Allmaras and SST k-@w turbulence model are investigated near stagnation points and subsequent not yet fully developed turbulent flow, and in regions of adverse pressure gradient before separation. These are compared with the corresponding turbulence model specific universal wall-functions and suggestions for the design of wall-function methods for non-equilibrium flows are given. Regions of non-equilibrium flow are detected by a flow based sensor and near-wall grid adaptation is then made possible due to the hybrid character of the wall-functions.

MEGAFLOW: Parallel complete aircraft CFD

Abstract As a consequence of the worldwide competition in the aircraft market requirements for the accurate prediction of aerodynamic performance and optimization of configurations have increased very much. More sophisticated wind tunnel as well as high quality CFD techniques have become necessary and essential tools for aircraft industry aerodynamic development groups. This requires an ongoing struggle for efficiency improvements where parallel computing is one of the major issues. This paper considers the MEGAFLOW activities in the area of parallel flow solvers including their application and use in the industrial framework. It describes the parallelization principles for the structured multi-block Navier–Stokes code FLOWer and the unstructured Navier–Stokes code TAU. Principle results for industrial applications are given with respect to efficiency, speed-up, load balancing, etc. Computational examples demonstrate the quality of the solvers for 3D flow over wing/body configurations as well as complete transport aircraft.

The MEGAFLOW Project — Numerical Flow Simulation for Aircraft

Some years ago the national CFD project MEGAFLOW was initiated in Germany, which combined many of the CFD development activities from DLR, universities and aircraft industry. Its goal was the development and validation of a dependable and efficient numerical tool for the aerodynamic simulation of complete aircraft which met the requirements of industrial implementations. The MEGAFLOW software system includes the block-structured Navier-Stokes code FLOWer and the unstructured Navier-Stokes code TAU. Both codes have reached a high level of maturity and they are intensively used by DLR and the German aerospace industry in the design process of new aircraft. Recently, the follow-on project MEGADESIGN was set up which focuses on the development and enhancement of efficient numerical methods for shape design and optimization. This paper highlights recent improvements and enhancements of the software. Its capability to predict viscous flows around complex industrial applications for transport aircraft design is demonstrated. First results concerning shape optimization are presented.

ATAAC – An EU-Project Dedicated to Hybrid RANS/LES Methods

This paper presents the European collaborative project “Advanced Turbulence simulation for Aerodynamic Application Challenges”, i.e. its background, objectives, approach and status. As an example of the outcome of this project, results obtained by some partners for one of the test cases employed are compared and discussed highlighting the status of today’s hybrid RANS/LES approaches.

Numerical Aerodynamics at DLR

Abstract Some years ago the national CFD project MEGAFLOW was initiated in Germany to combine many of the CFD development activities from DLR, universities and aircraft industry. Its goal was the development and validation of a dependable and efficient numerical tool for the aerodynamic simulation of complete aircraft which met the requirements of industrial implementations. The MEGAFLOW software system includes the block-structured Navier-Stokes code FLOWer and the unstructured Navier-Stokes code TAU. Both codes have reached a high level of maturity and they are intensively used by DLR and the German aerospace industry in the design process of new aircraft. Recently, the follow-on project MEGADESIGN and MEGAOPT were set up which focus on the development and enhancement of efficient numerical methods for shape design and optimization. This article highlights recent improvements of the software and its capability to predict viscous flows for complex industrial aircraft applications.

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.

Improved Embedded Approaches

In contrast to the non-zonal, DES-like, hybrid approaches, in which a transition from RANS to LES relies upon a natural instability of separated shear layers in massively separated flows, the zonal RANS-LES (actually, RANS – Wall Modelled LES or RANS-WMLES) hybrids imply the presence of a sharp interface between the flow regions treated by RANS and LES. The location of this interface may be arbitrarily specified by the user based on their understanding of the flow physics, available computational resources or the objectives of the simulation, e.g., a need for unsteady flow characteristics.

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.

Chimera Simulations of Transported Large-Scale Vortices and Their Interaction with Airfoils

A Chimera technique for moving grids is applied to simulate the transport of large-scale vortices convected by a mean velocity field over large distances and their interaction with an airfoil. While keeping the numerical dissipation at a minimum, the Chimera approach allows to resolve the vortex on a local fine grid whereas the unstructured global background grid can be relatively coarse. Having examined the vortex dissipation rate numerically, the interaction of a Rankine-like type vortex with a NACA 0012 airfoil and an ONERA-A airfoil near stall, respectively, is simulated. The interaction can be interpreted as a time-dependent variation in the effective angle of attack. A subsequent computation of a flapping NACA 0012 airfoil turns out to be an insufficient approximation of the vortex-airfoil interaction.

DLR’s Contribution to FLOMANIA — Methods, Models, Applications

Here we briefly review the two numerical methods used by DLR during the FLOMANIA project as well as some remarks on the turbulence models available in these codes including some remarks on the implementation of DES and RSM. Furthermore some additional results obtained with the latter models are presented which are not mentioned elsewhere in this book.