Date and Place: Thursdays in Room 32-349. For detailed dates see below!
In the Scientific Computing Seminar we host talks of guests and members of the SciComp team as well as students of mathematics, computer science and engineering. Everbody interested in the topics is welcome.
List of Talks
11:30SC Seminar Room 32-349
Patrick Mischke, TU Kaiserslautern
Implementation of a POD Based Surrogate Model for Aerodynamic Shape Optimization
Computational Fluid Dynamics (CFD) simulations are often used with aerodynamic shape optimization in mind. There exist several approaches to perform optimization, for example employing one shot methods or using adjoint computations by utilizing algorithmic differentiation techniques while running the flow solver. However, it may sometimes be preferred to inspect the flow fields across the whole design space, for instance if those strategies seem to operate too local for the problem on hand. This issue can be targeted by the use of surrogate models. A surrogate model predicts the flow field for a given design input in a computationally cheaper way, but also with less accuracy than the full fluid dynamics simulation. The surrogate model uses a set of training data to find reasonable model parameters for future design inputs. The goal of my bachelor thesis, that I present in this talk, was the implementation of an surrogate model for CFD using Proper Orthogonal Decomposition (POD) and Kriging regression. The challenges encountered are discussed, and the NACA 0012 airfoil at transonic flow conditions is presented as test case. The characteristic shock front and the high number of design parameters describing the airfoil geometry of its flow field lead to a rather expensive training process for the surrogate model. However, the discussed approach may still be valuable for other geometries or as foundation for surrogate models using other techniques.
11:30SC Seminar Room 32-349
Dr.-Ing. Bernhard Eisfeld, DLR Braunschweig, Institut für Aerodynamik und Strömungstechnik, C²A²S²E Center for Computer Applications in AeroSpace Science and Engineering
Reynolds-Stress Modelling – Concepts, Advances and Challenges
The Reynolds-Averaged Navier-Stokes (RANS) equations are still the backbone of numerical flow simulations in industrial applications. Hence, a turbulence model is required for closure, which decides about the accuracy of the predictions.
Many models are based on the assumption of a flow dependent eddy viscosity added to the molecular viscosity of the fluid. While agreeing with the observation of enhanced momentum transfer due to turbulent fluctuations, this is a significant simplification of the physics of turbulent flow, limiting the predictive accuracy in complex flow situations.
Improvement is expected by Reynolds-stress modelling based on the transport equation for the individual components of the Reynolds-stress tensor and for an additional length-scale providing variable. In this case, the modelling is restricted to the different terms of the Reynolds-stress transport equation and the length-scale equation that is usually taken over from corresponding eddy-viscosity models and considered the weakest link of the approach.
The presentation will introduce the Reynolds-stress transport equation, explain the physical significance of its terms and outline the corresponding modelling approaches.
Recent advances have been achieved by developing a length-scale correction. The underlying idea will be presented and its improvement on the prediction of separated flows will be demonstrated.
Turbulence modelling is challenged by the variety of flow phenomena that need to be treated. This will be underlined by a theoretical analysis of self-similar free-shear flows, predicting a layer of constant Reynolds-stress anisotropy. Experimental data confirm its existence, revealing differences in the turbulence structure between different flows. Hence, a self-adaptive modelling strategy is required, applying tailored models to automatically identified regions of the flow field. An example will be given, demonstrating the potential of such tailored modelling.
11:30SC Seminar Room 32-349
Matthias Freimuth, TU Kaiserslautern/MTU Aero Engines
The multiphysics coupling tool preCICE in the context of adjoint-based aeroelastic designs
In the emerging field of coupled numerical simulations including two or more physical fields the capabilities of the multiphysics coupling tool preCICE are discussed with respect to aeroelastic design in this talk. With an adapter for the structural solver within the multiphysics solver SU2 a new link between SU2 and preCICE is presented. The development process and the key ingredients for multiphysics coupling are explained and the result is shown with a simple testcase. A smaller aspect also addressed in this talk is the discrete adjoint method for the gradient computation to efficiently optimize in a fluid structure interaction framework. The talk is based on the content of my masters thesis at MTU Aero Engines in Munich and will be given in english.
14:30SC Seminar Room 32-349
Minimization by Successive Abs-Linearization: Recent Developments
For finite dimensional problems that are unconstrained and piecewise smooth the optimization based on successive abs-linearisation is well analysed yielding for example linear or even quadratic convergence under reasonable assumptions on the function to be optimised. In this talk we discuss the extension of this approach to the more general class of nonsmooth but still Lipschitz continuous functions covering also the Euclidean norm. For this purpose, we introduce the so-called clipped root linearisation and present first numerical results.
Furthermore, we sketch the extansion of this approach to the infinite dimensional setting.
11:30SC Seminar Room 32-349
Manfred Schneider, retired senior confirmed advisor Flight-Physics, Airbus Defense & Space Deutschland GmbH
Noise Simulation at FTEG high-lift airfoil using hybrid RANS/LES Model
This study focuses on the development, validation and application of the interdisciplinary computational fluid dynamics/computational aeroacoustics (CFD/CAA) method with the name Flight-Physics Simulator AEOLus (FPS-AEOLus). FPS-AEOLus is based on enhanced conservative, anisotropic, hybrid Reynolds-averaged Navier-Stokes/ Large-Eddy Simulation (RANS/LES) techniques to solve an aerodynamic flow field by applying the unsteady, compressible, hyperbolic Navier–Stokes equations of second order. The two-layer SSG/LRR-ω differential Reynolds stress turbulence model presented, combining the Launder-Reece-Rodi (LRR) model near walls with the Speziale-Sarkar-Gatski (SSG) model further apart by applying Menter’s blending function F1. Herein, Menter’s baseline ω-equation is exploited for supplying the length scale. Another emphasis is put on the anisotropic description of dissipation at close distance to the solid wall or in wake area for describing the friction-induced surface-roughness behaviour in viscous fluid physics and swirling wake effects. For that purpose, the SSG/LRR-ω seven-equations Reynolds stress turbulence model with anisotropic extension was realized, therefor the theory is described in general. Beyond that, a modified delayed detached-eddy simulation (MDDES) and a scale adaptive simulation (SAS) correction to capture the stochastic character of a large-eddy-type unsteady flow with massive flow separations in the broad band is implemented. To demonstrate the time-dependent noise propagation having wave interference a linearized Euler equation (LEE) model using a combined Momentum- and Lamb-vector source have been applied into the CFD/CAA – method.
The DLR 15 wing, a High-Lift device in landing configuration having a deployed slat and landing flap is studied experimentally and numerically. The first part of the application deals with the steady flow investigation; however, the same turbulence model is used for the unsteady flow case without the enclosed time derivatives. The second part concentrates on unsteady modelling for the Navier–Stokes and Linearized Euler field. With this new combined CFD/CAA – method, steady and unsteady numerical studies for the high-lift wing configuration for discovering the aerodynamic and –acoustic propagation effects are shown, discussed and when experimental data were available validated. The High-Lift wing has a constant sweep angle of Λ=30° to investigate possible cross-flow; to realize this, periodic boundary conditions were set in spanwise direction.