Numerische Mathematik und Wissenschaftliches Rechnen
Seminar Numerische Mathematik / Numerical mathematics seminars
aktuelles Programm / current program
Donnerstag, 14. 02. 2019, 14:30 Uhr (ESH)
Philipp Schroeder (Georg-August-Universität Göttingen)
Building bridges: Pressure-robust FEM, Beltrami flows and structure preservation in incompressible CFD
In this talk, an attempt is made to explain why pressure-robust FEM are (by construction) superior when it comes to simulating incompressible flows with a large amount of large-scale/coherent structures. We show that flows with large-scale structures are frequently dominated by large gradient (curl-free) forces. Pressure-robust methods are designed in exactly such a way that they can treat those forces more accurately than non-pressure-robust methods. Furthermore, the class of generalised Beltrami flows is introduced and placed in context with structure preservation and pressure-robustness. Several numerical examples, both in 2D and in 3D, of laminar and turbulent flows, are shown which underline our statements. All pressure-robust computations make us of (high-order) exactly divergence-free H(div)-(H)DG methods.
Mittwoch, 23. 01. 2019, 15:00 Uhr (ESH)
Prof. Vladimir A. Garanzha (Russian Academy of Sciences)
Moving adaptive meshes based on the hyperelastic stress deformation
We suggest an algorithm for the time-dependent mesh deformation based on the minimization of hyperelastic quasi-isometric functional without introducing time derivatives. The source of deformation is a time-dependent metric tensor in Eulerian coordinates. Due to the nonlinearity of the Euler-Lagrange equations we can not assume that the norm of its residual is reduced to zero at each time step. Thus, time continuity of the moving mesh is not guaranteed since iterative minimization may result in considerable displacements even for infinitely small time steps. To solve this problem, we introduce a special variant of factorized representation of Lagrangian metric tensor and nonlinear interpolation procedure for factors of this metric tensor. Continuation problem with respect to interpolation parameters is similar to the hypoelastic deformation with a controlled stress relaxation. At each time step we start with a Lagrangian metric tensor which completely eliminates internal elastic stresses and makes current mesh an exact solution of the Euler-Lagrange equations. The continuation procedure gradually introduces internal stresses back while forcing the deformation to follow the prescribed Eulerian metric tensor. At each step of the continuation procedure the functional is approximately minimized using a few steps of the parallel preconditioned gradient search algorithm. We derive an auxiliary discrete evolution equation for target shape matrices (factors of Lagrangian metric tensor) which resembles stress relaxation equations in hypoelasticity. We present 2d and 3d examples of moving deforming meshes which serve to represent moving bodies for parallel immersed boundary flow solver.
Donnerstag, 10. 01. 2019, 14:00 Uhr (ESH)
Dr. Naveed Ahmed (Lahore University of Management Sciences, Pakistan)
Numerical comparisons of finite element stabilized methods for a 2D vortex dynamics simulation at high Reynolds number
In this talk, I will present an up-to-date and classical Finite Element (FE) stabiliz ed methods for time-dependent incompressible flows. All studied methods belong to the Variational MultiScale (VMS) framework. So, different realizations of stabilized FE-VMS methods are compared in a high Reynolds number vortex dynamics simulation. In particular, a fully Residual-Based (RB)-VMS method is compared with the classical Streamline-Upwind Petrov--Galerkin (SUPG) method together with grad-div stabilization, a standard one-level Local Projection Stabilization (LPS) method, and a recently proposed LPS method by interpolation. These procedures do not make use of the statistical theory of equilibrium turbulence, and no ad-hoc eddy viscosity modeling is required for all methods. Applications to the simulation of a high Reynolds numbers flow with vortical structures on relatively coarse grids are showcased, by focusing on a two-dimensional plane mixing-layer flow. Both Inf-Sup Stable (ISS) and Equal Order (EO) (H^1)-conforming FE pairs are explored, using a second-order semi-implicit Backward Differentiation Formula (BDF2) in time. Based on the numerical studies, it is concluded that the SUPG method using EO FE pairs performs best among all methods. Furthermore, there seems to be no reason to extend the SUPG method by the higher order terms of the RB-VMS method.