Johan Hoffman

Computation of Mean Drag for Bluff Body Problems Using Adaptive DNS/LES

We compute the time average of the drag in two benchmark bluff body problems: a surface mounted cube at Reynolds number 40000, and a square cylinder at Reynolds number 22000, using adaptive DNS/LES. In adaptive DNS/LES the Galerkin least-squares finite element method is used, with adaptive mesh refinement until a given stopping criterion is satisfied. Both the mesh refinement criterion and the stopping criterion are based on a posteriori error estimates of a given output of interest, in the form of a space-time integral of a computable residual multiplied by a dual weight, where the dual weight is obtained from solving an associated dual problem computationally, with the data of the dual problem coupling to the output of interest. No filtering is used, and in particular no Reynolds stresses are introduced. We thus circumvent the problem of closure, and instead we estimate the error contribution from subgrid modeling a posteriori, which we find to be small. We are able to predict the mean drag with an estimated tolerance of a few percent using about

Adaptive Finite Element Methods for Incompressible Fluid Flow

10^5

Framework for Massively Parallel Adaptive Finite Element Computational Fluid Dynamics on Tetrahedral Meshes

mesh points in space, with the computational power of a PC.

Adaptive simulation of turbulent flow past a full car model

We present recent work on the following issues of CFD: (i) discretization of the non-stationary incompressible Navier-Stokes equations, (ii) solution of the discrete system at each time step, (iii) hydrodynamic stability, (iv) adaptive error control and a posteriori error estimates, (v) transition to turbulence and (vi) turbulence modeling.

On Duality-Based A Posteriori Error Estimation in Various Norms and Linear Functionals for Large Eddy Simulation

In this paper we describe a general adaptive finite element framework for unstructured tetrahedral meshes without hanging nodes suitable for large scale parallel computations. Our framework is designed to scale linearly to several thousands of processors, using fully distributed and efficient algorithms. The key components of our implementation, local mesh refinement and load balancing algorithms, are described in detail. Finally, we present a theoretical and experimental performance study of our framework, used in a large scale computational fluid dynamics computation, and we compare scaling and complexity of different algorithms on different massively parallel architectures.

UNIFIED CONTINUUM MODELING OF FLUID-STRUCTURE INTERACTION

The massive computational cost for resolving all turbulent scales makes a direct numerical simulation of the underlying Navier-Stokes equations impossible in most engineering applications. We present recent advances in parallel adaptive finite element methodology that enable us to efficiently compute time resolved approximations for complex geometries with error control. In this paper we present a LES simulation of turbulent flow past a full car model, where we adaptively refine the unstructured mesh to minimize the error in drag prediction. The simulation was partly carried out on the new Cray XE6 at PDC/KTH where the solver shows near optimal strong and weak scaling for the entire adaptive process.

A computational study of turbulent flow separation for a circular cylinder using skin friction boundary conditions

We derive a posteriori error estimates for the filtered velocity field in a large eddy simulation in various norms and linear functionals. The a posteriori error estimates take the form of an integral in space-time of a discretization residual and a modeling residual times a dual weight. The discretization residual is directly computable, and the modeling residual is estimated by a scale similarity model. We approximate the dual weight by solving an associated linearized dual problem numerically. Computational examples from transition to turbulence in Couette flow are presented.

Unicorn: a unified continuum mechanics solver

In this paper, we describe an incompressible Unified Continuum(UC) model in Euler (laboratory) coordinates with a moving mesh for tracking the fluid-structure interface as part of the discretizatio ...

Adaptive Computation of Aeroacoustic Sources for a Rudimentary Landing Gear Using Lighthill's Analogy

In this paper we present a computational study of turbulent flow separation for a circular cylinder at high Reynolds numbers. We use a stabilized finite element method together with skin friction boundary conditions, where we study flow separation with respect to the decrease of a friction parameter. In particular, we consider the case of zero friction corresponding to pure slip boundary conditions, for which we observe an inviscid separation mechanism of large scale streamwise vortices, identified in our earlier work. We compare our computational results to experiments for very high Reynolds numbers. In particular, we connect the pattern of streamwise vorticity in our computations to experimental findings of spanwise 3d cell structures reported in the literature.

On the Stability of the Dual Problem for High Reynolds Number Flow Past a Circular Cylinder in Two Dimensions

This chapter provides a description of the technology of Unicorn focusing on simple, efficient and 10597 general algorithms and software for the Unified Continuum (UC) concept and the adaptive General 10598 Galerkin (G2) discretization as a unified approach to continuum mechanics.