## Berichte der Arbeitsgruppe Technomathematik (AGTM Report)

### Refine

#### Year of publication

- 1993 (14) (remove)

#### Keywords

- Low-discrepancy sequences (1)
- Monte Carlo method (1)
- Van Neumann-Kakutani transformation (1)
- discrete measure (1)
- distribution (1)
- low discrepancy (1)
- particle method (1)

- 87
- An Analysis of Baganoff" s Shuffle Algorithm (1993)
- The paper presents the shuffle algorithm proposed by Baganoff, which can be implemented in simulation methods for the Boltzmann equation to simplify the binary collision process. It is shown that the shuffle algborithm is a discrete approximation of an isotropic collision law. The transition probability as well as the scattering cross section of the shuffle algorithm are opposed to the corresponding quantities of a hard-sphere model. The discrepancy between measures on a sphere is introduced in order to quantify the approximation error by using the shuffle algorithm.

- 88
- Tensor Spherical Harmonics and Tensor Spherical Splines (1993)
- In this paper, we deal with the problem of spherical interpolation of discretely given data of tensorial type. To this end, spherical tensor fields are investigated and a decomposition formula is described. Tensor spherical harmonics are introduced as eigenfunctions of a tensorial analogon to the Beltrami operator and discussed in detail. Based on these preliminaries, a spline interpolation process is described and error estimates are presented. Furthermore, some relations between the spline basis functions and the theory of radial basis functions are developed.

- 89
- Construction of Particlesets to Simulate Rarefied Gases (1993)
- In this paper a new method is introduced to construct asymptotically f-distributed sequences of points in the IR^d. The algorithm is based on a transformation proposed by E. Hlawka and R. Mück. For the numerical tests a new procedure to evaluate the f-discrepancy in two dimensions is proposed.

- 90
- Domain Decomposition: Linking Kinetic and Aerodynamic Descriptions (1993)
- We discuss how kinetic and aerodynamic descriptions of a gas can be matched at some prescribed boundary. The boundary (matching) conditions arise from requirement that the relevant moments (p,u,...) of the particle density function be continuous at the boundary, and from the requirement that the closure relation, by which the aerodynamic equations (holding on one side of the boundary) arise from the kinetic equation (holding on the other side), be satisfied at the boundary. We do a case study involving the Knudsen gas equation on one side and a system involving the Burgers equation on the other side in section 2, and a discussion for the coupling of the full Boltzmann equation with the compressible Navier-Stokes equations in section 3.

- 91
- A Comparison of Simulation Methods for Rarefied Gas Flows (1993)
- Simulation methods like DSMC are an efficient tool to compute rarefied gas flows. Using supercomputers it is possible to include various real gas effects like vibrational energies or chemical reactions in a gas mixture. Nevertheless it is still necessary to improve the accuracy of the current simulation methods in order to reduce the computational effort. To support this task the paper presents a comparison of the classical DSMC method with the so called finite Pointset Method. This new approach was developed during several years in the framework of the European space project HERMES. The comparison given in the paper is based on two different testcases: a spatially homogeneous relaxation problem and a 2-dimensional axisymmetric flow problem at high Mach numbers.

- 92
- An Adaptive Wavelet Galerkin Algorithm for one and two Dimensional Flame Computations (1993)
- This paper is concerned with the development of a self-adaptive spatial descretization for PDEs using a wavelet basis. A Petrov-Galerkin method [LPT91] is used to reduce the determination of the unknown at the new time step to the computation of scalar products. These have to be discretized in an appropriate way. We investigate this point in detail and devise an algorithm that has linear operation count with respect to the number of unknowns. It is tested with spline wavelets and Meyer wavelets retaining the latter for their better localisation at finite precision. The algorithm is then applied to the one dimensional thermodiffusive equations. We show that the adaption strategy merits to be modified in order to take into account the particular and very strong nonlinearity of this problem. Finally, a supplementary Fourier discretization permits the computation of two dimensional flame fronts.

- 93
- Fast Generation of Low-Discrepancy Sequences (1993)
- The paper presents a fast implementation of a constructive method to generate a special class of low-discrepancy sequences which are based on Van Neumann-Kakutani tranformations. Such sequences can be used in various simulation codes where it is necessary to generate a certain number of uniformly distributed random numbers on the unit interval.; From a theoretical point of view the uniformity of a sequence is measured in terms of the discrepancy which is a special distance between a finite set of points and the uniform distribution on the unit interval.; Numerical results are given on the cost efficiency of different generators on different hardware architectures as well as on the corresponding uniformity of the sequences. As an example for the efficient use of low-discrepancy sequences in a complex simulation code results are presented for the simulation of a hypersonic rarefied gas flow.

- 94
- 3D Eddy-Current Computation Using Krylov Subspace Methods (1993)
- This paper considers the numerical solution of a transmission boundary-value problem for the time-harmonic Maxwell equations with the help of a special finite volume discretization. Applying this technique to several three-dimensional test problems, we obtain large, sparse, complex linear systems, which are solved by using BiCG, CGS, BiCGSTAB resp., GMRES. We combine these methods with suitably chosen preconditioning matrices and compare the speed of convergence.

- 96
- Modelling and Numerical Simulation of Collisions (1993)
- In these lectures we will mainly treat a billard game. Our particles will be hard spheres. Not always: We will also touch cases, where particles have interior energies due to rotation or vibration, which they exchange in a collision, and we will talk about chemical reactions happening during a collision. But many essential aspects occur already in the billard case which will be therefore paradigmatic. I do not know enough about semiconductors to handle collisions there - the Boltzmann case is certainly different but may give some idea even for the other cases.