Les Investigation of Coherent Structures in Boundary Layers and Wakes. Volume III: Development of a Parallel Unstructured Grid Les Code

摘要

The purpose of the present investigation is to assess the feasibility of simulating and studying coherent structures in turbulent shear layers, making use of Large Eddy Simulations (LES). Volumes I and II were devoted to the investigation of coherent structures in LES solutions obtained using a structured-grid finite-difference code, first in an attached turbulent flow, and then in the wake behind a square cylinder. The structured grid code used in Parts I & II, despite its multi-domain capability, is limited in the domain geometries it can handle. The third part of the work was therefore devoted to the development of an unstructured grid LES code able to compute turbulent flows over arbitrary two-dimensional geometries. A combined finite-element/Fourier spectral space discretization scheme was selected, as it combines optimally the geometrical flexibility provided by the finite element scheme and the computational efficiency resulting from the decomposition in Fourier modes in the outof-plane direction. Indeed, thanks to this decomposition, and a suitable treatment of the nonlinear convective terms, the 3D flow problem is transformed into a series of 2D problems in Fourier space, completely decoupled within each time step. In addition, the decoupling allows for an easy parallelization by partitioning the work in Fourier space rather than physical space. Also, the stabilized finite element technique selected for the in-plane discretization provides an accuracy superior to its finite volume counterpart on the same unstructured grid and for the same discretization stencil.