Les Investigation of Coherent Structures in Boundary Layers and Wakes. Volume II: Wake Around Square Cylinder

摘要

Purpose of the present investigation is to assess the feasibility of simulating and studying coherent structures in turbulent shear layers, malting use of Large Eddy Simulations (LES). Volume I of present report has described the general background, the main development and the results obtained for wall bounded turbulent flow, particularly the turbulent charnel flow at equilibrium. The present Volume II is devoted to the study of the flow around an obstacle, in particular a cylinder with square cross section and infinlte length in span- wise direction. This test case belongs to a completely different class of flow respect the one analyzed in Volume I, and it is a rather severe test case for the LES code. Most of the advanced features of the code were exploited, and different simulations were performed, to collect, whenever possible, sufficient data to draw conclusions and remarks. Differently from the case of channel flow, were the code has been extensively validated and analyzed in detail, down to the level of turbulent kinetic energy budget, the present flow constitutes a still active area of basic research. The higher complexity of the flow, the absence of a second direction of homogeneity, beside the span-wise direction, the existence of a dominant frequency in the flow, the necessity of employing upwinding discretization for advection terms and other numerical difficulties have led to such complexities of execution that only the very basic set of statistics (statistical moments of first and second order) were obtained. At the same time, limits and draw-backs for current LES code implementation have been outlined. Most of the efforts were devoted to reach a sufficient similarity in behavior between LES simulation and reference experimental data; this attempt has led to build grids not perfectly suited to analyze structures applying the criteria based on the gradient of velocity tensor introduced in Volume L.