Study On Data Processing Methods For Extraction Of Turbulent Coherent Structures In The Bluff Near Wakes

The flow past a bluff body, as a typical turbulent shear flow, exists extensively in many industrial applications such as the water conservancy engineering, aerospace and architecture etc. Investigation on turbulent wakes behind a bluff body helps in understanding of many dynamical processes, which is of significant practical importance. Study on turbulent wakes, either experimentally or numerically, could afford abundant turbulent data. However, how to introduce effective mathematical post-processing methods into calculating turbulent properties more precisely and extracting turbulent structures more vividly becomes one of many researchers’ long-term focuses.In this paper, investigation of data processing methods for two typical bluff bodies (circular cylinder and disk) are mainly discussed based on PIV and LES data. Chapter1introduces mathematical backgrounds of the involved methods in our study and an extensive literature review of study on the two bluff bodies’ turbulent wakes. Chapter2conducts an experimental measurement of a circular cylinder’s near wake with a conventional PIV system in a wind tunnel at Re=7100. Considering the insufficiency of PIV results’ spatial resolution, the method Large-eddy PIV is applied to calculation of turbulent dissipation rate. Turbulent dimensional analysis is firstly presented to verify satisfaction of this method, and its dependence on grids is discussed. Finally, the distribution of turbulent dissipation rate in near wake of a circular cylinder is calculated and compared to those in mediate and far wakes. Results infer that when spatial resolution is located within inertial subrange, Large-eddy PIV is qualified to estimate distribution of turbulent dissipation rate in a circular cylinder’s near wake well.Chapter3is mainly about LES simulation of the flow over a disk at high Reynolds number (Re=22,000). Firstly numerical results are compared to those in PIV experiments in terms of turbulent statistical first and second moments, and spectral analysis is performed using FFT to discuss the vortex shedding mechanisms in the near wake of a circular disk. Large-scale structure’s location in azimuthal direction is investigated through two-point correlated analysis in different cross sections of wake.In Chapter4,2D POD decomposition is applied to study LES data in the near wake of a circular disk at Re=170and22,000. Through comparison of mode patterns at two Reynolds numbers, large-scale structure of a disk’s wake at high Reynolds number is analysed. The reconstructed field of3D POD modes is given and helical structure is extracted from the near wake of a circular disk to support the discussion in2D-POD decomposition.