Large Scale Vortex Structures and Intermodal Energy Transfer in the Wake of Wall-Mounted Bluff Bodies

Abstract

The turbulent wake of wall-mounted finite bluff bodies is a heuristic model for the study of large-scale coherent structures and vortex dynamics processes. This thesis presents: (i) a methodology for educing the topology of coherent structures from planar velocity data and simultaneous wall pressure, and (ii) a quantitative analysis for the energetic interactions between the scales of motions, in the non-homogeneous turbulent wake. The proposed methodology addresses the shortcomings of the traditional phase averaging approach in capturing the non-harmonic contributions to account for the cycle-to-cycle variations of the coherent fluctuations. The Extended Proper Orthogonal Decomposition technique is modified to incorporate the maximum pressure-velocity correlations by taking into account the pressure sensor time history. The method is used to construct the three-dimensional coherent fluctuations and the global modes in the highly modulated turbulent wake, for which a Galerkin-based energy flow analysis is subsequently conducted. The results are used to identify the sequence and direction of the energy transfer among dominant fluctuating scales. The transitional states are characterized with sequential energetic interactions of the unstable harmonic pair with higher harmonics and a non-oscillatory mode representing base flow variations. It is found that the couplings of the unstable harmonic pair and the non-oscillatory mode are described through the mean-field manifold approximation.