Prediction and Measurement of the Dynamics of Multi-Element Wake Flows

Wake flows can display a global instability that results in large-scale, anti-symmetric vortex shedding. Several prior studies have shown that the dynamics of this wake shedding can vary significantly in multi-element flows, where multiple wakes are placed side by side. The frequency, symmetry, and intermittency of wake shedding are highly dependent on the spacing between wakes and the number of wakes. The goal of this study is to map the dynamics of multi-element wake systems as a function of the number of wakes, wake spacing, and Reynolds number using two methods. First, we use a simplified linear stability analysis of piecewise flow profiles to predict the frequency, growth rate, and symmetry of the oscillations over a large range of these parameters. A similar parameter study is also done experimentally in a new facility. Stereo high-speed particle image velocimetry is used to measure the flow, and modal analyses, like spectral proper orthogonal decomposition and wavelet analysis, are used to capture the flow dynamics. Finally, the trends from the linear stability analysis and the experiments are compared to help us determine whether the linear approximations are able to correctly capture the complex phenomena present in multi-element flows.