Master's Thesis

Preliminary three-dimensional visualization of the coherent structures within the turbulent, chaotic wake behind the three-dimensional axisymmetric bluff body is explored. Various modal decomposition methods such as Proper Orthogonal Decomposition (POD), Dynamic Mode Decomposition (DMD) and Optimal Mode Decomposition (OMD) were used to extract the coherent realizations. From 19 local mean Centre of Pressure (COP) locations, \(\Theta_i\), the dominant structures with the Strouhal number ≈ 0.2 and 0.06 were extracted utilizing OMD. For the ascending Θi with a concentrated local mean COP region of \(\Theta_i > π\), the sequence of modes that are color-corrected and image-matched, \(S_{eq}\), is obtained with the remainder matrix, \(R_{em}\), to describe the dynamics of the realization. Finally, \(S_{eq}\) and \(R_{em}\) are put into polar coordinates with their corresponding \(\Theta_i\) locations to three-dimensionally visualize the modes. The phenomena depicted by the three-dimensional realizations obtained concur with the general understanding of the global oscillatory and the bubble-pumping modes at their corresponding Strouhal number of 0.2 and 0.06.

3D reliazation of the real (top) and imaginary (bot) \(St \approx 0.4\), the first harmonic of the shedding mode. Red an blue indicate the positive and negative velocity regions. Depending on the flow direction, the depicted 3D mode is subjected to change.