Effect of aspect ratio on the flow over a wall-mounted hemispherical turret

The effect of aspect ratio on the flow over a hemispherical turret was investigated using Time Resolved Particle Image Velocimetry (TRPIV) in a water tunnel at a Reynolds number of 33,000, based on the turret diameter (d). The height of the pedestal (h) was varied to obtain four aspect ratios (AR = h/d) of 0.5, 1, 1.5 and 2. For all cases, the boundary layer separated upstream of the models to form a classical horseshoe vortex that wrapped around the base of the turret models as it convected downstream. A bifurcation streamline resulted in the formation of a half saddle point near the forward attachment line that divided the flow into inward turning recirculating flow near the end wall, and the outer flow. The vertical displacement of the half saddle point continued to increase with increasing aspect ratio. Spectral analysis and Dynamic Mode Decomposition (DMD) was performed on the velocimetry data that showed existence of multiple peaks whose amplitudes increased with increasing aspect ratio and subharmonic distribution of energy. Results of power spectra were corroborated by DMD analysis which showed the gradual shift in the highest energy modes from high frequencies to lower ones and reduction of decay rates of lower energy modes with increasing aspect ratio. Surface flow visualization tests in the wind tunnel also showed the strong influence of aspect ratio on the near wall flow upstream and in the wake of the models vividly observed from the trajectories of the limiting streamlines.