THE FLUCTUATING VELOCITY FIELD ABOVE THE FREE END OF A SURFACEMOUNTED FINITE-HEIGHT SQUARE PRISM

Abstract

This study investigates the features of the fluctuating velocity field above the free end of a surface-mounted finite-height square prism, at a Reynolds number of Re = 4.2×10, using two state-of-the-art post-processing methodologies, namely Proper Orthogonal Decomposition (POD) and the swirling strength criterion. Instantaneous velocity measurements were obtained from Particle Image Velocimetry (PIV) experiments conducted in a low-speed wind tunnel for the flow over finite square prisms of aspect ratios AR = 9, 7, 5 and 3 (AR = H/D where H is the prism height and D is the prism width). The prisms were immersed in a thin, turbulent boundary layer with a dimensionless thickness of δ/D = 1.6. The Reynolds number based on the freestream velocity U∞ was Re = 4.2×10. POD modes were used to extract the dominant flow features, which revealed a pair of shear sub-layers above the free end, flow entrainment from the freestream above, as well as several vortex structures in the mean shear flow region. At lower aspect ratios, the shear layers become more pronounced. Combined with the swirling strength results, this represents evidence of Kelvin-Helmholtz instabilities within the shear layer region. These instabilities increase moving away from the vertical symmetry plane. Some structures in the near wake above and behind the prism trailing edge are also seen due to the interactions of the reverse flow downstream with the downwash flow.