An Experimental Investigation of Aspect Ratio and Incidence Angle Effects for the Flow Around Surface Mounted Finite Height Square Prisms

Abstract

The flow around a surfacemounted finiteheight square prism was investigated using a lowspeed wind tunnel. The experiments were conducted at a Reynolds number of Re = 7.3 أ— 104 for prism aspect ratios of AR = 3, 5, 7, 9, and 11 and incidence angles from خ±â€‰= 0 deg to 45 deg. The thickness of the boundary layer on the ground plane relative to the side length was خ´/D = 1.5. Measurements of the vortex shedding frequency were made with a singlecomponent hotwire probe, and measurements of the mean drag and lift forces were obtained with a force balance. For all aspect ratios and incidence angles, the mean drag coefficient and Strouhal number were lower than those of an infinite prism, while the mean lift coefficient was of nearly similar magnitude. As the aspect ratio was increased from AR = 3 to 11, the force coefficients and Strouhal number slowly approached the infinitesquareprism data. The mean drag coefficient and Strouhal number for the finite prism were less sensitive to changes in incidence angle compared to the infinite square prism. The critical incidence angle, corresponding to minimum mean drag coefficient, minimum (most negative) mean lift coefficient, and maximum Strouhal number, shifted to a higher incidence angle compared to the infinite square prism, with values ranging from خ±critical = 15 deg to 18 deg; this shift was greatest for the prisms of higher aspect ratio. The behavior of the force coefficients and Strouhal number for the prism of AR = 3 was distinct from the other prisms (with lower values of mean drag coefficient and mean lift coefficient magnitude, and a different Strouhal number trend), suggesting the critical aspect ratio was between AR = 5 and AR = 3 in these experiments. In the wallnormal direction, the power spectra for AR = 11 and 9 tended to have weaker and/or more broadbanded vortex shedding peaks near the ground plane and near the free end at خ±â€‰= 0 deg and 15 deg. For AR = 7 to 3, welldefined vortex shedding peaks were detected along the entire height of the prisms. For AR = 11 and 9, at خ±â€‰= 30 deg and 45 deg, vortex shedding peaks were absent in the power spectra in the upper part of the wake.