Characteristics of the flow-induced noise from rectangular rods with different aspect ratios and edge geometry

Abstract

The dynamic lift force, wake characteristics and the susceptibility to self-excited acoustic resonance is experimentally investigated for rectangular rods with aspect ratios of l/h=0.5, 1, and 2, where l is the rod length in the flow direction and h is the rod length perpendicular to the flow. The effect of rounding different edges on the flow-sound interaction mechanism is also considered. Rectangular rods with an aspect ratio of l/h=0.5 showed a significant escalation of the acoustic pressure and the dynamic lift force during acoustic resonance excitation. However, when the aspect ratio of the rod increased to l/h=2, an early excitation of the third acoustic mode is observed accompanied with a significant reduction in the dynamic lift force compared to the values observed before the onset of resonance excitation. It is revealed that the wake structure around the rectangular rod with an aspect ratio of l/h=2 has significant dependence on the edge geometry and this results in different shedding modes. The first shedding mode is the typical vortex shedding with a Strouhal value of Stl≈0.16−0.18 and it occurs for rods with aspect ratios of l/h=2 when either all the edges are sharp or the two upstream edges are sharp. Although the vortex shedding is dominant for this case, a masked shear layer instability along the lateral faces of the rectangular rod is what triggers the early excitation of the third acoustic mode. When either all the edges are rounded or the two upstream edges are rounded, the shear layer instability mode becomes dominant with a Strouhal value of Stl≈0.5−0.52 and the dynamic lift force decreases substantially. This mode classically excites the first and the third acoustic modes at their frequency coincidence and no typical single-street vortex shedding is detected downstream of the rod. The work presented in this paper reveals that rounding the upstream edges of a rectangular rod with an aspect ratio of l/h=2 can switch the shedding pattern making the shear layer instability mode dominant and hence a reduction of the dynamic loading and an attenuation of the acoustic energy is obtained.