Flow Visualization and Force Measurements on Accelerated Revolving Flat Plates at Low Reynolds Numbers

Abstract

The study explores the evolution of flow field structure and forces of low-aspect-ratio flat plates undergoing an accelerated revolving motion from rest. The measurements were performed in a water tank at angles of attack ranging from 15 to 75 in a Reynolds number range of 5,000 – 25,000, based on the chord length and terminal velocity at 75 % span. A tomographic Particle Image Velocimetry technique was used in order to capture threedimensional velocity fields at different phases of the revolving motion, in combination with direct force measurements with a six-component water-submergible force sensor. Comparison of force histories for different Reynolds number cases reveals an increasing trend of force generation with increasing Reynolds number, yet as evident from the analysis of the mean values, the effect is stronger at relatively low Reynolds numbers. Comparison of flow fields yields similar features at different stages of the motion albeit having more vorticity generated at higher Reynolds number cases. Nevertheless, the normalized LEV circulation and LEV trajectory display similar behavior regardless of the Reynolds number. On the other hand, the angle of attack has a pronounced effect both on the magnitude and temporal variation of forces. Although the behavior of the vortical structures is similar at different angles of attack, their morphology is significantly altered by the promoted separation at higher angles. Moreover, enhanced spanwise flow patterns are observed at higher angles of attack, which indicates the effect of separation-induced spanwise pressure gradient in the generation of spanwise flow.