Trends in Early Vortex Formation on a Wall-to-wall Plate in Pure Plunge

Abstract

Discernible trends in early vortex formation have been in inter vortex-plate distance, circulation, and vortex maximum azimuthal velocity when plunging a wall-to-wall flat plate at fixed angles of attack ranging from 15 to 90 with two different accelerative profiles. Vortex formation and shedding continues to play an important role in unsteady aerodynamics with applications ranging from flapping wings to maneuvering flight to helicopter rotors. Above a relatively low angle of attack when a flat plate is plunged in a fluid a leading edge vortex (LEV) is formed close to the leading edge and a trailing edge vortex (TEV) is formed close to the trailing edge. The formation, growth and convection of the LEV and TEV strongly influence the pressure field surrounding the flat plate and ultimately the forces experienced by the plate. Experiments were performed at the United States Air Force Research Labs Horizontal Free Surface Water Tunnel (AFRL/HFWT) with linear and sinusoidal acceleration profiles. The formation of the LEV was investigated for both acceleration profiles using Particle Image Velocimetry (PIV). Trends were identified in both the LEV distance to the plate as a function of convective distance and with angle of attack. Similarly, trends were identified in maximum vortex azimuthal velocity. The LEV normalized azimuthal velocity profiles were compared with several vortex models in the literature. The existing models were unable to reproduce the asymmetric azimuthal velocity distributions resulting from vortex proximity to the plate. A new model based on experimental results is proposed for the LEV core azimuthal velocity distribution inclusive of plate proximity effects.