Tomographic PIV Study of the Low Re Number Flow Around a Pitching Plate with a Ramp Time History

摘要

Unsteady aerodynamics in the low Reynolds number domain is of great interest in applications to Micro Air Vehicles (MAVs), especially with respect to flow over, and in the wake of, pitching airfoils. The massively separated flow over the upper surface of the pitching aerofoil allows for the production of large transient peaks of lift at this scale. This study extends our previous work to multi-dimensional (3D), multi-component (3C) measurements of this phenomenon by using Tomographic Particle Image Velocimetry (Tomo-PIV). For this purpose the three-component, three-dimensional (3C-3D) velocity fields in the wake of a flat plate undergoing a pitch-hold-return motion is investigated. The experiments are conducted in a water tunnel at a Reynolds number of 7,500 and a dimensionless pitching rate of Kc = 0.93. The instantaneous and phase averaged evolution of the trailing edge vortices is investigated, highlighting the three- dimensional organization of the coherent vortex structure and its spanwise variation. These experiments represent the first effort in the 3C-3D velocity field mapping of a flat plate undergoing a pitch-hold-return motion. Further optimization of the Tomo-PIV experiments is necessary to extract the full details of the complex vortical flow structure. It has become apparent from this study that the three dimensional flow structure observed in this nominally 2D 'infinite' flat plate is quite different to flow visualizations of unsteady flapping airfoil of low aspect ratio, which suggests that the more relevant case of a low aspect ratio flat plate undergoing a pitch-hold-return motion is a more relevant candidate for a Tomo-PIV study and should be pursued in any future investigation.