PIV studies of the unsteady vortex dynamics of a two-dimensional pitching airfoil.

摘要

The unsteady vortex dynamics of flow around a NACA 0015 airfoil pitching about its quarter chord axis at constant angular velocity has been studied in the light of experimental velocity field measurements obtained by the Particle Image Velocimetry (PIV) technique, and previously obtained unsteady surface pressure measurements. Both measurements were conducted on the same model in a free-surface water channel. The nondimensional pitch rate (defined as {dollar}alphasp{lcub}+{rcub}=Omegasb{lcub}z{rcub}c/Usbinfty{dollar} with c = airfoil chord length, {dollar}Usbinfty{dollar} = free-stream velocity and {dollar}Omegasb{lcub}z{rcub}{dollar} = angular velocity of pitching motion) was 0.072 and experiments were carried out at three different Reynolds numbers, namely {dollar}Re=1.8times 10sp4, 5.4times 10sp4{dollar} and 1.5{dollar}times 10sp5{dollar}.; The vorticity fields and streamline patterns deduced from the instantaneous PIV velocity data, in conjunction with the surface pressure gradient data, were used to understand the development of the unsteady flow field. It was found that local adverse pressure gradient in the front region infused counter-clockwise vorticity which forced the shear layer emanating from the leading edge to detach from the surface to form the so-called Leading Edge Vortex (LEV). This was also confirmed by computing the vorticity diffusion from the surface both from a vorticity flux balance (using the velocity data), and independently from the surface pressure gradients. As long as the vortex stays close to the surface, the aerodynamic lift continues to increase. The LEV eventually augments to form the dynamic stall vortex which is convected and washed away resulting in loss of lift and dynamic stall. The vortex structure was found to be more compact and better defined at the higher Reynolds numbers. The PIV data were also used to study the details of the vortex dynamics in the unsteady flow. The experimental results agreed well with the previous observations from flow visualizations and numerical analyses.; The thesis also includes a detailed description of the PIV system developed by the author for use in the airfoil studies. All the original PIV data and the important derived results are archived on computer file for use by any interested reader.