Low Reynolds Number Aerodynamics of Finite Wing at Low Reduced Pitch Rates

This paper presents experimental results of dynamic flow past an aspect ratio four wing pitching from zero-degree to 45-degree and transient flow at maximum angle of attack at Reynolds number range between 0 and 13k. The wing is a flat plate with rectangular planform and leading edge pivot axis at reduced pitch rates in a range of 0-0.13. The measurements were conducted using direct force measurement, dye flow visualization, and lens-shift stereo PIV. The measurement force data show that the wing at the phase of pitch acceleration in still water produces prominent normal force; insignificant forces are observed as the wing in constant pitch rate motion. The change of constant pitch rate of the wing gives promising forces under the strike of incoming uniform flow, consequently, causing effects similar to that of camber line change. The measured lift coefficient at a given reduced pitch rate are in good agreement with the results from a quasi-steady potential flow theory before dynamic stall. The theory fails to predict the drag coefficient due to the presence of dynamic wingtip vortex formation. Effect of Reynolds number in consideration on the pitching wing is insignificant. The occurrence of force stall at high angle of attack is associated with the evolution of spanwise flow in the wake, as revealed from dye flow visualization. Held at 45-degree angle of attack the wing experiences force bumps in transition to steady state; this transient behavior depends on reduced pitch rate and the shedding of swirling flow about streamwise direction.