Unsteady Force Measurement of SD7003 Foil Under Pitch-Up, Hold and Pitch-Down Motion at Re = 1.2x104 for Micro Aerial Vehicle (MAV) Applications

Abstract

The effect of rapid pitch-up, hold and pitch-down motion for SD7003 hydrofoil to lift coefficient was investigated. Sudden pitch-up leads to the development of large-scale leading-edge vortex (LEV), which produces non-linear lift, and effect subsequent dynamic stall. The faster pitch-up rate results in the stronger lead-edge vortex and deeper dynamic stall. Two pitch-up rate patterns: sudden and gradual were used and at non-dimensional pitch rate (Ω+) of 1.4 and 2.8 (reduced frequency, k, of 2.8, and 5.6). The lift coefficients, CL of linear sudden pitch-up motion are higher than those of gradual ones by 20%. The different ramp-up acceleration also causes the shifting in CL curve peak v.s. angular position curve peak, and the different CL curve gradient at the end of the cycle. The sudden pitch-up, the CL curve peak lags behind the angular position curve peak, whereas the gradual pitch-up produces opposite trend. The effect of the longer of hold time after the pitch-up motion is that, the flow could reattach on the hydrofoil, hence recovery of CL at later half of the motion cycle. The measured CL can be considered as the sum of the non-circulatory part (Theodorsen’s) and the circulatory part. For mildly unsteady i.e. Ω+ = 0.2 (k = 0.4) the circulatory part can be approximated by Polhamus’ vortex lift theory.