Relationship between leading-edge vortex and scaling laws of pitching foil in periodic flow

Scaling laws of thrust and lift coefficients generated by two-dimensional pitching foil in periodic flow were derived in our previous work. The periodic flow is a freestream velocity oscillating in the streamwise direction, and the oscillation frequency was set equal to the frequency of the foil. These scaling laws were derived by considering the added mass and circulatory forces. In the previous work, we found that the scaling laws agree well with the experimentally obtained thrust and lift coefficients at small pitch amplitude. We hypothesised that the experiment and scaling laws did not agree at large amplitudes because we assumed no separated flow. In this study, we measured the velocity fields around the foil by particle image velocimetry (PIV) to reveal the relationship between separated flow and the discrepancy of scaling laws. We find that there is a correlation between the occurrence of a leading-edge vortex and the discrepancy of the scaling laws when pitch amplitude is changed.