Experimental Investigation of Unsteady Tip Vortex in Plunging and Pitching Motions

This experimental study showcases the changes to tip vortex evolution caused by unsteady plunging and pitching motions. Three cases of motion are tested on a NACA 0012 airfoil using time-resolved stereoscopic particle image velocimetry (PIV). Tests are taken while the airfoil is subjected to sinusoidal plunging and sinusoidal pitching motions. A control test is also taken for a static airfoil for the baseline comparisons. Changes to the tangential velocity, axial velocity, turbulence intensity, and Reynolds shear stress are analyzed for the duration of each cycle of motion and compared to the static results. For the plunging and pitching cases, the swirl velocity magnitude during the downstroke is significantly greater than static values, exhibiting greater levels of turbulence intensity and large Reynolds shear stress peaks about the vortex core. In contrast, data from the upstroke phase yields minimal turbulence contents, due to the loss of lift. The vortex grows weaker during the upstroke plunge and nose-up pitch from the lift decrease for the plunging and pitching cases, even dispersing before reaching the test section at z/c = 5 for a period of the oscillation. While the tangential and axial velocity magnitude are similar for the plunging and pitching cases, the local velocity maxima come at different times during the cycle, with the plunge cycle reaching its maximum first earlier in the downstroke phase. The static, plunging, and pitching cases follow a similar Richardson number trend, where the outer region of the vortex falls below the stratification threshold to become turbulent. However, the unsteady motions introduce variations in the Richardson number’s stratification radius, and relaminarization is observed outside the core radius. The turbulence intensity exhibited by the plunging and pitching cases during the downstroke is greater than for the static case, but lesser during the upstroke.