A comprehensive study on vortex dynamics during parallel blade-vortex interaction (BVI) was conducted in a subsonic wind tunnel. A vortex was generated by applying a rapid pitch-up motion to an airfoil through a pneumatic system, and then interacted with an unloaded target airfoil placed downstream. Both Particle Image Velocimetry (PIV) and unsteady pressure measurements were performed to investigate the vortex dynamics during BVI, as well as its impact on the induced pressure and load fluctuations. The Reynolds number ranges of PIV and pressure measurements are 80,000 to 115,000, and 170,000 to 250,000, respectively.
Based on the PIV data, the interaction was classified into three categories in terms of vortex behavior: close interaction, very close interaction and direct interaction. For each type of interaction, the vortex trajectory and strength variation were obtained from phase-averaged PIV data. The PIV results revealed the mechanisms of vortex decay and the effects of several key parameters on vortex dynamics, including separation distance (h/c), Reynolds number, vortex strength (G) and vortex sense. Generally, BVI has two main stages: interaction between vortex and leading edge (vortex-LE interaction), and interaction between vortex and boundary layer (vortex-BL interaction). Vortex-LE interaction with small separation distance is usually dominated by inviscid effects, in which the decay in vortex strength is due to pressure gradients near the leading edge. Therefore, the decay rate is determined by separation distance and vortex strength, but it is relatively insensitive to Reynolds number. Vortex-LE interaction will become a viscous-type interaction if there is enough separation distance. Vortex-BL interaction is inherently dominated by viscous effects, so the decay rate is dependent on Reynolds number. Vortex sense also has great impact on vortex-BL interaction because it changes velocity field and shear stress near the surface.
Unsteady pressure data on the target airfoil were acquired for each combination of Reynolds number, vortex strength and separation distance, yielding the magnitudes of pressure and load fluctuations in each case. Both pressure and load fluctuations show asymmetric distribution about h/c = 0, with generally larger fluctuations for h/c < 0 (clockwise vortex below the target airfoil). The maximum fluctuation occurs when a clockwise vortex is slightly below the target airfoil due to the high velocity on the lower side of the leading edge induced by the vortex. This peak in fluctuation level biases more toward h/c < 0 with a stronger vortex, due to its larger size. The load fluctuation level is dependent on both the velocity fields near the leading edge and the decay in vortex strength. However, the role of vortex decay becomes insignificant at higher Reynolds number due to less decay in the viscous-type interaction. The above findings suggest that the local velocity field around the leading edge of the target airfoil is critical in both the vortex dynamics and the induced pressure and load fluctuations. The magnitudes of fluctuations can be reduced by passing the airfoil below a clockwise vortex or above a counter-clockwise vortex.