Computational/Experimental Study of Hovering Rotor Tip Vortex Formation

The formation and roll-up of a tip vortex trailed from a hovering helicopter rotor blade is studied in detail using both computations and measurements. The compressible Reynolds-Averaged Navier–Stokes (RANS) equations were computationally solved on an overset mesh system. The measurements were made using particle image velocimetry (PIV). The high resolution in both the numerics and measurements revealed, for the first time, multiple coherent structures in the evolving rotor tip vortex flow field. Secondary and tertiary vortices that result from cross-flow separations near the blade tip were identified using both techniques. These vortices, along with a part of the trailed wake, are ultimately entrained into the tip vortex downstream of the trailing edge. The simulations clearly demonstrate the resolution required to accurately represent the flow field. The advantage of PIV, which is the ability to make planar measurements at a given instant of time, has been fully utilized to validate the CFD predictions. Good qualitative and quantitative agreement is achieved between the CFD predictions and the PIV measurements. The sources of computational and measurement uncertainties are also discussed.