Force estimates in turbulent vortex wakes of accelerating propulsors: The effects of edge undulation on vortex formation

The effects of edge undulation on separated flows and vortex formation are investigated with various geometries of accelerated, low aspect-ratio propulsors. In addition to force measurements, multi-camera planar particle image velocimetry (PIV) is applied for a large field of view of approximately 1.9 m × 0.3 m. Edge undulations with a wavelength λ, that is significantly larger than the thickness δ of the separated shear layer are identified to enhance the propulsion force during the stable vortex growth and before the vortex detaches. Edge undulation leads to a more turbulent vortex core and a faster vortex growth. The application of different approaches to recover the acting forces from PIV data lead to the conclusion, that the three-dimensionality of the turbulent vortex wake leads to both: a loss of out-of-plane vorticity and an underestimation of the kinetic energy in the vortex.