Large Eddy Simulation of the Tip Leakage Flow in a Ducted Propulsor

Large-Eddy Simulation (LES) is performed to study the tip vortex flow in a ducted propulsor geometry replicating the experiments of Chesnakas and Jessup (2003); Oweis et al. (2006b) and Oweis et al. (2006a). Inception of cavitation in these marine propulsion systems is closely tied to the unsteady interactions between multiple vortices in the tip region. LES is used to shed insight into the structure of the tip vortex flow across a range of operating conditions. Simulation results are able to predict propeller loads within experimental scatter and show agreement with LDV measurements of the mean flow in the blade wake at design advance ratio, J = 0.98. The pressure differential between the pressure and suction sides of the blade is seen to produce a reversed flow in the tip gap which separates off the blade suction side tip, upstream of the blade trailing edge. The separated leakage flow rolls up into a coherent leakage vortex. Additional vorticity is fed to the primary leakage vortex by a separation sheet connecting it to the blade tip boundary layer. The separation sheet is seen to take the form of a skewed shear layer which produces a complex arrangement of unsteady vortices co- and counter-rotating with the primary vortex. Vortices arising in the blade boundary layer are seen to be perpendicular to the leakage flow while streamwise oriented vortices are seen to originate in the separation sheet itself. Vortices oriented parallel to the leakage flow are seen to produce instantaneous low-pressure regions wrapping helically around the primary vortex core. Such low pressure regions are seen both upstream and downstream of the propeller blade trailing edge. Statistics of instantaneous low pressures below the minimum mean pressure are found to be concentrated in the region of the blade’s trailing edge wake merging with the primary vortex core. The same qualitative flow behavior is found to persist for advance ratios ±14% of the design condition. The primary vortex core pressure is seen to reduce substantially for lower advance ratios. When the same rotor geometry is simulated without the duct at design advance ratio, the leakage vortex is seen to become more coherent and separate very near the blade trailing edge. The open rotor tip vortex has a lower mean pressure than the ducted rotor at the same thrust which is tied to changes in the chordwise loading distribution at the blade tip.

PRESENTER

Leasca, Theo
theo.j.leasca.civ@us.navy.mil
443-710-7743

Naval Surface Warfare Center Carderock Division; University of Michigan

CO-AUTHOR

Krishnan Mahesh
krmahesh@umich.edu

CATEGORY

Computational Fluid Dynamics

SYSTEMS USED

Warhawk

SECRET

No