An Analysis of the Effects of Vortex Shedding on Time Averaged Lift

Abstract

The use of propeller blade shapes with new trailing edge details has proven eective in delaying cavitation inception, but resulted in inconsistent prediction of average forces. Two-dimensional studies indicate that the cause of these inconsistencies can be found in the unmodeled vortex street that exists behind foils operating at high Reynolds Numbers. Flow unsteadiness due to vortex shedding can in principle be treated via Reynolds averaging; however, current turbulence models address only classical shear-driven turbulence and make no claim to represent vortex-street flows. Unsteady Reynolds Averaged Navier Stokes (RANS) modeling of the two-dimensional flow around various blade shapes indicates that the unmodeled vortex street will cause over prediction of lift coecient. Although vortex shedding can be fully modeled with unsteady RANS, it is computationally intensive and unrealistic for most engineering applications. A parameter model based on new computations of over 1,000 previously published experiments has been developed to allow a simple lift coecient correction to steady flow viscous calculations to improve engineering predictions. This lift coecient correction has been found to be small for typical low drag foils with traditional trailing edges.