On the enhancement of coupling potential flow models to RANS solvers for the prediction of propeller effective wakes

Abstract

The calculation of the effective wake within the CFD context is usually made by combining a potential-flow method for modeling the propeller forces with a RANS equation solver for simulating the viscous flow around the hull and possible appendages. The different assumptions and/or simplifications made in the potential flow model relative to the viscous flow solver may result in significant errors in the prediction of the effective wake particularly for high loadings. This is especially troublesome for ships with full forms where large differences are expected between the nominal and effective wake, and for special propulsion applications such as contra-rotating units. Such errors are responsible within the hydrodynamic design problem for an unadjusted prediction of the propeller pitch, and within the hydrodynamic analysis problem for a deficient prediction of self-propulsion point. This paper presents an approach based on correction factors which converts propeller-induced velocities approximately estimated via potential flow theory into viscous induced velocities on the basis of a viscous flow RANS analysis. The correction factors are calculated for one reference advance number and work accurately in a neighboring region where the propeller loading may change about ±50 %. This procedure allows controlling one of the errors present in the calculation of effective wakes, namely the error derived from coupling a potential-flow method for the representation of the propeller with a RANS solver. Consequently, it permits calculating the effective wake more precisely. The approach is illustrated for a simple case in which the potential flow model representing the propeller is an actuator disk.