Propeller Flow Analysis using FreSCo+
Propeller-flow analysis is concerned with both the open-water (OW) characteristics of a propeller and the integrated propeller in behind condition.
Methodology
FreSCo+ offers a number of VOF-based (mass-transfer) cavitation models for the analysis of propeller flows. The below given figure outlines the attainable predictive accuracy for an OW-diagram refering to a non-cavitating case (red=FreSCo+, blue=experiments by Marintek).
Cavitating propeller flows are difficult to simulate since they are governed by the influence of concentrated vortices, e.g. hub and tip vortices. The predictive accuracy particularly hinges on a fair simulation of the downstream evolution of the primary vortices. Traditional Boussinesq-viscosity RANS procedures are the most viable and widespread approaches to turbulent industrial simulations. The related turbulence closures are however known to introduce an unrealistic amount of vortex-diffusion through an overestimated eddy-viscosity, which motivates the use of a modified computational model based on vorticity-confinement (VC).
The basic idea of vorticity confinement is to introduce a synthetic body-force vector to the RHS of the momentum equation that neutralises the vorticity change due to an artificial viscosity. The artificial viscosity can either be attributed to discretisation errors and the employed finite-approximation scheme or errors induced by the utilised physical models. With respect to RANS-based simulations of industrial marine propeller flows, modelling errors dominate discretisation errors. Accordingly, a new turbulent vorticity-confinement strategy has been developped.The strategy is applied to the simulation of the INSEAN E779A propeller. Significant improvements of the tip and hub vortex evolution (cf. below) and the prediction of force coefficients are experienced in conjunction with the turbulent vorticity-confinement (the error in thrust- and torque-predictions has been reduced to less than 1%).
