Coupling to RANS Solvers
Coupling to RANS solvers - what does that mean? The idea is quite simple: The investigation of propeller hull interaction is an important application for RANS methods, because they are able to capture the important viscous effects of the flow in the stern region. Nowadays high-capacity processors allow for the investigation of the flow around a whole ship with these methods including the propeller and the rudder, but this is far away from practice. The main problems are the high computational effort and the time- and cost-intensive grid generation. Especially the propeller with its complex geometry causes many problems.
To overcome these difficulties the geometrically resolved propeller can be replaced by a propeller model as shown in Figure 1. The model is based on a body force approach, i.e. the impact of the propeller on the flow is simulated by adding certain forces to the source terms of the Navier-Stokes equations. The distribution of the forces is calculated by panMARE using the ship's wake field as input. Both solvers, the RANS solver and panMARE, run in parallel. Thus, the unsteady interaction of propeller and hull is covered.
Figure 2 left: Influence of the Body Force Distribution on the Pressure Field, right: Twisted Streamlines behind Virtual Propeller.
Two interfaces have been developed: One interface for the in-house viscous flow solver FreSCo+, and one for the commercial software ANSYS CFX. The coupling algorithm takes into consideration the entire blade geometry, ref. Figure 2. The propeller model reduces the computational effort drastically. Meshing the propeller only takes a few minutes. Numerical studies showed that the results calculated by the model widely agree with those calculated by a fully RANS computation with geometrically resolved propeller.