Chapter 4 - Phase 3: Steady Turn Predictions

In this chapter, CFD computations for a steady turn manoeuvre are compared to experiments performed in the QinetiQ rotating arm basin. The validation data provided to the group consists of forces and moments measured on the appended hull during the turn. As for the other chapters, a huge amount of data is collected given the number of institutions, computational codes and turbulence models or grid methodologies. The comparison between the CFD and the experiments is then the largest collaborative validation database known to date and provides the state of the art for CFD methodologies on submarines in complex manoeuvres. The validation exercise concludes that the dominant forces and moments Y , K and M are predicted consistently with the experiments but that more accurate information on the uncertainties in the measurements is needed to strengthen the conclusions. It is also concluded that the discrepancies observed in the comparisons are originating from the presence (or absence) of stall on the aft control surfaces. These differences in the local flows around the control surfaces are mostly attributed to the turbulence models and, as concluded in the previous chapters, the complete knowledge of the implementation of the models is found necessary to understand the results. Further validation studies on this turning manoeuvre would require aft control surfaces dedicated measurements of forces and wall shear stress to fully validate the computational methodologies. The analysis of the flow variables in the vortices confirms previous conclusions in the variability of the results due to the differences in the local grid refinements, and experimental data is necessary to achieve validation of the numerical prediction of these flow features.