Computation of transom-stern flows using a steady free-surface fitting RANS method

This paper discusses a detailed computational study of the flow off a 2D transom stern. The steady RANS solver PARNASSOS is used, using a surface-fitting technique and the steady iterative solution algorithm for the free surface. For dry-transom flow, ‘inviscid’ computations using this RANS solver show excellent agreement with nonlinear potential-flow solutions available in the literature. Viscous effects are shown to cause a substantial reduction of the trailing wave length, and a scale effect on that wave length. The transom immersion is systematically increased to investigate the limits of the dry-transom regime. A local vanishing of the longitudinal velocity at the wave surface near the first wave crest is used as an indication of wave breaking. The critical Froude number at which this happens is substantially increased by viscous effects, and much more so for model scale than for full scale. Therefore, in a range of transom Froude numbers a smooth flow will occur at full scale, but a spilling breaker just aft of the transom at model scale. It is shown that the width of this range may depend on the hull form, as it results from two opposite scale effects.
For wetted transom flows, the predictions at model scale show good agreement with experimental data for the water level at the transom and the trailing wave system. The transom immersion is systematically decreased to investigate the clearance of the transom. It is shown that at a given transom immersion the clearance is larger at full scale than at model scale. Thus there is a range of conditions where the transom is just cleared at full scale, with a spilling breaker downstream, while the flow is still attached to the transom at model scale.