Scale effects on cavitating flows due to surface roughness and laminar separation

Roughness and viscous scale effects are an important consideration when using model data to predict limited cavitation on a prototype, since in many cases the limited cavitation number of a model is different from that of the prototype. In this paper it is shown that fixed patch cavitation, which is observed on axisymmetric headforms and sheet cavitation observed on hydrofoils, may be controlled by surface roughness. An analysis predicts the same general characteristics as that shown by the experimental data; namely, the cavitation number increases with Reynolds number Re for a given size and increases with decreasing size for a given value of Re. The results of a theoretical analysis of bubble-ring cavitation on a hemispherical nose are also presented. The analysis, which indicates that both Reynolds number and Weber number are important scaling parameters, is employed in an attempt to correlate extensive experimental data.