Artificial thickening and thinning of cavitation tunnel boundary layers

• Use of an array of transverse jets for thickening liquid turbulent boundary layers.
• Jet mass flux is most the significant parameter in terms of thickness increase.
• Jet array geometry affects mixing and hence velocity profile development.
• Jet velocity must be minimised to delay cavitation inception.

Measurements of natural, thickened and thinned boundary layer mean velocity profiles on the ceiling of a cavitation tunnel test section are presented. The method of thickening investigated is via an array of transverse injected jets and for thinning via ingestion of the natural boundary layer fluid through a perforated plate. Several jet arrays of different geometric configuration and open area were tested over a range of jet to freestream velocity ratios, Reynolds numbers and cavitation numbers. The thickened and thinned velocity profiles are compared with the laws of the wall and wake using parameters derived from the natural boundary layer profiles. The most significant parameter controlling the degree of thickening is the open area, as predicted by one-dimensional mass and momentum conservation, with improvements achievable depending on the jet array configuration. Of the configurations tested an array of intermediately spaced jets was found more effective for thickening than a single row or either sparsely or closely spaced arrays. The profiles of all configurations were found to compare favourably with the laws of the wall and wake to varying degrees, depending upon the geometry, jet velocity and the streamwise length in terms of the number of boundary layer thicknesses for profile development. The results showed that boundary layers could be artificially thickened from momentum Reynolds numbers of about 30,000 to values of about 100,000, or friction Reynolds numbers from about 10,000 to 35,000. Jet velocity was shown to have a significant effect on cavitation inception and generated noise, demonstrating this must be minimised to optimise cavitation limits. Overall the results suggest that a jet array of large open area, to increase thickness and minimise the jet to freestream velocity ratio, with jets distributed with sufficient spacing to promote mixing provides an idealised configuration.