Interaction Between Boundary-Layer Transition and Cavitation Phenomena on a Yawed Cylinder

The turbulent transition process in three-dimensional (3-D) bounday layers is known to be greatly different from and more complicated than that in two-dimensional (2-D) cases. As a result, the transition process in 3-D boundary layers is not well understood. When such a flow field is also critical to cavitation inception, which is often the case in existing fluid machineries, flow structures become further complicated. Transition phenomena might influence the cavitation inception and its behavior ; therefore, investigation of cavitation phenomena in relation to boundary-layer transition is essential in order to refine existing fluid machineries. However, in spite of such industrial importance, such problems have not yet been investigated. Cavitation first appears at the positions where pressure is minimum. Therefore, the vortices in boundary layers developing in wall surfaces of fluid machineries are possible candidates for the cavitation inception. In the transition regions of general3-D boundary layers, streamwise vortices often appear. It is interesting to investigate such vortices from the standpoint of cavitation phenomena. In the present investigation, the relationship between cavitation phenomena and boundary-layer transition on a 45-degree yawed cylinder is investigated using both wind and water tunnels. The yawed cylinder is an appropriate model for the purpose of such investigations while there appears cross-flow instability, and streamwise vortices will appear in the boundary layer. The transition process in the case of a yawed cylinder is often investigated in relation to a swept=wing boundary-layer transition study. Appearance of the cavitation streaks, which have a regularly aligned fine structure in the streamwise direction, on the cylinder surface show that cavitation selectively appears along each cross-flow vortex where a low-pressure zone exists in the transition region of the boundary layer. The wavelengths of the cross-flow vortices measured in a wind tunnel model and of cavitation streaks measured in a water tunnel model are in good agreement. Since the comparison of other physically measurable flow properties between the 3-D boundary-layer transition and cavitation phenomena came out reasonably well, it can be considered, in general, that the cavitation inception is enhanced by cross-flow vortices appearing in a 3-D boundary layer on a 45-degree yawed cylinder.