Examination of the flow near the leading edge of attached cavitation. Part2. Incipient breakdown of two-dimensional and axisymmetric cavities

Local and global three-dimensionality of cavity interfaces near detachment was examined. Cavities forming on hydrophilic test objects at higher Reynol ds numbers (Re > 10(5)) exhibited a local how structure in the cavity inter face called 'divots'. Divots resulted from a local breakdown of the two-dim ensional laminar boundary separation. Divots did not form on hydrophobic te st objects. Instead, at higher Reynolds numbers (Re > 105), the cavity at d etachment was composed of a series of wedge shaped structures. Flows with s trong adverse pressure gradients upstream of cavity detachment exhibited on ly local three-dimensionality near one cavity detachment. Flows with weak a dverse pressure gradients upstream of cavity detachment were more susceptib le to breakdown into global three-dimensionality. This was the case for cav itation on the hydrofoils. Holographic particle imaging velocimetry (HPIV) was used to examine the spanwise and streamwise variation of the flow upstr eam of the cavity detachment. Three-dimensionality of the cavity detachment was associated with strong variations of the how upstream of the cavity in the direction perpendicular to the mean flow direction.