Water Quality Effects on Cavitation Inception in a Trailing Vortex

Tip vortex cavitation studies were made with a hydrofoil that was elliptical in planform, with an aspect ratio of 3, and having a modified NACA 662-415 profile. LDV measurements of the tangential velocity component in the vortex were used to determine that the minimum pressure in the vortex varies with lift coefficient squared, i.e., that the incipient cavitation number σi should follow a Cl2 relation (σi ≈ Cl2). This is in contradiction to previous observations (Arndt et al. 1991) that the tip vortex cavitation index varied approximately with lift coefficient to the power 1.4. By carefully monitoring the tensile strength of the water, i.e., its susceptibility to cavitation, the discrepancy was traced to the capability of the test water to sustain a tensile stress. Cavitation in “weak” water (no tensile strength) does follow the Cl2 relationship, whereas observations in “strong” water (rupture considerably below vapor pressure) more closely followed the previously observed variation, i.e., σi ≈ Cl1.4. Since the structure of the vortex cannot be affected by changes in the water quality, the discrepancy can be explained only by the amount of tension that can be sustained by the test water before inception occurs. Apparently a relatively larger value of tension can be sustained in the vortex is the strength of the vortex is increased (i.e., increasing Cl). This would explain the observed deviation from the expected Cl2 law for water with measurable tensile strength.