Mechanism of Tip Vortex Cavitation Suppression by Polymer and Water Injection.

Abstract

Tip vortex cavitation (TVC) is the form of cavitation inception occurs when a bubble is trapped in to the low-pressure region located in the center of the vortex from the tip of a hydrofoil. Researchers have shown that TVC can be suppressed by polymer or water injection, but the physics of this suppression is not understood. At the beginning of this study, the TVC suppression effect was surveyed experimentally as mass was injected into the tip region of an elliptic hydrofoil. The series of TVC desinence tests revealed that TVC desinence was delayed with increasing injectant flow rate and polymer concentration, and that the suppression effect became saturated beyond a maximal flow rate and a polymer concentration. In the next step, the flow field near a hydrofoil tip was investigated using Stereo Particle Image Velocimetry (SPIV). The estimated average static pressure for non-injection condition and mass injection conditions did not reveal significant difference. Furthermore, the estimated average static pressure derived from the SPIV measurements for the baseline (non-injection) was not low enough to cause a cavitation at the observed inception pressure. In the end of the study, close observations were carried out. The high-speed video images taken for the baseline (non-injection) cases suggested the existence of unsteady flow structures near the hydrofoil tip, which can lead local reduction in static pressure and accompanying TVC inception. Fluorescence dye visualization showed filamentation of the injected polymer near the tip region, and this suggests that a viscoelastic interaction between the polymer and the tip flow. Flow unsteadiness in the tip region can be introduced by several possible mechanisms (e.g. vortex-vortex interaction or the entrainment of turbulent shear flow around a vortex core), and vortex fragmentation and merger can increase the likelihood of pressure fluctuations in the tip region. The addition of mass appears to disrupt these processes, which is enhanced by the presence of the polymer. Further study is needed to reveal precisely how the mass injection reduces unsteadiness in the tip region. Moreover, we still need to obtain the knowledge to scale up the suppression effect for the ship scale propellers.