MEchanics GAthering -MEGA- Seminar: Physical mechanism and flow control of tip vortex cavitation

Occurrence of cavitation in hydraulic machines is a challenging issue, because cavitation is often associated with loss of efficiency, noise emissions, vibrations, and erosion damages. Tip vortices, which are an inherent characteristic of finite-span lifting surfaces, form an ideal site for the inception of cavitation as the static pressure at their cores usually drops much below the freestream pressure due to the rotational motion. In the first part of this talk, we focus on the effect of dissolved gas content on Tip Vortex Cavitation (TVC). The inception and desinence thresholds of TVC measured at different flow conditions for various gas contents reveal that TVC often disappears at cavitation indices much higher than the inception thresholds. Our measurements show that TVC desinence pressure increases with the gas content and, under specific flow conditions, may reach to atmospheric pressure. When the pressure of the cavitating core is below the initial saturation pressure of the dissolved gases, water flowing adjacent to the interface becomes supersaturated and diffuses the air molecules into TVC. The extent of the delay in desinence due to outgassing is, however, dictated by the bulk flow parameters. Owing to flow visualizations, we assert that the formation of a laminar separation bubble at the hydrofoil tip is a necessary condition for a delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core. We show that the hysteresis is suppressed once the laminar separation bubble is destroyed.
In the second part of the talk, we present our recent results on the effectiveness of winglets in suppressing TVC. In this study, an elliptical hydrofoil is selected as the baseline geometry and various winglets are realized by bending the last 5 or 10% of the span at ±45° and ±90° dihedral angles. Lift-and-drag force measurements demonstrate that the hydrodynamic performances of the winglet-equipped hydrofoils are not different from the baseline. Nevertheless, cavitation inception-desinence tests reveal that undeniable advantages are achieved by the winglets. It is found that the 10%-bent 90° winglets are more effective than the 45° cases, with -90° (bent down towards the pressure side) performing superior to +90°. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestream velocity and 14° incidence angle. Stereo-PIV measurements show that for the most effective winglet (10%-bent 90°-downward), the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the vortex core size increases significantly (by almost 70%). These effects are accompanied by a tangible reduction in the axial velocity at the vortex core leading to further mitigation of TVC.