Numerical Investigation of the Cavitation Effects on the Vortex Shedding from a Hydrofoil with Blunt Trailing Edge

Abstract

Vortex cavitation can appear in the wake flow of hydrofoils, inducing unwanted consequences such as vibrations or unstable behaviors in hydraulic machinery and systems. To investigate the cavitation effects on hydrofoil vortex shedding, a numerical investigation of the flow around a 2D NACA0009 with a blunt trailing edge at free caviation conditions and at two degrees of cavitation developments has been carried out by means of the Zwart cavitation model and the LES WALE turbulence model which permits predicting the laminar to turbulent transition of the boundary layers. To analyze the dynamic behavior of the vortex shedding process and the coherent structures, two identification methods based on the Eulerian and Lagrangian reference frames have been applied to the simulated unsteady flow field. It is found that the cavitation occurrence in the wake significantly changes the main vortex shedding characteristics including the morphology of the vortices, the vortex formation length, the effective height of the near wake flow and the shedding frequency. The numerical results predict that the circular shape of the vortices changes to an elliptical one and that the vortex shedding frequency is significantly increased under cavitation conditions. The main reason for the frequency increase seems to be the reduction in the transverse separation between the upper and lower rows of vortices induced by the increase in the vortex formation length.

Understanding the Influence of Wake Cavitation on the Dynamic Response of Hydraulic Profiles under Lock-In Conditions

“…It can be observed that the vertical red line is displaced downstream when the cavitation number is reduced, indicating that the vortex shedding formation location takes place further downstream in these conditions. This phenomenon was also observed by Chen et al [22] and Ausoni et al [6].…”

“…On the other hand, the horizontal red line descends along the vertical Y-axis when wake cavitation occurs, indicating a decrease of the vortex shedding formation location, which results in an increase in the vortices alignment. Again, Chen et al [22] also observed this phenomenon numerically. More specifically, Figure 10 shows the streamwise velocity distribution with vertical and horizontal red lines, added on the contour plots, pointing to the locations where vortices occurred.…”

“…On the other hand, the horizontal red line descends along the vertical Y-axis when wake cavitation occurs, indicating a decrease of the vortex shedding formation location, which results in an increase in the vortices alignment. Again, Chen et al [22] also observed this phenomenon numerically.…”

“…Ausoni et al [6] experimentally confirmed that the wake cavitation is responsible for the increase in the vortex shedding frequency behind a truncated NACA 0009 hydrofoil. Chen et al [22] simulated the cavitating flow around a 2D NACA 0009 hydrofoil. They predicted an increase in the vortex shedding frequency of about 14% for a cavitation number around 40% of the incipient one.…”

“…In summary, most of the literature related to the study of hydrofoils under vortex shedding flows deals with: (i) the prediction of the vortex shedding frequency for free-wake cavitation conditions [12,15,16,[29][30][31][32], (ii) the influence of the wake cavitation on the vortex shedding dynamic behavior [4,[20][21][22][23], and (iii) the prediction of the vibration amplitudes of hydrofoil under lock-off and lock-in conditions when no wake cavitation occurs [17,[24][25][26][27][28]. However, solely Ausoni et al [6] experimentally studied the interaction between wake cavitation and the dynamic response of a hydrofoil under a lock-in condition.…”