Interaction of Traveling Bubbles with the Boundary Layer and Attached Cavitation.

The inception and appearance of attached cavitation can be significantly influenced by the presence of traveling cavitation bubbles. Traveling bubbles may induce transient regions of incipient attached cavitation. Furthermore, traveling bubbles may sweep away regions of developed attached cavitation. The number and size of traveling cavitation bubbles in a flow is strongly dependent on the freestream nuclei distribution. Consequently, the interaction of traveling bubbles with the boundary layer and attached cavitation provide a mechanism whereby the freestream nuclei distribution can significantly influence the inception and appearance of attached cavitation. Experiments were performed on a two-dimensional hydrofoil to investigate this phenomena. Individual traveling cavitation bubbles were examined as they interacted with the flow over the hydrofoil. Each bubble was produced from a single nucleus. The nucleus was generated artificially by focusing a pulse laser slightly upstream of the hydrofoil. The processes that traveling bubbles induce transient attached cavitation and sweep developed attached cavitation were then observed through photographs and high speed video. Furthermore, the resulting boundary layers from the interaction with traveling bubbles were visualized using single-frame, double -pulsed, planar Particle Image Velocimetry (PIV). Traveling bubbles were observed to generate a local region of turbulence as they passed closely over an unstable laminar boundary layer. By producing a locally turbulent region, the bubbles could temporarily sweep away a portion of attached cavitation at the foil midcord. Also, the local turbulent region can entrain traveling bubbles to induce transient attached cavitation upstream of traveling bubbles. The bubbles were observed to strongly interact with a turbulent boundary layer, producing local regions of partial cavitation. Mechanisms of the interaction of traveling bubbles with the boundary layer and attached cavitation are discussed.