The effect of nuclei and gap height on cavitation in tip leakage flow

Cavitation in tip leakage flow is investigated experimentally in a variable pressure water tunnel using a single stationary hydrofoil analogy to that of rotating blading. The clearance between the hydrofoil tip and ceiling of the test section is varied for a gap height to maximum thickness ratio of between τ = h/tmₐₓ = 0.1–2 for two polydisperse freestream nuclei populations at a chord-based Reynolds number of Re = 3 × 10⁶. The two populations are representative of the limits of nuclei populations experienced in practical flows. The first population is comprised of strong nuclei with large negative critical pressures that occur in low concentrations within the flow. For the second nuclei population the flow is abundantly seeded with microbubbles of low tensile strength. The hydrofoil model is similar to a NACA66 thickness form with a NACA a = 0.8 series camber curve and maintains a flat pressure distribution along most of the chord. The hydrofoil is tested for cavitation numbers from σ = 1–6, where σ = (p − pv)/0.5ρU ² with p is the static pressure at the tunnel ceiling, and pv is the water vapour pressure. Tests are repeated in one degree increments for α = 0–10◦. Data recorded are of simultaneous high-speed photography and hydrophone measurements, and are used to relate the cavitation topology to acoustic data. Variation in the leakage vortex trajectory with gap height and incidence are discussed, as visualised by bubble accumulation in the leakage vortex for the heavily nucleated flow. Developed cavitation dynamics are shown to vary greatly with the nuclei availability as well as tip clearance, with inception events becoming particularly violent for intermediate tip clearances.