Abstract
Flow features, due to air injection through multiple vents on the surface of a hydrofoil inclined at an angle with respect to the free-stream in a cavitation tunnel, are presented here. The hydrofoil, with a chord length, c, is oriented at the angle of inclination, α = 3.5°. The Froude number, Fn, based on the free-stream velocity, V∞, and air injection vent diameter, dh, is 30.30, 50.51 and 70.71. Air is injected through multiple vents on the hydrofoil at the non-dimensional air injection coefficient, Cq∼16−8917. The air bubble packing per unit area is quantified using spatial density, SD, at various combinations of Fn,Cq based on a high-speed video from the side view. The time-averaged spatial density, <SD>, is observed to increase in a logarithmic manner with an increase in the air injection rate, Q, at various Froude numbers. There is an increase in the mean spatial density of the bubbles with the increase in Cq at all Fn. A power–law relation is shown to exist between the time-averaged spatial density, <SD>, and the non-dimensional flow variables, Reynolds number, Reair, Fn and Cq based on a regression analysis. By tracking individual finite volume bubbles flowing with the free-stream, the bubble dimensions in pixels are quantified using quantities such as the deformation rate, ϵ, and standardization, ϵS, from the side-view videos. It is observed that ϵ and ϵS change with time, even as they become advected with the free-stream. Through high-speed imaging from the top view, we characterize the bubbly flow features’ time-averaged thickness, t, at various combinations of Fn,Cq at α = 3.5°. We obtain a power-law relation between the non-dimensional time-averaged jet thickness, t/c, and the non-dimensional flow parameters such as, Reair, Fn,Cq and the non-dimensional streamwise distance, x/xref, based on a regression analysis, where xref is a reference distance. The results are relevant to engineering applications where the air–water bubbly flow in a free-stream is important.
Highlights
The two-phase flow of air injection in water in engineering applications affects drag reduction and noise attenuation benefits
Experiments were performed with a view to characterize the bubbles issuing through multiple vents on the inclined hydrofoil at various air injection rates and free-stream velocities
We characterized bubbles flowing through multiple vents on the surface of an inclined hydrofoil in a free-stream
Summary
The two-phase flow of air injection in water in engineering applications affects drag reduction and noise attenuation benefits. Yanuar [2] measured the total drag coefficient exerted on a model ship in a towing tank using a load cell transducer with air injection underneath the surface in two different air injection configurations. They demonstrated a decrease in the total drag coefficient upon air injection for the two configurations, when compared against the noinjection case at several Froude numbers. Advisory Committee for Aeronautics (NACA) 4412 hydrofoil in the free-stream flow of two-phase air and water was studied by Ohasi [3]. The air–water void fraction distribution in the wall-normal direction around a hydrofoil was quantified at several streamwise distances from the leading edge at a known angle of inclination and free-stream velocity
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