Effects of dissolved gas on surface pressure of a hydrofoil under cavitation condition

Dissolved gas in water is known as one of the factors which affect cavitation. The present study focuses on the effect of dissolved gas on cavitation around a hydrofoil, more specifically on the hysteresis of cavitation and surface pressure of the hydrofoil. The pressure measurement on a hydrofoil of Clark Y-11.7% was carried out under various dissolved gas conditions. Pressure in the test section was reduced stepwise from an atmospheric pressure to that in a super-cavitation condition, then was increased stepwise to the atmospheric pressure condition. Amount of dissolved oxygen (DO) was used as a parameter representing the degree of air content, and three DO conditions were set at low DO: under 30%, middle DO: around 50%, and high DO: over 70% of saturation under atmospheric pressure. Also, two angles of attack, 8 and 20 degrees were selected. Although there is no significant difference of the surface pressure and its hysteresis at the angle of attack of 8 degrees, dissolved gas seems to affect the behavior of cavitation. At the angle of attack of 20 degrees, it was found that the appearance of cavity was changed by DO conditions, and it would affect surface pressure of the foil. Under high DO condition, when the cavity oscillated with large amplitude, dissolved gas seemed to enhance the growth of the cavity, and the cavity tended to grow as covering the suction surface of the foil; it would make the time-averaged surface pressure of high DO condition lower than that of the other DO condition.