Hydrofoil Cavitation
Pressure in the flow is distributed in a certain way along the foil profile when it is moving. Under some conditions, pressure in water flow becomes equal to the pressure of saturated vapour p_d. In this case, the continuity of liquid is violated, and cavitation appears. In the presence of cavitation, the lift-drag ratio of a foil decreases, and erosive destruction of the foil surface may take place. When cavitating foils cross the water surface, air can entrain onto the upper side of a foil and diminish the lift rather abruptly. That's why the cavitation of foils near the free surface is not permitted at all. According to Bernulli equation, pressure on the foil prodile at water depth h is given by (1). Similarity condition for cavitating flows is the equality of cavitation numbers (2). Each foil profile has a certain pressure distribution. The local pressure coefficient is defined by (3). Cavitation appears when p = p_d. Coefficient of pressure on the upper side of a segment profile can be estimated by Lavrentiev's formula (4) for q = 1. For foils near the water surface, Vaganov's coefficient (5) is included.
For each profile, experimental cavitation diagram is generated (as the dependence of cavitation onset on the attack angle and cavitation number).
To calculate the critical speed when cavitation happens, it is convenient to use equation (7), where decompression correction fi is defined by expression (6). At given speed the maximum reduced foil thickness for subcavitating regimnes is determined by equation (8).
For approximate estimations, formula of Kru (9) for segment profiles and formula of Valhner (10) for simmetric profiles can be applied.
To increase the ship speed without cavitation appearance, the application of swept and sliding foils is recommended. If the normal profile of a sliding foil is the same as the profile of the straight foil, then the speed of the cavitation onset can be expressed through the corresponding speed of the straight foil (11), where 'thetta' is the foil sweep angle. The critical speed of the cavitation onset of the sliding foil for 'thetta' ~30-50 grad and for cavitation number ~0.1-0.4 can be expressed by (12).
