Structure of Flow Field around Sheet Cavity on Foil Section

Abstract

Flow fields around stable short sheet cavities are experimentally investigated for two foil sections. One is burst type whose leading edge separation bubble is long and significantly affecting the flow structure around the foil even in non-cavitating condition. The other is non-burst type whose separation bubble is short. Cavity shape and velocity distribution around the foil are measured in detail in both cavitating and non-cavitating condition using a Laser Doppler Velocimeter. Time-averaged velocity and turbulence distributions are investigated near the cavity surface and in the wake of the cavities. The conclusions can be summarized as follows :
(1) The cavity generated on the burst type foil is much thicker than that on the non-burst type foil. The cavity shape of the burst type foil strongly correlates with the shape of the separation bubble in non-cavitating condition. The cavity of the non-burst type foil is more unstable than that of the burst type foil. This is due to the difference in the separation bubbles in non-cavitating condition.
(2) The boundary layer separated near the foil leading edge affects the flow structure on the cavity surface for both foils. As the cavity becomes unstable, the turbulence intensity increases on the cavity surface and the time-mean velocity decreases.
(3) The flow structure behind the cavity is completely different between the burst type and non-burst type foils. In the case of the formar, the cavity does not affect the time-averaged velocity and widen the turbulence distribution. This is because the cavity of this foil consists of a cluster of bubbles that grow and collapse in the long separation bubble. As the cavity on non-burst type foil grows, on the other hand, the velocity and momentum loss increases rapidly and the turbulence distribution becomes wider having two peaks. This is considered to be due to the U-shape vortices shed from the cavity. From those studies it is clear that the cavity flow modeling should be made in different ways for the two types of foil.