Cavitation développée sur des parois à courbure continue : phisyque du détachement

Abstract

During the 1970s, a substantial research effort was carried out to explain the disparities traditionally described as "scale effects" found at the thresholds and on the figures of incipient cavitation ; it revealed firstly the role played by the population of microbubbles of air in the water and secondly the relationship between the state of the boundary layer and the cavitation figures. In the case of developed cavitation, the viscous effects are often overlooked. The present article describes recent experiments on continuous-curve bodies (circular and elliptical cylinders), which have however shown that an interaction exists between the boundary layer and the developed cavitation attached to a body. This interaction occurs in particular in the zone where the free boundary of the cavity becomes detached from the body : in tlis zone, the drop in ambient pressure leads to an extension of the cavitation and consequentiy to a change in the longitudinalpressure gradients that control the development of the boundary layer. In the case of the circular cylinder, the position and direction of detachment was studied by statistical analysis of a large number of photographs. With regard to the position of the detachment, the results show strong influence of the Reynolds number alongside the cavitation parameter ; they match to a large extent the more isolated results obtained by OBA, but show significant difference compared with ARAKERI's semi empirical modelization. The fast films show that the detachment zone is strongly affected by the non-permanent nature of the wake close to the cylinder. In the case of the two elliptical test bodies (with axis ratios 1/4 and 1/8), we examined in particular the following phenomena : the position of the points of detachment of the cavity, the forces (lift and drag) bearing on the body, and lastiy the state of the boundary layer, visualized by injection of a coloured stream and calculated by the method developped at O.N.E.R.A. The pressure coefficients introduced into the calculation were themselves obtained from a purely hydrodynamic modelization using the method of discretized singularities. The parameters taken account of in this study were the cavitation and Reynolds numbers and the geometry of the profiles. The principal results wer as follows : 1) The influence of the Reynolds number on the detachment position was more significant in the case of the thickest profile. There is an angle of attack close to 3 degrees for which cavitation diseppears almost completely, when the cavitation number is moderate ; at the same time the point of transition to turbulence moves further upstream. If the cavitation number is small, the cavity attaches itself to the profile in an almost unstable manner, depending on the three-dimensional configurations. At small angles of attack, detachment takes place behind the body and is preceded by laminary separation of the boundary layer. At larger angles, the cavity detaches close to the leading edge. It may be preceded either bytransition or by laminary separation. 2) The lift coefficient is sensitive to the Reynolds number, especially when the angle of attack deviates from the ideal angle, for which the boundary layers develop symmetrically on the under and upper surfaces of the profiles. 3) Calculation of the boundary layer gives the point of laminary separation before detachment of the cavity in the cases of large or small angles of attack ; it shows, in the cases of medium angles, the presence of transition to turbulence. To sum up, this general study reveals that cavity detachment and the characteristic points of the boundary layer (laminary separation, the beginning of transition) develop analogously when the angle of attack of the body varies.