Boundary-layer flow along a flat plate with uniform suction

Abstract

Summary.--Experiments have been carried out in the closed-circuit wind-tunnel at Cambridge University to determine the effectiveness of distributed suction as a means of controlling and stabilizing the flow in a boundary layer. These experiments have shown that the laminar exponential suction profile can be established and retained, provided the boundary layer is in an undisturbed laminar condition at the start of the suction region. Good agreement has been obtained between the measured velocity profiles and the theoretical exponential form. It has also been shown that the laminar suction profile, when once established, is able to surmount small disturbances which would normally be sufficient to promote transition in the absence of suction. There is, however, no evidence whatever to suggest that laminar flow can be re-established if transition once occurs. The variation with rate of suction of the total effective drag of a flat plate has been investigated. It has been established that, from the point of view of drag reduction, the optimum rate of suction is the minimum rate which is sufficient to maintain laminar flow under the prevailing conditions of stream turbulence and surface finish. A suction velocity ratio of approximately 0.0010 has proved necessary in order to ensure the preservation of laminar flow with the conditions prevailing in the wind-tunnel at Cambridge, although a lower figure may be adequate under the steadier air conditions of free flight. As far as turbulent flow is concerned, it has been shown that distributed suction provides an effective method of thinning a turbulent boundary layer. Some evidence has also been accumulated to show that an asymptotic turbulent suction profile may be closely approached at sufficient values of suction velocity. A theoretical basis has been suggested for this type of boundary-layer flow, using the vorticity transfer theory, which has given good agreement with the experimental results.