Improvements in and relating to solid boundary surfaces for fluid streams

摘要

610,622. Reducing surface friction; boundary layer control. PRESTON, J. H., and THWAITES, B. April 11, 1946, No. 11197. [Class 4] Separation of the boundary layer of a fluid stream moving over a solid surface is prevented by forming the surface of porous material, the linear size and spacing of whose pores are small compared with the thickness of the fluid boundary layer, means being provided for applying suction to the fluid through the pores of the material. The pore size and spacing are of the order of one-hundredth of the boundary layer thickness and do not exceed onefiftieth thereof. The invention may be applied to aerofoils, propeller blades, blading for fans, compressors or turbines, aircraft bodies, engine nacelles, submarine hulls, or ducts or channels in which fluid flow is confined, such as diffusers for compressors and pumps, or the walls of wind tunnels. Suitable surface materials are sintered metal sheet or fine metal gauze bent to the desired shape, or a mould of the desired form may be sprayed to produce a porous metal layer. Fig. 1 shows a thick-section symmetrical aerofoil in which the upper and lower surfaces 5 of the rearward quarter-chord are of finely porous material, a space 6 being formed below the porous covering by an internal partition 7, the space being connected to a suction pump P by ducts D. In the embodiment according to Fig. 3, the porous surface layer 12 extends between the points 10 and 11 at the leading edge of a thin symmetrical aerofoil. In order to preserve laminar flow, the porous areas of the surfaces of the aerofoils may be more extensive than illustrated, and an aerofoil may even be provided with a porous surface throughout. Fig. 5 shows a symmetrical rectangular-section channel or duct in which the portions S of the walls W over which the velocity gradient is steeply negative are of porous material, to which suction is applied. Variation of the applied suction head may be obtained by partitioning the space below the porous area and applying different heads to the several compartments, or the degree of porosity of the surface material may be varied appropriately. Specification 479,598 is referred to, and the Provisional Specification refers to Specification 578,763.