Exploratory Experiments on Machined Riblets on Compressor Blades

Abstract

During the last decades, riblets have shown a potential for viscous drag reduction. Several investigations and measurements of skin-friction have been completed in the boundary layer over a flat plate carrying riblets, mostly on thin foils. The purpose of the present study is to investigate machined riblet structures which are fabricated by grinding and laser machining. First tests dealing with machined riblets on flat plates are presented in this paper. Future investigations will examine the aerodynamic behaviour of machined riblets on NACA compressor blades. The specimens, flat plates and compressor blades, are tested in a linear cascade wind tunnel. Perfect trapezoidal riblets have been designed specifically to the flow parameters in this wind tunnel. Foils carrying the ideal riblets provide a basis for comparison with the riblets created by grinding and by laser machining. Parameters describing the geometry and the deviation from ideal riblets have been developed. It turned out that it is quite difficult to produce riblets satisfying the ideal geometrical requirements by machining. However, measurements in the wake of flat plates show that the laser shaped riblets as well as the ground riblets reduce pressure loss coefficients by up to 3.6%.