Microstructured Surfaces for Drag Reduction Purposes: Experiments and Simulations on Rectangular 2D Riblets
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Microstructured Surfaces for Drag Reduction Purposes: Experiments and Simulations on Rectangular 2D Riblets Håkan Rapp, Igor Zorić and Bengt Kasemo Department of Applied Physics Chalmers University of Technology 412 96 Göteborg, Sweden ABSTRACT It is well established that properly structured surface exhibits a lower friction drag, when exposed to a turbulent boundary layer, than a smooth surface under the same flow conditions. The observed drag decrease is usually attributed to an increased thickness of the viscous sublayer. In this work we examine the friction drag reducing mechanism. Two parallel approaches towards achieving this goal are presented. Photolithography was used to manufacture rectangular riblets in the 10∝m range on a standard 4” silicon wafer. A special compact plane channel system was designed and used for measurements of the frictional drag on structured surfaces in the turbulent flow covering a wide Reynolds number range. Navier-Stokes equation, for the examined drag reducing geometry, was solved in the laminar regime with appropriate boundary conditions. The resulting velocity field was used to extract the protrusion heights difference for streamwise and spanwise flows over the structured surface. The latter was then related to the experimentally measured drag reduction slope. We show that in case of a rectangular riblet, with a size of the order of one wall unit, the observed drag reduction can be accounted for within the above model. INTRODUCTION It is well established that riblets or grooves, lying in the streamwise direction in a turbulent boundary layer, influence viscous drag [1]. Since the details of the drag reduction mechanism are not well understood the bulk of the work has been concentrated, so far, on optimizing the geometry of surface structures, i.e. finding a combination of riblet spacing, s, height, h, and shape, that leads to a maximum drag reduction [2]. In particular, a drag reduction of up to ~10% has been reported for blade like riblets with the optimal configuration h/s=0.5 [2]. As far as the geometry of the drag reducing structure is concerned it is generally accepted that a significant drag reduction is only possible when the riblets are thin and sharp [1-4]. In particular, the maximum drag reduction for sharp tipped riblets occurs when the riblet spacing s+ is about 15 wall units1 [1]. The mechanism of the drag reduction, by the above passive structures, is related to the changed condition for both parallel flow and cross flow of a fluid moving along the structured surface. The changed conditions result in an increase in thickness of the viscous sublayer [5]. 1
A viscous wall unit is an inner length scale used near the wall in a turbulent boundary layer and is defined as l+= . /u* where . is the kinematic viscosity and u* =(τw/. )½ is the friction velocity, τw is the wall shear stress, and . is the fluid density. When the Reynolds number ( . velocity) increases, a wall unit becomes smaller and the size of the structure, e.g. riblet spacing s, expressed in
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