Drag Reduction Mechanism Research And Optimal Design Of Surface Micro Structures

Drag reduction is a fundamental technical issue in development of largecivil aircraft. The drag has been the key problem since the beginning ofnavigating by air. The reduction of drag can lead the enhancement of aircrafts’performance and improve the economical efficiency of the aircrafts.According to the demand of reducing the drag and improving the efficiency ofaircrafts, this thesis does some research on a passive flow control method toreduce the drag. This method is to reduce the drag by configuring microstructures on the surface of wall.This thesis systematically studies the drag reduction effects of laterallyconfigured riblets and grooves on flat plate, low-speed and high-speed airfoilusing a combination of RANS and LES methods. The RANS code is used tocalculate the drag values while the LES is used to investigate the detailed flowpatterns in the boundary layers due to the presence of the surface microstructures in order to reveal the mechanism of drag reduction. On this basis,the effects of different shapes, geometry parameters and number of surfacemicro structures are analyzed and optimized using the response surfacemethod. Moreover, the non-dimensional parameters of the literal riblets arestudied. Finally, the micro structures are reordered and dismissed on the flatplate to increase the potential of practical application.Optimal parameters are obtained using response surface method basedon CFD results. It can be seen from the numerical study that3.986%and4.675%drag reduction can be achieved comparing to flat plate by using theoptimized riblets and grooves respectively. The non-dimensional study indicate that, drag reduction could be achieved when the non-dimensionalheight of riblets, h~+≤5, and there is a better performance when h~+=2~3. Theresult of discrete micro structure shows this kind of configuration can reducethe drag to a certain extent, but the efficiency of drag reduction drops. Thecalculation of the micro structure on the low-speed and high-speed airfoildemonstrate that the presence of micro structure change the flow field and thepressure distribution of the airfoil massively. In the meantime, the microstructure lead the small reduction on viscous drag and a moderate increase inpressure drag which lead the increase of total drag on both low-speed andhigh-speed airfoil.From the analysis of the flow field, the mechanism of drag reduction isrevealed, that is, the existence of micro structure could generate some lateralvortexes which function like the roller in the mechanic bearings, so they canbe called as Micro Air Rollers (MARS). These rollers change the interactionof fluid and the wall, and change the velocity profile in the near-wall region,reduce the turbulent friction drag and then reduce the total drag. This researchprovides a basis for further research on turbulent drag reduction using microstructure.