Airfoil Drag Force
Hi! I am a PhD student who is studying the large WingInGround vehicles(the socalled Ekranoplans in Russia), and I am interested in the calculation of airfoil drag force when subsonic high Reynolds number flow. I think the part of drag force due to the surface shear stress plays a more and more important role as comparing to the part due to the surface pressure when the Reynolds number is increasing. And doublting that, as I know, the calculation of shear stress conponent is several order less then the pressure one and hence is negligible in several reports. Does it mean nothing to calculate the shear stress force along the airfoil? Could anybody talk about that or give any hints? I appreciate.
Best Regards, wowakai 
Re: Airfoil Drag Force
The governing eqs. for the flow ur talking about are the boundary layer eqs. The shear force is almost the only contributor to drag as long as there is no flow seperation. It is my understanding that the numerical solution of the boundary layer eqs. should give an accurate value of the drag. Ref: Boundary layer theory BY Schlihting..H

Re: Airfoil Drag Force
Even without the ground effect, there is always a component of the lift force which is in the free stream direction ( negative value), drag due to lift. The lift force comes from the pressure on the airfoil surfaces. With the ground effect, the pressure distribution on the lower surface of the airfoil will be modified as if there is a variable width channel flow in between ( airfoil and the ground, or water surface ). If the angle of attack is large, the drag due to lift will be the dominate factor. When the drag due to lift is zero, the only drag is the viscous drag. ( assuming that you are not flying at transonic speed where the wave will create additional drag.)

Re: Airfoil Drag Force
Dear Krishnakumar, Thanks for your graet mentions! You have helped me to pass some mistakes that bothered me a lot. Really, the magnitude of shear force in a flat plate decreases as increasing the Reynolds number. This result can be found in the book ˇ§Boundary layer theoryˇ¨ by Schlichting H. with good consistences between experimental and theoretical results. The same results are also found in an AIAA paper, called ˇ§High Reynolds Number Analysis of Flat Plate and Separated Afterbody Flow Using NonLinear Turbulence Modelsˇ¨ by John R. Carlson who solved the Reynoldsaveraged simplified NavierStokes equations (RANS) with kepson model. To well calculate the ratio of the lift and drag coefficients for an airfoil, I will pay more attention to the prediction of the transition and the separation zones , and together with the motion of the eddies near the airfoil.
Best Regards, wowakai 
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