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-   -   Causes for Drag over prediction in 2D flow (http://www.cfd-online.com/Forums/fluent/91321-causes-drag-over-prediction-2d-flow.html)

josip76 August 7, 2011 10:13

Causes for Drag over prediction in 2D flow
 
In examining a 2D transonic airfoil I've found my lift is dead on (error of 0.55%) but my drag in error by 18% (over predicted). Now from my understanding the drag in FLUENT is calculated by directly integrating the pressure and skin friction forces. Because the lift is being accuratly calculated and the shock position being accurately predicted im assuming that the wave drag is acutely predicted. So I'm attributing my error to the lack of resolution of the viscous drag (pressure and skin friction drag). But because the lift is mainly due to the pressure difference which again I gather is calculated by integrating the pressure forces on the upper and lower surfaces and differenceing, the pressure drag which is due to pressure imbalances would be correct indicating an error in the calculation of the shear stress?

I attributed these errors in turn to the high aspect ratio and skew in the grid. So with the high Re of the flow the boundary layer is tiny causing high aspect ratio cells near the wall. The dissimilar scales of motion in the normal and streamwise directions introduces numerical stiffness into the algorithm. Additionally, the discretization I used is second-order accurate, which is actually only formally second order accurate on smooth grids, the grid i used had stretching and skewness which I said produced high truncation errors leading to excessive dissipation?

Am I on the right track here in understanding where my drag error came from and the root cause of them?
Thanks

Centurion2011 September 20, 2011 09:18

komplikuješ Josipe brate :)


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