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Improving Airfoil Lift Prediction
Gepubliceerd door: Publicatie datum:
ECN Windenergie 26-1-2017
ECN publicatienummer: Publicatie type:
ECN-M--17-003 Conferentiebijdrage
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An improved formulation for lift estimation for integral boundary layer (IBL) methods (i.e. RFOIL, XFOIL) for thick airfoils is presented. Lift over-prediction (5 - 10% around (l=d)max) in RFOIL (similarly in XFOIL) is observed for thick airfoils for a wide range of angles of attack. The lift slope is over-predicted resulting in the increasing error in lift with increasing angle of attack. The wake geometry in RFOIL and XFOIL is determined from the inviscid calculations which seems to give rise to the above problem. A scheme has been developed for IBL methods and implemented in RFOIL, to include the effects of viscous flow on the wake geometry which lead to improved lift prediction for thick airfoils. New insights were obtained regarding the discrepancy in predicting the maximum lift. It is observed that lift over-prediction persists in case of thick trailing edge (TTE) airfoils even with the improved method. The cause for this behavior is identified and discussed in order to evaluate the possibilities of improvement. High Reynolds number and increased free-stream turbulence intensity have been observed to inhibit the onset of flow separation and delay stall. These effects are not yet captured accurately in RFOIL indicating a need for further investigation.

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