Studies on Eppler 61 Airfoil with Gurney Flaps at Low Reynolds numbers

Sajeer, Ahmed and Suriyanarayanan, P and Madhavan, KT and Ramesh, G Studies on Eppler 61 Airfoil with Gurney Flaps at Low Reynolds numbers. Project Report. NAL.

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Experimental studies on Eppler61 airfoil were carried out in the 0.55m Low speed wind tunnel in the low Reynolds number range of 46,000 to 164,000. Surface pressures on the airfoil and total pressures in the wake were measured simultaneously and using these pressures, the lift and drag coefficients were computed. Lift and drag coefficients variations with incidence showed nonlinearity in the form of sudden jump at incidence of 8° at Reynolds number of 46,000 and as the Reynolds number is increased the degree of non linearity decreases and the angle at which this occurs shifts to lower incidence angles. Analysis of surface pressure data showed that the nonlinearity is due to changes in the flow features on the airfoil from the separated flow over a large part of airfoil extending upto trailing edge to the reattachment of flow on the surface in the form of bubble. Comparison of lift coefficient with the data in the literature showed differences in both pre-stall and post stall characteristics; these are attributed to the measurement technique used to get the lift and drag coefficients, flow quality of tunnel, roughness on the airfoil surface and the accuracy of the airfoil profile. To improve the aerodynamic performance of the airfoil in particular to the lift enhancement capability, experimental studies were made with Gurney flaps of varying height from 4% to 7.4% of the airfoil chord. Surface pressure measurements and wake surveys were carried out in the Reynolds number range of 46000 to 164,000 and incidence range of -7 to +16 left and drag coefficients were computed for different angles of incidence from these data. In the presence of Gurney flaps, the lift curve shows an increase in lift coefficient in the incidence range tested, and the sudden jump seen due to formation of Laminar Separation Bubble was observed to occur at much lower incidence angle with no significant change in stall angle. An increase in flap height shows an increase in left coefficient at all incidence angles (prior to stall). A maximum lift coefficient of about 2.2 was observed at Reynolds number of 46,000. An increase in Reynolds number for a given flap height showed the gradual disappearance of sudden jump with no change in maximum lift. The mechanism for increase in lift coefficient due to flap was observed to be the declaration of flow on the lower surface causing an increase in pressure and attachment of flow on the upper surface. Another interesting feature observed was the LSB was found to occur on the upper surface only after the peak pressure coefficients near the leading edge exceed the value of -3.5. Apart from increase in lift coefficient, the variation with incidence is similar to that observed on basic airfoil indicating that the presence of Gurney flap has shifted the flow features on the airfoil to occur at lower angles.

Item Type: Monograph (Project Report)
Uncontrolled Keywords: Low Reynolds Number, Eppler 61, Gurney flaps, Laminar separation bubble, flow visualization
Subjects: AERONAUTICS > Aerodynamics
Depositing User: Mr P Suriyanarayanan
Date Deposited: 25 Jan 2011 06:32
Last Modified: 25 Jan 2011 06:32

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