Unsteady lift and sound produced by an airfoil in a turbulent boundary layer

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Unsteady lift and sound produced by an airfoil in a turbulent boundary layer

Howe, M.S.

An analysis is made of the unsteady lift exerted on a stationary rigid body immersed in an incompressible, plane-wall turbulent boundary layer. The lift is expressed as a surface integral over the body involving the upwash velocity induced by the 'free' vorticity Omega (found by taking explicit account of the interaction of the body with the flow and excluding the bound vorticity) and a harmonic function X₂ that depends only on the shape of the body. The upwash velocity is the free-field velocity given in terms of Omega by the Biot-Savart formula, augmented by the velocity field of a conventional distribution of image vortices in the wall. The function X₂ can be interpreted as the velocity potential of flow past the body, produced by motion of the wall at unit speed towards the body. Detailed predictions are made of the lift on a slender airfoil placed in the outer region of the boundary-layer. When the airfoil chord is large compared to the boundary-layer thickness, vortex shedding into the wake causes the magnitude of the net upwash velocity near the trailing edge to be small. The main contributions to the surface integral are then from the nose region, where the upwash velocity rnay be estimated independently of the fluctuations near the trailing edge. Analytical results for a thin plate airfoil of chord '2a' at distance 'h' from the wall show that the lift increases as 'a/h' increases; it is ultimately independent of 'a' and scales with the ratio of 'h' to the hydrodynamic wavelength. Application is made to determine the sound generated by the airfoil in a weakly compressible boundary layer flow over a finite elastic plate.