An experimental study of a three-dimensional pressure-driven turbulent boundary layer

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Virginia Tech

A three dimensional, pressure driven turbulent boundary layer created by an idealized wing-body junction flow is experimentally studied. The body used is a 3 : 2 elliptical nosed NACA 0020 tailed symmetric profile which has a chord length of 30.5 cm (12 inches), maximum thickness of 7.17 cm (2.824 inches) , height of 22.9 cm (9.016 inches). The body was sitting on a flat plate. The nominal reference velocity of the flow is 27 m/sec and the Reynolds number based on the momentum thickness at 0.75 chord upstream of the body on the centerline of the tunnel is ≃ 5936. The data presented include time-mean static pressure, skin friction magnitude and direction on the wall, as well as the mean velocity and all Reynolds stresses at several stations on a line determined with the mean velocity vector component parallel to the wall in the layer where the u²¯ normal stress is maximum. The mean velocity and stress data were obtained both with hot-wire ( HW ) and laser-Doppler-velocimeter ( LDV ) techniques. The LDV measurements were taken twice due to the differences observed between the HW and LDV data, which is also shown with the present study. This gave a chance to study the uncertainties on the mean velocity and the stresses extensively. Pressure distributions on the wing and the on the bottom plate were obtained with a Scanivalve and an inclined manometer. Skin friction vectors at several locations on the wall were measured in another study done by Allinger ( 1990 ) with a laser interferometer technique. The data show that the eddy viscosity of the flow is not isotropic, but the ratio of eddy viscosities perpendicular and parallel to the direction of the mean velocity vector component parallel to the wall at the point in the layer where u²¯ is maximum is close to unity, and the shear-stress vector direction in the flow lags behind the flow gradient vector direction. A₁, Townsend's structural parameter is not a constant of 0.15 as expected. The production of the turbulent kinetic energy and shear stresses are important below the logarithmic regions of the U axial velocity component profiles. The skin friction velocity is not the scale of the turbulence in such a flow. Further, a collection of 3-D turbulent boundary layer data including the present study is used to investigate the concept of the Law of the Wall velocity profile and the limitations of eddy-viscosity turbulence models in 3-D flows. For this purpose, several Law-of-the-Wall velocity profile models and eddy-viscosity models were tested. Johnston's Law-of-the-Wall relation and, for the pressure-driven flows the Johnson-King eddy-viscosity model and for the shear-driven flows Patel's eddy-viscosity model are most promising.