Effect of Freestream Turbulence on Adverse Pressure Gradient Turbulent Boundary Layers

Turbulent boundary layers exposed to high freestream turbulence and adverse pressure gradients appear in a variety of aerodynamic applications such as the suction side of airfoils within gas turbine engines, and on aircraft wings in turbulent ocean environments. For low freestream turbulence, an adverse pressure gradient is known to influence the development of near-wall turbulent structures and the turbulence behavior in the outer region of the boundary layer due to the resilience of large scale superstructures. For boundary layers with no pressure gradient, freestream turbulence tends to increase transport of momentum from freestream eddies into the boundary layer. However, it is not clear how freestream turbulence will interact with an adverse pressure gradient to influence turbulent boundary layer behavior. In this study, laser Doppler velocimetry is used to obtain mean profiles and turbulent statistics at three locations along a developing turbulent boundary layer over a flat plate that is subjected to low and high freestream turbulence, at zero and high adverse pressure gradient conditions. In addition, the influence of pressure gradient history is examined for low freestream turbulence by subjecting the boundary layer to different divergence shapes for the tunnel upper wall. The results indicate that freestream turbulence tends to inject momentum into the boundary layer to reduce the effect of an adverse pressure gradient on the wake region and inner region wall-normal and shear turbulent quantities, although streamwise turbulence is always increased. For a low freestream turbulence boundary layer, a sustained adverse pressure gradient results in significant outer region turbulence activity relative to a pressure gradient that has a larger peak but is not sustained as long.