Combined Effects of Surface Roughness and Pressure Gradient on Turbulent Boundary Layers

The effects of pressure gradients applied to both a flat and heterogenously rough surface on turbulent boundary layer development is not well understood. The current study experimentally investigates this fundamental interaction by applying favorable and adverse pressure gradients to a flat plate with strips of different roughesses, each 0.10 m wide by 3.7 m long, characterizing the development of a turbulent boundary layer over them. The study is performed in the Portland State University wind tunnel with a test section of 5 m in length, 1.2 m in width, and 0.8 m in height. Boundary layer development for each case is characterized based on particle image velocimetry (PIV) data which is used to develop momentum, vorticity and shear stress contours for each of the pressure gradient cases. The pressure gradients are characterized by several boundary layer parameters, including various length scales such as momentum, displacement thicknesses and the shape factor. The law of wall (log law) based on u* is used to distinguish the different boundary layer regions within the flow for each pressure gradient. The effects of varying pressure gradients on skin friction is also explored. Understanding these parameters for each pressure gradient case has many practical applications, including local wind turbine blade flows where significant pressure gradients are present on the blade.