The influence of a cylinder placed within the logarithmic region of a turbulent boundary layer

Flow physics in the wake of cylinders positioned within the logarithmic region of a turbulent boundary layer are investigated using planar PIV in a water channel at a friction Reynolds number of 1320. Most previous studies focus on cylinders mounted perpendicular to surfaces or outside of the boundary layer, but in our experiments the cylinders are located parallel to the wall and oriented across the freestream. Instantaneous and averaged velocity fields are used to discuss the impact of cylinder diameter (D) and the gap between the cylinder and the wall (G) relative to the boundary layer thickness (δ). The range of D and G are smaller than most examples in the literature, with 0.01 < D/δ < 0.1 and G/δ < 0.2. Regardless of diameter, the characteristics of the boundary layer are changed significantly by the cylinder wake. Larger diameter cylinders (D/δ ~ 0.1) generate a wake coherent over a downstream distance longer than δ, while smaller diameter cylinders (D/δ ~ 0.01) produce a narrower and weaker wake. Results facilitate understanding how an obstacle of specific size affects existing turbulence characteristics and organization within the boundary layer in an instantaneous and time-averaged sense.