Structure of 2-D and 3-D Turbulent Boundary Layers with Sparsely Distributed Roughness Elements

: The present study deals with the effects of sparsely distributed three-dimensional elements on two-dimensional (2-D) and three-dimensional (3-D) turbulent boundary layers (TBL) in three parts: Part 1 with isolated cylinders in the turbulent boundary layers, thus considering the effect of a single perturbation on the TBL; Part 2 when the same individual elements were placed in a sparse and regular distribution, thus showing the response of the flow to a sequence of perturbations; and Part 3, with the distributions subjected to 3-D turbulent boundary layers, thus examining the effects of streamwise and spanwise pressure gradients on the same perturbed flows as considered in Part 2. The 3-D turbulent boundary layers were generated by an idealized wing-body junction flow. Detailed 3-velocity-component Laser-Doppler Velocimetry (LDV) and other measurements were carried out to understand and describe the rough-wall flow structure around the elements. The measurements include mean velocities, turbulence quantities (Reynolds stresses and triple products), skin friction, surface pressure and oil flow visualizations in 2-D and 3-D rough-wall flows for Reynolds numbers, based on momentum thickness, greater than 7000. For the 2-D rough-wall flows, the roughness Reynolds numbers, k', based on the element height (k) and the friction velocity (Ur), range from 26 to 131. When these elements are placed in a distribution, the roughness elements create a large region of back flow behind them which is continuously replenished by faster moving fluid flowing through the gaps in the rough-wall. The fluid in the back flow region moves upward as low speed ejections where it collides with the inrushing high speed flow, thus, leading to a strong mixing of shear layers. This is responsible for the generation of large levels of turbulent kinetic energy (TKE) in the vicinity of the element height.