Total Mechanical Energy Transport Lines and Attractors in Separating Turbulent Boundary Layers

In order to provide new insights into the energetics of turbulent boundary layer flows with separation, we compute and characterize energy transport lines, which are defined as the lines tangent to the total mechanical energy transport vector field. Separation is induced in a Re-theta = 490 turbulent boundary layer by imposing two types of transpiration velocity profiles at the top boundary of the computational domain: one with suction and blowing, and the other with suction only. For both separation bubbles, we find that energy transport lines can exhibit attractors, which are a manifestation of local dissipation in the flow. We identify several attracting sets along the bottom wall and attracting spiral nodes inside the separation bubbles and their corresponding basins of attraction. The size and positions of the energy basins of attraction help us understand where the available total mechanical energy is transported to due to the mean flow, and turbulent Reynolds and viscous stresses, before it is fully dissipated. The suction-and-blowing separation bubble leads to slightly smaller losses of total mechanical energy than those for a reference nonseparated boundary layer, while the suction-only case shows larger losses than the nonseparated boundary layer case.