Inertia-viscoelastic meandering motion in a backward-facing step flow

Our particle–image-velocimetry experiment on the turbulent flow of a viscoelastic fluid through a backward-facing step reveals three different flows in low-, middle-, and high-diffusivity states, which depend on the balance between Weissenberg and Reynolds numbers. Although the middle-diffusivity state is characterized by a high-speed flow with eddy diffusivity similar to the Newtonian counterpart, the low-diffusivity state exhibits a straight flow in the high-speed region without eddy diffusivity. The Reynolds shear stress observed in the high-diffusivity state was higher than that of the Newtonian fluid, indicating active momentum transport. This is caused by the temporal–spatial meandering motion of the streamwise wave number corresponding to one-third of the channel half-height only in the wall-normal direction. The three states are determined by the disequilibrium state of the production and dissipation rates in the turbulent kinetic energy. The meandering motion became prominent when the turbulent production rate surpassed the dissipation rate. Besides, heat transfer enhancement was observed in the high-diffusivity state.