Simultaneous measurements of fiber-turbulence coupling past a backward-facing step*

Anisotropic particles suspended in turbulent flow may show preferential concentration and alignment. In turn, their orientation and motion prompt forces and torques that alter the morphology and intensity of the turbulence. To explore the impact of particles' inertia and concentration on such particles-turbulence interplay, we studied the case of fiber-inseminated flow past a backward-facing step, whose produced turbulence in the unladen case is a classical paradigm for turbulent flows. Experiments have been conducted in a channel with Reynolds numbers ranging between 10000 and 16000. Low and high Stokes number cases are compared using two sets of fibers with different lengths and diameters. Tests were repeated for fibers' volume fraction from 10^-6 to 10^-4. For each dataset, simultaneous Digital Holography (DH) and PIV measurements could resolve the 3D fibers orientation dynamics and tumbling and characterize the flow field. Special attention is given to implementing DH and understanding its limits when increasing the fibers' concentration. Finally, we performed the modal analysis of the PIV images. By correlating the most energetic particle and flow structures for the different cases, we obtained a more thorough picture of how fibers modulate the turbulent flow morphology.