Enhanced large-scale motions and suppressed very-large-scale motions in a rotating annular flume

This study presents an experimental investigation into how Prandtl's first-kind secondary currents, induced by streamline curvature, modulate large- and very-large-scale motions (LSMs and VLSMs). While previous research has shown that Prandtl's second-kind secondary currents (driven by turbulence anisotropy) can suppress VLSMs and promote secondary current instabilities (SCIs), the effects of Prandtl's first-kind secondary currents on these motions have remained unexplored. We address this gap using a rotating annular flume to subject LSMs and VLSMs to controlled secondary flows. Our experiments reveal a dual and competing role for these currents: they enhance the energy of LSMs, while simultaneously suppressing that of VLSMs. Counterintuitively, the characteristic streamwise length of the attenuated VLSMs increases with the strength of the secondary circulation. We attribute this elongation to a rotationally induced inverse energy cascade, which reorganizes the turbulent structures. Furthermore, the VLSMs are observed to span the entire flow depth, indicating that the secondary currents, while disrupting the energetic coherence of VLSMs, concurrently redistribute their momentum across the water column. These findings demonstrate that curvature-induced secondary flows fundamentally alter the multiscale dynamics of turbulent flow, not by simple disruption but by actively reorganizing turbulent structures into longer, more vertically coherent patterns.