Maoxing Wei

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.

The oscillating water column (OWC) is among the most promising concepts for harvesting wave energy, yet large vortices at the chamber wall often limit its efficiency, dissipating wave energy and obstructing bidirectional flow. The sawtooth-wall configuration has been proposed to weaken these undesirable vortices and enhance energy conversion efficiency; however, the governing mechanisms and optimal geometries remain poorly understood. This study employs high-resolution particle image velocimetry (PIV) to reveal how a sawtooth wall modifies internal flow patterns and thereby augments energy conversion. Three sawtooth lengths and heights were tested across a representative range of incident-wave periods. Phase-averaged velocity fields in the sawtooth crest, flank midpoint, and trough were compared with those in a conventional straight-wall OWC, and vortices were quantified via Q-criterion. The results show non-monotonic effects of sawtooth geometry: wide-sawtooth arrays induce strong trough jets but generate three-dimensional vortices that hinder inflow, whereas narrow-sawtooth arrays increase the flow resistance and fail to achieve the goal of vortex suppression. An intermediate sawtooth length achieves an optimal balance between jet momentum and vortex breakup, yielding smoother outflow and weaker inflow vortices over the entire wave period range. Appropriate sawtooth height, matched to the characteristic jet and vortex scales, is likewise critical. The findings of the present study offer concise design guidelines for sawtooth-wall OWCs, thereby facilitating further optimization of OWC designs for research, prototyping, and practical deployment.

Oscillating water column (OWC) converters represent a promising technology for wave energy extraction, with the flow dynamics playing a critical role in optimizing their performance. This study employs particle image velocimetry to investigate the vortex-induced flow field dynamics in an OWC wave energy converter with various front wall drafts. The effects of the front wall draft on water surface amplitude, vortex evolution, and oscillation patterns are analyzed under various wave periods, highlighting the decisive role of vortices near the front wall. The results reveal that increasing draft reduces water surface amplitude, particularly under short-period wave conditions. A strengthening leeward vortex is observed with increasing draft, contributing to enhanced mixing and turbulence near the front wall. The influence of front wall draft variations on the oscillation patterns is more pronounced under short-period wave conditions, while the impact diminishes for longer wave periods. The increasing front wall draft leads to an increased proportion of outflow near the front wall and decreased inflow in the deeper region. These findings provide crucial insights into the mechanisms by which draft variations impact OWC performance, offering a framework for the design and optimization of OWC converters.