Connections between propulsive efficiency and wake structure via modal decomposition

TL;DR

This study investigates how wake structures identified via modal decomposition relate to propulsive efficiency in oscillating hydrofoils, revealing key modal features associated with different wake patterns and efficiency peaks.

Contribution

It introduces the use of optimized dynamic mode decomposition to connect wake modal structures with propulsive performance in oscillating hydrofoils.

Findings

Modal components correlate with wake instabilities and vortex development.

Different wake patterns are linked to specific modal structures.

Peak efficiency conditions are associated with particular modal features.

Abstract

We present experiments on oscillating hydrofoils undergoing combined heaving and pitching motions, paying particular attention to connections between propulsive efficiency and coherent wake features extracted using modal analysis. Time-averaged forces and particle image velocimetry (PIV) measurements of the flow field downstream of the foil are presented for a Reynolds number of Re=11$\times$10$^3$ and Strouhal numbers in the range St=0.16-0.35. These conditions produce 2S and 2P wake patterns, as well as a near-momentumless wake structure. A triple decomposition using the optimized dynamic mode decomposition (opt-DMD) method is employed to identify dominant modal components (or coherent structures) in the wake. These structures can be connected to wake instabilities predicted using spatial stability analyses. Examining the modal components of the wake provides insightful explanations into the transition from drag to thrust production, and conditions that lead to peak propulsive efficiency. In particular, we find modes that correspond to the primary vortex development in the wakes. Other modal components capture elements of bluff body shedding at Strouhal numbers below the optimum for peak propulsive efficiency and characteristics of separation for Strouhal numbers higher than the optimum.