The transient flow induced by an array of wall-mounted flexible blades performing impulsive metachronal rowing, i.e., the sudden onset of coordinated oscillations of wall-mounted flexible blades with phase differences among them in an initially quiescent fluid, is numerically investigated across a wide range of the Cauchy number ($\mathrm{Ca}$), which quantifies the ratio of fluid inertial to the blade elastic force. Two distinct flow regimes emerge depending on the ratio of the natural frequency ($f_n$) to the rowing frequency ($f$). For stiff blades ($f_n>f$), the mean deflection angle scales linearly with $\mathrm{Ca}$, and the rescaled flow rate remains high. In contrast, for soft blades ($f_n<f$), the deflection saturates, and the rescaled flow rate decays rapidly as ${\mathrm{Ca}}^{-2.35}$. The maximum fluid transport is achieved near $\mathrm{Ca}=1$ and at shorter rowing wavelength (six blades per wavelength and unit spacing), where tip-shed concentrated vortices are (1) optimally positioned to reinforce the flow around power-stroke blades and (2) subsequently captured by those blades, thereby enhancing thrust. These findings provide physical insights into the hydrodynamic optimization principles underlying bio-inspired metachronal propulsion.

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• Flow-structure interactions
• Vortex dynamics