Wake characteristics induced by bending oscillation of a cantilevered needle in confined planar channel flow

Abstract

Understanding the wake dynamics of oscillating fuel-injector needles is critical for optimizing injection performance. To isolate the underlying mechanisms, this study employs an idealized two-dimensional model of a cantilever needle oscillating in planar channel flow. The effects of oscillation parameters, inflow velocity, fluid properties, needle-tip geometry, and bending mode on wake dynamics are systematically investigated, together with the resulting hydrodynamic forces and damping. Increasing oscillation intensity enhances near-tip vorticity and promotes asymmetric shear-layer roll-up, driving the wake from a Kármán-type to a reverse-Kármán-type pattern, whereas increasing inflow velocity or viscosity favors the opposite transition. The combined effects are characterized by the modified Strouhal number (<i>St_A</i>) and Reynolds number (<i>Re</i>), with a power-law transition boundary identified in the <i>St_A</i>-<i>Re</i> regime map. The transverse hydrodynamic damping changes from positive to negative as inflow velocity increases, indicating a reversal in fluid-structure energy transfer from energy dissipation to energy input. Variations in tip geometry and bending mode do not fundamentally alter the wake mechanism. These findings provide a mechanistic basis for understanding oscillation-induced wake dynamics, and may inform future assessment and optimization of vibration and flow stability in fuel-injector systems.