This study numerically investigates the dynamics of flexible filaments which are free to move laterally in a uniform flow. The effects of Reynolds number ($Re$) and bending stiffness ($K$) on the flow patterns and filament motion are considered. Results indicate that the filament can achieve spontaneous translation or vibration in most cases due to symmetry breaking, without any external energy input. Five distinct motion modes are identified in the $Re–K$ plane, i.e., static, quasistatic, translational, vibrational, and chaotic modes. The translation Reynolds number ($R{e}_V$), which scales with the filament's lateral velocity, and Strouhal number ($St$), a measure of the vibration frequency, are adopted to quantitatively distinguish these modes. For small $K, R{e}_V$ is found to follow a simple scaling law with $Re$. A reduced-order parametrized model is developed that accurately describes the local curvature and equilibrium configuration of the filament. Two key parameters, i.e., the effective bending stiffness ($K_{\mathrm{eff}}$) and effective Reynolds number ($R{e}_{\mathrm{eff}}$), defined based on the effective relative fluid velocity and the filament's lateral projection length, are introduced, successfully collapsing deformation and force data. Moreover, energy characterization shows fluid kinetic energy conversion rates of up to 15.5% to bending energy and 81.3% to filament kinetic energy. These findings provide insights into the dynamics of flexible bodies in uniform flow.

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• Drag reduction
• Flow-structure interactions
• Vortex shedding