Hydrodynamic Force Reduction in Square-like Cylinders Through Corner Rounding and Side Curvature: A Numerical Study at Low Reynolds Number

Natural aquatic organisms operating in low-velocity flow environments have converged on streamlined cross-sectional profiles characterised by rounded leading edges and convex lateral surfaces: geometries that minimise hydrodynamic drag and suppress flow-induced vibration. Drawing on this biological principle, the present study quantifies the hydrodynamic benefit of progressive corner rounding and side curvature on square cylinder cross-sections at Reynolds number 200, using two-dimensional simulations in ANSYS-FLUENT with the k-ω SST closure. Six configurations combining sharp and rounded corners with straight, concave and convex sides are examined, together with eleven aspect-ratio variations in the minimum-drag geometry. Among the configurations tested, the rounded-corner, convex-sided cylinder at H/R2 = 0.67 gives the lowest mean drag coefficient (C¯D = 1.285), a reduction of 9.1% relative to the sharp-cornered baseline (C¯D = 1.413). The sharp-concave section is the worst performer (C¯D = 1.547), so the tested geometries span a 20.3% range in mean drag; C¯L,rms rises from 0.104 for the sharp baseline to 0.208 at H/R2 = 1.25, and St rises from 0.100 to 0.133. Continuous wavelet analysis of the lift signal additionally resolves shedding onset time and spectral coherence, identifying the sharp-concave section as the only configuration whose shedding fails to reach a stationary state, which time-averaged coefficients alone would not reveal. Least-squares fits over the monotonic branch yield linear correlations for both coefficients in H/R2 (R2 = 0.978 and 0.996), which are combined with published Reynolds-number scaling to propose a separable approximation. The results provide a quantitative basis for the low-Reynolds-number design of bluff cross-sections.