Large eddy simulation of fillet radius effects on flow characteristics in a square-section diffuser

This study employs large‐eddy simulation to investigate the internal flow characteristics and three‐dimensional separation mechanisms in a square‐section diffuser with four fillet‐radius (r=0%D, 10%D, 20%D, and 30%D). Time-averaged, instantaneous, and fluctuating flow fields are analyzed to elucidate the effects of fillet radius on diffuser performance, corner separation, vortex evolution, and Reynolds stresses. The results show that increasing r leads to a quadratic increase in pressure recovery efficiency η_p, while both the total-pressure loss coefficient ζ and mean separation volume ratio exhibit exponential decay. At r=0%D, the diverging section exhibits a localized separation bubble and large-scale three-dimensional corner separation, giving rise to strong secondary flows. As r increases, the bubble size remains nearly unchanged, but corner separation is progressively suppressed—from significant reduction at r=10%D to full elimination at r=30%D. The associated secondary-flow intensity and turbulence levels are substantially reduced, as evidenced by decreased Reynolds normal stresses. In contrast, the Reynolds shear stresses exhibit limited variation, indicating that fillet radius primarily alters turbulence production rather than velocity fluctuation coupling. These findings elucidate the physical mechanism by which corner geometry governs three-dimensional separation and turbulence, providing high-fidelity reference data for diffuser optimization, corner-flow control, and turbulence-model benchmarking.