Research Note «Numerical Study of the Effect of Ratio of Radius to Width on Flow Pattern in a 90 Degree Bend

Flow pattern in river bends is very complicated and fully three dimensional in nature with high turbulence intensity. This type of flow has a great influence on the river morphology by eroding the outer bank which results in deposits of sediment at the inner bank. Owing to the importance of the study of such flow، in this study flow in bend was simulated using FLOW-3D software. FLOW-3D utilizes a volume of fluid (VOF) method for free surface simulation. RNG turbulence model was used for closing the 3-D Reynolds-averaged Navier-Stokes equations. Experimental data from a laboratory study of flow in a rigid bed channel with a 90 degree bend were used to verify the results from the numerical model. The central radius of the bend and ratio of radius of the bend to width were 2. 4 m and 4، respectively. Numerical results showed good agreement with the experimental results. Then four ratios of radius to width of the bend: R/B=2، 3 4 and 5 were simulated to study the effect of ratio of radius to width on flow pattern. Based on the results of the longitudinal velocity component، at the start of the bend the cross-section maximum velocity occurred adjacent to the inner bank for all ratios of radius to width. Toward the end of the bend، in the sharpest bend (R/B=2)، maximum velocity occurred adjacent to the inner bank، but it shifted to the area close to the outer bank for other ratios of radius to width. The transverse slope of the free surface increases as the ratio of radius to width decreases.