Numerical Analysis of Developing Turbulent Structure in S-Shaped Duct.

Abstract

Numerical simulation has been performed for developing turbulent flow in S-shaped duct with a square section at a Reynolds number of 4×104. The S-duct was formed from two 22.5 degree bends with 40 mm hydraulic diameter and 280 mm mean radius of curvature. Straight ducts with hydraulic diameters of 7.5 and 50 are attached to the inlet and outlet planes of the S-duct, respectively. In calculation, an algebraic Reynolds stress model was adopted in order to predict anisotropic turbulent flow precisely, and a boundary-fitted coordinate system was introduced for coordinate transformation. The calculated results were compared with the experimental data using a laser Doppler velocimeter. In particular, the distributions of Reynolds stresses are compared in this report. As a result of comparison with the experimental data, it was found that the present method can accurately predict contours of streamwise normal stress which show the active generation of turbulence near the outer wall of the second bend. Moreover, it was clarified by estimating the production term that the high value of streamwise normal stress near the outer wall of the second bend is responsible for the velocity gradient of streamwise mean velocity with respect to the radial direction. The profiles of other normal stresses and shear stress are also predicted using the anisotropic turbulent model without great discrepancy.