Numerical Analysis of Developing Turbulent Flow in S-Shaped Duct

Abstract

A numerical analysis has been carried out of developing turbulent flow in S-shaped duct with a square cross 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 turbulence precisely, and boundary-fitted coordinate system was introduced as the method of coordinate transformation. Calculated results were compared with the experimental data obtained using a laser Doppler velocimeter. As a result of the calculation, it was predicted without conflict with the experimental data that the flow at the inlet shows a "core" flow located nearer the inner wall of the first bend and a region of low-velocity fluid accumulated at the exit of the S-duct is observed. Moreover, the transition from the secondary flow of the first kind to that of the second kind is examined in the outlet straight duct. It was found that after reaching a maximum value near the S-duct outlet, the secondary flow decays and gradually transforms to the secondary flow of the second kind at a hydraulic diameter of about 18 from the bend outlet.