Numerical Analysis of Developing Turbulent Flow in a Square-Sectioned 90 Degree Bend.

Abstract

The present study has carried out a numerical analysis for developing turbulent flow in a square-sectioned 90 deg bend at Reynolds number of 4×104. The ratio of bend mean radius of curvature to hydraulic diameter is 2.3, and straight ducts of 45 and 30 hydraulic diameters long are attached to the inlet and outlet planes of the bend, respectively. In calculation, an algebraic Reynolds stress model was adopted in order to predict anisotropic turbulence precisely, and the boundary-fitted coordinate system was introduced as the method of coordinate transformation. Calculated results were compared with the experimental data measured separately by two research groups. Moreover, the transitions from the secondary flow of the second kind to that of the first kind and vice versa are examined. It was found that the present method predicted well the distributions of mean flow velocity and pressure-driven seconary flows. In the straight duct of the bend outlet, the secondary flow of the first kind maintains its value until about 1.0 hydraulic diameter from the bend outlet. After remaining constant in value up to this point, the secondary flow decays and gradually transforms to the secondary flow of the second kind at the about 10 hydraulic diameters from the bend outlet.