Prediction of Turbulent Flow Structure in a Fully Developed Rib-Roughened Narrow Rectangular Channel

Fully developed turbulent water-flow structure over one-side repeated-ribs in narrow two-dimensional rectangular channels was investigated experimentally by Particle Image Velocimetry (PIV) and analytically by the standard kappa-epsilon and nonlinear kappa-epsilon turbulent models. Two rib-pitch to height ratios (p/k) of 10 and 20 were investigated while the rib height was held constant at 4 mm. The rib height-to-channel equivalent diameter ratio (k/De) was 0.1. The streamwise mean velocity and turbulent kinetic energy distributions at six selected axial stations from the center rib for the two Reynolds number (Re) of 7,000 and 20,000 were obtained and compared with the predicted one. The performance ability in predicting separating and reattaching turbulent water-flow between the standard x-e and nonlinear kappa-epsilon models had yielded no clear conclusion. A large-scale turbulent eddy was generated by the rib promoter and then propagated into the mainstream flow, which led to the deformation of the velocity profile. The turbulent kinetic energy was increased about two times higher at p/k = 20 than that at p/k = 10 under the two Reynolds numbers. The effect of the p/k value and the Reynolds number (Re) on reattachment length (XR) was investigated and showed that the p/k and Re had no significant effect on the reattachment length beyond a critical value of Re = 15,000 where XR was found to be approximately 4 times of the rib height under water-flow condition.