Physical characteristics of flow in a square duct bend

Laser-Doppler measurements of the velocity in a 90 deg curved square duct are reported both for developed laminar flow and turbulent flow. Special attention has been given to the velocity distribution in sublayer. Pressure distribution along the wall was obtained through wall tapping by manometer. The Reynolds number of the flow ranges form 6,000 to 40,000. Due to the fine resolution ability of the LDV used, measurements of velocity at points only 20 microns away from the solid surface of duct wall have been executed successfully. The linear law of velocity distribution in sublayer has been verified, and the shear stresses on the duct wall were evaluated. By comparison of local coefficient of skin friction of duct with the pressure gradient, the synchronization of these two parameters was discovered or revealed. Total bend-loss coefficients for different flow regime have been calculated. It was noticed that effect of curvature is much stronger in low Reynolds number flow. With the help of flow visualization an interesting picture of the generation and development of spiral structure was demonstrated.