Numerical Calculations for the Length of the Transitory Zone in Partially Filled Circular Pipes with Steep Slope

Three-dimensional turbulent free-surface flow through smooth and corrugated circular pipes with steep slope is simulated by means of Computational Fluid Dynamics (CFD). For this purpose, the three-dimensional Reynolds Averaged Navier-Stokes equations are solved using the ANSYS Fluent solver, while interface between air and water is calculated using the Volume of Fluid (VOF) method. Effect of inlet conditions regarding the Froude and Reynolds numbers, channel slope and filling ratio on the length of flow development is investigated while both sub-critical and super-critical inlet conditions are considered. Results of the numerical calculations show that uniform open channel flow is guaranteed roughly 110 and 60 diameters downstream the pipe inlet, for smooth and corrugated pipes, respectively. The transitory length shows a tendency to decrease with the Reynolds number, contrary to the entrance length in pipe flow.