Author(s): Sahar Najmeddin; S. Samuel Li
Linked Author(s): S. Samuel Li
Keywords: Channel expansion; Turbulent flow; Flow separation; Energy loss; CFD
Abstract: Channel expansions are needed to provide a transition from a relatively narrow to a wide channel-section. In the transition, flow tends to separate from its diverging sidewalls and hence create turbulent eddies, if the angle of divergence exceeds a threshold value. This phenomenon can cause undesirable flow energy losses and erosion to the sidewalls locally and even further downstream. Previously, researchers have tried to optimise the transition’s horizontal shape in order to reduce flow separation; the results are inconclusive. The purpose of this paper is to extend earlier investigations about fitting a hump in the vertical to eliminate flow separation. This paper uses the numerical simulation approach. This approach permits an efficient and systematic exploration of the effects of different angles of divergence, crest height of the hump and the Froude number of subcritical flow. The model results are validated using existent analytical solutions under simplified conditions and available experimental data for a limited number of cases. Flow quantities presented in this paper include detailed velocities, vorticity, eddy structures, and cross-sectional area of flow reversal. These quantities are distributed at selected vertical and horizontal planes, and are available for the cases of with a hump and without. It is shown that the use of a hump effectively reduces flow separation and eddy motion in the transition. This is because the flow is forced to accelerate over the hump. As a result, the otherwise adverse pressure gradient, which is known to be responsible for flow separation, diminishes. A hump in the vertical can easily be incorporated into the bed of existent channel expansions, and would be less expensive to construct than to modify the horizontal shape (or the sidewalls) of existent expansions. The results presented in this paper are of practical values for the optimal design of humps.
Year: 2015