STUDY OF TURBULENT FLOW DUE TO EXPANSION IN OPEN CHANNEL USING ANSYS FLUENT

Abstract

Expansions in channel enable us to provide transition from a narrow to a considerably wide – section, which is important in the design of many hydraulic structures. In the transition, there is tendency of flow to separate from its diverging side walls and form turbulent eddies. This happens if the angle of divergence exceeds the limiting threshold value. This phenomenon leads to undesirable energy losses and erosion to the walls of the channel and also on the downstream side of the channel. Earlier, Researchers have done work on the optimization of horizontal shape of the transition so as to avoid flow separation but no concrete conclusion was derived from it. This project study extends the earlier investigation by means of fitting a hump in the vertical to restrict flow separation. This study makes use of CFD modelling approach. This approach allows systematic exploration of the variation of number of parameter like Divergence angle, Crest Height of hump and Froude number of subcritical flow. Flow quantities studied in this project are velocity, Eddy structure and vorticity. These quantities are available for cases with and without a hump and are distributes at selected vertical and horizontal planes. From the studies, we find that the tendency of flow separation and eddy motion are considerably reduced with the use of hump. These occur because here the flow is accelerated over the hump and as a consequence, the adverse pressure gradient which could have cause flow separation diminishes. A hump in the vertical can be easily introduced into the bed of existing channel expansion, and would economical from construction point of view as compared to that of modification of horizontal shape of existing expansions. The results presented in this thesis are of practical values for the optimal hump design.