CFD Modelling of Turbulent Flow in Open-Channel Expansions

摘要

Channel expansions provide a transition from a narrow to a relatively wide channelsection,udwhich is necessary in many hydraulic structures. In the transition, flow tends to separateudfrom its diverging sidewalls and create turbulent eddies, if the angle of divergence exceeds audthreshold value. This phenomenon can cause undesirable flow energy losses and erosion to theudsidewalls locally and even further downstream. Previously, researchers have tried to optimise theudtransition’s horizontal shape in order to reduce flow separation; the results are inconclusive. Theudpurpose of this study is to extend earlier investigations about fitting a hump in the vertical toudeliminate flow separation. This study uses the CFD modelling approach. This approach permitsudan efficient and systematic exploration of the effects of different angles of divergence, crestudheight of the hump and the Froude number of subcritical flow. The model results are validatedudusing existent analytical solutions under simplified conditions and available experimental dataudfor a limited number of cases. Flow quantities presented in this thesis include details of theudvelocity, vorticity, eddy structure, and cross-sectional area of flow reversal; these quantities areuddistributed at selected vertical and horizontal planes, and are available for the cases of with andudwithout a hump. It is shown that the use of a hump effectively reduces flow separation and eddyudmotion in the transition. This is because the flow is forced to accelerate over the hump, and as audresult, the otherwise adverse pressure gradient, which is known to be responsible for flowudseparation, diminishes. A hump in the vertical can easily be incorporated into the bed of existentudchannel expansions, and would be less expensive to construct than to modify the horizontaludshape (or the sidewalls) of existent expansions. The results presented in this thesis are ofudpractical values for the optimal design of humps.