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EXPERIMENTAL AND COMPUTATIONAL MODELLING OF 3-D FLOW AND BED SHEAR STRESSES DOWNSTREAM FROM A MULTIPLE DUCT TIDAL BARRAGE
[Thesis]. Manchester, UK: The University of Manchester; 2013.
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Abstract
The near-field depth-varying velocities and resulting bed stresses downstream from a tidal barrage have not previously been studied. The flow through and downstream of a row of seven open draft tubes in a barrage has been investigated through laboratory experiment in a wide flume, 3-D RANS CFD simulation and 2-D depth-averaged computation. When there is no turbine representation and hence negligible swirl in the draft tubes, agreement between the experiments and 3-D modelling is shown to be good, including the prediction of an asymmetric Coandă effect. With the addition of bulb bodies and vanes creating swirl in the draft tubes the velocity profiles are changed, with increased swirl directly downstream from the draft tubes and throughout the entire flume cross-section further from the barrage. The addition of rotors did not significantly alter the flow field patterns, for the stator/rotor combinations presented here. 3-D CFD could not accurately predict the velocity profiles resulting from the swirl in the ducts. The experiments and 3-D model shows that bed shear stress can be magnified markedly near the barrage particularly where the jets become attached to the bed. At full-scale this would result in a fully mobile bed with sand of typical grain size 1mm. One aim was to determine the distance downstream where depth-averaged modelling gives reasonable prediction and this is shown to occur around 20 tube diameters (20D) downstream of the barrage. Upstream of this, the depth-averaged modelling inaccurately predicts water level and bed shear as well as the 3-D flow field. The addition of swirl cannot be modelled using 2-D modelling, but by 20D downstream there is minimal velocity variation transversely and throughout the depth, indicating depth-averaged modelling would be applicable from this distance.