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**Prof. P. K. Stansby & Dr. D. A. Apsley University of Manchester**

Near-field Flow Downstream of a Tidal Barrage: Experiments, 3-D CFD and Depth-averaged Modelling Penny Jeffcoate Prof. P. K. Stansby & Dr. D. A. Apsley University of Manchester Welcome

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**Presentation Outline Introduction Research Aims Modelling Comparison**

Experimental, 3-D and depth-averaged modelling Swirl Assessment Swirl with bulb and stators Conclusions Future Work Swirl with bulb, stators and propellers Bed Shear stress and sediment transport Brief overview of the content of this presentation La Rance, France Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Tidal Barrage Sites Large tidal range Potential sites in UK:**

Solway Firth Morecambe Bay Mersey Dee Severn High initial investment Environmental impact Unknown flow effects Key feature for tidal barrages is large tidal range These are potential sites for UK Not been implemented due to... This project focuses on determining the flow effects, and thus other effects, such as sediment transport, which could affect both habitats and the efficiency of the barrage Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Project Motivation Previous Modelling Required Modelling 2-D**

Depth-averaged Large-scale 5-10m Whole estuary 3-D Depth-variation Small-scale 10-20cm Immediately downstream of barrage 20 duct diameters (20D) Previous modelling has been conducted – two-dimensional modelling of an entire estuary or the Irish Sea But smallest grid size is 5m, since turbine is 9m diameter, quite low resolution Want to conduct three-dimensional modelling, so look at variation throughout the depth and how differs from depth-averaged Min cell size... Within 20 duct diameters which is equivalent to approx 200m From these modelling requirements can clarify the key research aims Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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Research Aims 1. What is the limit of applicability of 2-D modelling at predicting close-to-barrage flow? 2. Are the results affected by the incorporation of swirl? 3. How is the bed stress, and thus the sediment transport, affected? Limit of 2-D – at what distance downstream does depth-averaged modelling give reasonable results Swirl – add inclined stators and propellers Bed stress – see where sediment scour and deposition occurs and how this will affect habitats as well as the barrage efficiency Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**1. How accurate is 2-D modelling?**

Experiments Scale factor = 1 in 143 D = 0.11m Uin = ms-1 hup = m hdown = m Vectrino ADV Weir Created barrage to fit in experimental flume by using a Severn proposal and scaling by factor of 1 in 143 This gave duct diameter of 0.11m, so 20 duct diameter region extends up to 2.2m from barrage Inlet velocity and downstream dam gave head difference across the barrage equivalent to 2.43m (deemed acceptable for generation by Wu 2010) NDV and ADV record velocities Barrage ducts Barrage walls Inlet Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**1. How accurate is 2-D modelling?**

Three-Dimensional Modelling StarCCM+ - Upstream tank, ducts and downstream tank Unstructured polyhedral mesh Base cell size ~0.02m Boundary conditions Velocity Inlet Pressure Outlet Walls Symmetry plane lid Standard k-ε model Convergence criteria Momentum and continuity residuals 10-4 Experimental conditions used to generate 3-D mesh using StarCCM+ Modelled entire flume region Base cell 0.02m but refinement close to barrage and close to bed, plus conducted mesh refinement studies Boundaries matched experiments Standard k-e used but k-omega and SST didn’t affect results Convergence reached, but model run for longer so that ensured divergence did not occur Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**1. How accurate is 2-D modelling?**

Two-Dimensional Modelling FORTRAN In-house Stansby SW2D model Downstream tank Cell size = ~0.01 – 0.02m Boundaries conditions 7 velocity inlets Fixed depth boundary outlet Vertical slip walls 2nd order, time-stepping model 2-D modelling used... Modelled downstream tank only with 7 velocity inlets representing duct outlets Comparable cell size, though no refinement Boundaries 2nd order... Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Probe and Profile Locations**

Velocities recorded at various depths and distances downstream as can see in diagram Velocities at 1 duct diameter, or 1D, 2 diameters, 5 diameters, 10 diameters, 20diameters recorded Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**StarCCM+ Velocity Vectors**

Here can see the velocity vectors produced by StarCCM+ at duct mid-height Acceleration into ducts as area decreases Jets form in downstream tank directly downstream from duct exits Spreading occurs as distance from barrage increases leading to more uniform velocity flow at the end of the region Eddies form either side of the jets, with asymmetry occurring due to the Coanda effect pulling the main body of the flow to one side of the tank The stream-wise velocity profiles analysed at 1 diameter, 2D, 5D, 10D, 20D downstream and compared with expt results 1D 2D 5D 10D 20D Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Depth-varying Velocity Profiles**

1D – jets evident at depth of ducts but not close to surface - velocity variation throughout depth - eddies at tank edges - if depth-averaged would lose the flow detail close to surface and jets may be under-predicted 5D – large variation across depth in velocity magnitude - asymmetry close to surface - asymmetric eddies 20D – very constant across whole profile and throughout depth - slight reduction in velocity on one side due to lingering effects of the Coanda High compatibility at all distances and depths Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**SW2D Velocity Vectors 1D 2D 5D 10D 20D**

Vel vectors produced by SW2D Jets form downstream from inlets high amount of spreading and flow becomes uniform quite quickly Eddies form but not asymmetric Superimpose half vectors from StarCCM and can see that jets extend further in CCM and eddies are much larger Amount of spreading in SW2D much higher Now look at depth-averaged velocity profiles 1D 2D 5D 10D 20D Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Depth-averaged Velocity Profile**

Vel profiles at 1 diameter show jets and eddies forming, but the profile close to the surface is completely different - Eddies predicted by SW2D are smaller than expt and CCM At 5D the high asymmetry still shown in depth-averaged expt and CCM, but not in SW2D. Also there is no reversed flow in SW2D , whereas the eddy is at it’s largest at this location At 20D v similar profiles, with the slight decrease on one side caused by Coanda, but negligible Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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Conclusions From 1 duct diameter (1D) to 10D downstream At 20D downstream Asymmetrical flow Symmetrical flow Variation across depth No variation across depth Large eddies in three-dimensional (3-D) model, small eddies in 2-D model No eddies formed Little similarity between 3-D and 2-D results High compatibility between 3-D and 2-D results In conc, from 1diameter to 10 diameters... From 20 diameters... At 20D, 2-D modelling provides accurate flow representation, but until 20D 3-D results are more accurate Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**2. Are the results affected by stator swirl?**

Experiments StarCCM+ Bulb included in ducts Swirl generated by body force: Constant* [-x, -(z - zref), (y - yref)] Next look at the effect of swirl on the results – going to do in two ways, firstly with bulb turbine and stators, then with propellers First added stators inclined at 30 degrees to a bulb turbine which was placed in each duct In order to maintain the head difference the velocity was reduced Vectrino ADV used to record 3 component velocities In the StarCCM+ model a body was placed in each duct to represent the turbine body and a swirl force was added to simulate the rotation and restriction to the flow caused by the stators Can see that this generates swirl by looking at vector plot Uin = ms-1 hup = m hdown = m Blades inclined at 30° Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Velocity Vectors - Experimental**

1D 5D 20D 1D – high amounts of rotation directly downstream from each duct - Some asymmetry, especially on the left where there is strong vertical component, which produced a rise in the fluid surface, evident in the expt - also circulation throughout whole tank, because close to the bed the predominant cross flow direction is one way, whilst close to the surface it’s the other - large variations in velocity throughout the depth that could not be depicted by depth-averaged modelling This is even stronger at 5D... There is no longer circulation about each duct, due to spreading, but there is full flume circulation If depth-averaged the cross-tank components would cancel and the flow would be assumed to be uniform in the streamwise direction At20D, this is no longer true, as the spanwise and vertical components of the flow have significantly reduced Next if we look at the vectors at each depth... Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Velocity Vectors - Experimental**

4cm 12cm 18.5cm Close to the bed at 4cm the flow tends to one side, due to the rotation from the guide vanes At 20D there is no longer this tending to one side Wakes also form directly downstream from each duct location, especially at 1D At 12cm from the bed, near the top of the duct exits, the spanwise velocity component is in the reversed direction, with asymmetry in the velocity magnitude occurring Close to the surface at 18.5cm the jets/wakes near the barrage are no longer evident and the main velocity component is across the flume The flow only becomes uniform across the depth at 20D where there is no longer a spanwise component to the flow We’ve tried to replicate this using StarCCM+ and we can see the streamlines produced by the program here Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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Streamlines Can see uniform flow into ducts, then acceleration and rotation as flow goes round the bulbs and the body force is applied. The amount of swirl can be altered by altering the body force and this produces greatly differing results – click So the amount of swirl and the velocity magnitude in the ducts is greatly reduced by relaxing the body force constant, as is the circulation in the downstream tank. The amount of swirl that is required to best match the experiments however is still a work in progress. The difference between the results, and how they will affect how much they match the expt is more easily seen in the midheight velocity vectors Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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**Velocity Vectors - Computational**

As you can see the high amount of swirl creates a wake downstream from the bulbs, with jets forming on either side of the duct exits. The amount of spreading is very high, leading to very low velocity flow quite close to the barrage However the lower swirl constant creates jets downstream from the ducts with only small decreases in the velocity directly downstream from the bulbs. These jets extend further downstream and cause higher velocity flow in the region close to the barrage. The body force constant must be refined so that the amount of swirl in the ducts and the downstream tank and the size of the bulb wake are more accurately predicted. This is the next stage of the project. Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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Conclusions What is the limit of applicability of 2-D modelling at predicting close-to-barrage flow? Acceptable further downstream than 20 diameters 3-D modelling is required for close-to-barrage modelling Are the results affected by the incorporation of swirl? Experimental results show large variations in flow and flow circulation Amount of swirl in computation must be refined to match experiments Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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Future Work Are the results affected by the incorporation of swirl? Altering the swirl constant Comparison with experimental results Incorporation of propeller How is the bed shear, and thus sediment transport, affected? Analysis of the close-to-bed experimental velocities Comparison with computational results Assessment of scour and deposition based on threshold of motion Scaling assessment Introduction – Research Aims – Modelling – Swirl – Conclusion – Future Work

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Bed Shear Stress Introduction – Research Aims – Modelling – Swirl – Bed Stress - Conclusion

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Shields Parameter Introduction – Research Aims – Modelling – Swirl – Bed Stress - Conclusion

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**Any Questions? Penny Jeffcoate**

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