1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Velocity Vectors - Experimental1D 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

17. 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

18. 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

19. 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

20. 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

21. 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

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

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

24. Any Questions? Penny Jeffcoate