Sediment transport by coherent structures in a horizontal open channel flow experiment

1. Sediment transport by coherent structures in a horizontal open channel flow experiment Lorentz Workshop 2006 Leiden W.A. Breugem and W.S.J. Uijttewaal Environmental Fluid Mechanics

2. Overview • Introduction • Sediment transport in open channel flow • Experimental setup • Results • Concentration profile development • Drift velocity histogram • Spatial drift velocity structure • Conclusions Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

3. ? ? ? ? ? ? Flow direction ? ? ? ? ? ? ? ? Sediment transport in open channel flow • Focus • Suspended sediment, hence outer region • Dilute flow (for now) g Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

4. Sediment feeder Oslim Sediment feeder Oslim Peristaltic Pump Peristaltic Pump Constant head vessel Constant head vessel Baffle Baffle Mixer Mixer Peristaltic Pump Peristaltic Pump Mixing vessel 23.50 m 23.50 m Mixing vessel Measurement section Measurement section 14.25 m 14.25 m Nozzle Main sieve Nozzle Main sieve 0.05 m 0.05 m Weir Weir 75 h 16 h Pump Buffer reservoir Return pipes (3 x) Pump Buffer reservoir Return pipes (3 x) Flume setup Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

5. Particle requirements • Comparison with DNS • Be suspended at Re =10,000 (i.e. u*/ws≈ 5) • dp/lk < 1 (assumption in Maxey & Riley eq.) • Rep < 1 (assumption in Maxey & Riley eq.) • Experimental procedure • Phase discrimination: dp >3 dtracer • Materials • Tracer: hollow glass (dtracer = 15 mm) • Sediment: polystyrene (dp = 375 mm, rp/rf≈1.035) Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

6. Instantaneous result • Re* ≈ 500 • Polystyrene particles • Flow from left to right • 0.8 ucl subtracted Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

7. Concentration profiles Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

8. Drift velocity • Drift from average Stokes drag force (Simonin et al., 1993): • I.e. The drift velocity is the deviation of the average fluid velocity seen by a particle. • Vertical momentum equation: Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

9. X=16 h X=75 h PDF vT=0.2u* Drift velocity profiles Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

10. Q2 Q2 Q2 Q2 Q1 Q1 Q1 Q1 Q3 Q3 Q3 Q3 Q4 Q4 Q4 Q4 Drift velocity PDF (Predominantly settling, x=16h) y/h = 0.55 vT=0.2u* Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

11. Q2 Q2 Q2 Q1 Q1 Q1 All particles V+ Q3 Q3 Q3 Q4 Q4 Q4 u+ Upgoing particles Downgoing particles V+ V+ u+ y/h = 0.55 u+ Drift velocity, deviation from random sampling (predominantly settling, x = 16 h) • More particles in Q4, less in Q2 • Downgoing particles in Q4 • Upgoing particles in Q1 and Q2 vT=0.2u* Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

12. Q2 Q2 Q2 Q2 Q1 Q1 Q1 Q1 Q3 Q3 Q3 Q3 Q4 Q4 Q4 Q4 Drift velocity histogram (fully developed, x =75 h) y/h = 0.55 vT=0.2u* Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

13. Q2 Q2 Q2 Q1 Q1 Q1 All particles • More particles in Q2 and Q3, less in Q4 • Downgoing particles in Q3 and Q4 • Upgoing particles in Q2 V+ Q3 Q3 Q3 Q4 Q4 Q4 u+ Upgoing particles Downgoing particles V+ V+ u+ y/h = 0.55 u+ Drift velocity, deviation from random sampling (fully developed, x = 75h) vT=0.2u* Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

14. v’ Q2 Q1 Q4 (fast) Q4 (fast) u’ Q2&Q3 (slow) Q2 (slow) Q3 Q4 Conceptual picture of particle transport Fully developed situation Settling situation Velocity Quadrants u’ Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

15. Spatial structure • The spatial structure is studied with conditional averages • Conditional averages are determined with Linear Stochastic Estimation (LSE). • With LSE, conditional averages can be calculated from two-point correlations. • A vortex head (rotating with the mean shear) at three different reference heights is used as condition. • Vortex identification is done with swirling strength. Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

16. Vectors Streamlines Free surface Bottom Shear layer Fluid flow structure (vortex near the wall) Q3 Q2 Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

17. Predominantly settling Fully developed Free surface Bottom Drift velocity structure (vortex near the wall) Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

18. Vectors Streamlines Free surface Bottom Fluid flow structure (vortex at 0.5 h) Q2 Q3 Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

19. Predominantly settling Fully developed Free surface Bottom Drift velocity structure (vortex at 0.5 h) Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

20. Flow Q2 Q4 Conceptual model (hairpin vortex) Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

21. Trapped particles (how does it get here?) Not so heavy particle in a vortex u2/r rp=O(rf) St < 1 p g Settling situation Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

22. HPV ISL HPV ISL HPV Q3 Q2 Q3 Conceptual picture fully developed situation Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

23. Spatial relation between Q2 & Q3 Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

24. Spatial relation between Q2 & Q4 Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion

25. Conclusion • The increased concentration in either fast (settling situation) or slow (fully developed situation) flow structures, cause the mean particle velocity to differ from the mean fluid velocity. • Downward transport occurs in sweeps (Q4) and inward interactions (Q3), upward transport in ejections. • In predominantly settling, particles are found less in ejections (Q2) and more often in sweeps (Q4). • In fully developed situation, particles are found less sweeps, and more often in both ejections and inward interactions. This is due to the alignment of several hairpin vortices. Introduction - Experimental setup – Drift velocity - Flow structures - Conclusion