Investigation on the influence of inlet conditions on the scour phenomenon

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Presentation of the conclusions of a research internship at TUM carried out in 2010.
The scour phenomenon is the fact that around marine structures embedded into the sediment bed, vortices are created which "scour", that is, erode the sediment bed and thus make the structure vulnerable. This study shows that experiments and empirical formulas produced by different researchers around the world show biases linked to their different experimental settings, relatively to the water inlet conditions.

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Investigation on the influence of inlet conditions on the scour phenomenon

  1. 1. 1Investigation on the influence ofthe inlet conditions on the scouringprocessStudent : Cédric Bellet ENPC 2010School tutor: Michel Benoit
  2. 2. 2 ENPC 2010Introduction• What is the scouring process?• Research on the scouring process• What is an inlet condition?
  3. 3. 3 ENPC 2010Introduction• What is the scouring process?• Research on the scouring process• What is an inlet condition?
  4. 4. 4 ENPC 2010Introduction
  5. 5. 5 ENPC 2010Introduction Scour at marine structures, Richard Whitehouse
  6. 6. 6 ENPC 2010Introduction• What is the scouring process?• Research on the scouring process• What is an inlet condition?
  7. 7. 7 ENPC 2010Introduction• Empirical formulas• Simulations: Direct Numerical Simulation, Large Eddy Simulation, etc.• Measurement: hot-film velocimetry, Laser Doppler Velocimetry, etc.
  8. 8. 8 ENPC 2010Introduction• What is the scouring process?• Research on the scouring process• What is an inlet condition?
  9. 9. 9 ENPC 2010Introduction
  10. 10. 10 ENPC 2010Introduction
  11. 11. 11 ENPC 2010Introduction: objectives• Draw up the state of the question of the influence of the inlet conditions on the scouring process• Quantify this influence in the framework of the experiments conducted at the laboratory
  12. 12. 12 ENPC 2010Introduction: stakes• Critique of the experiments conducted in the laboratory• Critique of the methods employed by other researchers• Focus on a parameter neglected by the researchers
  13. 13. 13 ENPC 2010Technische Üniversität München• Founded in 1868 by Ludwig II• 23000 students each year• Since 2006, belongs to the Elite Universities of Germany
  14. 14. 14 ENPC 2010Fachgebiet Hydromechanik• First chair of the TUM• Around ten researchers ▫ Numerical simulation ▫ Laboratory experiments on scour ▫ Cooperation with the city of Munich (Isar) ▫ Hydromechanic courses and study trips
  15. 15. 15 ENPC 2010Fachgebiet HydromechanikProf. Dr.-Ing. habil. M. Dipl.-Ing. Florian Pfleger Dr.-Ing. ChristophManhart Research Associate RappHead of Chair Head of Laboratory
  16. 16. 16 ENPC 2010Outline1. Influence of the inlet condition on the scouring process: starting point2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)3. Measurements, discussion, conclusion
  17. 17. 17 ENPC 2010Outline1. Influence of the inlet condition on the scouring process: starting point2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)3. Measurements, discussion, conclusion
  18. 18. 18 ENPC 2010Influence of the inlet condition on the scouring process: starting point
  19. 19. 19 ENPC 2010Hydromechanics elementsReynolds number:• Ratio of the inertial forces to the viscous forces• is the hydraulic radius,• the flow depth,• the channel width ;• is the flow mean velocity of the flow,• the kinematic viscosity of the fluid
  20. 20. is the velocity of the flow at one point 20 ENPC 2010 Hydromechanics elements Froude number: • Ratio of the inertial forces to the gravitational forces • is the velocity of the flow at one point • the flow depth; • the gravity acceleration
  21. 21. 21 ENPC 2010Hydromechanics elements• Froude number is equal to 0.33  subcritical flow• Reynolds number is equal to 176000  fully turbulent flow
  22. 22. 22 ENPC 2010The turbulent boundary layer
  23. 23. 23 ENPC 2010The sediment• Non-cohesive sand• Mean size 1.9mm• Natural angle of repose: 30°
  24. 24. 24 ENPC 2010The scouring process
  25. 25. 25 ENPC 2010Predictive formulas• Thirteen formulas• Seven input parameters  The flow depth,  The pier diameter ,  The mean velocity ,  The critical velocity ,  The mean size of the sediment ,  The non-uniformity of the sediment ,  The natural angle of repose• One output: the scour depth
  26. 26. 26 ENPC 2010Predictive formulas
  27. 27. 27 ENPC 2010Measurements under different inletconditions
  28. 28. 28 ENPC 2010Plexiglas pan on the water surface
  29. 29. 29 ENPC 2010Flow straighteners
  30. 30. 30 ENPC 2010Flow straighteners and a weir
  31. 31. 31 ENPC 2010Plates with holes
  32. 32. 32 ENPC 2010Plates and a weir
  33. 33. 33 ENPC 2010Observations• The inlet condition has a significant influence on the scouring process• Both dynamics and equilibrium are affected• The measurements range within the predictions• Inlet conditions can be classified
  34. 34. 34 ENPC 2010Observations Scour depth after two hours Flow straighteners 5cm Flow straighteners 8-9cm plus weir Grids 9-10cm Grids plus weir 11cm
  35. 35. 35 ENPC 2010Observations Scour dynamicsInlet condition ? ? and equilibrium
  36. 36. 36 ENPC 2010Outline1. Influence of the inlet condition on the scouring process: starting point2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)3. Measurements, discussion, conclusion
  37. 37. 37 ENPC 2010Measurement technique: the Laser Doppler Velocimetry (and a first measurement)
  38. 38. 38 ENPC 2010Laboratory configuration
  39. 39. 39 ENPC 2010Laser Doppler Velocimetry• A flow is seeded with small buoyant particles that scatter light
  40. 40. 40 ENPC 2010Laser Doppler Velocimetry• Two Laser beams cross at a point called the measurement volume, and create a set of fringes
  41. 41. 41 ENPC 2010Laser Doppler Velocimetry• Particles crossing the measurement volume emit a signal whose frequency is proportional to their velocity.
  42. 42. 42 ENPC 2010Laser Doppler Velocimetry• The signal is processed by a computer so as to get this velocity.
  43. 43. 43 ENPC 2010Laser Doppler Velocimetry• Frequency shift:• Detected frequency with and without frequency shift:
  44. 44. 44 ENPC 2010Processing of the data • Fast Fourier Transform • Determination of the main frequency • Determination of the corresponding velocity
  45. 45. 45 ENPC 2010Processing of the data • Around 2000 bursts: reliable information • Mean velocity component: expected value • Root mean square fluctuation component: variance
  46. 46. 46 ENPC 2010A first measurement Velocity field? •One measurement= one velocity component •2D velocities= two measurements per point
  47. 47. 47 ENPC 2010A first measurement • 95 points • 2 measurement directions per point • 2000 bursts per measurement
  48. 48. 48 ENPC 2010A first measurement
  49. 49. 49 ENPC 2010A first measurement • Presence of the horseshoe vortex • The approaching flow is bent by the presence of the pier • But: some oddities
  50. 50. 50 ENPC 2010Problems encountered• Fast Fourier samples number too small: peak-locking
  51. 51. 51 ENPC 2010Problems encountered• Measurement directions too close one to anotherSmall input variations result in very different outputs.
  52. 52. 52 ENPC 2010Outline1. Influence of the inlet condition on the scouring process: starting point2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)3. Measurements, discussion, conclusion
  53. 53. 53 ENPC 2010Measurements, discussion, conclusion
  54. 54. 54 ENPC 2010Measurement of four different velocityprofiles •Four different vertical velocity profiles •Measured under four different inlet condtions
  55. 55. 55ENPC 2010
  56. 56. 56 ENPC 2010Comment •The red profile cannot be used, because the flow was assymetric •The other profiles have identical mean velocities •And show significant differences of distribution in the upper region The inlet conditions have an influence on the vertical velocity distributions
  57. 57. 57 ENPC 2010Comparison of the measured profiles Wall shear stress of the different profiles: (Sagaut, 2006)
  58. 58. 58 ENPC 2010Comparison of the measured profiles Scour depth Wall shear after two stress hours Flow 5cm 0.32Pa straighteners Flow 8-9cm ? straighteners plus weir Grids 9-10cm 0.35Pa Grids plus weir 11cm 0.40Pa
  59. 59. 59 ENPC 2010Ettema’s Theory (1980) “The higher the shear parameter, the deeper the scour depth”
  60. 60. 60 ENPC 2010 Influence of the inlet condition Vertical velocity Wall shear Shear Scour dynamicsInlet condition Ettema’s theory distribution stress parameter and equilibrium
  61. 61. 61 ENPC 2010Introduction: objectives• Draw up the state of the question of the influence of the inlet conditions on the scouring process  OK• Quantify this influence in the framework of the experiments conducted at the laboratory OK
  62. 62. 62 ENPC 2010But…
  63. 63. 63 ENPC 2010Why is the influence of the inletconditions hardly ever mentioned?
  64. 64. 64 ENPC 2010The researchers’ assumption Universal logarithmic profile Vertical velocity Wall shear Shear Scour dynamicsInlet condition Ettema’s theory distribution stress parameter and equilibrium
  65. 65. 65 ENPC 2010How can someone get a universalvelocity profile?• After a certain free flow length, a boundary layer is fully developed universal logarithmic velocity profile• But there is no acknowledged criterion establishing this free flow length. This length is case-specific, and is not reached in our case
  66. 66. 66 ENPC 2010Reference Flow Channel Channel Free flow Hydraulic Free flow depth width length length radius length/hydr aulic radiusLink 2006 20-40cm 2m 30m 15m 17-29cm 51-88Carette 25cm 1.17m 11m 6.5m 17cm 382008Yanmaz 4-16.5cm 0.67m 10.9m 6.9m 4-11cm 62-1721991Zanke 1982 42-55cm 0.6m 15m 7.5m 17.5- 38-43 20cmDey 1995 3-5cm 0.81m 10m 5m 3-5cm 100-166Oliveto 4-60cm 1m 30m 15m 4-27cm 55-3752007Melville 10-24cm 0.44m 11.8m 6m 7-12cm 50-851987This report 15cm 1.17m N/A 6.5m 12cm 54
  67. 67. 67 ENPC 2010Conclusion• The inlet condition affects the scour phenomenon through the value of the shear parameter, in line with Ettema’s theory• Researchers tackle this problem by assuming that the boundary layer is fully developed which is the case when the free flow length is big enough• In our case, the free flow length is not big enough. Flumes of comparable dimensions might have the same problem
  68. 68. 68 ENPC 2010Conclusion• A criterion should be created to determine a correct inlet condition; else, the shear parameter should be explicitly mentioned.• Experiments conducted under comparable configuration should be considered with precaution.
  69. 69. 69Merci de votre attention ENPC 2010

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