investigation on the influence of inlet conditions on the scour phenomenon
DESCRIPTION
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. This work is licensed under a Creative Commons Attribution 3.0 Unported License.TRANSCRIPT
Investigation on the influence of
the inlet conditions on the scouring
process
Student : Cédric Bellet
School tutor: Michel Benoit
1
ENPC 2010
Introduction
• What is the scouring process?
• Research on the scouring process
• What is an inlet condition?
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Introduction
• What is the scouring process?
• Research on the scouring process
• What is an inlet condition?
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Introduction
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Introduction
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Scour at marine structures, Richard Whitehouse
Introduction
• What is the scouring process?
• Research on the scouring process
• What is an inlet condition?
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Introduction
• Empirical formulas
• Simulations: Direct Numerical Simulation, Large Eddy Simulation, etc.
• Measurement: hot-film velocimetry, Laser Doppler Velocimetry, etc.
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Introduction
• What is the scouring process?
• Research on the scouring process
• What is an inlet condition?
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Introduction
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Introduction
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Introduction: 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
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Introduction: 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
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Technische Üniversität München
• Founded in 1868 by Ludwig II
• 23000 students each year
• Since 2006, belongs to the Elite Universities of Germany
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Fachgebiet 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
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Fachgebiet Hydromechanik
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Dipl.-Ing. Florian PflegerResearch Associate
Prof. Dr.-Ing. habil. M. ManhartHead of Chair
Dr.-Ing. Christoph RappHead of Laboratory
Outline
1. Influence of the inlet condition on the scouring process: starting point
2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)
3. Measurements, discussion, conclusion
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Outline
1. Influence of the inlet condition on the scouring process: starting point
2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)
3. Measurements, discussion, conclusion
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Influence of the inlet condition on the
scouring process: starting point
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Hydromechanics elements
Reynolds 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
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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
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Hydromechanics elements
• Froude number is equal to 0.33
subcritical flow
• Reynolds number is equal to 176000
fully turbulent flow
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The turbulent boundary layer
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The sediment
• Non-cohesive sand
• Mean size 1.9mm
• Natural angle of repose: 30°
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The scouring process
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Predictive 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
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Predictive formulas
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Measurements under different inlet
conditions
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Plexiglas pan on the water surface
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Flow straighteners
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Flow straighteners and a weir
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Plates with holes
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Plates and a weir
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Observations
• 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
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Observations
Scour depth after
two hours
Flow straighteners 5cm
Flow straighteners
plus weir
8-9cm
Grids 9-10cm
Grids plus weir 11cm
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Observations
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Inlet condition ? ?Scour dynamics and equilibrium
Outline
1. Influence of the inlet condition on the scouring process: starting point
2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)
3. Measurements, discussion, conclusion
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Measurement technique: the Laser
Doppler Velocimetry (and a first
measurement)
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Laboratory configuration
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Laser Doppler Velocimetry
• A flow is seeded with small buoyant particles that scatter light
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Laser Doppler Velocimetry
• Two Laser beams cross at a point called the measurement volume, and create a set of fringes
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Laser Doppler Velocimetry
• Particles crossing the measurement volume emit a signal whose frequency is proportional to their velocity.
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Laser Doppler Velocimetry
• The signal is processed by a computer so as to get this velocity.
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Laser Doppler Velocimetry
• Frequency shift:
• Detected frequency with and without frequency shift:
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Processing of the data• Fast Fourier
Transform
• Determination of the main frequency
• Determination of the corresponding velocity
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Processing of the data
• Around 2000 bursts: reliable information
• Mean velocity component: expected value
• Root mean square fluctuation component: variance
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A first measurement
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Velocity field?•One measurement= one velocity component•2D velocities= two measurements per point
A first measurement
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• 95 points
• 2 measurement directions per point
• 2000 bursts per measurement
A first measurement
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A first measurement • Presence of the horseshoe vortex
• The approaching flow is bent by the presence of the pier
• But: some oddities
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Problems encountered• Fast Fourier samples number too small: peak-locking
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Problems encountered
• Measurement directions too close one to another
Small input variations result in very different outputs.
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Outline
1. Influence of the inlet condition on the scouring process: starting point
2. Measurement technique: the Laser Doppler Velocimetry (and a first measurement)
3. Measurements, discussion, conclusion
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Measurements, discussion, conclusion
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Measurement of four different velocity
profiles
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•Four different vertical velocity profiles•Measured under four different inlet condtions
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Comment
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•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
Comparison of the measured profiles
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Wall shear stress of the different profiles: (Sagaut, 2006)
Comparison of the measured profiles
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Scour depth
after two
hours
Wall shear
stress
Flow
straighteners
5cm 0.32Pa
Flow
straighteners plus
weir
8-9cm ?
Grids 9-10cm 0.35Pa
Grids plus weir 11cm 0.40Pa
Ettema’s Theory (1980)
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“The higher the shear parameter, the deeper the scour depth”
Influence of the inlet condition
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Inlet conditionVertical velocity
distributionWall shear
stressShear
parameterEttema’s theory
Scour dynamicsand equilibrium
Introduction: 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
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But…
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Why is the influence of the inlet
conditions hardly ever mentioned?
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The researchers’ assumption
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Inlet conditionVertical velocity
distributionWall shear
stressShear
parameterEttema’s theory
Scour dynamicsand equilibrium
Universallogarithmic
profile
How can someone get a universal
velocity profile?• After a certain free flow length, a boundary layer is
fully developeduniversal 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
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Reference Flow
depth
Channel
width
Channel
length
Free flow
length
Hydraulic
radius
Free flow
length/hydr
aulic radius
Link 2006 20-40cm 2m 30m 15m 17-29cm 51-88
Carette
2008
25cm 1.17m 11m 6.5m 17cm 38
Yanmaz
1991
4-16.5cm 0.67m 10.9m 6.9m 4-11cm 62-172
Zanke 1982 42-55cm 0.6m 15m 7.5m 17.5-
20cm
38-43
Dey 1995 3-5cm 0.81m 10m 5m 3-5cm 100-166
Oliveto
2007
4-60cm 1m 30m 15m 4-27cm 55-375
Melville
1987
10-24cm 0.44m 11.8m 6m 7-12cm 50-85
This report 15cm 1.17m N/A 6.5m 12cm 54
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Conclusion
• 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
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Conclusion
• 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.
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Merci de votre attentionENPC 2010
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