sand motion over vortex ripples induced by surface waves jebbe j. van der werf water engineering...
DESCRIPTION
Surface waves and oscillatory flow backgroundexperimentsflowsand dynamicstransport modellingconclusions shoreface surf zone wave boundary layerTRANSCRIPT
Sand Motion over Vortex Ripples induced by Surface Waves
Jebbe J. van der WerfWater Engineering & Management, University
of Twente, The Netherlands
Outline
1. Background2. Laboratory experiments3. Flow over ripples4. Sand dynamics over ripples5. Practical sand transport modelling6. Conclusions & further research
background experiments flow sand dynamics transport modelling conclusions
Surface waves and oscillatory flow
background experiments flow sand dynamics transport modelling conclusions
shoreface
surf zone
wave boundary layer
Wave-generated ripples
• Cover large part shoreface bed• η = 0.01-0.1 m and λ = 0.1-1.0 m• Vortex shedding if η/λ > 0.1
λ η
background experiments flow sand dynamics transport modelling conclusions
Sand transport processes over vortex ripples
Vortex ripples strongly influence wave boundary layer structure, near-bed turbulence intensity and sand transport mechanisms
z ≈ 2 η
η
Lower layer: organised convective processes dominant
Upper layer: turbulent processes dominant
background experiments flow sand dynamics transport modelling conclusions
Ph.D. research
1. New full-scale laboratory experiments2. Improvement ripple predictors3. Improvement practical models to predict
time-averaged concentration profile4. Development new practical sand
transport model5. Improvement 1DV-RANS sand transport
model
background experiments flow sand dynamics transport modelling conclusions
Experimental facilities
• Oscillatory flow tunnels• Flow equivalent to near-bed horizontal flow
generated by full-scale surface waves
background experiments flow sand dynamics transport modelling conclusions
Measurements
• Bed elevation using laser displacement sensor
• Particle velocities using ultra-sonic velocity profiler and PIV
• Net sand transport rates by mass conservation technique using measured masses in traps and volume changes
• Suspended sand concentrations
background experiments flow sand dynamics transport modelling conclusions
Suspended sand concentration measurement
• Transverse suction system
background experiments flow sand dynamics transport modelling conclusions
Suspended sand concentration measurement
• Transverse suction system• Optical concentration meter
background experiments flow sand dynamics transport modelling conclusions
Suspended sand concentration measurement
• Transverse suction system• Optical concentration meter• Acoustic backscatter system
background experiments flow sand dynamics transport modelling conclusions
Experimental conditions
• Regular and irregular asymmetric flow with T = 5.0-10.0 s and u = 0.4-1.3 m/s
• Uniform sand with D50 = 0.22-0.44 mm
timeonshore
offshore
u
background experiments flow sand dynamics transport modelling conclusions
Instantaneous flow field
background experiments flow sand dynamics transport modelling conclusions
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
Instantaneous flow field
background experiments flow sand dynamics transport modelling conclusions
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
Time-averaged flow field
background experiments flow sand dynamics transport modelling conclusions
Time- and ripple-averaged flow
background experiments flow sand dynamics transport modelling conclusions
Instantaneous suspended concentration field
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
background experiments flow sand dynamics transport modelling conclusions
Instantaneous suspended concentration field
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
background experiments flow sand dynamics transport modelling conclusions
Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
Horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
Horizontal suspended sand fluxes
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),('),('),(~),(~),(),(),(),,(),,(),,(
),('),(~),(),,(
),('),(~),(),,(
zxczxuzxczxuz
zxczxuzxczxuzxczxuzxtzxctzxutzx
zxczxczxctzxc
zxuzxuzxutzxu
current-related wave-related
background experiments flow sand dynamics transport modelling conclusions
Net horizontal suspended sand fluxes
background experiments flow sand dynamics transport modelling conclusions
D50 = 0.44 mm
T = 5.0 s
η = 0.08 m
λ = 0.41 m
Bedload transport
• Near-bed (mm’s) transport where grain-grain interactions are important
• Net bedload in the onshore direction due to flow asymmetry
• Forcing mechanism for onshore ripple migration (?)
background experiments flow sand dynamics transport modelling conclusions
Net sand transport ratebedload transport
dominant
suspended load transport dominant
background experiments flow sand dynamics transport modelling conclusions
Net sand transport rate
background experiments flow sand dynamics transport modelling conclusions
50DP
bedload transport dominant
suspended load transport dominant
Practical sand transport modelling
• Implemented in larger morphological modelling systems
• Current practical sand transport models– Quasi-steadiness: qs(t) = m |u|n-1 u
– <qs> onshore (> 0) for asymmetric oscillatory flows with larger onshore velocities
– Not valid in vortex ripple regime where net transport is generally offshore (< 0)
background experiments flow sand dynamics transport modelling conclusions
• Phase-lag effects schematically included• Four transport contributions F(θ’c,θ’t,P)
New practical sand transport model
onshore flow offshore flow
background experiments flow sand dynamics transport modelling conclusions
New practical sand transport model
background experiments flow sand dynamics transport modelling conclusions
10-2
10-1
100
101
-101
-100
-10-1
-10-2-10
1
-100
-10-1
-10-2
pred
icte
d no
n-di
men
sion
al s
and
trans
port
measured non-dimensional sand transport10
-210
-110
010
1
quasi-steady modelnew model
Conclusions
1. Flow and suspended sand dynamics controlled by vortex generation and ejection
2. Net sand transport controlled by offshore-directed suspended fluxes and onshore-directed near-bed transport
3. New practical sand transport model
background experiments flow sand dynamics transport modelling conclusions
Future research
• Comparison detailed data with more sophisticated models, 2DV-RANS models, …?
• Development of a general practical model to predict sand transport in coastal waters (Dutch/UK SANTOSS project)
background experiments flow sand dynamics transport modelling conclusions