The presentation Prof E. Toorman gave during the workshop. Subject is the discussion on how to define the nautical bottom.

1. 2/17/2011
Mud:
more complex than you think!
Revising the concept of nautical depth in the light of
micro-
micro-structure dynamics and implications for in-situ surveying
in-
by
Erik A. Toorman
Hydraulics Laboratory
Civil Engineering Department
HSB Workshop “Harbours and Specific Survey Problems”
“Harbours
8 December 2010, Flanders Hydraulics, Borgerhout (B)
SEDIMENT MECHANICS RESEARCH
@
K.U.LEUVEN
Research Unit Coordinator:
E-mail: erik.toorman@bwk.kuleuven.be
http://www.kuleuven.be/hydr/SedMech.html
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2. 2/17/2011
Understanding “mud”
and its behaviour under shear
Defining Mud
Mud is a mixture of water and fine sediments
(clay, silt and sand) and organic matter,
which behaviour is characterized as cohesive,
when it is dominated
by the clay fraction (>15%).
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3. 2/17/2011
Mud particles
Formed by flocculation
= aggregation (+ break-up) in a suspension
after Krone
© van Leussen (1993)
Mud layer micro-structure
after Partheniades
3D network structure for ρ > 1100 kg/m3 (gel point)
“card house”
© van Olphen primary kaolin particles (SEM image)
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4. 2/17/2011
Micro-structure in mud-sand mixture
Clay bridges glue the sand skeleton
Scanning-electron microscopy images (left: air-dried – right: oven dried)
© KULeuven (Torfs , PhD, 1995)
“Fluid” Mud
= “dense”, flocculated suspension with
concentration (slightly) above the gel point
(i.e. the structure formation threshold)
(soil at rest liquefied when sheared)
Mud rheology = macroscopic description of
its deformation and flow behaviour
Time-dependence due to (micro-)cracks and
restructuring
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5. 2/17/2011
Experimental equilibrium flow curve
total
viscous
SHEAR STRESS (Pa)
retained
SHEAR RATE (s-1)
bentonite suspension (9.91% by weight)
Weissenberg Rheogoniometer + parallel plates (Wright & Krone, 1989)
Experimental mud viscosity
“Shear thinning”
Apparent viscosity for River Parrett (UK) mud at different densities
Carri-Med CS rheometer data by Jones & Golden (1990)
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6. 2/17/2011
Thixotropy: conceptual
Barnes (J. Non-Newtonian Fluids, 1997)
Thixotropy
Time-dependent behaviour under shear of dense
flocculated suspensions:
– structural break-down when sheared (liquefaction),
– structural recovery at rest (aggregation).
(+ self-weight consolidation)
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7. 2/17/2011
Thixotropy @ microscale
Complex floc motion and interaction during simple shear flow in a 2D dispersion of PS particles at decane/water interface
Masschaele & Vermant (2008)
Thixotropy @ macroscale
Placement of ADV instruments on mud bank along the Surinam coast
© Hydraulics Laboratory, KULeuven
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8. 2/17/2011
Measurement: Rheometry
“Shear-rate-step-change” experiment
Surinam Coast mud (diluted to ρ = 1130 kg/m3 )
100
90
80
60
70 30
12
Viscosity(%)
60
6
3
50
1.5
0.6
40
0.3
30
20
10
0
17:16:48 17:24:00 17:31:12 17:38:24 17:45:36 17:52:48
Tim e(hh:m m :ss)
Brookfield rotoviscometer + vane spindle
Equilibrium flow curve: Bingham
8
7
TOTAL STRESS
SHEAR STRESS (Pa)
6
5
4
3
2 RESIDUAL STRESS
1
0
0 20 40 60 80 100 120 140
SHEAR RATE (s-1)
ρ = 1130 kg/m3 – after shear rate correction
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9. 2/17/2011
Rheological models
2.5
SHEAR STRESS / BINGHAM YIELD STRESS
2
Worrall-Tuliani
Bingham
1.5
Herschel-Bulkley
µ = τ / γ&
1
0.5 “true” yield stress Newtonian
effective viscosity shear thinning
0
0 10 20 30 40 50 60 70 80 90 100
SHEAR RATE (s-1)
Thixotropy Model
(Toorman, 2005)
• Constitutive equation:
τ (λ , γ& ) = (µ ∞ + λη 0 )γ& + λτ 0
λ
= µ ∞ γ& + τ 0
λe
• Structural kinetics (1st order):
dλ
= a (γ& )(1 − λ ) − b(γ& )λγ&
dt
1
• with: λe (γ& ) = and β = b / a = η0 /τ 0
1 + βγ&
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10. 2/17/2011
Fluid mud rheology
constant structure curves
equilibrium flow curve
bentonite
data Cheng & Evans (1965)
Fluid mud rheology
hysteresis
hectorite
data Joye & Poehlein (1971)
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11. 2/17/2011
Implications for
the definition of
Nautical Bottom
Navigability
• Thickness & viscosity of disturbed mud
= function of speed & ac-/deceleration
• Drag force unevenly distributed
disturbed mud
• Interface between disturbed and undisturbed =
artificial (and temporal!) “rheological transition”
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12. 2/17/2011
CFD of drag on a ball in a Bingham fluid
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13. 2/17/2011
Implications for
surveying
Problems
• Intrusive instruments (profiling or towed)
disturb the original microstructure.
• The degree of disturbance is determined by
the speed of intrusion and the size and
shape of the device
• No equilibrium data can be obtained
• Usually the device moves too fast such that
the structure is cut and not truly sheared
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14. 2/17/2011
Conclusions
Conclusions (1)
• Mud is a thixotropic gel (a “structured” fluid).
• Its instantaneous strength and apparent viscosity
depend on its micro-structure.
• A rheological transition indicates a discontinuity in
micro-structure (i.e. in history).
• The state of the micro-structure requires knowledge
of the shear history.
• The actual nautical depth depends thus also on the
state of the mud layer (which varies along the ship!)
and the shear history caused by the movement of
the ship and that of previous passages.
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15. 2/17/2011
Conclusions (2)
• Nautical depth cannot be defined in terms of
rheological parameters (and composition) alone!
• It also depends on the manoeuvre and ship
characteristics.
• Mud rheology can be characterized by dedicated
laboratory experiments.
• A rheological closure for CFD applications is
available.
• 3D CFD studies are needed to understand the
problem of nautical depth for each ship.
Thank you
Questions?
E-mail: erik.toorman@bwk.kuleuven.be
SELECTED PUBLICATIONS
Berlamont, J., Ockenden, M., Toorman, E. & Winterwerp, J. (1993). The characterisation
of cohesive sediment properties. Coastal Engineering, Vol.21:105-128.
Toorman, E.A. (1996). Sedimentation and self-weight consolidation: general unifying
theory. Géotechnique, Vol.46(1):103-113.
Toorman, E.A. (1998). Sedimentation and self-weight consolidation: general unifying
theory. Discussion. Géotechnique, Vol.48 (2):295-298.
Toorman, E.A. (1997). Modelling the thixotropic behaviour of dense cohesive sediment
suspensions. Rheologica Acta Vol.36 (1):56-65.
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