nautical bottom2010b
DESCRIPTION
The presentation Prof E. Toorman gave during the workshop. Subject is the discussion on how to define the nautical bottom.TRANSCRIPT
2/17/2011
1
Mud: Mud: more complex than you think! more complex than you think!
Revising the concept of nautical depth in the light of Revising the concept of nautical depth in the light of
micromicro--structure dynamics and implications for instructure dynamics and implications for in--situ surveyingsitu surveying
byby
Erik A. ToormanErik A. Toorman
Hydraulics LaboratoryHydraulics Laboratory
Civil Engineering DepartmentCivil Engineering Department
HSB Workshop “HSB Workshop “HarboursHarbours and Specific Survey Problems”and Specific Survey Problems”8 December 2010, Flanders Hydraulics, 8 December 2010, Flanders Hydraulics, BorgerhoutBorgerhout (B)(B)
SEDIMENT SEDIMENT MECHANICS RESEARCH MECHANICS RESEARCH
@ @
K.U.LEUVENK.U.LEUVEN
Research Unit Coordinator:
E-mail: [email protected]
http://www.kuleuven.be/hydr/SedMech.html
2/17/2011
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Understanding “mud”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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Mud particles
Formed by flocculation
= aggregation (+ break-up) in a suspension
© van Leussen (1993)
after Krone
Mud layer micro-structure
3D network structure for ρ > 1100 kg/m3 (gel point)
“card house”
after Partheniades
© van © van OlphenOlphen ���� primary kaolin particles (SEM image)
2/17/2011
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Micro-structure in mud-sand mixture
Clay bridges glue the sand skeletonScanning-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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Experimental equilibrium flow curve
bentonite suspension (9.91% by weight)
Weissenberg Rheogoniometer + parallel plates (Wright & Krone, 1989)
retained
viscous
total
SHEAR RATE (s-1)
SH
EA
R S
TR
ES
S (
Pa)
Experimental mud viscosity
Apparent viscosity for River Parrett (UK) mud at different densities
Carri-Med CS rheometer data by Jones & Golden (1990)
“Shear thinning”
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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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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)
ThixotropyThixotropy @ @ macroscalemacroscalePlacement of ADV instruments on mud bank along the Surinam coastPlacement of ADV instruments on mud bank along the Surinam coast
© Hydraulics Laboratory, © Hydraulics Laboratory, KULeuvenKULeuven
2/17/2011
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0
10
20
30
40
50
60
70
80
90
100
17:16:48 17:24:00 17:31:12 17:38:24 17:45:36 17:52:48
Time(hh:mm:ss)
Vis
co
sit
y(%
)
60
30
12
6
3
1.5
0.6
0.3
Measurement: Rheometry
“Shear-rate-step-change” experiment
Surinam Coast mud (diluted to ρ = 1130 kg/m3 )
Brookfield rotoviscometer + vane spindle
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
SHEAR RATE (s-1
)
SH
EA
R S
TR
ES
S (
Pa)
TOTAL STRESS
RESIDUAL STRESS
Equilibrium flow curve: Bingham
ρ = 1130 kg/m3 – after shear rate correction
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Rheological models
γτµ &/=
0
0.5
1
1.5
2
2.5
0 10 20 30 40 50 60 70 80 90 100
SHEAR RATE (s-1
)
SH
EA
R S
TR
ES
S /
BIN
GH
AM
YIE
LD
ST
RE
SS
Newtonian
Herschel-Bulkley
Worrall-Tuliani
Bingham
“true” yield stress
effective viscosity ���� shear thinning
Thixotropy Model
• Constitutive equation:
• Structural kinetics (1st order):
• with: and
( ) γλγλγλ
&&& )(1)( badt
d−−=
00 // τηβ == abγβ
γλ&
&
+=
1
1)(e
( )
0
00),(
τλ
λγµ
λτγληµγλτ
e
+=
++=
∞
∞
&
&&
(Toorman, 2005)
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Fluid mud rheology
equilibrium flow curveequilibrium flow curve
constant structure curvesconstant structure curves
�������� �������� ddata Cheng & Evans (1965)ata Cheng & Evans (1965)
bentonite
Fluid mud rheology
ddata ata JoyeJoye & & PoehleinPoehlein (1971)(1971)
hysteresis
hectorite
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Implications Implications for for
the definition of the definition of
Nautical Nautical BottomBottom
Navigability• Thickness & viscosity of disturbed mud
= function of speed & ac-/deceleration
• Drag force unevenly distributed
• Interface between disturbed and undisturbed = artificial (and temporal!) “rheological transition”
disturbed muddisturbed mud
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CFD of drag on a ball in a Bingham fluid
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Implications for Implications for
surveyingsurveying
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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ConclusionsConclusions
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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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: [email protected]
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.