concentrated non-brownian suspensions in viscous fluids. numerical simulation results, a...
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Concentrated non-Brownian suspensions in viscous fluids.
Numerical simulation results,
a “granular” point of view J.-N. Roux, with F. Chevoir, F. Lahmar, P.-E. Peyneau, S.
Khamseh
Laboratoire Navier, Université Paris-Est, FranceGranular materials : • quasistatic response : gradually apply shear stress or impose small shear rate on isotropic equilibrated state• steady shear flow with inertial effectsExtension to dense suspensions :Large hydrodynamic forces in narrow gaps between grains, role of contactsApproaches similar to dry granular case
Granular materials : initial density and critical stateGranular materials : initial density and critical state
Under large strain, critical state independent of initial state, characterized by a ‘’flow structure’’ (density + nb of contacts, anisotropy…)
Mon
od
isperse
sph
ere
asse
mb
ly, frictio
n co
effi
cien
t = 0
.3
Severa
l sam
ple
s with
40
00
bead
s, pre
pare
d a
t diff
ere
nt so
lid fra
ction
Triaxial test
Similar response in simple shear test !
Critical state of dry granular materials, 2D and3D results
Internal friction coefficient(actually somewhat different between shear tests and other load directions, e.g. triaxial, see e.g., Peyneau & Roux, PRE 2008)
Critical solid fraction
c = RCP for = 0
Compilation of simulation literature, in Lemaître, Roux & Chevoir, Rheologica Acta 2009
c and c do not depend on contact stiffness if large enough
Quasistatic rheology :
•interest of critical state concept (flow structure) c minimum solid fraction in flow
•Friction coefficient in contacts determines c and *c. Small rolling friction also quite influential
• using contact law and applied pressure P, define stiffness number (such that deflection is prop. to -1), assess approach to rigid grain limit
Inertial flows :
•Study shear flow under controlled normal stress rather than fixed density : non-singular quasistatic limit
• use internal friction * as an alternative to viscosity
What we know from simulations of dry grains (I)
23
(linear elasticity, 3D) (Hertz contacts, 3D) diameter ; NK Ea
aP P
Simulation of steady uniform shear flow
Fixed shear strain rate
Normal stress is imposed
22P
Material state in shear flow ruled by one dimensionless parameter, the inertial number
What we know from studies on dry grains (II) :
Inertial number and constitutive relations
2 3 D D
m mI I
P aP
Generalization of critical state to I-dependent states with inertial effects Useful constitutive law, applied to different geometries (Pouliquen, Jop,
Forterre…)
Monodisperse spheres, no intergranular
friction
(Peyneau & Roux, Phys Rev E 2008)
RCP
Steady shear flow of dry, frictionless beads at low I number : normal stress-controlled vs.
volume controlled(P.-E. Peyneau)
Same behavior, very large stress fluctuations if is imposed (4000 grains)
Ratio 12/22 = * expresses material rheology
• in simulation literature : nothing for > 0.6 (spheres), ad-hoc repulsive forces used to push grains apart, very few published results with N >1000…
• sharp contrast with simulations of dry grains!
• lubrication singularities believed to lead to some dynamic jamming phenomenon below RCP (related to possible origin of shear thickening)
Ball and Melrose, 1995-2004: no steady state (!?)
• Here : control normal stress rather than density, control lubrication cutoff and contact interaction stiffness
Simulations of dense suspensions
Experiments on dense suspensions
• Attention paid to possible density inhomogeneities local measurements
• Boyer et al. control of particle pressure !
Simulation of very dense suspensions
Simplified modeling approach, fluid limited to near-neighbor gaps and pairwise lubrication interactions (cf. Melrose & Ball)
Lubrication singularity cut off at short distance. Contact forces (or short–range repulsion due to polymer layer)
Stokes régime : contact, external and viscous forces balance
Questions
Divergence of at < RCP ? Sensitivity to repulsive forces ? To alone ? Effective viscosity, non-Newtonian effects, normal stresses…
Lubrication and hydrodynamic resistance matrix
NNvN VhF )(
h
RhN
2
2
3)(
Normal hydrodynamic force :
For 2 spheres of radius R, with perfect lubrication :
No contact within finite time !
Cut-off for narrow interstices h<hmin
(asperities), contact becomes possible
Without cutoff: both physically irrelevant and computationally untractable
dominant forces transmitted by a network of ‘quasi-contacts’
Choice of systems and parameters
D systems of identical spheres, diameter a :
tobeads
hmax/a = 0.1 or 0.3
hmin/a = 10-4 ; = 105
Vi from 0.1 down to 5.10-4
= 0 in solid contacts
+ alternative systems with repulsive forces, no lubrication cutoff
D systems of polydisperse disks, diameter d,
disks
3D lubrication
hmax/a = 0.5
hmin/a = 10-4 or 10-2 ; = 104
Vi down to 10-4 or 10-6
= 0.3 in solid contacts
0.7a d a
• assemble hydrodynamic resistance matrix (similar to stiffness matrix in elastic contact network)
• non-singular tangential coefficient
• add up ‘ordinary’ contact forces (elasticity + friction) to viscous hydrodynamic ones when grains touch (simple approximation)
cF
Model, computation method
0 extvc FFF
VFv W
vF
with
W
Wand Fc depend on grain positions
Solve extc FFV
W
Lees-Edwards boundary conditions
+ variable height,ensuring constant yy
Measurements in steady state :
Check for stationarity of measurements
Obtain error bar from ‘blocking’ technique
Request long enough stationary intervals
10 at least ...t Regression of fluctuations
Some technical aspects about simulations
N -1/2N -1/2
Constant volume and/or
constant shear stress conditions should produce
same system state in large N limit
Choice of time step, integration
t such that matrix and r.h.s. do not change `too much’…
( ( )) ( ) ( ( ))
( ) ( ) . ( )c extX t V t F X t F
X t t X t t V t
W
(1)
(1) (2) (1)
(1) (2)
( ( )) ( ) ( ( ))
( ( ) ( ) ) ( ) ( ( ) ( ) )
( ) ( ) . ( ) ( )2
c ext
c ext
X t V t F X t F
X t V t t V t F X t V t t F
tX t t X t V t V t
W
W
Euler (explicit) :
(error ~t2)
‘Trapezoidal’ rule : (error ~ t3)
A crucial test on the numerical integration of equations of motion
Relative difference between variation of h and integration of normal relative velocity in various interstices, 2 different numerical schemes : Euler (dotted lines), trapezoidal (continuous lines)
Keep it below 0.05 !
Control parameter for dense suspensions : viscous number Vi
PVi
P
aVi
D
D
3
2
aP
mI
P
mI
D
D
3
2Plays analogous role to inertia parameter
defined for dry grains
Vi = (decay time of h(t) in compressed layer within gap) / (shear time)
I = (acceleration time) / (shear time)
Acceleration (inertial) time replaced by a squeezing time in viscous layer
Cassar, Nicolas, Pouliquen, Phys. of Fluids 2005 (similar Vi, with drainage time)
Steady shear flow of lubricated beads at low Vi number : normal stress-controlled vs. volume
controlled
Vi = 10-3
Same behavior, large stress fluctuations if is imposed (1372 grains). Ratio 12/22 = * expresses material rheology
3D results : *and as functions of Vi
= 0 : difficultcase ! Approach to * = 0.1, =0.64 …
ViVi
identical spherical grains1372N
Back to more traditional (constant ) approach
Effective viscosity
*min, , ,
N
h af
a K
* *
12 22 Vi
From and
0 Vi one
gets:
* *
1
cVi
if* *
0 (not satisfied in our case !)
Shear rate effects ?
In rigid grain limit replace 4th argument by zero. No influence
of on effective viscosity 0
Exponent : 2.5 to 3 ?
Effective viscosity, as a fonction of solid fraction
(hc = hmax)
Polymer layer thicknesslb = 0.01 or 0.001(open/filled symbols )
Force F0 ratio F0/a2P (0.1, 1, 10) = (diamond, square, circle)
Influence of repulsive force
Adsorbed polymer layer, short-range repulsion, as
*
cVi Vi
cVi Vi
Ordered structurepolydispersity < 20%
34.10Vi
Shear-thinning within studied parameter range
Pair correlations in plane yz
5
40[ 1]bF l h
(Fredrickson & Pincus, 1991)
Vic ~ 3.10-2
Shear thinning with repulsive forces
2*
0
*
02
, , ,b
N
l af
a F
a ViFKa P
Shear thinning due to
change in reduced shear rate
Results correspond to
varying from 5.10-5 to 10-1
*
No shear thinning for smaller lb
Network of repulsive forces carry all shear stress as Vi decreases
Other ‘granular’ features: force distribution, coordination number…
Vi
xy
yy
Viscosity of random, isotropic suspensions
•Assume ideal hard sphere particle distribution at given
•Measure instantaneous shear viscosity
Comparison with Stokesian Dynamics results: encouraging agreement at large densities, although treatment of subdominant terms not entirely innocuous
2D disk model
3D frictionless spherical beads :
Analogy with dry granular flow
Difficulty to approach quasistatic limit
No singularity below RCP density, a steady-state can be reached
Importance of non-hydrodynamic interactions
Purely hydrodynamic model approached with stiff interactions
Effects of additional forces: 2-parameter space to be explored
Easier system, introduction of tangential forces, friction, faster approach to quasistatic limit
Show coincidence of quasistatic limits for dry grains and dense suspension
2D results : internal friction coefficient versus Vi
(analogous to function of I in dry inertialcase)
Viscous case hmin/a = 10-2 and hmin/a = 10-4 , =0,3
Dry inertial case (right) : =0.3 and =0
Coincidence I=0 / Vi=0
Viscous Dry, inertial
2D results : solid fraction versus Vi (in paste)(analogous to function of I in dry inertial case)
Viscous case hmin/a = 10-2 and hmin/a = 10-4 (896 grains), =0.3
Dry inertial case(right) : =0.3 and =0
Coincidence I=0 /Vi =0
ViscousDry,
inertial
* *c cI
* *c cVi 1 1
( )c
e Vi
1 1
c
eI
Constitutive laws : dry grains (2D) = 0 or 0.3
Constitutive laws, granular suspensions (2D), = 0.3
Same constant terms (quasistatic limit), different power laws
Effective viscosity, as a function of solid fraction
Divergence of effective viscosity as critical solid fraction c is approached
(once again) no accurate determination of exponent (2 ? 2.5 ?)
Little influence of roughness length scale
Importance of direct contact interactions
Pressure due to solid contact forces / total pressure
(Case hmin/a = 10-2)
Some conclusions on dense suspensions
interesting to study dense suspensions under controlled normal stress
importance of contact interactions : solid friction, asperities…) Contact or static forces dominate at low Vi
relevance of critical state as initially introduced in soil mechanics. Viscosity diverges as approaches c
With =0 in contacts, no jamming or viscosity divergence or any specific singularity below RCP, except in small systems
If is large enough, * independent of with elastic (-frictional) beads (but… Re ? Brownian effects ?)
introduction of repulsive interaction with force scale shear-thinning (or thickening)
Perspectives
bridge the gap between real (and difficult) hydrodynamic
calculations and ‘conceptual models’
Improve performance of numerical methods
Continuum fluid !(Keep separate treatment of lubrication singularities ?)