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HYDRODYNAMIC FORCES IMPLEMENTED INTO
LAMMPS THROUGH A LATTICE-BOLTZMANN
FLUID
Colin Denniston, Frances Mackay, Santtu Ollila, Tyson WhiteheadUniversity of Western Ontario
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Continuum fluid with thermal noise
o Navier-Stokes equations with thermal noise (Landau & Lifshitz):
o Note that the thermal noise appears in the stress tensor so will conserve mass and momentum. It should also obey the fluctuation-dissipation theorem (Landau & Lifshitz):
Noise in LB ref: S.T.T.Ollila, CD, M. Karttunen, T. Ala-Nisslia, J.Chem. Phys. 134, 064902 (2011).
0 ut
FusPuut )()(
''2',', ttTktsts B rrrr
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Point particle result: Segre & Siilberberg (1961), Ho & Leal (1974)
Does inertia matter for small Re?
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Simple Lattice Boltzmann Algorithm• fi = partial densities (9 in 2d, 15 in 3d)• { fi }i = a discrete probability distribution• Moments of these distributions are the
physical variables of interest:
• They evolve via the equation:
,, i
iii
i ff eu
})){,,(),(()( keqjjijiit ftftff xxe
e1
e2
e4
e3e0
e5e6
e7 e8
where
and Ai, Bi, Ci, and Di are chosen so that
,ee,, uupfff ii
ieq
ii
ieq
ii
eqi ue
,2iiiiii
eqi DCBAf euuueu
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• Each node represents a fixed area Ai• Nodes are distributed onto the lattice
• weights proportional to the opposite enclosed area within the cell. Eg. i1=A1/dx2
• Easily generalized to 3-D (use volume instead of area).• Peskin’s Immersed boundary method is similar. With compact support spreading 2 lattice sites from nodes lattice effects can be almost eliminated.
• First done for non-point objects in LB by Duenweg & Lobaskin, NJP (2004).
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Modelling:• Particles live off-lattice and evolve using molecular dynamics (written as a package for LAMMPS). • Particles are mapped to the mesh using NDA algorithm and hydrodynamic forces on each particle computed from:
• is “drag” coefficient (to be determined), vp is the particle velocity, and ui is the interpolated fluid velocity at node i. The resulting torque is also computed for rotational motion.•The fluid experience an equal and opposite force distributed back onto lattice in a matched way.
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Drag Force
Brinkman Theory: Felderhoff et al., Bhatt & Sacheti, S.T.T. Ollila, T. Ala-Nissila, CD, JFM 709, 129 (2012).
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In discrete time this can be conservative!
Consider the collision of two point particles
uivi
If the collision conserves momentum and kinetic energy then:
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i.e. something that looks like a discrete time drag force with:
mv=node massmu=fluid mass interacting with node via interpolation stencil
We also take: Ref: F.E. Mackay, CD, J. Comp. Phys. 237, 289 (2013).
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Hydrodynamics Radius:
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Picking in this way also give consistent particle-particle hydrodynamic interactions…
Fast particles
Slow particles
Fast particles
Slow particles
and diffusion with no additional adjustable parameters (other schemes have required Langevin noise on force coupling to get correct diffusive motion)
B=2.837 (Kremer & Dunweg,1993)
B
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Velocity auto-correlation function
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Two-particle diffusion:
rTkB
2
Theory curve: Crocker et al.,PRL 85, 888 (2000)
r
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Implementation in LAMMPS: MPI domain decomposition taken same as
particles in LAMMPS Implemented as fixes:
fix lb_fluid – applies force of particle on fluid
fix viscous_lb – applies force of fluid on particles
along with fix_momentum_lb Hook into update.cpp : units
Otherwise just a regular user packageRef: J. Comp. Phys. 237, 289 (2013)
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Sheared Colloids
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Polymer dynamics:Point particles
Particles withextended size
Ref: JCP 134, 064902 (2011)
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Polymer Diffusion Analytic value known = 2.837
A=1.630.01 from bead-spring calculation1
1. Dünweg et al., JCP 117, 914 (2002).
Expected:
We do not expect a slip term as in Kirkwood model because:• we imposed no-slip
Analytic value known = 2.837
A=1.630.01 from bead-spring calculation1
Slope givesB=2.80.05
Asymptotic valueA=1.67 0.05
Ref: JCP 134, 064902 (2011)
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Polymer in a channel
z
G
LR
C ,Lz
Periodic in x and y
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Diffusion
Ref: Soft Matter 9, 3478 (2013)
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Colloids in a Liquid Crystal
Ref: EPL 94, 66003 (2011)
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Colloids in LC in a lattice:
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To Do:
GPU package – preliminary version working (speedup ~ 75 for LB but now need to make LB domain decomposition bigger than particle domain decomposition)
Lubrication forces at close distances (allows coarser mesh)
Multiphase flow – have preliminary version of binary fluid and liquid crystal
Multiscale within one simulation
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Conclusions Fluctuations and particles were included in a lattice-
Boltzmann model with a conservative coupling between MD and LB
Inertia can matter at small Re for particles in flow Particles included in a way that guaranteed
conservative coupling and allows the LB fluid to act as the thermostat for the particles
Implemented in LAMMPS as fixes
Funding: NSERC, Ontario ERA, SharcNet