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<title>fix lb/fluid command — LAMMPS 15 May 2015 version documentation</title>
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<div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article">
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<div class="section" id="fix-lb-fluid-command">
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<span id="index-0"></span><h1>fix lb/fluid command<a class="headerlink" href="#fix-lb-fluid-command" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="syntax">
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<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID lb/fluid nevery LBtype viscosity density keyword values ...
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
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<li>lb/fluid = style name of this fix command</li>
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<li>nevery = update the lattice-Boltzmann fluid every this many timesteps</li>
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<li>LBtype = 1 to use the standard finite difference LB integrator,
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2 to use the LB integrator of <a class="reference internal" href="#ollila"><span>Ollila et al.</span></a></li>
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<li>viscosity = the fluid viscosity (units of mass/(time*length)).</li>
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<li>density = the fluid density.</li>
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<li>zero or more keyword/value pairs may be appended</li>
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<li>keyword = <em>setArea</em> or <em>setGamma</em> or <em>scaleGamma</em> or <em>dx</em> or <em>dm</em> or <em>a0</em> or <em>noise</em> or <em>calcforce</em> or <em>trilinear</em> or <em>D3Q19</em> or <em>read_restart</em> or <em>write_restart</em> or <em>zwall_velocity</em> or <em>bodyforce</em> or <em>printfluid</em></li>
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</ul>
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<pre class="literal-block">
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<em>setArea</em> values = type node_area
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type = atom type (1-N)
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node_area = portion of the surface area of the composite object associated with the particular atom type (used when the force coupling constant is set by default).
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<em>setGamma</em> values = gamma
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gamma = user set value for the force coupling constant.
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<em>scaleGamma</em> values = type gammaFactor
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type = atom type (1-N)
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gammaFactor = factor to scale the <em>setGamma</em> gamma value by, for the specified atom type.
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<em>dx</em> values = dx_LB = the lattice spacing.
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<em>dm</em> values = dm_LB = the lattice-Boltzmann mass unit.
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<em>a0</em> values = a_0_real = the square of the speed of sound in the fluid.
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<em>noise</em> values = Temperature seed
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Temperature = fluid temperature.
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seed = random number generator seed (positive integer)
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<em>calcforce</em> values = N forcegroup-ID
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N = output the force and torque every N timesteps
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forcegroup-ID = ID of the particle group to calculate the force and torque of
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<em>trilinear</em> values = none (used to switch from the default Peskin interpolation stencil to the trilinear stencil).
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<em>D3Q19</em> values = none (used to switch from the default D3Q15, 15 velocity lattice, to the D3Q19, 19 velocity lattice).
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<em>read_restart</em> values = restart file = name of the restart file to use to restart a fluid run.
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<em>write_restart</em> values = N = write a restart file every N MD timesteps.
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<em>zwall_velocity</em> values = velocity_bottom velocity_top = velocities along the y-direction of the bottom and top walls (located at z=zmin and z=zmax).
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<em>bodyforce</em> values = bodyforcex bodyforcey bodyforcez = the x,y and z components of a constant body force added to the fluid.
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<em>printfluid</em> values = N = print the fluid density and velocity at each grid point every N timesteps.
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</pre>
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</div>
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<div class="section" id="examples">
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<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-python"><div class="highlight"><pre>fix 1 all lb/fluid 1 2 1.0 1.0 setGamma 13.0 dx 4.0 dm 10.0 calcforce sphere1
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fix 1 all lb/fluid 1 1 1.0 0.0009982071 setArea 1 1.144592082 dx 2.0 dm 0.3 trilinear noise 300.0 8979873
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
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<p>Implement a lattice-Boltzmann fluid on a uniform mesh covering the LAMMPS
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simulation domain. The MD particles described by <em>group-ID</em> apply a velocity
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dependent force to the fluid.</p>
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<p>The lattice-Boltzmann algorithm solves for the fluid motion governed by
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the Navier Stokes equations,</p>
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<img alt="_images/fix_lb_fluid_navierstokes.jpg" class="align-center" src="_images/fix_lb_fluid_navierstokes.jpg" />
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<p>with,</p>
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<img alt="_images/fix_lb_fluid_viscosity.jpg" class="align-center" src="_images/fix_lb_fluid_viscosity.jpg" />
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<p>where rho is the fluid density, u is the local fluid velocity, sigma
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is the stress tensor, F is a local external force, and eta and Lambda
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are the shear and bulk viscosities respectively. Here, we have
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implemented</p>
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<img alt="_images/fix_lb_fluid_stress.jpg" class="align-center" src="_images/fix_lb_fluid_stress.jpg" />
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<p>with a_0 set to 1/3 (dx/dt)^2 by default.</p>
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<p>The algorithm involves tracking the time evolution of a set of partial
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distribution functions which evolve according to a velocity
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discretized version of the Boltzmann equation,</p>
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<img alt="_images/fix_lb_fluid_boltzmann.jpg" class="align-center" src="_images/fix_lb_fluid_boltzmann.jpg" />
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<p>where the first term on the right hand side represents a single time
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relaxation towards the equilibrium distribution function, and tau is a
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parameter physically related to the viscosity. On a technical note,
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we have implemented a 15 velocity model (D3Q15) as default; however,
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the user can switch to a 19 velocity model (D3Q19) through the use of
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the <em>D3Q19</em> keyword. This fix provides the user with the choice of
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two algorithms to solve this equation, through the specification of
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the keyword <em>LBtype</em>. If <em>LBtype</em> is set equal to 1, the standard
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finite difference LB integrator is used. If <em>LBtype</em> is set equal to
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2, the algorithm of <a class="reference internal" href="#ollila"><span>Ollila et al.</span></a> is used.</p>
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<p>Physical variables are then defined in terms of moments of the distribution
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functions,</p>
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<img alt="_images/fix_lb_fluid_properties.jpg" class="align-center" src="_images/fix_lb_fluid_properties.jpg" />
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<p>Full details of the lattice-Boltzmann algorithm used can be found in
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<a class="reference internal" href="fix_lb_viscous.html#mackay"><span>Mackay et al.</span></a>.</p>
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<p>The fluid is coupled to the MD particles described by <em>group-ID</em>
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through a velocity dependent force. The contribution to the fluid
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force on a given lattice mesh site j due to MD particle alpha is
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calculated as:</p>
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<img alt="_images/fix_lb_fluid_fluidforce.jpg" class="align-center" src="_images/fix_lb_fluid_fluidforce.jpg" />
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<p>where v_n is the velocity of the MD particle, u_f is the fluid
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velocity interpolated to the particle location, and gamma is the force
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coupling constant. Zeta is a weight assigned to the grid point,
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obtained by distributing the particle to the nearest lattice sites.
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For this, the user has the choice between a trilinear stencil, which
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provides a support of 8 lattice sites, or the immersed boundary method
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Peskin stencil, which provides a support of 64 lattice sites. While
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the Peskin stencil is seen to provide more stable results, the
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trilinear stencil may be better suited for simulation of objects close
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to walls, due to its smaller support. Therefore, by default, the
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Peskin stencil is used; however the user may switch to the trilinear
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stencil by specifying the keyword, <em>trilinear</em>.</p>
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<p>By default, the force coupling constant, gamma, is calculated according to</p>
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<img alt="_images/fix_lb_fluid_gammadefault.jpg" class="align-center" src="_images/fix_lb_fluid_gammadefault.jpg" />
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<p>Here, m_v is the mass of the MD particle, m_u is a representative
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fluid mass at the particle location, and dt_collision is a collision
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time, chosen such that tau/dt_collision = 1 (see <a class="reference internal" href="#mackay2"><span>Mackay and Denniston</span></a> for full details). In order to calculate m_u, the
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fluid density is interpolated to the MD particle location, and
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multiplied by a volume, node_area*dx_lb, where node_area represents
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the portion of the surface area of the composite object associated
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with a given MD particle. By default, node_area is set equal to
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dx_lb*dx_lb; however specific values for given atom types can be set
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using the <em>setArea</em> keyword.</p>
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<p>The user also has the option of specifying their own value for the
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force coupling constant, for all the MD particles associated with the
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fix, through the use of the <em>setGamma</em> keyword. This may be useful
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when modelling porous particles. See <a class="reference internal" href="fix_lb_viscous.html#mackay"><span>Mackay et al.</span></a> for a
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detailed description of the method by which the user can choose an
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appropriate gamma value.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">while this fix applies the force of the particles on
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the fluid, it does not apply the force of the fluid to the particles.
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When the force coupling constant is set using the default method,
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there is only one option to include this hydrodynamic force on the
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particles, and that is through the use of the
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<a class="reference internal" href="fix_lb_viscous.html"><em>lb/viscous</em></a> fix. This fix adds the hydrodynamic
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force to the total force acting on the particles, after which any of
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the built-in LAMMPS integrators can be used to integrate the particle
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motion. However, if the user specifies their own value for the force
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coupling constant, as mentioned in <a class="reference internal" href="fix_lb_viscous.html#mackay"><span>Mackay et al.</span></a>, the
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built-in LAMMPS integrators may prove to be unstable. Therefore, we
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have included our own integrators <a class="reference internal" href="fix_lb_rigid_pc_sphere.html"><em>fix lb/rigid/pc/sphere</em></a>, and <a class="reference internal" href="fix_lb_pc.html"><em>fix lb/pc</em></a>, to solve for the particle motion in these
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cases. These integrators should not be used with the
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<a class="reference internal" href="fix_lb_viscous.html"><em>lb/viscous</em></a> fix, as they add hydrodynamic forces
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to the particles directly. In addition, they can not be used if the
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force coupling constant has been set the default way.</p>
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</div>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">if the force coupling constant is set using the
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default method, and the <a class="reference internal" href="fix_lb_viscous.html"><em>lb/viscous</em></a> fix is NOT
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used to add the hydrodynamic force to the total force acting on the
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particles, this physically corresponds to a situation in which an
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infinitely massive particle is moving through the fluid (since
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collisions between the particle and the fluid do not act to change the
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particle’s velocity). Therefore, the user should set the mass of the
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particle to be significantly larger than the mass of the fluid at the
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particle location, in order to approximate an infinitely massive
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particle (see the dragforce test run for an example).</p>
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</div>
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<hr class="docutils" />
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<p>Inside the fix, parameters are scaled by the lattice-Boltzmann
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timestep, dt, grid spacing, dx, and mass unit, dm. dt is set equal to
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(nevery*dt_MD), where dt_MD is the MD timestep. By default, dm is set
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equal to 1.0, and dx is chosen so that tau/(dt) =
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(3*eta*dt)/(rho*dx^2) is approximately equal to 1. However, the user
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has the option of specifying their own values for dm, and dx, by using
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the optional keywords <em>dm</em>, and <em>dx</em> respectively.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">Care must be taken when choosing both a value for dx,
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and a simulation domain size. This fix uses the same subdivision of
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the simulation domain among processors as the main LAMMPS program. In
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order to uniformly cover the simulation domain with lattice sites, the
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lengths of the individual LAMMPS subdomains must all be evenly
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divisible by dx. If the simulation domain size is cubic, with equal
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lengths in all dimensions, and the default value for dx is used, this
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will automatically be satisfied.</p>
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</div>
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<p>Physical parameters describing the fluid are specified through
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<em>viscosity</em>, <em>density</em>, and <em>a0</em>. If the force coupling constant is
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set the default way, the surface area associated with the MD particles
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is specified using the <em>setArea</em> keyword. If the user chooses to
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specify a value for the force coupling constant, this is set using the
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<em>setGamma</em> keyword. These parameters should all be given in terms of
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the mass, distance, and time units chosen for the main LAMMPS run, as
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they are scaled by the LB timestep, lattice spacing, and mass unit,
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inside the fix.</p>
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<hr class="docutils" />
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<p>The <em>setArea</em> keyword allows the user to associate a surface area with
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a given atom type. For example if a spherical composite object of
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radius R is represented as a spherical shell of N evenly distributed
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MD particles, all of the same type, the surface area per particle
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associated with that atom type should be set equal to 4*pi*R^2/N.
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This keyword should only be used if the force coupling constant,
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gamma, is set the default way.</p>
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<p>The <em>setGamma</em> keyword allows the user to specify their own value for
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the force coupling constant, gamma, instead of using the default
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value.</p>
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<p>The <em>scaleGamma</em> keyword should be used in conjunction with the
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<em>setGamma</em> keyword, when the user wishes to specify different gamma
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values for different atom types. This keyword allows the user to
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scale the <em>setGamma</em> gamma value by a factor, gammaFactor, for a given
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atom type.</p>
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<p>The <em>dx</em> keyword allows the user to specify a value for the LB grid
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spacing.</p>
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<p>The <em>dm</em> keyword allows the user to specify the LB mass unit.</p>
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<p>If the <em>a0</em> keyword is used, the value specified is used for the
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square of the speed of sound in the fluid. If this keyword is not
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present, the speed of sound squared is set equal to (1/3)*(dx/dt)^2.
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Setting a0 > (dx/dt)^2 is not allowed, as this may lead to
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instabilities.</p>
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<p>If the <em>noise</em> keyword is used, followed by a a positive temperature
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value, and a positive integer random number seed, a thermal
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lattice-Boltzmann algorithm is used. If <em>LBtype</em> is set equal to 1
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(i.e. the standard LB integrator is chosen), the thermal LB algorithm
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of <a class="reference internal" href="#adhikari"><span>Adhikari et al.</span></a> is used; however if <em>LBtype</em> is set
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equal to 2 both the LB integrator, and thermal LB algorithm described
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in <a class="reference internal" href="#ollila"><span>Ollila et al.</span></a> are used.</p>
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<p>If the <em>calcforce</em> keyword is used, both the fluid force and torque
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acting on the specified particle group are printed to the screen every
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N timesteps.</p>
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<p>If the keyword <em>trilinear</em> is used, the trilinear stencil is used to
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interpolate the particle nodes onto the fluid mesh. By default, the
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immersed boundary method, Peskin stencil is used. Both of these
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interpolation methods are described in <a class="reference internal" href="fix_lb_viscous.html#mackay"><span>Mackay et al.</span></a>.</p>
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<p>If the keyword <em>D3Q19</em> is used, the 19 velocity (D3Q19) lattice is
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used by the lattice-Boltzmann algorithm. By default, the 15 velocity
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(D3Q15) lattice is used.</p>
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<p>If the keyword <em>write_restart</em> is used, followed by a positive
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integer, N, a binary restart file is printed every N LB timesteps.
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This restart file only contains information about the fluid.
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Therefore, a LAMMPS restart file should also be written in order to
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print out full details of the simulation.</p>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
|
|
<p class="last">When a large number of lattice grid points are used,
|
|
the restart files may become quite large.</p>
|
|
</div>
|
|
<p>In order to restart the fluid portion of the simulation, the keyword
|
|
<em>read_restart</em> is specified, followed by the name of the binary
|
|
lb_fluid restart file to be used.</p>
|
|
<p>If the <em>zwall_velocity</em> keyword is used y-velocities are assigned to
|
|
the lower and upper walls. This keyword requires the presence of
|
|
walls in the z-direction. This is set by assigning fixed boundary
|
|
conditions in the z-direction. If fixed boundary conditions are
|
|
present in the z-direction, and this keyword is not used, the walls
|
|
are assumed to be stationary.</p>
|
|
<p>If the <em>bodyforce</em> keyword is used, a constant body force is added to
|
|
the fluid, defined by it’s x, y and z components.</p>
|
|
<p>If the <em>printfluid</em> keyword is used, followed by a positive integer, N,
|
|
the fluid densities and velocities at each lattice site are printed to the
|
|
screen every N timesteps.</p>
|
|
<hr class="docutils" />
|
|
<p>For further details, as well as descriptions and results of several
|
|
test runs, see <a class="reference internal" href="fix_lb_viscous.html#mackay"><span>Mackay et al.</span></a>. Please include a citation to
|
|
this paper if the lb_fluid fix is used in work contributing to
|
|
published research.</p>
|
|
</div>
|
|
<hr class="docutils" />
|
|
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
|
|
<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
|
|
<p>Due to the large size of the fluid data, this fix writes it’s own
|
|
binary restart files, if requested, independent of the main LAMMPS
|
|
<a class="reference internal" href="restart.html"><em>binary restart files</em></a>; no information about <em>lb_fluid</em>
|
|
is written to the main LAMMPS <a class="reference internal" href="restart.html"><em>binary restart files</em></a>.</p>
|
|
<p>None of the <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> options are relevant to this
|
|
fix. No global or per-atom quantities are stored by this fix for
|
|
access by various <span class="xref std std-ref">output commands</span>. No
|
|
parameter of this fix can be used with the <em>start/stop</em> keywords of
|
|
the <a class="reference internal" href="run.html"><em>run</em></a> command. This fix is not invoked during <a class="reference internal" href="minimize.html"><em>energy minimization</em></a>.</p>
|
|
</div>
|
|
<div class="section" id="restrictions">
|
|
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
|
|
<p>This fix is part of the USER-LB package. It is only enabled if LAMMPS
|
|
was built with that package. See the <a class="reference internal" href="Section_start.html#start-3"><span>Making LAMMPS</span></a> section for more info.</p>
|
|
<p>This fix can only be used with an orthogonal simulation domain.</p>
|
|
<p>Walls have only been implemented in the z-direction. Therefore, the
|
|
boundary conditions, as specified via the main LAMMPS boundary command
|
|
must be periodic for x and y, and either fixed or periodic for z.
|
|
Shrink-wrapped boundary conditions are not permitted with this fix.</p>
|
|
<p>This fix must be used before any of <a class="reference internal" href="fix_lb_viscous.html"><em>fix lb/viscous</em></a>, <a class="reference internal" href="fix_lb_momentum.html"><em>fix lb/momentum</em></a>, <a class="reference internal" href="fix_lb_rigid_pc_sphere.html"><em>fix lb/rigid/pc/sphere</em></a>, and/ or <a class="reference internal" href="fix_lb_pc.html"><em>fix lb/pc</em></a> , as the fluid needs to be initialized before
|
|
any of these routines try to access its properties. In addition, in
|
|
order for the hydrodynamic forces to be added to the particles, this
|
|
fix must be used in conjunction with the
|
|
<a class="reference internal" href="fix_lb_viscous.html"><em>lb/viscous</em></a> fix if the force coupling constant is
|
|
set by default, or either the <a class="reference internal" href="fix_lb_viscous.html"><em>lb/viscous</em></a> fix or
|
|
one of the <a class="reference internal" href="fix_lb_rigid_pc_sphere.html"><em>lb/rigid/pc/sphere</em></a> or
|
|
<a class="reference internal" href="fix_lb_pc.html"><em>lb/pc</em></a> integrators, if the user chooses to specifiy
|
|
their own value for the force coupling constant.</p>
|
|
</div>
|
|
<div class="section" id="related-commands">
|
|
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
|
|
<p><a class="reference internal" href="fix_lb_viscous.html"><em>fix lb/viscous</em></a>, <a class="reference internal" href="fix_lb_momentum.html"><em>fix lb/momentum</em></a>, <a class="reference internal" href="fix_lb_rigid_pc_sphere.html"><em>fix lb/rigid/pc/sphere</em></a>, <a class="reference internal" href="fix_lb_pc.html"><em>fix lb/pc</em></a></p>
|
|
</div>
|
|
<div class="section" id="default">
|
|
<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
|
|
<p>By default, the force coupling constant is set according to</p>
|
|
<img alt="_images/fix_lb_fluid_gammadefault.jpg" class="align-center" src="_images/fix_lb_fluid_gammadefault.jpg" />
|
|
<p>and an area of dx_lb^2 per node, used to calculate the fluid mass at
|
|
the particle node location, is assumed.</p>
|
|
<p>dx is chosen such that tau/(delta t_LB) =
|
|
(3 eta dt_LB)/(rho dx_lb^2) is approximately equal to 1.
|
|
dm is set equal to 1.0.
|
|
a0 is set equal to (1/3)*(dx_lb/dt_lb)^2.
|
|
The Peskin stencil is used as the default interpolation method.
|
|
The D3Q15 lattice is used for the lattice-Boltzmann algorithm.
|
|
If walls are present, they are assumed to be stationary.</p>
|
|
<hr class="docutils" />
|
|
<p id="ollila"><strong>(Ollila et al.)</strong> Ollila, S.T.T., Denniston, C., Karttunen, M., and Ala-Nissila, T., Fluctuating lattice-Boltzmann model for complex fluids, J. Chem. Phys. 134 (2011) 064902.</p>
|
|
<p id="mackay"><strong>(Mackay et al.)</strong> Mackay, F. E., Ollila, S.T.T., and Denniston, C., Hydrodynamic Forces Implemented into LAMMPS through a lattice-Boltzmann fluid, Computer Physics Communications 184 (2013) 2021-2031.</p>
|
|
<p id="mackay2"><strong>(Mackay and Denniston)</strong> Mackay, F. E., and Denniston, C., Coupling MD particles to a lattice-Boltzmann fluid through the use of conservative forces, J. Comput. Phys. 237 (2013) 289-298.</p>
|
|
<p id="adhikari"><strong>(Adhikari et al.)</strong> Adhikari, R., Stratford, K., Cates, M. E., and Wagner, A. J., Fluctuating lattice Boltzmann, Europhys. Lett. 71 (2005) 473-479.</p>
|
|
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