Select the Transport Mechanism by checking the Advection and/or Diffusion boxes in the upper left part of the Experiment tab.

Checking Simulate Diffusion activates the simulation of random effects in particle movement. If the button is unchecked, the diffusivity is set to in the particle momentum equation.

Checking Simulate Advection activates the computation of drag forces from the fluid. If it is unchecked, the velocity of the fluid is zero and therefore the particles move by diffusion only. In this case the Flow Boundary Conditions and the Flow Solver tab are set to inactive.

In the Simulated Time panel, the simulated End Time can be given. At each Time Step, the particle positions are evaluated for their spatial distribution, their breakthrough curve, and their residence times.

By clicking the Approximate Time button, the simulated time is automatically set such that, for the given average fluid Velocity (defined in the Flow Boundary Conditions panel), most particle trajectories will pass through the whole geometry and a full breakthrough curve is computed. This works only if the advection is the dominant transport process. The Time Step value is automatically set to be 1% of the End Time.

In this panel, three Dimensionless Numbers are automatically estimated based on the entered parameter values, provided a structure is loaded. They give you an idea of how strongly advection and diffusion contribute to transport before the simulation is started. After the simulation, more precise values are listed in the report.

The Courant number quantifies the extent to which particles, molecules, or concentration fields are transported through the structure by advection during the simulation. For instance, if , then the solute (particles, molecules, or the concentration field) is advectively transported by the flowing solvent across half of the domain. The Courant number is defined as:

where is the characteristic velocity estimated from the flow boundary conditions, is the simulation time (which equals the End Time, given that the initial time is zero), and is the length of the structure in the through direction.

In contrast, the Fourier number quantifies the extent to which the solute (particles, molecules, or the concentration field) is transported through the structure by diffusion during the simulation. The Fourier number is defined as:

where is the characteristic diffusivity, taken as the maximum effective diffusivity across all existing materials.

Finally, the Péclet number is the ratio of advective transport to diffusive transport. A Péclet number of infinity implies purely advective transport, whereas a value equal to zero corresponds to purely diffusive transport. The Péclet number is defined as:

In the Particle Boundary Conditions panel, the user can choose what happens when a particle arrives at the domain boundary. This can be done individually for each of the six domain sides. In the Periodic case, a particle that leaves through one side of the domain, enters on the opposite side of the domain. In the Reflective case, a particle is reflected at the boundary. In the Open case, the particle leaves the domain and is no longer tracked. Those particles are reported in the breakthrough curve.

In the Flow Boundary Conditions panel, either the Pressure Drop, the mean flow Velocity, or the Flow Rate can be defined. Note that this part of the dialog is accessible only if Simulate Advection is selected.

From the Flow PDE menu, you can specify the equations to be solved for computing the fluid flow field. The flow field is required to simulate the particle movements and trajectories or transport of the concentration field. (Navier-)Stokes equations describe fluid flow in structures containing only solid materials and pores. (Navier-)Stokes-Brinkman equations describe the flow through structures containing porous materials, solids, and pores. Use the Stokes equation when fluid velocities are slow or the viscosities are high, i.e., at low Reynolds numbers. The Navier–Stokes equation should be used for high fluid velocities when inertial effects cannot be neglected.

The used flow boundary conditions in X and Y direction are reported. For the Track Particles & Molecules command the boundary conditions in X and Y direction are set automatically to fit to the chosen boundary conditions for the particle movement:

• Reflective particle behavior leads to Symmetric flow boundary conditions.
• Periodic particle movement requires Periodic flow boundary conditions.

For the Transport Concentration Field and Adsorption commands the boundary conditions in X and Y direction are fixed to Symmetric.

The boundary condition in Z direction, can be Periodic, Symmetric, or VinPout. Please refer to the FlowDict user guide for more details. For the Track Particles & Molecules command, using the Stokes(-Brinkman) equation only allows for using Periodic or Symmetric boundary conditions, while the Navier-Stokes(-Brinkman) equation uses the VinPout boundary condition

Additionally, the Slip Length can be defined in the Flow Boundary Conditions panel. The Slip Length allows including sliding effects in the simulation. The default Slip Length of zero corresponds to a flow velocity of zero along the solid surface. A non-zero Slip Length simulates the sliding of the fluid along the structure’s solid parts, increasing the fluid mean flux and thus, the permeability. This option might be used when it is realistic for a given physical material, e.g., for gases when the mean free path is comparable to the pore size. The same slip length must be set for all materials in the structure. Find more details on the Slip Length here.

For the No-Slip boundary condition two additional options are available:

• Use Second Order No-Slip (default): This option sets the tangential velocity to zero at the center of the voxel surfaces and use a second order approximation of the velocity field.
• Use Sharp Corner: This option sets the tangential velocity to zero at the voxel corners.

Find more details on these settings in the FlowDict user guide.