# Miscible Fluids¶

## Basic Concepts¶

This functionality is used to model interactions between miscible fluids with user-defined density and rheology. Examples of miscible fluids include glycerin/water, brine/salt water, and most biomass/water combination. Common applications of these models include fluid dilution, miscible fluid blending, and simulations of cleaning/washing.

Miscible fluids can enter the fluid domain via a background concentration, injection within a child geometry, a non-zero concentration at an inlet, mass transfer from a bubble or particle, or via a reaction. Miscible fluids can be removed from a system via outlets, mass transfer to a bubble or particle, or via a reaction The viscosity of the system can be a function of miscible fluid volume fraction and is defined using a custom viscosity model.

Miscible fluid output files are created for each fluid, and record the time-evolution of the mean miscible fluid volume fraction, the standard deviation of the miscible fluid volume fraction across the tank volume and the total miscible fluid volume. The volume fraction of each miscible fluid at each inlet and outlet are also recorded in the boundary condition output files.

Miscible fluids are characterized by a parent-child relationship. Within this relationship, the parent defines the properties of the miscible field and any user-specified initial background volume fraction in the fluid. The children define the shape/geometry/topology of any initial injection conditions, with a user-specified volume fraction for the children.

## General Properties¶

- Unit of Measure
Indicates how the initial background concentration is defined.

Volume Fraction: Define the initial condition a volume fraction within the child geometry

Volume of Fluid (m^3): Define the total volume of the fluid contained inside the child geometry

- Initial Background Concentration
Initial background concentration

This is the uniform background concentration of the species at the start of the simulation. Units defined by the Unit of Measure (defined above).

- Diffusion Coefficient
Diffusion coefficient [m^2/s]

Diffusion coefficient of the miscible through the base fluid.

- Density Option
Indicates if the miscible field density is to be modeled explicitly.

Off: Miscible field has no affect on local fluid density

On: Miscible field informs the local fluid density via a user-provided density

The effective fluid density at each point in the tank is taken to be the volume-fraction averaged density. This relationship does not need to be specified by the user.

- Density
Only presented when the

*Density Option*is active. This value represents the density of the miscible fluid, [kg/m^{3}]

## Child Geometry Injection Conditions¶

- Injection Geometry
Inject In Volume: the miscible fluid boundary condition is imposed on all points contained inside the geometry.

Inject At Surface: the miscible fluid boundary condition is imposed on the surface of the child geometry.

These conditions are identical for children geometries that are thin relative to the lattice spacing.

- Unit of Measure
Volume Fraction: User-specified miscible fluid volume fraction in the child geometry

Volume (m^3): User-specified miscible fluid volume in the child geometry

- Start Time
Injection start time [s].

Time at which to adding miscible fluid into child geometry

- Injection Time Span
Characterizes when miscible fluid is added to the system

- Entire Simulation
Child injection concentration is maintained at user-defined value over the entire simulation. In the a closed and non-reacting system, the mean miscible volume fraction will continuously increase in the system.

- Impulse
Child injection concentration is instantaneously set to user-defined at this impulse injection time. In the a closed and non-reacting system, the mean miscible volume fraction to will remain constant over time.

- Time Window
Child injection concentration is maintained at user-defined over the specified time window. In the a closed and non-reacting system, the mean miscible volume fraction will increase during the time window, then remain constant after the time window.

- Delayed Start
Child injection concentration is maintained at user-defined beginning at this time until the end of the simulation. In the a closed and non-reacting system, the mean miscible volume fraction will continuously increase once the simulation reaches this delayed start time.

## Boundary Conditions¶

- Boundary Condition Type
For each boundary condition present in the simulation, the following inputs are available

- Specified Value
The specified value defines the miscible field value at boundary condition, as measured in volume fraction. The specified value can be a function of time and position across the inlet face.

- Zero Gradient
A zero gradient is a Neumann boundary condition, which implies no variation in the miscible field volume fraction at the boundary condition in the direction of flow. Most outlets are assigned a zero gradient boundary condition.

- Zero Flux
A zero flux implies no miscible fluid transport though the boundary.

- Recirculation
For recirculation inlets, the volume fraction of the inlet in a function of the miscible fluid volume fraction exiting through the suction side of the coupled boundary condition.

If this value is set to

`c`

and Recirculation Link Flow Ratio is 1, the volume flow rate of the miscible fluid returning to the vessel through the recirculation loop will be equal to the volume flow rate leaving the vessel. This behavior represents a system were all of the miscible fluid leaving through suction side of the recirculation loop returns to the inlet.If this value were set to

`0.5c`

, the volume flow rate of the miscible fluid returning to the vessel through the recirculation loop will be 50% than the volume flow rate leaving the vessel through the suction side of the recirculation line. Under these settings, the concentration in the tank will decrease. This behavior represents a system were some of the miscible fluid is removed from the recirculation line and replace with base fluid prior to returning to the vessel.If this value were set to

`0`

, the volume flow rate of the miscible fluid returning to the vessel through the recirculation loop will be zero. This behavior represents a system were all of the miscible fluid is removed from the recirculation line prior to returning to the vessel.If this value were set to

`2c`

and Recirculation Link Flow Ratio is 1, the volume flow rate of the miscible fluid returning to the vessel through the recirculation loop will 2x larger than volume flow rate leaving the vessel through the suction side of the recirculation line. This behavior represents a system were miscible fluid replaces base fluid in the recirculation line prior to returning to the vessel.If this value were set to

`1`

, the volume flow rate of the miscible fluid returning to the vessel through the recirculation loop will be equal to the volume flow rate of the return inlet.Et cetera

Tip

Consistent with most outlet boundary conditions, the suction side of the recirculation inlet is typically set to zero gradient.

Note

If multiple recirculation inlets are coupled to a single suction-side boundary condition, the ratio of miscible fluid entering through each coupled inlet will be proportional to the surface area of each inlet to the total surface area across all coupled inlets.

## Advanced¶

- Stencil
Stencil used to define advection [Lattice, Finite Volume] The finite volume stencil considers fluxes in the +/- x,y, and z-directions. The lattice stencil considers fluxes in the directions of the fluid lattice

- Limiter
Flux limiter used to contain convection [VanLeer, Minmod, MUSCL, Superbee, DonorCell, LaxWendroff, DonorCell]

The Donor Cell is an upwind difference technique valid for systems with a Peclet number greater than 2 (a condition typically satisfied in turbulent systems). The other schemes are total variation diminishing and necessary in laminar systems.

## Output Data¶

- Statistics Output Frequency
For each miscible fluid, the instantaneous total miscible fluid volume, the mean volume fraction, the volume fraction standard deviation, and volume fraction percent relative standard deviation is printed to a unique output text file.

- Slices Output Frequency
The spatial variation in the miscible fluid volume fraction, fluid density, and fluid viscosity are reported on each slice.

- Volume Output Frequency
The spatial variation in the miscible fluid volume fraction, fluid density, and fluid viscosity are reported on the fluid volume output.