Boundary Conditions

General boundary conditions

The following inputs are defined using the bc prefix.

	Description	Type	Default
regions	Regions used to define boundary conditions.	String	None

The type of the boundary conditions in the BC region must be defined.

Description

Type

Default

[region]

Used to define boundary condition type. Available options include:

‘pi’ for pressure inflow BC type
‘po’ for pressure outflow BC type
‘mi’ for mass inflow BC type
‘nsw’ for no-slip wall BC type
‘eb’ for inhomogeneous Dirichlet BCs of temperature or fluid velocity (mass inflow) on the contained EBs

String

None

po_no_par_out

Let particles exit (default) or bounce-back at pressure outflows

Int

0

Fluid settings

For each boundary condition region, the fluid inputs are defined using the bc.[region].[fluid] prefix.

	Description	Type	Default
volfrac	Volume fraction [required if bc_region_type=’mi’]	Real	0
density	Fluid density [required if bc_region_type=’mi’ or ‘pi’]	Real	0
pressure	Fluid pressure [required if bc_region_type=’po’ or ‘pi’]	Real	0
temperature	Fluid temperature [required if bc_region_type=’mi’ or ‘pi’]	Real	0
velocity	Velocity components [required if bc_region_type=’mi’]	Reals	0 0 0
delp_dir	Direction for specified pressure drop. Note that this direction should also be periodic.	Int	0
delp	Pressure drop (Pa)	Real	0
species.[species0]	Species ‘species0’ mass fraction [required if solve_species=1 and bc_region_type=’mi’ or ‘pi’].	Real	0

Below is an example for specifying boundary conditions for fluid (fluid).

bc.regions = inflow outflow

bc.inflow = mi
bc.inflow.fluid.volfrac     =  1.0
bc.inflow.fluid.density     =  1.0
bc.inflow.fluid.velocity    =  0.015  0.0  0.0
bc.inflow.fluid.temperature =  300
bc.inflow.fluid.species.O2  =  0.0
bc.inflow.fluid.species.CO  =  0.5
bc.inflow.fluid.species.CO2 =  0.5

bc.outflow = po

bc.outflow.fluid.pressure =  0.0

Transient boundary conditions

Velocity, temperature, and pressure boundary conditions may also be specified as a function of time simply by adding a new column. The time value is entered in the new first column. We can make the mi boundary condition above time-dependent by replacing:

bc.inflow.fluid.velocity    =  0.0  0.0    0.0  0.0
bc.inflow.fluid.velocity    =  3.0  0.015  0.0  0.0

bc.inflow.fluid.temperature =  0.0  300
bc.inflow.fluid.temperature =  2.99 300
bc.inflow.fluid.temperature =  3.0  500
bc.inflow.fluid.temperature =  4.0  500
bc.inflow.fluid.temperature =  4.01 300

In the above example, the inflow velocity is accelerated from zero to its final value over a period of three seconds. Linear interpolation is used in between discrete time values and held constant at the last time value. The temperature sees an abrupt spike from 300 up to 500 at t = 3s and then back down again after 4s. Note that the timestep is not adjusted to sync with transient BCs.

Embedded boundary options

In MFIX-Exa it is possible to set boundary conditions on the embedded boundaries. For instance, it is possible to set inhomogeneous Dirichlet boundary conditions for the fluid temperature variable on the subpart of the embedded boundaries which is contained in the BC region (which in this case has to be tridimensional). We recall that, on the remaining part of the EBs, homogeneous Neumann boundary conditions are assumed by default.

In the following table there is a list of the possible entries for EB boundary conditions. Each entry must be preceded by bc.[region0].

	Description	Type	Default
eb.temperature	Inhomogeneous Dirichlet BC value for temperature on EBs contained in the (tridimensional) region [required if advect_enthalpy=1 and bc_region_type=’eb’].	Real	0.0

Below is an example for specifying boundary conditions for a fluid myfluid.

bc.regions = hot-wall

bc.hot-walls = eb
bc.hot-walls.eb.temperature = 800

In addition to the temperature, it is possible to set an inflow condition for fluid on an embedeed boundary. We recall that, on the remaining part of the EBs, no slip velocity conditions are assumed by default.

In the following table there is a list of the possible entries for inflow EB boundary conditions. Each entry must be preceded by bc.[region0]. Like traditional mass inflows, the fluid temperature, pressure, and species composition must be provided when appropriate.

	Description	Type	Default
fluid.velocity	(Required if not volflow) Inflow fluid velocity on EB faces contained in the (tridimensional) region. Note that if only one value is specified, that is assumed to be the magnitude in the direction of the EB face’s normal.	Reals	0 0 0
fluid.volflow	(Required if not velocity) Inflow BC for fluid volumetric flow rate in the (tridimensional) region. The flow is assumed to be normal to the EB surface in the region.	Real	0
fluid.volfrac	(Required) Volume fraction.	Real	0
eb.normal	(Optional) When specified, only cells with EB face normal that is parallel and opposite in direction to the specified values are imposed with the inflow velocity.	Reals	0 0 0
eb.normal_tol	(Optional) Used in conjunction with eb.normal. It determines the tolerance (in degrees) for choosing cells with a specific normal.	Real	0

Below is an example for specifying a normal inflow velocity magnitude for a region eb-flow.

bc.regions = eb-flow

bc.eb-flow = eb

bc.eb-flow.my_fluid.volfrac  = 1.0
bc.eb-flow.my_fluid.velocity = 0.1

Below is an example where only specific cells are imposed a velocity in the x-direction.

bc.regions = eb-flow

bc.eb-flow = eb

bc.eb-flow.eb.normal_tol = 3.0
bc.eb-flow.eb.normal =  0.9848  0.0000  0.1736  # 10 deg

bc.eb-flow.my_fluid.volfrac  = 1.0
bc.eb-flow.my_fluid.velocity = 0.1  0.0  0.0