Impose Regions

There are two categories to define: geometric shape and effect type.

Available geometric shapes:

• PARALLELEPIPED
• CYLINDER (with or without hemispherical caps)

Available effect types:

• Velocity (corresponding command suffix: *vel)
• Acceleration (corresponding command suffix: *acc)
• Porous media (corresponding command suffix: *porous)
• Temperature (corresponding command suffix: *temp)

Any combination of shapes and effects is permitted, making for total of eight possible scenarios (or 12 if you count cylinders with hemispherical caps as a separate shape). The impose region type name is composed by specifying the geometric shape and adding the effect suffix in capital letters, for example: PARALLELEPIPEDVEL, PARALLELEPIPEDPOROUS, CYLINDERACC, or CYLINDERTEMP.

The fluid velocity region imposes a velocity on all fluid particles which are within the prescribed region. The easiest analogy is a pump mechanism (momentum source), except that there are no moving parts involved. The acceleration region imposes a defined acceleration on all the particles which are within the defined region. It also can be thought of as a momentum source region, except that by defining the acceleration instead of hard-setting the velocity of the particles, the region is likely to behave more stably. Porous media definition follows the well known Darcy-Forchheimer model and allows for definition of volume averaged isotropic or non-isotropic porous media. Finally, the temperature region assigns a temperature to all fluid or wall particles (optional) which are within the prescribed region, and is essentially a heat source/sink.

In addition to shape and effect types, there are specific commands which are applicable to a specific subset of desired shape-effect combinations. For example, as already mentioned, cylinder shape has a possibility to include hemispherical caps at the ends of the cylinder, such that if you specify the height of the cylinder as zero, you would end up with a spherical shape. More obviously, parameters which define the geometry are different between PARALLELEPIPED and CYLINDER impose regions.

There is also a possibility to SET a magnitude of a desired field value inside the impose region or use the ADD the command to add the specified value to any instantaneous field value the particle might have inside the impose region.

One other variable which allows flexibility is the LOOSE/STRICT option, which allows for either hard (strict) imposing of the specified value, or soft (loose) imposing of the specified value. If we take an example of specifying velocity in X direction - STRICT definition will impose exactly the vector which the user provides, with Y and Z components being equal to zero. If we use the LOOSE option, the code will impose the X direction velocity, but will allow the solver to naturally accommodate (calculate) Y and Z velocity components.

The final option available is to define inertial (INERT) or body (BODY) frame of reference for velocity effect regions. If INERT is selected, the velocity/acceleration definition in the region assumes that the general (default) simulation coordinate system is used, which is the common case. This means that whatever velocity/acceleration is specified, it will be used as such. In case the user specifies the velocity and enables BODY frame of reference, the resulting velocity/acceleration vector in the region will be specified vector plus the velocity/acceleration of the body. This can be useful in specific scenarios.

Figure 1. Commands supported for each effect type

Commands

The regions are defined through a separate parameter section called imposeRegions.

When specifying the geometry of the shape type commands in the imposeRegions, a specific nomenclature is followed. Each command begins with the shape definition, followed by a suffix which defines the effect, followed by an underscore and key suffix (which is effectively unique to each command). For example:

more specifically resulting in parallelepipedvel_A_vec or cylinderporous_axis.

Additionally, when defining time series (varying parameters in time), additional suffixes are used for file, offset and latch commands. These suffixes are _tvs_ (time velocity series), _tas_ (time acceleration series) and _tts_ (time temperature series).

On the other hand, effect definitions are universal, such that once the impose region shape parameters are defined (using appropriate suffixes), you can add effect parameters.

We are emphasizing these definition principles in order to concisely (modularly) present the set of commands, instead of listing all eight (optionally 12) possible versions of the impose region definitions.

imposeRegions
{
	imposeRegion
	{
		;NOTE:
		;<shape>  = {parallelepiped, cylinder}
		;<effect> = {vel, acc, porous, temp}
		;<time_series> = {_tvs_, _tas_, _tts_}
		
		; UNIVERSAL PARAMETERS
		imposeRegion_type                       PARALLELEPIPEDTEMP 
		t_start                                 0.0
		t_damping                               0.1 
		t_end                                   20.0
		imposeregion_motphs                     1
		
		<shape><effect>_mode                    SET
		<shape><effect>_constraint              LOOSE
		<shape><effect>_frame                   BODY

		<shape><effect><time_series>file        time-dependent-series.txt
		<shape><effect><time_series>offset      0.1
		<shape><effect><time_series>latch       false

		; PARALLELEPIPED PARAMETERS
		parallelepiped<effect>_corner           "0.0 0.0 0.0"
		parallelepiped<effect>_A_vec            "1.0 0.0 0.0"
		parallelepiped<effect>_A_len            2           
		parallelepiped<effect>_B_vec            "0.0 1.0 1.0"
		parallelepiped<effect>_B_len            4            
		parallelepiped<effect>_C_vec            "1.0 0.0 1.0"
		parallelepiped<effect>_C_len            3            
		
		; CYLINDER PARAMETERS
		cylinder<effect>_axis                   "0.2 0.2 0.4"
		cylinder<effect>_cntr                   "0.2 0.2 0.4"
		cylinder<effect>_hght                   0.8
		cylinder<effect>_rad                    1.0
		cylinder<effect>_caps                   false
		
		; VELOCITY PARAMETERS
		<shape>vel_unv                          "1.0 2.0 0.0"
		<shape>vel_vel                          12.0
		use_prtl_reuni                          false

		; ACCELERATION PARAMETERS
		<shape>acc_acc                          100           
		<shape>acc_unv                          "0.0 0.0 1.0" 

		; TEMPERATURE PARAMETERS
		<shape>temp_temp                        150.0
		<shape>temp_fluidonly                   false

		; POROUS MEDIA PARAMETERS
		porous_principal_ax_x                   "1.0 0.0 0.0" 
		porous_principal_ax_y                   "0.0 1.0 0.0"
		porous_principal_ax_z                   "0.0 0.0 1.0"

		porous_inert                            "1000.0 10.0 1000.0"
		porous_inert_offdiag                    "1000.0 10.0 1000.0" 
		porous_visc                             "100.0  100.0 10.0"
		porous_visc_offdiag                     "100.0  100.0 10.0"
    }
}

Definitions

Parallelepiped Specific Commands

In Figure 2, note the three vectors which define the parallelepiped: parallelepiped<effect>_A_vec, parallelepiped<effect>_B_vec and parallelepiped<effect>_C_vec. The origin where the three vectors meet is the parallelepiped<effect>_corner.

Figure 2. Schematic View of a Generic Parallelepiped

Command	Contents	SI Unit Example
parallelepiped<effect>_corner	Coordinates of the defining the origin corner of the parallelepiped. This is a vector variable of the format: "X Y Z".	[m]
parallelepiped<effect>_A_vec	Three components of the vector A, which is defining one side of the parallelepiped. Note: This is a vector variable of the format: "X Y Z".
parallelepiped<effect>_A_len	Length of the side A (magnitude of the vector A). Note: This value is a scalar.	[m]
parallelepiped<effect>_B_vec	Three components of the vector B, which is defining one side of the parallelepiped. Note: This is a vector variable of the format: "X Y Z".
parallelepiped<effect>_B_len	Length of the side B (magnitude of the vector B). Note: This value is a scalar.	[m]
parallelepiped<effect>_C_vec	Three components of the vector C, which is defining one side of the parallelepiped. Note: This is a vector variable of the format: "X Y Z".
parallelepiped<effect>_C_len	Length of the side C (magnitude of the vector C). Note: This value is a scalar.	[m]

Cylinder Specific Commands

Velocity Region Specific Commands

Acceleration Region Specific Commands

Temperature Region Specific Commands

Porous Region Specific Commands

It is generally assumed (default) that the principal axes of the porous region align with the base simulation X, Y, and Z coordinates (inertial reference frame). If that is not the case, a separate coordinate system can be defined and the values defining porosity will be considered in that new coordinate system. The commands which define the custom principal axes of the porous region are: