<sources>

This category specifies parameters for volumetric sources (and sinks) of the following kinds: <porous>, <turbulence>, <mrf>, and <acoustic>.

<sources> - <porous>

This category contains all porous regions where a volumetric body force should be applied to the fluid according to:

\frac{- \partial p}{\partial x} = ρ R_{v} u + ρ R_{i} u^{2}

with,

$R_{v}$: Viscous coefficient.
$R_{i}$: Inertial resistance coefficient.

These regions must be contained in the STL file as a closed volume and all normals of the volume must point outwards.

CAUTION: If the volume is included in the STL and not specified as porous region, it is treated as a solid by ultraFluidX.

For each porous medium, the text files uFX_porousMassFlow_Inst.txt and uFX_porousMassFlow_Avg.txt in the uFX_coefficientsData folder contain the instantaneous and averaged mass flow rates and the corresponding cross sections per porous medium (output at every coarsest iteration in $[\frac{kg}{s}]$ ).

If a porous region is defined as isotropic (<porous_axis> is set to zero), the region is included in the file, but the mass flow rate cannot currently be evaluated. It is reported as zero.

The category can have an arbitrary number of children named <porous_instance>, each with the following parameters:

<name>: Name of the instance (optional).
<inertial_resistance>: Inertial resistance of the instance in $[\frac{1}{m}]$ .
<viscous_resistance>: Viscous resistance of the instance in $[\frac{1}{s}]$ .
<porous_axis>: Flow direction through the instance. Crossflow that is not aligned with the porous axis is blocked. Its components are specified via the child parameters <x_dir>, <y_dir> and <z_dir>. If all components are set to zero, no preferred direction is used and the instance acts as an isotropic porous region, for example, bug screens.
<parts>: The closed volume(s) to which the volumetric body force should be applied and which are contained in the STL file must be specified via the child parameter <name> within the <parts> category, several volumes can be specified via repeated usage of <name> to assign the same properties to all of them. Additionally, if a closed volume consists of separate parts, they can also be specified via repeated usage of <name> to effectively form the closed volume. The specified names must exactly match the respective part names in the STL file.

<sources> - <turbulence>

This category contains all areas where synthetic turbulence is generated according to the so-called “vortex method.” ^¹

Currently, the turbulence generator adds turbulent structures for flows in positive x-direction only. It is set up by an x-position and the extents in y- and z-direction.

ultraFluidX automatically selects the corresponding voxel layer with a thickness of one voxel where the synthetic turbulence is added. All voxels of each turbulence instance are refined to the finest voxel size that is found in the instance.

The category can have an arbitrary number of children named <turbulence_instance>, each with the following parameters:

<name>

(Optional) Name of the instance.

<point>

<x_pos>: Global x-position of the turbulence generation zone.

<bounding_box>

<y_min>, <z_min>, <y_max>, <z_max>: Extents of the turbulence generation zone in the global y- and z-direction.

<num_eddies>

Number of eddies that will be synthesized within the region in a pseudo-random manner. Should be in the order of 100-1000.

<length_scale>

Characteristic length scale for the synthetic turbulence in

[m]

. Should be in the order of (or identical to) the voxel size in the turbulence generator region.

<turbulence_intensity>

Intensity of the synthetic turbulence, that is, ratio between root-mean-square of the turbulent velocity fluctuations and the mean velocity.

<sources> - <mrf>

This category contains all the regions in which ultraFluidX applies body forces to account for a moving reference frame.

Note: The ultraFluidX MRF model is applied on top of the wall velocity <boundary_conditions>, which must be specified separately in the <boundary_conditions> section of the input deck.

The MRF regions must be contained in the STL file as a closed volume and all normals of the volume must point outwards.

Note: If the volume is included in the STL and not specified as an MRF region, it will be treated as a solid by ultraFluidX.

The <mrf> category can have an arbitrary number of children named <mrf_instance>, each with the following parameters:

<name>: (Optional) Name of the instance.
<rpm>: Rotational speed of the instance in [rpm] (revolutions per minute).
<center>: Center of rotation for each instance. Specified via the child parameters <x_pos>, <y_pos> and <z_pos> in $[m]$ .
<axis>: Rotational axis of the instance. Its direction (right-hand rule applies) must be specified via the child parameters <x_dir>, <y_dir> and <z_dir>.
<parts>: The closed volume(s) in which the MRF body force terms should be applied and which are contained in the STL file must be specified via the child parameter <name> within the <parts> category.; Several volumes can be specified via repeated usage of <name> to assign the same properties to all of them.; Additionally, if a closed volume consists of separate parts, they can also be specified via repeated usage of <name> to effectively form the closed volume. The specified names must exactly match the respective part names in the STL file.

<sources> - <acoustic>

This category specifies acoustic sources to create single tone or white noise monopolar sources. The category can have an arbitrary number of children named <acoustic_instance> with the following parameters:

<type>: single_tone, white_noise.
<amplitude>: Amplitude of the noise source in $[Pa]$ .
<frequency>: Frequency of the single tone source in $[Hz]$ . Not applicable to broadband source.
<num_avg_iterations>: Specifies the number of iterations over which the white noise signal is averaged. You can specify fractions of 1 to obtain a signal at the desired sub-time step level corresponding to the resolution level in which the source is defined. Not applicable to single tone source.
<position>: Position of the acoustic source with the child parameters <x_pos>, <y_pos> and <z_pos> in $[m]$ .

<sources> - <momentum>

This category specifies parameters that represent a momentum source therm. A momentum source zone can be characterized by either a constant velocity or a constant mass flow rate within a defined volume or part, which may have an arbitrary shape. Additionally, the mass flow rate can be linked to either a constant, user-defined density or the instantaneous spatially-averaged (scalar) value within the designated volume. Hence, three choices for the momentum source type are available for selection:

Velocity
Mass Flow Rate
Fixed Density Mass Flow Rate

The (vectorial) mass flow rates are converted into (vectorial) volume flow rates, which are then correlated to the projected (scalar) front surface of the volume to obtain the corresponding velocity vector likewise.

The complete default user interface for the momentum source is described as follows:

Any <momentum_instance> is defined by the following parameters:

<name>: Optional name of the momentum source.
<type>: The type of the momentum source: Velocity, mass-flow-rate, fixed-density-mass-flow-rate.

For the <type> fixed_density_mass_flow_rate:

<density>: The (fixed) fluid density within the entire momentum source volume.
<velocity>: 3D velocity vector, that is, specified through <[x;y;z]_dir> in [m/s].
<mass_flow_rate>: 3D mass flow rate vector, that is, specified through <[x;y;z]_dir> in [kg/s].
<num_ramp_up_iterations>: The number of discrete iterations (related to RL0) for which the ramp-up is active.
<parts>: The parts defining the momentum source zone/volume.

Note: The <type>fixed_density_mass_flow_rate should be used with caution, as it resembles an “experimental” implementation. Results of this type must always be critically scrutinized.

<sources> - <virtual_fan>

This category specifies parameters that represent a momentum source term replacing a rotating fan geometry. Specifically, you need to indicate a flow region and the coefficients of a third-order-polynomial reconstruction of the fan’s P-Q curve. Any <virtual_fan_instance> is described by the following parameters:

<name>: (optional) Name of the instance.
Note: Considering tangential velocity components is optional. Including or disabling of tangential velocity components is controlled by the parameters <rpm>, <radius> and <depth>. Setting any one of these parameters to zero will disable tangential velocity components. Valid values of all three parameters result in including tangential velocities.
<rpm>: (optional: Controls tangential velocity).; Rotational speed of the instance in [rpm] (revolutions per minute).
<radius>: (optional: Controls tangential velocity).; Radius of the source zone in $[m]$ .
<depth>: (optional: Controls tangential velocity and is required if <initial_pressure_rise> is used).; Depth of the fan in axial direction in $[m]$ .
<center>: Center of the fan in $[m]$ given by a vector of <x_pos>, <y_pos>, <z_pos>.
<axis>: Axis of rotation of the fan specified as a vector <x_dir>, <y_dir>, <z_dir>.
Note: The input will be normalized to unit length in ultraFluidX.
<axial_coefficients>: The four axial coefficients of the fan’s P-Q curve. The input parameters are normalized by the fan’s depth and have the following SI units:; <zeroth_order> in [m/s²]; <first_order> in [1/s]; <second_order> in [1/m]; <third_order> in [s/m²]
<parts>: <name>; This item indicates the names of parts contained in the STL file that form the closed volume(s) in which the body force terms of the virtual fan should be applied. Several volumes can be specified for the same <virtual_fan_instance> via repeated usage of <name> to assign the same properties to all of them. Additionally, if a closed volume consists of separate parts, they can also be specified via repeated usage of <name> to effectively form the closed volume. The specified names must exactly match the respective part names in the STL file.
Note: Ramping up the contribution of the virtual fan source can be challenging when starting from a flow field with zero velocity, more precisely when the zeroth order axial coefficient is negative. You can facilitate the ramp-up process by indicating the following values (beta feature):
<initial_pressure_rise>: (optional) The targeted/estimated initial pressure rise of the fan in $[Pa]$ .
Note: This functionality requires a valid value for the parameter <depth> to be set; otherwise, the initial pressure rise cannot be taken into account.
<num_ramp_up_iterations>: (optional) The number of discrete iterations, based on the coarsest refinement level, for which the ramp-up is active. (If a valid pressure rise value is given, <num_ramp_up_iterations> will be set to 200 by default; otherwise to zero).

¹ Mathey, F., Cokljat, D., Bertoglio, J. P., & Sergent, E. (2006). Assessment of the vortex method for large eddy simulation inlet conditions. Progress in Computational Fluid Dynamics, An International Journal, 6(1-3), 58-67.

² Mathey, F., Cokljat, D., Bertoglio, J. P., & Sergent, E. (2006). Assessment of the vortex method for large eddy simulation inlet conditions. Progress in Computational Fluid Dynamics, An International Journal, 6(1-3), 58-67.