# <simulation>

<simulation> - <general> - <num_ramp_up_iterations>
The inlet velocity is increased from zero to the value requested by <reference_velocity> over a certain amount of ramp-up iterations and physical time. By default, the number of ramp-up iterations is computed to achieve an average acceleration of 200 m/s2. For a case with reference velocity 40 m/s, the ramp-up phase covers the first 0.2 s of the simulation, and the number of ramp-up iterations is calculated accordingly.
Alternatively, you can explicitly specify a fixed number of coarsest iterations for the ramp up of a simulation.
Default = 0
<simulation> - <general> - <convergence_detection>
Activates the ultraFluidX convergence detection algorithm output (still in beta mode). The algorithm analyzes the transient drag and lift signals and estimates if and when the initial transient phase has ended and when the overall simulation has converged; the respective iteration steps are reported in the uFX_summary.txt file, together with averaged force coefficients that are determined between these two points in time.
<simulation> - <geometry> - <triangle_mesh_export>
true
The triangulated surface used in the code (after triangle_splitting, if applicable) is exported to uFX_meshData/uFX_surfaceMesh.stl.
This surface mesh contains all "physical” surfaces including passive parts, while surfaces needed for the mesh setup, for example, those defining sources or custom zones, are not displayed.
false (default)
<simulation> - <wall_modeling>
<wall_model_intensity>
The wall model intensity parameter is used together with the <coupling> “one-way” option and determines the strength of the link between the velocity taken as input to the wall model and the information fed to the first near wall cell. For a value of 0.0, the wall model is effectively switched off and a standard no-slip boundary condition is applied.
<donor_interpolation>
The donor position is the point where the wall model is coupled to the flow field, in a sufficiently well resolved part of the boundary layer. This donor position does not necessarily coincide with a voxel center; the information required at this position will be interpolated/derived from the data available in the 27 voxels (3x3x3) around the donor position.
<donor_position_factor>
This factor is applied to a reference length (see below) to determine the exact distance to the wall where the wall model is extracting information from the flow. The default value of 3.5 together with the default reference length corresponding to the near wall voxel size leads to a setup where the donor position is placed in the third layer of voxels above the wall.
<donor_position_use_local_reference_length>
The reference length is coupled to the voxel size at the wall. This parameter controls if a local reference length per refinement level or a global reference length is used.
If set to true (default), the reference length is evaluated per refinement level, which moves the donor position closer to the wall in regions of finer grid.
If set to false, in case the wall touches voxels of different refinement levels, the coarsest voxel size at the wall will be used as a reference length, also in regions of finer grid.
<donor_position_reference_length>
Using this parameter, you can also set the reference length to a specific value in $\left[\text{m}\right]$ , which should, however, be of the order of magnitude of the near wall voxel size. If set to 0.0 (default), the parameter is effectively switched off.
Note: This parameter is only effective in the case that you request a global reference length at the same time (see above). Otherwise, it will be ignored.
<simulation> - <initialization> - <seeding>
To accelerate convergence, you can initialize ("seed") the ultraFluidX simulation with velocity information from a previous run or from fallback velocity. Standard ultraFluidX output files (*.geo and *.data) with transient or time-averaged velocity information can be used for the initialization.
The following parameters are available for this feature:
<method>
(String)
velocity
Initialization with velocity field.
uniform
Uniform initialization with <fallback_velocity>, no ramp-up.
off (default, fallback)
Uniform initialization with 0.0, subsequent ramp-up to the <reference_velocity>.
<scale_factor>
(Double) A user-defined factor with which the initialization velocity from the EnSight file can be scaled in the entire domain. For example, you can rescale a velocity field based on an inlet velocity of 20 m/s to initialize a simulation with inlet velocity 25 m/s using a <scale_factor> of 1.25.
The scale factor is only applied to the Ensight initialization values, not to the <fallback_velocity>.
Default = 1.0
<fallback_velocity>
<x_dir>, <y_dir>, <z_dir>
(3x double) The velocity used for all voxels without valid initialization data in the EnSight data sets specified in the <source_files> category. This velocity is also used for the initialization if the <method> is set to uniform. The default for the <fallback_velocity> is the reference velocity of the simulation in x-direction and zero in y- and z-direction.
Default = Value of <simulation> <general> <reference_velocity> (X), 0.0 (Y), 0.0 (Z)
<source_files>
<default>
<file_set>
<geometry>
(String)
<data>
(String)
You can specify one or more sets of files. Each file includes one *.geo and one *.data file that must be specified via the <geometry> and <data> tags. The files must contain the same number of elements and the *.data file must contain a three-dimensional velocity field.
<overset>
<file_set>
<geometry>
(String)
<data>
(String)
In case of an overset simulation, the files defined in the <default> category are used for initialization of the background mesh. The files defined in the <overset> category are used for initialization of the foreground mesh.
If the source and target overset mesh voxels are not fully compatible due to a mismatch in the angle of rotation, the <fallback_velocity> is applied for the affected voxels.
In case of non-overset simulations, only the <default> files are used.