# Control the Variables that are Written to the FV-UNS Files

AcuTrans can be used to restrict the number of variables that are written into each FV-UNS file.

This feature provides you with a high level of control over the size of the FV-UNS files that are created. By default, AcuSolve will write all variables associated with the NODAL_OUTPUT command. However, you can add to this list by turning on additional command line options, or reduce the size of this list by specifying the desired variables individually. Consider, for example, a turbulent thermal flow simulation. The simplest AcuTrans command: acuTrans -out -to fieldview.

produces the following output:
...
acuTrans:            Nodal output vars = velocity,pressure,
temperature,eddy_viscosity,surface_y_plus,surface_film_coefficient,
wall_shear_stress
This list represents the nodal output variables that are written to disk as a result of the NODAL_OUTPUT command. However, there are also a number of additional variables that can be computed based on this list. For example, AcuTrans can compute the spatial derivatives of these variables and other quantities such as the CFL number. To trigger this behavior, the extended nodal output should be toggled on using the -extout command line option. When this is done, the list of variables grows. For example:
acuTrans -out -to fieldview -extout
produces the following output:
...
acuTrans:            Nodal output vars = velocity,
pressure,temperature,eddy_viscosity,surface_y_plus,
strain_rate_invariant_2,velocity_magnitude,
vorticity,cfl_number
When other output commands are present in the input file, such as DERIVED_QUANTITY_OUTPUT and NODAL_RESIDUAL_OUTPUT, these values are also added to the list of variables that are written to the FV-UNS file when the -extout option is active. For the case of a turbulent thermal simulation with DERIVED_QUANTITY_OUTPUT and NODAL_RESIDUAL_OUTPUT active, the list of extended output variables increases to the following:
acuTrans:            Nodal output vars = velocity,pressure,
temperature,eddy_viscosity,surface_y_plus,surface_film_coefficient,
residual_eddy_viscosity
acuTrans -out -to fieldview -extout -outv velocity,
pressure,residual_velocity,residual_pressure
...
residual_velocity,residual_pressure