# Time-averaging Isosurface Output

Time-averaging Isosurface Output in Flow360 lets you visualize the time-averaged flow field variables on surfaces of constant value within the computational domain. This is particularly useful for identifying and analyzing average flow structures like vortices, shock waves, or temperature contours.

Note: Time-averaging outputs are only available when using unsteady time stepping.

# Available Options

Option	Description	Applicable
Output fields	Flow variables to include in the output	always
Output format	Format for saving isosurface data	always
Start step	Physical time step from which to start the time average	always
Save interval	When to save outputs	always
Frequency	How often to save outputs	when Save interval is `Custom`
Frequency offset	Time step at which to start the output animation	when Save interval is `Custom`
Isosurfaces	Define one or more isosurfaces	always

# Detailed Descriptions

# Output fields

Select the flow variables to include in the isosurface output.

Default: None (user must select at least one field)
Example: Mach, pressure, velocity

Note: Select only the fields you need for your analysis. See Available Output Fields for a complete list of supported variables.

# Output format

The file format used to save the isosurface output data.

Default: paraview
Example: both
Options:
- paraview
- tecplot
- both

Notes:

Choose the format that best suits your post-processing workflow.

Select paraview for .vtu format, tecplot for .plt format, or both to save in both formats.

# Start step

The physical time step at which time-averaging begins.

Default: -1 (at end of simulation)
Example: 100 (begin averaging from the 100th physical time step)

Note: Use positive integers to start averaging from a specific time step.

# Save interval

Choose the points in the simulation where the results are saved.

Default: Save at end
Options:
- Save at end
- Custom

Notes:

Choose Save at end to save only the final results of the simulation.

Choose Custom to save the results in given intervals.

# Frequency

How often to save outputs, in number of physical time steps.

Default: -1 (only at the end of simulation)
Example: 100

Note: Higher frequencies provide better temporal resolution but increase storage requirements.

# Frequency offset

The time step at which to start the output animation.

Default: 0 (beginning of simulation)
Example: 1000

Note: Useful when you want to skip initial transient flow development.

# Isosurfaces

Define one or more isosurfaces by specifying a field variable and iso-value.

Default: None (user must select at least one field)
Definition parameters:
- Name: A unique identifier for the isosurface
- Field: The flow variable used to define the isosurface
- Iso-value: The constant value of the field variable that defines the isosurface
Examples:
- name: qcrit-iso, Field: qcriterion , Iso-value: 0.001
- name: atm-pressure, Field: pressure_with_units , Iso-value: 1013000 (Unit system: SI)

💡 Tips

# When to Use Time-Averaged Outputs

Use time averaging when you need statistical flow behavior rather than instantaneous snapshots
Particularly useful for unsteady flows with periodic or chaotic behavior
Effective for filtering out noise and revealing underlying flow patterns

# Selecting the Right Start Step

Set Start Step after initial transients have passed for more meaningful averages
Monitor convergence of your unsteady simulation to determine appropriate Start Step
Too early: averages may include non-physical startup transients
Too late: may miss important physical phenomena

# Setting Appropriate Frequency

For long simulations, use higher Frequency values to reduce storage requirements
For detailed time evolution of averages, use lower Frequency values
Balance between data resolution and file size

❓ Frequently Asked Questions

When should I use time-averaged outputs instead of regular outputs?

Time-averaged outputs are ideal for unsteady simulations where you want to analyze the statistical behavior of the flow rather than instantaneous values. They help filter out noise and reveal underlying flow patterns.
Can I have both time-averaged and instantaneous outputs for the same isosurface?

Yes, you can define both regular isosurface outputs and time-averaged isosurface outputs for the same points to monitor both instantaneous and time-averaged values.
How does the averaging calculation work?

Flow360 calculates an average from the Start Step specified until the end of the simulation. The average is computed and saved at the frequency specified.
Does time-averaging increase computational cost?

Time-averaging adds minimal computational overhead as it's primarily a post-processing operation, but it does require additional memory to store the averaged values.
What happens if I set Start Step to -1?

Setting Start Step to -1 means the time-averaged output will only be calculated and saved at the end of the simulation, providing a single averaged value over the entire simulation (after any specified start step).
Can I use time-averaging with steady simulations?

No, time-averaged outputs are only available when using unsteady time stepping methods, as they require time evolution to calculate meaningful averages.

🐍 Python Example Usage

# Example of configuring isosurface output using Flow360 Python API
import flow360 as fl
from flow360 import u

# Define multiple isosurfaces with different fields and values
my_temp = fl.UserVariable(name="my_temp", value=fl.solution.temperature).in_units(new_unit='K')
time_averaging_isosurfaces_output = fl.TimeAverageIsosurfaceOutput(
  isosurfaces=[
      fl.Isosurface(name="q_criterion_avg", field="qcriterion", iso_value=0.0004128),
      fl.Isosurface(name="q_criterion_avg", field=my_temp, iso_value=300*fl.u.K)
  ],
  start_step=420,
  output_fields=["velocity_magnitude"],
  output_format="both",
)

# Add your time-averaged isosurface output to simulation parameters
simulation_params = fl.SimulationParams(
    # ... other simulation parameters ...
    outputs=[time_averaging_isosurfaces_output]
)

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