# Time-averaging Surface Output

Time-averaging Surface Output in Flow360 allows you to collect statistical data for unsteady simulations, providing time-averaged values of flow field variables on geometry surfaces. This is essential for analyzing periodic or turbulent flows where instantaneous values may fluctuate significantly.

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

# Available Options

Option Description
Output fields Flow variables to time-average
Output format File format for the output files
Start step Time step at which to begin averaging
Write single file Option to write all surfaces to a single file
Save interval When to save outputs
Frequency How often to save the time-averaged output
Frequency offset Time step at which to begin saving output
Assigned boundaries Geometry surfaces to apply time-averaging to

# Detailed Descriptions

# Output fields

The flow variables that will be time-averaged in the output files.

  • Example: Cp, velocity, pressure

Note:

  • See detailed field descriptions in the Surface Output page.
  • Time-averaging is applied to each selected field independently.

# Output format

Specifies the file format for the output files.

  • Default: paraview
  • 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

Specifies the time step at which to begin the time-averaging process.

  • Default: -1 (at end of simulation)
  • Example: 5000 (start averaging at time step 5000)

Note: Set this to a time step after initial transients have settled for more meaningful statistics.

# Write single file

Controls whether all surface outputs are written to a single file instead of one file per surface.

  • Default: false
  • Options:
    • true
    • false

Note: This option currently only supports Tecplot output format.

# Save interval

Choos the points in the simulaton 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

Controls how often the time-averaged output files are generated.

  • Default: -1 (only at end of simulation)
  • Example: 100 (save time-averaged output every 100 time steps)

Note: Setting to -1 will only produce output at the end of the simulation.

# Frequency offset

Specifies the time step at which to begin saving the time-averaged output.

  • Default: 0 (at beginning of simulation)
  • Example: 1000 (start saving output at time step 1000)

Note: This setting works in conjunction with Frequency to control the time steps at which output is produced.

# Assigned boundaries

The geometry surfaces where time-averaging will be applied.

  • Example: wing, fuselage, all_walls

Note: You can specify individual surfaces or use pattern matching (e.g., "wall*") to select multiple surfaces.

💡 Tips
  • Averaging window: Ensure your simulation runs long enough after the Start Step to collect meaningful statistics
  • Transient effects: Set Start Step beyond the initial transient period
  • Periodic flows: For flows with known periodicity, average over multiple complete cycles
  • Flow stability: Monitor convergence of time-averaged quantities to ensure statistical stability

# When visualizing time-averaged surface output, consider:

  • Comparing time-averaged and instantaneous fields to identify regions of high unsteadiness
  • Using time-averaged fields to calculate stable performance metrics
  • Examining the difference between time-averaged and steady-state solutions for similar conditions
  • Creating streamlines based on time-averaged velocity fields to reveal mean flow patterns

❓ Frequently Asked Questions

  • How is time-averaging calculated?

    Time-averaging is calculated using an average formula:

    avg_new = avg_old + (new_value - avg_old) / n

    Where:

    • avg_new is the updated time-average
    • avg_old is the previous time-average
    • new_value is the instantaneous value at the current time step
    • n is the number of samples since the start of averaging

  • What's the difference between time-averaging and animation?

    • Animation: Captures instantaneous "snapshots" of the flow at specific time steps (controlled by Frequency)
    • Time-averaging: Produces statistical averages over many time steps, reducing noise and revealing mean flow patterns

  • When should I use time-averaging?

    Time-averaging is particularly useful for:

    • Turbulent flows where instantaneous values fluctuate significantly
    • Periodic flows where cycle-to-cycle statistics are of interest
    • Calculating mean aerodynamic performance metrics in unsteady simulations
    • Identifying quasi-steady flow features in dynamically changing simulations

🐍 Python Example Usage

 
# Example of setting up time-averaged surface output in Python
time_avg_output = fl.TimeAverageSurfaceOutput(
    name="Wing and Fuselage Time Avg",  # Descriptive name for this output
    output_format="paraview",
    output_fields=["primitiveVars", "Cp"],
    entities=[
        volume_mesh["wing"],
        volume_mesh["fuselage"],
    ],
    start_step=1000,  # Start averaging after time step 1000
    frequency=100,    # Output every 100 time steps
    frequency_offset=1500,  # Start outputting at time step 1500
)

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