# 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-averaged outputs are only available when using unsteady time stepping methods.

# 📋 Available Options

Option Description Unit
Name Name identifier for the time-averaging output
Start Step Time step at which to begin averaging time step
Frequency How often to save the time-averaged output time steps
Frequency Offset Time step at which to begin saving output time step
Output Format File format for the output files
Output Fields Flow variables to time-average
Surfaces Geometry surfaces to apply time-averaging to
Write Single File Option to write all surfaces to a single file

# 🔍 Detailed Descriptions

# Name

An identifier for the time-averaging output configuration.

  • Default: "Time average surface output"
  • Example: "Wing Time-Avg Output"
  • Notes: A descriptive name helps identify this specific output in logs and results.

# 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)
  • Notes: Set this to a time step after initial transients have settled for more meaningful statistics.

# 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)
  • Notes: 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)
  • Notes: This setting works in conjunction with Frequency to control the time steps at which output is produced.

# Output Format

Specifies the file format for the output files.

  • Default: paraview
  • Options: paraview, tecplot, or both
  • Notes: Choose the format that best suits your post-processing workflow.

# Output Fields

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

  • Example: Cp, velocity, pressure
  • Notes: Time-averaging is applied to each selected field independently.

# Surfaces

The geometry surfaces where time-averaging will be applied.

  • Example: wing, fuselage, all_walls
  • Notes: You can specify individual surfaces or use pattern matching (e.g., "wall*") to select multiple surfaces.

# Write Single File

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

  • Default: false
  • Options: true or false
  • Notes: This option currently only supports Tecplot output format.

💡 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 a running 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
)

← Slice Output →