Time-averaging Volume Output

Time-averaging Volume Output#

Time-averaging Volume Output in Flow360 allows you to calculate and visualize time-averaged flow variables throughout the entire computational domain. This is essential for statistical analysis of unsteady flows and understanding mean flow characteristics.

Available Options#

Option

Description

Applicable

Output fields

Flow variables to include in the output

always

Output format

Format for saving volume data

always

Start step

When to begin time-averaging

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

Global Time Stepping in Child Cases#

When working with child cases (cases forked from a parent simulation), it’s important to understand that the Frequency, Frequency offset, and Start step parameters refer to the global time step, which is transferred from the parent case.

Example: If the parent case finished at time_step=174, the child case will start from time_step=175. If Frequency=100 is set in the child case, the output will be saved at global time steps 200 (25 time steps into the child simulation), 300 (125 time steps into the child simulation), etc. Frequency offset also refers to the global time step, meaning that if in the previously mentioned child case, Frequency offset=50 was set (with Frequency=100), the output would be saved at global time steps 250 (75 time steps into the child simulation), 350 (175 time steps into the child simulation), etc.

Detailed Descriptions#

Output fields#

Select the flow variables to include in the volume output.

  • Default: None

  • Example: Mach, pressure, velocity

Notes:

  • See detailed field descriptions in the Volume Output page.

  • Only select fields you need to analyze to keep file sizes manageable.

Output format#

The file format used to save the volume output data.

  • 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 physical time step to start calculating time averaging.

  • Default: -1 (automatic detection)

  • Example: 50

Notes:

  • Set this to begin averaging after initial transients have died out. When set to -1, the solver will automatically determine when to start averaging based on flow convergence.

  • Important for child cases - this parameter refers to the global time step, which gets transferred from the parent case (see Global Time Stepping).

Save interval#

Choose 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#

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

  • Default: 1

  • Example: 100 — saves output every 100 physical time steps.

    • Standalone case: If you start a simulation from time_step=0 with frequency=100, outputs are saved at time steps 100, 200, 300, etc.

    • Parent-child case: If the parent finished at time_step=174, the child starts from time_step=175. With frequency=100 in the child, outputs are saved at global time steps 200 (25 steps into child), 300 (125 steps into child), 400 (225 steps into child), etc.

Notes:

  • -1 saves only at end of simulation.

  • Higher frequencies provide better temporal resolution but increase storage requirements.

  • Important for child cases - this parameter refers to the global time step (see Global Time Stepping).

Frequency offset#

The time step at which to start the output animation.

  • Default: 0 (beginning of simulation)

  • Example: 1000 — with frequency=100, outputs are saved at time steps 1000, 1100, 1200, etc.

    • Standalone case: If you start a simulation from time_step=0 with frequency=100 and frequency_offset=1000, outputs are saved at time steps 1000, 1100, 1200, etc.

    • Parent-child case: If the parent finished at time_step=174, the child starts from time_step=175. With frequency=100 and frequency_offset=200 in the child, outputs are saved at global time steps 200 (25 steps into child), 300 (125 steps into child), 400 (225 steps into child), etc.

Notes:

  • Useful when you want to skip initial transient flow development.

  • Important for child cases - this parameter refers to the global time step (see Global Time Stepping).

💡 Tips

Time-averaged volume output is particularly useful in the following scenarios:

  1. Analyzing unsteady flows: For flows with inherent unsteadiness (like vortex shedding, turbulent wakes, or separated flows), time-averaging provides mean flow statistics.

  2. Reducing storage requirements: Instead of saving many instantaneous snapshots, you can capture the statistical behavior with a single time-averaged result.

  3. Flow stability assessment: Time-averaged results help determine if a flow has reached a statistically steady state, even if instantaneous values are fluctuating.

  4. Turbulence analysis: For turbulent flows, time-averaging allows you to distinguish between mean flow patterns and turbulent fluctuations.

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

Performance Considerations

Time-averaged volume outputs offer several advantages over series of instantaneous outputs:

  1. Reduced storage requirements: Instead of storing many time steps, you can capture statistical behavior in a single file.

  2. Better statistical representation: Time-averaging naturally filters out numerical noise and transient features.

  3. Computational overhead: Time-averaging requires additional memory during the simulation to accumulate statistics, but this is typically minimal compared to the overall simulation cost.

  4. Convergence monitoring: Monitor the convergence of time-averaged quantities to determine when your statistics have become stable.

❓ Frequently Asked Questions

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

    Time-averaged outputs are ideal when:

    • You’re analyzing statistically steady turbulent flows

    • You need to filter out transient features or numerical noise

    • You want to reduce storage requirements while still capturing overall flow behavior

    • You’re interested in mean flow characteristics rather than instantaneous snapshots

    • You need statistical quantities like mean or RMS values for performance analysis

  • How does the Start Step parameter affect my results?

    The Start Step parameter controls when the solver begins accumulating statistics for time-averaging:

    • Setting it too early may include unwanted transient effects in your averages

    • Setting it too late may not give enough time for statistical convergence

    • The automatic setting (-1) detects when force coefficients begin to oscillate around a mean value

    • For better control, monitor your force history and set it manually once transients have subsided

  • Can I get fluctuation intensity or RMS values from time-averaged outputs?

    Currently, Flow360 time-averaged outputs include mean values only, not RMS or fluctuation intensities. To obtain these:

    • Export both instantaneous and time-averaged data

    • Post-process using ParaView/Tecplot to calculate RMS values

    • For turbulence quantities, consider using the k and omega fields which represent turbulent kinetic energy and specific dissipation rate

  • How long should I run my simulation to get good time-averaged results?

    For statistically meaningful time-averaged results:

    • Continue at least 5-10 characteristic time periods after starting the averaging

    • For external aerodynamics, typically 5-10 flow-through times past your geometry

    • For periodic flows (vortex shedding), at least 20-30 shedding cycles

    • Monitor key quantities (like forces) to ensure their time averages have converged

  • Do time-averaged outputs increase memory usage?

    Time-averaged outputs require additional memory during the simulation:

    • For each field being averaged, an additional array must be stored in memory

    • The memory overhead scales linearly with the number of time-averaged fields

    • On most modern systems, this is negligible compared to the overall simulation memory requirements

    • If memory is constrained, be selective about which fields to time-average

  • What’s the difference between “Start Step” and “Frequency Offset”?

    These parameters serve different purposes:

    • Start Step: When to begin accumulating data for the time average (affects the averaging calculation)

    • Frequency Offset: When to begin writing output files (affects only output timing, not the calculation)

    • Typically set Start Step based on flow physics (after transients)

    • Set Frequency Offset based on output management preferences

🐍 Python Example Usage

 
# Example of configuring time-averaged volume output
import flow360 as fl

# Define time-averaged volume output settings
time_avg_output = fl.TimeAverageVolumeOutput(
    name="Time Averaged Flow",
    output_format="paraview",
    output_fields=["Mach", "pressure", "velocity"],
    frequency=10,           # Save results every 10 time steps
    frequency_offset=100,   # Start saving at time step 100
    start_step=50           # Begin averaging at time step 50
)

# Add to simulation parameters
simulation_params = fl.SimulationParams(
    # ... other simulation parameters ...
    outputs=[time_avg_output]
)
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