Probe¶
The Probe statistics provide time-dependent data sampled at a user-defined location within the simulation domain. Probe outputs are used to monitor the temporal evolution of local flow quantities at a point in space, enabling analysis of transient behavior, local fluctuations, steady-state convergence, and time-averaged conditions. The probe location may be fixed or moving. All reported quantities are evaluated at the instantaneous probe location. A tab-separated ASCII .txt file is created for each Probe. The name of this file is Probe_{DynamicName}.txt, where the dynamic name corresponds to name of the Probe in the Model Tree. The output files are updated at the Statistics Output Write Interval.
The reported quantities fall into several categories:
Kinematic Metrics: Velocity and vorticity (including component-wise values) describe the local motion and rotation of the fluid at the probe location.
Turbulence and Dissipation Metrics: Energy dissipation rate, turbulent kinetic energy, and sub-grid turbulent viscosity characterize resolved and modeled turbulence behavior.
Stress and Deformation Metrics: Strain rate and resolved shear stress quantify local deformation and stress within the fluid.
Thermodynamic and Material Properties: Pressure, density, viscosity, and temperature describe the physical state of the fluid.
Multiphase and Particle Metrics: Fluid, particle volume fractions, and particle-set kLa provide information on local phase distribution and interphase transport.
Scalar and Custom Variable Metrics: Scalar field values and user-defined global variables allow tracking of transported quantities and custom model outputs.
Spatial and Probe Motion Metrics: Probe position and probe velocity describe the location and motion of the sampling point when probes are attached to moving geometries.
Age Metrics: Mean age tracks residence time behavior of the fluid at the probe location.
Time-Averaged Metrics: Time-averaged quantities provide smoothed representations of local behavior for steady-state or statistically converged analysis.
Statistics Table¶
The index table below shows the statistics that can appear in the Probe output file. Within this table, each statistic corresponds to a column in the output table that evolves with the time column.
Statistics |
Units |
Details |
When Appears |
|---|---|---|---|
Time |
s |
simulation time |
|
Age |
s |
fluid mean age |
|
Avg Turb KE |
J/kg |
time-averaged turbulent kinetic energy |
|
Custom Variable |
[dynamic] |
custom variable magnitude |
|
Custom Variable |
[dynamic] |
custom variable magnitude |
|
Custom Variable X |
[dynamic] |
custom variable value |
|
Custom Variable Y |
[dynamic] |
custom variable value |
|
Custom Variable Z |
[dynamic] |
custom variable value |
|
Density |
kg/m^3 |
density after accounting for multiphase, particles, bubbles, and scalar fields |
|
Energy Dissipation Rate |
W/kg |
energy dissipation rate including both resolved and unresolved components |
|
Fluid Viscosity |
m^2/s |
fluid kinematic viscosity |
|
Fluid Volume Fraction |
vf |
fluid volume fraction |
|
Particle Set kLa |
1/s |
kLa for particle set |
|
Particle Set Volume Fraction |
vf |
volume fraction for particle set |
|
Position X |
m |
position |
|
Position Y |
m |
position |
|
Position Z |
m |
position |
|
Pressure |
Pa |
pressure |
|
Probe Velocity Magnitude |
m/s |
probe velocity magnitude |
|
Probe Velocity X |
m/s |
probe velocity |
|
Probe Velocity Y |
m/s |
probe velocity |
|
Probe Velocity Z |
m/s |
probe velocity |
|
Resolved Shear Stress |
Pa |
resolved shear stress magnitude |
|
Resolved Strain Rate |
1/s |
strain rate magnitude not including unresolved strain |
|
Scalar Field |
[dynamic] |
scalar field value |
|
Sub-Grid Turbulent Viscosity |
m^2/s |
sub-grid turbulent viscosity from LES model |
|
Temperature |
K |
fluid temperature |
|
Time-Avg Energy Dissipation Rate |
W/kg |
time-averaged energy dissipation rate including both resolved and unresolved components |
|
Time-Avg Pressure |
Pa |
time-averaged pressure |
|
Time-Avg Resovled Shear Stress |
Pa |
time-averaged resolved shear stress magnitude |
|
Time-Avg Strain Rate |
1/s |
time-averaged strain rate magnitude |
|
Time-Avg Velocity |
m/s |
time-averaged fluid velocity magnitude |
|
Time-Avg Velocity Magnitude |
m/s |
time-averaged fluid velocity magnitude |
|
Time-Avg Velocity X |
m/s |
time-averaged fluid velocity |
|
Time-Avg Velocity Y |
m/s |
time-averaged fluid velocity |
|
Time-Avg Velocity Z |
m/s |
time-averaged fluid velocity |
|
Velocity |
m/s |
magnitude of fluid velocity |
|
Velocity Magnitude |
m/s |
magnitude of fluid velocity |
|
Velocity X |
m/s |
fluid velocity |
|
Velocity Y |
m/s |
fluid velocity |
|
Velocity Z |
m/s |
fluid velocity |
|
Vorticity |
1/s |
vorticity magnitude |
|
Vorticity Magnitude |
1/s |
vorticity magnitude |
|
Vorticity X |
1/s |
vorticity |
|
Vorticity Y |
1/s |
vorticity |
|
Vorticity Z |
1/s |
vorticity |
Usage and Interpretation¶
A Probe is most useful when the objective is to follow how a quantity changes in time at a specific point rather than how it varies in space. The Probe statistics provide time-dependent data sampled at a user-defined location within the simulation domain. The probe location may be fixed or moving. These outputs are used to track the temporal evolution of local flow quantities at a point in space. This enables analysis of transient behavior, local fluctuations, steady-state convergence, and time-averaged conditions. Probe data are also commonly used for lifeline analysis, where quantities are monitored along the trajectory of a moving probe to understand the local environment experienced by a fluid parcel or particle over time. All reported quantities are evaluated at the instantaneous probe location. This is in contrast to an Output Line, which samples data along a spatial path at each write time.
The Probe output includes instantaneous fluid quantities, custom variables, and time-averaged fluid values. It can also include particle-related statistics, such as local particle-set \(kLa\), as well as probe velocity quantities when the probe itself is moving.
Because a Probe represents a single point sample, it should be interpreted as a localized measurement. Sharp gradients, nearby interfaces, multiphase boundaries, or transient structures can cause the probe signal to vary significantly over time. For that reason, probe placement matters. In practice, probes are often used in groups so that multiple locations can be monitored simultaneously and compared over the same simulation interval. Time histories from several probes can reveal circulation patterns, propagation delays, mixing progression, or local differences in turbulence intensity and dissipation. This is especially useful when diagnosing whether a system has reached steady or statistically steady behavior.