Thermal Field¶

The Thermal Field statistics provide time-dependent, reduced-order data describing the evolution of thermal fields within the simulation domain. These statistics describe the evolution of thermal energy within the fluid and quantify how heat is generated, transferred, and distributed throughout the system. These quantities capture both internal sources of heating—such as viscous dissipation—and external heat transfer mechanisms—including user-defined sources, regional heating, and surface fluxes. The name of this file is ThermalField.txt. For immiscible two-fluid systems with separate thermal fields defined for each fluid phase, however, the solver produces two independent thermal statistics files, ThermalField1.txt and ThermalField2.txt. These files correspond to Fluid 1 and Fluid 2, respectively.

The output files are updated at the Statistics Output Write Interval. These outputs complement 3D spatial Plane (Slice) and Volume datasets by providing time-resolved summaries suited for convergence analysis, system-level characterization, and comparison across operating conditions.

This file should be used in tandem with the Thermodynamics Statistics output page, which reports global thermal energy quantities such as total power input, viscous dissipation, and volumetric heating contributions. Additional heat transfer detail can also be obtained from the individual Static Body Statistics, which report the convective heat transfer rate associated with each static body family that has an active thermal boundary condition. For multiphase systems using the Immiscible Two-Fluid Model with interfacial heat transfer enabled, users should additionally examine the Immiscible Thermal Transfer Statistics, which quantify heat exchange occurring across phase interfaces.

Statistics Table¶

The index table below shows the statistics that can appear in the Thermal Field Output file. Within this table, each statistic corresponds to a column in the output table that evolves with the time column.

Statistics	Units	Details
Time	s	simulation time
Dissipation Heating	W	total fluid energy dissipation transferred to thermal field
Global Heat Transfer	W	total heat transfer rate from global source/sink
Local Heat Transfer	W	total heat transfer rate from the local heat transfer UDF
Free Surface Heat Transfer	W	total heat transfer rate from surface flux UDF
Temperature	K	spatial mean of fluid temperature

Usage and Interpretation¶

The Thermal Field statistics should be interpreted as a global energy balance, where the temperature evolution of the fluid is governed by the net heat transfer into or out of the system. The simulation time \(t\) provides the temporal reference for all reported quantities.

The most important quantity is the Global Heat Transfer, which represents the total net heat transfer to or from the fluid. This term directly drives changes in the bulk temperature of the system. The relationship between heat transfer and temperature change is given by

\[\dot{Q}_{\text{global}} = \frac{d}{dt} \int_{\Omega} \rho(x)\, c_p\, T(x)\, dV,\]

where \(𝜌\) is density, \(c_p\) is specific heat, and \(T\) is temperature. In a well-mixed system, this simplifies to

\[\dot{Q}_{\text{global}} \approx \rho\, c_p\, V \, \frac{d\langle T \rangle}{dt},\]

showing that global heat transfer directly controls the rate of change of the mean temperature. The contributions to this term from each individual static body are reported separately in the Static Body Output files.

The Region Heat Transfer describes the heat transfer within a specific Heating Region, computed as

\[Q_{\text{region}} = \int_{\Omega_{\text{region}}} q_{\text{region}}(x)\, dV,\]

and is used for isolating contributions from each specific heating region.

The Dissipation Heating term represents internal heat generation due to viscous effects, computed as

\[\dot{Q}_{\text{diss}} = \int_{\Omega} \rho(x)\, \varepsilon(x)\, dV,\]

and contributes positively to the global heat balance. In high-shear or turbulent systems, this can be a significant source of heating.

The Local Heat Transfer represents heat addition or removal defined through the Local Heat Transfer UDF. It is computed as

\[Q_{\text{local}} = \int_{\Omega} q_{\text{local}}(x)\, dV.\]

The Surface Heat Transfer is the total heat exchange across the free surface,

\[Q_{\text{surface}} = \int_{A} \mathbf{q}''(x)\cdot \mathbf{n}\, dA.\]

The Temperature metric represents the spatially averaged fluid temperature,

\[\langle T \rangle = \frac{1}{V} \int_{\Omega} T(x)\, dV,\]

which reflects the cumulative effect of all heating and cooling processes.

This file should be used in tandem with the Thermodynamics Statistics output page.