Initial Conditions

The Initial Conditions constraint defines approximate or known initial conditions for a specific part of the model at the start of an analysis. For the selected geometry, the value you define with an Initial Conditions constraint overrides any corresponding value you define for the whole model.

Note:

To define conditions for the whole model, use the one or more options on the Initial Conditions page in the Solution dialog box.

Create an Initial Conditions constraint to define localized initial thermal or flow conditions for a region of your model at the start of the analysis. Use this constraint in conjunction with initial thermal or flow settings defined globally on the Initial Conditions page in the Solution dialog box. The values you specify for the selected elements using the Initial Conditions constraint override any corresponding values you define globally.

You can define different initial thermal or flow conditions by selecting one of the options from theType list. For example, for modeling two-phase, immiscible fluids, use the Initial Immiscible Fluid Mixture — 3D Flow type to define the initial tracked fluid region.

Constraining initial conditions in steady-state and transient analyses

• In a steady-state analysis, the solver uses the specified initial conditions of the selected geometry or elements as a starting point for the iterative solution process. The initial conditions that you specify will not affect the final results but can greatly influence solution time. By providing the iterative solver with a good guess as to the final conditions, you can dramatically reduce the number of iterations required to reach convergence, particularly for large, nonlinear models.

• In a transient analysis, constraining initial conditions defines the physical conditions for the selected geometry or elements at the start of the simulation. These conditions appear in the analysis results as the conditions at time zero.

Selecting geometry for applying Initial Conditions

The following detail provides information on the types of geometry supported with the Initial Conditions constraint.

Geometry	Selection	Comments
Polygon body	Polygon body meshed with 3D thermal or fluid elements	Not available for the Initial Water Content — 3D Flow type
Polygon face	Polygon face meshed with 2D thermal elementsFace of a polygon body meshed with 3D thermal or fluid elements	Not available for the Initial Immiscible Fluid Mixture — 3D Flow type
Polygon edge or curve	Curve or polygon edge meshed with 1D thermal beam elements or 1D fluid duct elementsEdge of body face meshed with 2D thermal elements
Vertex or point	Mesh point or point meshed with a 0D thermal element
Non-geometric element	Non-Geometric Element modeling object	Not available for initial 3D flow conditions

Supported variables and functions

The following table lists the independent variables and auto-generated expressions for the Initial Conditions quantities that you can define using fields and expressions.

Type	Quantity	Variables	Auto-generated expressions
Initial Temperature	Temperature	Spatial variables	fluid_temperatureheat_flow_ratemass_flow_ratenxnynzpressureradiusrotational_speedtemperaturetemperature_differencethermal_capacitancethicknesstimevolume_flow_ratexyz
Initial Fluid Pressure — 3D Flow*	Pressure
Initial Fluid Velocity — 3D Flow*	Velocity
Initial Humidity — 3D Flow*	**Relative Humidity (%)**Water Vapor to Dry Air Mass Ratio
Initial Water Content — 3D Flow*	Film ThicknessTotal MassSurface Density

Note:

*The auto-generated expressions are not available for these types.

In expressions, you can also use all built-in functions that are listed in Mathematical functions in expressions and most built-in functions that are listed in Thermal-flow functions in expressions.

Where do I find it?

Application	Pre/Post
Command Finder	Initial Conditions
Simulation Navigator	Right-click the Constraint Set container node → New Constraint → Initial Conditions

How do I

Model water sloshing in a bottle

Learn more

Simulating two-phase, immiscible fluids

Thermal initial conditions

3D flow initial conditions

Inputs to expressions

Look up more details

Auto-generated expressions

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Simcenter 3D Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal boundary conditions

Thermal/Flow, Electronic Systems Cooling, and Space Systems Thermal

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Initial Conditions, Simcenter 3D 2021.1 Series

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