Understanding ambient conditions

In fluid modeling, ambient conditions define flow properties for the flow that enters and exits the flow domain, including humidity, tracer fluid, and mixture.

In radiative modeling, ambient conditions define the heat exchange environment for a radiation domain.

You specify the ambient conditions for fluid and radiative modeling on the Ambient Conditions page in the Solution dialog box.

Ambient pressure

When you use the ambient pressure as the external condition for the Opening type of the Flow Boundary Condition simulation object, the flow solver models:

• The fluid entering the flow domain through an opening at the total specified pressure.

• The fluid leaving the flow domain through an opening at the specified static pressure.

You can define ambient pressure either as absolute pressure for liquids and gases or in terms of the altitude to analyze the flow of gases at a specific altitude.

You can define a constant or a time-varying ambient absolute pressure and use fields to do this. Select an existing field, or use a linked field or table field. To use expressions to define the relationship between pressure and time, use a formula field. For example, you can define the absolute pressure as a symbolic expression that contains condition sequence parameters.

For the ambient pressure defined in terms of altitude:

• The solver assumes that the density of the ambient fluid is the density at sea level, and adjusts the pressures according to the specified altitude value using the ideal gas law. You specify the density of the ambient fluid as a material property.

• The solver adjusts all fan curve pressures for fan curves that are not defined at an opening. You should still define fan curves at standard temperature and pressure.

Ambient fluid temperature

When you define the fluid ambient temperature:

• The fluid enters the flow domain through an inlet or an opening at the specified ambient fluid temperature, unless you define another temperature for the external condition of the inlet or the opening.

• Fluid that leaves the domain is unaffected.

You can also use the ambient fluid temperature as the environment temperature for:

• A Convection to Environment constraint

• A Simple Radiation to Environment constraint

Ambient radiative environment temperature

The ambient radiative environment temperature defines the temperature that the thermal solver uses to calculate heat exchange with the environment, when you define the following:

• A Radiation simulation object

• A Convection to Environment constraint

• A Simple Radiation to Environment constraint

Altitude varying pressure and temperature

You can define absolute pressure, fluid temperature, and radiative environment temperature as functions that vary with altitude. When you also define the altitude that varies with time, the software converts the functions for absolute pressure, fluid temperature, and radiative environment temperature to vary with time instead of altitude.

Ambient conditions and customer defaults

You can use customer defaults to set the following default values in the Solution dialog box.

• Altitude sets the default altitude for your analysis.

• Absolute Pressure sets the default absolute pressure.

• Fluid Temperature sets the default ambient fluid temperature.

• Radiative Environment Temperature sets the default ambient radiative environment temperature.

Tip:

To find a customer default, choose File→Utilities→Customer Defaults, and click Find Default .

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