9.4.4 Floating Operating Pressure

ANSYS FLUENT provides a "floating operating pressure'' option to handle time-dependent compressible flows with a gradual increase in the absolute pressure in the domain. This option is desirable for slow subsonic flows with static pressure build-up, since it efficiently accounts for the slow changing of absolute pressure without using acoustic waves as the transport mechanism for the pressure build-up.

Examples of typical applications include the following:

• combustion or heating of a gas in a closed domain

• pumping of a gas into a closed domain

Limitations

The floating operating pressure option should not be used for transonic or incompressible flows. In addition, it cannot be used if your model includes any pressure inlet, pressure outlet, exhaust fan, inlet vent, intake fan, outlet vent, or pressure far field boundaries.

Theory

The floating operating pressure option allows ANSYS FLUENT to calculate the pressure rise (or drop) from the integral mass balance, separately from the solution of the pressure correction equation. When this option is activated, the absolute pressure at each iteration can be expressed as

where $p$ is the pressure relative to the reference location, which in this case is in the cell with the minimum pressure value. Thus the reference location itself is floating.

$p_{\rm op,float}$ is referred to as the floating operating pressure, and is defined as

where $p^0_{\rm op}$ is the initial operating pressure and $\Delta p_{\rm op}$ is the pressure rise.

Including the pressure rise $\Delta p_{\rm op}$ in the floating operating pressure $p_{\rm op,float}$, rather than in the pressure $p$, helps to prevent roundoff error. If the pressure rise were included in $p$, the calculation of the pressure gradient for the momentum equation would give an inexact balance due to precision limits for 32-bit real numbers.

Enabling Floating Operating Pressure

When time dependence is active, you can turn on the Floating Operating Pressure option in the Operating Conditions dialog box.

Boundary Conditions Operating Conditions...

(Note that the inputs for Reference Pressure Location will disappear when you enable Floating Operating Pressure, since these inputs are no longer relevant.)

The floating operating pressure option should not be used for transonic flows or for incompressible flows. It is meaningful only for slow subsonic flows of ideal gases, when the characteristic time scale is much larger than the sonic time scale.

Setting the Initial Value for the Floating Operating Pressure

When the floating operating pressure option is enabled, you will need to specify a value for the Initial Operating Pressure in the Solution Initialization task page.

Solution Initialization

This initial value is stored in the case file with all your other initial values.

Storage and Reporting of the Floating Operating Pressure

The current value of the floating operating pressure is stored in the data file. If you visit the Operating Conditions dialog box after a number of time steps have been performed, the current value of the Operating Pressure will be displayed.

Note that the floating operating pressure will automatically be reset to the initial operating pressure if you reset the data (i.e., start over at the first iteration of the first time step).

Monitoring Absolute Pressure

You can monitor the absolute pressure during the calculation using the Surface Monitor dialog box (see Section  26.13.4 for details). You can also generate graphical plots or alphanumeric reports of absolute pressure when your solution is complete. The Absolute Pressure variable is contained in the Pressure... category of the variable selection drop-down list that appears in postprocessing dialog boxes. See Chapter  31 for its definition.