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# Error: Invalid Initial and Boundary Conditions for Pressure

For compressible flows:

Warning

Found boundary conditions with pressure values that are likely invalid: Pressure boundary condition. Absolute pressure should be provided for this analysis type and settings (compressible). Please review the pressure boundary conditions because the simulation might fail.

Warning

Pressure initial conditions are likely given in gauge pressure, whereas this analysis type and settings (compressible) require absolute pressure. Please review the pressure initial conditions because the simulation might fail.

For incompressible flows:

Warning

Found boundary conditions with pressure values that are likely invalid: Pressure boundary condition. Gauge pressure should be provided for this analysis type and settings (Boussinesq). Please review the pressure boundary conditions because the simulation might fail.

Warning

Pressure initial conditions are likely given in absolute pressure, whereas this analysis type and settings (Boussinesq) require gauge pressure. Please review the pressure initial conditions because the simulation might fail.

## What Happened?

The pressure levels for the boundary or initial conditions are likely invalid for the type of analysis.

## What Could Be the Possible Reason?

This warning message may occur for the Convective heat transfer, Conjugate heat transfer and Conjugate heat transfer v2.0 analysis types.

Before going through the causes and troubleshooting practices, it’s important to remember the concept of incompressible flow. In this type of flow, as the fluid element moves in the domain, its density won’t change.

In CFD, whenever we are dealing with incompressible flows, we always define Gauge pressure values. Conversely, when modeling compressibility, the pressure levels are Absolute. It’s important to follow this principle when setting up initial and boundary conditions.

Therefore, the user will receive a warning message if one of these criteria are met for boundary or initial conditions:

• Conjugate heat transfer: $$P < 100\ Pa$$
• Convective heat transfer with Compressible toggled off: $$P \geq 100\ Pa$$
• Convective heat transfer with Compressible toggled on: $$P < 100\ Pa$$
• Conjugate heat transfer v2.0: $$P \geq 100\ Pa$$

## What Can I Do Now?

• Please review the initial and boundary conditions for pressure, making sure they are correct.
• In the case of a conventional Conjugate heat transfer analysis, always define absolute pressure levels since it always models compressibility.
• The Conjugate heat transfer v2.0 analysis type requires gauge pressure levels.
• For Convective heat transfer, there’s an option to toggle compressibility on and off in the global settings, as seen in Figure 1. This will dictate what kind of pressure values users should define.
• For incompressible flows, a usual approach is to set Gauge pressure to 0 $$Pa$$ at the outlets.
• If you are confident that the pressure levels are correct but you are still receiving the warning message, you can still proceed to run the simulation by clicking on ‘OK’:

Important Information