Diverging Solutions—Boundary Condition Issues

Issue

This solution describes two commonly-occurring issues regarding the placement of pressure boundary conditions at the model outlet. Both can lead to solution divergence, and careful attention should be applied to how boundary conditions are applied to prevent these issues.

Problem 1

Issue:

Pressure boundary conditions allow the fluid pass both into and out of the model. 

This can become problematic when the pressure boundary condition is specified close to geometry features that cause the flow to swirl or recirculate. If the condition is too close to the recirculation, it can grow unbounded and lead to solution divergence. 

Solution:

To resolve this, place an extension on the model at the opening. The extension, while not a part of the physical device, should be long enough for flow recirculation to be completely resolved, and the flow to exit through the pressure condition in a uniform manner.

Problem 2

Issue: 

Flow recirculation zones that can lead to solution divergence occur when pressure boundary conditions are applied to adjacent surfaces that meet at a corner.

Solution: 

All nodes on a surface with a specified pressure boundary condition are constrained at the specified pressure. This is not physically possible across the sharp corner of adjoining surfaces.

Extend the openings to eliminate the recirculation and apply the pressure boundary condition to only one surface of the extension.

Typically the pressure loss due to the extension is negligible when compared to the entire model. If this is of concern, extract pressure values from the actual physical opening (instead of at the end of the extension) using a results plane and the Bulk Calculator.