What is a “Vortex Across a Pressure Boundary?”

In Flow Simulation, a vortex is a region in the fluid domain which causes a swirl in a region where there is asymmetric drag in the flow field.  The vortex itself is an expected phenomenon which itself is not problematic.  When that vortex is allowed to generate across a theoretical boundary within a CFD analysis that can cause the results to deviate from reality in the immediate vicinity of the boundary or also cause the solver to fail to produce results at all.  For that reason, it is important to note where this is happening in an analysis and take steps to avoid it.

How can this be fixed?

The vortex itself is generating because of the local solid geometry near the pressure boundary of a CFD setup.  If the flow through the boundary is not symmetric, a low-pressure region can generate in front of the boundary allowing fluid to pass the wrong direction through the boundary as intended.  The fix for this is to “build out” the model geometry.  What does this mean? The solid model needs to have more real life geometry added to the setup so the flow field can be allowed to have the vortex and then transition into a unidirectional flow.

Solution 1: Add Geometry

An example of a vortex across a boundary would be directly from the first Flow Simulation tutorial in SOLIDWORKS.

(The tutorials can be found under ‘Help’, ‘SOLIDWORKS Simulation’, ‘Flow Simulation Online Tutorial’ once the Flow Simulation add-in is turned on).  The ball valve, as it is setup in the tutorial, has two lids that are positioned closely to the ball of the valve.  In situations where the ball valve is not set completely open the flow through the valve is forced to be asymmetric as it passes through the pressure outlet.

The asymmetric flow out the pressure boundary allows fluid to backflow through the theoretical pressure boundary and creates the vortex that is seen below. (more…)