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Extending the Inlets and Outlets for a CFD Simulation

When performing computational fluid dynamics (CFD) simulations, properly defining the inlet and outlet boundary conditions is crucial to the success of the simulation. Why is extending the inlets and outlets so important?


There are a few main benefits of extending the inlets and outlets of a geometry before running a simulation. Extensions help ensure that the flow is fully developed as it is entering and exiting the simulation domain. Getting a uniform velocity profile along the inlet and outlet area helps with numerical convergence and more accurate results.

Around sharp turns, it is crucial to make sure the outlet boundary condition is placed far enough downstream. If not, the simulation can diverge and give incorrect results. If the outlet is placed too close to an obstacle (geometrical features causing flow expansion or contraction) the flow might undergo recirculation and will spiral back in through the outlet leading to divergence.

In the following example, we can see the benefits of extending the inlets and outlets. The velocity profile is drastically different in the first geometry but stays nearly the same as we continue to extend the faces.

CFD simulation with no extensions
Figure 1: An example CFD simulation with no inlet and outlet extension.
CFD simulation with moderate extensions
Figure 2: The same example simulation with a moderate inlet and outlet extension.
CFD simulation with large extensions
Figure 3: This time, the simulation has a large inlet and outlet extension.

Generally, ensuring an inlet length of 5 times the inlet width and an outlet length of 10 times should be sufficient. If there are still unnatural results or convergence issues, extending these faces further won’t negatively affect the final result of the simulation.

For more information about CFD simulations, be sure to check out the following pages:


Last updated: April 1st, 2021