CFD Calculation Instability Due to High Velocity and Small Diffuser Areas

In certain CFD simulations, instability can occur when discharge velocities are high (eg. 8 m/s) and diffuser areas are small (eg 0.04 m2). 

This instability is typically caused by rapid changes in dependent variables, which can lead to convergence issues or oscillations in the results. 

- Recommended Solution:

A common way to mitigate instability in these situations is to slow down the rate at which dependent variables change. This can be achieved by reducing the velocity false time steps parameter in the solver settings.

In one tested case, reducing the velocity false time steps value to 0.005 significantly improved stability. The cost of increased stability is a reduction in the speed of solution. 

- Steps to Apply the Fix:

1. Start a CFD calculation by clicking the Run CFD Calculation toolbar icon 
2. At the CFD Calculation Options, open the Dependent Variable Control Settings header 
3. On the X-Axis Velocity set the False time step to 0.005 (or lower if still unstable). 
4. Repeat the step above for the Y-Axis and Z-Axis
5. Re-run the simulation and monitor stability and convergence.
6. Adjust further if necessary, balancing stability with computational time.

As a general rule of thumb, the grid line merge tolerance should not exceed 10% of the mean cell dimension, which helps improve model stability and calculation accuracy. Eg. a 0.3m grid size will be set with a 0.03m merge tolerance.