This topic describes some of the common problems in turbomachinery simulations.
Results do not match test data
- Verify that the analysis settings (pressure, flow, rpm, fluid) represent test conditions.
- Verify that the simulation geometry represents the actual geometry.
- Verify that torque, pressure, and flow have reached a steady-state solution (stopped changing). If not, run additional time steps.
- Refine the mesh throughout the model, and reduce time step size.
The maximum velocity exceeds the impeller tip speed or the solution diverges
In the typical pump or fan analysis, the highest velocity should be the tip velocity of the impeller (v = radius x rotational speed). If the fluid velocity exceeds the tip velocity, there may be a problem in the simulation which can lead to divergence.
Assess the problem
The first step is to identify where the peak velocity is and how much fluid is moving at this velocity. To do this, create an iso surface showing Velocity Magnitude, and move the slider to the right. If the maximum velocity is localized to a small region, it may not affect the rest of the results field, and can be ignored.
If there is a large region of high velocity flow, check the model carefully:
Check the mesh
- Ensure that nodal aspect ratios within the rotation region are below 100. To check aspect ratio, enable Stream Function in the Result quantities dialog (on the Control tab of the Solve dialog), and run 0 iterations. Create an iso surface to visualize Nodal Aspect Ratio.
- Inspect the mesh in critical areas such as blade leading edges, the volute tongue, and the blade suction surfaces. Refine the mesh as necessary.
Change the solution settings
- On the Control tab of the Solve quick edit dialog, click Solution control.
- Slide the Velocities and Pressure sliders to 0.25.
- Try running again.
- If problem still persists, change Compressibility to Compressible on the Physics tab of the Solve quick dialog.
- This helps to stabilize the solution by adding an additional term to the flow equations.
- If the fluid density is constant, the flow is considered incompressible.