[Wolfram]

Dear all,

i am simulating a mixed flow fan with a diameter of approx. 250mm, around 3100RPM, steady simulation, frozen rotor and k-epsilon turbulence model. The domains and boundaries are shown below:
https://www.bilder-upload.eu/bild-26...25032.jpg.html
I am comparing the simulation results with experimental results from our test bench. The fans inlet is installed at the test chamber, and the exhaust is at free air.

Here is the issue:
When i start the simulation with low volume flow rates (200m^3/hr) the simulation does not converge with a timestep of 1. Especially within the rotating region i can find high "MAX Residuals". After playing around with a higher timestep of around 30, the simulation starts converging and the pressure rise of 418Pa show a good matching with the performance curve. Using this converged solution for runs with a higher volume flow rate (1600m^3/hr), the simulation is also converging and the pressure rise with 36Pa is close to the test bench result. You can find the mentioned operation points as blue dots in the graph below.

When i start the simulation with high volume flow rates (1600m^3/hr) without the previous converged solution, the simulation is not converging at at. Especially within the rotating region the high "MAX Residuals" are not decreasing and the pressure rise 116Pa is far away from the test bench results. Interestingly the "not converged" solution with high "MAX Residuals" in the rotating region shows a good matching with the performance curve relating to the pressure rise for low volume flow rates (200m^3/hr).You can find the mentioned operation points as red dots in the graph below.
https://www.bilder-upload.eu/bild-23...26090.jpg.html

The flow pattern is different, too. With high volume flow rates the fluid velocity in the "not converged solution" is much high in the outer spans than in the "converged solutions". The bigger tip vortex for the "not converged" solution is shown below on the right side.


Here you can see the residuals with 1600m^3/hr for the "not converged" solution:
https://www.bilder-upload.eu/bild-e1...28098.jpg.html

Here you can see the residuals with 1600m^3/hr for the "converged" solution (same setup and mesh as above. The only difference are the inital values as discribes at the beginning of this post):
https://www.bilder-upload.eu/bild-b1...28200.jpg.html


I have tried tons of different meshes from 5 mio. elements to 70 mio elements. Furthermore als kinds of plausible boundaries and a huge range of domain sizes.

I would be really grateful for your advices to fix this issue.

Kind regards from germany

Wolfram