Great that you are interested in this. It should interesting to work on this together. Great questions to ask, lets see if we can deduce anything else from the geometry from @robnaz’s answers.
I initially saw that the blades are touching the MRF and I believe they are. From the mesh it seems like a cross-section of the blade profile can be seen at the MRF. This is workable but of course, not good. If however, the MRF diameter is larger than the fan blade diameter then it is ok. But it should also be smaller than the duct diameter. Based on what you measured, it should be ok. Though it would be best to give more margin between the MRF and the fan blade diameter as you have correctly pointed out, SnappyHexMesh tends not be happy with small margins like that (4mm).
I do think your geometry fix in Rhino would help the simulation plenty. I might do some modifications myself and re-run the mesh if needed.
The length of the MRF acts similarly to that of the diameter. As long as it is lengthwise longer than that of the length of the rotors then it is fine. With some margin of course.
Yes it would, unless the support is supposed to rotate with the fan or if the support is very thin/insignificant. Then again, you can have the supports inside the MRF. Just have to assign a moving wall velocity 0 in all directions to prevent the MRF from “rotating” the supports.
MRFs provide a rotational value to the geometry within its zone (hence the need for internal meshing). With no geometry, the MRF will not be able to apply a rotation and the flow will not rotate. So in essence, the MRF doesn’t interact with the flow but rather the geometry which then rotates the flow. No worries! I had ran into alot of problems with MRFs from another project. Quite an interesting parameter.
Ok, so we just make sure that the rear circular face of the MRF falls between (with space for Snappy) the rotor and the missing ‘afterbody’ and support members. For now, I would just ignore any CAD that is the shaft which the rotor is mounted on so that we have no structure passing through the MRF.
We can just ignore the support members and have the blades and center rotating cone encased within the MRF. Results can be fine tuned with a geometry update with the supports if needed. Otherwise, magic floating prop will do
Also, for the sake of completeness, could you confirm that your results chart was actually for a rotor speed of 21.476 radians/sec, inlet flow rate was 6.75 m^3/sec and the outlet pressure was 0Pa?
We only have a margin of 0.007 (diameter wise) to work with so maybe somewhere in the middle would be a good place to start. So 1.4775? Not much of a difference though, we might need to adjust the meshing settings to try and get snappyhexmesh to not screw around.
Please pay attention to coordinates of MFR zone. I experimented for a while with multiple MRF zones and discovered ‘half-rotating’ or even ‘rotating in both (+ / -) directions’ MRF zones. Setting probes points and observing velocity vectors convinced me about reality of those ‘impossible’ objects. For the moment, you can handle only one MRF zone and it should be centered on one of axes. Problem is reported and Ruben repeats that they are working on it.
I would rather say ‘fortunately’, as currently multiple MRF zones seem unmanageable. Correct setting of one MRF zone ‘just’ needs to aline center of rotation on ‘0’ of one of the axes. In you big setup it may be a bit tedious.
Moreover, when running simulation, I suggest you start to ‘load’ your turbine gently, using CVS file with progressive rad/s definitions. Otherwise you will have ‘exploding’ behaviour, such an amount of initial energy will dissipate slowly and reaching a steady flow would need 5-10 times more time of simulation.
Perhaps I should say ‘center of rotation’ ALONG 0 of one of axes. Example: base of your MRF cylinder can be at Z 0 m, top of cylinder at Z 2.0 m (if rotation axe is ‘Z’). You can, but do not need to align ‘geometric center’ of MRF zone to 0,0,0.
For ‘load’, try to go with ‘parabolic’ curve, which is good enough to not break you virtual setup.
I created a new project and created my own mesh and simulations from scratch after I modified and imported the original ‘axial’ geometry file in RHINO.
Geometry changes made in Rhino (named ‘axial-DaleFan1p466m-new0’):
Two of the rotor blades had many naked edges near their tips so I deleted those two blades and copied the good blade so that the rotor was a good closed solid (not sure this was needed since SimScale seemed to think the original rotor was a watertight solid anyway)
I trimmed the rotor tips shorter to 1.466m diameter (not sure this was needed since the results did not change much but I was hoping that the MRF mesh would show in SimScale without the longitudinal streaks on the surface but streaks remained)
I moved all the geometry so that the 0,0,0 coordinate was at the tip of the rotor spinner as per @Retsam suggestion.
In the SimScale project:
I created a mesh on my new geometry using the same refinements as Mesh 19 in the original project.
I made 2 simulation runs in my simulation named ‘Incompressible’.
Run 1 had simulation parameters that matched ‘6.75 - 100% > Real run’ except that the I moved the ‘MRF rotating zone 1’ origin and the ‘Forces and moments 1’ center of rotation so that they fell on the rotors axis of rotation. In the OP’s simulation parameters, these were off the axis of rotor rotation by 11 mm. I also made a number of ‘Numerics’ changes that I have come to use regularly which help me debug convergence issues.
Run 2 was only changed from Run1 to implement a 200s parabolic rise in inlet flow rate to 6.75 m^3/s as per suggestion by @Retsam.
Results:
Run 1: No significant change in Head, head is still 285,000 Pa with this convergence and pressure plots:
As far as all moments about the center point of the MRF volume (that is the point I chose for the ‘Forces and Moments 1’ calculations), I would only expect to see a moment about the rotational axis (x) and the others should be 0 (I think). So why is the total pressure and viscous moment about the x axis direction ~97,000 Nm while the total z axis moments is ~200,000 Nm and the y axis ~1,600 Nm?
You have two sources of ‘inertial load’ into your pump (I did not know it before going to the project, now). You did apply ‘pacing load’ on your inlet but you started your turbine at full speed. It is an obvious mismatch, you can fix. By chance your setup keeps now for 430 seconds and blows afterward. So please disregard everything after 430 seconds of simulation, as it is completely off and cannot recover (so question about total pressure on other axes after 430 seconds of simulation is irrelevant) .
You will always find fluctuations on other axes than a rotation one. Those pressure forces should be < 0.1 %. I see pressure force on Z axes develop to over 0.2 % from 380 seconds on of simulation but one cannot be sure about it’s origine.
May I bring your attention also to ‘Forces and moments’ you defined which are centered on Y axe, ‘-0.55669’, for all blades, that seems incorrect, as you rotate on X axe.
Please correct me if I am wrong, but my understanding is that the point which you choose for ‘Center of Rotation’ in the ‘Force and moments’ results is that it can be defined anywhere in the whole coordinate system. I think of the force and moments results that we get are simply the force and moments that would have to be applied at that point to hold the geometry selected in ‘Forces and moments’ at that time interval. I know this would require some invisible, rigid, mass-less structure that connected that point to the geometry selected, but once you have the results for that point, you can just translate the moments to some other point that actually holds those faces in real structure that is missing from the CAD (no support structure for rotor).
I put that point at the ‘center point of the MRF volume’ and in doing so it led me to my ‘Where to go from here’ question about moments that have to be applied about that ‘center of rotation’ and my query about why there are z and y axis moments when intuitively, my brains says those should be close to zero about that center point…
These moments are not fluctuations but are pretty stable through the last few hundred time steps as you can see in my images.
I will also additionally bring the rotational speed up parabolically in a yet to be, Run 3…
I see your point, but perhaps somebody would need to jump in that "Forces and moments’ setup on multiple, not connected bodies. I also make possibly invalid assumption about ‘geometric center’ of that assembly of blades.
One more point is that I would add separate ‘sensors’ to inlet and outlet in order to be able to observe the discrepancies…
