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One half propeller simulation with MRFSimpleFoam crashing |
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May 16, 2019, 04:12 |
One half propeller simulation with MRFSimpleFoam crashing
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#1 | ||
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Dear OpenFOAM Users,
After successfully completing the 2 wing UAV propeller simulation using MRFSimpleFoam and pimpleDyMFOAM I am now trying to reduce the computation time by carrying out simulation with only 1 wing instead of 2 with MRF method. The procedure which I am following is illustrated below. 1) Generation of separate mesh for static domain and the dynamic domain 2) Merging the two regions by using mergeMeshes 3) Generation of required topoSets 4) Definition of appropriate Initial and boundary conditions 5) Simulation using MRFSimpleFoam The above steps mentioned were well accomplished while simulating the complete propeller. But with single propeller and half of the domain my, simulation is crashing at the beginning. A close investigation in this direction revealed that the min patch weight at few regions are '0', which means there is no interaction between the static and quasi-rotating meshes. I have attached the mesh below, along with a sample definition of initial and boundary conditions. Also, I have attached the output file at the end. AMI_Dynamic.png AMI_Static.png Dynamic_Domain.png Static_Domain.png With my experience so-far with OpenFOAM, I think it is happening because of the marked region which is not present in the stationary domain. Please let me know how to overcome these problems. If at all I have missed any thread with respect to above mentioned problem, please do let me know. I thank the forum members for lending their hands to teach me OpenFOAM this far. Thank you. Quote:
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May 17, 2019, 03:39 |
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#2 |
Member
Thomas Sprich
Join Date: Mar 2015
Posts: 76
Rep Power: 11 |
Hi Krao,
As we had discussed (Periodic cyclic AMI + MRF crashing on pressure), try using the tutorial case to get good weightings on your AMI. That should solve that problem. Let me know if that solves your weighting issue. It might be worth trying for the full domain. In that tutorial, the rotating domain and stationary domain are meshed at the same time, so its a little bit easier. In addition I think you are not using symmetry incorrectly. Your domain is not symmetrical about a plane. Your geometry is, but the flow is not. Think about if your impeller rotates out of the symmetry plane. The symmetry plane will make it as if the impeller rotates into the domain at that point. That is probably not what your are simulating. You would want the impeller to come in at the top half, i.e. 180 degrees about the rotation axis. To model it correctly, instead of using symmetry you need to use cyclicAMI. Note the red: Code:
boundaryField { rotor_periodic_1 { type cyclicAMI; inGroups 1(cyclicAMI); matchTolerance 0.1; transform rotational; neighbourPatch rotor_periodic_2; rotationAxis (0 0 1); rotationCentre (0 0 0); nFaces 1628; startFace 171370; } } Thus you will make use of two pairs of cyclicAMI, one pair for 'transform rotational' and one pair for 'transform noOrdering'. So, in your last image, the face closest to the view. Split this face in half along the rotational axis. The upper half will be one of the rotational AMI faces and the lower half, the other. Essentially, you need to use a periodic symmetry. This might be worth googling to see how its done. It is also possible to use MRF without cyclicAMI altogether and this generates pretty good results as well. I use this technique extensively. Note this models the full domain, but it is possible to use the periodic rotational symmetry. In my snappyHexMesh, I still call the interface between the rotating domain and stationary domain AMI and in "refinementSurfaces" I use: Code:
AMIBottom { level (2 2); cellZone rotatingDomainBottom; faceZone rotatingDomainBottom; cellZoneInside inside; } Thomas |
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May 17, 2019, 03:56 |
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#3 | |
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Hi Thomas,
Thank you very much for the quick reply, I had forgot to mention my boundary file, which I have attached for the future reference. As you have mentioned I am using the symmetry in a wrong way. I will try the new simulation with the suggestions made by you and post my findings. Quote:
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May 17, 2019, 08:53 |
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#4 | |
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Hi Thomas,
As you have suggested, I revisited my mesh and improved its quality. For that I made use of surfaceAutoPatches. This has solved the problem of AMI weight going to zero. As suggested by you I have changed the boundary conditions of rotating patches which I am attaching for the future users. Only thing I did not understand is the wrong usage of symmetry patches! Which boundary condition, can I use at the center of the domain? Can I use the normal wall boundary condition at the center of the stationary domain? Quote:
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May 20, 2019, 07:40 |
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#5 |
Member
Thomas Sprich
Join Date: Mar 2015
Posts: 76
Rep Power: 11 |
Hi Krao,
Can you upload a sketch of your geometry and how you have named the patches? This might make it easier to help. Thomas |
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May 21, 2019, 04:01 |
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#6 | |
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Hi Thomas,
thank you once again for your reply. I have attached the image of my computational domain with the names of the patches. Also I have edited few of my boundary patches. 1) As you have pointed the wrong use of symmetry, I came to know that the usage of 'symmetry patch' as shown in the figure is improper. I think I need to split this patch (in gray) into two halves and maybe name it like AMI 5 & AMI 6. Before proceeding with that, I would like to know whether the AMI (rotation transformation) can be applied to a stationary domain, which is my bounding box(computation area). 2) I am using noOrdering for AMI 1 & AMI 2. This is the half cylinder portion which is the MRF region, where the propeller blade is present. For the face of the same half-cylinder region marked AMI 3 & AMI 4, I am using rotational transformation. This usage of patches are due to the fact that the half cylinder region in static and dynamic domain have had difference in geometry, as shown in the last two figures. The static region does not have any face and contains only half cylinder, because of the snapping effect. The dynamic region have the half cylinder with a face. Therefore I have split this face into 2 patches and given AMI with rotational transformation. For meshing I am using cfMesh and for combining the static and dynamic regions I am using mergeMesh utility. Domain_Nomenclature.jpg AMI_Static.png AMI_Dynamic.png In the above figure the highlighted circular region demonstrates the MRF region. The relevant boundary file is as follows. Quote:
Regards, Krao |
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May 21, 2019, 04:44 |
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#7 | |||
Member
Thomas Sprich
Join Date: Mar 2015
Posts: 76
Rep Power: 11 |
Hi Krao,
Quote:
Look at this post Periodic cyclic AMI + MRF crashing on pressure I was modelling only a quarter of the domain, whereas you are modelling half. In my case, each side of the wedge was a rotational AMI (or according to your naming: AMI5 and AMI6). The only difference is that your rotational planes are actually in the same plane verses mine that were 90 degrees apart. Quote:
Code:
AMI1 { type cyclicAMI; inGroups 1(cyclicAMI); nFaces 101643; startFace 6046206; matchTolerance 0.0001; transform noOrdering; neighbourPatch AMI2; method faceAreaWeightAMI; } AMI2 { type cyclicAMI; inGroups 1(cyclicAMI); nFaces 84970; startFace 6147849; matchTolerance 0.1; transform noOrdering; neighbourPatch AMI1; method faceAreaWeightAMI; } The amount of faces in AMI3 and AMI4 appear to be more similar, I think you are OK there. Quote:
This should get your simulation going. Let me know how it goes. Regards, Thomas |
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May 21, 2019, 05:53 |
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#8 | ||
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Hi Thomas,
Thank you very much for your time and clearing all my queries. Quote:
Quote:
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May 22, 2019, 10:44 |
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#9 | |||
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Hi Thomas,
Greetings! I have successfully completed my propeller simulation and I would like to discuss few results here. The AMI weights of different patch-pairs are as follows Quote:
The results obtained for the half propeller simulation are as follows, Quote:
Quote:
Regards, Krao |
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May 23, 2019, 04:59 |
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#10 | |
Member
Thomas Sprich
Join Date: Mar 2015
Posts: 76
Rep Power: 11 |
Hi Krao,
Congratulations on getting it to work! Well done. Your AMI weightings look good. Quote:
You don't need to be concerned about your results but you do need to understand where they come from, they are not errors and you can't ignore them. Imagine you had modeled your full case, but instead of modelling your propeller as one part, you modeled it as two parts, one for each blade. You would then get force results for each blade that look similar to the half-domain case that you just modeled. Think about what forces and moments results you would get. Firstly your moment about y-y axis. For each half impeller you would get a moment and the total would some to the total moment you have already calculated. But now think about the force required to produce a moment about y-y. if your blade is in the x direction, the force for the one blade would be positive y, and the force on the other blade would be in the negative y direction. The sum of forces would be close to zero, which is what you are seeing in your complete case, but if you only look at half the case, then you will have a force. If you applied the same thinking to all the forces and moments, you will understand the origin of each force. For my interest, did you use cfMesh's hexahedral or polyhedral mesh? Thomas |
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May 23, 2019, 06:03 |
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#11 | |
Senior Member
Kmeti Rao
Join Date: May 2019
Posts: 145
Rep Power: 8 |
Hi Thomas,
Thank you very much for the detailed explanation. Got a better understanding of how the forces function works as well. Just for curiosity I will try the simulation for the second half whenever I get some time. It should yield same result as expected. Quote:
I appreciate your time and invaluable inputs throughout the simulation process. Kind regards, Krao |
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