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#1 |
Senior Member
Attesz
Join Date: Mar 2009
Location: Munich
Posts: 367
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Hi,
I'm simulating a quad rotor helicopter's propeller (only one). During my work, the simulation reaches the convergence very strange, I think. The RMS residuals, domain imbalances are converged, but the monitor points: force on the propellers and the mass flow seems to be not. How can it be, that the imbalances are under 0.0001 but the mass flow "curve" is not constant? Information: I'm using ICEM to generate mesh. Cell number: about 800.000. In boundary layer I'm using 5 cells, and the yplus value are under 2, at most under 1. The mesh is fine around the blades and the leading and trailing edges, and of course between the blade tip and wall. Simulation properties: Steady run, SST turbulence model, 5000 RPM rotational velocity, Frozen Rotor interface type, inlet: total pressure with zero gradient turb, outlet stat pressure with medium intensity, each pressures are 1 bar (0 relative). Timescale: Automatic with Agressive lenght scale option. Initialization: velocity and pressure, with estimated values. My problem is, that why cannot reach the simulation convergence under 1000 step, and how can I speed up the simulation? It seems to be, that that the massflow and forces can't converge, but RMS values are good. RMS.jpg massflow.jpg force.jpg If you need more information, let me know! Thanks for any advice! Regards, Attesz |
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#2 |
Member
Join Date: Nov 2009
Posts: 49
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Hi,
I have some observations about your work, maybe it can help, 1- You used 5 cells in the boundary layer but in the CFX help it is mentioned that a boundary layer should be resolved with at least of 15 nods for Low-R model. 2- To speed up your simulation convergence a high quality of mesh is required. 3- The stabilization of force and mass flow is not only function of RMS but also of MAX residual. 4- I see that error on those quantities is below 1% after 400 iterations, is this a problem?????!!!! |
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#3 |
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SanS
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Posts: 41
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#4 | |||
Senior Member
Attesz
Join Date: Mar 2009
Location: Munich
Posts: 367
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Hi Abou ali, thank you for your answer!
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And one more question: what is the best way to measure thrust in simulation? So far, a plane was used, and the axial force acting on this figured out. Is it better, to calculate an average speed, and using the equation: m*c+A*(p-p0)? Thank you once again, Attesz Last edited by Attesz; January 9, 2010 at 15:25. |
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#5 | |
Senior Member
Attesz
Join Date: Mar 2009
Location: Munich
Posts: 367
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Quote:
Thank you, Attesz |
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#6 |
Member
SanS
Join Date: Mar 2009
Posts: 41
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With an autotimescale you can take for ever to reach convergence. Ramp it by factor of 10 and then monitor your residuals and variables of interest. You could ramp it up as high as you can get away with.
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#7 |
Senior Member
Attesz
Join Date: Mar 2009
Location: Munich
Posts: 367
Rep Power: 14 ![]() |
Hi sans,
I've started a simulation with Timescale Factor 10, it seems to be good. But this timescale wouldn't cause inaccuracy in results? After finish, should I run a few iterations with conservative timescale and Factor 1? Thank you, Attesz |
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#8 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Are you sure your simulation has converged? It looks like you only have loose convergence to me and you could easily converge tighter. That may help things.
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#9 |
Senior Member
Attesz
Join Date: Mar 2009
Location: Munich
Posts: 367
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Thanks Glenn, but ramping up with Timescale Factor, the RMS residuals have started to decrease rapidly, and also the mass&force values have stabilized, so sans's advice is working!
Attesz |
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#10 |
Senior Member
George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
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__________________
Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials |
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#11 |
Senior Member
Attesz
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Thank you ckleanth!
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#12 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Regardless, have you checked that your convergence is tight enough? You need to do a sensitivity check on it. (Run a tighter and looser convergence tolerance and see if the differences are significant for you)
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#13 |
Member
DB
Join Date: Apr 2011
Posts: 86
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Hi,
I hope you have tried Delunay method of volume meshing in ICEM, if not I think you should cause it has HUGE effect on convergence as i have seen in some of my rotating machinery cases. Also I agree with ghorrocks, your simulation seems a relatively simple one so you should have a tighter convergence criteria like RMS/MAX= 10-6, dont concentrate so much on imbalance, your monitors should be your primary criteria.
__________________
-D.B |
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#14 |
New Member
Thomad Berdicd
Join Date: Dec 2017
Posts: 2
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now, i'm simulating it ( 2 baldes moving ) if you have any tutorial about it. please send me via
hoangha050709@gmail.com thanks a lot |
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#15 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,643
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There are tutorials on modelling rotating machinery in the ANSYS customer website.
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#16 |
New Member
Allyssa Grey
Join Date: Nov 2020
Posts: 1
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I think these simulators are useful learned a lot while training for my license. I also have to say that before I started training for a real pilot license I was a huge fan of drones and I still am. I would build them from scratch and my parents would yell at me for doing that. After a while they just decided to buy me one. I was really happy about it and honestly I made sure that I took great care of it. After I got older and learned how to fly my first drone properly I started looking into more advance models. I was lucky enough to find a really cool blog that has a list of the best drones that are on the market right now and they also talk about the technical aspects of certain models, which is also very important for me. You guys can check it here: https://buyersimpact.co.uk/ I ordered one of the models of that list and I have had been waiting for the delivery for a few days already. Honesty my family is being weird about my drone obsession since I can fly regular helicopters but hey it's been in my heart from day one. I will always love it I think
Last edited by lysagrey; January 4, 2021 at 13:36. |
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Tags |
convergence, propeller, quadrotor |
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