Hydraulic cylinder-pressure development

Galahad · May 9, 2015, 13:51

Hello everyone,

I recently started using Ansys CFD software and i am trying to model a hydraulic cylinder with a moving piston. Specifically, i want it to move under pressure from the fluid, and NOT separately. For example, I want 300 bars of pressure on the one side of the piston, while the inlet is on the other side. When pressure on inlet exceeds 300 bars the piston should move.

I am currently using CFX, although i think Fluent might be the right one. I have already tried using immersed solid, it failed, but i do not know if my set up is correct. I also tried using a surface to model the piston but the pre-proc will not accept it. I also used a fluid domain, combined with a solid domain (the two domains intercepted), but it seemed to me that Ansys would not understand that there are two separate volumes of fluid.

Could someone please point me to the right direction ? I would really appreciate it. Thank you in advance for your time.

fresty · May 10, 2015, 07:36

Hi,

I believe you would have to post some images indicating what exactly are you trying to model and also the CCL file.

Galahad · May 10, 2015, 09:10

I am trying to investigate the pressure development in the beginning of the piston's movement.

Galahad · May 14, 2015, 15:46

Anybody knows how to create a moving fluid domain? I want to apply translation but i only find rotation ....

fresty · May 17, 2015, 16:17

HI.

Have you tried using rigid body and moving mesh combination. I believe the system can simply be modeled as fluid domain and if i am not wrong and can recall from top of my head, the parallel sides of the piston can be nominated as rigid body with translation DOF only. Meshing would be slightly tricky with moving mesh features. Try comparing your system with Tutorial#22 (Moving a ball...) of CFX, it would hopefully resolve it for you.

Cheers.

Galahad · May 18, 2015, 04:17

Fresty,

first, thank you for the reply. I did try to model it this way, but i am not sure the setup is correct. I guess you mean i should use a fluid domain from the inlet up to the piston. Unless i am doing something wrong with the mesh, which is not unlikely as i have never used moving mesh before, this setup is not working. I think the reason is that the fluid domain is increasing in volume, it is not just deforming internally, like in the tutorial. So there is need for new mesh creation, not just a remeshing. I also used a fluid domain from the inlet up to the outlet, overlapped by the immersed solid domain. That did not work either, CFX did not understand that there are two separate volumes separeted by the piston. So, if I set inlet pressure at 300 bar and outlet at 0 bar, solution would just create a ultrahigh speed fluid jet to create that pressure drop, with linear and concentrated losses.
Again, thans for the reply, means a lot to me.

fresty · May 18, 2015, 12:02

Cheers, thanks.
I understand and reckon that yes a mesh formation is required rather than deformation. Have you tried adjusting Momentum Source Scaling factor while modelling the Piston as Immersed Solid? I am pretty sure Immersed Solids is not the best approach especially when you have disjoint fluid sections but occasionally you might get something out of it by increasing the scaling factor.

Quote:

..., this setup is not working.

Just curious, did you receive any error or just unexpected results?

fresty · May 18, 2015, 12:59

Just simulated your model with a quick & rough immersed solid approach (with slightly higher momentum source scaling factor). Is this what you get (below image)? And is this undesirable? Here i found that CFX did detect the solid body (not really a solid as immersed solids are just sources of momentum stagnation by principle) and it does move the piston solely by virtue of flow (Pressure difference). Does this conform to your system..?

Galahad · May 18, 2015, 17:50

No, I haven't actually tried that. But I will.

I got an error about the moving mesh. Which I think indicates the problem is not in the immersed solid but on the moving mesh setup.

I think your results are those expected. Did you use two separate fluid domains or just one ? And which were your boundary conditions? Pressure on both sides? Thanks you so much, again.

fresty · May 19, 2015, 01:35

HI.
Alright.
I just used one fluid domain and overlapped the immersed solid (Rigid Body Solution) domain. The momentum source scaling factor i used was 50 for a start. Boundary conditions, yes pressure on both sides; used were 30 Bar on inlet (left end of the picture) and 0.5 Bar at the other end. You could increase the source scaling factor and insert your desired boundary conditions and check if it works for you, ideally it should. Let me know if i could support further. Moreover, i believe using both moving mesh and immersed solid together for this simulation is extravagant (probably not applicable as well; can only confirm if have some images etc. of your model.)

Cheers.

Galahad · May 19, 2015, 14:50

It runs but the results are not correct. Using rigid body solution, it just ejects the piston out of the fluid domain, unless i use like a huge mass, but even then sometimes. It also creates a high speed jet to create the losses, like i mentioned earlier. That kept happening with the momentum source scaling factor increased. Thanks for taking the time to reply mate, I deeply appreciate it .

Galahad · May 19, 2015, 15:01

What is your velocity size ?

fresty · May 19, 2015, 16:50

What time step are you using? I would suggest to start by using a very low time step in the order of 10^-4 or so. I could easily imagine the results you may be getting, that's where immersed solids have their limitation. You may want to employ UDFs for restriction of piston at a certain displacement (before it gets pushed out of the flow domain. Secondly, using a slightly larger dia piston would help restricting the escaping flow streamlines at the peripheries of the piston.
Not sure if comparing the velocity of my arbitrary attempt would help but it's around 800m/s or so.
And if all of this doesn't change things then fluid domain model would be the best and probably the only way, would just need to figure out the approach.
Thanks and hope it helps.

May 9, 2015, 13:51	Hydraulic cylinder-pressure development	#1
Galahad New Member Join Date: May 2015 Posts: 7 Rep Power: 10	Hello everyone, I recently started using Ansys CFD software and i am trying to model a hydraulic cylinder with a moving piston. Specifically, i want it to move under pressure from the fluid, and NOT separately. For example, I want 300 bars of pressure on the one side of the piston, while the inlet is on the other side. When pressure on inlet exceeds 300 bars the piston should move. I am currently using CFX, although i think Fluent might be the right one. I have already tried using immersed solid, it failed, but i do not know if my set up is correct. I also tried using a surface to model the piston but the pre-proc will not accept it. I also used a fluid domain, combined with a solid domain (the two domains intercepted), but it seemed to me that Ansys would not understand that there are two separate volumes of fluid. Could someone please point me to the right direction ? I would really appreciate it. Thank you in advance for your time.

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May 10, 2015, 07:36		#2
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	Hi, I believe you would have to post some images indicating what exactly are you trying to model and also the CCL file.

May 14, 2015, 15:46		#4
Galahad New Member Join Date: May 2015 Posts: 7 Rep Power: 10	Anybody knows how to create a moving fluid domain? I want to apply translation but i only find rotation ....

May 17, 2015, 16:17		#5
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	HI. Have you tried using rigid body and moving mesh combination. I believe the system can simply be modeled as fluid domain and if i am not wrong and can recall from top of my head, the parallel sides of the piston can be nominated as rigid body with translation DOF only. Meshing would be slightly tricky with moving mesh features. Try comparing your system with Tutorial#22 (Moving a ball...) of CFX, it would hopefully resolve it for you. Cheers.

May 18, 2015, 04:17		#6
Galahad New Member Join Date: May 2015 Posts: 7 Rep Power: 10	Fresty, first, thank you for the reply. I did try to model it this way, but i am not sure the setup is correct. I guess you mean i should use a fluid domain from the inlet up to the piston. Unless i am doing something wrong with the mesh, which is not unlikely as i have never used moving mesh before, this setup is not working. I think the reason is that the fluid domain is increasing in volume, it is not just deforming internally, like in the tutorial. So there is need for new mesh creation, not just a remeshing. I also used a fluid domain from the inlet up to the outlet, overlapped by the immersed solid domain. That did not work either, CFX did not understand that there are two separate volumes separeted by the piston. So, if I set inlet pressure at 300 bar and outlet at 0 bar, solution would just create a ultrahigh speed fluid jet to create that pressure drop, with linear and concentrated losses. Again, thans for the reply, means a lot to me.

May 18, 2015, 12:59		#8
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	Just simulated your model with a quick & rough immersed solid approach (with slightly higher momentum source scaling factor). Is this what you get (below image)? And is this undesirable? Here i found that CFX did detect the solid body (not really a solid as immersed solids are just sources of momentum stagnation by principle) and it does move the piston solely by virtue of flow (Pressure difference). Does this conform to your system..?

May 18, 2015, 17:50		#9
Galahad New Member Join Date: May 2015 Posts: 7 Rep Power: 10	No, I haven't actually tried that. But I will. I got an error about the moving mesh. Which I think indicates the problem is not in the immersed solid but on the moving mesh setup. I think your results are those expected. Did you use two separate fluid domains or just one ? And which were your boundary conditions? Pressure on both sides? Thanks you so much, again.

May 19, 2015, 01:35		#10
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	HI. Alright. I just used one fluid domain and overlapped the immersed solid (Rigid Body Solution) domain. The momentum source scaling factor i used was 50 for a start. Boundary conditions, yes pressure on both sides; used were 30 Bar on inlet (left end of the picture) and 0.5 Bar at the other end. You could increase the source scaling factor and insert your desired boundary conditions and check if it works for you, ideally it should. Let me know if i could support further. Moreover, i believe using both moving mesh and immersed solid together for this simulation is extravagant (probably not applicable as well; can only confirm if have some images etc. of your model.) Cheers.

May 19, 2015, 14:50		#11
Galahad New Member Join Date: May 2015 Posts: 7 Rep Power: 10	It runs but the results are not correct. Using rigid body solution, it just ejects the piston out of the fluid domain, unless i use like a huge mass, but even then sometimes. It also creates a high speed jet to create the losses, like i mentioned earlier. That kept happening with the momentum source scaling factor increased. Thanks for taking the time to reply mate, I deeply appreciate it .

May 19, 2015, 15:01		#12
Galahad New Member Join Date: May 2015 Posts: 7 Rep Power: 10	What is your velocity size ?

May 19, 2015, 16:50		#13
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	What time step are you using? I would suggest to start by using a very low time step in the order of 10^-4 or so. I could easily imagine the results you may be getting, that's where immersed solids have their limitation. You may want to employ UDFs for restriction of piston at a certain displacement (before it gets pushed out of the flow domain. Secondly, using a slightly larger dia piston would help restricting the escaping flow streamlines at the peripheries of the piston. Not sure if comparing the velocity of my arbitrary attempt would help but it's around 800m/s or so. And if all of this doesn't change things then fluid domain model would be the best and probably the only way, would just need to figure out the approach. Thanks and hope it helps.