Stopping a rigid body in motion

wade_doll · October 5, 2012, 17:11

I have seen the question asked several times, but never an answer given.

I am using the 6DOF rigid body solver to model a steel ball in a pipe of flowing water. I wish to have the moving fluid move the ball a designated distance (the green x in the attachment) then stop, but for the analysis to continue running. I am interested in what happens during the ball movement AND after it has stoppped.

Is there a way to stop the rigid body after it has moved X distance and continue?

I have simplified the problem for discussion purposes. I realize I could model the stationary ball as a steady state.

ghorrocks · October 6, 2012, 07:51

There should be lots of ways to model this. Probably the easiest is to do the ball motion as a normal 6DOF model, then stop the simulation and run a fixed mesh simulation using the 6DOF model as an initial condition.

msaeedsadeghi · June 22, 2013, 10:50

Hi,
I am trying to model a moving wing in a fluid. Would you please help me in modeling it using Rigid Body Motion in CFX?

Here is a photo of the project.

Thanks a lot for your kind help.

ghorrocks · June 23, 2013, 06:33

What type of motion does it have? Does the wing deform? Does the wing move due to aerodynamic forces?

msaeedsadeghi · June 23, 2013, 07:24

Wing is not deforming. I am using Rigid Body Motion.

Isn't better to model it using Immersed Solid? It is too simple, but don't know if it is reliable or no.

ghorrocks · June 23, 2013, 07:29

Let's not jump to choosing models yet. I still need to understand what you are doing more before we can talk about approriate models.

We Re and Ma number is the flow at? What is the working fluid?

What motion does the body have? Is it purely rotational? Or translational? Or some complex motion?

Is the motion coupled to the fluid flow, or can it be prescribed in advance?

Also your image is very strange, there is nothing which looks like an airfoil on it. Can you explain your geometry better?

msaeedsadeghi · June 23, 2013, 07:35

It's a simple model. Just want to know how should it be modeled.

Blade is the part removed from the circle. Fluid is oil. Just want to rotate fluid. I may add a channel at the middle of blade to connect two sides of the blade, fluid could move through the channel and came to other side because of momentum.

ghorrocks · June 23, 2013, 07:39

It might be a simple mode, but you still have not explain what this simple model is actually doing. I cannot see any inlets or outlets, so is it a closed domain? And what is moving? I really have no idea what you are modelling, so please explain it as if I am an idiot (I won't be offended

).

msaeedsadeghi · June 23, 2013, 07:51

Thanks for rapid reply.
I have changed the approach and trying it using Immersed Solid.
Just want to know if it is reliable.

ghorrocks · June 23, 2013, 07:57

Yes, it is reliable - it would not be in the software if it was not - but it has important restrictions. And your model might be better modelled with other approaches, that is why I am trying to understand what you are modelling.

But if you have already made the decision to use immersed solids then go for it.

msaeedsadeghi · June 28, 2013, 09:52

Quote:

Originally Posted by ghorrocks

Yes, it is reliable - it would not be in the software if it was not - but it has important restrictions. And your model might be better modelled with other approaches, that is why I am trying to understand what you are modelling.

But if you have already made the decision to use immersed solids then go for it.

Would you please share your email to send you my CFX file? I have tried with solver option but couldn't get a good result. It is not realistic. Viscosity is so high but it still rotates so fast.

ghorrocks · June 29, 2013, 07:28

I do not give out my email on the forum. Also please do not PM me. If you have a CFD question post it on the forum.

msaeedsadeghi · June 29, 2013, 14:28

OK dear Glenn

I have set Torque as input, run the simulation using Immersed Solid model. Have gotten rotation speed in CFD-post, but the angular velocity was so high, not realistic. I decreased the Torque to see the effect, but nothing changed, the speed was still high.

What is the problem?

ghorrocks · June 30, 2013, 07:47

For many applications immersed solid is not an accurate way to model rotating machinery. Use the rotating frame of reference models for this.

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October 6, 2012, 07:51		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	There should be lots of ways to model this. Probably the easiest is to do the ball motion as a normal 6DOF model, then stop the simulation and run a fixed mesh simulation using the 6DOF model as an initial condition.

June 23, 2013, 06:33		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What type of motion does it have? Does the wing deform? Does the wing move due to aerodynamic forces?

June 23, 2013, 07:24		#5
msaeedsadeghi Senior Member SSL Join Date: Oct 2012 Posts: 226 Rep Power: 14	Wing is not deforming. I am using Rigid Body Motion. Isn't better to model it using Immersed Solid? It is too simple, but don't know if it is reliable or no.

June 23, 2013, 07:29		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Let's not jump to choosing models yet. I still need to understand what you are doing more before we can talk about approriate models. We Re and Ma number is the flow at? What is the working fluid? What motion does the body have? Is it purely rotational? Or translational? Or some complex motion? Is the motion coupled to the fluid flow, or can it be prescribed in advance? Also your image is very strange, there is nothing which looks like an airfoil on it. Can you explain your geometry better?

June 23, 2013, 07:35		#7
msaeedsadeghi Senior Member SSL Join Date: Oct 2012 Posts: 226 Rep Power: 14	It's a simple model. Just want to know how should it be modeled. Blade is the part removed from the circle. Fluid is oil. Just want to rotate fluid. I may add a channel at the middle of blade to connect two sides of the blade, fluid could move through the channel and came to other side because of momentum.

June 23, 2013, 07:39		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	It might be a simple mode, but you still have not explain what this simple model is actually doing. I cannot see any inlets or outlets, so is it a closed domain? And what is moving? I really have no idea what you are modelling, so please explain it as if I am an idiot (I won't be offended ).

June 23, 2013, 07:51		#9
msaeedsadeghi Senior Member SSL Join Date: Oct 2012 Posts: 226 Rep Power: 14	Thanks for rapid reply. I have changed the approach and trying it using Immersed Solid. Just want to know if it is reliable.

June 23, 2013, 07:57		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Yes, it is reliable - it would not be in the software if it was not - but it has important restrictions. And your model might be better modelled with other approaches, that is why I am trying to understand what you are modelling. But if you have already made the decision to use immersed solids then go for it.

June 29, 2013, 07:28		#12
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I do not give out my email on the forum. Also please do not PM me. If you have a CFD question post it on the forum.

June 29, 2013, 14:28		#13
msaeedsadeghi Senior Member SSL Join Date: Oct 2012 Posts: 226 Rep Power: 14	OK dear Glenn I have set Torque as input, run the simulation using Immersed Solid model. Have gotten rotation speed in CFD-post, but the angular velocity was so high, not realistic. I decreased the Torque to see the effect, but nothing changed, the speed was still high. What is the problem?

June 30, 2013, 07:47		#14
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	For many applications immersed solid is not an accurate way to model rotating machinery. Use the rotating frame of reference models for this.