# Forces Acting on a Rotating Cylinder (Moving Mesh)

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April 14, 2015, 20:44
Forces Acting on a Rotating Cylinder (Moving Mesh)
#1
Member

Mike
Join Date: Jun 2012
Posts: 58
Rep Power: 13
Hi,

I am doing a moving mesh simulation of a spinning cylinder (2D) in a multiphase simulation (oil and air). Everything above the the center of the cylinder is specified as air while everything below is specified as oil with viscosity of .05 Pa.s.

The idea is that this spinning cylinder will generate a pressure gradient between its rotating surface and the bottom wall thus generating lift. As a result, it should come to an equilibrium distance from the wall with the gravity force. (Pic 1)

*Problems to clarify
1) I am trying to understand the forces acting on the cylinder. I know I have gravity (so massxgravity) acting downwards. Since the cylinder is rotating at a velocity I now have some some force acting on the cylinder as it approaches the bottom wall.

I am not sure if I should have buoyancy enabled or not. Right now, I do have any heat transfer enabled so I assume I should not have buoyancy enabled. The way I understand it, is that as the rotating cylinder approaches the wall, a pressure will be generated which will lift the cylinder. I assume this force does not depend on the density of the fluid but only the viscosity. I have attached a picture from a textbook which shows the buoyancy force balanced by a spring force and mg.

2)I am not sure what the difference is if I specify the whole domain with oil and do a single phase simulation instead of doing one with oil and air?? I assume the generated pressure in the gap will be the same regardless of a multitask simulation or single phase.

3)The other question I have is that the objective of my simulation is to design a spring that dampens the oscillations of the spinning cylinder. (Pic 3. To do this simulation I specify a spring with equilibrium position at the center of the cylinder. I have an equation that tells me what the equilibrium steady state pressure should be based the distance between the rotating cylinder and wall. Using the pressure I was able to come up with the spring constant needed to dampen the oscillations of the cylinder (pic 3).
Does this logic at all makes sense? In my graph, the cylinder oscillations are dampened.

I appreciate any insight you can provide!

Dreamchaser
Attached Images
 SpinningCylinder.jpg (17.8 KB, 18 views) ForcesonCylinder.PNG (7.5 KB, 17 views) SpherewithSpring.jpg (15.3 KB, 22 views) dampedoscillationsofcylinder.jpg (33.5 KB, 18 views)

 April 15, 2015, 06:06 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143 You will need gravity to get the air/oil interface to act correctly I suspect. As you state, this simulation should also work single phase. I strongly recommend you try this first - start simple and get that working. Then add the complexity of multiphase. Ideal springs have no damping - so why do you think adding a spring will damp the vibrations? The oscillations are damped due to physical and non-physical factors. Physical factors: the vibration will cause the fluid to move about and viscosity will damp this. Non-physical factors: numerical dissipation from things like mesh size, time step size, advection scheme and many others adds additional damping to the simulation which will dampen the vibrations over time.

April 15, 2015, 22:30
#3
Member

Mike
Join Date: Jun 2012
Posts: 58
Rep Power: 13
Hi,

Thank you for the response. Yes, I see what you are saying.
I have switched to a single phase simulation as you suggested.

What I am trying to do as a first step in my simulations is to spin the cylinder at an rpm with gravity enabled. As a result, the cylinder should come to a steady state position where the gravity force will balance the generated pressure in the gap. First I disabled the gravity. As a result, the cylinder goes down initially and then moves upward until it hits the top wall.

When I enable gravity, the cylinder slams into the bottom wall (pic attached).

I am trying to validate a case from a paper. I am not sure why the cylinder is slamming into the bottom wall. Some possible reasons I think:

1) my domain might be wrong. (pic attached). I have a big box with the oil everywhere.

2) I have no buoyancy force enabled. However this should not matter since naivier stokes should include gravity.

3) when I analyzed the force acting per time on the cylinder it was constantly increasing?? Based on my mass for the rigid body and gravity the rigid body my force should be .04N. Not sure why the force is going above.

Thanks

Quote:
 Originally Posted by ghorrocks You will need gravity to get the air/oil interface to act correctly I suspect. As you state, this simulation should also work single phase. I strongly recommend you try this first - start simple and get that working. Then add the complexity of multiphase. Ideal springs have no damping - so why do you think adding a spring will damp the vibrations? The oscillations are damped due to physical and non-physical factors. Physical factors: the vibration will cause the fluid to move about and viscosity will damp this. Non-physical factors: numerical dissipation from things like mesh size, time step size, advection scheme and many others adds additional damping to the simulation which will dampen the vibrations over time.
Attached Images
 rotatingcylinder.jpg (21.7 KB, 17 views) collapsingcylinder.jpg (20.6 KB, 15 views) domain.jpg (18.3 KB, 12 views)

 April 16, 2015, 03:50 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143 If you are doing this with the rigid body model you are going to need very fine time steps to resolve the cylinder motion - or it will do what you just report.

April 24, 2015, 14:16
#5
Member

Mike
Join Date: Jun 2012
Posts: 58
Rep Power: 13
Quote:
 Originally Posted by ghorrocks If you are doing this with the rigid body model you are going to need very fine time steps to resolve the cylinder motion - or it will do what you just report.
Hi Glenn,

Its works now!! Thanks so much for the advice!!
I reduced the time step to 1E-6 and there is lift on the cylinder.
Thank you!!

If you were in Florida, I would buy you dinner!

Take care

 April 25, 2015, 06:01 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143 Next time I pass by Florida you can buy me a beer