Single Rotating Domain

fluentnoob · June 22, 2009, 07:24

Hi everyone!

I was trying to model a rotating domain shaped like the region in between two concentric cylinders. Here is a picture of the domain.
http://picasaweb.google.co.in/lh/pho...eat=directlink

I tried setting the inlet boundary condition as mass flow inlet and outlet as pressure outlet. And I'm rotating the domain about an axis along the +ve Y direction, and passing through the center of the cylinders. And the pressure distribution I expected was concentric isobaric lines throughout the domain. But I cannot seem to get this distribution from the boundary conditions I have set. Can you please help me out with the kind of boundary conditions I am supposed to use?

I specified the fluid domain as rotating reference frame with and angular velocity of 0.1 rad/s, specified the mass flow rate as 4.01e+06 kg/s. The rotation axis origin as X=-100, Y=0, Z=0. Rotations axis direction as X=-100, Y=1, Z=0.The dimensions of the inlet and the outlet are 20mx20m.
And specified the static pressure as zero at the pressure outlet.

Can you please tell me what am I doing wrong?

Thank you very much.

fluentnoob · June 22, 2009, 13:34

OK. I found that I was specifying the rotation axis direction incorrectly. I specified it as [0 1 0] and got the isobaric pressure lines as expected. But something is still wrong. The velocity profile on the side walls is as expected, highest on the wall with the smaller radius, and more on the wall with the larger radius. But away from the walls, the velocity profile goes awry. Have a look at this please...
http://picasaweb.google.co.in/lh/pho...eat=directlink
Can you please please help me out? What am I doing wrong? Why is the velocity profile on the Inlet so different? Why doesnt it vary like (omega*R)? That is, increase as the radius increases (It is increasing, but as you can see in the pic, in a very strange fashion)? Please please please help me out. Thank you.

fluentnoob · June 23, 2009, 07:20

Ok. I have another doubt now. When the whole domain is rotating, should there be concentric iso-velocity lines, or both iso-velocity and isobaric lines(both concentric)?

What should be the boundary conditions? Please please help me out..

fluentnoob · June 23, 2009, 11:20

Ok, So I got that part sorted out. Could anyone please just help me with the following? I'd be extremely grateful to the person who can give me advice on the following, any advice. Thank you.

I am trying to simulate a sphere rotating about a point, submerged in water. I have defined a domain around the sphere as fluid. Now I proceeded to solve for the forces etc. on the sphere in the following two ways:

Approach 1:
At the Inlet, defined the fluid velocity as angular, of the appropriate magnitude, and with the apt rotation axis. All this in absolute terms.

Approach 2:
At the Inlet, defined the fluid velocity as angular, of the appropriate magnitude, and with the apt rotation axis. Also, defined the sphere as rotating about the rotation axis. All this in absolute terms.

Could you please help me out over here and tell me which approach is correct? Would all the extra terms due to the circular motion (centrifugal force etc) be included in approach 2 and not 1? Please can anyone help me out with this?

Thank you..

fluentnoob · June 23, 2009, 11:48

By approach 2, wouldnt the viscous force increase? Because the velocity of the fluid relative to the sphere will increase and become the (velocity of the sphere + Tangential velocity of the fluid) right? So does that make the first approach correct?

June 22, 2009, 07:24	Single Rotating Domain	#1
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 17	Hi everyone! I was trying to model a rotating domain shaped like the region in between two concentric cylinders. Here is a picture of the domain. http://picasaweb.google.co.in/lh/pho...eat=directlink I tried setting the inlet boundary condition as mass flow inlet and outlet as pressure outlet. And I'm rotating the domain about an axis along the +ve Y direction, and passing through the center of the cylinders. And the pressure distribution I expected was concentric isobaric lines throughout the domain. But I cannot seem to get this distribution from the boundary conditions I have set. Can you please help me out with the kind of boundary conditions I am supposed to use? I specified the fluid domain as rotating reference frame with and angular velocity of 0.1 rad/s, specified the mass flow rate as 4.01e+06 kg/s. The rotation axis origin as X=-100, Y=0, Z=0. Rotations axis direction as X=-100, Y=1, Z=0.The dimensions of the inlet and the outlet are 20mx20m. And specified the static pressure as zero at the pressure outlet. Can you please tell me what am I doing wrong? Thank you very much.

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June 22, 2009, 13:34		#2
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 17	OK. I found that I was specifying the rotation axis direction incorrectly. I specified it as [0 1 0] and got the isobaric pressure lines as expected. But something is still wrong. The velocity profile on the side walls is as expected, highest on the wall with the smaller radius, and more on the wall with the larger radius. But away from the walls, the velocity profile goes awry. Have a look at this please... http://picasaweb.google.co.in/lh/pho...eat=directlink Can you please please help me out? What am I doing wrong? Why is the velocity profile on the Inlet so different? Why doesnt it vary like (omega*R)? That is, increase as the radius increases (It is increasing, but as you can see in the pic, in a very strange fashion)? Please please please help me out. Thank you.

June 23, 2009, 07:20		#3
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 17	Ok. I have another doubt now. When the whole domain is rotating, should there be concentric iso-velocity lines, or both iso-velocity and isobaric lines(both concentric)? What should be the boundary conditions? Please please help me out..

June 23, 2009, 11:20		#4
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 17	Ok, So I got that part sorted out. Could anyone please just help me with the following? I'd be extremely grateful to the person who can give me advice on the following, any advice. Thank you. I am trying to simulate a sphere rotating about a point, submerged in water. I have defined a domain around the sphere as fluid. Now I proceeded to solve for the forces etc. on the sphere in the following two ways: Approach 1: At the Inlet, defined the fluid velocity as angular, of the appropriate magnitude, and with the apt rotation axis. All this in absolute terms. Approach 2: At the Inlet, defined the fluid velocity as angular, of the appropriate magnitude, and with the apt rotation axis. Also, defined the sphere as rotating about the rotation axis. All this in absolute terms. Could you please help me out over here and tell me which approach is correct? Would all the extra terms due to the circular motion (centrifugal force etc) be included in approach 2 and not 1? Please can anyone help me out with this? Thank you..

June 23, 2009, 11:48		#5
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 17	By approach 2, wouldnt the viscous force increase? Because the velocity of the fluid relative to the sphere will increase and become the (velocity of the sphere + Tangential velocity of the fluid) right? So does that make the first approach correct?