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Rotating a sphere

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Old   April 30, 2010, 08:41
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this is what I did, I'll give you some more images on the results
Attached Images
File Type: jpg Method.jpg (43.7 KB, 26 views)
File Type: jpg inlet.jpg (47.1 KB, 24 views)
File Type: jpg cl cd.jpg (33.0 KB, 20 views)
File Type: jpg y+.jpg (71.2 KB, 23 views)
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Old   April 30, 2010, 09:56
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This is the velocity (vector, pathline and contour) and the pressure contour with the inlet velocity=1 and the moving wall=5 rad/s
Attached Images
File Type: jpg velvec.jpg (49.9 KB, 19 views)
File Type: jpg velpath.jpg (58.9 KB, 21 views)
File Type: jpg velcontour.jpg (65.6 KB, 17 views)
File Type: jpg prescontour.jpg (53.5 KB, 15 views)
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Old   April 30, 2010, 11:53
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Sorry that was a bit unclear. That part is the results. Where you can set 1st or 2nd upwind under solution.

Anyway, with a rotating wall you should be getting some lift here, one way or the other.

You don't by any chance have a 'specified shear' wall? That could explain your results.
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Old   April 30, 2010, 12:21
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Hi, when I looked at your results again, I saw they actually show a slight asymmetry. This is good news, as it indicates that your moving wall boundary condition is actually at work (assuming you're calculating steady).

I'm not sure about your version of Fluent, but in the Monitor->Forces dialogue, you might want to check that the cl is in the correct direction (probably the y-direction).

This might be the answer, since the flow seems unsymmetric but still you don't get any lift.

Another thing you might try is to speed up the rotation to, say, 50 rad/s and see what happens?

I'm interested what is going on so please let me know if this works, Jack
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Old   April 30, 2010, 13:22
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I've checked the axes as shown in the picture and changed it to Y, the force was already 1 in Y, as for the rotation I changed it to 137 rad/s and left the inlet as 1 m/s, cl=-5,75e-3 and cd=2,96e-2, I also put other pics of results

Quote:
Originally Posted by jack1980 View Post
Hi, when I looked at your results again, I saw they actually show a slight asymmetry. This is good news, as it indicates that your moving wall boundary condition is actually at work (assuming you're calculating steady).

I'm not sure about your version of Fluent, but in the Monitor->Forces dialogue, you might want to check that the cl is in the correct direction (probably the y-direction).

This might be the answer, since the flow seems unsymmetric but still you don't get any lift.

Another thing you might try is to speed up the rotation to, say, 50 rad/s and see what happens?

I'm interested what is going on so please let me know if this works, Jack
Attached Images
File Type: jpg lift.jpg (60.9 KB, 13 views)
File Type: jpg 1ms 137rads velcont.jpg (69.7 KB, 15 views)
File Type: jpg 1ms 137rads velcontzoom.jpg (61.9 KB, 14 views)
File Type: jpg 1ms 137rads prescontzoom.jpg (44.2 KB, 14 views)
File Type: jpg cl and cd.jpg (78.1 KB, 14 views)
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Old   April 30, 2010, 15:12
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Hi, it seems your in business now. There is definitely an effect in the velocity and pressure plots, and you are getting lift. The lift might seem a bit small, but it could be ok for this case. As a last check of the mechanism you could reverse the rotation and see if the lift is inverted.

Your residuals are 0.001. May I suggest you lower them to 1e-6? This will lead to nicer convergence of the cd and cl.

There should be enough experimental data on rotating cylinders to check your simulation.

Good luck!
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Old   May 1, 2010, 07:36
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I'm just having problems with defining the velocity, in a real situation do you have any ideia about the velocity and angular velocity I should define on the ball? I've tried searching for real values but all I can find is articles with wind tunnels...

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Originally Posted by jack1980 View Post
Hi, it seems your in business now. There is definitely an effect in the velocity and pressure plots, and you are getting lift. The lift might seem a bit small, but it could be ok for this case. As a last check of the mechanism you could reverse the rotation and see if the lift is inverted.

Your residuals are 0.001. May I suggest you lower them to 1e-6? This will lead to nicer convergence of the cd and cl.

There should be enough experimental data on rotating cylinders to check your simulation.

Good luck!
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Old   May 3, 2010, 03:16
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There is some data in:

Sarah Barber, Matt Carre, Soccer Ball Aerodynamics

which is a chapter of the book:

Martin Peters, Computational Fluid Dynamics for Sport Simulation, Springer-Verlag (2009)
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Old   May 3, 2010, 11:37
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In this case, when I input rotation to the ball, it's the cl coefficient that's equal to the cs coefficient that's equal to the cm magnus coefficient, right?

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This is what I thought too - that it was just another name for 'lift'?
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Old   May 3, 2010, 11:48
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I tried contacting the both of them, only Matt Carre responded, he told me it's very difficult to simulate the airflow around a sphere in CFD and that he wasn't an expert on CFD, I think Sarah Barber is the one that did the simulation part, but she didn't respond, I'm trying now to do the 3D part, my only problem is with the velocity and angular velocity, I know that to have the spin I have to put the angular velocity much higher than the inlet velocity, but I really don't think that the angular velocity of a football in flight can reach 500 rad/s (which is the higher value I had to put for it to show the magnus effect) and that's the main problem I'm having


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Originally Posted by jack1980 View Post
There is some data in:

Sarah Barber, Matt Carre, Soccer Ball Aerodynamics

which is a chapter of the book:

Martin Peters, Computational Fluid Dynamics for Sport Simulation, Springer-Verlag (2009)
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Old   May 3, 2010, 15:36
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Hi, here I have a plot that gives experimental values for a soccer ball. The spin parameter (R omega / u) ranges 0.00 to 0.20. The magnus coefficient is almost linear and coincidentally appears to be roughly as large as the spin parameter, it ranges from 0.00 to 0.22. These are rough values and I haven't really read this chapter so I'm not into the details.

See if this information can get you going. If you want I can send you a copy of the plot tomorrow, I'll have to look for a digital version.

Good luck
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Old   May 3, 2010, 15:37
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where Re=2.1e5
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Old   May 3, 2010, 16:08
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Ok I'd really appreciate if you could send me those results, by the way what were the values you used to obtain the spin parameter and magnus coefficient?

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Originally Posted by jack1980 View Post
Hi, here I have a plot that gives experimental values for a soccer ball. The spin parameter (R omega / u) ranges 0.00 to 0.20. The magnus coefficient is almost linear and coincidentally appears to be roughly as large as the spin parameter, it ranges from 0.00 to 0.22. These are rough values and I haven't really read this chapter so I'm not into the details.

See if this information can get you going. If you want I can send you a copy of the plot tomorrow, I'll have to look for a digital version.

Good luck
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where Re=2.1e5
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Old   May 3, 2010, 16:26
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Hi, as far as I understand these are experimental values for a scale model soccer ball, from a book I have here. The spin parameter is defined:

radius X angular_velocity / flow_velocity

The magnus coefficient is taken from the lift force, measured in the experiment.

I will upload the plot tomorrow. I'm afraid it has a reference to yet another reference. So I hope you can find the original data ...
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Old   May 3, 2010, 16:44
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So the spin parameter was calculated, right? it wasn't input in fluent?tnhx for the help, I'll be awaiting the plot tomorow


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Originally Posted by jack1980 View Post
Hi, as far as I understand these are experimental values for a scale model soccer ball, from a book I have here. The spin parameter is defined:

radius X angular_velocity / flow_velocity

The magnus coefficient is taken from the lift force, measured in the experiment.

I will upload the plot tomorrow. I'm afraid it has a reference to yet another reference. So I hope you can find the original data ...
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Old   May 4, 2010, 07:40
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Hi,

There is a lot of information on: http://www.grc.nasa.gov/WWW/K-12/airplane/beach.html Here they have a ideal lift for both the cylinder and the sphere. It's all in lb/ft etc. so beware of the conversion. This might perhaps be a first verification for (low Re?) results.

Then, the next step might be some validation. Here, you might also start with low Re (so you could have a Laminar model??). Then there is some very nice experimental data in:

B. Oesterlé and T. Bui Dinh, Experiments on the lift of a spinning sphere in a range of intermediate Reynolds numbers, Experiments in Fluids, 1998

For example they have the following plot (gamma is the spin parameter, read article or abstract for definitions):



So if you would like you could try to reproduce this plot, by changing Re and spin parameter.

For the soccer ball model I'm afraid I don't have a digital image of the plot. Anyway, it accumulates to the following:

Re = 2.1e5
spin_parameter = 0.00 - 0.20
cl = 1.1 * spin_parameter (roughly)

However this is for a soccer ball which is not really a sphere...

Hope this helps, good luck!
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Old   May 4, 2010, 08:30
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Right now my main difficulties are in the fluent part, where I now see that according to the spin parameter where the range is 0-0,20:

Sp=r*ang_vel/flow_vel

If I input a velocity flow of 30 m/s on the inlet for example, what value do I consider on the rotation angular velocity speed of the ball?(I did a conversion by doing v=w.r where w=v/r but I don't think that's right), or can I input a value of Sp in fluent?



Quote:
Originally Posted by jack1980 View Post
Hi,

There is a lot of information on: http://www.grc.nasa.gov/WWW/K-12/airplane/beach.html Here they have a ideal lift for both the cylinder and the sphere. It's all in lb/ft etc. so beware of the conversion. This might perhaps be a first verification for (low Re?) results.

Then, the next step might be some validation. Here, you might also start with low Re (so you could have a Laminar model??). Then there is some very nice experimental data in:

B. Oesterlé and T. Bui Dinh, Experiments on the lift of a spinning sphere in a range of intermediate Reynolds numbers, Experiments in Fluids, 1998

For example they have the following plot (gamma is the spin parameter, read article or abstract for definitions):



So if you would like you could try to reproduce this plot, by changing Re and spin parameter.

For the soccer ball model I'm afraid I don't have a digital image of the plot. Anyway, it accumulates to the following:

Re = 2.1e5
spin_parameter = 0.00 - 0.20
cl = 1.1 * spin_parameter (roughly)

However this is for a soccer ball which is not really a sphere...

Hope this helps, good luck!
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Old   May 4, 2010, 08:43
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ang_vel = flow_vel * Sp / radius
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Old   May 4, 2010, 08:54
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I obtained an angular velocity of 17,4 rad/s, considering sp=0,2 and v=30m/s, if I input these velocities on fluent I won't obtain lift, when I input a velocity higher than the angular velocity I can't obtain lift, I tried inputing for example w=500 rad/s and v=12m/s, then I obtained lift but it was just testing

QUOTE=jack1980;257507]ang_vel = flow_vel * Sp / radius[/QUOTE]
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Old   May 4, 2010, 08:57
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Have you set the direction of the spin vector and lift vector perpendicular?
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