Quick question - Can I use rotating wall for a spinning rugby ball (with seams)??

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March 1, 2017, 07:54
Quick question - Can I use rotating wall for a spinning rugby ball (with seams)??
#1
New Member

Jake McPherson
Join Date: Jan 2017
Posts: 6
Rep Power: 6
Hi,

I'm simulating a rotating Rugby ball to look at spin effects. I'll be spinning it along it's longitudinal axis. I have seen that the rotating wall can only be used for symmetrical geometries, does this rule the ball out?

I've added a picture so you can see the seams.
Attached Images
 ball with seams.jpg (99.3 KB, 28 views)

 March 1, 2017, 14:22 #2 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 By itself you can do a spinning ball (i.e. there's no wind). The problem is when you want it to spin and translate. It's doable in some cases depending on your boundary conditions. The question is whether or not the seams actually move, if the mesh needs to move or not.

March 1, 2017, 14:29
#3
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Jake McPherson
Join Date: Jan 2017
Posts: 6
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Quote:
 Originally Posted by LuckyTran By itself you can do a spinning ball (i.e. there's no wind). The problem is when you want it to spin and translate.
I'm not sure what you mean. I planned on making a fluid domain with a velocity inlet, then have the ball spin in there. The wall motion, rotating boundary condition works for spheres etc but will it work with a seemed ball?

I've put another pic in showing where it'll be rotating. If I can't use the moving wall I'm really stuck on how to model it
Attached Images
 showing axis.jpg (98.6 KB, 13 views)

 March 1, 2017, 14:53 #4 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 The problem is that your mesh is fixed in space, unless you do a dynamic mesh (which is the general solution). If the velocity is coming along the axis of the ball then it can work. If you go into a rotating reference frame, then it's possible to solve it using a static mesh as long as you can specify the correct boundary conditions. If there's an angle of attack, then it doesn't work. This is also a hint. Nothing happens if the ball is spinning on its longitudinal axis and moving in the same direction. If there's an angle of attack, you get the magnus effect because a fixed point on the ball sees a time-varying velocity.

March 1, 2017, 15:17
#5
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Jake McPherson
Join Date: Jan 2017
Posts: 6
Rep Power: 6
Quote:
 Originally Posted by LuckyTran The problem is that your mesh is fixed in space, unless you do a dynamic mesh (which is the general solution). If the velocity is coming along the axis of the ball then it can work. If you go into a rotating reference frame, then it's possible to solve it using a static mesh as long as you can specify the correct boundary conditions. If there's an angle of attack, then it doesn't work. This is also a hint. Nothing happens if the ball is spinning on its longitudinal axis and moving in the same direction. If there's an angle of attack, you get the magnus effect because a fixed point on the ball sees a time-varying velocity.
I'm trying to avoid the dynamic mesh at the minute. The model I showed was just going to be my comparison to current work, to show I'm in the right range for drag.

I was going to create 6 different models, each with a different angle of attack (0-90 degrees), then set the spin axis accordingly.

So i.e at 15 deg, rotational origin =(0,0,0)

Then use trig to find xy coordinates the would relate to 15 deg.

The entire scope of my project really is looking at magnus effects on the ball.

P.s thanks for the replies

 March 1, 2017, 15:24 #6 New Member   Jake McPherson Join Date: Jan 2017 Posts: 6 Rep Power: 6 I've just figured it out for 15 as an example. If I assume the vector has a length of 1 and an angle of 15 deg, y=1sin(15) and x=1cos(15) This gives rotation axis direction (0.9659,0.25882,0). Would that work?