# A circular cylinder in simple shear flow

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October 5, 2021, 23:15
A circular cylinder in simple shear flow
#1
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Abbas ali
Join Date: Oct 2021
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The motion of circular cylinder with density ratio 10 (density of cylinder =10* density of fluid) free to rotate about centre axis placed at the centreline of the fluid field with the upper and lower walls moving in opposite directions at velocity Uw /2 was studied for a wide range of Reynolds numbers.
I want to find variation of angular speed ω for the confinement ratio of H/a=4, where G is the shear rate (2Uw/H), a is the radius of the cylinder and H is the channel height, with Re=Ga2/ν.

i tried this dynamic mesh .
but my cylinder is not rotating.
can someone help how should i proceed it. do i need to use the udf for this
Attached Files
 problem statement_compressed.pdf (52.4 KB, 10 views)

 October 7, 2021, 02:52 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,671 Rep Power: 65 I have no idea what you would even use a UDF for, so no =) I don't follow what is this cylinder. Is it just a circular region of fluid with a different density? Then you don't need a dynamic mesh. You just need to specify the proper intitial conditions. If it's a solid cylinder, then I don't get why the cylinder has a density. You still wouldn't need a dynamic mesh. But you could model this with slip walls with 0 stress and find the fluid velocity near the wall.

 October 19, 2021, 06:28 #3 New Member   Abbas ali Join Date: Oct 2021 Posts: 2 Rep Power: 0 It is solid cylinder. I want to know the angular velocity with which solid cylinder will rotate. I used periodic beoundary condition between inlet and exit but then cylinder do not rotate. When i used same pressure at inlet and outlet still cylinder do not rotate. Upper and bottom wall are moving in +x and -x respectively with some velocity. Do you suggest any method/boundary conditions by which cylinder can rotate

 October 19, 2021, 11:39 #4 Senior Member   Lorenzo Galieti Join Date: Mar 2018 Posts: 373 Rep Power: 12 I think the domain looks something like this —————————-> V_wall Cylinder O <————————— -V_wall So the shear layer should indeed promote the rotation of the cylinder. The density of the cylinder then has some effect on the moment of inertia Having said that, no clue on how to solve this with fluent

 October 19, 2021, 12:21 #5 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,671 Rep Power: 65 I mean the way to model this would be to set the wall rotation speed for the cylinder. Link it to a parameter and iterate it until the integrated wall shear stress is zero (kind of like in the old days when we used to iterate the pressure until the target flow rate is achieved). It still doesn't involve a dynamic mesh & I'm still confused where the density/inertia of the cylinder comes in for a steady problem. With that said, maybe that info wasn't even relevant in the first place, except to say that the cylinder is solid. Having also said that, what is even being done in this simulation that we have no details about that is supposedly not rotating? All we have is a problem statement.

October 19, 2021, 12:40
#6
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Lorenzo Galieti
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Quote:
 Originally Posted by LuckyTran It still doesn't involve a dynamic mesh & I'm still confused where the density/inertia of the cylinder comes in for a steady problem.
Well, I kind of assumed that he wants to do unsteady with the cylinder initially at rest. If he wants steady, then no clue, at the end of the day it is not my thesis :P

 October 19, 2021, 16:30 #7 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,671 Rep Power: 65 If this is an unsteady problem I would expect to see something (anything) concerning initial conditions.

 Tags ansys, cylinder, fluent, shear, simple