# Swirl-Velocity Specification of 2-D Fan Boundary Condition

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 September 21, 2018, 03:04 Swirl-Velocity Specification of 2-D Fan Boundary Condition #1 New Member   Join Date: Sep 2018 Posts: 6 Rep Power: 6 I opened a relevant thread before at the student community forums of ANSYS but did not find a closure about the subject (i.e. https://studentcommunity.ansys.com/t...ary-condition/ ). I'll appreciate any help from the cfd-online community. I am defined a 2-D fan model using the fan boundary condition on a square surface in a 3-D simulation of a room. I am puzzled by how Fluent interprets the tangential velocity since the tangential velocity I get as a result is always less than what I input, and typically half of the constant value I input as tangential velocity (i.e. when I use values on the order of axial velocity). The results below correspond to the case with a constant tangential velocity of 1 m/s. According to the plot of tangential velocity as a function of y direction along the midline of the fan surface, the values plateaued at around 0.5 m/s outside hub (half of the input). How should I interpret this? I'll greatly appreciate your help in clarifying how Fluent uses inputs for tangential velocity in the 2-D fan boundary condition? Why don't I see the tangential velocity imposed between the hub radius and outer dimension of the boundary surface as I define? Is there a physical reasoning behind this?

 September 21, 2018, 11:26 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,146 Rep Power: 61 Which boundary condition are you using? Fluent doesn't have a "fan boundary condition" Just post a screenshot of your boundary condition input.

September 24, 2018, 01:43
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Quote:
 Originally Posted by LuckyTran Which boundary condition are you using? Fluent doesn't have a "fan boundary condition" Just post a screenshot of your boundary condition input.
Here is the screenshot of what I use.
Attached Images
 bc.jpg (90.4 KB, 64 views)

 September 28, 2018, 15:25 #4 New Member   Join Date: Sep 2018 Posts: 6 Rep Power: 6 Any help from the community would be appreciated.

 October 2, 2018, 10:13 #5 Member   yanki cob Join Date: Jan 2018 Posts: 30 Rep Power: 7 Can you check velocities slightly downstream/upstream of the fan surface?

October 2, 2018, 13:58
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Quote:
 Originally Posted by yanki Can you check velocities slightly downstream/upstream of the fan surface?
1. Tangential velocity
2. X velocity
3. Z velocity

at the velocity inlet and fan bc
Attached Images
 swirl_Z_axis_1.0_inlet_fan.jpg (120.1 KB, 34 views) swirl_Z_axis_1.0_inlet_fan_xvelocity.jpg (119.7 KB, 23 views) swirl_Z_axis_1.0_inlet_fan_zvelocity.jpg (120.2 KB, 20 views)

October 3, 2018, 08:49
#7
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yanki cob
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OK, but can you change the contour plane to another plane that is slightly downstream/upstream of the fan plane? Like, 5mm away from the fan.

Also, why do you have a velocity inlet BC?

Quote:
 Originally Posted by erdenh 1. Tangential velocity 2. X velocity 3. Z velocity at the velocity inlet and fan bc

Last edited by yanki; October 3, 2018 at 08:51. Reason: added

October 3, 2018, 09:23
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Quote:
 Originally Posted by yanki OK, but can you change the contour plane to another plane that is slightly downstream/upstream of the fan plane? Like, 5mm away from the fan. Also, why do you have a velocity inlet BC?
No special reason for using velocity inlet BC. I just used a uniform velocity to be able to see the effect of swirl function in the fan BC.

Attached are contours for 5mm upstream, fan and 5mm downstream with the same scale.

Thanks
Attached Images
 swirl_Z_axis_1.0_5mm_upstream.jpg (123.0 KB, 16 views) swirl_Z_axis_1.0_fan.jpg (123.4 KB, 14 views) swirl_Z_axis_1.0_5mm_downstream.jpg (150.8 KB, 18 views)

October 3, 2018, 09:38
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yanki cob
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So, the max. velocity 5mm downstream is higher than max. velocity on fan plane, right?

Can you find a plane with the velocity value that you input (1m/s I guess)?

Try to turn OFF the node values.

Quote:
 Originally Posted by erdenh No special reason for using velocity inlet BC. I just used a uniform velocity to be able to see the effect of swirl function in the fan BC. Attached are contours for 5mm upstream, fan and 5mm downstream with the same scale. Thanks

October 3, 2018, 10:11
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Quote:
 Originally Posted by yanki So, the max. velocity 5mm downstream is higher than max. velocity on fan plane, right?
Correct.

Quote:
 Originally Posted by yanki Can you find a plane with the velocity value that you input (1m/s I guess)? Try to turn OFF the node values.
I have attached the contour at 25 mm downstream of the fan, where the tangential velocity becomes 1 m/s (node values off). I guess I understand where you are going. Does the fan BC impose the tangential velocity within the first cell downstream of the fan, since the cell size is 25 mm?
Attached Images
 swirl_Z_axis_1.0_25mm_downstream.jpg (188.5 KB, 10 views)

October 3, 2018, 10:52
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yanki cob
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I don't know how the BC works, and couldn't find any explanation on the theory/user guide. But there might be an averaging problem, that's why I asked for turned off nodes.

As I understand, this is not a "problem" for you, you just wanted to make sure that the fan BC works the way it intended to.

Quote:
 Originally Posted by erdenh Correct. I have attached the contour at 25 mm downstream of the fan, where the tangential velocity becomes 1 m/s (node values off). I guess I understand where you are going. Does the fan BC impose the tangential velocity within the first cell downstream of the fan, since the cell size is 25 mm?

 Tags 3-d simulation, fan boundary condition, swirl, tangential velocity