Lift Vs Drag?

Vamsi · May 1, 2016, 02:18

Hello everyone,

In ANSYS FLUENT: for flow over a object, Could anyone tell me why the coefficient of drag reaches a constant value whereas the Coefficient of lift oscillates over time?

Thanks in advance for the reply.

davidwilcox · May 2, 2016, 04:28

it simply means your solution is not converged. This is of course if you are running a steady state case.

Vamsi · May 2, 2016, 05:39

Quote:

Originally Posted by davidwilcox

it simply means your solution is not converged. This is of course if you are running a steady state case.

No. I am running a transient case.

Hamidzoka · May 3, 2016, 03:38

Hi
If there is no separation around your object constant drag coefficient can be related to viscousity which acts as a damper and can suppress the falucctuations. While the lift comes from pressure distributions on the surfaces of the object and can be subject to change when you run a time dependet simulation. Wakes and other time dependent flow structures can simply affect pressure distributions as time passes.

Regards

Vamsi · May 3, 2016, 22:12

Quote:

Originally Posted by Hamidzoka

Hi
If there is no separation around your object constant drag coefficient can be related to viscousity which acts as a damper and can suppress the falucctuations. While the lift comes from pressure distributions on the surfaces of the object and can be subject to change when you run a time dependet simulation. Wakes and other time dependent flow structures can simply affect pressure distributions as time passes.

Regards

Hi. Thanks for the reply. I do have flow separation on the square cylinder. So does this effect the Cd Vs time plot?

ys12 · May 4, 2016, 11:55

If you are doing a transient analysis with turbulence options checked, there is no reason to expect a constant C_l value. At each instant in time the separated flow leaves a chaotic turbulent wake (no matter how small).

I think in an unsteady situation like this, a steady C_l / C_d value doesn't mean much.

michujo · May 4, 2016, 11:57

Hello,

I am no expert in the subject but it could be that your simulation is capturing the right behaviour.

You might want to check the Strouhal number of the vortex shedding from your simulations against experimental and numerical data from the literature. There are plenty of papers on the matter, just google "flow past a square cylinder".

Cheers,
Michujo.

lazzouni · May 8, 2016, 16:00

how to have the drag coef in fluent thanks

May 1, 2016, 02:18	Lift Vs Drag?	#1
Vamsi New Member VAMSI YERRAGURAVAGARI Join Date: Mar 2016 Location: Houston-TX Posts: 16 Rep Power: 10	Hello everyone, In ANSYS FLUENT: for flow over a object, Could anyone tell me why the coefficient of drag reaches a constant value whereas the Coefficient of lift oscillates over time? Thanks in advance for the reply. Last edited by Vamsi; May 1, 2016 at 14:03.

May 8, 2016, 16:00	drag coef	#8
lazzouni New Member lazzouni rima Join Date: Apr 2016 Posts: 3 Rep Power: 10	how to have the drag coef in fluent thanks

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May 2, 2016, 04:28		#2
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	it simply means your solution is not converged. This is of course if you are running a steady state case.

May 3, 2016, 03:38		#4
Hamidzoka Senior Member Hamid Zoka Join Date: Nov 2009 Posts: 282 Rep Power: 18	Hi If there is no separation around your object constant drag coefficient can be related to viscousity which acts as a damper and can suppress the falucctuations. While the lift comes from pressure distributions on the surfaces of the object and can be subject to change when you run a time dependet simulation. Wakes and other time dependent flow structures can simply affect pressure distributions as time passes. Regards

May 4, 2016, 11:55		#6
ys12 New Member Join Date: Dec 2014 Posts: 3 Rep Power: 11	If you are doing a transient analysis with turbulence options checked, there is no reason to expect a constant C_l value. At each instant in time the separated flow leaves a chaotic turbulent wake (no matter how small). I think in an unsteady situation like this, a steady C_l / C_d value doesn't mean much.

May 4, 2016, 11:57		#7
michujo Senior Member Join Date: Dec 2011 Location: Madrid, Spain Posts: 134 Rep Power: 15	Hello, I am no expert in the subject but it could be that your simulation is capturing the right behaviour. You might want to check the Strouhal number of the vortex shedding from your simulations against experimental and numerical data from the literature. There are plenty of papers on the matter, just google "flow past a square cylinder". Cheers, Michujo.