capturing vortex shedding

alinik · February 24, 2014, 14:13

Hey guys,

I need to capture vortex shedding after the flow is passed over a turbine blade. I have resolved time step to the point that it is 0.001 sec and my mesh is very refined. I do not know what else I have to vary to capture this. Strouhal number should be 0.2 and I chose my timestep so it is way below period of vortex shedding. Theoretically everything should work now but I do not know what is the problem.

Thank

ghorrocks · February 24, 2014, 17:12

Sounds like an FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

alinik · February 24, 2014, 18:11

Glenn,

These are basic and rudimentary type of things that every CFD researcher is well aware of that. I have done all these steps before and could not capture the vortices.

Any specific and detailed solution is much appreciated.

flotus1 · February 24, 2014, 18:16

Then first of all we need a specific and detailed description of the problem.

ghorrocks · February 24, 2014, 18:18

They are basic - but you made no comment about covering the basics. Regardless, I bet the problem will be in that list somewhere.

For instance your comments about time step size suggest you have not done a time step sensitivity study - this is essential. Even better, set it to adaptive time stepping homing in on 3-5 coeff loops per iteration and let it find its own time step size (and don't limit the min and max time step size artificially, if it needs a very small time step then let it use it).

Also please post your CCL and an image of your geometry, including the mesh.

alinik · February 26, 2014, 12:42

This is the mesh picture. I am going to upload the CCL in a minute.

alinik · February 26, 2014, 12:47

And this is the CCL.

Thank you guys

ghorrocks · February 26, 2014, 17:49

You have fixed time stepping. As previously stated, you really need to check this, or go to adaptive time stepping.

You have viscous work on. It looks pretty unlikely this is important so turn it off.

You are also setting the advection scheme for each equation individually. I would not do this, at least while you are debugging. I would remove this and leave them at the defaults.

February 24, 2014, 14:13	capturing vortex shedding	#1
alinik Senior Member ali Join Date: Oct 2009 Posts: 318 Rep Power: 18	Hey guys, I need to capture vortex shedding after the flow is passed over a turbine blade. I have resolved time step to the point that it is 0.001 sec and my mesh is very refined. I do not know what else I have to vary to capture this. Strouhal number should be 0.2 and I chose my timestep so it is way below period of vortex shedding. Theoretically everything should work now but I do not know what is the problem. Thank

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February 24, 2014, 17:12		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Sounds like an FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

February 24, 2014, 18:11		#3
alinik Senior Member ali Join Date: Oct 2009 Posts: 318 Rep Power: 18	Glenn, These are basic and rudimentary type of things that every CFD researcher is well aware of that. I have done all these steps before and could not capture the vortices. Any specific and detailed solution is much appreciated.

February 24, 2014, 18:16		#4
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,427 Rep Power: 49	Then first of all we need a specific and detailed description of the problem.

February 24, 2014, 18:18		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	They are basic - but you made no comment about covering the basics. Regardless, I bet the problem will be in that list somewhere. For instance your comments about time step size suggest you have not done a time step sensitivity study - this is essential. Even better, set it to adaptive time stepping homing in on 3-5 coeff loops per iteration and let it find its own time step size (and don't limit the min and max time step size artificially, if it needs a very small time step then let it use it). Also please post your CCL and an image of your geometry, including the mesh.

February 26, 2014, 17:49		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	You have fixed time stepping. As previously stated, you really need to check this, or go to adaptive time stepping. You have viscous work on. It looks pretty unlikely this is important so turn it off. You are also setting the advection scheme for each equation individually. I would not do this, at least while you are debugging. I would remove this and leave them at the defaults.