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August 18, 2008, 00:47 
2D Transient flow around a cylinder

#1 
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Hi, I am trying to simulate the vortex shedding process behind a cylinder but have been unable to do so. Just wondering if anyone know how to solve the problem?
I've set up the problem as a very thin plate with symmetry on the top and bottom side of the cylinder. I am solving it using the LES Smagorinsky model, and have also applied wall roughness to the cylinder. In the workbench mesh, I have also set up 5 boundary layers with first thickness of 5E8m. I've run several simulations, some running over 24 hours but have been unable to produce vortex shedding. Anyone encountered the same problem or know how to fix this? Any help will be greatly appreciated. Cheers, Ben 

August 18, 2008, 01:59 
Re: 2D Transient flow around a cylinder

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Hi,
You can't run LES in 2D  but it should shed vorticies regardless. What Re are you modelling? You will need second order differencing in time and space. How many flow time scales did your 24 hour go for? A small asymmetry should start when the flow has travelled about 1 diameter or so, and build up into a vortex as the flow progresses. Glenn Horrocks 

August 18, 2008, 05:16 
Re: 2D Transient flow around a cylinder

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I'm looking at a cylinder in 3D with the LES model.
The fluid domin is rectangular (length=60m, height=width=20m) and the cylinder diameter=1m and sapns the domain. The centre of the cylinder is 10m downstrean of the inlet of the rectangle. At Mach=0.3 and ISA sealevel gives a Reynolds no.=7million. I want to try the "define first layer by y+" option. I've set it to y+=1 so a 1st layer is 3.9x106 and I've put in 20 layers at 1.2 expansion factor. Not sure if this is a good y+ value. With more layers the memory was maxed out. I'll have to set up a few Controls at get more element detail in the wake and around the cylinder to merge neatly with the large elements on the domain boundaries. Is this good enough for the boundary layer? Thanks. 

August 18, 2008, 05:16 
Re: 2D Transient flow around a cylinder

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Cheers for your post Glenn,
My Reynolds number is in the order of ~50000, and I have used physical timescales of 0.02seconds. I stopped the last simulation at 0.6seconds. (I have a flow velocity of 20m/s so by 0.6 seconds the flow would have travelled more than 1diameter down the cylinder) Yes, I have used second order backward euler for my transient scheme and a specified blend factor of 0.8 for my advection scheme. Glenn, is there a way of introducing flow perturbation in ANSYSCFX? I've just read somewhere on another forum that this might have to be done to induce vortex shedding..... When you mentioned that LES can't be run in 2D, should I used kepsilon model instead? Thanks for your help! Ben 

August 18, 2008, 08:33 
Re: 2D Transient flow around a cylinder

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August 18, 2008, 21:06 
Re: 2D Transient flow around a cylinder

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Hi,
Yes, for 2D you should use a standard RANS turbulence model. SST or keps are typical choices. When you go 3D you should consider the SAS model. It should be ideal for these types of flows. Generally you don't need to perturb the flow, enough numerical noise exists to get it to start. Regards, Glenn Horrocks 

August 18, 2008, 21:07 
Re: 2D Transient flow around a cylinder

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Hi,
Meshing to y+=1 at such a high Reynolds number is difficult. Sounds like you should consider the SAS turbulence model. Glenn Horrocks 

August 18, 2008, 21:57 
Re: 2D Transient flow around a cylinder

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Do u have these files of that simulation (http://br.youtube.com/watch?v=jMmPYuKqKCU)? (.blk, .tin, .ccl, and .cfx files)
very nice this movie, congratulations! 

August 19, 2008, 02:24 
Re: 2D Transient flow around a cylinder

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Thanks Glenn I'll try that aswell. Just sort of trying different options in CFX.
How do you work out an appropriate y+ value? I've seen the equation but to me it seems like guess work/experience. 

August 19, 2008, 23:50 
Re: 2D Transient flow around a cylinder

#10 
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Het, that's a great video! Cheers for that!


August 19, 2008, 23:51 
Re: 2D Transient flow around a cylinder

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Yeap, finally got it in the end. Simply changing from LES to kepsilon and cutting down the timestep solved the problem. Cheers for your help!
Ben 

August 20, 2008, 02:03 
Re: 2D Transient flow around a cylinder

#12 
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Hi,
Only mesh to y+=1 if you need the sublayer or transition models. For most high Re flows normal wall functions are as good as anything and allow you to use y+ from about 15 to 100 or maybe higher. You have to do a sensitivity check to find out for your case what mesh you need here. Glenn Horrocks 

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