# Flow around an circular cylinder

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 March 25, 2014, 06:45 Flow around an circular cylinder #1 Member     Christian Join Date: Sep 2013 Location: Germany Posts: 86 Rep Power: 5 Hello everbody, im working in a project at my university. And a part of this project is to simulate a flow around a circular cylinder. My main problem is that i can not reach the values for the drag coefficent that i read in Schlichting. cd(Re=300) = 0.729 | stationary cd(Re=300) = 1.32 | instationary 1. The mesh I Meshed everything in ICEM as octree mesh and put some prism elements around the cylinder. 2. CFX Steady State ref Pressure = 1 bar T.Model = SST Boundarys: Inlet - v = 1,46 m/s Outlet = 0 bar Sides = Symetry Zylinder = Wall no slip Top and Bottom = Wall free slip Transient same as in steady state and i calculated with an Stroudhal number of 0.2 for the transient Setup: Simulation Time 0.01 s Timestep Size 0.0041 s (~25 steps) Now the Problem: For Steady state i get a cd value of 0.83 And for Transient i get an cd value of 1.1 I think both of the simulations are setup wrong because when i take a look at the pressure distribution it doesnt look like something i expect. Where did i make the mistake? Why is there no karman street like in this video http://www.youtube.com/watch?v=BVyu4UEiB1k ? Is maybe just my simulation time not long enougth? Or is my reference Re Value wrong? I calculatet the Re and cd Value with a specific length = (diameter * length) Last edited by Chris_321; March 25, 2014 at 08:33.

 March 25, 2014, 17:47 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99

 March 25, 2014, 17:48 #3 Senior Member   Bruno Join Date: Mar 2009 Location: Brazil Posts: 273 Rep Power: 13 A few things: - Your boundary layer mesh looks very coarse. You should start by checking the Y-plus on the cylinder wall (should be <1-2 for SST). - Regenerate the mesh using a smaller growth ratio for the boundary layers (try 1.1-1.2). - Make sure you have enought layers around the cylinder to cover the whole height of the boundary layer. Cheers.

 March 26, 2014, 18:04 #4 Member     Christian Join Date: Sep 2013 Location: Germany Posts: 86 Rep Power: 5 I think this looks better But there is still the possibillity to improve, maybe behind the cylinder. Is my y+ ok for SST with auto wall treatment?

 March 26, 2014, 20:57 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 Yes, that looks much better. But you might still be a bit coarse - you will need to do a sensitivity check to find out. Your y+ means that the automatic wall treatment will be integrating to the wall at all locations. You should be OK here, but again, a sensitivity check is the best way to be sure.

 March 27, 2014, 07:47 #6 Member     Christian Join Date: Sep 2013 Location: Germany Posts: 86 Rep Power: 5 In a tutorial they initilaize the u velocity with (step(y/1[m])+ 0.5)*xvelociy I Know what the step function do but what means the y and why Do they use this for initilaizing?

 March 27, 2014, 07:57 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 The y/1[m] bit converts y as a value with units of metres to a value with no units. The step function requires an argument which is unitless.

 March 27, 2014, 08:56 #8 Member     Christian Join Date: Sep 2013 Location: Germany Posts: 86 Rep Power: 5 Is the value of y the max. y value of my domain in meter? They dont specify y in one of their expressions. Why do they set up the Initial value with a step function? Whats the benefit of it? Is it wrong to initilize just with my x_velocity? - Im confused!

 March 27, 2014, 17:43 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,726 Rep Power: 99 No, y is a field variable and takes the value of the y coordinate of that point. So it varies at each point of the domain. I suspect they use this initial condition to give the flow a strong asymmetry to start the vortex street off. This is probably not required and I would start as you have assuming just a constant X velocity. I would only artificially introduce an asymmetry if it is really required.

 March 28, 2014, 04:26 #10 Member     Christian Join Date: Sep 2013 Location: Germany Posts: 86 Rep Power: 5 thank you very mutch

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