# Incorrect Drag and Drag Coefficient for flow over a cylinder

 Register Blogs Members List Search Today's Posts Mark Forums Read

 June 29, 2010, 12:13 #2 Senior Member   Join Date: Jul 2009 Posts: 230 Rep Power: 11 At a Re of 100, would the flow not be laminar? Have you tried turning off the turbulence model? Also, are you comparing apples to apples, i.e. are you comparing total drag to total drag, viscous drag to viscous drag, etc?

 June 29, 2010, 14:48 #3 Senior Member   jeff osborne Join Date: Mar 2010 Posts: 108 Rep Power: 8 Hi, thanks for the reply. Looking at the solvers, I guess I would need to use icoFoam. Is there anything that is used for laminar steady-state problems? Does that even exist? Thanks -J

 June 30, 2010, 03:45 #4 New Member   Mohammd Aghakhani Join Date: May 2009 Posts: 12 Rep Power: 9 In Re=100 you need u unsteady simulation because of vortex shedding behind cylinder therfore, Cd will be time averaged CD over simulation time

 June 30, 2010, 06:49 #5 Senior Member   jeff osborne Join Date: Mar 2010 Posts: 108 Rep Power: 8 I believe it calculates it at every iterations, and then from there I can take the average of them to get the cd of the cylinder, right?

 June 30, 2010, 10:37 #6 Senior Member   jeff osborne Join Date: Mar 2010 Posts: 108 Rep Power: 8 Also for my drag coefficient output, why does it start at a high value, then continuously decreases, appearing to asymptotically approach a value (the value I'm assuming is the actual drag coefficient)? Thanks

June 30, 2010, 11:17
#7
Senior Member

jeff osborne
Join Date: Mar 2010
Posts: 108
Rep Power: 8
Quote:
 Originally Posted by agd At a Re of 100, would the flow not be laminar? Have you tried turning off the turbulence model? Also, are you comparing apples to apples, i.e. are you comparing total drag to total drag, viscous drag to viscous drag, etc?
agd:

In response about the Reynolds number. The reason i chose to use turbulent modelling was because I read that for flow around an object, purely laminar flow exists only until Re=0.1. So, to be safe, I decided to use a turbulence model. Does this sound correct?

Thanks

 June 30, 2010, 12:40 #8 Senior Member   Join Date: Jul 2009 Posts: 230 Rep Power: 11 I don't know where you read the condition on the Re, but for practical purposes the flow around a cylinder at a Re = 100 will be laminar, and probably massively unsteady (I can't remember the cutoff where the separation bubble collapses into a vortex street.) I am unfamiliar with icoFoam, so I cannot answer any questions about that code. The behavior you describe for the drag coefficient is not unexpected, since the initial drag calculation will use a velocity gradient at the surface much larger than what the actual solution is - after one iteration or timestep, the approximate gradient at the surface is Ufreestream/ds, where ds is the distance of the first node off the wall. This assumes a no-slip wall, and is obviously much larger than the final velocity gradient, and resultant shear stress, will be.

 June 13, 2012, 06:24 #9 New Member   Malhar Malushte Join Date: May 2012 Posts: 16 Rep Power: 6 hello every1, i am also new to openfoam. i have done similar sim ulation for flow acrodss a cylinder. i get drag n lift forces correctly, i.e accor ding to vortex shedding i am getting variation in lift forces. but the coeff of drag n lift Cd n Cl, i am getting them constant throughout. i am unable ti digest this contrasting behavour. please guide me thr this. thanks n regards malhar.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post MFerris CFX 44 May 11, 2015 18:27 Jwolf CFX 11 February 4, 2010 06:08 lin OpenFOAM Running, Solving & CFD 3 April 16, 2009 10:50 vinz OpenFOAM Running, Solving & CFD 98 October 27, 2008 09:43 Avijit FLUENT 11 March 29, 2004 14:04

All times are GMT -4. The time now is 12:27.