# How to compute the viscous dissipation term

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

 December 17, 2013, 03:14 How to compute the viscous dissipation term #1 Member   yehanyu Join Date: Mar 2012 Location: Beijing, China Posts: 46 Rep Power: 7 Hi, everybody. I am performing 2D unsteady CFD simulations on a cross-flow fan. I am using ANSYS FLUENT 12.1. I adopt the standard k-omega model to simulate turbulence. I want to compute the viscous dissipation term. I cannot find this variable in FLUENT. Hence I exported the solution to Tecplot, and then write some equations to compute it. I use the following equation to compute the viscous dissipation term: (This equation can be found in ANSYS FLUENT 12.0 Theory Guide, page 4-24 and 4-25) viscous dissipation= where I use [Data]|[Alter]|[Specify Equations…] in Tecplot to implement these equations. The expressions which I type into Tecplot are listed as follows: {duxdx}=ddx({x-velocity}) {duxdy}=ddy({x-velocity}) {duydx}=ddx({y-velocity}) {duydy}=ddy({y-velocity}) {viscous_heating2}=ddx({x-velocity}*({viscosity-eff}*({duxdx}*2)-2.0/3.0*{viscosity-eff}*({duxdx}+{duydy}))+{y-velocity}*({viscosity-eff}*({duxdy}+{duydx}))) + ddy({x-velocity}*({viscosity-eff}*({duydx}+{duxdy}))+{y-velocity}*({viscosity-eff}*({duydy}*2)-2.0/3.0*{viscosity-eff}*({duxdx}+{duydy}))) Finally, I display contour of the variable “viscous_heating2”. However, to my surprise, the minimum value of this variable is -679152, and the maximum value is 352578. This is weird – because the dissipation should be always positive. How could negative values occur?

 December 17, 2013, 03:33 #2 Senior Member   cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 13 One possible cause of your troubles is that you are using the ddx functions in tecplot to compute derivatives. That's among the worst methods you can use, they are first oder accurate (at most second), and depending on your grid, that can be way to inaccurate. Can you compute your derivatives in your code? On the original mesh maybe? Then there's at least the chance that you are not hunting tecplot ghosts with this....

 December 17, 2013, 23:26 #3 Member   yehanyu Join Date: Mar 2012 Location: Beijing, China Posts: 46 Rep Power: 7 Thank you very much. I'll try it.

 Tags dissipation, tecplot, turbulence, viscous

 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 santiagomarquezd OpenFOAM Programming & Development 1 December 13, 2011 21:44 siw CFX 1 December 6, 2010 07:41 newbee OpenFOAM Running, Solving & CFD 4 August 25, 2006 14:33 Joseph CFX 0 October 1, 2004 08:52 yann Phoenics 0 June 5, 2003 10:23

All times are GMT -4. The time now is 18:18.