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December 17, 2013, 03:14 
How to compute the viscous dissipation term

#1 
Member
yehanyu
Join Date: Mar 2012
Location: Beijing, China
Posts: 41
Rep Power: 6 
Hi, everybody. I am performing 2D unsteady CFD simulations on a crossflow fan. I am using ANSYS FLUENT 12.1. I adopt the standard komega 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 424 and 425) 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({xvelocity}) {duxdy}=ddy({xvelocity}) {duydx}=ddx({yvelocity}) {duydy}=ddy({yvelocity}) {viscous_heating2}=ddx({xvelocity}*({viscosityeff}*({duxdx}*2)2.0/3.0*{viscosityeff}*({duxdx}+{duydy}))+{yvelocity}*({viscosityeff}*({duxdy}+{duydx}))) + ddy({xvelocity}*({viscosityeff}*({duydx}+{duxdy}))+{yvelocity}*({viscosityeff}*({duydy}*2)2.0/3.0*{viscosityeff}*({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: 12 
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: 41
Rep Power: 6 
Thank you very much. I'll try it.


Tags 
dissipation, tecplot, turbulence, viscous 
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