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-   -   Help to understand these two images (http://www.cfd-online.com/Forums/main/10963-help-understand-these-two-images.html)

Freeman March 11, 2006 07:58

Help to understand these two images
 
Hi all!

I've been reading successful cases in Fluent web site about external aerodynamics developements in the shape of cars to reduce the drag. Particulary, I read about the Eco-Roadster by Opel and an Opel Astra and in both texts it is showed a plot of an iso-surface of ptot=0 coloured by turbulent kinethic energy (TKE) that I don't understand very well:

Eco-Roadster: http://img83.imageshack.us/img83/903...oadster0dm.jpg

Opel Astra: http://img90.imageshack.us/img90/1708/astra8rr.jpg

The Eco-Roadster text tells that this plot helped the engineers to see where and how to make aerodynamic improvements to the shape of the car; the fact is that I'm not able to explain to myself how the iso-surface of ptot=0 coloured by the TKE could help me to realize where and how to make aero improvements. Why ptot=0 and not static pressure=0 or any other? And what has to do here the TKE coloured on the iso-surface of ptot=0? Perhaps the question is how interact TKE in the total pressure to make an increase of drag?

As you can see in the Opel Astra image, it is showed again that this particular plot seems to be helpful to engineers to make areo improvements in the final styling shape of their cars. Anybody could enlighten me with this a bit?

Thanks a lot!

Regards, Freeman

Mani March 12, 2006 13:42

Re: Help to understand these two images
 
the term 'total pressure' is ambiguous, here, to say the least. it cannot be the absolute total pressure, because that cannot be zero. so it must be some kind of normalized or relative total pressure. i would assume that pressure zero really means "undisturbed freestream pressure", consistent with incompressible nomenclature (they should really learn that the world is compressible and name things in an unambiguous way). if that's true, then total pressure zero simply means zero velocity. zero velocity contours can help you identify regions of recirculation (separated flow) which are associated with drag, especially if the flow in those regions is very turbulent (much energy is dissipated). you will want to minimize those separations.

Freeman March 12, 2006 15:00

Re: Help to understand these two images
 
Many thanks for your reply, Mani!

I think that this total pressure is perhaps the static+dynamic pressure (without the atmosferic), so a ptot=0 is the zone where fluid has a zero velocity as you said. But I don't know what is the benefit of plotting the ptot=0 colouring it by the turbulent kinethic energy (TKE): is there any influence of the TKE on a fluid with zero velocity (ptot=0)?

Thanks a lot!

Lars Ola Liav├ąg March 14, 2006 09:11

Re: Help to understand these two images
 
Viscous losses makes ptot fall along a streamline. If the Opel people have used ptot=0 as the reference value at the inlet, ptot < 0 means you've had losses.

Freeman March 15, 2006 10:21

Re: Help to understand these two images
 
Thanks Lars, very clear your last post!

But is there any reason to colour this contour plot by the turbulent kinethic energy?I mean, the TKE has influence in the pressure to make these losses or it has nothing to do?

Thanks a lot!


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