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Old   December 22, 2010, 21:14
Default question about turbulent kinetic energy
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fanwei
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i have a Question about turbulent kinetic energy.

I did a CFD Simulation of Car, i have this Picture of turbulent kinetic

energy

Can somebody explain, what the turbulent kinetic energy mean and the

large turbulent kinetic energy in the Area of the car bring which result??

tks everybody for helping me

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Old   December 23, 2010, 04:55
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Irshad
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Turbulence kinetic energy (TKE) is the mean K.E per unit mass associated with eddies in turbulent flow. For the car, it is loss of ke from mean flow in order to continue eddies which are formed due to pressure difference.
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Old   December 23, 2010, 07:43
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Dear Junker;
when flow becomes turbulent, transport mechanisms like diffusion, highly increases beyond the molecular diffusion (which is encountered in laminar flows). it is believed that turbulent flows structure includes parcells with highly transient characteristics called eddies. in fact, eddies movements intensifies flow transportation mechanisms like diffusion. they can be as large as flow integral scale and they can also be very small.
Turbulence kinetic energy (TKE) is the energy content of eddies in turbulent flows. larger the size, the higher is energy content of eddies.
TKE is extracted from mean flow to larger eddies, from larger eddies to smaller ones and finally it is going to be dissipated in very small eddies where viscous effects defeats the kinetic energy.
high TKE points to regions in which high amount of turbulent energy is extracted from mean flow.

i hope it helps
Ragards
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Old   December 23, 2010, 13:55
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tks for your both Help to explain, what the TKE mean. I understand it a bit

more.

But now my question ist: the Area with highly TKE, just like the Underbody

and the Backside of car in Picture. Do this Area bring some negative

effects of the Aerodynamik because of this eddies



Merry chrismas
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Old   December 23, 2010, 14:15
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At the rear end there is low pressure because of flow seperation. Because of low pressure at rear and high pressure at front, presure drag produced which is major in Aerodynamics of road vehicles. In the low pressure region large eddies formed, hence it extracts more k.e from mean flow. similarly, at frond end curvature, flow departs into two ways and because of low ground clearance, high speed flow with low pressure region will form at the underside. That is why TKE dissipation is more.
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Old   December 23, 2010, 14:23
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can you pls explain, why there is low pressure in Large Eddies? just because of Movement?
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Old   December 23, 2010, 14:49
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Whenever the flow seperation occurs, the vaccum space will be created because of unablity to attach. Later flow try to fill the vaccum as flow moves from high preesure to low pressure. In taht process, reversal flow occurs as well eddies form by interaction with mean flow which have sufficient energy. since flow reversal occurs very faster, low pressure will be created and vice versa as per Bernoulli's principle.
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Old   December 24, 2010, 01:16
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I had an idea of a somewhat different flow seperation phenomenon ocuuring behind 'bluff bodies' and also the logic behind formation of the eddies.
Pls correct me if i am wrong.
1) First of all, there is no vaccum created, (I know that you must have meant a low pressure zone, but just for the heck of mentioning)
2) When the flow across the bluff bodies occur, the total energy of the flow ( I mean relative to the body ), especially total pressure over the body is higher than the zone of stagnant air behind it.
3) Hence when the flow over the body reaches the turn of high gradient in the aft region of the bluff body, the boundary layer can no loger sustain such a pressure difference and if you see the classical boundary layer seperation picture you cn see, due the seperation itself very high vorticity is induced in the flow, Hence whe this flow seperates it 'curls' , leading to formation of eddies.

Hence the answer to Junkers Ques would be that, the low pressure at the core is one of the reasons for formation of eddies

My intention here is to only get a clearer picture by having an open discussion,

ANYBODY HAVING A GOOD IDEA OF THIS PHENOMENON PLEASE SHED SOME LIGHT ON IT AND CORRECT ME IF I AM MISTAKEN.


As far as your question on ill effects of high TKE is concerned, one obvious drawbacks is greater skin friction drag due to increased tubulence because of seperation, but I dont know statistically how much does this matter.

-Dinesh
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Old   December 24, 2010, 04:06
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Quote:
Originally Posted by junker4236 View Post
Can somebody explain, what the turbulent kinetic energy mean and the
In turbulent flows instantaneous velocity is presented as
U=<U>+u
where <..> stands for the mean part of any value and u is the velocity fluctuation part. Mean part is calculated as average within time period T which is large in comparison with typical fluctuations.

Average fluctuations <u> is equal to zero but <u^2> is not.

Value \sqrt{<u^2>} is named turbulent intensity.
Half of sum of square intensities in all 3 directions is named turbulent kinetic energy (k):
k=\frac{1}{2}(<u_x^2>+<u_y^2>+<u_z^2>)
see for example Hinze's book "Turbulence" or other.
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