CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

Inviscid Turbulent flow

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

Like Tree10Likes
  • 1 Post By arizwan
  • 1 Post By FMDenaro
  • 1 Post By duri
  • 2 Post By cfdnewbie
  • 1 Post By FMDenaro
  • 1 Post By agd
  • 1 Post By FMDenaro
  • 2 Post By FMDenaro

Reply
 
LinkBack Thread Tools Display Modes
Old   August 28, 2013, 02:12
Post Inviscid Turbulent flow
  #1
Member
 
Vino
Join Date: Mar 2013
Location: India
Posts: 70
Rep Power: 3
Vino is on a distinguished road
I have a strange question. Is there any possibility that an inviscid flow can be a turbulent flow and do we use any inviscid turbulent model for our practical application?
Vino is offline   Reply With Quote

Old   August 28, 2013, 03:15
Default Inviscid Turbulent flow
  #2
New Member
 
rizzu
Join Date: Jun 2013
Posts: 2
Rep Power: 0
arizwan is on a distinguished road
Yes, If the fluid has no viscosity, the viscous force will be small compared to inertial force. So the value of the Reynold’s number will be high. So we will have turbulent flow. But one thing practically, there is no invisid fluid, as every fluid has some viscosity. But for our calculation purpose if the fluid has low viscosity, we assume the viscosity is zero.
Vino likes this.
arizwan is offline   Reply With Quote

Old   August 28, 2013, 03:51
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,247
Rep Power: 17
FMDenaro will become famous soon enough
inviscid turbulence is a theoretical situation: for zero viscosity (infinite Reynolds number) the turbulence features an energy cascade extending up to an infinite wavenumbers since the Kolomogorov scale is zero. No dissipative part of the energy spectrum exists.
In such case, no DNS is realizable and LES can be performed with some care.
See this LES:
http://www-personal.engin.umd.umich....nn_Dhanak2.pdf
Vino likes this.
FMDenaro is offline   Reply With Quote

Old   August 28, 2013, 11:19
Default
  #4
Senior Member
 
duri
Join Date: May 2010
Posts: 130
Rep Power: 6
duri is on a distinguished road
Quote:
Originally Posted by Vino View Post
Is there any possibility that an inviscid flow can be a turbulent flow and do we use any inviscid turbulent model for our practical application?
Euler equation can only convect rotation and can't generate it without viscosity. Viscosity is necessary for turbulence. Perfect slip condtion at wall and at shear will not develop turbulence unless it is convected from inlet condition.
There is no such inviscid tubulent model, what ever dissipation a cfd solver gives is purely numerical. Since high reynolds number flow can be approximated to inviscid flow except near boundary layer, it cannot be used for many cases except were viscous effects dominates.
Vino likes this.
duri is offline   Reply With Quote

Old   August 28, 2013, 11:54
Default
  #5
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 10
cfdnewbie is on a distinguished road
Quote:
Originally Posted by duri View Post
Euler equation can only convect rotation and can't generate it without viscosity. Viscosity is necessary for turbulence. Perfect slip condtion at wall and at shear will not develop turbulence unless it is convected from inlet condition.
There is no such inviscid tubulent model, what ever dissipation a cfd solver gives is purely numerical. Since high reynolds number flow can be approximated to inviscid flow except near boundary layer, it cannot be used for many cases except were viscous effects dominates.

Turbulence is generated through a vortex stretching mechanism, which only required three-dimensionality. Viscosity is NOT needed to produce a scale cascade, only to limit the cascade to a finite bandwidth.
FMDenaro and Vino like this.
cfdnewbie is offline   Reply With Quote

Old   August 28, 2013, 11:58
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,247
Rep Power: 17
FMDenaro will become famous soon enough
Quote:
Originally Posted by duri View Post
Euler equation can only convect rotation and can't generate it without viscosity. Viscosity is necessary for turbulence. Perfect slip condtion at wall and at shear will not develop turbulence unless it is convected from inlet condition.
There is no such inviscid tubulent model, what ever dissipation a cfd solver gives is purely numerical. Since high reynolds number flow can be approximated to inviscid flow except near boundary layer, it cannot be used for many cases except were viscous effects dominates.
This is not exactly true... inertial energy cascade is determinated by the non-linear term that produces higher and higher wavenumbers components of the flow.
Vorticity equation exists too for inviscid flows and stretching term does not vanish for Euler equation.
The contribution of the viscosity is in the appearance of the end of the inertiale energy cascade due to the dissipation.
Vino likes this.
FMDenaro is offline   Reply With Quote

Old   August 28, 2013, 12:44
Default
  #7
agd
Senior Member
 
Join Date: Jul 2009
Posts: 176
Rep Power: 7
agd is on a distinguished road
While it is true that turbulence is generated by vortex stretching, which is present even in inviscid flows, the question still presents itself: where does the vorticity arise. Since the original poster stated that his interest is in practical applications, this question is not a trivial one. In any application with solid walls, the absence of viscosity not only eliminates a source of vorticity production, but also removes a strong local damping function of turbulence. Substituting numerical dissipation for physical viscosity has always seemed a little off to me, and I'm still skeptical of results from such simulations. Of course, I had a colleague once who said that all turbulence models are just rough approximations, and maybe an inviscid approach is just a little rougher than most.
Vino likes this.
agd is offline   Reply With Quote

Old   August 28, 2013, 12:57
Default
  #8
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,247
Rep Power: 17
FMDenaro will become famous soon enough
Quote:
Originally Posted by agd View Post
While it is true that turbulence is generated by vortex stretching, which is present even in inviscid flows, the question still presents itself: where does the vorticity arise. Since the original poster stated that his interest is in practical applications, this question is not a trivial one. In any application with solid walls, the absence of viscosity not only eliminates a source of vorticity production, but also removes a strong local damping function of turbulence. Substituting numerical dissipation for physical viscosity has always seemed a little off to me, and I'm still skeptical of results from such simulations. Of course, I had a colleague once who said that all turbulence models are just rough approximations, and maybe an inviscid approach is just a little rougher than most.
Your comment is wellcome, further to the vorticity equation for inviscid flows, the problem must be closed by BC.s and initial condition for vorticity. If no initial vorticity exists and no vorticity flows into the domain then you are right. At least excluding the case of internal production of vorticity that can be modelled by some device producing circulation (I was thinking something like MVG).
Vino likes this.
FMDenaro is offline   Reply With Quote

Old   August 29, 2013, 08:14
Default
  #9
Member
 
Vino
Join Date: Mar 2013
Location: India
Posts: 70
Rep Power: 3
Vino is on a distinguished road
Quote:
Originally Posted by cfdnewbie View Post
Turbulence is generated through a vortex stretching mechanism, which only required three-dimensionality. Viscosity is NOT needed to produce a scale cascade, only to limit the cascade to a finite bandwidth.

My doubt again is, as our friend said above, is there any possibility that an inviscid flow can generate turbulence? even when we stir inviscid flow with spoon for example, we can not generate vorticity, because of slip condition.( i agree that the situation is different for practical idealization which was my original question )
Vino is offline   Reply With Quote

Old   August 29, 2013, 08:48
Default
  #10
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,247
Rep Power: 17
FMDenaro will become famous soon enough
Ok, let's stop for a moment thinking about vorticity.
Consider the 1D inviscid Burgers equation

du/dt + d/dx (u^2/2) =0

This model produces what is called "Burgulence", see http://arxiv.org/pdf/nlin/0012033.pdf

Now consider a very smooth initial condition, for example u(x,0) = u0*sin(x), you will simply see that the Fourier component exp(ikx) when inserted into the non linear term will produce exp(2ikx) and so on for time passing...
This is the generation of a scale cascade adn is a model to understand the born of turbulence.
The issue is that even a very smooth initial field can produce an infinite number of wavelenghts.
Of course, if the initial is exactly zero you can not generate nothing but if you have some "small fluctuations" superimposed this generates a cascade.

in 2 and 3D cases, the scenario is the same, the non linearity in the momentum equation generates the scale cascade that can be seen by the vorticity in the stretching action. Viscosity will only dissipate vortices at the Kolmogorov scale (well, the dissipation starts at the Taylor microscale and continues up to the Kolmogorov scale)

In conclusion, any practical application in which velocity gradients are present and interact by means of the non linear convection, produce scale cascade despite of the absence of viscosity.
cfdnewbie and Vino like this.
FMDenaro is offline   Reply With Quote

Old   August 29, 2013, 15:07
Default
  #11
Member
 
Vino
Join Date: Mar 2013
Location: India
Posts: 70
Rep Power: 3
Vino is on a distinguished road
Thank you very much. Now i got some better clarity regarding this.
Vino is offline   Reply With Quote

Reply

Thread Tools
Display Modes

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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
reversed flow at pressure inlet and turbulent viscosity is limited.... cfdiscool FLUENT 9 September 20, 2014 12:53
Turbulent Flow in Straight Square Duct - k-ep Validation mcclud STAR-CCM+ 2 August 7, 2012 16:56
Turbulent flow + VOF CK FLOW-3D 3 January 13, 2009 07:30
Natural convection - Inlet boundary condition max91 CFX 1 July 29, 2008 20:28
Inviscid flow solver luca_g OpenFOAM Running, Solving & CFD 0 December 21, 2006 12:55


All times are GMT -4. The time now is 15:13.