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March 31, 2003, 04:07 
About low Re number turbulent flows

#1 
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Hi,
As far as I know, a flow should be very turbulent (very high Reynolds number) to have the usual features explained by the turbulence theory: the energy cascade by means of vortex stretching, the energy spectrum with the inertial subrange, production and dissipation equilibrium etc. Turbulence models used in the RANS approach are constructed accepting that the flow is very turbulent and thus the latter features would take place. However, it is well known that these models have limitations even when the flow is very turbulent indeed. Therefore, can we expect the turbulence models to perform well in the calculation of low Reynolds number turbulent flows (i.e. when the mentioned features don't take place)? Can we handle (fully) low Reynolds number turbulent flows with the socalled Low Reynolds number Turbulence Models? Or, are they suitable only to handle near wall effects in very turbulent flows? Thanks in advance, Gorka 

March 31, 2003, 04:59 
Re: About low Re number turbulent flows

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You can only expect lowRe turbulence models to handle nearwall effects in fully turbulent flows.
The term "lowRe" in these models does not refer to the global Renumber, it refers to the local Reynolds number close to the wall. Hence, a lowRe model can not predict transitional flows with the features you describe (nonequilibrium, no separation of scales, coherent structures, anisotropy, ...) 

March 31, 2003, 05:25 
Re: About low Re number turbulent flows

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I was under the assumption that lowRe models generally have two aspects, near wall low Re effects and also a bulk flow modification. Actually I suppose I'm really talking about ke models. I know the term f_mu is used to reduce the effective viscosity, f_mu being a smooth function related to the local turbulent Reynolds number. A model with no great finesse but one that at least attempts to reduce mu_t away from the wall when Re_t <50.


March 31, 2003, 08:47 
Re: About low Re number turbulent flows

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f_mu, together with the other damping functions and constants (f1, f2, Ceps1 and Ceps2), are used to enforce the correct behaviour in the boundary layer. Constants and functions in lowRe models are normally tuned based on channel and boundary layer data to correctly predict the boundary layer while reproducing the original "highRe" model away from walls.
I have not seen any papers where lowRe models are tuned based on "global lowRe bulk flow effects". Have you got any references on this? (I have been out of the turbulence modeling business for 5 years so I might be wrong). The f_mu function specifically is used to correct for the effect of molecular viscosity on the shear stress close to walls where the turbulent fluctutations are damped. Some models use f_mu functions based on Re_t (Jones Launder, Launder Sharma, ...) while others use f_mu functions based on y+ (Chien, Shih Mansour, ..)and some use a combination of both (Lam Bremhorst, Nagano Tagawa, ...). The goal when devising all of these models as far as I know have been to reproduce *boundary layer* properties correctly close to walls in fully turbulent flows. Sometimes you see transitioncorrections to these models where they in addition are tuned to correctly predict the diffusion of turbulent energy into the boundary layer and thereby predict bypass transition (boundary layer transition caused by the high level of freestream turbulence). 

April 1, 2003, 05:28 
Re: About low Re number turbulent flows

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Thanks Jonas and Harry for your responses,
As Jonas, I don't know any turbulence model that would be tuned based on "global lowRe bulk flow effects". Then, can we spect to obtain good results if we use a turbulence model in a global lowRe bulk flow? Would the results be better (more realistic) if the flow was more turbulent? Regarding to LES, can we spect to obtain good results using the LES approach in these flows if actually no separation of turbulent scales takes place?? I suppose the separation of the scales is the main hypothesis of LES...!! Thanks in advance, Gorka 

April 1, 2003, 07:17 
Re: About low Re number turbulent flows

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Interesting! My experience was with the Launder Sharma model in what is now CFX (FLOW3D back then). My assumption (due to either my inexperience or over reliance on the user manual) was that f_mu was imposed in the bulk flow and that the other 3 modifications were wall specific.
Am I right in saying then that there is no turbulence model that even attempts to model low Re effects in the bulk flow!!?? Has any one investigated the validity of applying f_mu in the bulk flow? 

April 1, 2003, 08:38 
Re: About low Re number turbulent flows

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For simplicity f_mu is most often applied in the whole flow domain. However, outside boundary layers it normally does not have any effect (it is equal to 1). The f_mu function in the Launder Sharma model has not been tuned for any lowRe bulk flow effects. However, since f_mu in the Launder Sharma model is only dependent on the turbulent Reynolds number and does not contain any explicit walldistance it will have an effect also in the bulk region if you do get very low turbulent Reynolds numbers there. As far as I know the effect is untuned and unknown though. Also note that the turbulent Reynolds number (k^2/(molecular_kinematic_viscosity*epsilon)) used in the f_mu function is not at all the same as the global Reynolds number (velocity*length_scale/molecular_kinematic_viscosity).


April 1, 2003, 09:31 
Re: About low Re number turbulent flows

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>> Also note that the turbulent Reynolds number
:> (k^2/(molecular_kinematic_viscosity*epsilon)) used in :> the f_mu function is not at all the same as the global :> Reynolds number :> (velocity*length_scale/molecular_kinematic_viscosity). Not totally true I think! k/e is the length scale that is used in the eddy viscosity assumption, k is the velocity scale. The only difference then between Re_t and Re is the 'nature' of the velocity and length scales. The former uses turbulent quantities, the latter uses mean velocity and geometric quantities. What this means I don't know (apart from the fact that using f_mu in the bulk flow might not be so unrealistic after all?) 

April 1, 2003, 09:47 
Re: About low Re number turbulent flows

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Of course both are Reynolds numbers. What I meant is that the turbulent Reynolds number is not at all representative of the bulkflow global Reynolds number.


April 1, 2003, 10:12 
Re: About low Re number turbulent flows

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I wonder what the physical sense of these two Reynolds number is then.
If Re is the ratio of intertial forces to viscous forces then isn't Re_t the ratio of intertial eddy forces to viscous forces? There inherent assumption in Re_t is that the flow is already turbulent. Can't really find any physical use for Re_t. Anyone got any ideas?? 

April 1, 2003, 12:31 
Re: About low Re number turbulent flows

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Just some though :
turbulent length scale : l_t ~ k*sqrt(k)/eps turbulent velocity : u_t ~ sqrt(k) so turbulent Reynolds number : Re_t = l_t*u_t / nu For laminar flow Re_t = 0. Turbulence Reynolds number has to deal with energy containing eddies (size l_t, velocity u_t). One also may consider dissipative scale, characterized by the Kolmogorov length scale : eta = (nu^3/eps)^(1/4) to compare eta and l_t : l_t/eta = Re_t^(3/4) So Re_t characterized the separation between the energy containing eddies and the dissipative eddies. When Re_t tends to zero, the dissipation occurs at the same scale than the production. keps models are based on the hypothesis of spectral equilibrium. That's mean that all and only the energy tranfert throught the energy cascade is dissipated. This can only append when l_t and eta are clearly separated, so when Re_t is hight. To "correct" keps model for low turbulent Reynolds number : 1) correct the c_eps_2 in the epsilon equation, since this constante drives the turbulence decay, which is modify for low turbulent Reynolds number (Mansour and Wray 1992  Decay of isotropic turbulence at low Reynolds number  Phys. of Fluids, 6(2) pp 808814) 2) Correct the production terms for k and eps. Since production occurs directly at the dissipative scale. This could be done using a third turbulent variable  see Lumley... Sylvain 

April 1, 2003, 16:19 
Re: About low Re number turbulent flows

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sylvain's interpretation is one way to look at it. Another interpretation that I can think of is this:
Re_t = k^2/(epsilon * nu) nu_t = Cmu * k^2/epsilon Hence Re_t ~ nu_t / nu ~ turbulent "viscous" forces / molecular viscous forces Where nu = molecular kinematic viscosity nu_t = turbulent kinematic viscosity In words this means that the turbulent Reynolds number is propotional to the ratio between the turbulent viscosity and the molecular viscosity. 

April 1, 2003, 16:25 
Re: About low Re number turbulent flows

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Very interesting, thanks sylvain! Do you know how successful these corrections for low turbulent Reynolds numbers flows are? For what kind of applications are they used?
I'd imagine that there are few lowRe_t cases where you have enough wellbehaved (scale separation, isotropy etc.) turbulence to succefully use a k  epsilon model. 

April 2, 2003, 05:19 
Re: About low Re number turbulent flows

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I apologize if it seems selfish to take up again my previous questions ...
How can we handle global Low Renumber (turbulent) flows, if 1)Turbulence Models (RANS) are tuned for High Renumber flows. 2)LES supposes the scales of the turbulence to be separated, wich is not true in Low Renumber flows. Is DNS the only reliable approach? Thanks, Gorka 

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