# Is this understanding of turbulence models correct?

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 January 30, 2011, 19:16 #2 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 230 Rep Power: 11 A couple of things: - Cigarette plumes will be driven by temperature (buoyancy). The constant thermodynamic properties assumption (viscosity, cp) would need to be checked. - Mesh and time resolution has an impact: if you solve for the transient laminar equations on a very fine grid, you might be able to resolve turbulent structure (think intrinsic LES and pseudo-DNS). The numerical algorithms will have a role to play. - There is an aspect that your interpretation of transient turbulent model that is incorrect (on the basis of RANS turbulence model). The RANS turbulence model aims to model the whole turbulence spectra, not the <<"smaller scale" disturbance>>. The turbulence structure that the model presents are the coherent turbulent ones, which should correspond to natural instabilities frequency and should have higher energy than the modeled turbulence. Your interpretation would be correct for a LES model. __________________ --- Julien de Charentenay

 January 31, 2011, 12:56 #3 New Member   Join Date: Jan 2011 Posts: 3 Rep Power: 8 Thank you Julien for your patience and help 1. Yes, I see what you mean. I was implicitly assuming a Boussinesq-approximation like case, but I understand that in a general case all material properties may vary. 2. Yes, this was something I missed. 'Numerical viscosity' would play an important role (anisotropically at that!) in messing up the solution. I am intrigued at your suggestion of looking at a fine grid transient constant viscosity method as a pseudo DNS. I will read up further on this. 3. This is critical: What would happen if the scale of the grid wasn't small enough to represent the higher frequencies of the spectrum? Is it that the field will be simply "numerically chopped" ? If that is so, what exactly changes when we change the parameters of a RANS model? Suppose we have a very fine grid, so that numerical filtering of the field is not of concern. We change the values of a k and epsilon in such a model. What do we expect will change? (Since the full spectrum is modeled anyway) Once again, thanks for your patience!!

 January 31, 2011, 22:31 #4 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 230 Rep Power: 11 I will try to clarify a couple of points: 1) DNS vs transient laminar: - The equations are the same (the Navier-Stokes equations without averaging); - DNS would need to be 3D, whereas 2D laminar is possible; - DNS would minimise numerical diffusion using high order time and space discretisation; - DNS would use a fine mesh to resolve turbulent structure up to the Kolmogorov scales. 2) RANS: modeled vs resolved turbulence In very short, RANS models aim to resolve the turbulence. In theory, mesh resolution should not impact your solution - at least in a steady-state fashion - and definitively not the turbulent energy distribution. Playing with the turbulence model coefficients are going to affect the behavior of the model: Take the k-epsilon model (http://www.cfd-online.com/Wiki/Stand...psilon_model): - Increase C_mu will increase turbulent dispersion (artificially?); - Changing C_1epsilon and C_2epsilon will affect the balance of turbulence dissipation (which would be equivalent to changing the shape of the spectrum IMO). Usually, one would change the coefficient to match the behavior of the turbulence model to a specific configuration (which has been investigated experimentally). This may relates to different turbulence behavior: mixing layer versus jet for example, or it may not. Hope it helps more than it confuses. In practice, I would personally recommends to use the default turbulence coefficients and try to get convergence (tough enough). And do occasional mesh sensitivity analysis. __________________ --- Julien de Charentenay

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