# What are the differences between an LES-SGS model and a RANS based turbulence model?

I'm kind of new to the Large Eddy Simulation (LES) modeling world and I wondered what is the difference between an LES subgrid scale model like the Smagorinsky–Lilly model and a one-equation Reynolds-Averaged Navier-Stokes (RANS) based model like the Spalart-Allmaras. They are both based on the Boussinesq eddy viscosity ($\mu$) hypothesis and both using a one-equation approach for modelling $\mu$. However, no one is using the Smagorinsky–Lilly model for RAS or the Spalart-Allmaras for LES.

What is the difference between these two models and what kind of physical processes are they applied to?

• @IRO-bot yes, sorry for that. Where i work simulations based on the RANS equations are sometimes called RAS ("RANS-simulation") and thanks for the clean up :). – Julian Carpenter Dec 1 '14 at 14:16
• Also what is LES? – AtmosphericPrisonEscape Dec 2 '14 at 20:49
• @AtmosphericPrisonEscape i'm sorry if my question was to much of an acronym battlefield ... LES means Large Eddy Simulation – Julian Carpenter Dec 2 '14 at 20:51
• Ah thanxs. Of course no one not knowing the acronyms will be able to answer your question, it is however interesting to read about this, as I see in Isopycnal's answer that in fact it can be relevant for various people. – AtmosphericPrisonEscape Dec 2 '14 at 20:55

## 1 Answer

The biggest difference between LES and RANS is that, contrary to LES, RANS assumes that $\overline{u'_i} = 0$ (see the Reynolds-averaged Navier–Stokes equations). In LES the filter is spatially based and acts to reduce the amplitude of the scales of motion, whereas in RANS the time filter removes ALL scales of motion with timescales less than the filter width.

From that paper, specifically the section 'Structural similarity of LES and RANS equations', you can see that the equations being solved are essentially the same for LES and RANS, however, the physics are different. The main difference being that in RANS the unclosed term is a function of the turbulent kinetic energy and the turbulent dissipation rate whereas in LES the closure term is dependent on the length scale of the numerical grid. So in RANS the results are independent of the grid resolution!

A model qualifies as an LES model if it explicitly involves in one or the other way the step size of the computational grid. RANS models, in contrast, only depend on physical quantities, including geometric features like the wall distance.

As far as typical processes, this figure summarizes it pretty well. DNS resolves all scales of motion, all the way down to the Kolmogorov scale. LES is next up and resolves most of the scales, with the smallest eddies being modeled. RANS is on the other end of the spectrum from DNS, where only the large-scale eddies are resolved and the remaining scales are modeled.

The figure above is from André Bakker's lectures: http://www.bakker.org/dartmouth06/engs150/10-rans.pdf

DNS: Very small scale flow (ex:turbulent boundary layers). Currently computationally intractable for most problems.

LES: Aims to solve the computational cost that DNS poses and reveals the eddies hidden behind the mean in RANS. Good for coastal scale scale 2D simulations and possibly lab-scale 3D simulations with a highly optimized parallel code.

RANS: It is the least computationally expensive method that is used for turbulent modeling, but it is really not very good when certain phenomena cannot be averaged, such as instabilities. Acoustic waves are also incorrectly modeled because they are inherently unsteady processes which can't be averaged, so typically modelers will crank up the turbulent and numerical viscosity to remove acoustic waves from the system.

This shows the main difference between LES and RANS.

• thanks for your answer, yes that is completely true ... my question was maybe not clear enough. i'm aware of the differences concerning their decomposition method. but i was unable to understand why an LES SGS model takes an different approach to model Eddy Viscosity compared to an RANS based turbulence model. In the end they both parameterize the same parameter ... – Julian Carpenter Dec 2 '14 at 20:49
• that's a pretty cool figure ! could you tell me where it comes from ? i would like to use for an upcoming talk – Julian Carpenter Dec 2 '14 at 20:58
• @JulianCarpenter I added the reference and I will try to answer your specific question soon. – Isopycnal Oscillation Dec 2 '14 at 21:38
• @JulianCarpenter Hope that makes more sense now. – Isopycnal Oscillation Dec 2 '14 at 22:16
• thank you! this is a great answer and helps me a lot .. I will checkout this reference you gave – Julian Carpenter Dec 2 '14 at 22:21