Understanding Turbulence Modeling Methods: LES, RANS, and DNS Explained

[_shankybro_]

Can anybody please provide me any information on the LES, RANS and DNS? What is the basic vice of each and what are the differences? Why use one over the other? And additionally, are there any online lectures/videos that would help me understand the turbulence modelling methods better? Please try to use as simple language as possible. I am a Mechanical Engineer, not a physicist.

 

[boneh3ad]

Please try to use as simple language as possible. I am a Mechanical Engineer, not a physicist.

What is your background in fluids, i.e. how in-depth can the answer be?

 

[_shankybro_]

I am a Mechanical engineering grad student..I just do not want you to be using terms a fluid dynamicist would be using teaching a physics grad student :) Your answer can be as in-depth as it could get without confusing me ..

 

[Vadar2012]

Funnily enough, I learned this stuff from a physicist during my Masters thesis. I would go and read the articles about them on the CFD wiki, they give you a pretty good overview. In my experience, there's no real rules on when to use each method. It's easy if you have experiemental data to compare to though...

 

[boneh3ad]

I can't really tell you a whole heck of a lot about turbulence models because, quite honestly, I hate the core idea behind turbulence modeling so I don't even bother.

As far as the difference between RANS, LES and DNS, it really comes down to trade offs between how much physics is captures and how long the computations take.

A DNS solves the Navier-Stokes equations directly (or any other set of equations for that matter). Because of that, they are capable of capturing pretty much 100% of the physics in the flow and are limited only be computational power and the assumptions you make in setting up the simulations. That is why they are sometimes called numerical experiments. Turbulent flows, as you know, have a variety of scales ranging from the inertial scales down to Kolmogorov scales. Unfortunately, that means that the mesh for a DNS must be incredibly dense to capture all that physical content, so the time to converge on a solution is extraordinarily large. It scales approximately with Re³.

RANS averages the Navier-Stokes equations in order to simplify the equations and make them less computationally intensive to solve. It averages out a lot of the smaller scales, which are the ones that drive up the computational time for the most part. This means you don't need nearly as fine a mesh. That is nice when you don't need the fine detail and a turbulence model like k-ε will do for you just fine. It does mean that you lose a lot of the finer physics of the flow, though. Generally, even complex problems at high Reynolds numbers can be solved in fairly short amounts of time though.

LES falls between the two and is closer in physical accuracy to a DNS than to RANS. It is slightly faster than a DNS and captures slightly less physics. It captures much smaller scales than RANS does, but not nearly those of a DNS. The equations themselves differ from those of RANS but are still not the full Navier-Stokes equations.

There is also a new technique called the partially-averaged Navier-Stokes equations, or PANS. That uses a constant whose value can be used to set the fidelity of the simulation anywhere between that of RANS and LES.

Unfortunately though, I am not a turbulence guy. I work in the area of boundary-layer stability and transition, not turbulence modeling, so I don't know a whole lot about RANS, PANS or LES beyond what I have said here. Certainly not enough that I could take the place of a few journal papers.

 

[_shankybro_]

Thanx bro! That does help a lot... I also asked some of my professors, and they did help me as well...Now except for the mathematics of these techniques, I think I have a fair enough idea of what they are. Thanks again!

 

[Chronos]

This is fluid mechanics. It is application specific.

 

[_shankybro_]

Chronos, your point being . . .

 

[boneh3ad]

This is fluid mechanics. It is application specific.

Nothing I said in my reply is application specific. On top of that, I fundamentally disagree with the concept that everything in fluid mechanics is application specific. Lots of stuff is, and lots of stuff isn't.