# Time required for RANS vs. LES vs. DNS

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 April 2, 2017, 05:58 Time required for RANS vs. LES vs. DNS #1 Member   William Join Date: Aug 2016 Posts: 56 Rep Power: 9 Hi, I know that RANS is computationally more efficient than LES and DNS. And I also know that LES is computationally more efficient than DNS. I've read allot about the difference between the methods. But I've never seen how much the computational time difference is. Let's say I've modeled an Airfoil using RANS (kw-SST). The solution has converged and I've ensured grid independence. The solution took exactly one hour. - How much time could I expect for the LES simulation/modeling? - How much time could I expect for the DNS simulation?

 April 2, 2017, 08:12 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,793 Rep Power: 71 It it not comparable, several order of magnitude of difference...just think that RANS is a steady formulation (in statistical sense) and can be run in 2D. Conversely, LES/DNS are 3D and unsteady formulations.

 April 2, 2017, 09:02 #3 Super Moderator     Alex Join Date: Jun 2012 Location: Germany Posts: 3,406 Rep Power: 47 Plus it depends on the Reynolds Number. If it is too high, DNS might be completely out of reach for any computer available today. Without any information about the case, "several orders of magnitude higher" is really the only reasonable estimate. lcarasik and Chris2337 like this.

 April 6, 2017, 18:02 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,683 Rep Power: 66 There are Reynolds number comparisons you can do to get the scaling for LES and DNS. But for RANS vs a time-resolved simulation it is extremely hard to give a general answer except to say several orders of magnitude. One reason is it depends on the separation of time-scales, i.e. Reynolds number. Laminar flows can be solved with an unsteady laminar solver and this is cheap (RANS and DNS are the same thing and they are equally cheap/expensive). With higher Reynolds numbers, you hit turbulence, and there's separation, which increases with Re and it becomes increasingly more costly to solve with LES/DNS with higher Re. Another reason it's very tough to compare is that "my RANS took 1 hour to solve" is not a good benchmark. In principle, if there is no non-linear coupling between equations, you can solve a RANS problem using only a few iterations. That is, the solution time is highly dependent on your initial guess. So even for RANS, there is probably a 3-4 order of magnitude variation in time to solve depending on the user-skill (i.e. between 1 hr and 1000 hours). LES & DNS typically run for compute years, decades, or centuries. The reason they stop is because they can't afford to run any longer.

September 25, 2021, 08:28
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Quote:
Hi
Thank you for the clear statement.
If there is LES simulation for the jet flow in Re=10^4, where the total injection time is about 200 ms, and it needs to be solved for the total duration of 30s to 1 min:

Can you estimate the run-time duration of turbulent jet flow with time step of 1e-6?
I mean on a normal system with 32GB RAM and 16 core, is it possible to get any result?
If the inlet injection is about 2cm in diameter, what is the best size of dx?
I will really appreciate to have your idea.

 Tags dns, les, rans, time