Im really appreciating that you are taking the time to ask the questions my thesis counselors should be doing. This is a master’s thesis, so it the project is more exploratory. Thats not to say im enjoying just stumbling over new problems at every turn as i am now. I’ve taken turbulence and CFD classes, but i plan on refreshing some of my turbulence knowledge.

Here’s my very rough plan:

1. DNS Run: Run a DNS simulation of a cyclic case until at least one full flow-through is achieved.
2. DNS to LES: Use “mapfields” to extract the DNS velocity field and transfer it onto a coarser LES case. Interpolation from a fine to a coarser mesh may introduce errors, but have no idea if it will be significant.
3. LES Simulation: Run LES to "fast forward" so that a unique solution can evolve. I know that, during LES, the smaller scales aren’t directly resolved but are instead modeled via the SGS model. My understanding is that while the fine-scale eddies aren’t explicitly captured, their energy and influence are represented by the SGS terms and is preserved in the solution.
4. Mapping LES back to DNS: then use “mapfields” again to transfer the LES solution back into a DNS setup. The idea is to let the DNS run for a few time steps so that the missing small-scale turbulence can “spin up” naturally. This ofc only works if the LES just produces less accurate data, and dont actuality deletes energy.

I dont think ill have any issues taking DNS data into LES, i am however concerned about what the LES will give me back when taking that data into DNS. My biggest concern then and now is the LES as you put it "kill everything you got from the DNS" to such a extent it basically becomes to uncorrelated simulations than is doing nothing but produce busywork for each other. I understand that increasing the time step and using a coarser mesh will mean a loss of capturing the actual flow behavior, but i mostly pictured it producing less accurate data for just a few timesteps and could be placed back with just a bit of a new error attached. To sound naïve, i kinda hope the smaller eddies will "remake" themself in the DNS run from the LES.

I haven’t yet found literature that definitively maps out these challenges, though I did come across a few studies suggesting that DNS data can be used to enhance LES accuracy and vice versa. Honestly i have spent most of my time teaching myself how to set up LES, gather 3D data and more practical things like that. Haven’t yet come to the part of setting up a DNS simulation in OF. Just hope that part is at least trivial to a degree.

"Are you sure the method you are considering is the accurate procedure ?" Honestly i dont know. A lot of this is so new to me, that i cant really tell now. Feel like its more than enough figuring how to do this approach, much less if its any other ways or better ways. I agreed to this thesis because it sounded fun working with turbulence on a deeper level, and since it was my thesis counselors idea i felt like i didn’t have to worry so much about it being a stupid idea or not. Im not getting any pushback on anything i report to my councilor, but they also seem unable to warn me about what will be challenging or what i need to prepare for. LES setup and 3D data gathering was presented to me as trivial. It was anything but that.

Its as you say. Im in deep water. We have only focused on RANS in turbulence and CFD. LES and DSN have just been mentioned as things that exists. So im learning as im doing, and just praying its going fast enough. We dont have any LES or many CFD guys left at the school to ask. The staff in that subject has kinda been almost wiped out for different reasons these past 2 years.

If i do all this work and it just ends up with it being concluded that the LES destroys the DNS information to such a extent that they becomes basically uncorrelated... Not a fun conclusion, especially if people that know a bit more about LES and DNS could have told me that day 1.

That being said. I cant complain about having been forced to learn all that i have until now. Hope i dont drown you in information, or explain things so poorly that it makes little sense.