[Mr.Goodcat]

Goodmorning, everyone.

I'm a neophyte of Fluent and i need to simulate a flow case which requires the use of moving reference frames.

I'm using the pressure based coupled solver (steady state) with the SST k-w turbulence model and i'm trying to simulate the flow field for a helicopter rotor blade in hover.

Hoping to avoid convergence problems, i thought that for the first 150 (more or less) iterations i would use a first order upwind discretization, switch off the energy equation and use very low URF factor (as suggested in the manual).
Since that wasn't enough, following the suggestion of a tutorial i found online i'd like to try to gradually increase the rotational speed of the moving reference frame, instead of starting immediately with the 40rad/s of the reference case.

If anyone of you has ever used such methodology, could you please explain to me how to apply it? I mean:

- should i start with low rotational velocity and increase it gradually every (for example) 40 iterations KEEPING switched off energy equation/using first order upwind AND WHEN I REACH 40 RAD/S switch on the energy equation and change to second order ?

-OR should i "complete" the simulation (from first order/energy off to second order/energy on) for EVERY STEP of increased rotational speed?

Much obliged to anyone who might enlighten me...



-------Almost forgot:

I used the "final"rotational speed of 40rad/s when i estimated the wall distance needed to achieve a y+=1.
I don't need to modify the mesh for every step of speed, do i???

[duri]

No need to turn on second order, energy etc for every rpm increment similarly no need to turn off either. I mean you can start with your final settings and look for convergence and decide based on that.

[LuckyTran]

I would check the box, higher order term relaxation. This adds a urf for the second order terms so it behaves like 1st order until it stabilizes then gradually becomes closer to second order with each iteration. This saves you from having to manually switch on/off.

What equation of state are you using? If constant density where temperature plays no role then yes you can disable it until you reach your final speed. But if you are using something like ideal gas law I would always leave energy on because you want to gradually bring in the pressure and temperature dependence rather than a hard switch.

It sounds like you haven't done anything yet. These tricks are useful for avoiding convergence problems, you should run your simulation with default settings regardless. If it converges without any issues then great. Only when you actually have a problem should you start tinkering, otherwise you are just wasting time. Doing it this way also helps you learn when to expect convergence issues, the types of problems that lead to convergence issues, etc. If you always start a simulation with a super low urf then you'll never learn when problems arise.

[Mr.Goodcat]

Thank you for the pointers to both of you, duri and Lucky Tran. I'll put them immediately to use.

Also, Lucky Tran, I particularly appreciated your advice:

Only when you actually have a problem should you start tinkering, otherwise you are just wasting time. Doing it this way also helps you learn when to expect convergence issues, the types of problems that lead to convergence issues, etc. If you always start a simulation with a super low urf then you'll never learn when problems arise

I do have a tendency to make my life harder than it has to be..

Ps: as a matter of fact I am making an ideal gas assumption (with constant viscosity).