[Edoardo1993]

Hello, I am newbie in CFD and I need help to choose the right boundary conditions for my case.

Regarding to the attached image, I am simulating the 3D gas expansion in free atmosphere through a convergent nozzle that is connected to the hydrogen tank. Due to the high pressure (500 bar), the flow is certainly sonic and the tutorial I am working with is reactingFoam/RAS/chokedNozzle/ because it allows both no combustion and combustion case. I don't want to model the tank and nozzle, therefore my domain is only the yellow region.




What are the right BCs to choose for p,U,T in the inletFuel and outlet surfaces?


I used the following BCs but I think they are wrong because results have no physic sense:


InletFuel:
p zeroGradient;
U fixedValue
T fixedValue (how do I calcolate the exact value?)


Outlet:
p fixedValue Uniform 101235;
U pressureInletOutletVelocity;
T inletOutlet;



Thank you for your help

[Edoardo1993]

Please, can anyone help me?

[Tobermory]

My first comment, and please don't take this the wrong way, is that you've picked a hell of a difficult case to model as a CFD "newbie"! Not immediately helpful, but I guess that what I am trying to say is - be prepared for a lot of work.

My second comment is that it's always much easier to simulate a problem if you already know what the flow physics are, and what the rough result will be. For example, here you've already spotted that the release is sonic ... but have you thought through what that means in terms of solver type and BCs? Have you researched this thoroughly before starting to try and run it in OpenFOAM?

My guess to the latter is probably not. You'll have to deal with a very agressive shock structure downstream of the nozzle for this highly under-expanded jet. I doubt whether reactingFoam will manage to run this high pressure ratio case, but I may be wrong ... you'd probably be better off starting with one of the solvers that are designed for highly compressible flows (rhoSimpleFoam etc), although these will need extending to allow for the hydrogen/air mix.

There are LOADS of posts on under-expanded jet simulations on this forum - have a search and read, and you will probably pick up some useful info, incl on what solvers others have had success with. Also, do the legwork and find some PhD or Masters theses online for similar flow cases, and read up how they did their simulations ... and also read up on the physics involved. That will help you, for example, with your question on what the throat temperature should be (spoiler - read up on isentropic flow equations, eg the NASA pages).

A final comment - I have usually found that I have better success for sonic jets by modelling the plenum upstream rather than trying to apply the boundary condition at the throat.

Good luck!

[Edoardo1993]

Tobermory thank you so much for your answer. It helped me to think more openly.

Originally Posted by Tobermory

it's always much easier to simulate a problem if you already know what the flow physics are, and what the rough result will be. For example, here you've already spotted that the release is sonic ... but have you thought through what that means in terms of solver type and BCs? Have you researched this thoroughly before starting to try and run it in OpenFOAM?

I have basic engineering knowledge of thermofluid dynamic and choked nozzle theory, so I should be able to roughly predict the physics.



Please, from now interrupt me whenever I say something wrong. I would go step by step:

1)CHOICE OF TUTORIAL
I want these key words: turbulent, RANS, compressible, combustion.
I am understanding that rhoCentralFoam (https://www.openfoam.com/documentati...ntralFoam.html) can capture the shock wave while reactingFoam (https://www.openfoam.com/documentati...ctingFoam.html) can not. But, on the other hand, reactingFoam can handle the combustion which is the goal of my study because I am focused on the chemical properties of the mix.

I need to simulate the gas leak without ignition and then with ignition and I would like to use one tutorial only (reactingFoam can switch off and on the chemistry) rather than rhoCentralFoam plus reactingFoam.

Do you advice to use only reactingFoam for both cases though that means missing the ability to see the shock waves?