# How to describe the momentum equation?

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 November 11, 2021, 09:35 How to describe the momentum equation? #1 New Member   Gao Shangya Join Date: Oct 2021 Posts: 18 Rep Power: 3 Hello everyone, I am currently working on my masters thesis which is about writing a code in Matlab to describe the simulation of a gas exchange in an hydrogen combustion engine.And now I am trying to writing a code( compressible and dimensional).But I am really confused with the gas viscosity. I want to neglect the viscosity because the viscosity of air ambient temperature is so small.When I look for some information to support my idea but I found nothing. The compression and heating of gas are not considered. Air enters the engine cylinder at a certain speed (20m/s). I would be very grateful if you could give me some advice

 November 12, 2021, 03:09 #2 Senior Member   Arjun Join Date: Mar 2009 Location: Nurenberg, Germany Posts: 1,221 Rep Power: 31 Based on my experience with such methods, i would just leave the viscosity out for the start. You will get very reasonable results in this case without it too. Then once you see everything working, add the viscosity. aerosayan and aero_head like this.

 November 12, 2021, 08:13 #3 Senior Member   Sayan Bhattacharjee Join Date: Mar 2020 Posts: 464 Rep Power: 7 As @arjun said, viscosity won't even matter there too much. You might get away with only writing a compressible inviscid NS solver, which is basically a Euler solver. I hope someone corrects me if I'm wrong, but you might not even require to code turbulence models, since turbulence models are needed to make the set of linear equations solvable by having enough equations for enough unknowns. Since Euler equations don't have any left over unknowns (i.e they have enough equations for the unknown variables), they don't require more equations like the turbulence models to make them solvable. If it's true, you don't even need velocity-pressure coupling methods like SIMPLE. So, your work might get reduced if my assumptions are scientifically correct. But I'm not sure if Euler solvers are stable for subsonic conditions. I only used them for supersonic/hypersonic flows. If I'm wrong, someone has to kindly correct me.

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