What's the difference between velocity inlet and mass flow inlet?
 

Hi fellows..
I know that we should use mass flow B.c for compressible flows and if we use velocity inlet for that matter we get unphysical results according to the Fluent tutorial , but i see some papers used velocity inlet for ideal gas and i was wondering is it ok?
i talked to an expert and he said this warning that you see when you use velocity inlet for compressible situation doesn't mean that you can't use it!..it just means that you might get a wrong answer for your simulation...it depends on your simulation....it might lead's to a correct answer...
i wanna know your opinion...

At the velocity inlet you set a fixed velocity as boundary condition. If you do that for compressible flow and the density changes, also the mass flow through that inlet changes. If you don't care about that, because you actually want a fixed velocity and not a fixed mass flow then I don't see any problem.
For incompressible flow the fixed velocity and fixed mass flow are the same (except a factor) because you set the density as input parameter.
I think that's what the people, you are referring to, mean.

Thank you philipp for your answer..
another question about mass flow inlet B.c...
I entered 108 F for my inlet temperature....but when i initialize the temperature drops down to 70 F....it actually changes as i change my inlet velocity...
but when i use velocity inlet, i don't have this problem.....why is that?

It's because the input temp. In "mass flow" and "pre. Inlet" is total temperature and in "velocity inlet" you enter static temperature!

Thank you Amin for sharing your knowledge ..
I get this from your answer, If we have a problem which involves ideal gas and if we could assume that temperature is constant we should use static temperature and ergo velocity inlet...
my problem is ideal gas with constant temp at 104 F(it's a jet flow)....and i need to do an acoustic simulation....so i need to use unsteady solver ....but it seems "velocity inlet" has some problem with it....because it doesn't reach to an steady state....mass fluxes doesnt go near zero, which is strange because i have a simple inlet and out let...but "mass flow" reaches to that state....
So if i use "velocity inlet" i have that "steady" problem
and if i use "mass flow" i can't get a constant temperature
so what should i do?

No!!
TOTAL specifications of fluid are functions of physical states and velocity of flow in each position and could be changes cross the flow! So if you enter total temprature, it isn't mean that temp. is constant cross the domain! It's just fluid specification at inlet BC!
I guess for your case velocity inlet could not been a good BC!
For find out static temp. from total temp. a simple way is:
"Set your total temp. in your BC
Go to initialization tab and select your inlet! Now you can see static temp. here!"

Be careful because we don't know your case full, the suggestions maybe has some error!!