[Naresh_]

I'm doing project on fluid flow analysis of rocket nozzle ,kindly help me what is difference between gauge pressure and total pressure and what to be entered in it

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[LuckyTran]

They are different things.

There is pressure. You can refer to this pressure in absolute units or with respect to a reference gauge. If you use a gauge or reference, then that pressure is called a gauge pressure. In Fluent, nearly all pressures are gauge pressures for numerical robustness.

Total pressure is the stagnation pressure. Generally it is the static pressure that would result if the flow is brought to rest isentropically.
https://en.wikipedia.org/wiki/Stagnation_pressure

The total/stagnation pressure that you specify for your boundary condition, because you are using Fluent, should also be a gauge pressure. I.e. a total gauge pressure.

Then there is a supersonic / initial static pressure. This is also a gauge pressure (all pressures in Fluent are gauge pressures). The / is because it has 2 uses. If the inlet is supersonic, then you must specify the stagnation and static pressure (i.e. you are indirectly specifying the Mach number at the inlet). If the inlet is subsonic, the supersonic static pressure is ignored. Now it has another use. Some users like to use a special feature in Fluent to initialize their flow field called the "compute from" feature. If you use this compute from feature and select the particular boundary, it will specify the initial guess for the static pressure throughout your domain to be this initial static pressure.