Question regarding stagnation state and process

[jason.bourne]

question regarding stagnation state and process!

stagantion state: it is reference state defined as the thermodynamic state which would exist if the fluid were brought to zero velocity and zero potential.

this is the definition from the textbook fundamentals of gas dynamics b Zucker R.D.

and in order to accomplish stagnation process, the stagnation state must be reached

1) without any energy exchange (Q=0 and W=0)

2) without losses

that is the process has to be reversible.

how do i check whether the process is reversible or not?
is entropy the only way to determine whether the process is reversible or not?
why the process has to be reversible in case of determining the stagnation state?
suppose we consider aircrafts which has pitot static tubes, which helps in determining the stagnation state, but the flow before it, is it reversible? i don't think so then how does pitot tube help in determining the stagnation state?

please help to understand this stagnation state and process. Can you explain me it clearly?

 

[FredGarvin]

Another way to think of stagnation properties is to think of what the fluid properties would be if you were moving with the fluid. In that sense I think you can see that the zero relative velocity has to be accomplished without changing the properties themselves.

The entropy aspect is an approximation in practice. There is no way to completely eliminate its production when you look at something like a pitot tube. The engineering comes in when you design a tube in an effort to reduce the losses as much as possible and keep your measurement as true as you can to the actual value.

A process is reversible if there are a few things:
- No delta T or heat transfer across the system boundary
- Usually small changes in state must be made
- The changes happen slowly
- Friction can not be present

 

[jason.bourne]

Another way to think of stagnation properties is to think of what the fluid properties would be if you were moving with the fluid. In that sense I think you can see that the zero relative velocity has to be accomplished without changing the properties themselves.

can you please explain this frame of reference part. m not geting it. coz all i know is suppose if m traveling with the fluid and if i have a pressure measuring device(pressure gauge) and a temperature prob they will give me the value of static properties rather than the stagnation properties. So what reference frame has to be chosen so that i can determine the stagnation state?