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 text book 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 dont 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?