thanks for the reply s.m, yes this is the case in compressible flows and that is really true but usually how i calculate Cp value is just from VTK files in paraView and make a slice at particular span of blade , using spread sheet , just divide all the pressure values which you get in spread sheet with maximum +ve pressure value among all the values and here is Cp.(stagnation pressure technique). it also works. in some papers i found that some authors also do like this.