hi, i have a doubt, when ever flow takes place in a duct due to friction its velocity decreases , but if it a subsonic flow its velocity increases upto M=1 and it it is supersonic its velocity decreases . plz anyone tell the reason, thank u

I think you are talking about nozzles. in converging nozzle, due to decrease in area as you move in along its length, velocity increases. In diverging nozzle velocity decreases. Because, velocity is inversly proportional to c/s area. AND IN converging-diverging nozzle, the case will be interesting. go through fluid mechanics book.

thanks for ur reply, actually im talking about flow through constant area ducts with friction, not related to nozzles or diffuser or variable area.

With friction the boundary layer growth causes a reduction in the cross-sectional flow area. This area change is what causes the flow to accelerate if it is subsonic, and decelerate if supersonic. In a sense, the flow "thinks" it is in a nozzle, and if the duct is long enough, the flow approaches M = 1.

Hi! anitha

You should read the topic about Fanno line and Rayleigh line in fluid mechanics textbook.

I suggest the book of Anderson and the classical one of Shapiro.