diamond-shaped exhaust pattern

huangyhg

diamond-shaped exhaust pattern
i am a nuclear engineer going for another bachelor's in aerospace, and someone recently asked me a question regarding the diamond shaped exhaust pattern formed when a rocket lifts off. i've researched this on the net and even bought a fundamentals of aerodynamics book which is kicking my butt.
the information i got from the net was:
at engine start the flow is overexpanded, meaning the exhaust pressure is higher than supersonic isentropic pressure (whatever that is) but lower than ambient pressure, which causes an oblique shock to form at the exit plane of the nozzle. then it says that the flow that has passed thru the shock wave will be turned towards the centerline. after this i get lost in all the reflected shock waves, and prandtl - meyer wave explanations.
right now i would just like for someone to explain the first part to me, about the pressure differences. after that i'm sure i can learn the rest using my aero book. what is supersonic isentropic pressure?
start with a thermodynamic book
i have a thermo book and my problem really doesn't lie with understanding thermo concepts. i would just like someone to tell me what supersonic isentropic pressure is.
in a nutshell engines are designed for operation at altitude where, obviously, the ambient pressure is lower than at sea level. when the gases exiting the c-d nozzle at low altitudes is supersonic, the gas pressure continues to decrease from the choke point at the minimum cross section of the nozzle and the exit pressure can be lower than that of atmospheric (atmosphereic pressure acts as a back pressure on the nozzle). the "supersonic isentropic pressure" is simply the pressure of the supersonic gases exiting the nozzle and the flow is isentropic in the nozzle, i.e. no heat transfer with the surroundings and no friction.
i don't know which aero book you are using but i would strongly recommend the ones by john d. anderson such as fundamentals of aerodynamics or introduction to flight. they cover your topic in great detail and he starts from basic principles to give the reader maximum comprehension.
good luck