Why Does Flow Become Supersonic in the Diverging Section of a Nozzle?

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SUMMARY

The discussion centers on the behavior of flow in converging-diverging nozzles, specifically addressing why flow becomes supersonic in the diverging section. It is established that a converging section can only achieve subsonic flow, while the diverging section facilitates supersonic flow once Mach 1 is reached at the throat. The relationship between area change and flow velocity is defined by the equation dA/A = [M^2 - 1]dV/V, indicating that at Mach 1, the flow speed remains constant in the converging section. The discussion emphasizes the importance of nozzle design, particularly the shape and configuration of the divergent section, in achieving supersonic speeds.

PREREQUISITES
  • Understanding of compressible flow dynamics
  • Familiarity with Mach number and its significance in fluid mechanics
  • Knowledge of converging-diverging nozzle design principles
  • Basic grasp of the relationship between area and flow velocity in nozzles
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  • Study the principles of compressible flow theory
  • Learn about the design and analysis of converging-diverging nozzles
  • Explore the effects of nozzle shape on flow characteristics
  • Investigate the role of shock waves in supersonic flows
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Aerospace engineers, mechanical engineers, students studying fluid dynamics, and anyone interested in the design and function of nozzles and turbines.

ank_gl
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i just started reading about nozzles and turbines in my semester course. i went through converging diverging nozzles. for achieving a supersonic flow, a converging diverging nozzle is used. now a converging section will work as a nozzle only till the flow is subsonic and a diffuser at supersonic flow. that means the maximum velocity obtainable in converging section is speed of sound in the same medium.
also dA/A = [M^2 - 1]dV/V, at Mach 1, section ll not alter the speed of flow.
now consider a converging diverging section. suppose mach 1 is reached at the throat, at mach 1, section ll not alter the speed of flow, then why would the flow become supersonic in the diverging section. i am a little confused about this. since no part of stream can go over mach 1 in converging section, diverging section should act as a diffuser, right?? apparently i am wrong, but i don't know where..
also i was wondering what effect the shape of section might have, i mean concavity or convexity of the nozzle
 
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A properly designed CD nozzle will have it's throat at M=1. Compressible theory is where the divergent part starts to work. There are a few funny things that happen because of it. The acceleration of a flow in a divergent section is one of them. Keep reading!

http://exploration.grc.nasa.gov/education/rocket/nozzle.html
 
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