At supersonic speed, why does Mach number decrease?

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SUMMARY

At supersonic speeds, a decrease in flow area leads to a decrease in Mach number due to the generation of shock waves. When flow area decreases, static pressure increases and total pressure decreases, resulting in an irreversible process. This phenomenon occurs because the increase in density from the pressure rise outweighs the loss of flow area, thus conserving mass flow. The discussion highlights the importance of understanding the relationship between flow area, density, and Mach number in supersonic aerodynamics.

PREREQUISITES
  • Understanding of supersonic flow dynamics
  • Familiarity with shock wave theory
  • Knowledge of compressible fluid mechanics
  • Basic principles of mass flow conservation
NEXT STEPS
  • Study the effects of shock waves on supersonic aircraft design
  • Learn about the principles of compressible flow using the NASA website
  • Research the relationship between flow area and Mach number in nozzles
  • Explore the concept of expansion fans and their impact on flow dynamics
USEFUL FOR

Aerospace engineers, fluid dynamics researchers, and students studying supersonic aerodynamics will benefit from this discussion.

TimeRip496
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At supersonic speed, air become compressible. However why does Mach number decrease when flow area decrease? I understand that pressure will increase because the density will increase when flow area decrease. However shouldn't the Mach number increase in order to conserve mass flow? Unless the Increase in density is much higher than the loss in flow area.

As for expansion fan, the Mach number will increase due to increase in flow area. Again is it because just like the above, the loss in density is much greater than the increase in flow area, hence Mach number will increase to conserve the mass flow?
 
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Your question is not clear. What is the context? A supersonic airplane? A nozzle? Please provide a link to where you read about mach number decrease.
 
anorlunda said:
Your question is not clear. What is the context? A supersonic airplane? A nozzle? Please provide a link to where you read about mach number decrease.
Nozzle.
"When an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, shock waves are generated. Across a shock wave, the Mach number decreases, the static pressure increases, and there is a loss of total pressure because the process is irreversible."

https://www.grc.nasa.gov/www/k-12/airplane/expans.html
 

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