At supersonic speed, why does Mach number decrease?

In summary, at supersonic speeds, air becomes compressible and when there is a decrease in flow area, the Mach number decreases due to the generation of shock waves. This causes an increase in static pressure and a loss of total pressure. The same principle applies to expansion fans, where an increase in flow area leads to an increase in Mach number. The changes in density and flow area play a role in determining the Mach number and conserving mass flow.
  • #1
TimeRip496
254
5
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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  • #2
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.
 
  • #3
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
 

1. What is Mach number and how is it calculated?

Mach number is a unitless measurement of an object's speed relative to the speed of sound. It is calculated by dividing the object's velocity by the speed of sound in the surrounding medium.

2. Why does Mach number decrease at supersonic speeds?

Mach number decreases at supersonic speeds because as an object approaches and surpasses the speed of sound, the air particles in front of it are compressed and can't move out of the way fast enough. This creates a shock wave and increases the air pressure, which in turn causes the object to experience increased drag and resistance. As a result, the object's speed decreases.

3. How does air temperature affect Mach number?

Air temperature affects Mach number because the speed of sound is directly proportional to the temperature of the medium. As air temperature increases, the speed of sound also increases. This means that an object traveling at a constant speed will have a higher Mach number in warmer air compared to colder air.

4. Can Mach number be higher than 1 at subsonic speeds?

No, Mach number cannot be higher than 1 at subsonic speeds. Subsonic speeds are below the speed of sound, so the Mach number will always be less than 1. It is only at supersonic speeds that Mach number can exceed 1.

5. How does Mach number affect aircraft design?

Mach number plays a crucial role in aircraft design. At supersonic speeds, the shape of an aircraft must be carefully designed to minimize drag and reduce the effects of shock waves. Additionally, the materials used in construction must be able to withstand the high temperatures and pressures experienced at these speeds. Mach number also impacts the performance and stability of an aircraft, so it must be carefully considered during the design process.

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