Drag in Sueprsonic

  • #1

Main Question or Discussion Point

Hi
Can any one explain in detail about drag increase in supersonic speed


Why and how drag increases in Blunt shape nose cone to compare with Sharp nose cone at supersonic speed?


Ryan
 

Answers and Replies

  • #2
Well....just to clarify, do the two cones have the same diameter? Because even for subsonic flows, a blunt shaped object would increase the drag due to the earlier separation of the boundary layer.
 
  • #3
minger
Science Advisor
1,495
2
I believe one factor is due to the shock wave disconnecting from the body for the case of the blunt body.
 
  • #4
1,564
6
Explain in detail? No, you will need a text book for the that. What I will tell you is that at super sonic flows nothing becomes intuitive anymore. Nozzle become diffusers, diffusers become nozzle, and the aerodynamic of most geometry completely change. Supersonic flows are a petty neat subject but the concepts that follow them take some time and work to comprehend.
 
  • #5
7
0
Explain in detail? No, you will need a text book for the that. What I will tell you is that at super sonic flows nothing becomes intuitive anymore. Nozzle become diffusers, diffusers become nozzle, and the aerodynamic of most geometry completely change. Supersonic flows are a petty neat subject but the concepts that follow them take some time and work to comprehend.
I still can't intuitively grasp how a supersonic flow works in a converging-diverging nozzle works.

After the choke point, the area is increasing AND the velocity is increasing. To top it all of, the density is decreasing in a flow that is going fast enough to be considered compressible...sigh..
 
  • #6
I still can't intuitively grasp how a supersonic flow works in a converging-diverging nozzle works.

After the choke point, the area is increasing AND the velocity is increasing. To top it all of, the density is decreasing in a flow that is going fast enough to be considered compressible...sigh..


It can be explained through the manipulation of the mass conservation equation:

density*velocity*area = constant

Taking the derivative and manipulating it to get d density/density + d velocity/velocity + d area/area = 0, and using the definition of the speed of sound/Mach number...you end up with this equation:

d velocity/velocity * (mach number^2 - 1) = d area/area

This is known as the area-velocity relationship. So as you can see, for mach numbers less than 1, you get a negative sign for the value, thereby getting that relationship used in INcompressible flow where velocity slows down when area goes up, and vice versa.

When you start to go above Mach 1, the sign becomes positive, and both area and velocity essentially change in the same direction.
 
  • #7
2,985
13
I still can't intuitively grasp how a supersonic flow works in a converging-diverging nozzle works.

After the choke point, the area is increasing AND the velocity is increasing. To top it all of, the density is decreasing in a flow that is going fast enough to be considered compressible...sigh..
Why should you intuitively grasp supersonic flow? It's not something in your everyday tactile experience!

Do you have an intuitive understanding of low reynolds flow? Nope. You are way too large to experience 'atoms' of air hitting you. You can only feel a continuum.

Your 'intuition' will come through understanding of the mathematics of the flow.
 

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