Doppler

Raparicio

Hello,

There's a problem with the doppler efect that I don't understand.

When a object runs at a velocity a little minor than velocity of sound, appears in front of the emisor a group of high frecuence, that in simulators (applets) is increasing in time. This wave, is like a physical object, that acts like a wall?

In the exactly case that the object goes at mach 1, how can we calculate the doppler efect? in theory, there's a wave of infinite frecuency and power (???)

Dr.Brain

Doppler's effect is not applicable to ultrasonic sounds. Thats for your second one.Please clarify the first question....a bit corrugated..

Raparicio

Dear Dr. Brain,

Thanks for the first.

The second question is, how can we calculate the waves that are generated at the exactly velocity velocity of sound. In this, we see that waves are acumulated in front of the moving object, but in the clasical formulas it's a mathematical singularity.

best reggards.

OlderDan

At speeds a little less than Mach 1 the doppler effect does produce very short wavelength sound waves that propegate away from the source at the speed of sound. These would be heard by a stationary observer (relative to the air) as high frequency sound. This is just a normal doppler effect. To the object moving through the air, as Mach 1 is approached there is a very real and highly energetic disturbance in the atmosphere that does serve as a "barrier" that must be penetrated to achieve higher speeds. The first airplanes to achieve these speeds experienced a great deal of turbulence and drag associated with this barrier. Supersonic aircraft are designed to deal with this transition from subsonic to supersonic speeds. Swept wings are commonly used to keep the component of velocity perpendicual to the leading edge of the airfoil below the speed of sound, which considerably reduces the drag. At speeds above Mach 1, the shock wave tends to be formed toward the back edge of the wing, so the plane in a sense "breaks the sound barrier" and achieves stable flight conditions. There is an excellent photo of a jet at Mach 1 where the shock wave produces a vapor cloud at the following link. I have seen a video clip of this somewhere online, but I don't have the source for it handy

http://antwrp.gsfc.nasa.gov/apod/ap010221.html

When the source is moving at the speed of sound or greater, there is a shock wave produced that is modeled by the applets you refer to. The shock wave is not a simple sinusoidally varying waveform. It is a highly energetic wavefront of a large change in air pressure that is heard as a "sonic boom". It's frequency spectrum is much more complex than the frequency of the source, and cannot be determined from simple doppler considertions. When it arrives, it shakes buildings and rattles windows. After the initial shock passes, the ususal sound waves arrive with frequncy components determined by the doppler effect. Of course when those waves arrive at an observer, the object that produced the sound has long since moved away from where that sound was produced. Even at subsonic speeds you can often see a jet in the sky and hear the sound coming from a point far behind it.

Try this

http://faculty.rmwc.edu/tmichalik/movies/F-18.MPEG

If that doesn't work try going to this site to download it

http://faculty.rmwc.edu/tmichalik/physmov.htm

Raparicio

Dear OlderDan,

Thanks for your dedication to this question. This information is very interesting and just what I'm looking for.

You say: "as Mach 1 is approached there is a very real and highly energetic disturbance in the atmosphere that does serve as a "barrier" that must be penetrated to achieve higher speeds."

Id like to calculate it numerically.
It exists information about this wave, its formulation, characteristics, etc?

Thanks for all.
R. Aparicio.

OlderDan

You have your work cut out for you I'm certianly not an expert in this area. Here is a brief paper that might be of some help. I'll look a bit more and see if I can find anything else.

http://www.fas.org/sgp/othergov/doe/...s/00326956.pdf

Dr.Brain

Perhaps , you would like to know about the Cherenkov Radiation that is emitted when an onject reaches Mach1 speed. Google the term for more information .

Dr.Brain

Perhaps , you would like to know about the Cherenkov Radiation that is emitted when an object reaches Mach1 speed. Google the term for more information .

Raparicio

Really, your information has been very useful to me.

I've found that the equation of Rankine-hugoniot is that works with it.

Now the problem is to find information about this.

Thanks to all.

R. Aparicio.