4 ms·
The author appears to have confused group velocity (the speed of the wave) with particle velocity (the speed of the air molecules). This is a very basic notion
by shoyer 13y ago
The author appears to have confused group velocity (the speed of the wave) with particle velocity (the speed of the air molecules). This is a very basic notion of how waves work -- the waves move, not the medium.
In this case, the wave (and the vehicle) would be moving at the speed of sound, but the air particles they hit wouldn't. They're basically still stationary. There is no magical force to bring them up to the speed of sound before they hit the vehicle. Hence, the drag is not reduced.
I'm a physicist, but by no means an expert on sound. There are more details on how sound actually works on this page: http://mysite.du.edu/~jcalvert/waves/soundwav.htm http://mysite.du.edu/~jcalvert/waves/soundwav.htm. A relevant quote: "v [the particle velocity] is much less than the phase [wave] velocity of sound" (note: phase and group velocity are equal for sound in air).
Edit: This criticism is not right; see below.
- cma 13y agoYou read it wrong; at no point were any particles in the wave assumed to be going the speed of sound; that's why when you look from the vehicles reference you see a standing wave but at every point on it particles are still flowing towards the vehicle. The picture says it is in a sound-speed reference frame, but it probably confused you if you didn't read that. At no point is the air doing anything but apply drag to the vehicle's movement; there is just a sweet spot to minimize drag, but it isn't minimized to zero. Read through this thread a bit and you'll see me talking about the exact same difference of propagation of a disturbance vs displacement/velocity of individual particles...
- shoyer 13y agoOK, after looking at the diagram and re-reading the article you are right -- you did not in fact mix up group and particle velocity
- cma 13y agoIt is probably my fault cause I sort of wrote the post as I worked out the concept and that ended up muddying a lot of the writing. And I'm not a physicist so I'm sure I didn't quite state everything in a very traditional way...
- deleted 13y ago[deleted]