Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Midterm Question - Sort of



Concerning driven oscillations of bridges, James McLean asks, "Does the
frequency (and phase coherence) of the vortex departure really depend
on the motion of the bridge? What about in the case of an organ pipe
or recorder (the wind instrument) fipple?"
I don't know the answer to the first question, but I do for the second.
Instruments working on what I call the "whistle" mechanism do have
very strong feedback between the standing wave in the tube and
vortexes. That's what makes them work. Things in this category
include flutes, recorders, whistles, organ pipes, and even soda-pop
bottles.
The following explanation may be oversimplified, but it demonstrates
the idea. When you blow across the top of a pop bottle your air stream
splits, some going over the top and some going inside. The burst of
air going inside makes a pressure wave headed toward the bottom of the
bottle, i.e. a sound wave. When that wave reflects from the bottom of
the bottle and arrives back at the top, it puffs the split air stream
outward, sort of making a non-split stream going over the top. But
immediately afterward, the process starts over again. So neglecting
all the turbulence and vortexes that form, we can sort of view the air
stream as alternating between split and not-split. The air going into
the bottle gets turned on and off by interaction with standing wave in
the bottle even though the air source is providing a steady stream of
air.
Not only do all the instruments mentioned above work this way, it is
also interesting to note there is an electromagnetic analog, and this
is the klystron tube used to make microwaves. In this case the
standing electromagnetic waves in a cavity interact with a beam of
electrons going past an opening into the cavity much like the air going
over the opening of the pop bottle.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817