5 Experimental Fluid Mechanics
355
Fig. 5.14 Christian Doppler (1803–1853, Austrian physicist)
Fig. 5.15 Doppler effect
is
f =
a ± U 0
a ∓ V 0
f 0
where f 0 is the natural frequency of the wave source, a is the acoustic velocity
of the stagnant air, U 0 is the speed of the observer relative to the air, and V 0
is the velocity of the wave source relative to the air. When the observer moves
toward the source, a positive sign is taken in front of U 0 ; when the observer
moves away from the source (i.e., along the source), a negative sign is taken
in front of U 0 . When the wave source moves toward the observer, a negative
sign is taken in front of V 0 ; when the wave source moves away from the