354
P. Liu
equipment, the computer image processing system is used to complete the
image display and data processing, and then the change in color display
parameters is used to provide rich flow field information and high-quality
images.
2. Combined flow visualization and measurement technique
In view of the increasing complexity of the flow phenomenon, people
urgently need to rely on laser technology, computer technology, information
processing technology, etc., thus promoting the joint technique of flow visualization and measurement. For example, laser Doppler velocimetry (LDV),
particle image velocimetry (PIV), laser-induced fluorescence (LIF), computed
tomography (CT), and optical surface pressure measurement (ospmt) developed in recent decades are all visualization and measurement techniques. The
combination technique of quantity, which has the function of both qualitative display and quantitative measurement, has greatly promoted the research
progress of complex flow.
(1) Doppler effect of waves
The Doppler effect of waves is a phenomenon in which the observed
frequency differs from the source frequency due to the relative motion
between the wave source and the observer. This was discovered by Christian
Doppler in 1842 (Austrian physicist, 1803–1853, as shown in Fig. 5.14). A
famous example is: When a whistling train passes through the observer (as
shown in Fig. 5.15), he will find that when the train is approaching from
a distance, the sound of the whistle is changed from low to high; When
the train moves from near to far, the sound will change from high to low,
which is called Doppler effect. The reason is determined by the frequency of
sound wave vibration. If the frequency is high, the tone will sound high; if the
frequency is low, the tone will sound low. It can be seen that the wavelength
of the sound wave emitted by the whistle of a train moving at a constant speed
changes as it propagates. The result is that for trains coming from afar, the
wavelength of sound waves becomes shorter as if the waves are compressed.
On the contrary, when the train moves far away, the wavelength of the sound
wave becomes larger, as if the wave has been stretched. The frequency of the
wave received by the observer relative to the sound wave of the moving object
P. Liu
equipment, the computer image processing system is used to complete the
image display and data processing, and then the change in color display
parameters is used to provide rich flow field information and high-quality
images.
2. Combined flow visualization and measurement technique
In view of the increasing complexity of the flow phenomenon, people
urgently need to rely on laser technology, computer technology, information
processing technology, etc., thus promoting the joint technique of flow visualization and measurement. For example, laser Doppler velocimetry (LDV),
particle image velocimetry (PIV), laser-induced fluorescence (LIF), computed
tomography (CT), and optical surface pressure measurement (ospmt) developed in recent decades are all visualization and measurement techniques. The
combination technique of quantity, which has the function of both qualitative display and quantitative measurement, has greatly promoted the research
progress of complex flow.
(1) Doppler effect of waves
The Doppler effect of waves is a phenomenon in which the observed
frequency differs from the source frequency due to the relative motion
between the wave source and the observer. This was discovered by Christian
Doppler in 1842 (Austrian physicist, 1803–1853, as shown in Fig. 5.14). A
famous example is: When a whistling train passes through the observer (as
shown in Fig. 5.15), he will find that when the train is approaching from
a distance, the sound of the whistle is changed from low to high; When
the train moves from near to far, the sound will change from high to low,
which is called Doppler effect. The reason is determined by the frequency of
sound wave vibration. If the frequency is high, the tone will sound high; if the
frequency is low, the tone will sound low. It can be seen that the wavelength
of the sound wave emitted by the whistle of a train moving at a constant speed
changes as it propagates. The result is that for trains coming from afar, the
wavelength of sound waves becomes shorter as if the waves are compressed.
On the contrary, when the train moves far away, the wavelength of the sound
wave becomes larger, as if the wave has been stretched. The frequency of the
wave received by the observer relative to the sound wave of the moving object