The distance between the sound waves heard from the coming locomotive is compressed, just like a person closing the accordion. The result of this action is a significantly higher pitch. When the train leaves, sound waves spread and a lower sound appears-this phenomenon is called "Doppler effect".
This Doppler effect is also used by radar velocimeter to detect the speed of motor vehicles. Shoot a light from the speedometer, shoot it on the car, and then return to the speedometer. The microprocessor in the speedometer compares the returned wavelength with the original wavelength. The closer the return wavelength is, the faster the car will move forward-which proves that the driver is more likely to drive too fast.
LDV/PDPA system of TSI
Main devices and principles of LDV/PDPA
The laser Doppler velocimeter measures the Doppler signal of tracer particles through a laser probe, and then obtains the velocity according to the relationship between velocity and Doppler frequency. Because it is laser measurement, there is no interference to the flow field and the speed measurement range is wide. Moreover, because the Doppler frequency has a linear relationship with the speed, it has nothing to do with the temperature and pressure at this point, so it is the instrument with the highest speed measurement accuracy in the world at present.
The working principle of LDV/PDPA velocimetry can be explained by interference fringes. When the focusing lens converts two incident lights into? After angular convergence, due to the good coherence of the dry laser beam, light and dark interference fringes are formed at the convergence point, and the fringe spacing is proportional to the wavelength of the dry light wave and inversely proportional to the sine value of the dry half-intersection angle. When the particles in the fluid pass through the direction of the stripe region, they will scatter a series of scattered waves whose intensity changes with time in turn, which is called Doppler signal. The frequency of this series of light wave intensity changes is called Doppler shift. The higher the velocity of particles passing through the edge region, the higher the Doppler frequency shift. The linear relationship between Doppler frequency shift and fluid velocity can be obtained by dividing the particle velocity perpendicular to the fringe direction by the fringe spacing and considering the refractive index of the fluid. LDV/PDPA system uses the linear relationship between velocity and Doppler frequency shift to determine velocity. The phase difference of Doppler frequency in all directions is proportional to the particle diameter, and the particle size can be determined by using the monitored phase difference.
LDV/PDPA system is divided into optical path part and signal processing part according to its functions. Optical path: He-Ni laser or Ar ion laser are used, because they can provide high-power lasers with three wavelengths of 5 14.5 nm, 488 nm and 476.5 nm. Optical splitter with frequency shifting device divides the laser into two beams with equal intensity, which are sent to the laser emitting probe through single-mode polarization maintaining fiber and fiber coupler, and the laser is adjusted to focus on the same point at the waist to ensure the minimum measurement volume, which is the measuring body, that is, the optical probe. The receiving probe sends the received Doppler signal to the photomultiplier tube and converts it into an electrical signal, which is complicated to process, and then it is analyzed and processed by the Doppler signal analyzer and recorded by the computer. Support system software can process data. Under the condition of suitable tracer particles in the flow field, the velocity and particle diameter in three directions can be measured simultaneously.
TSI is the first company in the world to produce commercial LDV/PDPA system. At present, the LDV/PDPA system of TSI has four patented designs, which are widely used in the research of flow field, turbulence, mass transfer, heat transfer, flow pattern and combustion. FSA4000 can handle the Doppler frequency as high as 175MHz, plus the frequency shift of 40MHz, and can handle the flow field above1000 m/s..
Therefore, for the 3D PDPA system, the laser is required to be stable and work stably for a long time, and the energy of the three wavelengths is required to be as equal as possible to ensure the accuracy of 3D velocity measurement. Therefore, TSI Company chose the laser of Coherent Company, the world's first laser brand, which is expensive but stable in quality. In the design of optical path, it is required to ensure high signal-to-noise ratio, convenient adjustment and user's convenience. This requires the adjustment of the optical fiber probe, which requires a wide adjustment range and high adjustment accuracy. Moreover, in multi-dimensional measurement, multiple lasers need to be focused on the same point, and TSI company provides special adjustment tools to fundamentally ensure the quality of signals. Most articles, papers and test results about laser velocimetry in the world are obtained through TSI products.