CN1564021A - Simulaneous multipoint laser doppler velocity measuring method - Google Patents

Abstract

The invention discloses a simultaneous multi-point laser Doppler velocity measurement method, which belongs to the technical field of flow field testing. The method includes the following process: two acousto-optic driving sources respectively give and drive two acousto-optic modulators with signals of frequency f and f+Δf; The diffracted light separated by δ; the multi-level diffracted light generated by the two acousto-optic modulators passes through the focusing lens, and the corresponding diffracted light of the same order converges at a point in the measured flow field at an intersection angle of 2θ, which is a measurement point; A photodetector and a single signal processor simultaneously analyze the spectrum of Doppler signals generated by multiple measurement points. The light path structure of the device of the present invention is simple. Use different frequency shifts to distinguish the speed of different LDV measurement points, only need a single photodetector and a single signal processor to complete the simultaneous measurement of multi-point speed, which greatly simplifies the structure of the multi-point laser Doppler speed measurement device, making it more efficient Practical.

Description

Simultaneous Multipoint Laser Doppler Velocimetry Method

Technical field

The invention relates to a laser Doppler velocity measurement method, in particular to a simultaneous multi-point measurement laser Doppler velocity measurement method, which belongs to the technical field of flow field testing.

Background technique

Laser Doppler measurement velocimetry technology (LDV for short) has the remarkable characteristics of not disturbing the measured flow field, high spatial resolution, fast frequency response, and high precision. It is an important speed measurement technology in many fields such as medical engineering. LDV can provide flow velocity information at a point in space in time series. In order to obtain the velocity distribution on a certain section of space, it is necessary to move the measurement point of LDV point by point. For the steady flow field, the above method is allowed, but for the unsteady flow, the above method is not applicable. Particle Image Velocimetry (PIV for short) uses computer image processing technology to quantitatively obtain the magnitude and direction of the two-dimensional velocity of the flow field from the recorded particle images. Due to the limitation of image recording and image processing speed, the measurement frequency of the highest velocity field provided by PIV is not enough to meet the needs of turbulent flow measurement. In order to solve the multi-point measurement of unsteady flow field, people have developed scanning LDV, which uses mechanical or other beam scanning technology to quickly scan the flow field at the LVD measurement point to obtain the flow velocity distribution along the scanning trajectory. Scanning LDV is to sacrifice the time-domain information of the flow velocity in exchange for the spatial information, so the multi-point quasi-simultaneous measurement of the flow velocity is realized to a certain extent.

Contents of Invention

The purpose of the present invention is to provide a simultaneous multi-point laser Doppler velocity measurement method, which realizes the acquisition of spatial information of the flow field in the same time domain, and has the remarkable characteristics of fast frequency response and high precision.

The present invention is realized through the following technical scheme, adopting a speed measuring device composed of a laser, a beam splitter, two acousto-optic modulators, an acousto-optic drive source, a focusing lens, a reflecting mirror, a receiving lens, a photoelectric detector and a signal processor The device realizes the simultaneous multi-point laser Doppler velocity measurement method for the flow field, and is characterized in that it comprises the following process:

1. Two acousto-optic drive sources respectively drive the two acousto-optic modulators with signals of frequency f and f+Δf;

2. Each acousto-optic modulator generates at least two diffracted lights separated by a fixed angle δ in space;

3. After the multi-level diffracted light generated by the two AOMs passes through the focusing lens, the corresponding diffracted light of the same order converges at a point in the measured flow field at an intersection angle of 2θ, which is a measurement point;

4. Using a photodetector and a single signal processor to simultaneously analyze the frequency spectrum of the Doppler signal generated by at least two measurement points.

The detailed technique of the present invention is as follows: the laser beam generated by the laser 1 is divided into two parallel beams by the beam splitter 2, and then enters two acousto-optic modulators 3, 4 respectively. The multi-level diffracted lights 7 and 8 generated by the acousto-optic modulators 3 and 4 are converged by the focusing lens 9 to form a plurality of LDV measurement points 10 at the focal plane of the focusing lens 9 . The acousto-optic modulators 3, 4 are driven by the acousto-optic drive sources 5, 6 respectively. When the scattered particles pass through the LDV measurement point 10, they scatter the beam intersecting the measurement point. The backscattered light passes through the focusing lens 9, is reflected by the mirror 11, and is focused on the photosensitive surface of the photodetector 13 by the receiving lens 12. The signal generated by the photodetector 13 is connected to a signal processor 14 .

Assuming that the frequencies of the output drive signals of the acousto-optic drive sources 5 and 6 are f and f+Δf respectively, then the frequency shifts of the +1, +2, ... +n-order diffracted light generated by the acousto-optic modulator 3 are f, 2f , ..., nf; the frequency shifts of the +1, +2, ... +n order diffracted light generated by the AOM 4 are f+Δf, 2f+2Δf, ..., nf+nΔf. The multi-level diffracted light generated by the acousto-optic modulators 3 and 4 is separated in space by a fixed angle δ, and after passing through the focusing lens 9, the diffracted light of the corresponding diffraction order forms a plurality of LDV measurement points 10 on the focal plane of the focusing lens 9, namely The first-order diffracted light generated by the AOM 3 intersects with the first-order diffracted light generated by the AOM 4 (intersection angle is 2θ), forming an LDV measurement point. Since there is a frequency difference of Δf between the first-order diffracted light generated by the acousto-optic modulators 3 and 4, the interference fringes formed in the LDV measurement point will be at a speed of Δf d (d is the distance between the interference fringes), that is, the two beams The LDV measurement point formed by the first-order diffracted light has a frequency shift amount of Δf. Similarly, the corresponding LDV measurement points are formed by other diffracted lights of the same order, where the beam angles are all 2θ, and the frequency shifts are 2Δf, ..., nΔf. Since each LDV measurement point has a different frequency shift, the Doppler signals generated by different measurement points are separated from each other in frequency spectrum. Therefore, only one photodetector and a single signal processor can be used to realize multi-point LDV measurement, which is one of the remarkable features of the utility model.

The included angle δ between different orders of diffracted light produced by the AOM is determined by the frequency f of the drive signal, and the distance between adjacent LDV measurement points is F δ (F is the focal length of the focusing lens 9), so change The f value can adjust the distance between adjacent LDV measurement points.

The optical path structure of the device realizing the present invention is simple, different frequency shifts are used to distinguish the speeds of different LDV measurement points, and only a single photodetector and a single signal processor are required to complete the simultaneous measurement of multi-point speeds, which greatly simplifies the multi-point laser Doppler measurement. The structure of the Le speed measuring device makes it more practical. At the same time, by adjusting the central operating frequency f of the acousto-optic modulator, the distance between adjacent LDV measurement points can be adjusted, which is convenient to use. On the other hand, changing the frequency difference Δf of the driving signals of the two acousto-optic modulators can adjust the velocity measurement range of the multi-point laser Doppler velocity measuring device. Those skilled in the art know that the multi-point laser Doppler velocimetry device of the present invention can also be realized in other ways, for example, the forward receiving mode can be adopted, that is, the photodetector 13 is placed in the direction along the propagating direction of the incident light beam to receive The scattered light from the LDV measurement point 10 makes the LDV measurement easier to implement because the forward scattered light is stronger than the back scattered light all other things being equal.

Description of drawings

Accompanying drawing 1 is the structural diagram of realizing the multi-point laser Doppler velocimetry device of the present invention. In accompanying drawing 1, 1 is the laser device of continuous output, 2 is the beam splitter, 3, 4 is the acousto-optic modulator, 5, 6 is the drive source of the acousto-optic modulator, 7, 8 is the multiplicity that the acousto-optic modulator produces 9 is a focusing lens, 10 is a plurality of LDV measurement points formed on the focal plane of the focusing lens, 11 is a reflector, 12 is a receiving lens, 13 is a photodetector, and 14 is a signal processor. Accompanying drawing 2 is the spectrogram of the signal output by the measured three-point laser Doppler velocimetry device. In accompanying drawing 2, 15, 17, 19 are the frequency spectrums corresponding to the frequency shift amount 0, 500kHz, 1000kHz of three LDV measurement points respectively, 16, 18, 20 are respectively the frequency spectrums of the optical Doppler signal corresponding to three LDV measurement points .

Detailed ways

Realize in the simultaneous multi-point laser Doppler velocimeter of the present invention, laser 1 adopts the He-Ne laser of 5mW; The beam splitting distance of beam splitter 2 is 50mm; Acousto-optic modulator 3,4 uses lead molybdate crystal, Their central operating frequency is 60MHz; the focal length of the focusing lens 9 is 500mm. Under the above parameter conditions, the distance between adjacent LDV measurement points is 6.75mm.

Accompanying drawing 2 is the spectrogram of the signal output by the measured three-point laser Doppler velocimetry device under the above-mentioned parameter conditions. The analog signal output by the photodetector 13 is converted into a digital signal by the A/D converter in the signal processor 14, and then the frequency spectrum is obtained by FFT. On the frequency spectrum, since the signals from different LDV measurement points have different frequency shifts, their peaks 16, 18, 20 appear in different positions of the frequency spectrum without overlapping each other. The peaks 15 , 17 , and 19 appearing on the frequency spectrum are the frequency spectrum corresponding to the frequency shifts 0, 500 kHz, and 1000 kHz of the three LDV measurement points, respectively. By identifying and determining the positions of these peaks and calculating the frequency difference between peak 16 and peak 15, peak 18 and peak 17, peak 20 and peak 19, the speed of different LDV measurement points can be obtained, and simultaneous multi-point speed measurement can be realized .

Claims (1)

1. multiple spot laser Doppler velocity measurement method simultaneously, this method adopts the speed measuring device that comprises that laser instrument, beam splitter, two acousto-optic modulators, acousto-optic drive source, condenser lens, catoptron, receiver lens, photodetector and signal processor are formed, stream field tests the speed, and it is characterized in that comprising following process:

1). two acousto-optic drive sources are respectively that f and f+ Δ f signal give and drive two acousto-optic modulators with the frequency;

2). the diffraction light that every acousto-optic modulator produces two-stage at least time and separates with a fixed angles δ in the space;

3). by being that 2 θ converge a bit with the inferior diffraction light of one-level with the angle of cut accordingly behind the multi-level diffraction light process condenser lens of two acousto-optic modulators generation, be a measurement point in tested flow field;

4). simultaneously the frequency spectrum that the Doppler signal that at least two measurement points produce is arranged is analyzed with a photodetector and single signal processor.