阅读说明：本技术 一种激光诱导多点击穿标记测速方法 (Laser-induced multipoint breakdown marking speed measuring method ) 是由 李博 李晓峰 高强 于 2020-03-05 设计创作，主要内容包括：本发明公开一种激光诱导多点击穿标记测速方法,采用以下系统实现,所述系统包括飞秒激光器、纳秒激光器、聚焦透镜、二向色镜、轴锥镜以及CCD相机；测量过程中,产生的飞秒激光经过聚焦透镜后在目标流场中自聚焦产生一条一定长度的一维等离子体通道,该通道为预电离区域；产生的纳秒激光经二向色镜反射后与飞秒激光光束在空间上重合,再经过轴锥镜聚焦后在等离子体通道上产生零级贝塞尔光束诱发多点击穿产生用于测速的击穿等离子体亮斑,用CCD相机记录击穿等离子体亮斑的位置信息。本发明采用飞秒纳秒激光连用实现流场空间中多点击穿,通过追踪击穿位置的位移情况,进而实现流场中多点速度同时测量。(The invention discloses a laser-induced multipoint breakdown marking speed measurement method which is realized by adopting a system, wherein the system comprises a femtosecond laser, a nanosecond laser, a focusing lens, a dichroic mirror, an axicon and a CCD camera; in the measuring process, generated femtosecond laser passes through a focusing lens and then is self-focused in a target flow field to generate a one-dimensional plasma channel with a certain length, and the channel is a pre-ionization region; the generated nanosecond laser is reflected by the dichroic mirror and then coincides with the femtosecond laser beam in space, the nanosecond laser is focused by the axicon, the zero-order Bessel beam is generated on the plasma channel to induce multi-point breakdown to generate a breakdown plasma bright spot for speed measurement, and the position information of the breakdown plasma bright spot is recorded by the CCD camera. The invention realizes multi-point breakdown in the flow field space by femtosecond nanosecond laser, and further realizes simultaneous measurement of the speed of multiple points in the flow field by tracking the displacement condition of the breakdown position.)

1. A laser-induced multipoint breakdown marking speed measurement method is characterized by being realized by adopting a system, wherein the system comprises a femtosecond laser, a nanosecond laser, a focusing lens, a dichroic mirror, an axicon and a CCD camera; in the measuring process, generated femtosecond laser passes through a focusing lens and then is self-focused in a target flow field to generate a one-dimensional plasma channel with a certain length, and the channel is a pre-ionization region; the generated nanosecond laser is reflected by the dichroic mirror and then coincides with the femtosecond laser beam in space, the nanosecond laser is focused by the axicon, the zero-order Bessel beam is generated on the plasma channel to induce multi-point breakdown to generate a breakdown plasma bright spot for speed measurement, and the position information of the breakdown plasma bright spot is recorded by the CCD camera.

2. The laser-induced multipoint breakdown marking velocimetry method according to claim 1, further comprising the steps of:

the CCD camera captures the space displacement of the plasma bright spot in a fixed time interval by two-time shooting, the obtained image data is processed by a computer to obtain position change, namely displacement information, and finally the displacement is divided by the time interval of image shooting to obtain the speed information of the corresponding position of the flow field.

Technical Field

The invention relates to the field of laser spectroscopy and the technical field of flow field velocity measurement, in particular to a laser-induced multipoint breakdown marking velocity measurement method.

Background

The accurate measurement of parameters such as temperature, speed, component concentration, mass flow and the like of the flow field has great significance for the research and development of internal combustion engines, scramjet engines and the like, wherein the flow field speed is directly related to the motion state of the flow field, so that the measurement of the speed in the flow field is particularly important.

Current flow field velocity measurements can be broadly divided into two broad categories:

the first non-optical method, represented by an anemometer and a pitot tube, can accurately measure the velocity of the flow field, but can bring great interference to the flow field due to the need of invading the flow field for measurement.

One is to utilize Doppler effect, take laser Doppler Velocimetry (L DV) technology as the representative, the principle of measuring the speed is to calculate the particle movement speed through utilizing the laser to shine to the moving particle, the Doppler frequency shift of the scattered light that the moving particle sends out moves the particle, or combine the particle absorption line frequency shift with the intensity change of the fluorescence radiation, utilize the intensity of fluorescence radiation to realize the speed measurement of point to be measured, the laser Doppler Velocimetry is mostly used for the measurement of the high-speed flow field usually;

secondly, the velocity measurement is realized by utilizing the displacement of the tracer particles in unit time, and Particle Image Velocimetry (PIV) is used for particle tracer velocity measurement, wherein the PIV technology needs to add the tracer particles into a flow field to be measured, and obtains the velocity information of the flow field by calculating the displacement of the added tracer particles in a certain time.

Both techniques are limited by the difficulty of application of the followability and scattering properties of the added particles in hypersonic flow fields as well as combustion fields. In recent years, molecular marker tracing and speed measuring technology is widely applied, and comprises the following steps: HTV (hydroxyl labeling velocimetry) technology for labeling OH molecules in flow field, VENOM (hybridization adsorbed nitrile Oxide monitoring) technology for labeling NO, and labeling O₂RE L IEF (raman excitation plus laser-induced electronic fluorescence) etc. since the tracer particles are molecules in the flow field, the problem of followability is fundamentally solved, on the other hand, the optical signal mainly comes from the fluorescent signal of the tracer molecules, so the problem of scattering of the particles does not need to be considered, but the fluorescent signal of the tracer molecules is easily affected by stray light and temperature environment.

Disclosure of Invention

The invention aims to provide a laser-induced multipoint breakdown mark speed measurement method aiming at technical defects in the prior art, and the method is a method for measuring the flow field speed by utilizing the laser-induced multipoint breakdown mark.

The technical scheme adopted for realizing the purpose of the invention is as follows:

a laser-induced multipoint breakdown marking speed measurement method is realized by adopting a system, wherein the system comprises a femtosecond laser, a nanosecond laser, a focusing lens, a dichroic mirror, an axicon and a CCD camera; in the measuring process, generated femtosecond laser passes through a focusing lens and then is self-focused in a target flow field to generate a one-dimensional plasma channel with a certain length, and the channel is a pre-ionization region; the generated nanosecond laser is reflected by the dichroic mirror and then coincides with the femtosecond laser beam in space, the nanosecond laser is focused by the axicon, the zero-order Bessel beam is generated on the plasma channel to induce multi-point breakdown to generate a breakdown plasma bright spot for speed measurement, and the position information of the breakdown plasma bright spot is recorded by the CCD camera.

The laser-induced multipoint breakdown marking speed measuring method further comprises the following steps:

the CCD camera captures the space displacement of the plasma bright spot in a fixed time interval by two-time shooting, the obtained image data is processed by a computer to obtain position change, namely displacement information, and finally the displacement is divided by the time interval of image shooting to obtain the speed information of the corresponding position of the flow field.

The invention adopts femtosecond laser pre-ionization and uses nanosecond laser to induce breakdown to realize multipoint breakdown on one line, and realizes speed measurement by tracking the displacement of plasma at the broken point. The breakdown threshold value is effectively reduced in the preionization process of the femtosecond laser, the plasma channel generated by the femtosecond laser is overlapped with the Bessel beam generated by the nanosecond laser through the axicon lens, the generation area of the plasma is effectively enlarged, and the generated plasma can be bypassed in the Bessel beam focusing process, so that the multi-point breakdown phenomenon is realized.

The laser-induced breakdown is a phenomenon that plasma is generated by laser-induced avalanche ionization, the plasma signal is strong, the duration is long, the plasma is not influenced by a temperature environment, and the flow field does not need to consider the following problem, so that the laser-induced breakdown has obvious advantages as a marker for measuring the speed in the high-speed flow field.

Drawings

Fig. 1 is a schematic diagram of a laser-induced multi-point breakdown marker velocimetry system.

Fig. 2 is a schematic diagram of CCD imaging velocimetry.

Fig. 3 is a beam combiner light path diagram.

The reference numbers in the figures illustrate:

1. a femtosecond laser; 2. a nanosecond laser; 3. a focusing lens; 4. a dichroic mirror; 5. an axicon;

and 6, a CCD camera.

Detailed Description

The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

As shown in fig. 1, the laser-induced multi-point breakdown marking speed measurement method of the present invention is implemented by using the following speed measurement system, and the whole speed measurement system is shown in fig. 1 and includes: the device comprises a femtosecond laser 1, a nanosecond laser 2, a focusing lens 3, a dichroic mirror 4, an axicon 5 and a CCD camera 6.

In the measuring process, the femtosecond laser generates a plasma channel with the length of about several centimeters through the focusing lens, the nanosecond laser generates a Bessel light beam through the focusing of the axicon lens, and the focusing length is also several centimeters. The method comprises the steps that space coincidence is completed in focusing areas of two beams of laser under the assistance of a dichroic mirror, dense breakdown plasmas are generated in an overlapping area after the two beams of laser are focused, and then displacement of each breakdown plasma is measured at a determined time interval, so that speed information can be obtained.

Specifically, the femtosecond laser 1 is used for generating femtosecond pulses, and the femtosecond pulses are focused by the focusing lens 3 and then self-focused in a target flow field to form a one-dimensional plasma channel due to the nonlinear Kerr effect and the plasma defocusing effect, wherein the channel is a pre-ionization region; nanosecond pulses generated by a nanosecond laser 2 pass through a dichroscope 4, then coincide with femtosecond laser beams in space, are focused by an axicon 5 and then act on a preionization region formed by the femtosecond laser self-focusing filamentation, the specific focusing mode is shown as a beam combining light path in fig. 3, the nanosecond laser is reflected by the dichroscope, then is focused on a plasma channel by the axicon to generate zero-order Bessel light beams to induce multipoint breakdown to generate breakdown plasma bright spots for speed measurement, and finally, position information of the breakdown plasma bright spots is recorded by a CCD camera 6.

Taking a jet flow field as an example, the measurement process is as shown in fig. 2, a CCD camera 6 captures the spatial displacement of plasma bright spots in a fixed time interval by two times of shooting, finally the obtained image data is processed by a computer to obtain position change, i.e. displacement information, and finally the displacement is divided by the time interval of image shooting to obtain the speed information of the corresponding position of the flow field.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.