A flow field density measurement method based on femtosecond laser electron excitation marking

Abstract

The application provides a flow field density measurement method based on femtosecond laser electron excitation marking, which comprises the following steps: firstly, a laser pulse is emitted by a femtosecond laser, and a time delay generator is used to control the laser emission time; secondly, the laser pulse is focused on a measurement area by a frequency conversion module, a mirror group and a focusing lens device; thirdly, a camera with an added optical filter is arranged on the side of the measurement area and in a 90° direction with the laser transmission direction, and the camera is used to filter and collect the fluorescence signals generated by the femtosecond laser excited particles; finally, the density information of the flow field in the measurement area is deduced according to the fluorescence signal decay time information collected by the camera. The operation process of the application is simple, the measurement result is accurate, the flow field density can be measured while the influence on the flow field property is extremely small, the arrangement of the measurement device is simplified in the research on complex flow problems such as hypersonic flow, and the application is suitable for promotion and application.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of aerospace technology, and particularly relates to a flow field density measurement method based on femtosecond laser electronic excitation marking. BACKGROUND

[0002] With the continuous development of aerospace technology, in-depth understanding of the complex flow phenomena and mechanisms involved in the unsteady flow process plays an important role in the aerodynamic design of new aircraft. Fine measurement of flow field state parameters (such as velocity, temperature, pressure, density, composition, etc.) will help to understand these complex flow phenomena and provide effective experimental verification and comparison for unsteady flow models and calculations. Density is one of the most important parameters for characterizing the flow of the flow field. The change in the spatial distribution of density will cause the change in the refractive index of the flow field, and then affect the transmission characteristics of light in the flow field and possibly change the accuracy of target identification. Therefore, the development of fine density testing technology not only helps to understand the flow process, but also provides important data for the analysis of light transmission process in the unsteady flow field, especially the hypersonic aerodynamic optical effect under the effect of high-temperature real gas.

[0003] Traditional flow field parameter measurement methods mainly include Pitot tube, hot-wire anemometer and sensor contact technology. Among them, the Pitot tube and hot-wire anemometer need to be combined with the related gas state equation when measuring the flow field density, and the spatial distribution of the flow field density cannot be directly obtained. And this contact measurement method will have a certain impact on the flow field state. The commonly used non-contact density measurement methods at present mainly include three kinds: the first kind is the schlieren imaging method. Since light moves in different density media and will be refracted and curved, the refractive index represents the degree of bending. The refractive index gradient of light in the measured flow field is proportional to the air flow density, and the density is measured; the second kind is the laser focusing differential interference method. This method is based on the interference principle of coherent light. A parallel light beam with the same phase and polarization direction is emitted by a laser. After passing through a series of optical lens systems, it is divided into two beams with a certain separation angle, equal light intensity and perpendicular to each other. Then the two beams are focused in the measurement area to form two different focal points. After passing through the measurement area, the two beams are converged again by using optical lenses. The corresponding voltage signal is obtained by using a light intensity detector, and then the density fluctuation information of the flow field is obtained by backstepping the voltage signal; the third kind is the double-tracer particle fluorescence marking method. This method scatters two different tracer particles in the flow field. The concentration of the tracer particles is obtained by shooting the fluorescence intensity of the tracer particles. The concentration of the tracer particles and the quenching effect of the tracer particles in the flow field are used to obtain the concentration distribution of oxygen in the flow field. The density distribution of the entire flow field can be calculated from the concentration information of the flow field components.

[0004] The above flow field density test techniques have certain problems, for example, the schlieren method has high spatial resolution requirements, the laser focusing difference method has a relatively complex experimental device, and only density fluctuations in the flow field can be measured, and the specific density of the flow field cannot be obtained. The double-tracer particle fluorescence marking method needs to scatter tracer particles in the flow field, and when the flow speed is high, the following property and uniformity of the particles are limited, thereby affecting the structure and properties of the flow field itself. SUMMARY

[0005] In view of the technical problems in the above background art, the present application provides a flow field density measurement method based on femtosecond laser electron excitation marking, which has a simple operation process, accurate measurement results, can complete the measurement of the flow field density while having little effect on the flow field properties, simplifies the arrangement of the measurement device in the study of complex flow problems such as hypersonic flow, and is suitable for popularization and application.

[0006] To solve the above technical problems, the present application provides a flow field density measurement method based on femtosecond laser electron excitation marking, which first emits a laser pulse through a femtosecond laser, and simultaneously uses a delay generator to control the laser emission time; then focuses the laser pulse on the measurement area through a frequency conversion module, a mirror group and a focusing lens device; then arranges a camera with a filter on one side of the measurement area and in a direction of 90o with respect to the laser transmission direction, filters and collects the fluorescence signal generated by the femtosecond laser excited particles; and finally inversely calculates the density information of the flow field in the measurement area according to the fluorescence signal decay time information collected by the camera.

[0007] The flow field density measurement method based on femtosecond laser electron excitation marking, wherein the method specifically comprises the following steps:

[0008] 1) Emit a collimated laser beam from the femtosecond laser under the control of the delay generator;

[0009] 2) Change the wavelength of the output laser through the frequency conversion module, change the propagation direction of the laser through the mirror group, and focus the laser on the measurement area through the focusing lens;

[0010] 3) Collect the laser beam after being focused on the measurement area for a distance through the laser collector to avoid the influence of laser scattering on the signal-to-noise ratio of the imaging;

[0011] 4) Focus the laser beam to excite the tracer particles in the measurement area and generate a fluorescence signal through the focusing lens, and collect the fluorescence signal through the camera with a band-pass filter;

[0012] 5) Connect the camera with a band-pass filter to the computer, fit the signal decay lifetime of the collected fluorescence signal images at different times through the computer, and calculate the density information of the measurement area through the decay lifetime of the fluorescence signal.

[0013] The flow field density measurement method based on femtosecond laser electron excitation marking, wherein the femtosecond laser in step 1) emits laser pulses required for measurement under the control of a delay generator, and the emission of femtosecond laser pulses can be controlled to match the camera image acquisition in time sequence under the control of the delay generator.

[0014] The flow field density measurement method based on femtosecond laser electron excitation marking, wherein the laser beam in step 2) sequentially passes through a frequency conversion module, a mirror group and a focusing lens; the light beam focused by the focusing lens will excite the tracer particles in the measurement region, ionize, and then produce excited state molecules and decay to low energy level states, accompanied by the generation of fluorescence signals.

[0015] Taking N2 as an example, the generation of fluorescence signals mainly has the following reactions:

[0016]

[0017]

[0018] The fluorescence spectra released by the above reaction formulas (1)-(3) are located in the first positive band of N2 500-900 nm, the second positive band of N2 300-450 nm and the first negative band of N2 300-500 nm; among them, the fluorescence signal corresponding to the first negative band is relatively strong, and the measurement of the density in the flow field containing N2 relies on the fluorescence signal of the first negative band of N2, and the lifetime of the fluorescence signal is obtained by exponential fitting method, wherein the decay time of the fluorescence signal generated by the transition of N2 has the following relationship with the density of the measurement region:

[0019]

[0020] In the above formulas (4)-(6), τ is the decay time constant of the fluorescence signal; R and k B are the gas constant and the Boltzmann constant of the gas medium, respectively; k0 is the spontaneous emission rate of the excited state ion; k1 is an experimental measurement constant; k C is the relaxation rate of particle collision annihilation; C0 is a constant; n is the particle number density of the measurement region; Q L is the heat generated by laser focusing; c P is the specific heat capacity at constant pressure; and ρ is the gas flow density of the measurement region.

[0021] The flow field density measurement method based on femtosecond laser electron excitation marking, wherein step 5) is to convert the information in the picture into the time information of the fluorescence signal decay by a computer, and then obtain the density information of the measurement region according to the above formulas (4)-(6).

[0022] By adopting the technical scheme, the application has the following beneficial effects:

[0023] The flow field density measurement method based on femtosecond laser electron excitation marking has the following advantages: compared with the previous flow field density measurement, the application adopts the femtosecond laser electron excitation marking to generate a fluorescent signal for flow field density measurement, which belongs to non-invasive measurement, the operation process is simple, no tracer particles need to be scattered in the air medium flow field, only one laser is needed to generate femtosecond laser, the experimental device is simple, and the influence on the nature of the flow field is small, and the measurement result is more accurate.

[0024] In the hypersonic flow, the flow field has nonlinear properties, and a slight disturbance in the flow field will have a great influence on the thermodynamic properties of the flow field. Meanwhile, in the hypersonic flow, the flow field also faces the problem of low density, and many theoretical calculation formulas are no longer applicable. The application directly links the density of the flow field with the signal attenuation characteristics of the laser-induced fluorescence, and the density distribution of the flow field can be directly obtained from the attenuation characteristics of the fluorescent signal.

[0025] The application adopts the method of measuring density based on the attenuation characteristics of the fluorescent signal generated by femtosecond laser electron excitation marking. Since the ultrafast femtosecond laser pulse is used as an optical probe, the response speed is fast. The application provides a non-invasive flow field density measurement method, which can complete the measurement of the flow field density while having little influence on the nature of the flow field. In the study of complex flow problems such as hypersonic flow, the arrangement of the measurement device is simplified. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the drawings needed in the specific embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the application, and for those skilled in the art, other drawings can be obtained without creative labor.

[0027] Figure 1 The application relates to a structure diagram of an experimental device involved in the flow field density measurement method based on femtosecond laser electron excitation marking. DETAILED DESCRIPTION

[0028] The technical solutions of the application will be described in detail below with reference to the drawings. Obviously, the described embodiments are some of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0029] The present invention will be further explained below with reference to specific embodiments.

[0030] like Figure 1 As shown, this embodiment provides a flow field density measurement device based on femtosecond laser electronic excitation markers, including a femtosecond laser 1, a computer 2, an imaging camera 3, a frequency conversion module 4, and a laser beam gatherer 6.

[0031] The frequency conversion module 4 is matched and set in the laser output optical path of the femtosecond laser 1. It is used to receive the laser beam output by the femtosecond laser 1 and control the wavelength of the output laser beam; the laser beam output by the frequency conversion module 4 enters the measurement area 5.

[0032] The laser beam collector 6 is located on the light-emitting side of the measurement area 5. It is used to collect the focused laser light to prevent the laser from spreading further and causing damage to the experimental equipment and personnel.

[0033] The imaging camera 3 is matched and positioned outside the measurement area 5, and is used to acquire the fluorescence signal generated by the gas in the laser-excited flow field.

[0034] The computer 2 is electrically connected to the imaging camera 3 and is used to control the imaging camera 3's corresponding imaging settings such as gate width and imaging gain.

[0035] This invention relates to a flow field density measurement method based on femtosecond laser electron excitation markers. Based on the aforementioned flow field density measurement device based on femtosecond laser electron excitation markers, the method first emits a laser pulse from a femtosecond laser while simultaneously controlling the laser emission time using an autocorrelator. Then, the femtosecond laser pulse is focused onto the measurement area using a frequency conversion module, a mirror group, and a focusing lens device. Next, a camera equipped with a filter is arranged on one side of the measurement area at a 90° angle to the laser transmission direction to filter and collect the fluorescence signal generated by the femtosecond laser-excited particles. Finally, the flow field density information within the measurement area is deduced based on the decay time information of the fluorescence signal collected by the camera.

[0036] The flow field density measurement method based on femtosecond laser electron excitation markers of this invention can simultaneously measure the density information of multiple spatial points within the test area.

[0037] This invention relates to a flow field density measurement method based on femtosecond laser electron excitation marking, which specifically includes the following steps:

[0038] The S100 femtosecond laser emits a collimated laser beam under the control of a delay generator.

[0039] S200, the femtosecond laser beam is adjusted by the frequency conversion module, the mirror group and the focusing lens, and focused on the measurement area; specifically, the wavelength of the output laser is changed by the frequency conversion module, the propagation direction of the laser is changed by the mirror group, and the laser is focused on the measurement area by the focusing lens;

[0040] S300, the laser beam focused on the measurement area for a distance is collected by the laser concentrator to avoid the influence of laser scattering on the signal-to-noise ratio of imaging;

[0041] S400, the laser beam is focused by the focusing lens to excite the tracer particles in the measurement area and generate a fluorescence signal, and the fluorescence signal is collected by a camera containing a band-pass filter;

[0042] S500, the camera containing the band-pass filter is connected to the computer, the signal decay lifetime of the collected fluorescence signal images at different times is fitted by the computer, and the density information of the measurement area is calculated by the decay lifetime of the fluorescence signal.

[0043] The femtosecond laser in the above step S100 emits laser pulses required for measurement under the control of the delay generator, and the control of the delay generator can control the timing of the emission of femtosecond laser pulses and the image acquisition of the camera.

[0044] The laser beam in the above step S200 passes through the frequency conversion module, the mirror group and the focusing lens in turn; the focused beam after the focusing lens will excite the tracer particles (for example, N2) in the measurement area and ionize, and then produce excited state molecules and decay to low energy level state, accompanied by fluorescence signal generation.

[0045] The fluorescence signal generation involved in the present technology mainly includes the following reactions:

[0046]

[0047] The fluorescence spectra released by the above reaction formulas (1)-(3) are located in the first positive band (500-900nm), the second positive band (300-450nm) and the first negative band (300-500nm) of N2, respectively, wherein the fluorescence signal corresponding to the first negative band is strong, and in the present application, the measurement of the density in the flow field containing N2 mainly relies on the fluorescence signal of the first negative band of N2. The fluorescence signal lifetime can be obtained by exponential fitting method, wherein the decay time of the fluorescence signal generated by the transition of N2 has the following relationship with the density of the measurement area:

[0048]

[0049] In the above formulas (4)-(6), τ is the decay time constant of the fluorescence signal; R and k BR is the gas constant and the Boltzmann constant for the gaseous medium, respectively; k0 is the spontaneous emission rate of the excited state ions; k1 is an experimentally measured constant; k C is the relaxation rate for particle collision annihilation; C0 is a constant; n is the particle number density of the measurement region; Q L is the heat generated by the laser focusing; c P is the specific heat capacity at constant pressure; p is the air flow density of the measurement region.

[0050] In the step S400, the specific collection of the fluorescence signal is performed by using a camera with an optical filter.

[0051] In the step S500, the information in the picture is converted into time information of the fluorescence signal decay by using a computer, and the density information of the measurement region is obtained according to the formula (4)-(6).

[0052] The operation process of the application is simple, the measurement result is accurate, the measurement of the flow field density can be completed while the influence on the flow field property is very small, the arrangement of the measurement device is simplified in the research of the complicated flow problems such as the hypersonic flow, and the application is suitable for promotion and application.

[0053] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, and not to limit them; although the application has been described in detail with reference to the above embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the above embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A flow field density measurement method based on femtosecond laser electron excitation marking, characterized by :First, a laser pulse is emitted by a femtosecond laser, and a delay generator is used to control the laser emission time; then, the laser pulse is focused on a measurement area by a frequency conversion module, a mirror group and a focusing lens device; then, a camera with an added optical filter is arranged on the side of the measurement area and at 90 o degrees to the laser transmission direction to filter and collect the fluorescence signals generated by the femtosecond laser excited particles; finally, the density information of the flow field in the measurement area is inversely deduced according to the fluorescence signal decay time information collected by the camera. Specifically comprising the following steps: 1) a femtosecond laser emits a collimated laser beam under the control of a delay generator; 2) the wavelength of the output laser is changed by a frequency conversion module, the propagation direction of the laser is changed by a mirror group, and the laser is focused to a measurement area by a focusing lens; 3) the laser beam focused to a distance in the measurement area is collected by a laser concentrator to avoid the influence of laser scattering on the signal-to-noise ratio of imaging; 4) the laser beam is focused by a focusing lens to excite tracer particles in the measurement area and generate a fluorescence signal, and the fluorescence signal is collected by a camera containing a bandpass filter; 5) the camera containing the bandpass filter is connected to a computer, the collected fluorescence signal images at different times are fitted for signal decay lifetime by the computer, and the density information of the measurement area is calculated by the fluorescence signal decay lifetime; In the step 2), the laser beam passes through the frequency conversion module, the mirror group and the focusing lens in turn; the focused beam after the focusing lens will excite the tracer particles in the measurement area and ionize, and then generate excited state molecules and decay to low energy state, accompanied by fluorescence signal generation; With As a tracer particle, the following reaction takes place for the production of a fluorescent signal: ； ； ； The fluorescence spectra released by the above reaction formulas (1)–(3) are respectively located in The first positive band is 500–900 nm, the second positive band is 300–450 nm, and the first negative band is 300–500 nm; among them, the fluorescence signal corresponding to the first negative band is stronger, containing… The measurement of density within the flow field relies on The fluorescence signal lifetime of the first negative band was obtained by exponential fitting, where... The decay time of the fluorescence signal generated by the transition is related to the density of the measurement area as follows: ； ； ； In the above equations (4) - (6), τ is the decay time constant of the fluorescence signal; R and R and kB are the gas constant and the Boltzmann constant of the gaseous medium, respectively; is the spontaneous emission rate of the excited state ion; is an experimentally measured constant; is the relaxation rate of particle collisional annihilation; is a constant; n is the particle number density of the measurement region; is the heat generated by the laser focus; is the specific heat at constant pressure; p is the air flow density of the measurement region.

2. The femtosecond laser electron excitation tagging based flow field density measurement method of claim 1, wherein: The femtosecond laser in the step 1) emits the required laser pulses under the control of the delay generator, and the femtosecond laser pulse emission can be controlled to match the camera image acquisition in time sequence under the control of the delay generator.

3. The femtosecond laser electron excitation tagging based flow field density measurement method of claim 1, wherein: The step 5) is to convert the information in the picture into the time information of the fluorescence signal decay by the computer, and then obtain the density information of the measurement area according to the above formulas (4)~(6).

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