Flow field density measurement method based on femtosecond laser electron excitation mark

Abstract

The invention provides a flow field density measurement method based on a femtosecond laser electron excitation mark, which comprises the following steps of: emitting a laser pulse by using a femtosecond laser, and controlling the laser emission time by using a delay generator; the laser pulse is focused on a measurement area through a frequency conversion module, a reflector group and a focusing lens device; then, arranging a camera which is additionally provided with an optical filter on one side of the measurement area and forms a 90-degree angle with the laser transmission direction, and filtering and collecting fluorescence signals generated by femtosecond laser excitation particles; and finally, reversely deducing density information of the flow field in the measurement area according to fluorescence signal decay time information acquired by the camera. The method is simple and convenient in operation process and accurate in measurement result, can complete the measurement of the flow field density while generating minimal influence on the property of the flow field, simplifies the arrangement of a measurement device in the research of complicated flow problems such as hypersonic flow and the like, and is suitable for popularization and application.

Description

Technical Field

[0001] The invention relates to the field of aerospace technology, and in particular to a flow field density measurement method based on femtosecond laser electron excitation marking. Background Art

[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. Refined 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 to characterize flow field flow. Changes in the spatial distribution of density will cause changes in the refractive index of the flow field, which in turn affects the transmission characteristics of light in the flow field and may change the accuracy of target identification. Therefore, the development of refined density testing technology not only helps to understand the flow process, but also provides important data for the analysis of light transmission processes in unsteady flow fields, especially hypersonic aero-optical effects including high-temperature real gas effects.

[0003] Traditional flow field parameter measurement methods mainly include contact technologies such as pitot tubes, hot wire anemometers and sensors. Among them, when using pitot tubes and hot wire anemometers to measure flow field density, it is necessary to combine them with the relevant gas state equation, and the spatial distribution of flow field density cannot be directly obtained, and this contact measurement method will have a certain impact on the flow field state. At present, there are three main non-contact density measurement methods commonly used: the first is the schlieren imaging method. Since light will refract and bend when moving in media of different densities, the refractive index represents the degree of this bending. The density is measured by using the refractive index gradient of light in the measured flow field, which is proportional to the airflow density of the flow field; the second is the laser focusing differential interferometry method. This method is based on the interference principle of coherent light. A laser emits a parallel light beam with the same phase and polarization direction. After 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, and then the two beams are focused. In the measurement area, two different focal points are formed. After passing through the measurement area, the two beams of light are converged again using an optical lens, and the corresponding voltage signal is obtained using a light intensity detector, and then the density pulsation information of the flow field is inferred from the voltage signal; the third method is the double tracer particle fluorescence labeling method. This method spreads two different tracer particles in the flow field, and obtains the concentration of the tracer particles by photographing the fluorescence intensity of the tracer particles. The concentration distribution of oxygen in the flow field is obtained through the concentration of the tracer particles and the quenching effect of the tracer particles in the flow field, and then 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 technologies all have certain problems. For example, the spatial resolution of the schlieren method is relatively high, the experimental device of the laser focus difference method is relatively complex, and it can only measure the density pulsation in the flow field, and cannot obtain the specific density of the flow field. The double tracer particle fluorescent labeling method requires the spread of tracer particles in the flow field. When the flow velocity is high, the followability and uniformity of the particles will be limited, which will affect the structure and properties of the flow field itself. Summary of the invention

[0005] In view of the technical problems existing in the above-mentioned background technology, the present invention proposes a flow field density measurement method based on femtosecond laser electron excitation labeling, which has a simple operation process and accurate measurement results. It can complete the measurement of flow field density while having minimal impact on the flow field properties. In the study of more complex flow problems such as hypersonic flow, the layout of the measurement device is simplified, and the method is suitable for promotion and application.

[0006] In order to solve the above technical problems, the present invention provides a flow field density measurement method based on femtosecond laser electronic excitation labeling, which first emits a beam of laser pulses through a femtosecond laser, and uses a delay generator to control the laser emission time; then, the laser pulse is focused on the measurement area through a frequency conversion module, a reflector group and a focusing lens device; then, a camera equipped with a filter is arranged on one side of the measurement area and at 90o to the laser transmission direction, and the fluorescence signal generated by the femtosecond laser excited particles is filtered and collected; finally, the density information of the flow field in the measurement area is inferred based on the fluorescence signal decay time information collected by the camera.

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

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

[0009] 2) The wavelength of the output laser is changed through the frequency conversion module, the propagation direction of the laser is changed through the reflector group, and the laser is focused to the measurement area through the focusing lens;

[0010] 3) The laser beam focused to a certain distance from the measurement area is collected by a laser beam collector to avoid the influence of laser scattering on the imaging signal-to-noise ratio;

[0011] 4) focusing the laser beam through a focusing lens to excite the tracer particles in the measurement area and generate a fluorescent signal, and collecting the fluorescent signal through a camera containing a bandpass filter;

[0012] 5) Connecting the camera with the bandpass filter to a computer, fitting the signal decay lifetime of the fluorescence signal images collected at different times through the computer, and obtaining the density information of the measurement area through the decay lifetime calculation of the fluorescence signal.

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

[0014] The flow field density measurement method based on femtosecond laser electron excitation labeling, wherein: in the step 2), the laser beam passes through the frequency conversion module, the reflector group and the focusing lens in sequence; the light beam focused by the focusing lens will excite the tracer particles in the measurement area and ionize them, thereby generating excited state molecules and decaying to a low energy state, accompanied by the generation of fluorescent signals;

[0015] Taking N2 as an example, the fluorescence signal is mainly generated by the following reactions:

[0016]

[0017]

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

[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 Boltzmann constant of the gas medium respectively; k0 is the spontaneous emission rate of excited ions; k1 is the experimental measurement constant; k C is the relaxation rate of particle collision annihilation; C0 is a constant; n is the particle number density in the measurement area; Q L The heat generated by laser focusing; c P is the specific heat capacity at constant pressure; ρ is the air flow density in the measurement area.

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

[0022] By adopting the above technical solution, the present invention has the following beneficial effects:

[0023] The flow field density measurement method based on femtosecond laser electronic excitation labeling of the present invention is reasonably conceived. Compared with the previous flow field density measurement, the flow field density measurement of the present invention adopts femtosecond laser electronic excitation labeling to generate fluorescent signals, which is a non-invasive measurement with a simple operation process. There is no need to spread tracer particles in the flow field with air as the medium. Only one laser is needed to generate femtosecond laser. The experimental device is simple, and the influence on the properties of the flow field itself is small, and the measurement result is more accurate.

[0024] In hypersonic flow, the flow field has nonlinear properties, and slight disturbances in the flow field will have a great impact on the thermodynamic properties of the flow field. At the same time, in hypersonic flow, there is also the problem of low density of the flow field, and many theoretical calculation formulas are no longer applicable. The present invention directly links the density of the flow field with the signal attenuation characteristics generated by laser-induced fluorescence, and the density distribution of the flow field can be directly obtained from the attenuation characteristics of the fluorescence signal.

[0025] The present invention adopts a density measurement method based on the attenuation characteristics of the fluorescence signal generated by the femtosecond laser electronic excitation label. Since the ultrafast femtosecond laser pulse is used as an optical probe, it has a faster response speed; a non-invasive flow field density measurement method is provided, which completes the flow field density measurement while having a minimal impact on the flow field properties; and the arrangement of the measurement device is simplified in the study of more complex flow problems such as hypersonic flow. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 It is a schematic diagram of the structure of the experimental device involved in the flow field density measurement method based on femtosecond laser electron excitation labeling of the present invention. DETAILED DESCRIPTION

[0028] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The present invention is further explained below in conjunction with specific implementation modes.

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

[0031] The frequency conversion module 4 is matched and arranged on the laser output optical path of the femtosecond laser 1 , and 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 at the light-emitting side of the measurement area 5 and is used to collect the focused laser light to prevent the laser light from further propagating and causing damage to experimental equipment and personnel.

[0033] The imaging camera 3 is matched and arranged outside the measuring area 5 , and is used to collect 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 settings of the imaging camera 3 such as the corresponding door width and imaging gain.

[0035] The present invention is based on the flow field density measurement method of femtosecond laser electronic excitation labeling, based on the above-mentioned flow field density measurement device based on femtosecond laser electronic excitation labeling, firstly, a beam of laser pulses is emitted by a femtosecond laser, and the laser emission time is controlled by an autocorrelator; then, the femtosecond laser pulse is focused on the measurement area by a frequency conversion module, a reflector group and a focusing lens device; then, a camera equipped with a filter is arranged on one side of the measurement area and at a 90o direction with the laser transmission direction, and the fluorescence signal generated by the femtosecond laser excited particles is filtered and collected; finally, the density information of the flow field in the measurement area is inferred based on the decay time information of the fluorescence signal collected by the camera.

[0036] By using the flow field density measurement method based on femtosecond laser electron excitation marking of the present invention, the density information of multiple spatial points in the area to be measured can be measured simultaneously.

[0037] The present invention provides a flow field density measurement method based on femtosecond laser electron excitation labeling, which specifically comprises the following steps:

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

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

[0040] S300, collecting the laser beam focused to a certain distance of the measurement area through a laser beam collector to avoid the influence of laser scattering on the imaging signal-to-noise ratio;

[0041] S400, focusing the laser beam through a focusing lens to excite the tracer particles in the measurement area and generate a fluorescence signal, and collecting the fluorescence signal through a camera with a bandpass filter;

[0042] S500, connecting a camera with a bandpass filter to a computer, fitting the signal decay lifetime of the fluorescence signal images collected at different times through the computer, and obtaining the density information of the measurement area through the decay lifetime calculation 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. Under the control of the delay generator, the emission of femtosecond laser pulses and camera image acquisition can be controlled to match in timing.

[0044] In the above step S200, the laser beam passes through the frequency conversion module, the reflector group and the focusing lens in sequence; the light beam focused by the focusing lens will excite the tracer particles (taking N2 as an example) in the measurement area and ionize them, thereby generating excited state molecules and decaying to a low energy level, accompanied by the generation of fluorescence signals.

[0045] The fluorescence signals involved in this technology mainly include the following reactions:

[0046]

[0047] The fluorescence spectra released by the above reaction formulas (1)-(3) are respectively located in the first positive band (500-900nm), the second positive band (300-450nm) and the first negative band (300-500nm) of N2, wherein the fluorescence signal corresponding to the first negative band is relatively strong. In the present invention, the measurement of the density in the flow field containing N2 mainly relies on the fluorescence signal of the first negative band of N2, and the fluorescence signal lifetime can be obtained by the exponential fitting method, wherein the decay time of the fluorescence signal generated by the N2 transition 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 Bare the gas constant and Boltzmann constant of the gas medium respectively; k0 is the spontaneous emission rate of excited ions; k1 is the experimental measurement constant; k C is the relaxation rate of particle collision annihilation; C0 is a constant; n is the particle number density in the measurement area; Q L The heat generated by laser focusing; c P is the specific heat capacity at constant pressure; ρ is the air flow density in the measurement area.

[0050] In the above step S400, the fluorescence signal is specifically collected by using a camera equipped with an optical filter;

[0051] In the above step S500, the information in the image is converted into the time information of the fluorescence signal attenuation by a computer, and then the density information of the measurement area is obtained according to the above formulas (4) to (6).

[0052] The present invention has a simple operation process and accurate measurement results. It can complete the measurement of flow field density while having minimal impact on flow field properties. In the study of more complex flow problems such as hypersonic flow, it simplifies the layout of the measurement device and 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 present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A flow field density measurement method based on femtosecond laser electron excitation labeling, characterized in that :First, a beam of laser pulses 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 the measurement area through a frequency conversion module, a reflector group and a focusing lens device; then, a camera equipped with a filter is arranged on one side of the measurement area and at 90o to the laser transmission direction, and the fluorescence signal generated by the particles excited by the femtosecond laser is filtered and collected; finally, the density information of the flow field in the measurement area is inferred based on the decay time information of the fluorescence signal collected by the camera.

2. The flow field density measurement method based on femtosecond laser electron excitation labeling according to claim 1, characterized in that: The specific steps include: 1) The femtosecond laser emits a collimated laser beam under the control of a delay generator; 2) The wavelength of the output laser is changed through the frequency conversion module, the propagation direction of the laser is changed through the reflector group, and the laser is focused to the measurement area through the focusing lens; 3) The laser beam focused to a certain distance from the measurement area is collected by a laser beam collector to avoid the influence of laser scattering on the imaging signal-to-noise ratio; 4) focusing the laser beam through a focusing lens to excite the tracer particles in the measurement area and generate a fluorescent signal, and collecting the fluorescent signal through a camera containing a bandpass filter; 5) Connecting the camera with the bandpass filter to a computer, fitting the signal decay lifetime of the fluorescence signal images collected at different times through the computer, and obtaining the density information of the measurement area through the decay lifetime calculation of the fluorescence signal.

3. The flow field density measurement method based on femtosecond laser electron excitation labeling according to claim 2, characterized in that: The femtosecond laser in step 1) emits the laser pulses required for measurement under the control of the delay generator, and the emission of the femtosecond laser pulses and the camera image acquisition can be controlled to match in timing under the control of the delay generator.

4. The flow field density measurement method based on femtosecond laser electron excitation labeling according to claim 2, characterized in that: In the step 2), the laser beam passes through the frequency conversion module, the reflector group and the focusing lens in sequence; the light beam focused by the focusing lens will excite the tracer particles in the measurement area and ionize them, thereby generating excited state molecules and decaying to a low energy state, accompanied by the generation of fluorescence signals; Taking N2 as an example, the fluorescence signal is mainly generated by the following reactions: N2(C 3 P u )—N2(B 3 P g )+hν(2); The fluorescence spectra released by the above reaction equations (1)-(3) are located in the first positive band of N2 at 500-900nm, the second positive band at 300-450nm and the first negative band at 300-500nm respectively; the fluorescence signal corresponding to the first negative band is relatively strong. The density measurement in the flow field containing N2 relies on the fluorescence signal of the first negative band of N2. The fluorescence signal lifetime is obtained by the exponential fitting method. The decay time of the fluorescence signal generated by the N2 transition has the following relationship with the density of the measurement area: 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 Boltzmann constant of the gas medium respectively; k0 is the spontaneous emission rate of excited ions; k1 is the experimental measurement constant; k C is the relaxation rate of particle collision annihilation; C0 is a constant; n is the particle number density in the measurement area; Q L The heat generated by laser focusing; c P is the specific heat capacity at constant pressure; ρ is the air flow density in the measurement area.

5. The flow field density measurement method based on femtosecond laser electron excitation labeling according to claim 4, characterized in that: The step 5) is to convert the information in the image into the time information of the fluorescence signal decay through a computer, and then obtain the density information of the measurement area according to the above formulas (4) to (6).

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