High-speed flow field velocity disturbance measurement system and method
By using femtosecond lasers and time-domain stretching technology combined with focused differential interferometry, the problems of high system complexity and high cost in existing technologies are solved, and efficient and continuous measurement of high-speed flow field velocity disturbances is achieved.
Patent Information
- Application Number
- CN202510807138.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-17
- Publication Date
- 2025-09-12
AI Technical Summary
In existing high-speed flow field velocity disturbance measurement technology, the number of measurement points in a single frame and the system complexity are mutually constrained, resulting in a complex system structure and high cost, and the inability to achieve continuous and efficient measurement.
A femtosecond laser is used to generate femtosecond pulses, which are stretched into nanosecond pulses by a time domain stretching unit. A one-dimensional continuous point focusing differential interferometry unit is used to obtain interference signals. The signal acquisition and processing unit performs data processing to calculate the velocity disturbance of the high-speed flow field.
It achieves efficient and continuous measurement of velocity disturbances in high-speed flow fields, reduces system complexity and cost, and can accurately obtain the evolution law of fluid flow characteristics.
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Figure CN120628527A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of speed measurement, and more specifically, relates to a high-speed flow field velocity disturbance measurement system and method. Background Art
[0002] At present, hypersonic wind tunnels have become a key equipment for development. However, the research on unstable incoming flow in hypersonic wind tunnels has been hindered to a certain extent by the lack of testing technology. Therefore, the measurement of velocity and density fluctuations of hypersonic incoming flow has always been a concern. In hypersonic measurements, traditional contact measurement methods not only severely disrupt the flow field but also damage the measurement equipment. However, with the development of laser technology, several non-contact measurement methods have emerged. Currently, the more common laser velocimetry methods include particle image velocimetry, laser Doppler velocimetry, and femtosecond laser electron excitation labeling. Due to inertia, tracer particles have poor tracking in areas with dramatic density variations such as shock waves and boundary layers, significantly affecting the accurate measurement of velocity distributions. Doppler shifts can only capture the average velocity of the fluid flow and require multiple measurements. Measurement accuracy depends primarily on the flow field's stability and repeatability. Femtosecond laser electron excitation labeling eliminates the need to add tracer particles to the flow field under test and utilizes the naturally occurring nitrogen in the airstream for flow field labeling. However, in some fluids with low transparency or difficult to excite, the excitation efficiency can be low, resulting in weak signals and affecting measurement accuracy.
[0003] Focused differential interferometry, a method based on the principle of optical interference, has been widely used in the hypersonic field. It has a high response frequency and the ability to obtain various subtle fluid characteristics on the sub-microsecond timescale, such as turbulence intensity and boundary layer transition. In recent years, to obtain more information about fluid flow characteristics, some researchers have explored adding additional devices to the focused differential interferometry system to increase the number of detection points. The detection points have evolved from single points and two points to multi-point arrays. However, creating multiple points is very costly and increases the size and complexity of the system. The multiple measurement points are distributed discontinuously, and the information collected is limited. Only the velocity mean between two points can be calculated, making it difficult to obtain regional velocity distribution. Multiple high-speed photodetectors are also required to measure velocity disturbances in high-speed fluids. Overall, existing multi-point focused differential interferometry methods have high structural complexity and high cost. Existing focused differential methods suffer from the inability to measure continuously and the mutual constraints between the number of measurement points in a single frame and the system complexity. Summary of the Invention
[0004] The present invention provides a high-speed flow field velocity disturbance measurement system and method to solve the problem in the prior art that the number of single-frame measurement points and system complexity constrain each other, resulting in a complex system structure and high cost.
[0005] The present invention provides a method for measuring velocity disturbance of a high-speed flow field, comprising the following steps: Using a femtosecond laser to generate femtosecond pulses; The femtosecond pulse is stretched into a nanosecond pulse in the time domain by a time domain stretching unit, so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; A focusing differential interferometer unit is used to perform one-dimensional continuous point focusing differential interferometry on a high-speed fluid device as a detection object to obtain an interference signal; The interference signal is collected by a signal collection and processing unit, and velocity disturbance measurement information of the high-speed flow field is calculated based on the collected information.
[0006] Preferably, the time domain stretching unit includes a time domain stretching device, and the time domain stretching device adopts one of a multi-reflection glass rod, a single-mode optical fiber, a multi-mode optical fiber, and a free-space corner reflector.
[0007] Preferably, the time domain stretching unit further includes an amplifier device, and the amplifier device is used to amplify the intensity of the stretched nanosecond pulse.
[0008] Preferably, the focused differential interference unit is used to continuously focus the stretched pulse along the one-dimensional direction of the fluid cross section, and to generate spatially adjacent differential interference point pairs on the test area of the high-speed fluid device to obtain the interference signal.
[0009] Preferably, the focusing differential interference unit includes a first beam splitter, a detection optical path, a reference optical path, and a second beam splitter; the detection optical path includes a first reflective diffraction grating, a first focusing component, a second focusing component, and a second reflective diffraction grating; the reference optical path includes a delay component; and a high-speed fluid device is located between the first focusing component and the second focusing component. The first beam splitter is used to split the incident pulse into a detection light and a reference light; the detection light is spatially dispersed by the first reflective diffraction grating and is focused on the detection object in the test area of the high-speed fluid device by the first focusing component, wherein the flow direction of the detection object is parallel to the one-dimensional detection light after spatial dispersion and focus; after passing through the detection object and the second focusing component in sequence, the detection light is incident on the second reflective diffraction grating, and the second reflective diffraction grating is used to reaggregate the one-dimensional dispersed light pulse signal carrying the detection object information into a one-dimensional time domain spectrum; the reference light is incident on the delay component, and the delay component is used to make the optical path of the detection light path consistent with that of the reference light path; the outgoing light of the detection light path and the outgoing light of the reference light path are both incident on the second beam splitter and interfere with each other to obtain the interference signal.
[0010] Preferably, the focusing differential interference unit includes a first transmission diffraction grating, a first spectrometer, a third focusing assembly, a fourth focusing assembly, a second spectrometer, and a second transmission diffraction grating, which are sequentially arranged along the optical path; and the high-speed fluid device is located between the third focusing assembly and the fourth focusing assembly. The incident pulse is spatially dispersed by the first transmission diffraction grating, and each incident light ray is split into two beams at different angles by the first spectrometer. After passing through the third focusing component, two one-dimensional spectra that do not completely overlap on the same straight line are formed in the focal region. After passing through the detection object in the test area of the high-speed fluid device, the one-dimensional spectrum forms an optical signal carrying information about the detection object. After the optical signal passes through the fourth focusing component and the second spectrometer in sequence, light of the same wavelength is reconverged to form interference. The second transmission diffraction grating is used to rearrange the spatial spectrum information in the time domain to obtain the interference signal.
[0011] Preferably, the first light splitting device and the second light splitting device are symmetrically arranged with respect to the high-speed fluid device, and have the same device structure, both including a polarization beam splitter, a first reflector and a second reflector; The polarization beam splitter in the first spectroscopic device is used to split the incident pulse into two separate pulses with different vibration directions. After the two pulses are reflected by the first reflector and the second reflector respectively, two parallel pulses with a certain distance between them are formed, or two pulses with a certain angle are formed.
[0012] Preferably, the signal acquisition and processing unit includes a photoelectric detector, a high-speed oscilloscope and a computer connected in sequence; the interference signal is converted into a digital signal by the photoelectric detector, which is collected by the high-speed oscilloscope and then transmitted to the computer. The computer performs one-dimensional continuous point signal processing to obtain velocity disturbance measurement information of the high-speed flow field.
[0013] Preferably, the one-dimensional continuous point signal processing comprises the following sub-steps: performing a Hilbert transform on the interference signal at each spatial position and extracting the real and imaginary parts; Directly calculate the phase difference of the differential point; Performing cross-correlation operation on the phase differences of two pairs of differential points; Calculate the average speed based on the spacing: Output the velocity perturbation distribution of consecutive point pairs in one-dimensional space.
[0014] In another aspect, the present invention provides a high-speed flow field velocity disturbance measurement system, comprising: Femtosecond lasers, used to generate femtosecond pulses; A time domain stretching unit, configured to stretch the femtosecond pulse into a nanosecond pulse in the time domain so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; A focusing differential interferometry unit is used to perform one-dimensional continuous point focusing differential interferometry on a high-speed fluid device as a detection object to obtain an interference signal; A signal acquisition and processing unit, configured to acquire the interference signal and calculate velocity disturbance measurement information of the high-speed flow field based on the acquired information; The high-speed flow field velocity disturbance measurement system is used to execute the steps in the above-mentioned high-speed flow field velocity disturbance measurement method.
[0015] One or more technical solutions provided in the present invention have at least the following technical effects or advantages: The present invention proposes a high-speed flow field velocity disturbance measurement scheme that first uses a femtosecond laser to generate a femtosecond pulse; then the femtosecond pulse is stretched into a nanosecond pulse in the time domain by a time domain stretching unit, so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; then the high-speed fluid device as the detection object is subjected to one-dimensional continuous point focused differential interference measurement using a focused differential interference unit to obtain an interference signal; finally, the interference signal is collected using a signal acquisition and processing unit, and the velocity disturbance measurement information of the high-speed flow field is calculated based on the collected information. The present invention combines time domain stretching and focused differential interference to complete focused differential interference with one-dimensional continuous point distribution, which can reduce the multi-channel high-speed acquisition requirements required by traditional focused differential technology. The high-speed flow field velocity disturbance measurement system provided by the present invention has a simple structure and is easy to adjust, which can reduce the complexity and cost of the system, and solves the problem that the existing focused differential method cannot measure continuously, and the number of single-frame measurement points and the complexity of the system are mutually restricted. The present invention has the advantages of fast measurement speed, large number of measurement points, continuous measurement points, and the ability to obtain velocity fluctuation distribution. It can achieve accurate measurement of hypersonic fluid velocity fluctuations and effectively improve the measurement efficiency of hypersonic fluid dynamics pulse facilities. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1 A schematic diagram of a system corresponding to a high-speed flow field velocity disturbance measurement method provided in Example 1 of the present invention; Figure 2 A schematic diagram of a system corresponding to a high-speed flow field velocity disturbance measurement method provided in Example 2 of the present invention; Figure 3 This is a structural diagram of a spectrometer in a system corresponding to a high-speed flow field velocity disturbance measurement method provided in Example 2 of the present invention.
[0017] Among them, 101-femtosecond laser, 102-single-mode optical fiber, 103-erbium-doped optical fiber amplifier, 104-first beam splitter, 105-delay component, 106-second beam splitter, 107-first reflective diffraction grating, 108-first focusing component, 109-high-speed fluid device, 110-second focusing component, 111-second reflective diffraction grating, 112-photodetector, 113-high-speed oscilloscope, 114-computer; 201-Femtosecond laser, 202-Single-mode optical fiber, 203-Erbium-doped fiber amplifier, 204-First transmission diffraction grating, 205-First spectrometer, 206-Third focusing assembly, 207-High-speed fluid device, 208-Fourth focusing assembly, 209-Second spectrometer, 210-Second transmission diffraction grating, 211-Photodetector, 212-High-speed oscilloscope, 213-Computer; 301 - polarization beam splitter, 302 - first reflecting mirror, 303 - second reflecting mirror. DETAILED DESCRIPTION
[0018] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods.
[0019] Example 1: Example 1 provides a method for measuring velocity disturbance in a high-speed flow field, comprising the following steps: S1, generates femtosecond pulses using a femtosecond laser; S2, stretching the femtosecond pulse into a nanosecond pulse in the time domain by a time domain stretching unit, so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; S3, using a focusing differential interferometer unit to perform one-dimensional continuous point focusing differential interferometry on the high-speed fluid device as a detection object to obtain an interference signal; S4, using a signal acquisition and processing unit to acquire the interference signal, and calculating the velocity disturbance measurement information of the high-speed flow field based on the acquired information.
[0020] Among them, the time domain stretching unit in S2 includes a time domain stretching device, and the time domain stretching device uses a multi-reflection glass rod, a single-mode optical fiber, a multi-mode optical fiber, a free-space corner reflector, etc. to stretch the time domain spectrum.
[0021] The time-domain stretching unit in S2 may further include an amplifier device, which is used to amplify the intensity of the stretched nanosecond pulse. For example, the amplifier device may be an erbium-doped fiber amplifier.
[0022] In the present invention S3, the focusing differential interference unit is used to continuously focus the stretched pulse along the one-dimensional direction of the fluid cross section, and to generate spatially adjacent differential interference point pairs on the test area of the high-speed fluid device to obtain the interference signal.
[0023] The following describes a system corresponding to a high-speed flow field velocity disturbance measurement method provided in Example 1, taking a time domain stretching device using a single-mode optical fiber and an amplifier device using an erbium-doped optical fiber amplifier as an example.
[0024] See also Figure 1 In Example 1, a femtosecond laser 101 is used to generate a femtosecond pulse; the femtosecond pulse is stretched into a nanosecond pulse in the time domain through a single-mode optical fiber 102, so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship (that is, the time domain and frequency domain spectra are aligned to facilitate the subsequent mapping of the time domain signal into the spatial domain signal, and finally the optical signal changing at the nanosecond level can be collected by the photodetector), and is amplified by an erbium-doped fiber amplifier 103 (that is, the pulse power is enhanced); a focusing differential interferometer unit is used to perform one-dimensional continuous point focusing differential interferometry measurement on the high-speed fluid device as the detection object to obtain an interference signal.
[0025] The focusing differential interference unit in the present invention is mainly used to generate spatial dispersion and add phase information of the measured signal through interference, so as to finally obtain high-precision velocity disturbance measurement information of the object to be measured.
[0026] The focusing differential part in the focusing differential interference unit uses a spatial dispersion device to complete the mapping of the time domain and space domain of the stretched time domain pulse signal, and uses a symmetrical optical path structure so that the spectral information changing in the spatial domain can correspond one-to-one with the time domain signal finally received.
[0027] The interference part in the focused differential interference unit uses a beam splitter to split the nanosecond pulse signal into two paths, namely a signal detection optical path composed of a time domain-space domain-time domain conversion optical path and a signal reference optical path with the same optical path, to collect the phase signal of the detection object and achieve high-precision measurement of the detection area.
[0028] Specifically, the focusing differential interference unit includes a first beam splitter 104, a detection optical path, a reference optical path and a second beam splitter 106; the detection optical path includes a first reflective diffraction grating 107, a first focusing component 108, a second focusing component 110 and a second reflective diffraction grating 111; the reference optical path includes a delay component 105 (for example, a delay line with an adjustable optical path); and a high-speed fluid device 109 is located between the first focusing component 108 and the second focusing component 110.
[0029] The first beam splitter 104 is used to split the incident pulse into a probe light and a reference light. The probe light is spatially dispersed by the first reflective diffraction grating 107 and focused by the first focusing assembly 108 onto a probe object in a test area of the high-speed fluid device 109. The flow direction of the probe object is parallel to the spatially dispersed and focused one-dimensional probe light (i.e., the flow direction of the probe object is parallel to the one-dimensional spatial light pulse and is located at the working focal length of the focusing assembly). After passing through the probe object and the second focusing assembly 110, the probe light is incident on the second reflective diffraction grating 111. The second reflective diffraction grating 111 reaggregates the one-dimensional dispersed light pulse signal carrying information about the probe object into a one-dimensional time-domain spectrum. The reference light is incident on the delay assembly 105, which is used to ensure that the optical path of the probe light path is consistent with that of the reference light path. The outgoing light of the probe light path and the outgoing light of the reference light path are both incident on the second beam splitter 106 and interfere with each other to obtain the interference signal.
[0030] The signal acquisition and processing unit in S4 includes a photoelectric detector 112, a high-speed oscilloscope 113 and a computer 114 connected in sequence; the interference signal (i.e., the time-domain spectrum signal) is converted into a digital signal by the photoelectric detector 112, which is collected by the high-speed oscilloscope 113 and then transmitted to the computer 114. The computer 114 performs one-dimensional continuous point signal processing to obtain velocity disturbance measurement information of the high-speed flow field.
[0031] Specifically, the one-dimensional continuous point signal processing includes the following sub-steps: A Hilbert transform is performed on the interference signal at each spatial location, and the real and imaginary parts are extracted.
[0032] Directly calculate the phase difference at the differential point (without phase unwrapping).
[0033] Perform a cross-correlation operation on the two pairs of differential point phase differences.
[0034] Calculate the average speed based on the spacing.
[0035] Output the velocity perturbation distribution of consecutive point pairs in one-dimensional space.
[0036] The above signal processing part is further explained below with reference to the formula.
[0037] The intensity distribution of the interference signal is expressed as follows (that is, the intensity and spatial distribution of the phase signal of the measured area obtained by detection are expressed as follows):
[0038] Where, is the intensity distribution of the interference signal, that is, the one-dimensional spatial distribution of the intensity of the recorded phase signal in the measured area; x is the one-dimensional spatial position, that is, the different positions of the one-dimensional spot area; is the first parameter; is the position-dependent wavelength, i.e., the wavelength focused at different positions of the sample; is the one-dimensional spatial distribution of the signal intensity in the high-speed fluid region to be measured (for example, the one-dimensional spatial distribution of the signal intensity in the detection light path), is the one-dimensional spatial distribution of the signal intensity without external phase fluctuation (e.g., reference light path), is the optical path difference between two paths (for example, the detection light path and the reference light path), B is the conversion factor between time information and wavelength information, and C is the conversion factor between wavelength information and spatial information. is the one-dimensional spatial distribution of the phase information of the area to be measured.
[0039] in, 、 、 、 Information that changes over time, , B, C, Do not change with time.
[0040] Perform Hilbert transform on the obtained phase information and extract the real and imaginary parts of the last term in the above formula; that is, for each spatial position x The interference signal Perform the Hilbert transform and extract the real part and the imaginary part , which is expressed as follows:
[0041] The phase information of the area to be measured is deduced as follows:
[0042] Assume that the phase values of differential region 1 and differential region 2 are and , the phase value of the reference optical path does not change with time under ideal conditions, and is set to , then the phase difference between interference point 1 and interference point 2 is That is, the present invention directly calculates the differential point phase difference, which is expressed as follows:
[0043] Perform a cross-correlation operation on the two pairs of differential point phase differences (i.e., use the cross-correlation function to calculate the cross-correlation time delay of the phase difference between the differential points in the two interference regions):
[0044] Where, is the phase difference between interference point 3 and interference point 4.
[0045] The average velocity is calculated based on the spacing; that is, the average velocity between the two interference regions is calculated using the following formula:
[0046] Where, is the distance between adjacent differential points (i.e., two interference regions), is the cross-correlation time delay, is the average velocity between the two interference regions.
[0047] Compared with the traditional focused differential interference signal processing method, the calculation method proposed in Example 1 avoids the problem of inconsistent data response when different photodetectors are used to collect data at different positions. In the processing of phase signals, the unwrapping step is omitted, which can reduce the phase recovery error, reduce the amount of calculation, and directly perform cross-correlation operations on the two differential areas.
[0048] The solution provided in Example 1 is very suitable for focused differential interferometry measurement of smaller fluid structures.
[0049] Example 2: Example 2 provides a method for measuring velocity disturbances in a high-speed flow field. Example 2 differs from Example 1 in that the focusing differential interference unit in Example 2 includes a first transmission diffraction grating 204, a first spectrometer 205, a third focusing assembly 206, a fourth focusing assembly 208, a second spectrometer 209, and a second transmission diffraction grating 210, which are sequentially arranged along the optical path; a high-speed fluid device 207 is located between the third focusing assembly 206 and the fourth focusing assembly 208. Figure 2 .
[0050] The incident pulse is spatially dispersed by the first transmission diffraction grating 204. The first spectrometer 205 splits each incident light ray into two beams at different angles. After passing through the third focusing assembly 206, two one-dimensional spectra that do not completely overlap on the same straight line are formed in the focal region. After passing through the detection object in the detection area of the high-speed fluid device 207, the one-dimensional spectrum forms an optical signal carrying information about the detection object. After this optical signal passes through the fourth focusing assembly 208 and the second spectrometer 209, light of the same wavelength is reconverged to form interference. The second transmission diffraction grating 210 rearranges the spatial spectral information in the time domain to obtain the interference signal.
[0051] The first light splitting device 205 and the second light splitting device 209 are arranged symmetrically with respect to the high-speed fluid device 207, and have the same device structure, see Figure 3 , each comprising a polarization beam splitter 301, a first reflector 302, and a second reflector 303. The first light splitting device 205 and the second light splitting device 209 are both variable pitch angle light splitting devices.
[0052] The polarization beam splitter 301 in the first spectroscopic device 205 is used to split the incident pulse into two separate pulses with different vibration directions. After the two pulses are reflected by the first reflector 302 and the second reflector 303 respectively, two parallel pulses with a certain distance between them are formed, or two pulses with a certain angle are formed.
[0053] The polarization beam splitter 301 may be a Wollaston prism.
[0054] For example, the polarization beam splitter 301 in the first light splitting device 205 is recorded as a first Wollaston prism, and the polarization beam splitter 301 in the second light splitting device 209 is recorded as a second Wollaston prism. Then, the two beams of linear polarized light with mutually perpendicular vibration directions and different incident angles are first reflected by the first reflector 302 and the second reflector 303 in the second light splitting device 209 respectively and then enter the second Wollaston prism. Then, the second Wollaston prism is used to combine the two beams of linear polarized light with mutually perpendicular vibration directions and different incident angles into one.
[0055] That is, Example 2 uses a Wollaston prism to split the nanosecond pulse signal into two pulses with a smaller angle. After obtaining the high-speed fluid area information, the light of the same wavelength interferes again to achieve high-precision measurement of the area to be measured.
[0056] That is, the nanosecond light pulse signal in Example 1 passes through a beam splitter, and the stretched nanosecond light pulse is separated into a signal detection light path and a signal reference light path, while Example 2 uses a Wollaston prism to split the light, generating two beams of pulses with a very small angle, and adjacent differential points interfere.
[0057] The focusing assembly in both Examples 1 and 2 can be composed of a 4f system and an objective lens. The objective lens can be a pre-assembled lens or a single lens. The focusing assembly has the function of flexibly adjusting the spectral width and the spot size. The delay line is composed of several reflectors. The photodetector is a single-pixel high-speed photodetector.
[0058] The composition of the femtosecond laser, time domain stretching unit and signal acquisition and processing unit in Example 2 and Example 1 and the steps of their execution are the same. For example, in Example 2, a femtosecond laser 201, a single-mode optical fiber 202, an erbium-doped optical fiber amplifier 203, a photodetector 211, a high-speed oscilloscope 212 and a computer 213 can also be specifically set up. Therefore, they are not repeated here and can be understood by referring to the description of Example 1.
[0059] Example 2 uses a spectroscopic device with a variable spacing angle for time-domain stretched focused differential interferometry. Compared with the traditional focused differential interferometry structure, the spectroscopic device of Example 2 can achieve focused differential point interferometry with variable spacing and can be used to construct differential points with large spacing. Therefore, the solution of Example 2 is very suitable for focused differential interferometry measurements of larger fluid structures (such as wind tunnels and aircraft engines).
[0060] Example 3: Embodiment 3 provides a high-speed flow field velocity disturbance measurement system, comprising: Femtosecond lasers, used to generate femtosecond pulses; A time domain stretching unit, configured to stretch the femtosecond pulse into a nanosecond pulse in the time domain so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; A focusing differential interferometry unit is used to perform one-dimensional continuous point focusing differential interferometry on a high-speed fluid device as a detection object to obtain an interference signal; A signal acquisition and processing unit, configured to acquire the interference signal and calculate velocity disturbance measurement information of the high-speed flow field based on the acquired information; The high-speed flow field velocity disturbance measurement system is used to perform the steps in the high-speed flow field velocity disturbance measurement method described in Example 1 or Example 2.
[0061] Overall, the femtosecond pulse laser in Example 3 serves as a light source; the time domain stretching unit stretches the femtosecond pulse to the nanosecond level, aligning the time domain and frequency domain spectra; the focused differential interference unit maps the time domain spectrum to a spatial distribution, with light of different wavelengths corresponding to different spatial positions, and forms an interference signal, which helps to detect subtle density changes in high-speed flow fields; the signal acquisition and processing unit uses a single-pixel photodetector to convert the obtained interference signal (light signal) into a digital signal, and the computer therein uses a phase demodulation algorithm and a correlation estimation algorithm to calculate the velocity fluctuation of the fluid.
[0062] The optical path of the system corresponding to Example 1 is as follows: a femtosecond laser generates a pulsed laser signal, which passes through a time-domain stretching unit to align the time-domain spectrum with the frequency-domain spectrum. A beam splitter then splits the stretched signal into a detection optical path and a reference optical path. The detection optical path enters a focused differential section, with the focus area corresponding to the high-speed fluid flow location. The reference optical path and the detection optical path together form an interference section, generating an interference signal that detects subtle density variations. The interference signal is collected by a signal acquisition and processing unit, converted into a digital signal for storage and recording, and the velocity fluctuation information of the high-speed flow field is recovered through calculation.
[0063] The optical path of the system corresponding to Example 2 is as follows: a femtosecond laser generates a pulsed laser signal. The time-domain spectrum is aligned with the frequency-domain spectrum by a time-domain stretching unit. After being split by a diffraction grating and a Wollaston prism, two non-overlapping differential points are formed in the focal region. These differential points subsequently form an interference signal, capturing subtle density variations. The interference signal is collected by a signal acquisition and processing unit, converted into a digital signal for storage and recording, and the velocity fluctuation information of the high-speed flow field is recovered through calculation.
[0064] Since the functions of each device or unit in the high-speed flow field velocity disturbance measurement system provided in Example 3 correspond to the steps in the high-speed flow field velocity disturbance measurement method described in Example 1 or Example 2, you can refer to the description of Example 1 or Example 2 for understanding, and will not repeat them here.
[0065] In summary, the present invention combines the high sampling rate and low system complexity of time-domain stretching with selectively focused differential interferometry to measure regional velocities in high-speed flow fields and detect subtle changes. It can also achieve one-dimensional continuous measurement at hundreds of MHz. This solves the problem of the mutual constraints between the number of measurement points per frame and system complexity in traditional focused differential interferometry (current methods can only measure discontinuously, and the greater the number of measurement points, the greater the system complexity, which is prohibitively complex). It can thus reveal the evolution of fluid flow characteristics. The present invention provides a high-speed flow field velocity perturbation measurement scheme with the advantages of simple structure and low cost, and can achieve one-dimensional continuous point measurement, thereby improving the efficiency of focused differential interferometry.
[0066] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.
Claims
1. A method for measuring velocity disturbance in a high-speed flow field, characterized in that: The following steps are involved: Using femtosecond lasers to generate femtosecond pulses; The femtosecond pulse is stretched into a nanosecond pulse in the time domain by a time domain stretching unit, so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; A focusing differential interferometer unit is used to perform one-dimensional continuous point focusing differential interferometry on a high-speed fluid device as a detection object to obtain an interference signal; The interference signal is collected by a signal collection and processing unit, and velocity disturbance measurement information of the high-speed flow field is calculated based on the collected information.
2. The high-speed flow field velocity disturbance measurement method according to claim 1, characterized in that: The time domain stretching unit includes a time domain stretching device, and the time domain stretching device adopts one of a multi-reflection glass rod, a single-mode optical fiber, a multi-mode optical fiber, and a free-space corner reflector.
3. The high-speed flow field velocity disturbance measurement method according to claim 2, characterized in that: The time domain stretching unit further includes an amplifier device, which is used to amplify the intensity of the stretched nanosecond pulse.
4. The method for measuring high-speed flow field velocity disturbance according to claim 1, characterized in that: The focused differential interference unit is used to continuously focus the stretched pulse along the one-dimensional direction of the fluid cross section, and to generate spatially adjacent differential interference point pairs on the test area of the high-speed fluid device to obtain the interference signal.
5. The high-speed flow field velocity disturbance measurement method according to claim 4, characterized in that: The focusing differential interference unit includes a first beam splitter, a detection optical path, a reference optical path, and a second beam splitter; the detection optical path includes a first reflective diffraction grating, a first focusing component, a second focusing component, and a second reflective diffraction grating; the reference optical path includes a delay component; a high-speed fluid device is located between the first focusing component and the second focusing component; The first beam splitter is used to split an incident pulse into a probe light and a reference light. The probe light is spatially dispersed by the first reflective diffraction grating and focused by the first focusing assembly onto a probe object in a test area of the high-speed fluid device, wherein the flow direction of the probe object is parallel to the spatially dispersed and focused one-dimensional probe light. After sequentially passing through the probe object and the second focusing assembly, the probe light is incident on the second reflective diffraction grating. The second reflective diffraction grating reaggregates the one-dimensional dispersed light pulse signal carrying information about the probe object into a one-dimensional time-domain spectrum. The reference light is incident on the delay component, and the delay component is used to make the optical path of the detection light path consistent with the optical path of the reference light path; the outgoing light of the detection light path and the outgoing light of the reference light path are both incident on the second beam splitter and interfere with each other to obtain the interference signal.
6. The method for measuring high-speed flow field velocity disturbance according to claim 4, characterized in that: The focusing differential interference unit includes a first transmission diffraction grating, a first spectrometer, a third focusing assembly, a fourth focusing assembly, a second spectrometer, and a second transmission diffraction grating, which are sequentially arranged along the optical path; a high-speed fluid device is located between the third focusing assembly and the fourth focusing assembly; The incident pulse is spatially dispersed by the first transmission diffraction grating, and each incident light ray is split into two beams at different angles by the first beam splitting device. After passing through the third focusing component, two one-dimensional spectra that do not completely overlap on the same straight line are formed in the focal region. After passing through the detection object in the test area of the high-speed fluid device, the one-dimensional spectrum forms an optical signal carrying information about the detection object; after the optical signal passes through the fourth focusing component and the second spectroscopic device in sequence, light of the same wavelength is reconverged to form interference; and the second transmission diffraction grating is used to rearrange the spatial spectrum information in the time domain to obtain the interference signal.
7. The method for measuring high-speed flow field velocity disturbance according to claim 6, characterized in that: The first light splitting device and the second light splitting device are symmetrically arranged with respect to the high-speed fluid device, and have the same device structure, both including a polarization beam splitter, a first reflector and a second reflector; The polarization beam splitter in the first spectroscopic device is used to split the incident pulse into two separate pulses with different vibration directions. After the two pulses are reflected by the first reflector and the second reflector respectively, two parallel pulses with a certain distance between them are formed, or two pulses with a certain angle are formed.
8. The method for measuring high-speed flow field velocity disturbance according to claim 1, characterized in that: The signal acquisition and processing unit includes a photoelectric detector, a high-speed oscilloscope and a computer connected in sequence; the interference signal is converted into a digital signal by the photoelectric detector, which is collected by the high-speed oscilloscope and then transmitted to the computer. The computer performs one-dimensional continuous point signal processing to obtain velocity disturbance measurement information of the high-speed flow field.
9. The method for measuring high-speed flow field velocity disturbance according to claim 8, characterized in that: The one-dimensional continuous point signal processing comprises the following sub-steps: performing a Hilbert transform on the interference signal at each spatial position and extracting the real and imaginary parts; Directly calculate the phase difference of the differential point; Performing cross-correlation operation on the phase differences of two pairs of differential points; Calculate the average speed based on the spacing: Output the velocity perturbation distribution of consecutive point pairs in one-dimensional space.
10. A high-speed flow field velocity disturbance measurement system, characterized in that: include: Femtosecond lasers, used to generate femtosecond pulses; A time domain stretching unit, configured to stretch the femtosecond pulse into a nanosecond pulse in the time domain so that the time domain waveform and the frequency domain spectrum maintain a linear mapping relationship; A focusing differential interferometry unit is used to perform one-dimensional continuous point focusing differential interferometry on a high-speed fluid device as a detection object to obtain an interference signal; A signal acquisition and processing unit, configured to acquire the interference signal and calculate velocity disturbance measurement information of the high-speed flow field based on the acquired information; The high-speed flow field velocity disturbance measurement system is used to perform the steps in the high-speed flow field velocity disturbance measurement method according to any one of claims 1 to 9.