An OFDR-based linear optical fiber flow field probe, flow field measurement method and flow field measurement device

Abstract

This invention discloses a linear fiber optic flow field probe based on OFDR, comprising a capillary tube and a multi-core fiber. The multi-core fiber includes a probe segment and a transmission segment connected together. The transmission segment is fixedly disposed within the capillary tube, while the probe segment is freely disposed outside the capillary tube. The capillary tube defines the initial direction of the probe segment. One end of the transmission segment, away from the probe segment, extends outside the capillary tube for connection to an optical frequency domain reflectometer. The multi-core fiber includes at least three peripheral cores for measuring the flow direction of the fluid. Each peripheral core is uniformly distributed around the central axis of the multi-core fiber on the same circumference, and the central angle between any two adjacent peripheral cores is the same. This linear fiber optic flow field probe can measure the flow direction of fluid. This invention also discloses a flow field measurement method and a flow field measurement device based on the above-mentioned linear fiber optic flow field probe.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing, and more particularly to a linear fiber optic flow field probe based on OFDR, a flow field measurement method, and a flow field measurement device. Background Technology

[0002] A flow field probe is an instrument used to measure the flow state and distribution of related physical quantities of a fluid (such as a gas or liquid) within a spatial region.

[0003] Optical Frequency Domain Reflectometry (OFDR) is a fiber optic sensing and measurement technology. It works by utilizing the Rayleigh backscattering phenomenon of light propagating through an optical fiber. By analyzing the frequency changes of the light signal emitted from the light source as it travels back and forth through the fiber, it detects changes in physical parameters along the fiber, such as temperature and strain. OFDR plays a significant role in national and civilian perimeter security monitoring, oil and gas pipeline monitoring, wind turbine health monitoring, and deep-sea and submarine earthquake monitoring.

[0004] Chinese patent application CN201810969044.3 discloses an online monitoring system for drainage pipelines based on OFDR distributed optical fiber. The system includes a distributed optical fiber assembly, a light source generator, a data acquisition device, and a data processing center, connected sequentially. The distributed optical fiber assembly comprises multiple monitoring groups positioned at different fill levels within the drainage pipeline. Each monitoring group includes at least one distributed monitoring optical fiber, which is disposed on the inner wall of the drainage pipeline and arranged axially along the pipeline. This online monitoring system calculates the flow velocity distribution along the distributed monitoring optical fiber by performing wavelength strain demodulation at various locations on the distributed monitoring optical fiber, based on the different strains produced by fluids at different flow velocities acting on it.

[0005] However, the aforementioned patent can only measure the flow velocity of the fluid, but cannot measure the flow direction of the fluid. Summary of the Invention

[0006] To address the shortcomings of the prior art, this invention provides an OFDR-based linear fiber optic flow field probe, a flow field measurement method, and a flow field measurement device, which can measure the flow direction of fluids.

[0007] The technical problem to be solved by the present invention is achieved through the following technical solution:

[0008] A linear fiber optic flow field probe based on OFDR includes a capillary fixing tube and a multi-core fiber.

[0009] The multi-core optical fiber includes a fiber detection section and a fiber transmission section connected together. The fiber transmission section is fixedly disposed inside the capillary tube, and the fiber detection section is freely disposed outside the capillary tube. The capillary tube defines the initial direction of the fiber detection section. The end of the fiber transmission section away from the fiber detection section extends out of the capillary tube for connection with an optical frequency domain reflectometer.

[0010] The multi-core optical fiber includes at least three peripheral cores for measuring the flow direction of the fluid, and each peripheral core is uniformly distributed around the central axis of the multi-core optical fiber on the same circumference.

[0011] Furthermore, each peripheral fiber core within the optical fiber detection segment has multiple peripheral gratings evenly distributed along the optical fiber axis, and the peripheral gratings of each peripheral fiber core are aligned with each other.

[0012] Furthermore, the multi-core optical fiber also includes a central core for measuring the flow velocity of the fluid, and the central core is located on the central axis of the multi-core optical fiber.

[0013] Furthermore, the central fiber core within the optical fiber detection segment has multiple central gratings evenly distributed along the optical fiber axis.

[0014] A flow field measurement method for the aforementioned linear fiber optic flow field probe; the flow field measurement method includes the following steps:

[0015] Step 100: When the optical fiber detection segment is parallel to the initial direction, obtain the peripheral reference signal of each peripheral fiber core in the optical fiber detection segment; and when the optical fiber detection segment is in a fluid, obtain the peripheral measurement signal of each peripheral fiber core in the optical fiber detection segment.

[0016] Step 200: Demodulate the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position;

[0017] Step 300: Calculate the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position;

[0018] Step 400: Calculate the bending angle of the optical fiber detection segment based on the curvature of the optical fiber detection segment at each axial position;

[0019] Step 500: Determine the flow direction of the fluid based on the initial direction and bending angle of the optical fiber detection segment.

[0020] Furthermore, in step 200, when demodulating the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position, the following steps are included:

[0021] Step 210: Use Fast Fourier Transform to convert the peripheral reference signals and peripheral measurement signals of each peripheral fiber core into the distance domain;

[0022] Step 220: In the distance domain, using a first moving window and a first moving distance, the peripheral reference signal and peripheral measurement signal of each peripheral fiber core are locally extracted by sliding window method to obtain the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core at each axial position.

[0023] Step 230: Use inverse fast Fourier transform to convert the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core into the wavelength domain;

[0024] Step 240: In the wavelength domain, demodulate the local peripheral reference signal and the local peripheral measurement signal at the same axial position of the same peripheral fiber to obtain the wavelength offset of each peripheral fiber core at each axial position.

[0025] Furthermore, within the optical fiber detection segment, each peripheral fiber core is uniformly distributed with multiple peripheral gratings along the optical fiber axis, and the peripheral gratings of each peripheral fiber core are aligned with each other; the first moving window has the same grating length as the peripheral gratings, and the first moving distance has the same spacing distance as the peripheral gratings.

[0026] Furthermore, in step 300, when calculating the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position, the following steps are included:

[0027] Step 310: Compare the wavelength offsets of each outer fiber core at the same axial position to determine the maximum offset, minimum offset, and intermediate offset at each axial position;

[0028] Step 320: Subtract the maximum and minimum offsets at the same axial position from the intermediate offset to obtain the maximum and minimum bending offsets at each axial position;

[0029] Step 330: Based on the relationship curve between wavelength offset and strain, determine the maximum and minimum bending strain at each axial position;

[0030] Step 340: Calculate the curvature of the optical fiber probe segment at each axial position based on the maximum and minimum bending strain at the same axial position.

[0031] Furthermore, the multi-core optical fiber also includes a central core for measuring the flow velocity of the fluid, the central core being located on the central axis of the multi-core optical fiber; the flow field measurement method further includes the following steps:

[0032] Step 100: When the optical fiber probe segment is not in the fluid, obtain the center reference signal of the central fiber core in the optical fiber probe segment, and when the optical fiber probe segment is in the fluid, obtain the center measurement signal of the central fiber core in the optical fiber probe segment.

[0033] Step 200: Demodulate the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at each axial position;

[0034] Step 300: Calculate the strain distribution of the optical fiber detection segment at each axial position based on the wavelength offset of the central fiber core at the same axial position;

[0035] Step 400: Based on the relationship curve between strain and flow velocity, determine the flow velocity of the fluid at each axial position of the optical fiber detection segment.

[0036] Furthermore, in step 200, when demodulating the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at various axial positions, the following steps are included:

[0037] Step 210: Use Fast Fourier Transform to convert the center reference signal and center measurement signal of the central fiber core into the distance domain;

[0038] Step 220: In the distance domain, using a second moving window and a second moving distance, the center reference signal and center measurement signal of the central fiber core are locally truncated by sliding window method to obtain the local center reference signal and local center measurement signal of the central fiber core at each axial position;

[0039] Step 230: Use inverse fast Fourier transform to convert the local center reference signal and local center measurement signal of the central fiber core into the wavelength domain;

[0040] Step 240: In the wavelength domain, demodulate the local center reference signal and the local center measurement signal of the central fiber at the same axial position to obtain the wavelength offset of the central fiber core at each axial position.

[0041] Furthermore, the central fiber core within the optical fiber detection segment is uniformly distributed with multiple central gratings along the optical fiber axis; the second moving window has the same length as the central gratings, and the second moving distance has the same spacing as the central gratings.

[0042] Furthermore, within the optical fiber detection segment, each peripheral fiber core is uniformly distributed with multiple peripheral gratings along the optical fiber axis, and the central gratings of the central fiber core are aligned with the central gratings of each peripheral fiber core. In step 300, calculating the curvature distribution of the optical fiber detection segment at various axial positions based on the wavelength offset of each peripheral fiber core at the same axial position includes the following steps:

[0043] Step 310: Subtract the wavelength offset of each peripheral fiber core from that of the central fiber core at the same axial position to obtain the bending offset of each peripheral fiber core at each axial position.

[0044] Step 320: Based on the relationship curve between wavelength offset and strain, determine the bending strain of each outer fiber core at the same axial position;

[0045] Step 330: Calculate the curvature of the optical fiber detection segment at each axial position based on the bending strain at each position on the same axis.

[0046] A flow field measurement device includes the aforementioned linear fiber optic flow field probe, an optical frequency domain reflectometer, a signal acquisition unit, and a host computer. The fiber optic transmission section of the linear fiber optic flow field probe is connected to the optical frequency domain reflectometer, the optical frequency domain reflectometer is connected to the signal acquisition unit, and the signal acquisition unit is connected to the host computer.

[0047] The present invention has the following beneficial effects: The linear fiber optic flow field probe of the present invention is based on optical frequency domain reflection technology. It utilizes the distributed sensing formed by each peripheral fiber core in the fiber optic probe segment to measure the strain distribution formed by each peripheral fiber core under the action of fluid. Based on the strain distribution of each peripheral fiber core, the curvature distribution of the fiber optic probe segment is calculated, thereby determining the bending angle of the fiber optic probe segment and finally obtaining the flow direction of the fluid. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the structure of the linear fiber optic flow field probe provided by the present invention.

[0049] Figure 2 A radial plane schematic diagram of the fiber optic detection section in the linear fiber optic flow field probe provided by the present invention.

[0050] Figure 3 A schematic diagram of the axial plane of the fiber optic detection section in the linear fiber optic flow field probe provided by the present invention.

[0051] Figure 4 This is a schematic diagram of the bending of the linear fiber optic flow field probe provided by the present invention.

[0052] Figure 5 A flowchart illustrating the steps of the flow field measurement method provided by this invention.

[0053] Figure 6 This is a flowchart of step 200 in the flow field measurement method provided by the present invention.

[0054] Figure 7 This is a flowchart of step 300 in the flow field measurement method provided by the present invention.

[0055] Figure 8 This is a flowchart illustrating the steps of another flow field measurement method provided by the present invention.

[0056] Figure 9 This is a flowchart of step 200 in another flow field measurement method provided by the present invention.

[0057] Figure 10 This is a flowchart of step 300 in another flow field measurement method provided by the present invention.

[0058] Figure 11 A schematic diagram of the flow field measurement device provided by the present invention. Detailed Implementation

[0059] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, examples of which are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0060] In the description of this invention, it should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0061] Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include one or more of that feature. In the description of this invention, "multiple" means two or more, unless otherwise explicitly specified.

[0062] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixing," and "setting," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0063] Example 1

[0064] like Figure 1-4 As shown, a linear fiber optic flow field probe based on OFDR includes a capillary fixing tube 1 and a multi-core fiber 2.

[0065] The multi-core optical fiber 2 includes a fiber detection section 21 and a fiber transmission section 22 connected to each other. The fiber transmission section 22 is fixedly disposed inside the capillary tube 1, and the fiber detection section 21 is freely disposed outside the capillary tube 1. The capillary tube 1 defines the initial direction of the fiber detection section 21. One end of the fiber transmission section 22 away from the fiber detection section 21 extends out of the capillary tube 1 for connection with the optical frequency domain reflectometer.

[0066] The multi-core optical fiber 2 includes at least three peripheral cores 23 for measuring the flow direction of the fluid. Each peripheral core 23 is uniformly distributed around the central axis of the multi-core optical fiber 2 on the same circumference.

[0067] The linear fiber optic flow field probe of the present invention is based on optical frequency domain reflection technology. It utilizes the distributed sensing formed by each peripheral fiber core 23 in the fiber optic probe section 21 to measure the strain distribution formed by each peripheral fiber core 23 under the action of fluid. Based on the strain distribution of each peripheral fiber core 23, the curvature distribution of the fiber optic probe section 21 is calculated, thereby determining the flow direction of the fluid.

[0068] In this embodiment, the uniform distribution of each peripheral fiber core 23 means that the central angle between any two adjacent peripheral fiber cores 23 is the same.

[0069] The optical fiber detection segment 21 and the optical fiber transmission segment 22 can be made using the same multi-core optical fiber 2, or they can be made using different multi-core optical fibers 2 and then fused together.

[0070] Optical frequency domain reflection technology is based on the Rayleigh backscattering phenomenon of optical fiber. In order to locally enhance the Rayleigh backscattering signal of each peripheral fiber core 23 in the optical fiber detection section 21, preferably, each peripheral fiber core 23 in the optical fiber detection section 21 has multiple peripheral gratings 24 evenly distributed along the optical fiber axis, and the peripheral gratings 24 of each peripheral fiber core 23 are aligned with each other.

[0071] The axial position of the peripheral grating 24 is formed within the optical fiber detection section 21, which is the flow direction measurement point used to measure the flow direction. When measuring the flow direction, it is only necessary to analyze the Rayleigh backscattering signal corresponding to the axial position of the peripheral grating 24.

[0072] In this embodiment, the uniform distribution of the peripheral gratings 24 means that the spacing between any two adjacent peripheral gratings 24 in the same peripheral fiber core 23 is the same. The alignment of the peripheral gratings 24 between the peripheral fiber cores 23 means that the peripheral gratings 24 are provided at the same axial position in different peripheral fiber cores 23.

[0073] The multi-core optical fiber 2 also includes a central core 25 for measuring the flow velocity of the fluid, and the central core 25 is located on the central axis of the multi-core optical fiber 2.

[0074] The linear fiber optic flow field probe of the present invention utilizes the distributed sensing formed by the central fiber core 25 within the fiber optic probe section 21 to measure the strain distribution formed by the central fiber core 25 under the action of fluid, and calculates the strain distribution of the fiber optic probe section 21 based on the strain distribution of the central fiber core 25, thereby determining the flow velocity of the fluid.

[0075] Similarly, in order to locally enhance the Rayleigh backscattered signal of the central fiber core 25 within the optical fiber detection segment 21, preferably, the central fiber core 25 within the optical fiber detection segment 21 has multiple central gratings 26 uniformly distributed along the optical fiber axis.

[0076] The axial position of the central grating 26 within the optical fiber detection section 21 is the flow velocity measurement point for measuring flow velocity. When measuring flow velocity, it is only necessary to analyze the Rayleigh backscattered signal corresponding to the axial position of the central grating 26.

[0077] In this embodiment, the uniform distribution of each segment of the central grating 26 means that the spacing between any two adjacent segments of the central grating 26 in the central fiber core 25 is the same.

[0078] The central gratings 26 of the central fiber core 25 and the central gratings 26 of the peripheral fiber cores 23 can be aligned or staggered. That is, the velocity measurement points formed by the central fiber core 25 on the optical fiber detection section 21 and the flow direction measurement points formed by the peripheral fiber cores 23 on the optical fiber detection section 21 can be distributed at the same axial position or at different axial positions.

[0079] The capillary fixing tube 1 includes a first tube body 11 and a second tube body 12 connected to each other, with the first tube body 11 and the second tube body 12 arranged at a preset angle; the first tube body 11 is used to be placed in the fluid to define the initial direction of the optical fiber detection segment 21, wherein the initial direction of the optical fiber detection segment 21 is parallel to the first tube body 11; the second tube body 12 is used to be placed outside the fluid to lead the optical fiber transmission segment 22 out of the fluid.

[0080] In this embodiment, the capillary fixing tube 1 and the optical fiber transmission section 22 are fixed by filling with sealant.

[0081] Example 2

[0082] like Figure 5 As shown, a flow field measurement method is used for the linear fiber optic flow field probe described in Embodiment 1; the flow field measurement method includes the following steps:

[0083] Step 100: When the optical fiber detection segment is parallel to the initial direction, obtain the peripheral reference signal of each peripheral fiber core in the optical fiber detection segment; and when the optical fiber detection segment is in a fluid, obtain the peripheral measurement signal of each peripheral fiber core in the optical fiber detection segment.

[0084] In step 100, an optical frequency domain reflectometer is connected to the fiber transmission segment of the linear fiber optic flow field probe. The optical frequency domain reflectometer emits linearly swept light into the fiber optic probe segment and receives the Rayleigh backscattered light generated by the linearly swept light within the fiber optic probe segment. The Rayleigh backscattered signal is then obtained through photoelectric conversion. When the fiber optic probe segment is parallel to the initial direction, the Rayleigh backscattered signal obtained by the optical frequency domain reflectometer from each peripheral fiber core includes the peripheral reference signal. When the fiber optic probe segment is in a fluid environment, the Rayleigh backscattered signal obtained by the optical frequency domain reflectometer from each peripheral fiber core includes the peripheral measurement signal.

[0085] Each peripheral fiber core within the optical fiber transmission segment will also generate Rayleigh backscattered light. Based on the length of the optical fiber transmission segment and the flight time of the linearly swept light within each peripheral fiber core, the starting position of the optical fiber detection segment can be determined from the obtained Rayleigh backscattered signals. The Rayleigh backscattered signals before the starting position are then removed to form the peripheral reference signal and the peripheral measurement signal.

[0086] Step 200: Demodulate the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position.

[0087] In step 200, when the optical fiber probe segment is in a fluid, it will experience two types of strain under the influence of the fluid: bending strain caused by fluid impact and axial strain caused by fluid friction. The bending strain of each peripheral fiber core differs. At the same axial position, the peripheral fibers located outside the bending angle will be bent and stretched, causing a positive shift in the center wavelength of the peripheral measurement signal relative to the peripheral reference signal. The closer to the outside of the bending angle, the greater the bending and stretching, and the greater the positive shift. The peripheral fibers located inside the bending angle will be bent and compressed, causing a negative shift in the center wavelength of the peripheral measurement signal relative to the peripheral reference signal. The closer to the inside of the bending angle, the greater the bending and compression, and the greater the negative shift. The axial strain of each peripheral fiber core remains the same.

[0088] Specifically, such as Figure 6 As shown, in step 200, when demodulating the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position, the following steps are included:

[0089] Step 210: Use Fast Fourier Transform to convert the peripheral reference signals and peripheral measurement signals of each peripheral fiber core into the distance domain;

[0090] Step 220: In the distance domain, using a first moving window and a first moving distance, the peripheral reference signal and peripheral measurement signal of each peripheral fiber core are locally extracted by sliding window method to obtain the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core at each axial position.

[0091] Step 230: Use inverse fast Fourier transform to convert the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core into the wavelength domain;

[0092] Step 240: In the wavelength domain, demodulate the local peripheral reference signal and the local peripheral measurement signal at the same axial position of the same peripheral fiber to obtain the wavelength offset of each peripheral fiber core at each axial position.

[0093] In this embodiment, each peripheral fiber core is demodulated based on the peripheral reference signal and the peripheral measurement signal to obtain the wavelength offset at each axial position. This is a conventional signal demodulation method in optical frequency domain reflection technology and will not be described in detail here. It should be noted that the wavelength offset can be positive or negative. A positive wavelength offset corresponds to a peripheral fiber core that has been bent and stretched, while a negative wavelength offset corresponds to a peripheral fiber core that has been bent and compressed.

[0094] If each peripheral fiber core within the fiber detection segment has multiple peripheral gratings evenly distributed along the fiber axis, and the peripheral gratings of each peripheral fiber core are aligned with each other, then the first moving window has the same grating length as the peripheral gratings, and the first moving distance has the same spacing distance as the peripheral gratings. This ensures that when the peripheral reference signal and peripheral measurement signal are intercepted in step 220 using the first moving window and the first moving distance, the intercepted local peripheral reference signal and local peripheral measurement signal are Rayleigh backscattered signals enhanced by each peripheral grating segment.

[0095] Step 300: Calculate the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position.

[0096] In step 300, a wavelength offset corresponds to the strain of an outer fiber core at an axial position. By calculating the strain of each outer fiber core at the same axial position, the curvature corresponding to that axial position can be obtained. By repeating the above calculation for each axial position, the curvature distribution of the optical fiber detection segment at each axial position can be obtained.

[0097] Specifically, such as Figure 7 As shown, in step 300, when calculating the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position, the following steps are included:

[0098] Step 310: Compare the wavelength offsets of each outer fiber core at the same axial position to determine the maximum offset, minimum offset, and intermediate offset at each axial position.

[0099] In step 310, the maximum offset is the wavelength offset generated by the outermost fiber core located at the outermost bend angle in the optical fiber detection segment, the minimum offset is the wavelength offset generated by the outermost fiber core located at the innermost bend angle in the optical fiber detection segment, and the intermediate offset is the wavelength offset generated by the outermost fiber core located at the middle of the bend angle in the optical fiber detection segment.

[0100] Step 320: Subtract the maximum and minimum offsets at the same axial position from the intermediate offset to obtain the maximum and minimum bending offsets at each axial position.

[0101] In step 320, as described above, when the optical fiber probe segment is in a fluid, it experiences two types of strain under the influence of the fluid: bending strain caused by fluid impact and axial strain caused by fluid friction. Therefore, each wavelength offset actually includes both the bending offset caused by the bending strain and the axial offset caused by the axial strain. When measuring the flow direction, only the bending offset is needed, while the axial offset introduces an error into the flow direction measurement.

[0102] The outer fiber core, located at the midpoint of the bend angle, is closest to the central axis of the fiber optic probe segment and exhibits the greatest symmetry. The wavelength shift caused by the outer bending and stretching can largely cancel out the wavelength shift caused by the inner bending and compression. Therefore, the bending shift contained in the intermediate offset is minimal and can be considered as containing only axial shift. Thus, by using the intermediate offset as a reference value and subtracting the maximum and minimum offsets from the intermediate offset, the axial shift contained in the maximum and minimum offsets can be eliminated.

[0103] Step 330: Based on the relationship curve between wavelength offset and strain, determine the maximum and minimum bending strain at each axial position.

[0104] In step 330, the maximum bending strain ε is calculated using the following formula. max and minimum bending strain ε max ,

[0105]

[0106] Where, Δλ max and Δλ min These are the maximum and minimum bending offsets, λ0, respectively. max and λ0 min The initial center wavelengths corresponding to the maximum and minimum bending offsets are respectively, and P is the photoelastic coefficient of the optical fiber detection segment.

[0107] Step 340: Calculate the curvature of the optical fiber probe segment at each axial position based on the maximum and minimum bending strain at the same axial position.

[0108] In step 330, the curvature k of the optical fiber probe segment at each axial position satisfies the following formula:

[0109]

[0110] Where, ε max and ε min These are the maximum and minimum bending strains, θ, respectively. max and θ min They are respectively with ε max and ε min The corresponding angular offsets of the two outer cores in the radial coordinate system, where r is the distance between each outer core and the central axis, and Δθ is the angle relative to ε. max and ε min The corresponding central angle between the two outer fiber cores, The bending direction angle.

[0111] Step 400: Calculate the bending angle of the optical fiber detection segment based on the curvature of the optical fiber detection segment at each axial position.

[0112] In step 400, the curvature of the optical fiber probe segment at each axial position can be directly integrated to obtain the bending angle β of the optical fiber probe segment. The integration formula is as follows:

[0113]

[0114] Where l is the length of the optical fiber detection segment, and k(s) is the curvature of the optical fiber detection segment at each axial position.

[0115] Step 500: Determine the flow direction of the fluid based on the initial direction and bending angle of the optical fiber detection segment.

[0116] In step 500, the initial direction of the optical fiber detection segment is added to the measured bending angle to determine the bending direction of the optical fiber detection segment, and the bending direction of the optical fiber detection segment is parallel to the flow direction of the fluid.

[0117] Example 3

[0118] As an optimization of Embodiment 2, in this embodiment, such as Figure 8 As shown, the flow field measurement method further includes the following steps:

[0119] Step 100: When the optical fiber probe segment is not in a fluid, obtain the center reference signal of the central fiber core within the optical fiber probe segment; and when the optical fiber probe segment is in a fluid, obtain the center measurement signal of the central fiber core within the optical fiber probe segment.

[0120] In step 100, when the fiber optic probe segment is not in a fluid, the Rayleigh backscattered signal obtained by the optical frequency domain reflectometer from the central fiber core includes the central reference signal; when the fiber optic probe segment is in a fluid, the Rayleigh backscattered signal obtained by the optical frequency domain reflectometer from the central fiber core includes the central measurement signal.

[0121] The central fiber core within the optical fiber transmission segment also generates Rayleigh backscattered light. Based on the length of the optical fiber transmission segment and the flight time of the linearly swept light within each peripheral fiber core, the starting position of the optical fiber detection segment can be determined from the obtained Rayleigh backscattered signals. The Rayleigh backscattered signals before the starting position are then removed to form the central reference signal and the central measurement signal.

[0122] Step 200: Demodulate the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at each axial position.

[0123] In step 200, as described above, when the optical fiber probe segment is in a fluid, it will experience two types of strain under the influence of the fluid: bending strain caused by fluid impact and axial strain caused by fluid friction. Since the central fiber core is located on the central axis of the optical fiber probe segment and possesses perfect symmetry, the wavelength shift caused by bending and stretching on its outer side and the wavelength shift caused by bending and compression on its inner side can completely cancel each other out. Therefore, the wavelength shift of the central fiber core is caused only by axial strain.

[0124] Specifically, such as Figure 9 As shown, in step 200, when demodulating the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at various axial positions, the following steps are included:

[0125] Step 210: Use Fast Fourier Transform to convert the center reference signal and center measurement signal of the central fiber core into the distance domain;

[0126] Step 220: In the distance domain, using a second moving window and a second moving distance, the center reference signal and center measurement signal of the central fiber core are locally truncated by sliding window method to obtain the local center reference signal and local center measurement signal of the central fiber core at each axial position;

[0127] Step 230: Use inverse fast Fourier transform to convert the local center reference signal and local center measurement signal of the central fiber core into the wavelength domain;

[0128] Step 240: In the wavelength domain, demodulate the local center reference signal and the local center measurement signal of the central fiber at the same axial position to obtain the wavelength offset of the central fiber core at each axial position.

[0129] Demodulating the central fiber core based on the central reference signal and the central measurement signal to obtain the wavelength offset at each axial position is a conventional signal demodulation method in optical frequency domain reflection technology, which will not be elaborated on here. It is important to note that the wavelength offset can be positive or negative. A positive wavelength offset indicates that the central fiber core is axially stretched, while a negative wavelength offset indicates that the central fiber core is axially compressed. However, since the optical fiber probe section is freely positioned in the fluid, the central fiber core can only be axially stretched by the frictional force of the fluid, and cannot be axially compressed by the frictional force of the fluid. Therefore, the wavelength offset of the central fiber core can only be positive.

[0130] If the central fiber core within the fiber detection segment is uniformly distributed with multiple central gratings along the fiber axis, then the second moving window has the same length as the central grating, and the second moving distance has the same spacing as the central grating, so that when the central reference signal and central measurement signal are truncated in step 220 using the second moving window and the second moving distance, the truncated local wavelength domain central reference signal and local wavelength domain central measurement signal are Rayleigh backscattered signals enhanced by each central grating segment.

[0131] Step 300: Calculate the strain distribution of the optical fiber detection segment at each axial position based on the wavelength offset of the central fiber core at the same axial position.

[0132] In step 300, the wavelength offset Δλ and the strain force F satisfy the following formula:

[0133]

[0134] Wherein, λ0 is the initial center wavelength, P is the photoelastic coefficient of the optical fiber detection segment, and E is the elastic modulus of the optical fiber detection segment.

[0135] Step 400: Based on the relationship curve between strain and flow velocity, determine the flow velocity of the fluid at each axial position of the optical fiber detection segment.

[0136] In step 400, the strain force F and the flow velocity v satisfy the following formula:

[0137]

[0138] Where ρ is the fluid density, C d denoted as the fluid drag coefficient, b as the radial circumference of the optical fiber detection segment, and l as the axial length of the optical fiber detection segment.

[0139] Example 4

[0140] As an optimized embodiment of Example 3, in this embodiment, the central gratings of each segment of the central fiber core are aligned with the central gratings of each segment of the peripheral fiber cores. That is, the velocity measurement points formed by the central fiber core on the optical fiber detection segment and the flow direction measurement points formed by the peripheral fiber cores on the optical fiber detection segment are distributed at the same axial position.

[0141] In step 300, the wavelength offset of the central fiber core at each axial position is used instead of the intermediate offset of each peripheral fiber core at each axial position to calculate the curvature distribution of the optical fiber detection segment at each axial position.

[0142] Specifically, such as Figure 10 As shown, in step 300, when calculating the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position, the following steps are included:

[0143] Step 310: Subtract the wavelength offset of each peripheral fiber core from that of the central fiber core at the same axial position to obtain the bending offset of each peripheral fiber core at each axial position.

[0144] In step 310, as described above, the wavelength offset of each peripheral fiber core includes both the bending offset caused by bending strain and the axial offset caused by axial strain, while the wavelength offset of the central fiber core is caused only by axial strain. Subtracting the wavelength offset of the central fiber core at the same axial position from the wavelength offset of each peripheral fiber core eliminates the axial offset contained in the wavelength offset of each peripheral fiber core.

[0145] Step 320: Based on the relationship curve between wavelength offset and strain, determine the bending strain of each outer fiber core at the same axial position.

[0146] In step 320, the bending strain ε is calculated using the following formula.

[0147]

[0148] Wherein, Δλ is the bending offset, λ0 is the initial center wavelength corresponding to the bending offset, and P is the photoelastic coefficient of the optical fiber detection segment.

[0149] Step 330: Calculate the curvature of the optical fiber detection segment at each axial position based on the bending strain at each position on the same axis.

[0150] In step 330, three peripheral fiber cores are selected, and the curvature k of the fiber probe segment at each axial position satisfies the following formula:

[0151]

[0152] Where, ε a ε b and ε c The bending strain θ represents the bending strain of outer cores a, b, and c, respectively. b and θ c Let r be the central angle between outer fiber cores b and c and outer fiber core a, respectively, and r be the distance between each outer fiber core and the central axis. The bending direction angle.

[0153] Example 5

[0154] like Figure 11 As shown, a flow field measurement device includes a linear fiber optic flow field probe as described in Embodiment 1, an optical frequency domain reflectometer, a signal acquisition unit, and a host computer. The fiber optic transmission section of the linear fiber optic flow field probe is connected to the optical frequency domain reflectometer, the optical frequency domain reflectometer is connected to the signal acquisition unit, and the signal acquisition unit is connected to the host computer.

[0155] In some examples, the optical frequency domain reflectometer has multiple main interferometer arms, each main interferometer arm being connected to a peripheral fiber core or a central fiber core. Each main interferometer arm is responsible for receiving the Rayleigh backscattered signal output by the corresponding peripheral fiber core or central fiber core.

[0156] In some examples, the optical frequency domain reflectometer has only one main interferometer arm, which is connected to the linear fiber flow field probe via an optical switching switch. The main interferometer arm is switched sequentially to each peripheral fiber core or the central fiber core via the optical switching switch, so as to receive the Rayleigh backscattered signals output by each peripheral fiber core or the central fiber core in sequence.

[0157] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method of flow field measurement, characterized by, This is used for a linear fiber optic flow field probe; the linear fiber optic flow field probe includes a capillary tube and a multi-core fiber. The multi-core optical fiber includes a fiber detection section and a fiber transmission section connected together. The fiber transmission section is fixedly disposed inside the capillary tube, and the fiber detection section is freely disposed outside the capillary tube. The capillary tube defines the initial direction of the fiber detection section. The end of the fiber transmission section away from the fiber detection section extends out of the capillary tube for connection with an optical frequency domain reflectometer. The capillary fixing tube includes a first tube body and a second tube body connected to each other, with the first tube body and the second tube body set at a preset angle; the first tube body is used to be placed in the fluid to define the initial direction of the optical fiber detection segment. The second tube is positioned outside the fluid to lead the optical fiber transmission segment out of the fluid; the initial direction of the optical fiber detection segment is parallel to the first tube. The multi-core optical fiber includes at least three peripheral cores for measuring the flow direction of the fluid, and each peripheral core is uniformly distributed around the central axis of the multi-core optical fiber on the same circumference. The fiber optic detection segment is used to reflect the linearly swept light emitted by the optical frequency domain reflector, thereby forming a Rayleigh scattering signal. Under the action of fluid impact, it undergoes bending strain, and then the strain response generated by the outer fiber core inside it causes the Rayleigh scattering signal to shift its center wavelength, thereby sensing the flow direction of the fluid. The flow field measurement method includes the following steps: Step 100: When the optical fiber detection segment is parallel to the initial direction, obtain the peripheral reference signal of each peripheral fiber core in the optical fiber detection segment; and when the optical fiber detection segment is in a fluid, obtain the peripheral measurement signal of each peripheral fiber core in the optical fiber detection segment. Step 200: Demodulate the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position; Step 300: Calculate the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position; Step 400: Calculate the bending angle of the optical fiber detection segment based on the curvature of the optical fiber detection segment at each axial position; Step 500: Determine the flow direction of the fluid based on the initial direction and bending angle of the optical fiber detection segment; In step 300, when calculating the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position, the following steps are included: Step 310: Compare the wavelength offsets of each outer fiber core at the same axial position to determine the maximum offset, minimum offset, and intermediate offset at each axial position; Step 320: Subtract the maximum and minimum offsets at the same axial position from the intermediate offset to obtain the maximum and minimum bending offsets at each axial position; Step 330: Based on the relationship curve between wavelength offset and strain, determine the maximum and minimum bending strain at each axial position; the maximum bending strain amount and the minimum bending strain amount as follows: ， wherein and are the maximum and minimum bending offsets, respectively, and are initial center wavelengths corresponding to the maximum and minimum bending offsets, respectively, is the photoelastic coefficient of the fiber probe section. Step 340: Calculate the curvature of the optical fiber detection segment at each axial position based on the maximum and minimum bending strain at the same axial position; The curvature of the optical fiber detection segment at various axial positions It satisfies the following formula: in, and These are the maximum and minimum bending strains, respectively. and They are respectively with and The corresponding angular offsets of the two outer fiber cores in the radial coordinate system, where r is the distance between each outer fiber core and the central axis. To be respectively with and The corresponding central angle between the two outer fiber cores, The bending direction angle; In step 400, the curvature of the optical fiber probe segment at each axial position is integrated to obtain the bending angle of the optical fiber probe segment. The integral formula is as follows: Where l is the length of the optical fiber detection segment. The curvature of the optical fiber probe segment at each axial position; In step 500, the initial direction of the optical fiber detection segment is added to the measured bending angle to determine the bending direction of the optical fiber detection segment, which is parallel to the flow direction of the fluid.

2. The flow field measurement method according to claim 1, characterized in that, In step 200, when demodulating the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position, the following steps are included: Step 210: Use Fast Fourier Transform to convert the peripheral reference signals and peripheral measurement signals of each peripheral fiber core into the distance domain; Step 220: In the distance domain, using a first moving window and a first moving distance, the peripheral reference signal and peripheral measurement signal of each peripheral fiber core are locally extracted by sliding window method to obtain the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core at each axial position. Step 230: Use inverse fast Fourier transform to convert the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core into the wavelength domain; Step 240: In the wavelength domain, demodulate the local peripheral reference signal and the local peripheral measurement signal at the same axial position of the same peripheral fiber to obtain the wavelength offset of each peripheral fiber core at each axial position.

3. The flow field measurement method according to claim 2, characterized in that, Within the optical fiber detection segment, each peripheral fiber core is uniformly distributed with multiple peripheral gratings along the optical fiber axis, and the peripheral gratings of each peripheral fiber core are aligned with each other; the first moving window has the same grating length as the peripheral gratings, and the first moving distance has the same spacing distance as the peripheral gratings.

4. The flow field measurement method according to claim 1, characterized in that, The multi-core optical fiber also includes a central core for measuring the flow velocity of the fluid, the central core being located on the central axis of the multi-core optical fiber; the flow field measurement method further includes the following steps: Step 100: When the optical fiber probe segment is not in the fluid, obtain the center reference signal of the central fiber core in the optical fiber probe segment, and when the optical fiber probe segment is in the fluid, obtain the center measurement signal of the central fiber core in the optical fiber probe segment. Step 200: Demodulate the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at each axial position; Step 300: Calculate the strain distribution of the optical fiber detection segment at each axial position based on the wavelength offset of the central fiber core at the same axial position; Step 400: Based on the relationship curve between strain and flow velocity, determine the flow velocity of the fluid at each axial position of the optical fiber detection segment.

5. The flow field measurement method according to claim 4, characterized in that, In step 200, when demodulating the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at various axial positions, the following steps are included: Step 210: Use Fast Fourier Transform to convert the center reference signal and center measurement signal of the central fiber core into the distance domain; Step 220: In the distance domain, using a second moving window and a second moving distance, the center reference signal and center measurement signal of the central fiber core are locally truncated by sliding window method to obtain the local center reference signal and local center measurement signal of the central fiber core at each axial position; Step 230: Use inverse fast Fourier transform to convert the local center reference signal and local center measurement signal of the central fiber core into the wavelength domain; Step 240: In the wavelength domain, demodulate the local center reference signal and the local center measurement signal of the central fiber core at the same axial position to obtain the wavelength offset of the central fiber core at each axial position.

6. The flow field measurement method according to claim 5, characterized in that, The central fiber core within the optical fiber detection segment is uniformly distributed with multiple central gratings along the optical fiber axis; the second moving window has the same length as the central gratings, and the second moving distance has the same spacing as the central gratings.

7. A flow field measurement method, characterized in that, This is used for a linear fiber optic flow field probe; the linear fiber optic flow field probe includes a capillary tube and a multi-core fiber. The multi-core optical fiber includes a fiber detection section and a fiber transmission section connected together. The fiber transmission section is fixedly disposed inside the capillary tube, and the fiber detection section is freely disposed outside the capillary tube. The capillary tube defines the initial direction of the fiber detection section. The end of the fiber transmission section away from the fiber detection section extends out of the capillary tube for connection with an optical frequency domain reflectometer. The capillary fixing tube includes a first tube body and a second tube body connected to each other, with the first tube body and the second tube body set at a preset angle; the first tube body is used to be placed in the fluid to define the initial direction of the optical fiber detection segment. The second tube is positioned outside the fluid to lead the optical fiber transmission segment out of the fluid; the initial direction of the optical fiber detection segment is parallel to the first tube. The multi-core optical fiber includes at least three peripheral cores for measuring the flow direction of the fluid, and each peripheral core is uniformly distributed around the central axis of the multi-core optical fiber on the same circumference. The fiber optic detection segment is used to reflect the linearly swept light emitted by the optical frequency domain reflector, thereby forming a Rayleigh scattering signal. Under the action of fluid impact, it undergoes bending strain, and then the strain response generated by the outer fiber core inside it causes the Rayleigh scattering signal to shift its center wavelength, thereby sensing the flow direction of the fluid. The flow field measurement method includes the following steps: Step 100: When the optical fiber detection segment is parallel to the initial direction, obtain the peripheral reference signal of each peripheral fiber core in the optical fiber detection segment; and when the optical fiber detection segment is in a fluid, obtain the peripheral measurement signal of each peripheral fiber core in the optical fiber detection segment. Step 200: Demodulate the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position; Step 300: Calculate the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position; Step 400: Calculate the bending angle of the optical fiber detection segment based on the curvature of the optical fiber detection segment at each axial position; Step 500: Determine the flow direction of the fluid based on the initial direction and bending angle of the optical fiber detection segment; The multi-core optical fiber also includes a central core for measuring the flow velocity of the fluid, the central core being located on the central axis of the multi-core optical fiber; the flow field measurement method further includes the following steps: Step 100: When the optical fiber probe segment is not in the fluid, obtain the center reference signal of the central fiber core in the optical fiber probe segment, and when the optical fiber probe segment is in the fluid, obtain the center measurement signal of the central fiber core in the optical fiber probe segment. Step 200: Demodulate the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at each axial position; Step 300: Calculate the strain distribution of the optical fiber detection segment at each axial position based on the wavelength offset of the central fiber core at the same axial position; Wavelength offset The following formula applies to the relationship between the strain force F and the strain force: F=E* in, The initial center wavelength, Let E be the elastic modulus of the optical fiber detection segment; Step 400: Based on the relationship curve between strain and flow velocity, determine the flow velocity at each axial position of the fluid distribution in the optical fiber detection section; The strain force F and the flow velocity v satisfy the following formula: in, For fluid density, denoted as the fluid drag coefficient, b as the radial circumference of the optical fiber detection segment, and l as the axial length of the optical fiber detection segment.

8. The flow field measurement method according to claim 7, characterized in that, In step 200, when demodulating the peripheral reference signal and peripheral measurement signal of the same peripheral fiber core to obtain the wavelength offset of each peripheral fiber core at each axial position, the following steps are included: Step 210: Use Fast Fourier Transform to convert the peripheral reference signals and peripheral measurement signals of each peripheral fiber core into the distance domain; Step 220: In the distance domain, using a first moving window and a first moving distance, the peripheral reference signal and peripheral measurement signal of each peripheral fiber core are locally extracted by sliding window method to obtain the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core at each axial position. Step 230: Use inverse fast Fourier transform to convert the local peripheral reference signal and local peripheral measurement signal of each peripheral fiber core into the wavelength domain; Step 240: In the wavelength domain, demodulate the local peripheral reference signal and the local peripheral measurement signal at the same axial position of the same peripheral fiber to obtain the wavelength offset of each peripheral fiber core at each axial position.

9. The flow field measurement method according to claim 8, characterized in that, Within the optical fiber detection segment, each peripheral fiber core is uniformly distributed with multiple peripheral gratings along the optical fiber axis, and the peripheral gratings of each peripheral fiber core are aligned with each other; the first moving window has the same grating length as the peripheral gratings, and the first moving distance has the same spacing distance as the peripheral gratings.

10. The flow field measurement method according to claim 7, characterized in that, In step 300, when calculating the curvature distribution of the optical fiber detection segment at each axial position based on the wavelength offset of each peripheral fiber core at the same axial position, the following steps are included: Step 310: Compare the wavelength offsets of each outer fiber core at the same axial position to determine the maximum offset, minimum offset, and intermediate offset at each axial position; Step 320: Subtract the maximum and minimum offsets at the same axial position from the intermediate offset to obtain the maximum and minimum bending offsets at each axial position; Step 330: Based on the relationship curve between wavelength offset and strain, determine the maximum and minimum bending strain at each axial position; Step 340: Calculate the curvature of the optical fiber probe segment at each axial position based on the maximum and minimum bending strain at the same axial position.

11. The flow field measurement method according to claim 7, characterized in that, In step 200, when demodulating the center reference signal and center measurement signal of the central fiber core to obtain the wavelength offset of the central fiber core at various axial positions, the following steps are included: Step 210: Use Fast Fourier Transform to convert the center reference signal and center measurement signal of the central fiber core into the distance domain; Step 220: In the distance domain, using a second moving window and a second moving distance, the center reference signal and center measurement signal of the central fiber core are locally truncated by sliding window method to obtain the local center reference signal and local center measurement signal of the central fiber core at each axial position; Step 230: Use inverse fast Fourier transform to convert the local center reference signal and local center measurement signal of the central fiber core into the wavelength domain; Step 240: In the wavelength domain, demodulate the local center reference signal and the local center measurement signal of the central fiber core at the same axial position to obtain the wavelength offset of the central fiber core at each axial position.

12. The flow field measurement method according to claim 11, characterized in that, The central fiber core within the optical fiber detection segment is uniformly distributed with multiple central gratings along the optical fiber axis; the second moving window has the same length as the central gratings, and the second moving distance has the same spacing as the central gratings.

13. A flow field measuring device, characterized in that, The device is used to implement the flow field measurement method according to any one of claims 1-12; the flow field measurement device includes a linear fiber optic flow field probe, an optical frequency domain reflectometer, a signal acquisition device, and a host computer, wherein the fiber optic transmission section of the linear fiber optic flow field probe is connected to the optical frequency domain reflectometer, the optical frequency domain reflectometer is connected to the signal acquisition device, and the signal acquisition device is connected to the host computer.

Images