Linear optical fiber flow field probe based on OFDR, flow field measurement method and flow field measurement device

Abstract

The invention discloses an OFDR-based linear optical fiber flow field probe, which comprises a capillary fixing tube and a multi-core optical fiber, the multi-core optical fiber comprises an optical fiber detection section and an optical fiber transmission section which are connected, the optical fiber transmission section is fixedly arranged in the capillary fixing tube, the optical fiber detection section is freely arranged outside the capillary fixing tube, and the optical fiber transmission section is connected with the capillary fixing tube. The initial direction of the optical fiber detection section is defined by the capillary fixing tube; one end, far away from the optical fiber detection section, of the optical fiber transmission section extends out of the capillary fixing tube and is used for being connected with an optical frequency domain reflectometer; the multi-core optical fiber comprises at least three peripheral fiber cores and is used for measuring the flowing direction of fluid, the peripheral fiber cores are evenly distributed on the same circumference around the central axis of the multi-core optical fiber, and the central angles between any two adjacent peripheral fiber cores are the same. The linear optical fiber flow field probe can measure the flowing direction of fluid. The invention further discloses a flow field measuring method and a flow field measuring device based on the linear optical fiber flow field probe.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and in particular to a linear optical fiber flow field probe based on OFDR, a flow field measurement method, and a flow field measurement device. Background Art

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

[0003] Optical Frequency Domain Reflectometry (OFDR for short) is a fiber optic sensing and measurement technology. Its working principle is to utilize the Rayleigh backscattering phenomenon when light propagates in an optical fiber, and detect the changes in physical parameters along the optical fiber, such as temperature, strain, etc., by analyzing the frequency changes generated by the round-trip of the optical signal emitted by the light source in the optical fiber, and plays a major role in fields such as national and civilian perimeter security monitoring, oil and gas pipeline monitoring, wind turbine health monitoring, deep sea and submarine earthquake monitoring, etc.

[0004] Chinese Patent with the patent application number CN201810969044.3 discloses an online monitoring system for drainage pipes based on OFDR distributed optical fiber, including a distributed optical fiber component, a light source generating device, a data acquisition device, and a data processing center connected in sequence. The distributed optical fiber component includes a plurality of monitoring groups arranged at different filling degrees inside the drainage pipe. Each monitoring group includes at least one distributed monitoring optical fiber, and the distributed monitoring optical fiber is arranged on the inner wall of the drainage pipe and along the axial direction of the drainage pipe. This online monitoring system calculates the flow velocity distribution of the fluid along the distributed monitoring optical fiber by demodulating the wavelength strain at each position on the distributed monitoring optical fiber according to the different strains generated by fluids with different flow velocities acting on the distributed monitoring optical fiber.

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

[0006] In order to solve the deficiencies of the above prior art, the present invention provides a linear optical fiber flow field probe based on OFDR, a flow field measurement method, and a flow field measurement device, which can realize the measurement of the flow direction of a fluid.

[0007] The technical problems to be solved by the present invention are realized through the following technical solutions:

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

[0009] The multi-core optical fiber includes an optically fiber detection section and an optically fiber transmission section connected to each other. The optically fiber transmission section is fixedly arranged inside the capillary fixing tube, the optically fiber detection section is freely arranged outside the capillary fixing tube, and the capillary fixing tube defines an initial direction of the optically fiber detection section; One end of the optically fiber transmission section away from the optically fiber detection section extends outside the capillary fixing 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 a fluid, and each peripheral core is uniformly distributed on the same circumference around the central axis of the multi-core optical fiber.

[0011] Further, multiple sections of peripheral gratings are uniformly distributed along the optical fiber axis in each peripheral core inside the optically fiber detection section, and the peripheral gratings of each peripheral core are aligned with each other.

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

[0013] Further, multiple sections of central gratings are uniformly distributed along the optical fiber axis in the central core inside the optically fiber detection section.

[0014] A flow field measurement method is used for the linear optical fiber flow field probe described above; The flow field measurement method includes the following steps:

[0015] Step 100: When the optically fiber detection section is parallel to the initial direction, obtain the peripheral reference signals of each peripheral core inside the optically fiber detection section, and when the optically fiber detection section is in a fluid, obtain the peripheral measurement signals of each peripheral core inside the optically fiber detection section;

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

[0017] Step 300: Calculate the curvature distribution of the optically fiber detection section at each axial position according to the wavelength offset of each peripheral core at the same axial position;

[0018] Step 400: Calculate the bending angle of the optically fiber detection section according to the curvature of the optically fiber detection section at each axial position;

[0019] Step 500: Determine the flow direction of the fluid according to the initial direction and the bending angle of the optically fiber detection section.

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

[0021] Step 210: Use the fast Fourier transform to convert the peripheral reference signal and the peripheral measurement signal of each peripheral core into the distance domain;

[0022] Step 220: In the distance domain, use the first moving window and the first moving distance to respectively perform local interception on the peripheral reference signal and the peripheral measurement signal of each peripheral core in a sliding window manner to obtain the local peripheral reference signal and the local peripheral measurement signal of each peripheral core at each axial position;

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

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

[0025] Further, multiple segments of peripheral gratings are uniformly distributed along the fiber axis on each peripheral core within the fiber detection section, and the peripheral gratings of each peripheral core are aligned with each other; the first moving window is the same as the grating length of the peripheral grating, and the first moving distance is the same as the interval distance of the peripheral grating.

[0026] Further, in step 300, when calculating the curvature distribution of the fiber detection section at each axial position according to the wavelength offset of each peripheral core at the same axial position, the following steps are included:

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

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

[0029] Step 330: Determine the maximum bending strain and the minimum bending strain at each axial position according to the relationship curve between the wavelength offset and the strain;

[0030] Step 340: Calculate the curvature of the fiber detection section at each axial position according to the maximum bending strain and the minimum bending strain at the same axial position.

[0031] Further, the multi-core optical fiber further 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; the flow field measurement method further includes the following steps:

[0032] Step 100: When the optical fiber detection section is not in the fluid, obtain the central reference signal of the central core in the optical fiber detection section, and when the optical fiber detection section is in the fluid, obtain the central measurement signal of the central core in the optical fiber detection section;

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

[0034] Step 300: Calculate the strain force distribution at each axial position of the optical fiber detection section according to the wavelength offset at the same axial position of the central core;

[0035] Step 400: Determine the flow velocity magnitude of the fluid distributed at each axial position of the optical fiber detection section according to the relationship curve between the strain force and the flow velocity.

[0036] Further, in step 200, when demodulating the central reference signal and the central measurement signal of the central core to obtain the wavelength offset at each axial position of the central core, the following steps are included:

[0037] Step 210: Use the fast Fourier transform to convert the central reference signal and the central measurement signal of the central core into the distance domain;

[0038] Step 220: In the distance domain, use a second moving window and a second moving distance to locally intercept the central reference signal and the central measurement signal of the central core by means of a sliding window respectively, so as to obtain the local central reference signal and the local central measurement signal at each axial position of the central core;

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

[0040] Step 240: In the wavelength domain, perform a demodulation operation on the local central reference signal and the local central measurement signal of the central optical fiber at the same axial position to obtain the wavelength offset at each axial position of the central core.

[0041] Further, multiple segments of central gratings are evenly distributed along the optical fiber axis in the optical fiber detection section; the length of the second moving window is the same as that of the central grating, and the second moving distance is the same as the pitch of the central grating.

[0042] Further, multiple segments of peripheral gratings are evenly distributed along the optical fiber axis on each peripheral optical fiber in the optical fiber detection section, and the central gratings of each segment on the central optical fiber are aligned with the central gratings of each segment on each peripheral optical fiber; in step 300, when calculating the curvature distribution of the optical fiber detection section at each axial position according to the wavelength offset of each peripheral optical fiber at the same axial position, the following steps are included:

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

[0044] Step 320: Determine the bending strain of each peripheral optical fiber at the same axial position according to the relationship curve between the wavelength offset and the strain;

[0045] Step 330: Calculate the curvature of the optical fiber detection section at each axial position according to the bending strains at the same axial position.

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

[0047] The present invention has the following beneficial effects: The linear optical fiber flow field probe of the present invention is based on the optical frequency domain reflection technology, uses the distributed sensing formed by each peripheral optical fiber in the optical fiber detection section to measure the strain distribution formed by each peripheral optical fiber under the action of the fluid, calculates the curvature distribution of the optical fiber detection section according to the strain distribution of each peripheral optical fiber, and then determines the bending angle of the optical fiber detection section, and finally obtains the flow direction of the fluid. Description of the Drawings

[0048] Figure 1 It is a schematic structural diagram of the linear optical fiber flow field probe provided by the present invention.

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

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

[0051] Figure 4 Schematic diagram of the bending of the linear optical fiber flow field probe provided by the present invention.

[0052] Figure 5 Flow chart of the steps of the flow field measurement method provided by the present invention.

[0053] Figure 6 Flow chart of the steps of step 200 in the flow field measurement method provided by the present invention.

[0054] Figure 7 Flow chart of the steps of step 300 in the flow field measurement method provided by the present invention.

[0055] Figure 8 Flow chart of the steps of another flow field measurement method provided by the present invention.

[0056] Figure 9 Flow chart of the steps of step 200 in another flow field measurement method provided by the present invention.

[0057] Figure 10 Flow chart of the steps of step 300 in another flow field measurement method provided by the present invention.

[0058] Figure 11 Principle block diagram of the flow field measurement device provided by the present invention. Detailed implementation manners

[0059] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals indicate the same or similar elements or elements having the same or similar functions from beginning to end. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present invention, and should not be construed as a limitation to the present invention.

[0060] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0061] In addition, 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 quantity of the indicated technical features. Thus, features defined with "first", "second", and "third" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more unless otherwise specifically defined.

[0062] In the present invention, unless otherwise clearly specified and defined, terms such as "installed", "connected", "joined", "fixed", "set", etc. shall be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and may also be the communication inside two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0063] Embodiment 1

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

[0065] The multi-core optical fiber 2 includes a connected optical fiber detection section 21 and an optical fiber transmission section 22. The optical fiber transmission section 22 is fixedly arranged in the capillary fixing tube 1, the optical fiber detection section 21 is freely arranged outside the capillary fixing tube 1, and the capillary fixing tube 1 defines the initial direction of the optical fiber detection section 21; one end of the optical fiber transmission section 22 far from the optical fiber detection section 21 extends outside the capillary fixing tube 1 for connection with an 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, and each peripheral core 23 is uniformly distributed on the same circumference around the central axis of the multi-core optical fiber 2.

[0067] The linear optical fiber flow field probe of the present invention is based on optical frequency domain reflection technology. By using the distributed sensing formed by each peripheral core 23 in the optical fiber detection section 21, the strain distribution formed by each peripheral core 23 under the action of the fluid is measured, and according to the strain distribution of each peripheral core 23, the curvature distribution of the optical fiber detection section 21 is calculated, and then the flow direction of the fluid is determined.

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

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

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

[0071] The axial position where the peripheral grating 24 is made in the optical fiber detection section 21 is the flow direction measurement point for measuring the flow direction. When measuring the flow direction, it is only necessary to analyze the Rayleigh backscattering signal corresponding to the axial position where the peripheral grating 24 is made.

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

[0073] The multi-core optical fiber 2 further 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 optical fiber flow field probe of the present invention utilizes the distributed sensing formed by the central core 25 in the optical fiber detection section 21 to measure the strain distribution formed by the central core 25 under the action of the fluid, and calculates the stress distribution of the optical fiber detection section 21 according to the strain distribution of the central core 25, and further determines the flow velocity of the fluid.

[0075] Similarly, in order to locally enhance the Rayleigh backscattering signal of the central core 25 in the optical fiber detection section 21, preferably, multiple sections of central gratings 26 are uniformly distributed along the optical fiber axis in the central core 25 in the optical fiber detection section 21.

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

[0077] In this embodiment, the uniform distribution of each section of the central grating 26 means that in the central core 25, the interval distance between any two adjacent sections of the central grating 26 is the same.

[0078] Between each segment of the central grating 26 of the central core 25 and each segment of the central grating 26 of each peripheral core 23, they can be aligned with each other or staggered from each other. That is, each flow velocity measurement point formed by the central core 25 on the optical fiber detection section 21 and each flow direction measurement point formed by each peripheral core 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 connected first tube body 11 and a second tube body 12, and a preset angle is provided between the first tube body 11 and the second tube body 12; the first tube body 11 is used to be arranged in the fluid to define the initial direction of the optical fiber detection section 21, wherein the initial direction of the optical fiber detection section 21 is parallel to the first tube body 11; the second tube body 12 is used to be arranged outside the fluid to lead out the optical fiber transmission section 22 to the outside of the fluid.

[0080] In this embodiment, a sealant is filled between the capillary fixing tube 1 and the optical fiber transmission section 22 for fixation.

[0081] Embodiment Two

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

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

[0084] In this step 100, an optical frequency domain reflectometer is connected to the optical fiber transmission section of the linear optical fiber flow field probe, and a linearly swept-frequency light is emitted into the optical fiber detection section through the optical frequency domain reflectometer, and the Rayleigh backscattered light generated by the linearly swept-frequency light in the optical fiber detection section is received, and then the Rayleigh backscattered signal is obtained through photoelectric conversion. When the optical fiber detection section is parallel to the initial direction, the Rayleigh backscattered signals obtained by the optical frequency domain reflectometer from each peripheral core contain the peripheral reference signals, and when the optical fiber detection section is in the fluid, the Rayleigh backscattered signals obtained by the optical frequency domain reflectometer from each peripheral core contain the peripheral measurement signals.

[0085] Rayleigh backscattered light is also generated in each peripheral core within the optical fiber transmission section. Based on the length of the optical fiber transmission section and the flight time of the linearly swept optical signal in each peripheral core, the starting position of the optical fiber detection section can be determined from the obtained Rayleigh backscattered signal, and the Rayleigh backscattered signal before the starting position is removed to form the peripheral reference signal and the peripheral measurement signal.

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

[0087] In this step 200, when the optical fiber detection section is in a fluid, two types of strains will be generated in the optical fiber detection section under the action of the fluid. One is the bending strain generated by the impact of the fluid, and the other is the axial strain generated by the friction of the fluid. Among them, the bending strains of each peripheral core are different. At the same axial position, the peripheral optical fiber located outside the bending angle will be bent and stretched, so that the center wavelength of the peripheral measurement signal will have a positive offset relative to the peripheral reference signal, and the closer to the outside of the bending angle, the greater the bending and stretching, and the greater the positive offset. The peripheral optical fiber located inside the bending angle will be bent and compressed, so that the center wavelength of the peripheral measurement signal will have a negative offset relative to the peripheral reference signal, and the closer to the inside of the bending angle, the greater the bending and compression, and the greater the negative offset; while the axial strains of each peripheral core are the same.

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

[0089] Step 210: Use the fast Fourier transform to convert the peripheral reference signal and the peripheral measurement signal of each peripheral core into the distance domain;

[0090] Step 220: In the distance domain, use the first moving window and the first moving distance to locally intercept the peripheral reference signal and the peripheral measurement signal of each peripheral core by means of a sliding window to obtain the local peripheral reference signal and the local peripheral measurement signal at each axial position of each peripheral core;

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

[0092] Step 240: In the wavelength domain, perform demodulation operations on the local peripheral reference signal and the local peripheral measurement signal of the same peripheral optical fiber at the same axial position to obtain the wavelength offsets of each peripheral core at each axial position.

[0093] In this embodiment, demodulating each peripheral core according to the peripheral reference signal and the peripheral measurement signal to obtain the wavelength offsets at each axial position is a conventional signal demodulation method in optical frequency domain reflectometry, which will not be elaborated in detail here. It should be noted that the wavelength offsets have positive and negative values. A positive wavelength offset corresponds to a peripherally core that is bent and stretched, and a negative wavelength offset corresponds to a peripherally core that is bent and compressed.

[0094] If multiple peripheral gratings are uniformly distributed along the fiber axis in each peripheral core within the fiber detection section, and the peripheral gratings of each peripheral core are aligned with each other, then the length of the first moving window is the same as the grating length of the peripheral grating, and the first moving distance is the same as the interval distance of the peripheral grating, so that when the peripheral reference signal and the peripheral measurement signal are intercepted using the first moving window and the first moving distance in step 220, the intercepted local peripheral reference signals and local peripheral measurement signals are Rayleigh backscattering signals enhanced by each section of the peripheral grating.

[0095] Step 300: Calculate the curvature distribution of the fiber detection section at each axial position according to the wavelength offsets of each peripheral core at the same axial position.

[0096] In this step 300, a wavelength offset corresponds to the strain of a peripheral core at an axial position. By calculating the strains of each peripheral core at the same axial position, the curvature corresponding to this axial position can be obtained. Repeating the foregoing calculation for each axial position can obtain the curvature distribution of the fiber detection section at each axial position.

[0097] Specifically, as Figure 7 shown, when calculating the curvature distribution of the fiber detection section at each axial position according to the wavelength offsets of each peripheral core at the same axial position in step 300, the following steps are included:

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

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

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

[0101] In this step 320, as described above, when the optical fiber detection section is in a fluid, the optical fiber detection section will generate two kinds of strains under the action of the fluid. One is the bending strain generated by the impact of the fluid, and the other is the axial strain generated by the friction of the fluid. Therefore, each wavelength offset actually includes 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 needs to be used, and the axial offset will cause errors in the measurement of the flow direction.

[0102] The peripheral core located in the middle of the bending angle is closest to the central axis of the optical fiber detection section and has the greatest symmetry. The wavelength offset caused by the bending tension on the outside can be maximally offset by the wavelength offset caused by the bending compression on the inside. Therefore, the bending offset contained in the intermediate offset is the smallest and can be regarded as only containing the axial offset. Therefore, taking the intermediate offset as the reference value and subtracting the maximum offset and the minimum offset from the intermediate offset respectively can eliminate the axial offset contained in the maximum offset and the minimum offset.

[0103] Step 330: Determine the maximum bending strain and the minimum bending strain at each axial position according to the relationship curve between the wavelength offset and the strain.

[0104] In this step 330, the following formula is used to calculate the maximum bending strain ε max and the minimum bending strain ε max ,

[0105]

[0106] where, Δλ max and Δλ min are the maximum bending offset and the minimum bending offset respectively, λ0 max and λ0 min are the initial central wavelengths corresponding to the maximum bending offset and the minimum bending offset respectively, and P is the photoelastic coefficient of the optical fiber detection section.

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

[0108] In this step 330, the curvature k of the fiber optic detection section at each axial position satisfies the following formula:

[0109]

[0110] where ε max and ε min are the maximum bending strain and the minimum bending strain respectively, θ max and θ min are the angular offsets of two peripheral cores corresponding to ε max and ε min in the radial coordinate system, r is the distance between each peripheral core and the central axis, Δθ is the central angle between two peripheral cores corresponding to ε max and ε min respectively, and is the bending direction angle.

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

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

[0113]

[0114] where l is the length of the fiber optic detection section, and k(s) is the curvature of the fiber optic detection section at each axial position.

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

[0116] In this step 500, adding the initial direction of the fiber optic detection section to the measured bending angle can determine the bending direction of the fiber optic detection section, and the bending direction of the fiber optic detection section is parallel to the flow direction of the fluid.

[0117] Embodiment III

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

[0119] Step 100: When the fiber optic detection section is not in the fluid, obtain the central reference signal of the central core within the fiber optic detection section, and when the fiber optic detection section is in the fluid, obtain the central measurement signal of the central core within the fiber optic detection section.

[0120] In this step 100, when the fiber optic detection section is not in the fluid, the Rayleigh backscattering signal obtained by the optical frequency domain reflectometer from the central core contains the central reference signal. When the fiber optic detection section is in the fluid, the Rayleigh backscattering signal obtained by the optical frequency domain reflectometer from the central core contains the central measurement signal.

[0121] The central core within the fiber optic transmission section also generates Rayleigh backscattered light. According to the length of the fiber optic transmission section and the flight time of the linearly swept frequency light in each outer core, the starting position of the fiber optic detection section can be determined in the obtained Rayleigh backscattering signal, and the Rayleigh backscattering signal before the starting position can be removed to form the central reference signal and the central measurement signal.

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

[0123] In this step 200, as described above, when the fiber optic detection section is in the fluid, the fiber optic detection section will generate two kinds of strains under the action of the fluid. One is the bending strain generated by the fluid impact, and the other is the axial strain generated by the fluid friction. Since the central core is located on the central axis of the fiber optic detection section and has complete symmetry, the wavelength offset caused by the bending tension on the outer side can be completely offset by the wavelength offset caused by the bending compression on the inner side. Therefore, the wavelength offset of the central core is only caused by the axial strain.

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

[0125] Step 210: Use the fast Fourier transform to convert the central reference signal and the central measurement signal of the central core into the distance domain;

[0126] Step 220: In the distance domain, use the second moving window and the second moving distance to locally intercept the central reference signal and the central measurement signal of the central core respectively by means of a sliding window to obtain the local central reference signal and the local central measurement signal at each axial position of the central core;

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

[0128] Step 240: In the wavelength domain, perform a demodulation operation on the local central reference signal and the local central measurement signal of the central optical fiber at the same axial position to obtain the wavelength offset of the central core at each axial position.

[0129] Demodulating the central core according to the central reference signal and the central measurement signal to obtain the wavelength offset at each axial position is a conventional signal demodulation method for optical frequency domain reflectometry, which will not be elaborated in detail here. It should be noted that the wavelength offset has positive and negative values. A positive wavelength offset indicates that the central core is axially stretched, and a negative wavelength offset indicates that the central core is axially compressed. Since the optical fiber detection section is freely arranged in the fluid, the central 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 core only has positive values.

[0130] If multiple central gratings are uniformly distributed along the optical axis of the central core in the optical fiber detection section; then the length of the second moving window is the same as that of the central grating, and the second moving distance is the same as the spacing of the central grating, so that when the second moving window and the second moving distance are used to intercept the central reference signal and the central measurement signal in step 220, the intercepted local wavelength domain central reference signals and local wavelength domain central measurement signals are the Rayleigh backscattering signals enhanced by each section of the central grating.

[0131] Step 300: Calculate the strain force distribution of the optical fiber detection section at each axial position according to the wavelength offset of the central core at the same axial position.

[0132] In this step 300, the relationship between the wavelength offset Δλ and the strain force F satisfies the following formula:

[0133]

[0134] where λ0 is the initial central wavelength, P is the photoelastic coefficient of the optical fiber detection section, and E is the elastic modulus of the optical fiber detection section.

[0135] Step 400: Determine the flow velocity magnitude of the fluid distributed at each axial position of the optical fiber detection section according to the relationship curve between the strain force and the flow velocity.

[0136] In this step 400, the relationship between the strain force F and the flow velocity v satisfies the following formula:

[0137]

[0138] Among them, ρ is the fluid density, C d is the resistance coefficient of the fluid, b is the radial perimeter of the fiber optic detection section, and l is the axial length of the fiber optic detection section.

[0139] Example 4

[0140] As an optimized solution of Example 3, in this example, the central gratings of each section of the central core are aligned with the central gratings of each section of each peripheral core. That is, each flow velocity measurement point formed by the central core on the fiber optic detection section and each flow direction measurement point formed by each peripheral core on the fiber optic detection section are distributed at the same axial position.

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

[0142] Specifically, as Figure 10 shown, in step 300, when calculating the curvature distribution of the fiber optic detection section at each axial position according to the wavelength offset of each peripheral core at the same axial position, the following steps are included:

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

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

[0145] Step 320: Determine the bending strain of each peripheral core at the same axial position according to the relationship curve between the wavelength offset and the strain.

[0146] In this step 320, the following formula is used to calculate the bending strain ε,

[0147]

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

[0149] Step 330: Calculate the curvature of the fiber detection section at each axial position based on the bending strains at the same axial position.

[0150] In this step 330, three of the outer cores are selected, and the curvature k of the fiber detection section at each axial position satisfies the following formula:

[0151]

[0152] Where, ε a , ε b and ε c are the bending strains of the outer core a, the outer core b, and the outer core c respectively, θ b and θ c are the central angles between the outer core b and the outer core c and the outer core a respectively, r is the distance between each outer core and the central axis, is the bending direction angle.

[0153] Embodiment 5

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

[0155] In some examples, the optical frequency domain reflectometer has a plurality of main interference arms. One main interference arm is correspondingly connected to one outer core or the central core, and each main interference arm is responsible for receiving the Rayleigh backscattering signal output by the corresponding outer core or the central core.

[0156] In some examples, the optical frequency domain reflectometer has only one main interference arm, and is connected to the linear optical fiber flow field probe through an optical switch, so as to sequentially switch the main interference arm to each outer core or the central core through the optical switch to sequentially receive the Rayleigh backscattering signals output by each outer core or the central core.

[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 are not intended to limit them. Although the embodiments of the present invention have been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or equivalently replaced, and these modifications or equivalent replacements do not make the modified technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A linear optical fiber flow field probe based on OFDR, characterized in that, It includes a capillary fixing tube and a multi-core optical fiber. The multi-core optical fiber includes a connected optical fiber detection section and an optical fiber transmission section. The optical fiber transmission section is fixedly arranged inside the capillary fixing tube, and the optical fiber detection section is freely arranged outside the capillary fixing tube. The capillary fixing tube defines the initial direction of the optical fiber detection section; one end of the optical fiber transmission section far from the optical fiber detection section extends outside the capillary fixing tube and is used to be connected to an optical frequency domain reflectometer. The multi-core optical fiber includes at least three peripheral cores for measuring the flow direction of a fluid, and each peripheral core is uniformly distributed on the same circumference around the central axis of the multi-core optical fiber.

2. The linear optical fiber flow field probe according to claim 1, wherein On each peripheral core inside the optical fiber detection section, there are multiple sections of peripheral gratings uniformly distributed along the optical fiber axis, and the peripheral gratings of each peripheral core are aligned with each other.

3. The linear optical fiber flow field probe according to claim 1 or 2, characterized in that, The multi-core optical fiber further includes a central core for measuring the flow velocity of a fluid, and the central core is located on the central axis of the multi-core optical fiber.

4. The linear optical fiber flow field probe according to claim 3, wherein On the central core inside the optical fiber detection section, there are multiple sections of central gratings uniformly distributed along the optical fiber axis.

5. A flow field measurement method, characterized in that For the linear optical fiber flow field probe described in claim 1; the flow field measurement method includes the following steps: Step 100: When the optical fiber detection section is parallel to the initial direction, obtain the peripheral reference signals of each peripheral core inside the optical fiber detection section, and when the optical fiber detection section is in a fluid, obtain the peripheral measurement signals of each peripheral core inside the optical fiber detection section. Step 200: Demodulate the peripheral reference signal and the peripheral measurement signal of the same peripheral core to obtain the wavelength offset of each peripheral core at each axial position. Step 300: Calculate the curvature distribution of the optical fiber detection section at each axial position according to the wavelength offsets of each peripheral core at the same axial position. Step 400: Calculate the bending angle of the optical fiber detection section according to the curvature of the optical fiber detection section at each axial position. Step 500: Determine the flow direction of the fluid according to the initial direction and the bending angle of the optical fiber detection section.

6. The flow field measurement method according to claim 5, characterized in that In step 200, when demodulating the peripheral reference signal and the peripheral measurement signal of the same peripheral core to obtain the wavelength offset of each peripheral core at each axial position, it includes the following steps: Step 210: Use the fast Fourier transform to convert the peripheral reference signal and the peripheral measurement signal of each peripheral core into the distance domain. Step 220: In the distance domain, use a first moving window and a first moving distance to locally intercept the peripheral reference signal and the peripheral measurement signal of each peripheral core respectively in a sliding window manner to obtain the local peripheral reference signal and the local peripheral measurement signal of each peripheral core at each axial position. Step 230: Use the inverse fast Fourier transform to convert the local peripheral reference signal and the local peripheral measurement signal of each peripheral core into the wavelength domain. Step 240: In the wavelength domain, perform a demodulation operation on the local peripheral reference signal and the local peripheral measurement signal of the same peripheral optical fiber at the same axial position to obtain the wavelength offset of each peripheral core at each axial position.

7. The flow field measurement method according to claim 6, characterized in that Each of the peripheral cores in the fiber detection section is uniformly distributed with multiple sections of peripheral gratings along the fiber axis, and the peripheral gratings of each peripheral core are aligned with each other; the first moving window has the same length as the grating length of the peripheral grating, and the first moving distance is the same as the interval distance of the peripheral grating.

8. The flow field measurement method according to claim 5, characterized in that In step 300, when calculating the curvature distribution of the fiber detection section at each axial position according to the wavelength offsets of each peripheral core at the same axial position, the following steps are included: Step 310: Compare the wavelength offsets of each peripheral 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 offset and the minimum offset at the same axial position from the intermediate offset respectively to obtain the maximum bending offset and the minimum bending offset at each axial position; Step 330: Determine the maximum bending strain and the minimum bending strain at each axial position according to the relationship curve between the wavelength offset and the strain; Step 340: Calculate the curvature of the fiber detection section at each axial position according to the maximum bending strain and the minimum bending strain at the same axial position.

9. The flow field measurement method according to claim 5, characterized in that, The multi-core fiber further 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 fiber; the flow field measurement method further includes the following steps: Step 100: When the fiber detection section is not in the fluid, obtain the central reference signal of the central core in the fiber detection section, and when the fiber detection section is in the fluid, obtain the central measurement signal of the central core in the fiber detection section; Step 200: Demodulate the central reference signal and the central measurement signal of the central core to obtain the wavelength offset of the central core at each axial position; Step 300: Calculate the strain force distribution of the fiber detection section at each axial position according to the wavelength offset of the central core at the same axial position; Step 400: Determine the flow velocity magnitude of the fluid distributed at each axial position of the fiber detection section according to the relationship curve between the strain force and the flow velocity.

10. The flow field measurement method according to claim 9, wherein In step 200, when demodulating the central reference signal and the central measurement signal of the central core to obtain the wavelength offset of the central core at each axial position, the following steps are included: Step 210: Use the fast Fourier transform to convert the central reference signal and the central measurement signal of the central core into the distance domain; Step 220: In the distance domain, use a second moving window and a second moving distance to locally intercept the central reference signal and the central measurement signal of the central core respectively in a sliding window manner to obtain the local central reference signal and the local central measurement signal of the central core at each axial position; Step 230: Use the inverse fast Fourier transform to convert the local central reference signal and the local central measurement signal of the central core into the wavelength domain; Step 240: In the wavelength domain, perform demodulation operations on the local central reference signal and the local central measurement signal of the central optical fiber at the same axial position to obtain the wavelength offset of the central core at each axial position.

11. The flow field measurement method according to claim 10, characterized in that Multiple segments of central gratings are evenly distributed along the axial direction of the central core within the fiber detection section; the length of the second moving window is the same as that of the central grating, and the second moving distance is the same as the pitch of the central grating.

12. The flow field measurement method according to claim 11, wherein, Multiple segments of peripheral gratings are evenly distributed along the axial direction of each peripheral core within the fiber detection section, and the central gratings of each segment of the central core are aligned with the central gratings of each segment of each peripheral core; in step 300, when calculating the curvature distribution of the fiber detection section at each axial position according to the wavelength offsets of each peripheral core at the same axial position, the following steps are included: Step 310: Subtract the wavelength offset of each peripheral core from the wavelength offset of the central core at the same axial position to obtain the bending offset of each peripheral core at each axial position; Step 320: Determine the bending strain of each peripheral core at the same axial position according to the relationship curve between the wavelength offset and the strain; Step 330: Calculate the curvature of the fiber detection section at each axial position based on the bending strains at the same axial position.

13. A flow field measurement device, characterized in that It includes the linear optical fiber flow field probe described in claim 1, an optical frequency domain reflectometer, a signal collector, and a host computer. The optical fiber transmission section of the linear optical fiber flow field probe is connected to the optical frequency domain reflectometer, the optical frequency domain reflectometer is connected to the signal collector, and the signal collector is connected to the host computer.

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