CN115980386A - Seawater flow velocity measuring method based on panda optical fiber - Google Patents
Seawater flow velocity measuring method based on panda optical fiber Download PDFInfo
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Abstract
The invention discloses a seawater flow velocity measuring method based on panda optical fibers, which is characterized in that seawater flow velocity measurement is carried out through an optical fiber flow velocity sensor based on panda optical fibers and a target-type constant-strength cantilever beam, the optical fiber flow velocity sensor comprises the target-type constant-strength cantilever beam, a force transfer rod and a stressed target, wherein the first end of the cantilever beam is fixed, the second end of the cantilever beam is fixedly connected with the stressed target through the force transfer rod, and the stressed target is used for being placed in fluid to obtain the impact force of the fluid; the panda optical fiber with the end surface plated with the gold film is used for measuring the bending stress of the cantilever beam caused by water flow impact; the calculation module is used for acquiring the flow velocity of the fluid by acquiring the bending stress based on the relation between the strain of the cantilever beam and the flow velocity of the fluid; the optical fiber flow velocity sensor provided by the invention is expected to play an important role in the field of ocean current measurement due to the advantages of small volume, stable structure, high sensitivity and the like.
Description
Technical Field
The invention relates to the technical field of ocean current flow velocity measurement, belongs to an ocean current flow velocity sensing measurement technology based on panda optical fiber reflection type interference and a target type equal-strength cantilever beam, and particularly relates to a panda optical fiber-based seawater flow velocity measurement method.
Background
Ocean currents are one of the common forms of seawater motion, and direct and accurate sea surface flow field measurement is required for global climate change monitoring, oceanographic research, atmospheric ocean interaction and physical and biochemical interaction process research. The method has important significance for long-term monitoring of ocean surface flow fields, mastering and predicting of ocean current rules, fishery, shipping, pollution discharge, military and the like. The sea surface flow velocity is generally below 3m/s, and the average flow velocity in most regions is within 1m/s, so that the design of the current meter mainly aims at low-flow-velocity current measurement. Most of the current sensors for measuring the flow velocity of seawater mainly comprise an acoustic Doppler flow velocity meter, but the sensors are limited by the problems of electromagnetic interference, complex equipment, high cost and the like, and are exposed to the risk of electric leakage and water leakage. In recent years, optical measurement means represented by an optical fiber sensor provides a new idea for fluid flow rate measurement.
The application of the optical fiber sensor in the field of flow velocity measurement has been reported more, and the optical fiber sensor can be divided into a hot wire type and a strain type according to the sensing principle. The hot-wire optical fiber flow velocity sensor based on the heat balance principle realizes flow velocity measurement by means of the cooling effect of fluid. At present, many optical fiber flow velocity sensors based on a heat balance principle are reported in sequence, but a hot-wire optical fiber flow velocity sensor is limited by a mechanism problem and is mainly used for micro-fluidic or wind speed measurement.
The strain type optical fiber flow velocity sensor realizes the measurement of flow velocity by monitoring the acting force of fluid on an optical fiber sensing unit, wherein the strain type flow velocity sensor based on the coupling of a target type cantilever beam and the optical fiber sensor is a mature sensor in the type of the sensor. Compared with a hot wire type optical fiber flow velocity sensor, the strain type optical fiber flow velocity sensor has the advantage of great potential in the field of ocean current measurement without being limited by a measurement object.
In summary, a great deal of research on optical fiber flow velocity sensors is mainly oriented to wind tunnel testing, pipeline flow or microfluidic fields, and there is no research report on optical fiber sensors for measuring sea current flow velocity, and most of the reported optical fiber flow velocity sensors, especially the sensing units of the target type cantilever beam flow velocity sensors, are of optical fiber grating structures, and the measurement precision of the optical fiber flow velocity sensors cannot meet the requirements of ocean scientific research. Therefore, in order to meet the requirements of marine environment measurement and ocean current low-flow-rate and high-precision measurement, it is urgently needed to develop an ocean current flow rate sensing measurement method for ocean current flow rate measurement research.
Disclosure of Invention
In order to solve the problems, the invention aims to provide a method for sensing and measuring the flow rate of ocean current based on a reflective panda optical fiber and a target-type constant-strength cantilever beam.
In order to achieve the above technical object, the present application provides a method for measuring seawater flow velocity based on panda fiber, which measures seawater flow velocity through a fiber flow velocity sensor based on panda fiber and target-type constant-strength cantilever beam, the fiber flow velocity sensor comprising:
the target type cantilever beam with equal strength consists of a cantilever beam, a force transmission rod and a stressed target, wherein the first end of the cantilever beam is fixed, the second end of the cantilever beam is fixedly connected with the stressed target through the force transmission rod, and the stressed target is used for being placed in fluid to obtain the impact force of the fluid;
the panda optical fiber is used for measuring the bending stress of the cantilever beam caused by water flow impact;
and the calculation module is used for acquiring the flow velocity of the fluid by acquiring the bending stress based on the relation between the strain of the cantilever beam and the flow velocity of the fluid.
Preferably, the cantilever beam is a triangular constant strength cantilever beam.
Preferably, the cantilever beam has an aspect ratio of 22:5, the thickness of the cantilever beam is adjustable within 0.3-0.5 mm.
Preferably, the stressed target is a circular stressed target and is used for conducting fluid impact force to the triangular constant-strength cantilever beam through the force transmission rod to cause the triangular constant-strength cantilever beam to generate bending deformation.
Preferably, the radius of the circular force-bearing target is adjustable within the range of 2.5-3.5 cm.
Preferably, the panda fiber consists of a broadband light source, a circulator, a fiber polarizer and a panda polarization maintaining fiber which are sequentially connected through a single-mode fiber, wherein a direct-current magnetron sputtering technology is used for plating a gold film on the tip end face of the panda polarization maintaining fiber to serve as a reflector, the reflector is used for reflecting two beams of polarized light with different effective refractive indexes and generated by the double refraction effect of the panda polarization maintaining fiber to the fiber polarizer for interference, and the two beams of polarized light are received by the spectrometer after passing through the circulator;
the panda polarization maintaining fiber with gold-plated end surface is used as the fiber sensing probe and is welded to the central axis of the cantilever beam.
Preferably, the length of the optical fiber sensing probe is adjustable within 2.2-4.7 cm.
Preferably, the calculation module is further configured to obtain a relationship between the strain of the cantilever beam and the flow rate of the fluid, where the relationship is expressed as:
wherein FL represents the bending moment of the target rod, E represents the elastic modulus of the cantilever beam, b represents the length of the cross section of the cantilever beam, and h represents the width of the cross section of the cantilever beam.
Preferably, the fiber optic flow rate sensor is further configured to adjust the size of the target-type constant-intensity cantilever beam and the length of the fiber optic sensing probe of the panda fiber optic fiber according to the flow rate sensitivity, wherein the flow rate sensitivity is expressed as:
wherein, P e Expressed as a constant of strain-induced birefringence change, λ represents the probe wavelength, V represents the flow velocity, a represents the stressed target area, L/b is the cantilever beam aspect ratio, ξ is the local drag coefficient, and ρ is the fluid density.
Preferably, the constant for the strain-induced birefringence change is expressed as:
wherein, P 11 、P 12 、P 21 、P 22 Is the optical fiber stress optical tensor, upsilon is the Poisson ratio of the optical fiber, n s ,n f Respectively expressed as the effective refractive index of the fast and slow axes of the panda fiber.
The invention discloses the following technical effects:
compared with the prior art, the peculiar birefringence effect of the panda optical fiber enables the panda optical fiber to be more sensitive to strain than a common Bragg grating, so that the flow velocity sensitivity of the sensor can be effectively improved, and the selection of the constant-strength cantilever beam avoids the problem of limited measurement precision caused by uneven stress of the optical fiber sensing unit;
the optical fiber flow velocity sensor provided by the invention is expected to play an important role in the field of ocean current measurement due to the advantages of small volume, stable structure, high sensitivity and the like.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.
FIG. 1 is a schematic diagram of a target cantilever structure according to an embodiment of the present invention;
fig. 2 is a schematic view of the distribution of surface stress of the constant strength cantilever structure according to the embodiment of the present invention, wherein (a) represents the aspect ratio of 22:5, the surface stress distribution of the uniform strength cantilever beam structure with the thickness of 0.3mm and the radius of the circular stressed target of 2.5cm, and (b) the stress of each point on the central axis of the uniform strength cantilever beam;
FIG. 3 is a schematic diagram of a sensing principle of a reflective panda fiber Sagnac interferometer according to an embodiment of the invention;
FIG. 4 is a schematic view of a flow rate measurement system according to an embodiment of the present invention;
fig. 5 is a schematic diagram of the experimental effect of flow velocity sensing according to an embodiment of the present invention, in which (a) shows that the cantilever has an aspect ratio of 22, a thickness of 0.3mm, a radius of a circular stressed target of 2.5cm, and a reflection spectrum of a sensor at different flow velocities, (b) shows a 1491nm DIP flow velocity wavelength fitting curve and experimental and theoretical sensitivity curves, (c) shows a sensor sensitivity curve with flow velocity at different wavelengths, and (d) shows a change relationship between the experimental and theoretical sensitivities with the wavelength when the flow velocity is 0.09 m/s;
FIG. 6 is a schematic diagram showing the relationship between the thickness of the cantilever and the sensitivity of the sensor according to the embodiment of the present invention, wherein (a) the experimental sensitivity of the sensor varies with the flow rate when the thicknesses of the cantilever are 0.3mm,0.4mm and 0.5mm, respectively, and the inset is the interference spectrum of the sensor with the thickness of 0.3mm at different flow rates, (b) the variation relationship of the sensitivity of the sensor with the thickness at wavelengths of 1485nm,1525nm and 1612nm is shown, the solid line is the theoretical result, and the dots are the experimental result;
FIG. 7 is a schematic diagram showing the relationship between the area of the target and the sensitivity of the sensor according to the embodiment of the present invention, wherein (a) is a curve showing the variation of the sensitivity of the sensor with the flow rate when the radius of the target is 2.5cm, 3.0cm, and 3.5cm, respectively, (b) is a curve showing the variation of the sensitivity of the sensor with the thickness at wavelengths of 1400nm, 1470nm, and 1565nm, and the solid line is a theoretical result and the point is an experimental result;
fig. 8 is a schematic diagram of a relationship between the length of the panda optical fiber and the sensitivity of the sensor according to the embodiment of the present invention, where (a) shows a change curve of the sensitivity of the sensor with the flow rate when the thickness of the cantilever beam is 0.4mm, the radius of the stressed target is 2.5cm, and the lengths of the optical fibers are 2.2cm,3.7cm, 4.0cm, and 4.4cm, respectively, and (b) shows a change trend of the sensitivity of the sensor with the length of the panda optical fiber when the flow rate is 0.1 m/s;
FIG. 9 is a graphical representation of the accuracy of the sensitivity of a sensor according to an embodiment of the present invention, wherein (a) represents the experimental sensitivity of the sensor compared to a theoretical sensitivity value and (b) represents the absolute error of the experimental sensitivity;
FIG. 10 is a graph showing the flow rate measurements of an embodiment of the present invention, wherein (a) shows the fiber optic flow rate sensor compared to the ADV for water flow measurements in a tank, and the inset shows the interference spectra of the sensor at different flow rates, and (b) shows the flow rate measurement error of the fiber optic flow rate sensor.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all the embodiments. The components of the embodiments of the present application, generally described and illustrated in the figures herein, can be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present application, presented in the accompanying drawings, is not intended to limit the scope of the claimed application, but is merely representative of selected embodiments of the application. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present application without making any creative effort, shall fall within the protection scope of the present application.
As shown in fig. 1-10, the invention provides a method for measuring seawater flow rate based on panda fiber, which measures seawater flow rate through a fiber flow rate sensor based on panda fiber and a target-type constant-strength cantilever beam. The structural design and theoretical analysis of the optical fiber flow velocity sensor are as follows:
1. the structural design of the target cantilever beam:
the sensing structure for measuring the flow rate of water flow based on the coupling of the target cantilever beam structure and the optical fiber sensor is shown in figure 1. The upper end of the cantilever beam is fixed, the other end of the cantilever beam is connected with the round stressed target through the cylindrical dowel bar, the stressed target is immersed in fluid, water flow at a certain speed impacts the stressed target to cause the cantilever beam to generate bending deformation, the optical fiber sensing unit is adhered to the central axis of the cantilever beam and used for measuring the bending stress of the cantilever beam generated by water flow impact, and the measurement of the flow velocity of the fluid is realized according to the relation between the strain of the cantilever beam and the flow velocity. In order to avoid the reduction of the measurement precision caused by uneven stress distribution, the cantilever beam structure selects a triangular uniform-strength cantilever beam, namely the stress of each point on the axis of the isosceles triangle is equal in magnitude.
The impact force caused by the fluid impacting the stressed target is called dynamic pressure, the difference between the static pressure before the target and the static pressure after the target, which is generated by the separation of the fluid flow beam after the target, is called static pressure difference, and when the fluid passes through the target piece, friction force is generated at the boundary of the target piece. Defining the pressure and flow rate before the target as P 0 ，V 0 When water flows through the target, the pressure and the speed are respectively P and V, the density of the fluid is rho, and the local resistance coefficient is expressed as xi. According to the Bernoulli equation, in the flow of the fluid neglecting viscosity loss, the sum of pressure potential energy, kinetic energy and potential energy of any two points on a streamline is kept unchanged:
multiplying two sides of the formula (1) by the incident flow cross-sectional area A of the stressed target sheet at the same time:
the first term of the above equation is static pressure and the second term is dynamic pressure, and friction is ignored because the friction contact surface is small. The relationship between the fluid flow velocity V and the acting force F of the fluid on the stressed target is simplified as follows:
the constant-strength cantilever beam is subjected to bending deformation under the impact of fluid, and the bending normal stress strength condition is as follows:
wherein σ max The maximum positive stress generated by the bending of the cantilever beam is shown, E and epsilon are respectively the elastic modulus and the strain of the cantilever beam, M = FL is the bending moment of the cantilever beam, F is the force of fluid with certain speed impacting the stressed target,and F and L in the same formula (3) are the height of the triangular cantilever beam. W Z The bending section coefficient of the target rod, y represents the distance from a point on the cross section to the central axis, I Z The bending resistance section coefficient W is the moment of inertia of the cross section to the central axis and is used for the structure with the rectangular cross section Z ＝(bh 2 ) And 6, b is the length of the rectangular cross section, namely the length of the bottom side of the isosceles triangle, and h is the width of the rectangular cross section, namely the thickness of the triangular cantilever beam. The isointensity cantilever strain versus fluid force relationship is expressed as:
the strain of each point on the central axis of the triangular constant-strength cantilever beam is the same, the formula (5) represents the relation between the strain of the central axis of the cantilever beam and the structural parameters, and the stress of the structure under different stress conditions can be determined as long as the triangular structural parameters are determined.
However, not all structures meeting the isosceles triangle condition are equal-strength cantilever beams, and in order to determine the parameter range of the equal-strength cantilever beam structure, an optimal solution is selected, and the stress distribution of the equal-strength cantilever beams is solved by using a finite element algorithm. The method is characterized in that the selected material is polyvinyl chloride (PVC), the radius of a circular stress target is set to be 2.5cm, the stress distribution and the size of the central axis of the equal-strength cantilever beam under the conditions of different thicknesses and different height-width ratios are calculated, and the height-width ratio of the cantilever beam is finally determined to be 22 by combining the measurement environment of a laboratory water tank: 5. FIG. 2 (a) shows an aspect ratio of 22:5, the rectangular area at the upper end of the structure is a fixed end designed for facilitating the fixation of the structure in an experiment, and simulation results show that the existence of the fixed end has no influence on the stress distribution and the size of the structure. The impact force of the circular stressed target is set to be 50N, the stress at different positions on the axis of the cantilever beam structure is obtained as shown in a graph (b), the connecting point of the fixed end and the triangular cantilever beam is set to be a position 0, and therefore, the position has stress mutation, and stress abnormality occurs near the position 20 cm. In view of the factIn application, due to the influence of the fixed end and the force transmission point, the optical fiber sensing unit is required to be adhered to the central axis of the cantilever beam within the range of 5-15cm in the preparation of the sensor, and the stress on each point of the panda optical fiber is ensured to be equal as much as possible. The stress of the central axis of the constant-strength cantilever beam obtained by finite element algorithm solution is about 1.45 multiplied by 10 10 N/m 2 The theoretical stress of the cantilever beam structure calculated by the formula (5) is 1.46 multiplied by 10 10 N/m 2 The simulation result is consistent with the theoretical calculation result.
2. Sensing principle of reflective panda optical fiber Sagnac interferometer
In order to improve the stability of the sensor and facilitate experimental operation, the research uses a direct-current magnetron sputtering technology to plate a gold film on the end face of the tip of the panda fiber as a reflector, and the transmission spectrum of the reflector is equivalent to that of the traditional Sagnac ring. Fig. 3 is a schematic diagram of a sensing principle of a reflective panda fiber Sagnac interferometer, and a system transmission process is as follows: signal light emitted by a broadband light source (NKT Photonics Super KCompact) is transmitted to an optical fiber polarizer through a circulator, and all devices are connected through a Single Mode Fiber (SMF). The optical fiber polarizer converts signal light into linearly polarized light and injects the linearly polarized light into a panda Polarization Maintaining Fiber (PMF), and two beams of linearly polarized light with different effective refractive indexes are generated on the fast and slow axes of the optical fiber due to the double refraction effect of the panda optical fiber. The two orthogonal linear polarized lights are reflected by the gold film on the end face of the optical fiber, interfere after passing through the polarizer again, and the reflected lights are received by a spectrometer (Ando AQ 6370C) after passing through the circulator. Stress birefringence in the panda type polarization maintaining optical fiber is caused by photoelastic effect, in the process of drawing the polarization maintaining optical fiber, due to different linear thermal expansion coefficients of all parts in the optical fiber, the optical fiber generates anisotropic stress by cooling, the photoelastic effect enables the refractive index of the material to present anisotropy, high birefringence is generated, and the unique birefringence effect of the panda type polarization maintaining optical fiber enables the panda type polarization maintaining optical fiber to present excellent characteristics in the field of stress sensing.
Two orthogonal components transmitted on the fast and slow axes of the panda optical fiber are reflected by the gold film, and interfere after passing through the polarizer again, wherein the interference spectrum approximates to the periodic function of the wavelength:
the panda fiber birefringence effect produces phase difference:
wherein, B is the birefringence of the panda fiber, L' is the length of the panda fiber, and lambda is the wavelength of the input light. Panda fiber sensing unit pastes on the cantilever beam axis, and the fluid strikes the atress target and leads to the cantilever beam to take place to buckle, and optic fibre receives the bending stress effect, and the photoelastic effect arouses that the optic fibre refracting index changes, thereby the stress causes optic fibre tensile or compression to change optic fibre length, finally leads to the phase difference to change:
d represents the diameter of the panda optical fiber core. The first term represents the phase delay caused by the refractive index change of the fiber core due to the photoelastic effect, the second term is the phase delay caused by the length change of the optical fiber, and the third term represents the phase delay caused by the diameter change of the fiber core due to the waveguide effect, but the phase delay caused by the diameter change has little effect and can be generally ignored. For the integral of the above equation, the phase difference change is expressed as:
wherein, Δ B represents the change of birefringence of the panda fiber due to photoelastic effect, and Δ L' represents the change of length of the panda fiber due to stretching or compressing of the panda fiber due to bending deformation of the cantilever beam. The free spectrum is defined as:
according to the wavelength shift relation caused by phase change
Simultaneous (9) (10) and simplified to obtain the spectral wavelength shift and strain relation:
Δλ＝λ(1+P e )ε (11)
wherein the content of the first and second substances,
constant for the change in strain-induced birefringence, depending on the fiber material, P 11 、P 12 、P 21 、P 22 Is the fiber stress optical tensor, and upsilon is the poisson ratio of the fiber. As can be seen from the equation (11), the wavelength shift and the strain are linear, and the incident wavelength is defined as λ when no external strain is introduced 0 The wavelength λ at the position of destructive interference due to stress effects can then be expressed as:
λ＝λ 0 +λ(1+P e )ε (12)
the optical fiber sensing probe is used for measuring bending strain of the cantilever beam, when the cantilever beam is bent and deformed due to fluid impact, the reflection spectrum interference wavelength shifts, and the flow velocity of the fluid can be obtained by monitoring the shift of the interference wavelength. Simultaneous equations (3), (5) and (12) yield the wavelength at the position of destructive interference as a function of flow velocity:
the flow rate sensitivity of the sensor is derived from the above equation:
according to the theoretically derived sensor sensitivity formula (14), the flow velocity sensitivity of the sensor is influenced by the detection wavelength lambda, the flow velocity V and the structural parameters of the cantilever beam with equal strength, such as the area A of the force target, the thickness h of the triangle and the height-to-width ratio L/b. The cantilever beam structure aspect ratio has been determined by finite element analysis to be the optimal solution 22: other influencing factors will be discussed experimentally in relation to the sensor flow rate sensitivity.
3. Discussion of flow rate sensing experiments and results:
a flow velocity measuring system as shown in figure 4 is set up, the circulating water tank simulates the flow velocity change of seawater, the length, the width and the height of the glass water tank are respectively 130cm multiplied by 22cm multiplied by 30cm, a controller at the upper right end controls the flow velocity of the water tank, and the system can provide the flow velocity in the range of 0-0.8 m/s. An acoustic Doppler flowmeter (ADV) is used for monitoring the flow rate of the water tank and calibrating a sensor. The H-shaped support is designed as shown in an inset and used for fixing the cantilever beam and adjusting the position of the sensor, the rectangular fixed end of the cantilever beam is adhered to the short arm in the middle of the H-shaped support, the short arm can move up and down and be fixed so as to ensure that the circular stressed target is just completely immersed in water in the experimental process, and the two arms extending outwards from the two ends of the H-shaped support are lapped on the water tank for fixing the sensor, so that the measuring position of the sensor cannot be influenced by water flow impact at a high flow rate. And the lower right inset is a real object diagram of the optical fiber flow velocity sensor, according to a cantilever beam stress simulation result diagram in fig. 2 (b), due to the influence of the fixed end and the dowel bar, the optical fiber sensing unit is adhered to the position of 5-15cm of the central axis of the cantilever beam, and the polarization maintaining optical fiber probe and the single mode fiber are adhered to the PVC cantilever beam by 502 glue.
The H-shaped bracket for fixing the sensor is built on the water tank, so that the sensor is fixed in the middle of the water tank, the middle short arm of the H-shaped bracket is adjusted, and the circular stressed target is ensured to be just completely immersed in water in order to avoid unstable flow rate of the left-end water inlet and the right-end water outlet of the circulating water tank from influencing the measurement result. ADV (NortekAs, vector) measurement accuracy is +/-0.5% +/-1 mm/s of measurement value, the measurement range is 0-4m/s, and the device is fixed near the sensor so as to monitor the flow velocity around the sensor in real time. The flow rate of the water tank is increased through the control end, the ADV records the current flow rate, and after the flow is stable, the spectrometer records the reflection spectrum at different flow rates. As the flow rate of the water bath increases, the amount of cantilever deformation increases and the optical fiber experiences a bending stress with a spectral red shift, as shown in fig. 5 (a). To characterize the flow rate response of the sensor, DIP data at approximately 1491nm were fit and sensitivity was calculated, with a fit as high as 0.98 as shown in fig. 5 (b). The relation between the sensitivity of the sensor and the flow rate is obtained by derivation of the fitting curve, and the relation is like a blue solid line, and experimental results show that the sensitivity of the flow rate of the sensor is increased along with the increase of the flow rate and is consistent with theoretical derivation results. The aspect ratio of the cantilever beam structure used in the experiment is 22: and 5, the thickness is 0.3mm, the radius of the circular stress target is 2.5cm, the length of the panda optical fiber is 4.6cm, the detection wavelength is 1491nm, the detection wavelength is substituted into a formula (14), the change relation of the theoretical sensitivity of the structure along with the flow rate is obtained by solving and is shown as a black dotted line in the figure, the experimental result (blue solid line) and the theoretical result (black dotted line) are well matched, and the relative error is 3.4%.
Theoretical results (14) indicate that the sensor flow rate sensitivity is related to the detection wavelength, and that the sensitivity increases with increasing detection wavelength. In the experiment, a super-continuous laser is used as a broadband light source, a spectrometer records an interference spectrum with the wavelength within the range of 1200-1650nm, all DIP data in the figure 5 (a) are fitted and the sensitivity is solved, and the obtained sensor sensitivity change relation along with the flow speed under different wavelengths is shown in the figure 5 (c). The sensitivity of the sensor increases with the increase of the flow rate and the detection wavelength, and when the flow rate is 0.09m/s, the sensitivity of the flow rate is 355.55 nm/(m.s) when the wavelength is 1626nm at the maximum -1 ) A minimum wavelength of 270.54 nm/(m · s) at 1249nm -1 ). A curve showing the change of theoretical sensitivity with wavelength obtained from a flow rate of 0.09m/s is shown in a black solid line in FIG. 5 (d), blue data points indicate the sensitivity of different DIPs obtained from an experimental result in FIG. 5 (c) at a flow rate of 0.09m/s, the experimental result fluctuates up and down around the theoretical curve with an average relative error of 1.68% and an average absolute error of 5.23 nm/(m.s) -1 ) And the experimental sensitivity is better matched with the theoretical sensitivity.
4. Cantilever thickness discussion:
the flow velocity sensitivity of the sensor is related to the thickness in theory derivation, so that cantilever beams with different thicknesses are designed to carry out flow velocity sensing experiments, and the relationship between the sensitivity of the sensor and the thickness is discussed. The cantilever aspect ratio was designed to be 22: and 5, the radius of the round stressed target is 2.5cm, the thicknesses h are respectively 0.3mm,0.4mm and 0.5mm, three groups of target type cantilever beam structures are welded with three groups of panda optical fibers, the lengths of the three groups of panda optical fibers are 4.6cm, and the three groups of target type cantilever beam structures are respectively coupled with the three groups of cantilever beam structures to carry out flow rate sensing experiments. The curve of the sensitivity of cantilever beam sensors with different thicknesses, which is obtained through experiments, along with the change of the flow velocity is shown in fig. 6 (a), three groups of experiments are used for carrying out flow velocity sensing experiments according to the fact that only one variable of the thickness of the cantilever beam is used, three groups of experiments are respectively used for discussing DIP (double in-line detector) with the detection wavelength being close to 1485nm, the experiment results follow the change rule that the sensitivity of the sensors increases along with the increase of the flow velocity and decreases along with the increase of the thickness of the cantilever beam, and the theoretical derivation is consistent. It is noted that the cantilever thickness affects the flow rate measurement range of the sensor, and as the flow rate increases, the cantilever reaches the maximum deformation, and the spectrum no longer shifts with the increase of the flow rate, as shown in the inset of fig. 6 (a). For a structure with a thickness of 0.3mm, when the flow velocity is greater than 0.097m/s, the spectrum does not regularly move with the increase of the flow velocity any more, and the slight shift of the spectrum at high flow velocity is due to cantilever beam jitter caused by turbulence on the surface of water. With the increase of the thickness of the cantilever beam, the measurement range of the sensor is increased, and the maximum measurement range of the sensor with the thickness of 0.5mm in the experimental design is 0-0.218m/s, so that the measurement range of the sensor can be expanded by increasing the thickness of the cantilever beam to realize the measurement of the ocean current flow velocity in a wider range. Fig. 6 (b) shows a graph obtained by calculation according to the formula (14) to see whether the change relationship between the sensor sensitivity and the cantilever beam thickness still satisfies the theoretical rule under different DIPs in the three groups of experimental results, the change of the sensor sensitivity with the thickness is as the curve in the graph under the detection wavelengths of 1485nm,1525nm and 1612nm, the experimental data points change around the theoretical curve, and the change trend is consistent with the theoretical calculation. However, the experimental results have errors from the theoretical curves, because three independent experiments cannot ensure that the positions of the DIPs in each group are completely consistent, and only DIPs with similar wavelengths can be selected when discussing the thickness influence, as shown in fig. 6 (a).
5. Discussion of the stressed target area:
the sensitivity of the sensor is related to the area of the stressed target in a theoretical derivation manner, the circular stressed targets with different radiuses are designed for the experiment of flow velocity sensing, and the relation between the sensitivity of the sensor and the area of the stressed target is discussed. Three groups of sensor structures are designed, the radiuses of the stressed targets are respectively 2.5cm, 3.0cm and 3.5cm, other structural parameters are consistent, an experimental result is obtained as shown in a graph 7 (a), the sensitivity of the sensor is increased along with the increase of the flow velocity, the sensitivity is increased along with the increase of the radiuses of the stressed targets at the same flow velocity, and the variation trend is consistent with the theory. In addition, the area of the stressed target influences the flow velocity measurement range of the sensor, and the larger the area of the stressed target is, the larger the deformation amount of the cantilever beam at the same flow velocity is, so that the maximum deformation amount is more easily reached. When the radius of the circular stressed target is 2.5cm, the maximum measuring range of the sensor is 0-0.156m/s, so that for the ocean current measurement in the large flow velocity range, the measuring range of the sensor can be expanded by reducing the area of the stressed target in addition to increasing the thickness of the cantilever beam, but the sensitivity of the sensor is reduced along with the reduction of the area of the stressed target, and the expansion of the measuring range means that the sensitivity is sacrificed. The theoretical sensitivity is calculated according to the formula (14) under different wavelengths of the sensor, the relation of the theoretical sensitivity along with the change of the radius of the stressed target is shown as a curve in fig. 7 (b), data points are the results of the change of the experimental sensitivity along with the area of the stressed target under different wavelengths, and the experimental results change around the theoretical curve, so that the change trend that the sensitivity increases along with the increase of the area of the stressed target is met. The error in the experimental results from the theoretical curve in fig. 7 (b) is due to slight differences in DIP position wavelength for comparison in three sets of experiments.
6. Panda fiber length discussion:
panda optical fibers are used as sensing units of the flow velocity sensor, and whether the optical fiber length affects the sensitivity of the sensor is a concern. The theoretical formula (14) does not contain the length of the optical fiber, so that the sensors with panda optical fibers of different lengths are designed for a flow rate sensing experiment, and the influence of the length of the panda optical fiber on the flow rate sensitivity is verified. Four groups of sensing structures are designed in an experiment, the thickness of the cantilever beam is 0.4mm, the radius of the stressed target is 2.5cm, the lengths of the optical fibers are 2.2cm,3.7cm,4cm and 4.7cm respectively, the sensitivity of the four groups of sensors is obtained according to the change relation of the flow velocity, and the sensitivity curves of the structures with different optical fiber lengths are almost overlapped as shown in figure 8 (a). The sensitivity of the sensor at an extraction flow rate of 0.1m/s is shown in fig. 8 (b), and the experimental result shows that the sensitivity of the sensor is independent of the length of the panda fiber, and the positions of DIP used for comparison cannot be completely the same, so that an error exists. Theories and experiments verify that the flow velocity sensitivity of the sensor is irrelevant to the length of the optical fiber, so that a short panda optical fiber can be selected for fusion welding in the preparation process of the sensor, the sensor can be conveniently coupled with cantilever beam structures with different structural parameters, and according to a formula (10), the shorter the length of the optical fiber is, the larger the FSR is, and the increase of the free spectral range is more beneficial to the demodulation of the sensor and the improvement of the measurement precision of the sensor.
Ten sets of experiments are designed to discuss the relationship between the sensitivity of the sensor and the thickness of the cantilever beam, the radius of the stressed target and the length of the panda optical fiber, in order to verify the accuracy of the sensitivity of the sensor obtained by the experiments, the theoretical sensitivity value of different DIPs in each set of experiments is calculated by using a formula (14), and the experimental result is compared with the theoretical result to obtain a graph shown in fig. 9 (a). Correlation coefficient (R) of data points 2 ) 0.9608, root Mean Square Error (RMSE) 11.6, and FIG. 9 (b) shows that the average absolute error of the experimental results is 8.75 nm/m.s -1 The average relative error is 3.65%, and the sensitivity obtained by experiments is well consistent with a theoretical value.
7. And (3) flow velocity measurement:
the target type cantilever beam optical fiber flow velocity sensor is used for measuring the water flow velocity in the flow velocity measuring system shown in the figure 4, and compared with an ADV measuring result, the thickness of the cantilever beam of the sensor selected in the experiment is 0.4mm, the radius of the stress target is 3cm, and the length of the optical fiber is 2.2cm. And (3) increasing the flow rate of the circulating water tank, performing red shift on the spectrum, and recording an ADV measurement result and the output spectrum of the sensor after the flow rate is stable, wherein the spectrum is shown as an insert 10 (a). The measurement results of DIPA and DIPB are calculated by combining a sensitivity calibration curve of the sensor in a flow velocity sensing experiment, the measurement result of the target type cantilever beam optical fiber flow velocity sensor and the measurement result of ADV are compared with each other, for example, as shown in FIG. 10 (a), the correlation coefficient of the ADV and the flow velocity measurement result of the optical fiber sensor is 0.9162, the root mean square error is 0.0065, and the result shows that the measurement result of the sensor is well matched with the measurement result of the ADV, and the optical fiber flow velocity sensor has high reliability in the field of ocean current measurement. The two measurements have errors, and FIG. 10 (b) shows the absolute error of the optical fiber sensor measurement, and the average absolute error of DIPA is 0.0059 m.s -1 The average relative error was 5.7%, and the average absolute error of DIPB was 0.0049 mS -1 The average relative error was 4.5%. At the same flow rate, since the wavelength of DIPB is greater than that of DIPA, the sensitivity of DIPB is higherAnd larger, the measurement error is smaller. Along with the increase of the flow velocity, the error measured by the sensor also gradually increases, because when the flow velocity of the water tank increases, the deformation quantity of the cantilever beam increases, the stressed target gradually approaches the water surface, the turbulence of the water surface influences the stability of the sensor, and the cantilever beam has slight vibration, so that the deviation of the measurement result is relatively large. Besides, the panda fiber is sensitive to not only strain but also temperature, and the temperature sensitivity of the reflective panda fiber Sagnac interferometer is as high as 1.38 nm/DEG C, so that the temperature cross-sensitive effect caused by the ambient temperature and water flow is also an error source. However, in general, the optical fiber flow velocity sensor provided by the invention has relatively small error in water flow velocity measurement, and can meet the high-precision requirement of ocean current measurement.
8. And (4) conclusion:
the invention provides a novel method for coupling a reflective panda fiber sagnac interferometer and a target-type equal-strength cantilever beam for measuring the flow velocity of ocean current. The water flow impacts a stressed target immersed in water, a cantilever beam connected with the stressed target generates bending deformation, an optical fiber sensing unit pasted on the central axis of the cantilever beam is subjected to bending stress to cause spectrum deviation, and the measurement of the flow velocity of the water flow is realized by monitoring the deviation of an output spectrum. The invention discusses the influence factors of the flow velocity sensitivity of the sensor from two aspects of theory and experiment, the result shows that under the condition that the height-width ratio and the elastic modulus of the cantilever beam are determined, the sensitivity of the sensor is increased along with the reduction of the thickness of the cantilever beam, the increase of the area of a stressed target, the increase of the detection wavelength and the increase of the flow velocity, and is irrelevant to the length of the panda optical fiber, and the experiment shows that when the flow velocity is 0.09m/s, the maximum flow velocity sensitivity of the sensor is 355.55 nm/(m.s) -1 ). Secondly, the measuring range of the sensor is increased along with the increase of the thickness of the cantilever beam and the reduction of the area of the stressed target, and experiments show that when the thickness of the cantilever beam is 0.5mm, the water flow rate measurement in the range of 0-0.218m/s can be realized. In order to meet the requirement of measuring the flow velocity of ocean current, the high flow velocity measurement can be realized by increasing the thickness of the cantilever beam and reducing the area of the stressed target, and the range of the sensor can be enlarged by changing the material of the cantilever beam according to a theoretical derivation result. Evaluating experimental results, theoretical sensitivity and experiments based on theoretical derivation formulaSensitivity has consistency, correlation coefficient (R) 2 ) 0.9608, and an average relative error of 3.65%. Finally, the acoustic Doppler current meter is used for calibration, the high-precision measurement of the water tank flow speed is realized, and the average absolute error between the provided optical fiber flow speed sensor and the ADV measurement result is 0.0049 m.s -1 And the average relative error is 4.5 percent, and the high-precision requirement of ocean current measurement can be met. Compared with published research results, the panda optical fiber used by the structure provided by the invention is correspondingly changed and sensitized due to the birefringence efficiency, so that the flow velocity sensitivity of the sensor is effectively improved, the optical fiber flow velocity sensor has a great advantage in the ocean current measurement field due to the coupling with the equal-strength cantilever beam, and the optical fiber flow velocity sensor based on the target type cantilever beam is expected to play an important role in the ocean current measurement field due to the advantages of small volume, stable structure, high sensitivity and the like.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (10)
1. A seawater flow velocity measurement method based on panda optical fibers is characterized in that seawater flow velocity measurement is carried out through an optical fiber flow velocity sensor based on panda optical fibers and a target-type constant-strength cantilever beam;
the optical fiber flow velocity sensor comprises
The target type cantilever beam with equal strength consists of a cantilever beam, a dowel bar and a stressed target, wherein the first end of the cantilever beam is fixed, the second end of the cantilever beam is fixedly connected with the stressed target through the dowel bar, and the stressed target is used for being placed in fluid to obtain the impact force of the fluid;
the panda optical fiber with the end surface plated with a gold film is used for measuring the bending stress of the cantilever beam caused by water flow impact;
and the calculation module is used for acquiring the flow velocity of the fluid by acquiring the bending stress based on the relation between the strain of the cantilever beam and the flow velocity of the fluid.
2. The panda fiber-based seawater flow velocity measurement method according to claim 1, wherein:
the cantilever beam is a triangular constant-strength cantilever beam.
3. The panda fiber-based seawater flow velocity measurement method according to claim 2, wherein:
the aspect ratio of the cantilever beam is 22:5, the thickness of the cantilever beam is 0.3-0.5mm and is adjustable.
4. The panda optical fiber-based seawater flow velocity measurement method according to claim 3, wherein:
the stress target is a circular stress target and is used for conducting the fluid impact force to the triangular constant-strength cantilever beams through the dowel bar so as to cause the triangular constant-strength cantilever beams to generate bending deformation.
5. The panda fiber-based seawater flow velocity measurement method according to claim 4, wherein:
the radius of the round stressed target is 2.5-3.5cm and is adjustable.
6. The panda fiber-based seawater flow velocity measurement method according to claim 5, wherein:
the panda optical fiber with the end surface plated with the gold film consists of a broadband light source, a circulator, an optical fiber polarizer and a panda polarization maintaining optical fiber which are sequentially connected through a single mode optical fiber, wherein the panda polarization maintaining optical fiber is plated with the gold film on the tip end surface of the panda polarization maintaining optical fiber by using a direct current magnetron sputtering technology as a reflector, and the reflector is used for reflecting two beams of linearly polarized light with different effective refractive indexes, which are generated by the double refraction effect of the panda polarization maintaining optical fiber, to the optical fiber polarizer for interference and receiving the beams by a spectrometer after passing through the circulator;
the panda polarization maintaining fiber with the end surface plated with the gold film is used as an optical fiber sensing probe of the panda optical fiber and is welded on the central axis of the cantilever beam.
7. The panda optical fiber-based seawater flow velocity measurement method according to claim 6, wherein:
the length of the optical fiber sensing probe is adjustable within the range of 2.2-4.7 cm.
8. The panda fiber-based seawater flow velocity measurement method according to claim 7, wherein:
the calculation module is further configured to obtain a relationship between the strain of the cantilever beam and the flow rate of the fluid, where the relationship is expressed as:
wherein FL represents cantilever beam bending moment, E represents cantilever beam elastic modulus, b represents cantilever beam cross section length, and h represents cantilever beam cross section width.
9. The panda optical fiber-based seawater flow velocity measurement method according to claim 8, wherein:
the optical fiber flow velocity sensor is also used for adjusting the size of the target type constant-strength cantilever beam and the length of an optical fiber sensing probe of the panda optical fiber according to flow velocity sensitivity, wherein the flow velocity sensitivity is expressed as follows:
wherein, P e Expressed as constants for strain induced birefringence changes, λ represents the probe wavelength, V represents the flow rate, a represents the stressed target area, L/b is the cantilever beam aspect ratio, ξ is the local drag coefficient, and ρ is the fluid density.
10. The panda fiber-based seawater flow velocity measurement method according to claim 9, wherein:
the constant for the strain-induced birefringence change is expressed as:
wherein, P 11 、P 12 、P 21 、P 22 Is the optical fiber stress optical tensor, upsilon is the Poisson ratio of the optical fiber, n s And n f Respectively expressed as the effective refractive index of the fast and slow axes of the panda fiber.
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