JPH05312823A - Measuring method of fluid flow velocity distribution - Google Patents

Abstract

(57) [Summary] [Objective] The method of measuring the flow velocity from the frequency of Karman vortex using an optical fiber is extended to the method of measuring the flow velocity distribution of non-uniform flow. [Structure] Laser light from a laser light source 11 is polarized by a polarizer 12, and its linearly polarized wave (Y polarized wave) is divided by a coupler 13, and one optical path is adjusted by an optical path length adjuster 15. Λ / 2 of one optical path sent to two free optical paths
The polarization of this optical path is made into the orthogonal X polarization by the wave plate 17, and the other polarization is shifted in frequency to f + Δf by the ultrasonic frequency shifter 16 and both polarizations are transmitted through the coupler 18 to the polarization maintaining optical fiber. To the sensor. The optical fiber sensor 14 is fixed at a predetermined resolution distance, and is adjusted by the optical path length adjuster 15 so that the optical path difference between the X polarized wave and the Y polarized wave becomes 0 for each fixed section. By causing interference, the output light is received by the photodetector 20 via the analyzer 19, the signal is processed by the signal processing unit 21, and the frequency of the envelope of the amplitude of the beat signal obtained as a result is measured. Get the distribution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluid flow velocity distribution measuring method for measuring a flow velocity distribution having different velocities in the depth direction such as the ocean.

[0002]

2. Description of the Related Art As a method for measuring the flow velocity of a fluid using an optical fiber, for example, an optical fiber Fabry-Perot interferometer type velocity meter is an "optical fiber sensor" (edited by Takanori Ogoshi).
(Ohm Company, 61.7.30). 182-187
Is shown in. In this anemometer, as shown in FIG. 5, a mirror is attached to one end of an optical fiber inserted orthogonally to a pipe in which a fluid to be measured is inserted, and a beam splitter acting as a half mirror is arranged at the other end. It consists of an optical resonator formed between a mirror and a half mirror. 1 is a laser light source, 2 is a beam splitter, 3 and 8 are lenses, 4 is a single mode optical fiber, 5 is a tube, 6 is a mirror, 7 is a fixed point,
Reference numeral 9 is a light receiver, and 10 is a signal processing unit.

This velocity meter utilizes the vibration of the Karman vortex generated behind the optical fiber 4 when the fluid traverses the columnar body of the optical fiber 4 inserted in the conduit, and the optical fiber 4 uses the Karman vortex. Is subjected to lift force due to the vibration generated by, and thus the propagated light undergoes phase modulation due to a change in the refractive index, and the signal processor 10 detects the frequency of the phase change of the light received by the light receiver 9 through the beam splitter 2. Then, the flow velocity is measured by this.

[0004]

However, the above-described conventional velocity meter can measure only one point for the velocity of a pipe, and the velocity is continuous in the depth direction like the ocean current in the ocean, for example. There is a problem that it is not possible to measure the changing flow velocity as a distributed flow velocity.

In the present invention, paying attention to the problems in the above-mentioned conventional flow velocity measuring method, the method of measuring the frequency of the Karman vortex by using the optical fiber and measuring the flow velocity by this is expanded to obtain a non-uniform flow. An object of the present invention is to provide a measuring method capable of measuring the flow velocity distribution of.

[0006]

In order to solve the above-mentioned problems, the present invention measures the flow velocity distribution of a non-uniform flow with a required resolution. Therefore, the light from a light source having a predetermined coherence length corresponding to the resolution distance is measured. Is divided into two and sent to two optical paths each of which has an adjustable optical path length, and one of the light beams of the two optical paths is frequency-shifted with respect to the other, and one of them is the other. Polarization-maintaining optical fiber which is polarized so as to be orthogonal to the above and is input to the respective orthogonal axes (x-axis and y-axis) of the polarization-maintaining optical fiber and fixed at each resolution distance along the support rod supported by the flow. The length of the adjustable optical path is adjusted so that the mode coupling of both polarizations is generated in each section by the vibration lift received from the fluid by the sensor, and the flow is measured by measuring the frequency of the envelope of the output side beat signal. Than was the flow velocity distribution measuring method of the fluid consists of measuring it.

[0007]

The principle of the above-described measuring method according to the present invention will be described with reference to FIG. A laser light source is generally used as the light source, and a laser diode having a low coherence length corresponding to the resolution distance of the optical fiber sensor is used. The light from the light source is polarized by the polarizer to become linearly polarized light. If this linearly polarized light is Y-polarized, this light is split into two optical paths by the beam splitter BS and sent. As shown in the figure, a frequency shifter and a λ / 2 plate are provided in one optical path, and the frequency of the Y-polarized light is shifted to f + Δf by the frequency shifter, where f is the frequency.
And the X axis with the optical axis rotated 90 ° with respect to the Y polarization with the λ / 2 plate
It becomes polarized light, is reflected by mirror M, and is beam splitter B.
Sent to S.

An optical path length adjuster is provided on the other optical path, and the light of frequency f divided by the beam splitter is
The optical path length is adjusted by ± ΔL and sent to the beam splitter BS, and is incident on the polarization maintaining optical fiber sensor together with the light on the one optical path.

By the way, X polarization from the above two optical paths,
The Y-polarized light propagates independently in the polarization-maintaining optical fiber, and since the optical fibers have different refractive indexes in the X-axis and Y-axis directions, the propagation speed of the light is different. However, if the optical fiber receives vibration or the like on the way, the X polarized wave may leak into the Y polarized wave, and both polarized waves may be mixed and interfere with each other (mode coupling between both polarized lights).

Further, as shown in the figure, if there is vibration at a distance L from the incident end of the polarization maintaining optical fiber and the X polarization and the Y polarization are mixed, whether or not both polarizations interfere at this position is determined. It depends on whether the optical path length difference between the two lights after they leave the laser light source is less than or equal to the coherent length of the lights. Therefore, if the λ / 2 provided after the frequency shifter in the optical path shown
If there is no plate, since the optical path difference ΔL = 0, interference immediately occurs when the two optical paths are combined, and a beat signal with a constant amplitude Δf is output as output light.

When the λ / 2 plate is provided, the light passing through this optical path becomes X-polarized light and an optical path difference ΔL occurs at the position of the distance L. At this time, if ΔL <coherent length, interference also occurs at the position of this distance L, and a beat signal of Δf is generated.
However, if ΔL> coherent length, the coherence of light is lost, so that interference does not occur at the combined position. Therefore, if an optical path length difference of ΔL is given in advance to either one of the polarized waves by the optical path length adjuster, the optical path difference becomes 0 at a predetermined distance L, and interference is surely caused by vibration lift or the like. Since the optical fiber sensor is fixed in the distribution direction of the flow velocity of the flow at a predetermined length interval, the optical path length adjuster continuously or at a predetermined length pitch so as to cause interference in each section. If the length difference is changed, the flow velocity can be measured in each section.

When the optical path difference of ΔL is adjusted by the optical path length adjuster, it is desirable to make adjustment so that ΔL becomes completely 0. However, there may be some error in practice. If the error becomes larger than the coherent length of the light source, interference does not occur at a desired position. Therefore, the light source must inevitably have a low coherent length and a coherent length comparable to the error of the ΔL adjusting mechanism.

To measure the flow velocity by the optical fiber sensor,
A method of measuring the vibration of the Karman vortex generated behind the optical fiber sensor is used. Karman vortices are regularly generated behind the cylinder, and according to Karman, the frequency f of the Karman vortices is represented by f = St · U / d. St is the Strouhal number, U is the flow velocity, d is the channel width, and St is constant with respect to the Reynolds number Re in a predetermined range. Therefore, the flow velocity can be obtained by measuring the frequency of vibration.

In the case of the present invention, both the support rod for supporting the optical fiber sensor and the optical fiber itself behind it serve as the pillar. That is, when the optical fiber sensor behind the Karman vortex generated by the support rod vibrates,
And, the optical fiber itself may vibrate itself while generating a Karman vortex. In the former case, the optical fiber sensor is provided immediately behind the support rod, and in the latter case, the support rod is in a position that does not affect the optical fiber sensor, and the measurement can be performed by any method. Of course, the Karman vortex is generated and measurement is possible only by the optical fiber sensor without the supporting rod.

When the fluid hits the optical fiber sensor in each section, vibration of Karman vortex occurs in the optical fiber sensor,
When a lift is applied by the vibration, mode coupling between the X polarized wave and the Y polarized wave occurs at that portion, and as a result, the amplitude of the beat signal of the output light changes corresponding to the vibration frequency. Therefore, if the frequency of the envelope of the amplitude of the beat signal is measured, the flow velocity can be obtained.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an overall schematic view of a measuring system for carrying out the flow velocity measuring method according to the present invention. Laser light source 11
Is a low-coherent laser beam having a coherent length of about 1 cm, which is linearly polarized by a polarizer 12 and divided by a coupler 13 into two optical fibers 14, 1.
Send to optical path 4. Reference numeral 15 is an optical path length adjuster, 16 is an ultrasonic frequency shifter, 17 is a λ / 2 wavelength plate, and 18 is a coupler. Details of the optical path length adjuster 15 are shown in FIG.

Since the λ / 2 wave plate is provided so that the polarizations of one optical path and the other optical path are orthogonal to each other, it may be provided anywhere in the optical path. or,
The optical path length adjuster 15 and the ultrasonic shifter 16 may be provided in opposite positions as shown in the principle diagram of FIG. 4, or both may be provided in either optical path. X joined at coupler 18
The polarized light and the Y polarized light are sent to the sensor portion (below the sea surface) via the polarization maintaining optical fiber 14. The sensor portion below the sea surface of the optical fiber 14 is the anchor A on the seabed as illustrated.
It is fixed at a fixed point X at a predetermined interval to a support cable C supported between the buoy B and the buoy B. The details are shown in FIG.

The optical fiber 14 may be an optical fiber having a sufficient strength. That is, the optical fiber itself can be used as a vortex generator / detection sensor as well as when the Karman vortex generated by the support cable C is measured by the optical fiber behind the Karman vortex. The optical fiber 14 folded back from the anchor A is inserted into the support cable C, and the optical fiber 14 itself is not affected by the vibration due to the flow. The output light sent out through the folded optical fiber 14 is received by the light receiver 20 via the analyzer 19, converted into an electric signal, and sent to the signal processing unit 21. The signal processing unit 21 measures the frequency of the envelope of the beat signal amplitude.

The velocity distribution under the sea surface is measured as follows by the measuring system configured as described above. The laser light from the laser light source 11 is converted into Y-polarized linearly polarized light by the polarizer 12, and this is divided by the coupler 13 into two. 1
It is polarized to X polarization at 7. The other light is Y-polarized and its frequency is shifted to f + Δf, and the two polarized waves are merged by the coupler 18 and are incident on the polarization-maintaining optical fiber 14.

Since the polarization-maintaining optical fiber has different refractive indexes in the X-axis and Y-axis directions, the propagation speed of light is different. Now, the X-axis has a refractive index as large as 10 ⁻⁴ (speed). Is slow)
Then, at the position of L = 1 km, 1 km × 10 ⁻⁴ = 10
An optical path difference of cm occurs. Therefore, the optical path adjuster is used to
If an optical path difference of 10 cm is given, the optical path difference becomes 0 at a distance of 1 km and interference occurs.

In practice, the optical path difference between the X polarized wave and the Y polarized wave is not always exactly 0 at the desired position, but may include some errors. If this error is equal to or longer than the coherent length of the laser light, both polarizations will not interfere even if they are mixed at a desired position. For example, the coherent length is 1 mm,
The error given in advance by the optical path adjuster for the optical path difference of 10 cm is 1
If it is equivalent to cm, interference does not occur at the position of 1 km. For this reason, the laser light must have a coherent length of about the coherent length matching the optical path length adjustment error (about 1 cm in this example). Of course, if the coherent length is too long than the setting step of the optical path length adjuster, interference of a plurality of positions to be distinguished occurs at the same time and the position resolution deteriorates.

Since the sensor portions of the polarization-maintaining optical fiber are fixed at predetermined intervals in the flow velocity distribution direction of the fluid, the flow velocity distribution can be measured by measuring the flow velocity in each fixed section. .. In that case, for example, the optical path difference adjuster gives an optical path difference so that the optical path difference becomes 0 in the first section from the top, and while changing the optical path difference to a continuous value, the second section, the third section Measure the flow velocity in each section as follows. Alternatively, the optical path difference may be changed to a value at a predetermined interval corresponding to each section.

The flow velocity is measured by measuring the frequency of the Karman vortex generated behind the sensor portion of the optical fiber as described above. The frequency is measured from the frequency of the envelope of the amplitude of the beat signal Δf generated by the interference of the X and Y polarized waves for each section of the sensor portion of the optical fiber.

[0024]

As described in detail above, in the flow velocity distribution measuring method according to the present invention, one of the two optical paths is made adjustable in optical path length, and the light on both optical paths is polarized orthogonal to each other and the frequency of one is shifted. Then, both polarizations are sent to the polarization-preserving optical fiber sensor, and the optical fiber sensor is fixed at a predetermined interval in the flow to cause interference in each section, and the envelope of the amplitude change of the beat signal. Since the flow velocity distribution is measured by measuring the frequency of the flow velocity, it is possible to measure the flow velocity in a distributed state as compared with the flow velocity measuring method that conventionally only measured the flow velocity at one point. An extremely simple measurement method is used without using a circuit, and there is an effect that a nonuniform flow velocity distribution can be measured with high accuracy.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of a flow velocity measuring system according to an embodiment.

FIG. 2 is a schematic configuration diagram of an optical path length adjuster.

FIG. 3 is an explanatory view of a method for fixing a polarization maintaining optical fiber.

[Figure 4] Principle diagram of flow velocity measurement method

FIG. 5 is a schematic block diagram of a conventional flow velocity measuring device.

[Explanation of symbols]

 11 Laser Light Source 12 Polarizers 13 and 18 Coupler 14 Polarization-Maintaining Optical Fiber 15 Optical Path Length Adjuster 16 Ultrasonic Frequency Shifter 17 λ / 2 Wave Plate 19 Analyzer 20 Photoreceiver 21 Signal Processing Section

Front page continuation (72) Inventor Sosumi Sato 1-4 Umezono, Tsukuba-shi, Ibaraki Electronic Technology Research Institute, Industrial Technology Institute (72) Inventor Yoshikazu Murata 1-3-3 Shimaya, Konohana-ku, Osaka Sumitomo Denki Kogyo Co., Ltd. Osaka Works (72) Inventor Hideaki Nijima 1-3-3 Shimaya, Konohana-ku, Osaka Sumitomo Denki Kogyo Co., Ltd. Osaka Works

Claims (1)

[Claims] 1. In order to measure a flow velocity distribution of a non-uniform flow with a required resolution, light from a light source having a predetermined coherent length corresponding to a resolution distance is divided into two, and one of the optical path lengths is determined. The polarization-maintaining optical fiber is fed to two adjustable optical paths, one of the two optical paths is frequency-shifted with respect to the other, and one of them is polarized so as to be orthogonal to the other. A polarization-preserving optical fiber sensor, which is input to each unique orthogonal axis and fixed at each resolution distance along a support rod supported by the flow, receives a vibrational lift force from the fluid, which causes mode coupling between both polarization lights in each section. A flow velocity distribution measuring method for a fluid, which comprises measuring the flow velocity by adjusting the length of the adjustable optical path and measuring the frequency of the envelope of the output beat signal.