JPH0156382B2 - - Google Patents

Description

Detailed Description of the Invention Technical Field The present invention relates to a laser Doppler velocimeter that measures the flow velocity of a fluid based on a change in frequency difference caused by the Doppler effect exerted by the fluid between laser beams irradiating the fluid. Regarding the flow velocity measurement probe used for (LDV), it is particularly designed to be ultra-compact and to be able to stably and accurately measure the fluid flow velocity close to the flow velocity measurement point in the fluid in a quasi-non-contact manner. be.

Prior Art Regarding the observation of water currents, there has recently been a trend toward the introduction of optical methods in place of the traditionally used mechanical methods such as propeller current meters and electrical methods such as hot wire current meters. One of the optical methods for water flow observation is a water flow visualization method that has been used for a long time, and various improvements have been made from the standpoint of hydraulic research. It is difficult to say that this is the best method for visualizing the flow structure.

A newly developed laser Doppler velocimeter (LDV) is a newly developed method for optically measuring flow velocity. It uses the Doppler effect of flow velocity on laser light to optically precisely measure flow velocity, and is mainly based on signal processing technology. As a result of numerous improvements, it has reached the stage where, under certain conditions, it is possible to obtain data that far exceeds that of the hot wire anemometer, which is the mainstream method of current velocity measurement. However, conventional laser Doppler velocimeters aim to measure flow velocity in a completely non-contact manner, so they require modification of the water flow measurement device, such as making the side wall that crosses the optical path transparent, and they also require modifications to the water flow measurement device, such as making the side wall that crosses the optical path transparent, and the free surface is In addition to a number of problems, such as the inability to design a transverse optical path, they have traditionally been inferior to hot-wire anemometers in terms of ease of use, which, along with price issues, have hindered their widespread use.

The present inventor has proposed a system that can eliminate the drawbacks of the conventional laser Doppler velocimeter (LDV) and compensate for many of the points lost by making it a completely non-contact type, and moreover, the flow rate is substantially non-contact. We first developed a so-called quasi-non-contact type probe-type optical system that enables measurement, and by introducing such an LDV probe, we were able to conduct measurements near two-layer flow interfacial waves, which was difficult with conventional optical systems. Although it has also made it possible to measure flow velocity, conventional LDV probes have not yet achieved sufficiently satisfactory measurement performance.

In other words, by introducing an optical fiber into the laser Doppler velocimeter, which enables precise measurement of flow velocity by introducing a laser beam with extremely good coherence into the optical method of measuring fluid flow velocity, The LDV probe uses the Doppler effect of the fluid flow velocity on the crossed beams to perform precise measurements of fluid velocity while maintaining non-contact measurement at a distance from the measurement point. In , a multi-mode optical fiber was used, and in order to create a difference in the light intensity of the three laser beams formed by the optical mask for flow velocity measurement using the Doppler effect, the optical mask was equipped with a multi-mode optical fiber. Conventionally, the following serious problems have arisen in the performance of LDV probes due to the difference in pinhole diameters.

First, as the optical fiber that guides the laser beam from the light source to the LDV probe, a multimode optical fiber has traditionally been used, which is widely used, can be obtained relatively inexpensively, and is easy to introduce into optical systems. Multi-mode optical fibers are relatively easy to handle as the diameter of the core that forms the optical waveguide is relatively large, making it easy to inject laser light, but on the other hand, the diffusion angle of the diffused light emitted from the terminal aperture is Because of the large size, it is difficult to sufficiently miniaturize the probe, which mainly consists of optical systems, and the intersection angle at the convergence point of the three beams created from the diffused light with a large diffusion angle widens, resulting in stratified flow. It has been difficult to precisely measure the flow velocity near the interface. Furthermore, because it is multimode, the spatial intensity distribution of the light emitted from the optical fiber becomes uneven, creating a so-called speckle pattern, which degrades the quality of the optical signal and makes it difficult to obtain sufficient measurement accuracy. Not only is it not possible, but if the optical fiber bends or shakes,
The speckle pattern changes and becomes noise, etc.
Various problems arose.

Furthermore, since the fluid flow velocity is measured based on the frequency of the beat component caused by interference between two intersecting beams due to the Doppler effect exerted by the fluid flow velocity, conventionally, the optical mask in the LDV probe is
As described later, among the three pinholes placed at each vertex of a right-angled isosceles triangle, the diameter of the pinhole on the right-angled vertex that forms the light beam that will later become scattered light is determined to form the reference light beam. In a state where the pinhole diameter is about 5 times the diameter of other pinholes and has a significant difference, it becomes extremely difficult to optically explain or theorize the measurement principle described above, and as a result, the measurement data The problem has been that the reliability of the system is reduced.

Summary of the Invention The purpose of the present invention is to solve the above-mentioned conventional problems and eliminate their drawbacks, to make the conventional LDV probe much smaller and micron, and to improve the measurement of interfaces where the density suddenly changes, which has been difficult to measure in the past. Nearby flow velocity measurement,
The object of the present invention is to provide a micro LDV probe for flow velocity measurement that makes it possible to measure the velocity of a plasma flow that handles a minute fluid space and eliminates the ambiguity of conventional measurement principles.

That is, the micro LDV probe for flow rate measurement of the present invention includes an optical fiber that guides laser light from a light source, and a small hole that guides the laser light from the optical fiber, except for a portion that passes through the small hole. At each vertex of a right-angled isosceles triangle, the apex of which is perpendicular to the point of intersection of the light-reducing film and the extension of the optical axis of the optical fiber, the apex of which faces the small hole of the light-reducing film, respectively. an optical mask having three pinholes of approximately the same diameter located therein; and three pinholes each having a center on an extension of the optical axis and passing through each of the three pinholes in the optical mask.
A condensing lens that condenses the light flux of the book onto the flow velocity measurement point,
a prism that is interposed between the condenser lens and the flow velocity measurement point and refracts the three light beams at a substantially right angle; It is characterized in that it is configured to be able to measure the flow velocity of fluid at.

Principle of operation of the inventive device Before explaining the present invention in detail with reference to the drawings below, in order to facilitate understanding of the present invention, the principle of flow velocity measurement using the laser Doppler effect will be explained. put.

However, the conventional completely non-contact type laser Doppler velocimeter (LDV), in which the optical path of the laser beam is provided across the flow path of the fluid to be measured, has the following problems.

(1) When handling large-scale flows outdoors or in experimental water tanks, it is necessary to replace the side walls that partition the measurement area with transparent materials or use an expensive high-power laser light source.

(2) Even if the flow is small-scale, turbidity can cause light to attenuate, making measurements impossible.

(3) It was difficult to measure current velocity at remote and deep locations.

(4) When measuring flow velocity near interfacial waves, it is often impossible to detect the Doppler effect by heterodyne detection because the optical beam is refracted by the interfacial waves.

In order to solve the problems of the conventional completely non-contact type LDV, the present inventor first established the optical path of the laser light from the light source to the lens system placed close to the measurement point, and the collection of the laser light scattered by the fluid. We developed a flow velocity measurement system as schematically shown in Figure 1 using a quasi-non-contact type LDV probe that uses an optical fiber to prevent the attenuation of the laser beam. In the illustrated LDV probe type flow velocity measurement system, a laser beam from a laser light source 1 is injected into an optical fiber 3 via a lens system 2, and a probe 4 connected to the tip of the optical fiber 3 is used to measure the flow rate as described below. The two laser beams formed are taken out and irradiated to the measuring point P in the fluid 5, and the components of the two laser beams scattered in the same direction by the fluid 5 are guided to the detector 7 through the optical fiber 6. Detects the frequency difference due to the Doppler effect between the two laser beams corresponding to the flow velocity of
The processed results are displayed on the display 10 and recorded on the recorder 11.

The characteristics of the LDV probe type flow rate measurement system with this configuration are as described above.
In addition to guiding the laser beam through the optical fiber 3 to the LDV probe 4 without loss, as shown in Fig. is applied to an optical mask 13 provided as described below to form three laser beams, and these three laser beams are made to intersect with each other at a measurement point P at a small angle by a condenser lens system described later. This is what makes it so. Since each pinhole on this optical mask 13 is arranged at the same distance from the center of a condensing lens system to be described later, they intersect each other at a measurement point P located at the focal point of the condensing lens system. Furthermore, three pinholes are placed at each vertex of the right-angled isosceles triangle on the optical mask 13, and the amount of light of the laser beam passing through the pinhole located at the right-angled vertex is adjusted to the other two pinholes. When each is selected to be about five times the light intensity of the laser beam passing through, the laser beam 14 passing through the pinhole at the right angle vertex hits the fluid at the measurement point P and is scattered.
Since the amounts of light of the other laser beams 15a, 15b, which are used as reference beams and which travel straight through the other pinholes, and the scattered lights, which each travel in the same direction, are approximately equal, the reference beams 15a, 15b in the respective directions The scattered light and the optical fiber 16
a and 16b to the signal processing system, and the fluid flow velocity in the directions of the two orthogonal sides of the above-mentioned right-angled isosceles triangle can be measured by the so-called reference beam method based on the measurement principle described below.

Here, the principle of measuring unidirectional flow velocity using two laser beams 14 and 15 passing through two pinholes located at both ends of one of two orthogonal sides of a right-angled isosceles triangle will be explained with reference to FIG. explain.

Now, let the flow velocity vector be V, and the flow velocity in the direction perpendicular to the optical axis of the condensing lens system be U, and in the minute region where the two laser beams meet, they are scattered by solid particles in the fluid and undergo the Doppler effect. Let k ₁ and k ₂ be the wave number vectors of the scattered light from the laser beam 14 and the reference light 15 that has traveled straight without being scattered by solid particles, that is, the vectors taking into account the wave frequencies, then their wave number vectors k ₁ , k ₂ are almost equal as described above, so that mutual heterodyne detection is possible, and the light intensity detected by the detector 7 is expressed by the following equation (1).

I∝A ² ₁ +A ² ₂ +2A ₁ A ₂ COS {(k ₂ ~ k ₁ )Vt+}
...(1) Here, A ₁ and A ₂ are the amplitudes of wave number vectors k ₁ and k ₂ , the first and second terms on the right side represent the DC component, and the third term is the Represents a beat component that has a frequency difference between the wave number vector k ₁ of the laser beam 14 whose frequency has changed due to the Doppler effect due to changing direction and the wave number vector k ₂ of the reference beam 15 whose frequency does not change because it has traveled straight. It is something. The beat frequency exhibited by the beat component, that is, the frequency difference _d between the wave number vectors k ₁ and k ₂ is expressed by the following equation (2).

_d = (k ₂ ~ k ₁ ) V/2π ...(2) Therefore, by measuring this bead frequency _d , the flow velocity U represented by the flow velocity vector V can be determined by the following equation (3). .

U= _d・λ _p /(2n sinθ/2) ...(3) Here, λ _p is the wavelength of light in vacuum,
n is the refractive index of the fluid medium, and θ is the angle between the optical axis of the condenser lens system and the reference light beam 15.

Furthermore, as can be seen from the above equation (1), the frequency difference between the wave number vectors k ₁ and k ₂ , that is, the beat frequency _d
Since it is not possible to distinguish the polarity of the light intensity depending on whether the polarity is positive or negative, the magnitude of the unidirectional flow velocity can be measured, but the polarity of the flow velocity, that is, the flow direction cannot be determined.

According to the above-mentioned quasi-non-contact LDV probe type flow velocity measurement system, among the problems of the completely non-contact laser Doppler velocimeter mentioned above, problems (1) to (3) can be solved. is almost solved, for example,
Even if the flow velocity is located 1 km away, it can be measured without problems using a laser light source of about 5 mV.
It is possible to measure flow velocity even in turbid water equivalent to a river flood, and it is also possible to simultaneously measure flow velocity components in two orthogonal directions, making it easy to use and similar to conventional hot wire current meters. In addition, item (4) of the problems of the completely non-contact type LDV mentioned above, namely,
The problem of measuring flow velocity near interfacial waves is the conventional method described below.
Combined with the drawbacks of the LDV probe, a complete solution has not been reached.

Regarding the flow direction, which could not be determined with the LDV probe type flow velocity measurement system described above, the frequency difference between the wave number spectra k ₁ and k ₂ of the scattered light 14 and the reference light 15 described above also applies to a stationary medium. By making the original frequencies different between the two so that _d takes a finite value, it is possible to determine the flow direction.
In other words, the above frequency difference _d in the stationary medium is
Assuming _p , the flow velocity U is expressed by the following equation (4).

U = ( _d − _p ) λo / (2n sin θ / 2) ... (4) Here, the frequency difference _d is the frequency change in the light intensity detected by the detector 7 at the same time as in equation (2) above. be. Therefore, the flow direction can be determined as U=0 when _d = _p , U>0 when _d > _p , and U<0 when 0< _d < _p .
The relationship among these variables U, _d , _{and p} is shown in FIG.

However, the conventional LDV probe developed by the present inventor and used in the above-mentioned quasi-non-contact flow measurement system has the above-mentioned drawbacks, and cannot be used for water flow observation that is fully satisfactory for hydraulic research. The above properties were not obtained.

The micro LDV probe for flow velocity measurement of the present invention is
In addition to using a single-mode optical fiber as the optical fiber, which was the cause of the conventional drawbacks mentioned above, a pinhole of the same diameter was provided in the optical mask for light attenuation, which was also the cause of the conventional drawbacks, to reduce the concentration. The filter has been improved to provide a difference in light intensity, so that the desired measurement performance can be obtained.

EXAMPLES The present invention will be explained in detail below using examples with reference to the drawings.

First, the configuration of the entire flow velocity measurement system using the micro LDV probe of the present invention is shown in FIG. 6, based on the basic configuration shown in FIG. 1. In the illustrated overall configuration, a component 4 having an elongated cylindrical shape is the micro LDV probe of the present invention, and the tip of an optical fiber 3 connected to the laser light source 1 via a lens system is inserted as a part of the component. This is exactly the same as in the basic configuration shown in the first figure. After condensing the three laser beams emitted from the micro LDV probe 4 at a measurement point P in the fluid 5, they enter the light receiving optical fibers 6a and 6b arranged in the straight direction of each reference light beam. The resulting light component is guided to photomultiplier tubes 7a and 7b for amplification, together with a beat component generated based on the Doppler effect exerted by the fluid flow velocity due to mutual interference between the reference light and the scattered light, and then passed through bandpass filters 8a and 7b.
8b to extract only each beat component,
Further, the frequency-voltage converters 9a and 9b extract a voltage signal proportional to the beat frequency of each beat component, and the arithmetic unit 17 performs necessary data processing to calculate, for example, fluid flow velocity in both horizontal and vertical directions.

Next, each component in the above-mentioned overall configuration of the micro LDV probe type current velocity measurement system according to the present invention will be explained in more detail. A coherent light source is indispensable for measuring flow velocity, and in the prototype experiment of the probe of the present invention, a helium-neon gas laser was used to obtain an output of 15 milliwatts, but in general, the higher the output, the better the measurement accuracy. will improve. The laser beam from the light source 1 is focused by a lens system 2 acting as a convex lens and injected into the core of the optical fiber 3.

However, in order to solve the conventional problems caused by using a multimode optical fiber in the conventional LDV probe mentioned above, the micro LDV probe according to the present invention is relatively expensive and has a low laser beam flux. Although it is necessary to use extremely precise optical means for handling such as injection, a single mode optical fiber 3 is used, which is much thinner than a multimode optical fiber. As is well known, optical fibers are made by wrapping a core glass fiber, which serves as an optical path medium, with a glass sheath having a high refractive index.
It transmits a light beam with minimal attenuation through repeated total reflection at the interface, and a single mode optical fiber typically has a diameter of 10 μm.
Since single-mode total reflection is repeated within the extremely thin core section described below, from the terminal aperture, the diffusion angle is much smaller than that of multi-mode, and diffused light with an even spatial intensity distribution is emitted from the terminal aperture. can get. In the prototype experiment of the probe of the present invention, Fujikura Electric Cable's
SMC/10/125/10 type optical fiber was used, and this optical fiber has a transmission characteristic in which light with a wavelength of 1 μm is attenuated by 1 dB per 1 km, and its cross-sectional structural dimensions are shown in Figure 7 a. , b, with a core portion 3a having a diameter of 10 μm and an outer diameter of 125 μm.
The outer periphery of the cladding portion 3b is protected by a silicon coating layer 3c and a polyamide coating layer 3d, and the outer diameter is
It is finished to a diameter of 900μm, and the outer circumference is covered with a normal cable coating of 3mm in outer diameter.

Next, in the micro LDV probe 4,
As schematically shown in FIG. 8, the laser diffused light 12 emitted from the end aperture of the optical fiber 3 causes
For example, an optical mask 1 made of aluminum foil or the like with pinholes with a diameter of 0.1 mm arranged as described below.
3 to form three laser beams, which are focused on a measuring point P in the fluid 5 through a condensing lens system as will be described later, while the condensed optical axis is refracted almost at right angles by a prism 19. For example, a laser beam emitted from the tip of a probe 4 inserted perpendicularly from the water surface passes through the fluid 5 parallel to the water surface so that flow velocity measurement based on the Doppler effect can be easily performed. The micro LDV probe 4 is constructed by adhering a prism 19 to the tip of an elongated cylindrical case 20 that houses each component, and the three laser beams B ₁ - As described _above , the reference beams B ₁ and B ₃ that have traveled straight and the scattered light B ₂ that has been scattered in the same direction are guided to the data processing system by the receiving optical fibers 6a and 6b.

Therefore, _the important point about the micro LDV probe 4 of the present invention is that the three laser beams B ₁ to B ₃ formed by the optical mask 13
~ B ₃ arrangement and its strength. The three laser beams B ₁ to _{B 3} in the probe of the present invention are
The arrangement is as shown in the figure, and three laser beams B ₁ to B ₃ are focused by three pinholes located at each vertex of an isosceles right triangle, equidistant from the optical axis of the condenser lens system. Formation is similar to conventional LDV probes.

However, the laser beam B ₁ used as the reference beam,
In order to increase the light intensity of the laser beam _B2 , which is scattered light that is affected by the Doppler effect due to the fluid flow velocity, by approximately five times compared to the light intensity of _B3 , in conventional probes, as described above, each pinhole is As will be described later, a light-reducing film is used that provides an appropriate amount of attenuation to the passing light, as will be described later.

The principle of flow velocity measurement based on the relationship between the flow velocity and the Doppler effect of the fluid on the frequencies of the reference light and scattered light using the laser beams B ₁ to _{B 3} having such a configuration was described above with reference to FIGS. 2 and 3. As a supplementary explanation, the important point about the laser beam arrangement shown in Figure 9 is that the laser beam
The angle between the plane containing B ₁ and B ₂ and the horizontal plane of the fluid and the angle between the plane containing laser beams B ₂ and B ₃ and the horizontal plane of the fluid are both 45°, and the angle between the laser beams B ₁ and B ₂ and the intersection angles of B ₂ and B ₃ are equal to each other.

Considering the relationship between the frequency and flow velocity of one of the two combinations of reference light and scattered light mentioned above, the intersection of laser beams _B1 and _B2 , as shown in Fig. 10. , laser beams B ₁ and B ₂ intersecting at an angle θ are incident on the fluid at a flow rate U from the lateral direction, and these laser beams are caused by the movement of solid particles in the fluid at the measurement point P.
B ₁ and B ₂ are scattered as shown schematically. Therefore, the frequency ₁ of the laser beam B ₁ that travels straight, becomes the reference beam, and enters the receiving optical fiber 6 is as follows:
The measurement point P is independent of the Doppler effect caused by the fluid.
Of the laser beam _B2 scattered by solid particles in the fluid, the light intensity of the scattered light component traveling in the same direction as the reference beam _B1 is:
As described above, the light intensity is approximately the same as that of the reference beam _B1 , and its frequency ₂ exhibits a frequency change proportional to the flow velocity due to the Doppler effect based on the fluid flow velocity. Therefore, when the combined light component guided by the light-receiving optical fiber 6 is detected,
A detection output corresponding to the combined light intensity changing at a beat frequency _d equal to the frequency difference ₁ to ₂ between the two caused by the Doppler effect based on the fluid flow velocity is obtained, and | ₁ − ₂ |= _d ...(5) Become. This beat frequency _d can be sufficiently detected and easily measured by detecting the output of the photomultiplier tube 7, since the moving speed of solid particles in the fluid is extremely low compared to the speed of light. , Furthermore, based on the relationship shown in the following equation (6), the fluid flow velocity U
can be found.

U=λ _p | ₁ ~ ₂ |/2n (sin θ/2) ...(6) Here, U is the flow velocity of the fluid along the line segment mm' perpendicular to the focusing optical axis shown in Fig. 10. , n is the refractive index of the fluid medium, and _p is the wavelength of the laser light in vacuum. Therefore, the flow velocity U is the above-mentioned beat frequency _d with the known quantity as a proportionality constant = |
It can be easily determined from the proportional relationship between ₁ and ₂ |. Note that the fluid flow velocity in the direction perpendicular to the line segment mm' can also be determined in the same manner as described above by combining the laser beams B ₂ and B ₃ .

Next, we will discuss how to find the flow velocity of a fluid whose direction is actually constantly changing.
This will be explained with reference to the figures. That is, as shown in the figure, it is assumed that a solid particle P moving with the fluid is moving at a speed U in the direction of the arrow shown in the figure, and its flow velocity vector U is projected onto a plane containing laser beams B ₁ and B ₂ . The component vector U ₁ in the PP ₁ direction and the component vector U ₂ in the PP ₂ direction projected onto the plane including the laser beams B ₂ and B ₃ will be considered separately. In addition,
Projection directions PP ₁ and PP ₂ are both relative to the horizontal direction.
Assume that the slope is 45°. Moreover, between the flow velocity vector U and each component vector U ₁ , U ₂ is U=√ ² ₁
There is a relationship of +U ² ₂ , but in general, in fluid measurement, the horizontal component u is more important than the flow velocity U itself in the flow direction.
It is more important to know the vertical component v. Therefore, the horizontal flow velocity component u and the vertical flow velocity component v can be obtained from the following equation (7).

u=1/√2(U ₁ +U ₂ ), v=1/√2(U ₁ −U ₂ )...(7) Therefore, the above-mentioned projected flow velocity vector U ₁ ,
If U ₂ is found, horizontal and vertical components u and v of the flow velocity U are
can be obtained, and by extension, the direction of the flow velocity U,
In other words, the flow direction can also be determined.

On the other hand, if the pinholes of the optical mask 13 in the LDV probe are arranged as shown in Figure 9 to form each of the laser beams B ₁ to B ₃ , the component flow velocity vectors U ₁ and U ₂ described above can be obtained. , therefore,
Using the probe of the present invention, it is possible to determine the horizontal and vertical components u and v of the fluid flow velocity U, or to determine the direction of the flow velocity U, by using a pinhole mask with the configuration shown in Figure 9. becomes.

Note that the direction of the flow velocity U, that is, the flow direction, cannot be determined unless the following matters are known in advance. In other words, in FIG. 11, there is no significant change in the direction of the flow velocity U indicated by the arrow, and the direction of the fluid U is within the perpendicular region formed by both projection directions PP ₁ and PP ₂ that sandwich the horizontal direction. As long as there is a current, the direction of the flow can be determined. On the other hand, as can be seen from the above, the plane containing the laser beams B ₁ and B ₂ shown in FIG. 9 is set horizontally, and the plane containing the laser beams B ₂ and B ₃ is set vertically. Even if it is set in the direction, the horizontal and vertical components u and v of the flow velocity U described above cannot be directly detected.

Next, other components subsequent to the above-mentioned main parts in the overall configuration shown in FIG. 6 of the flow rate measurement system using the micro LDV probe of the present invention will be explained in detail. For example, single mode optical fibers 6a and 6b for receiving light are inserted so that the beams B ₁ and B ₃ are incident thereon, respectively, with an outer diameter of 1 mm.
The two metal pipes are supported by a support frame 18 with appropriate separation, and are moved in any direction in the fluid to be measured so that the flow velocity measurement data at each measurement point can be sequentially plotted. .

The light components guided by the light-receiving optical fibers 6a and 6b are amplified by photomultiplier tubes 7a and 7b, and then detected and the changes in light intensity are converted into voltage signals, but it is assumed that the multiplication characteristics of both are almost the same. Also, bandpass filters 8a and 8b extract the beat frequency components based on the Doppler effect in each converted output voltage signal, and each frequency converts the beat frequencies of these components into voltage signals. It is also preferable that the voltage converters 9a and 9b have substantially the same characteristics.

Furthermore, those converted output voltage signals are converted to the arithmetic unit 1
7 and perform data processing, suppose that the photomultiplier tube 7a is supplied with the received light component in the direction of the laser beam _B1 , and the photomultiplier tube 7b is supplied with the received light component in the direction of the laser beam _B3 . From the former, the projected flow velocity vector U ₁
Since the light intensity change component of the beat frequency proportional to U2 is detected, and from the _latter , the light intensity change component of the beat frequency proportional to the projected flow velocity vector U2 is detected. Each converted output voltage signal also has a projected flow velocity vector.
It exhibits a voltage proportional to U ₁ and U ₂ . Therefore,
Those converted output voltages V ₁ and V ₂ are respectively expressed by the following equations (8), and the horizontal and vertical components u and v mentioned above can also be found, and √ ²
The fluid flow velocity U itself can also be determined by setting +v ² =U.

Next, an example of a specific configuration of the micro LDV probe of the present invention is shown in FIG. 12 in a state where it is disassembled into each component and expanded in the optical axis direction. In the illustrated configuration example, the cable coating at the tip of the single mode optical fiber 3 having the above-described configuration is removed to expose the polyamide coating layer 3d, and the coating layers 3e and 3d at the tip are also removed. The cladding portion 3b is exposed. The tip of the optical fiber 3 in such a state is inserted into the connector 21, and the tip of the exposed polyamide coating layer 3d is fixed to the intermediate partition wall 21a, and the connector 2
The exposed cladding part 3b is inserted into the rear half of the cylinder 22 fitted to the front half of the cylinder 1, and its tip is fixed to the intermediate partition wall 22a. In this state, the diffused laser beam 12 emitted from the tip opening of the single-mode core portion 3a of the optical fiber 3 fixed in the cylinder 22 is emitted from the tip of the cylinder 22 before reaching the inner peripheral surface of the cylinder 22. A light-reducing film 23 attached as described below will be encountered. This light-attenuating film 23 is successively overlapped with the subsequent optical mask 13 and condensing lens system 24 in close contact with each other, housed in a cap 25 that fits into the tip of the cylinder 22, and bonded to the tip by pressure. Yes, and furthermore,
The aforementioned prism 19 is bonded to the tip of the cap 25 using, for example, an epoxy adhesive. Although the connector 21, cylinder 22, and cap 25 were made of brass in the prototype experiment, they may be made of any material as long as it does not cause a chemical reaction with the fluid medium to be measured. can.

As described above, the light attenuation film 23 irradiated with the diffused light emitted from the optical fiber 3 has the intensity of the laser beams B ₁ and B ₃ used as the reference lights relative to the laser beam B ₂ that becomes the scattered light at the measurement point. It acts as a density filter to attenuate the light by approximately 1/5, and it is preferable to develop a suitably exposed negative film to obtain the required amount of attenuation. Also, in this light attenuation film 4, among the three pinholes provided in the optical mask 13, which will be described later,
A small hole 23a is opened opposite the pinhole located at the right-angled vertex of the right-angled isosceles triangle, and the diffused light from the optical fiber 3 is passed through without being attenuated to form a laser beam.
Let it be B ₂ .

The optical mask 13 provided in contact with the above-described light-reducing film 23 is provided with three pinholes P ₁ to _{P 3} in the configuration shown in FIG. 13, as in the conventional case. As described above, in the optical mask 13 according to the present invention, all three pinholes P ₁ to P ₃ are formed to have the same diameter, for example, 0.1 mm, and are used to form the laser beams B ₁ to B ₃ , respectively. That's right. This optical mask 13 is formed to have an outer diameter of 1.6 mm, for example, and is housed in the cap 25.

The condensing lens system 24 provided in contact with the above-mentioned optical mask 13 functions as a convex lens that condenses the three laser beams B ₁ to B ₃ onto the measurement point P. For example, it can be used by cutting a commercially available objective lens. Then, the outer diameter is reduced and housed in the cap 25. In the prototype experiment, a lens with a focal length of 5.0 mm was used.

By combining each of the above components, for example, the outer diameter
As mentioned above, the small prism 19 is glued to the tip of the micro LDV probe 20 of the present invention, which is configured in a cylindrical shape with a length of 2.0 mm and a total length of 27.0 mm, to seal the entire body watertight and prevent the infiltration of the fluid medium to be measured. To prevent. The small prism 19 has a prism surface in contact with the cap 25 of, for example, 2 mm x 2 mm, and the reflecting slopes that refract the laser beams B ₁ to _{B 3} at approximately right angles are made into mirror surfaces by, for example, vapor deposition of aluminum. The reference beam and the scattered light are received by the light-receiving optical fibers 6a and 6b provided in the straight direction of the reference beams _B1 and _B3 emitted from the prism 19 and passed through the measurement point P, and guided to the data processing system. As mentioned above.

The micro LDV probe of the present invention configured as described above is sealed with adhesive, so it can be used in a fluid environment with a temperature range that does not differ significantly from room temperature, from normal pressure to 0.1 atm. In addition, the single mode optical fiber used is flexible enough that its function will not be impaired even if it is wrapped around a cylinder with a diameter of about 10 mm, making it easy to install the probe of the present invention in a fluid. The mode can be selected quite freely, but is limited in that it must be installed in combination with a light-receiving optical fiber.

Effects As is clear from the above explanation, according to the present invention, the following remarkable effects can be obtained.

In other words, when measuring the flow velocity near the interface of a laminar flow where the density changes rapidly, the conventional LDV probe could only get close to the interface by about 5 mm, but the probe of the present invention is much more compact. , the flow velocity of two orthogonal components can be measured without affecting the flow near the measurement point by approaching the boundary surface to within 1 mm. Moreover, since the measurement error caused by the probe of the present invention is less than the error caused in the position setting of the probe, the reliability of the measurement data is also improved.

On the other hand, in conventional LDV probes, in order to obtain the intensity difference between the reference laser beam and the scattered light laser beam, which is subject to the Doppler effect of the flow velocity,
Because the diameters of the beam forming pinholes were different, there was a problem that the measurement results were unclear, but in the probe of the present invention, all the pinhole diameters are the same and the diameter reduction film is used. Since the required strength difference is obtained, this conventional problem can be solved.

Furthermore, since conventional LDV probes use multimode optical fibers, the intensity distribution of the transmitted light changes due to bending or shaking of the optical fibers, causing noise generation. Since the probe uses a single mode optical fiber, it has not only been made smaller, but also the quality of the measurement signal, such as the signal-to-noise ratio, has been significantly improved.

Furthermore, the use of a single-mode optical fiber in the probe of the present invention not only improves the signal-to-noise ratio, but also makes it possible to miniaturize the LDV probe to a diameter of 2 mm that hardly disturbs the flow. It has also become possible to obtain flow velocity data that was previously impossible to measure, such as nearby flow velocity. As an example of actual measurement of such flow velocity data, FIG. 14 shows flow velocity data measured by installing the probe of the present invention at a position 4 mm above the density boundary surface in a stratified flow in which a freshwater layer flows above a saltwater layer. Of the flow velocity data shown, the upper row records interface fluctuations near the measurement point, the middle row shows the fluctuations in the flow velocity u in the mainstream direction due to the interface fluctuations, and the lower row shows the fluctuations in the flow velocity v in the direction perpendicular to the mainstream. It is shown in Note that the illustrated flow velocity data is obtained by removing noise from the original measurement data described later through a bandpass filter, and clearly measures the state of fluctuations in the flow velocities u and v in response to interface fluctuations. Measurement data of two orthogonal flow velocity components near such a density boundary has not been previously obtained, and the use of the probe of the present invention has made it possible to elucidate the manner in which turbulence occurs near the density interface. The availability of such hydrodynamically important and novel measurement data according to the present invention can also be expected for numerous other technical fields.

Another important effect of the ultra-miniaturization of the LDV probe according to the present invention is that the intersection angle between the reference light beam and the scattered light beam at the measurement point has become extremely small. There is no longer a risk that one of the light beams will deviate from the density interface region, for example, and the measurement accuracy of the LDV element has been significantly improved, and a high-quality optical signal can now be obtained. For example, the original waveform of the measured output voltage obtained by a conventional LDV probe and the original waveform of the measured output voltage obtained by the micro LDV probe of the present invention under the same measurement conditions are compared and shown in Figure 15a. b and are shown respectively. As is clear from the figure, the accuracy of the measurement data obtained by the probe of the present invention is much higher than that of the conventional probe, and the noise level is also much lower.

The ultra-miniaturization of the micro LDV probe of the present invention makes it possible to measure flow velocity in a narrow area, to measure flow velocity near an interface, and to obtain high-quality measurement data with high accuracy, which greatly improves flow velocity measurement results in all fields. It is expected that this will lead to major changes in measurement methods in the fields of civil engineering and oceanography, for example, and it will also be possible to measure currents of plasma and harmful gas flows, which were previously impossible, and to measure currents in the deep sea. It becomes possible to do this effectively.
For example, regarding the measurement of plasma flow velocity, conventionally it was difficult to obtain a good measurement signal because it was measured using a non-contact LDV through a glass plate covering the plasma in multiple layers, but with the present invention, For example, the LDV probe can be brought very close to the plasma to obtain high quality measurement signals.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the schematic configuration of the LDV probe type flow velocity measurement system, Figures 2 and 3 are perspective and side views schematically showing the principle of laser Doppler flow velocity measurement, and Figure 4 is Similarly, a vector diagram schematically showing the measurement principle, Fig. 5 is a front view showing the configuration of the optical mask for laser beam formation in the conventional LDV probe, and Fig. 6 is a diagram of the flow velocity measurement system using the micro LDV probe of the present invention. A block diagram showing the overall configuration; FIGS. 7a and 7b are a front view and a side view respectively showing a structural example of an optical fiber used in the micro-LDV probe of the present invention;
Fig. 8 is a perspective view schematically showing the action of the micro LDV probe of the present invention, Fig. 9 is a diagram showing an example of the configuration and arrangement of the laser beam for measuring the flow velocity, and Fig. 10 is the same operation for measuring the flow velocity. FIG. 11 is a diagram schematically showing the principle, FIG. 11 is a diagram schematically showing the configuration of the flow velocity vector as a result of the measurement, and FIG. 12 is a perspective view showing an expanded example of the detailed configuration of the micro LDV probe of the present invention. 13 is a front view showing an example of the configuration of the optical mask, and FIG. 14 is a signal waveform diagram showing an example of the flow velocity measurement results.
Figure 5 a and b are LDVs according to the conventional and the present invention.
FIG. 6 is a signal waveform diagram showing examples of flow velocity measurement results using a probe. DESCRIPTION OF SYMBOLS 1... Laser light source, 2... Lens system, 3... Optical fiber, 3a... Core part, 3b... Clad part, 3c... Silicon coating layer, 3d... Polyamide coating layer, 4... Micro LDV probe , 5...
...Fluid, 6, 6a, 6b... Optical fiber for light reception,
7... Wave detector, 8, 8a, 8b... Bandpass filter, 9... Signal analyzer, 9a, 9b... Frequency-voltage converter, 10... Display, 11... Recorder, 12... Diffused light, 13... Optical mask, 14
... Laser beam for scattering, 15a, 15b ... Laser beam for reference, 16, 16a, 16b ... Optical fiber for light reception, 17 ... Arithmetic unit, 18 ... Supporter, 19 ... Prism, 20 ... case, 21
...Connector, 21a, 22a...Bulkhead, 22...
...Cylinder, 23...Dimmer film, 23a...
Small hole, 24...Condenser lens system, 25...Cap, P...Measurement point.

Claims (1)

[Scope of Claims] 1. An optical fiber that guides laser light from a light source, and an attenuator that has a small hole and attenuates the diffused light from the end opening of the optical fiber except for a portion that passes through the small hole. Approximately equal diameters located at respective vertices of a right-angled isosceles triangle whose perpendicular vertices face the small holes of the light-reducing film at approximately equal distances from the intersection of the optical film and the extension of the optical axis of the optical fiber. No. 3
an optical mask having three pinholes, and a light condenser that focuses three beams of light that are centered on an extension of the optical axis and have passed through the three pinholes in the optical mask onto a flow velocity measurement point. The lens includes a prism that is interposed between the condensing lens and the flow rate measurement point and refracts the three light beams at approximately right angles, and the Doppler effect acting between the three light beams allows the A probe for measuring flow velocity, characterized in that it is configured to be able to measure the flow velocity of a fluid at a flow velocity measurement point. 2. The flow rate measuring probe according to claim 1, wherein the optical fiber is configured to operate in a single mode. 3 The light attenuation mask reduces the diffused light by approximately 1/5.
3. The flow rate measuring probe according to claim 1, wherein the probe is configured to reduce the light intensity.