Abstract:
A clamp-on type acoustic Doppler current profiler eliminates, among ultrasound echoes caused by two measurement lines of a longitudinal wave and a shear wave propagating in a piping, the ultrasound echo based on the longitudinal wave, thereby providing the measurement of a flow rate profile and/or a flow rate with a higher accuracy. The profiler includes a wedge mounted to the piping. The wedge includes an inclined surface at which an ultrasonic transducer can be mounted. The inclination is such that the ultrasound transducer receives only the ultrasound echo from the reflection of a shear wave component off the reflectors in the fluid.

Description:
BACKGROUND  
       [0001]     A clamp-on type ultrasound flow meter typically uses an ultrasound transducer attached to a part of an outer periphery of a tubular body, such as a piping, to measure, from the exterior of the tubular body, the flow rate of fluid moving in the tubular body. Clamp-on type ultrasound flow meters are mainly classified into ones that utilize the difference in propagation time and ones that utilize the Doppler effect.  
         [0002]     The former ones based on the difference in propagation time reciprocate ultrasound, applied slantedly across the tubular body to the fluid moving in the tubular body. The difference between the time in which the ultrasound propagates along the outward route and the time in which the ultrasound propagates along the return route is used to measure the fluid flow rate.  
         [0003]     On the other hand, the latter ones, based on the Doppler effect, relies on reflectors, namely suspended particles and/or air bubbles included in the fluid, which are assumed to move at the same speed as the fluid. The movement speed of the reflectors is used to measure the flow rate of the fluid. Specifically, this technique transmits ultrasound into the fluid being measured, and the frequency of the ultrasound is changed by the Doppler effect in accordance with the speed of the reflectors when the ultrasound is reflected off the same. The frequency of the reflected ultrasound is detected to measure the speed of the reflectors, thereby measuring the fluid flow rate profile and/or the fluid flow rate.  
         [0004]     A conventional Doppler ultrasound flow meter is disclosed, for example, in Japanese Laid-Open patent Publication No. 2000-97742.  FIG. 6  schematically illustrates a structure of such a flow meter. The Doppler ultrasound flow meter shown in  FIG. 6  includes an ultrasound velocity profile measurement unit (hereinafter referred to as a UVP unit)  10  that measures the flow rate of fluid  22  in a piping  21  in a non-contact or non-invasive manner. This UVP unit  10  includes an ultrasound transmission means  11  for transmitting to the fluid  22  an ultrasound pulse having a required frequency (basic frequency f o ) along a measurement line ML, a flow rate profile measurement circuit  12  for receiving the ultrasound echo reflected from the measurement region of the ultrasound pulse transmitted into the fluid  22  to measure the flow rate profile thereof in the measurement region, a computer  31  (e.g., microcomputer, CPU, MPU) for calculating the flow rate profile of the fluid  22  to integrate it in the radius direction of the piping  21 , thereby measuring the flow rate of the fluid  22  depending on time, and a display apparatus  32  for displaying the output from this computer  31  in chronological order.  
         [0005]     The ultrasound transmission means  11  includes, for example, a signal generator  15 , namely consisting of an transducer  13  for generating an electric signal having a basic frequency (e.g., 1 MHz, 2 MHz, 4 MHz) and an emitter  14  for outputting an electric signal from the transducer  13  as a pulse having a frequency F rpf  for each predetermined cycle (1/F rpf ). The signal generator  15  inputs a pulsed electric signal having the basic frequency F rpf  to the ultrasound transducer  16 . The ultrasound transducer  16  transmits an ultrasound pulse having the basic frequency f o  to the fluid  22  in the piping  21  along the measurement line ML. This ultrasound pulse is a straight beam having a beam width, for example, of 5 mm having very little dispersion.  
         [0006]     The ultrasound transducer  16  also works as a transmitter/receiver that is designed to receive an ultrasound echo generated when a transmitted ultrasound pulse is reflected off the reflectors in the fluid  22 . The reflectors can be air bubbles or suspended particles uniformly distributed in the fluid  22 , i.e., foreign matter having different acoustic impedance from that of the fluid  22 .  
         [0007]     The transducer  16  converts ultrasound echo received thereby into an electric echo signal. This electric echo signal is amplified by an amplifier  17  in the UVP unit  10  and digitized by an AD converter  18 . The digital echo signal is input into the flow rate profile measurement circuit  12 . The electric signal having the basic frequency f o  from the transducer  13  and digitized by the AD converter  18  is input to the flow rate profile measurement circuit  12 . Based on the difference of frequency between these signals, the flow rate based on Doppler shift is measured to calculate the flow rate profile of the fluid  22  in the measurement region, along the measurement line ML. The flow rate profile of this measurement region can be corrected by the oblique angle α of the ultrasound transducer  16  (oblique angle to the direction perpendicular to the longitudinal or axial direction of the piping  21 ), thereby measuring the flow rate profile of the fluid  22  in the cross section of the piping  21 .  
         [0008]     Next, how the Doppler ultrasound flow meter operates will be further described in detail with reference to  FIG. 7 . In section (A) of  FIG. 7 , the ultrasound transducer  16  is inclined by the oblique angle α to the direction along which the fluid  22  flows. The ultrasound transducer  16  transmits the ultrasound pulse having the basic frequency f o  into the piping. The ultrasound pulse collides with and is reflected off the reflectors (e.g., suspended particles uniformly dispersed in the fluid  22  on the measurement line ML), namely turning into an ultrasound echo “a,” which is received by the ultrasound transducer  16 , as shown in section (B) of  FIG. 7 .  
         [0009]     In section (B) of  FIG. 7 , reference numeral “b” denotes a multiple reflection echo reflected off the tubular wall of the piping  21  into which an ultrasound pulse is transmitted, and reference numeral “c” denotes a multiple reflection echo created at the tubular wall of the opposing section of the piping  21 . The ultrasound transducer  16  transmits an ultrasound pulse having a cycle of (1/F rpf ) as shown. The echo signal “a” received by the ultrasound transducer  16  is filtered and the Doppler shift method is used to measure the flow rate profile along the measurement line ML, thereby providing the display as shown in section (C) of  FIG. 7 . This flow rate profile is measured by the flow rate profile measurement circuit  12  of the UVP unit  10  and is displayed by a display apparatus  32  via the computer  31 .  
         [0010]     As described above, the Doppler shift method uses a mechanism in which, when an ultrasound pulse is transmitted to the fluid  22  flowing in the piping  21 , it is reflected off the reflectors mixed in or uniformly dispersed in the fluid  22 , which turns into an ultrasound echo. The frequency of the ultrasound echo is shifted in a magnitude proportional to the flow rate. The flow rate profile signal of the fluid  22  measured by the flow rate profile measurement circuit  12  is transmitted to the computer  31  and the flow rate profile signal can be integrated in the radius direction of the piping  21 , thereby calculating the flow rate of the fluid  22 . The flow rate “m(t)” of the fluid  22  at time “t” can be represented by the following mathematical expression (1): 
 
 m ( t )=ρ∫ v ( x·t )· dA    (1), 
 
 where “ρ” represents the density of the fluid, “v(x·t)” represents the velocity component (in direction “x”) at time “t” and “A” represents the sectional area of the piping. 
 
         [0011]     The above flow rate m(t) can also be calculated by the following mathematical expression (2): 
 
 m ( t )ρ∫∫ vx ( r·θ·t ) ·r·dr·dθ   (2), 
 
 where “vx(r·θ·t)” represents the velocity component at time “t” from the center on the cross section of the piping axis direction for distance “r” and angle “θ.”
 
         [0012]     To accurately determine the flow rate of the fluid  22  in both the steady state and the non-steady state by the above-described conventional Doppler ultrasound flow meter, the flow rate profile of the fluid  22  in the piping  21  must be detected accurately. As can be seen from the above-described measurement mechanism, the flow rate profile of the fluid  22  is obtained by subjecting the ultrasound echo off the reflectors in the fluid  22  to signal processing for calculation. For this reason, this ultrasound echo must contain only an acoustic signal. The acoustic and electric noise components must be eliminated.  
         [0013]     Acoustic noise having an influence on this ultrasound echo includes for example that caused by the reflection or scattering between the mediums having different acoustic impedances and that caused by longitudinal and shear waves generated in solid matter (e.g., piping material). Solid matter (e.g., metal) generally includes therein two types of acoustic waves. One is called a compressional wave, a longitudinal wave having a displacement in the same direction as the direction along which a wave propagates, and the other is called a shear wave, a shear wave having a displacement in the direction perpendicular to the direction along which the wave propagates.  
         [0014]     According to a publication entitled INTRODUCTION TO ELECTRIC ACOUSTIC ENGINEERING by SHOKODO Co., Ltd., pp. 247-251, when an acoustic wave is transmitted from a fluid into a solid matter in an oblique direction, the solid matter includes therein not only a longitudinal wave but also a shear wave. It is generally known that, when an acoustic wave propagates from one type of solid to another type of solid, then both the longitudinal and shear waves are caused along both the direction along which the acoustic wave is transmitted and the direction along which the acoustic wave is reflected.  
         [0015]     How an ultrasound echo is influenced by a longitudinal wave and a shear wave in solid matter will be described with respect to  FIG. 8 . As shown in  FIG. 8 , an acoustic wave propagates from medium  1  to medium  2 . In this case, the relation between a propagation angle θ in  (incidence angle at the interface between both mediums) and an angle θ out  (refraction angle or output angle at the boundary between both mediums) of the acoustic wave in mediums  1  and  2  can be expressed by the following mathematical expression (3): 
 
sin θ in   /c   1 =sin θ out   /c   2    (3), 
 
 where “c 1 ” represents the acoustic velocity in medium  1 , “c 2 ” represents the acoustic velocity in medium  2 , “θ in ” represents an angle at medium  1 (incidence angle), and “θ out ” represents an angle at medium  2  (refraction angle). 
 
         [0016]     When the acoustic wave from medium  1  is incident on medium  2  and the acoustic velocity c 2  in medium  2  is higher than acoustic velocity c 1  in medium  1  (c 1 &lt;c 2 ), there is a critical angle at which the acoustic wave is totally reflected at the interface between these mediums. This critical angle θ c  is represented by the following mathematical expression (4): 
 
θ c =sin −2 ( c   1   /c   2 )   (4), 
 
 where “c 1 ” represents the acoustic velocity in medium  1 , and “c 2 ” represents the acoustic velocity in medium  2 , and c 1 &lt;c 2 . 
 
         [0017]     The following section will describe the oblique angle of the ultrasound transducer  16  of the conventional Doppler ultrasound current profiler shown in  FIG. 6  (incidence angle of ultrasound to piping  21 ) in accordance with a publication (hereafter Publication 2) entitled DEVELOPMENT OF FLOW MEASUREMENT METHOD USING ULTRASONIC VELOCITY PROFILER (UVP) (6) NIST (US.), CALIBRATION: FLOW MEASUREMENT USING LOOPS—TEST RESULTS AND PRECISION VERIFICATION,” by Atomic Energy Society of Japan, 1999 autumn convention, 2001, and a publication (hereafter Publication 3) entitled DEVELOPMENT OF A NOVEL FLOW METERING SYSTEM USING ULTRASONIC VELOCITY PROFILE MEASUREMENT, Experiments in Fluids, vol. 32, 2003, pp. 153-160.  
         [0018]     Publication 2 describes an example in which a so-called clamp-on type Doppler ultrasound current profiler is provided at an outer wall of a stainless piping for measuring the fluid flow rate. In this example, the ultrasound transducer has an oblique angle of 5 or 10 degrees. Publication 3 describes that an ultrasound transducer driven with a frequency of 1 MHz to the piping at an oblique angle of 5 degrees while an ultrasound transducer driven with a frequency of 4 MHz to the piping at an oblique angle of 0 to 20 degrees, and also describes that the ultrasound transducer and the piping have therebetween an acrylic member having a thickness of 2 mm to be used as a wedge.  
         [0019]      FIG. 9  shows the structure in accordance with the measurement conditions described in Publication 3. Here, an acrylic wedge  42  is fixed with an ultrasound transducer  41  such that this ultrasound transducer  41  is inclined, at an angle θ in , relative to a direction perpendicular to the longitudinal direction of a piping  43 . Specifically, ultrasound from the wedge  42  to the piping  43  has an incidence angle of θ in . According to Publications 2 and 3, the fluid  44  (as shown in  FIG. 9 ) is water, while the piping  43  is made of stainless steel. The velocity of sound in water is about 1,500 m/s, while the velocity of the longitudinal wave in stainless steel is about 5,750 m/s and the velocity of the shear wave in stainless steel is about 3,206 m/s. The velocity of the longitudinal wave in acrylic is 2,730 m/s.  
         [0020]     The critical angles θ c  of the longitudinal wave and the shear wave are calculated based on the above-described mathematical expression 4. The critical angle of the longitudinal wave at the interface between the wedge  42  and the piping  43  is 28.3 degrees, and the critical angle of the shear wave at the interface between the wedge  42  and the piping  43  is 58.4 degrees. When the ultrasound transducer  41  transmits an acoustic wave having an oblique angle (incidence angle) θ in  of 20 degrees for example, the wedge  42  and the piping  43 , both of which are solids, have a longitudinal wave and a shear wave at the interface therebetween. The incidence angle θ in  at the above interface is equal to or lower than the critical angles of both of the longitudinal wave and the shear wave. Thus, the piping  43  has therein the propagation of both of the longitudinal wave and the shear wave.  
         [0021]     Furthermore, the longitudinal wave and the shear wave propagating in the piping  43  are transmitted into water while being refracted. This causes two measurement lines ML. In the piping  43  shown in  FIG. 9 , the longitudinal wave has a refraction angle (output angle) θ pl  of 46.1 degrees while the shear wave has a refraction angle θ ps  of 23.7 degrees. When the acoustic wave is transmitted from the piping  43  into water, the acoustic wave is converted to a longitudinal wave and the refraction angle θ fl  in water is 10.84 degrees. A publication (hereafter Publication 4) entitled ULTRASONICS MANUAL, Editorial Committee of the Ultrasonics Manual, Maruzen Co., Ltd., discloses the transmission rate of the acoustic wave when, as in the above case, the acoustic wave is transmitted from metal into water. Here, medium  1  is made of aluminum while medium  2  is water (referring to  FIG. 8 ).  
         [0022]      FIG. 10  illustrates the relation shown in Publication 4 when an aluminum plate corresponding to the medium  1  and water corresponding to the medium  2  have a shear wave at the interface therebetween, between the incidence angle and the energy reflection coefficient (reflectivity) and the energy transmission coefficient (transmission rate). In  FIG. 10 , the wave “SV” represents a shear wave and the wave “L” represents a longitudinal wave. As can be seen from  FIG. 10 , total reflection does not occur and the longitudinal wave is transmitted even when the incidence angle of the shear wave exceeds 28 degrees.  FIG. 11  shows the relation, when the aluminum plate and water have a longitudinal wave at the interface therebetween, between the incidence angle and the reflectivity and the transmission rate. As can be seen from  FIG. 11 , only the longitudinal wave is transmitted.  
         [0023]     Next,  FIG. 12  shows the behavior of the ultrasound echo in the structure of  FIG. 9 . The ultrasound echo from the reflector in water returns from water to the ultrasound transducer  41  via the same route as that through which ultrasound is transmitted from the piping  43  into water. The ultrasound echo has an incidence angle θ f  of 10.84 degrees when the ultrasound is transmitted from water into the aluminum piping  43 . Thus, both the longitudinal wave and the shear wave are generated, as can be seen from  FIG. 12 .  
         [0024]     As shown in  FIG. 12 , when the ultrasound echo is transmitted from water into the piping  43 , two measurement lines of the longitudinal wave and the shear wave are generated, and thus, the piping  43  has therein four ultrasound echoes. When the ultrasound echo is transmitted from the piping  43  into the wedge  42 , the acoustic wave has refraction in accordance with mathematical expression  3  but there is no critical angle because the wedge  42  is made of a material having a lower acoustic velocity than that of the piping  43 . As a result, no total reflection occurs and four ultrasound echoes progress in the wedge  42  in the direction of the ultrasound transducer  41 . Thus, the four ultrasound echoes propagating in the wedge  42  are transmitted into the ultrasound transducer  41  with a time difference in accordance with the acoustic velocity of the ultrasound transducer  41  in the propagation route.  
         [0025]     In  FIG. 12 , “θ pl ” represents the refraction angle of the longitudinal wave at the interface between the fluid (water)  44  and the piping  43 , “θ ps ” represents the refraction angle of the shear wave, “θ wl ” represents the refraction angle of the longitudinal wave at the interface between the piping  43  and the wedge  42 , and “θ ws ” represents the refraction angle of the shear wave. The ultrasound echo received by the ultrasound transducer  41  has a time axis corresponding to the position along the direction of the diameter of the piping  43 . The longitudinal wave and the shear wave in the piping  43  have different acoustic velocities. Thus, the ultrasound echo received by the ultrasound transducer  41  at a specific time is obtained by synthesizing the flow rate at point “A′” of the fluid  44  in the piping  43  measured by the shear wave in  FIG. 13  with the flow rate at point “A′” (which is at a different position from that of point “A” along the direction of the diameter of the piping  43 ) of the fluid  44  in the piping  43  measured by the longitudinal wave.  
         [0026]     Specifically, as schematically shown in  FIG. 14 , the flow rate calculated based on the ultrasound echo received by the ultrasound transducer  41  at a specific time is actually obtained by synthesizing the flow rates at point “A” and point “A′” (which are at different positions), and thus, the flow rate profile and consequently the flow rate of the fluid  44  in the piping  43  cannot be measured accurately.  
         [0027]     As described above, the Doppler ultrasound flow meter for calculating the flow rate by measuring the flow rate profile in the piping has a problem in that an acoustic wave transmitted from the ultrasound transducer generates a longitudinal wave and a shear wave in a piping and the two measurement lines are transmitted into the fluid, which causes the ultrasound echoes from the respective reflectors to be received by the Doppler ultrasound flow meter, thus causing the flow rate profile to be measured inaccurately.  
         [0028]     In view of the above problem discovered by the present inventors, there remains a need to provide an apparatus and a method for more accurately measuring the fluid flow rate profile and the fluid flow rate. The present invention addresses this need. Specifically, the above noted problems can be solved by eliminating or isolating, from the ultrasound echoes caused by two measurement lines of a longitudinal wave and a shear wave propagating in the tubular body (e.g., piping), the ultrasound echo caused by the longitudinal wave.  
       SUMMARY OF THE INVENTION  
       [0029]     The present invention relates to an apparatus and a method for measuring a fluid flow rate profile using a Doppler effect.  
         [0030]     One aspect of the present invention is the apparatus, such as a clamp-on type acoustic Doppler current profiler, for measuring the flow rate profile of fluid traveling through a tubular body, based on the frequency of ultrasound changed by the Doppler effect when ultrasound is reflected off reflectors existing in the fluid. The tubular body is made of material that allows an acoustic wave to propagate therethrough. The profiler includes a wedge that can be externally mounted to the tubular body and an ultrasound transducer mounted to the wedge. The wedge also is made of material that allows an acoustic wave to propagate therethrough. The ultrasonic transducer is fixed to the wedge at an inclination relative to the direction in which the fluid travels through the tubular body such that the ultrasound transducer receives only the ultrasound echo from the reflection of a shear wave component off the reflectors in the fluid.  
         [0031]     The inclination is such that when the velocities of a longitudinal wave and a shear wave of the ultrasound propagating through the tubular body are equal to or higher than the velocity of the longitudinal wave propagating through the wedge, the incidence angle of the ultrasound transmitted from the wedge into the tubular body is equal to or higher than the critical angle of the longitudinal wave that is determined by the velocity of the longitudinal wave propagating through the wedge and the velocity of the longitudinal wave propagating through the tubular body, and is equal to or lower than the critical angle of the shear wave that is determined by the velocity of the longitudinal wave propagating through the wedge and the velocity of the shear wave propagating through the tubular body.  
         [0032]     The inclination also can be such that when the velocities of a longitudinal wave and a shear wave of the ultrasound propagating through the tubular body are equal to or higher than the acoustic velocity in the fluid, the incidence angle of the ultrasound transmitted from the tubular body into the fluid is equal to or higher than the critical angle of the longitudinal wave that is determined by the acoustic velocity propagating through the fluid and the velocity of the longitudinal wave propagating through the tubular body, and is equal to or lower than the critical angle of the shear wave that is determined by the acoustic velocity propagating through the fluid and the velocity of the shear wave propagating through the tubular body.  
         [0033]     The wedge can be made of a resin or metal. The tubular body also can be made of a metal or resin. The resin for the wedge can be composed of any of acrylic, epoxy resin, polyvinyl chloride, and polyphenylene sulfide. The resin for the tubular body can be composed of any of polyvinyl chloride, acrylic, FRP, polyethylene, polytetrafluoroethylene, tar epoxy, and mortar. The metal for the wedge can be any of iron, steel, cast iron, stainless, copper, lead, aluminum, and brass. The metal for the tubular body can be any of iron, steel, ductile cast iron, cast iron, stainless, copper, lead, aluminum, and brass.  
         [0034]     The inclination is such that an incidence angle of ultrasound pulse at the interface between the wedge and the tubular body is 45 degrees.  
         [0035]     Another aspect of the present invention is the method of measuring a flow rate profile of fluid, traveling in the tubular body, based on the frequency of ultrasound changed by the Doppler effect when ultrasound is reflected by the reflectors existing in the fluid. The method can comprise the steps of mounting externally on the tubular body, the wedge previously mentioned, mounting the ultrasound transducer previously mentioned on the wedge at the inclination mentioned previously relative to the direction in which the fluid travels through the tubular body such that the ultrasound transducer only receives the ultrasound echo from the reflection of a shear wave component off the reflectors in the fluid. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]      FIG. 1  shows the structure of the main part illustrating the first embodiment of the present invention.  
         [0037]      FIG. 2  shows the relation shown in Publication 5 between the incidence angle of the ultrasound from water into the piping and the energy transmission coefficients of the longitudinal wave and the shear wave in the piping.  
         [0038]      FIG. 3  shows an example in which the flow rate to the position along the diameter direction of the piping is measured when the oblique angle of the ultrasound transducer is 15 degrees.  
         [0039]      FIG. 4  shows an example in which the flow rate to the position along the diameter direction of the piping is measured when the oblique angle of the ultrasound transducer is 45 degrees.  
         [0040]      FIG. 5  shows the comparison, with regards to the measurement errors of the flow rate output, between the first embodiment of the present invention and a conventional technique.  
         [0041]      FIG. 6  shows a conventional clamp-on type ultrasound Doppler current flow meter.  
         [0042]      FIG. 7  shows sections (A), (B), and (C) that illustrate the mechanism through which a Doppler ultrasound flow meter operates.  
         [0043]      FIG. 8  shows the propagation status of an acoustic wave when the acoustic wave propagates in different mediums.  
         [0044]      FIG. 9  shows the measurement conditions shown in Publications 2 and 3.  
         [0045]      FIG. 10  shows the relationship, shown in Publication 4, between the incidence angle and the transmission rate and the reflectivity.  
         [0046]      FIG. 11  shows the relationship, shown in Publication 4, between the incidence angle and the transmission rate and the reflectivity.  
         [0047]      FIG. 12  shows the behavior of the ultrasound echo in  FIG. 9 .  
         [0048]      FIG. 13  shows a further enlarged view of  FIG. 9 .  
         [0049]      FIG. 14  shows the flow rate profile for explaining the objective of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0050]      FIG. 1  shows the main part of the first embodiment. Although the structure of  FIG. 1  include substantially the same components as included in  FIG. 9 ,  FIG. 12 ,  FIG. 13 , etc., different reference numerals are used, for clarity. Reference numeral  51  denotes an ultrasound transducer, which generates an acoustic wave. This ultrasound oscillator or transducer  51  can be made of a piezoelectric material, such as PZT (e.g., zircon, lead titanate) and operates both as an ultrasound transmitter/receiver. Reference numeral  52  denotes a wedge made of a resin material in which an acoustic wave can propagate (e.g., acrylic, epoxy resin, polyvinyl chloride, polyphenylene sulfide). The wedge  52  has an inclined plane  52   a  at the upper end thereof, and the ultrasound transducer  51  can be fixed to the inclined plane  52   a  with an epoxy adhesive agent or the like. The inclined plane  52   a  is inclined such that the oblique angle of the ultrasound transducer  51  to the direction perpendicular to the longitudinal direction of the piping  53  (incidence angle of the ultrasound pulse at the interface between the wedge  52  and the piping  53 ) is equal to θ in . Reference numeral  54  denotes fluid.  
         [0051]     Still referring to  FIG. 1 , the velocity of sound in the piping  53  is higher than that in the wedge  52 , when the wedge  52  is made of acrylic while the piping  53  is made of aluminum, for example, and the fluid  54  is water. The speed of sound in acrylic is about 2,730 m/s, the velocity of the longitudinal wave in aluminum is about 6,420 m/s while the velocity of the shear wave therein is about 3,040 n/s, and the speed of sound in water is about 1,500 m/s. The piping  53  may be made of aluminum or other metal in which an acoustic wave can propagate (e.g., iron, steel, ductile cast iron, stainless steel, copper, lead, brass). There is a critical angle when an ultrasound pulse is transmitted from the wedge  52  into the piping  53  and when an ultrasound pulse is transmitted from the fluid  54  into the piping  53 . As is clear from Snell&#39;s law, the critical angle of any material has the relation as shown in the following mathematical expression (5): 
 
sin θ in   /c   w =sin θ pl   /c   pl =sin θ ps   /c   ps =sin θ f   /c   f    (5), 
 
 where c w  represents the acoustic velocity in the wedge  52 , c pl  represents the acoustic velocity of the longitudinal wave in the piping  53 , c ps  represents the acoustic velocity of the shear wave in the piping  53 , c f  represents the acoustic velocity in the fluid  54 , θ in  represents the oblique angle of the acoustic wave in the wedge  52  (incidence angle to piping  53 ), θ pl  represents the angle of the longitudinal wave in the piping  53  (refraction angle), θ ps  represents the angle of the shear wave in the piping  53  (refraction angle), and θ f  represents the incidence angle θ in the fluid  54 . 
 
         [0052]     When the wedge  52  is made of acrylic, the piping  53  is made of aluminum, and the fluid  54  is water, then the critical angle of the longitudinal wave is 25.2 degrees, while the critical angle of the shear wave is 63.9 degrees when ultrasound is transmitted from the wedge  52  into the piping  53 . Thus, when the oblique angle θ in  of the ultrasound transducer  51  (incidence angle at the interface between the wedge  52  and the piping  53 ) is within the above critical angle range (i.e., 25.2 degrees≦θin≦63.9 degrees), only the shear wave propagates in the piping  53  because the longitudinal wave is totally reflected at the interface between the wedge  52  and the piping  53 . As a result, only the ultrasound along one measurement line caused by the shear wave in the piping  53  is transmitted into water. Subsequently, only the ultrasound echo from the reflectors in water reflected by the shear wave component is received. Specifically, the ultrasound transducer  51  does not receive the ultrasound echo caused by the longitudinal wave, thus reducing the acoustic noise included in the measured flow rate. This improves the measurement accuracy of the flow rate profile and enables the flow rate to be calculated with a higher accuracy.  
         [0053]     Next, an example will be specifically described in which the wedge  52  shown in  FIG. 1  produces an acoustic wave of an incidence angle θ in  of 45 degrees. When the acoustic wave propagates from the wedge  52  into the aluminum piping  53 , the above incidence angle θ in  exceeds 25.2 degrees (which is the critical angle of the longitudinal wave). Thus, the longitudinal wave is totally reflected at the interface between the wedge  52  and the piping  53 , and does not propagate in the piping  53 . On the other hand, the shear wave propagates in the piping  53  with the refraction angle of 51.9 degrees.  
         [0054]     Next, when the acoustic wave is transmitted from the piping  53  into the fluid  54  (which is water), then only the longitudinal wave exits into the water. As a result, the longitudinal wave propagates in water at a refraction angle (θ fs  in  FIG. 1 ) of 22.8 degrees along one measurement line. The longitudinal wave reflected off the reflector, i.e., the ultrasound echo, is also transmitted into the piping  53  at an incidence angle of 22.8 degrees.  
         [0055]     With regards to the transmission of the acoustic wave from water into the aluminum piping, data is shown in  FIG. 2 , as provided in a publication (hereafter Publication 5) entitled ACOUSTIC WAVE” by Cordon S. Kino.  FIG. 2  shows the relation between the incidence angle of an ultrasound wave from water into the piping and the energy transmission coefficient (transmission rate) of the longitudinal wave and the shear wave in the piping.  
         [0056]     According to  FIG. 2 , the incidence angle to the piping  53  of 22.8 degrees is equal to or higher than the critical angle of the longitudinal wave, and thus, the longitudinal wave is totally reflected at the interface between water and the piping  53 . Specifically, the longitudinal wave does not propagate in the piping  53 . Thus, the piping  53  has therein only one measurement line of the ultrasound echo produced by the shear wave and the ultrasound transducer  51  receives the ultrasound echo of this shear wave, thus reducing the conventional acoustic noise caused by the longitudinal wave.  
         [0057]     As described above, the measurement accuracy of the flow rate profile can be improved over conventional cases by improving the oblique angle of the ultrasound transducer  51  (incidence angle to the piping  53 ) to eliminate the longitudinal wave in the piping  53 .  
         [0058]      FIGS. 3 and 4  show examples in which the flow rate to the position along the diameter direction of the piping  53  is measured when the oblique angle of the ultrasound transducer  51  is 15 degrees ( FIG. 3 ) and when the oblique angle of the ultrasound transducer  51  is 45 degrees ( FIG. 4 ). When the oblique angle of the ultrasound transducer  51  is set at 45 degrees, which is equal to or higher than the critical angle of the longitudinal wave, when the ultrasound is transmitted from the wedge  52  into the piping  53  (25.2 degrees) at an angle that is equal to or lower than the critical angle of the shear wave (63.9 degrees), then appropriate measurement values as shown in  FIG. 4  are obtained according to which the flow rate is continuously changed depending on the position along the diameter direction. When the oblique angle is set at 15 degrees, the piping  53  has therein both the longitudinal wave and the shear wave, and thus the ultrasound echo is received by the ultrasound transducer  51 . Because the ultrasound echo includes a large amount of acoustic noise, the measurement values of flow rate profile becomes unstable, which deteriorates the measurement accuracy.  
         [0059]      FIG. 5  shows the comparison, with regards to the measurement errors that occurred when the flow rate output of an electromagnetic flow meter was measured based on the flow rate profile, between a case in which the oblique angle of the ultrasound transducer  51  is similarly provided to be 45 degrees according to this embodiment, and a case in which the oblique angle of the ultrasound transducer  51  is provided to be 15 degrees, as in conventional cases. As can be seen from  FIG. 5 , this embodiment also significantly improves the measurement errors when compared to conventional cases.  
         [0060]     In the second embodiment of the present invention, only the longitudinal wave element of the ultrasound echo propagating in the piping  53  after being reflected by the reflector in the fluid  54  is eliminated. It is assumed that, when the fluid  54  is water, for example, the critical angle of the longitudinal wave in the ultrasound echo transmitted into the aluminum piping  53  after being reflected by the reflector in water is 13.5 degrees while the critical angle of the shear wave is 29.6 degrees when the acoustic wave in water is 1500 m/s.  
         [0061]     Thus, when the acoustic wave from the piping  53  into water has an incidence angle that is equal to or higher than 13.5 degrees and that is equal to or lower than 29.6 degrees, then only the shear wave element is transmitted into the piping  53  and the longitudinal wave element is eliminated when the ultrasound echo is transmitted from water into the piping  53 , thus reducing the acoustic noise caused by the longitudinal wave. As a result, the ultrasound transducer  51  receives only the ultrasound echo of the shear wave in the piping  53 , and this allows the piping  53  to have reduced acoustic noise caused by the longitudinal wave, provides the measurement of a flow rate profile with a higher accuracy, and improves the accuracy of the measurement of a flow meter.  
         [0062]     The wedge also can be made of a metal in which an acoustic wave can propagate (e.g., iron, steel, cast iron, stainless steel, copper, lead, aluminum, brass), and the piping may be made of a resin in which an acoustic wave can propagate (e.g., polyvinyl chloride, acrylic, FRP, polyethylene, polytetrafluoroethylene (also known as Teflon®), tar epoxy, mortar).  
         [0063]     According to the present invention, The longitudinal wave element of the ultrasound that is transmitted from an ultrasound transducer and that propagates in the tubular body or from the wedge to the tubular body can be eliminated. Thus, the fluid has therein only ultrasound along one measurement line, caused by the shear wave in the tubular body. As a result, only the ultrasound echo caused by reflection of the shear wave off the reflector in the fluid appears. Thus, the ultrasound echo caused by the longitudinal wave is not received by the ultrasound transducer, thus reducing the acoustic noise.  
         [0064]     Given the disclosure of the present invention, one versed in the art would appreciate that there may be other embodiments and modifications within the scope and spirit of the present invention. Accordingly, all modifications and equivalents attainable by one versed in the art from the present disclosure within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention accordingly is to be defined as set forth in the appended claims.  
         [0065]     The disclosure of the priority applications, JP 2003-396755, in its entirety, including the drawings, claims, and the specifications thereof, is incorporated herein by reference.