METHOD AND APPARATUS FOR MEASURING THE VELOCITY OF A FLUID

Abstract: A method and apparatus for measuring the velocity of a fluid in which a double-exposure image (2) of the speckle pattern or particle images of tracer particles (7a) in a fluid (7) is irradiated with a collimated beam (3) of coherent light, and is subjected to Fourier transformation (4) to produce Young's fringes (6) at its focal plane. A slit (10, 13) is rotated relatively with respect to the double-exposure image (2) to produce changes in the amount of light transmitted by the slit and determine the direction of the fringes from the changes thus produced. To determine the spacing of the fringes, a variable-spacing grating filter (20, 22) is used which is complementary to the image of the fringes and has a transmission distribution function of sin² or cos². The thus obtained direction and spacing of the fringes are used to determine the velocity of the fluid.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method and apparatus for measuring the velocity of a fluid, and more particularly to a method and apparatus for measuring the velocity of a fluid in which a double-exposure image of the speckle pattern or particle images of tracer particles in a fluid is irradiated with a collimated beam of coherent light, and is subjected to Fourier transformation through a lens to produce at its focal plane Young's fringes which are analyzed to determine its spacing and direction in order to measure the velocity of the fluid. Description of the Prior Art Methods of measuring the two-dimensional velocity component of a fluid include the laser speckle method described by T. D. Dudderar and P. G. Simpkins in "Laser speckle photography in a fluid medium," Nature Vol. 270 3 page 45 (November 1977), and the white-light speckle method described by E. Bernabeu, J. C. Amaré and M. P. Arroyo in "White-light speckle method of measurement of flow velocity distribution," Applied Optics Vol. 21 No. 14, 2583 (1982). These methods utilize the fact that when a fluid is irradiated by a beam of light, the speckle pattern produced by particles in the fluid or the particle image pattern exhibits a translational motion. The working principle of these methods is to record a speckle or particle image pattern from a fluid flow as a double-exposure image, and the amount of translational movement of the pattern is detected quantitatively by optical processing, using a laser beam. In this case, the shift vector of the scattered wave will correspond to the flow velocity. A pulsed laser light source or a chopped continuous light source is used to record the double-exposure image, and tracer particles are mixed into the fluid to scatter the light. The fluid containing these scatterers is then irradiated twice by a pulsed beam, and movement of an image of the particles themselves or movement of the speckle pattern is photographed to produce a double-exposure image. The Young's fringes method is one method used for optical processing of the double-exposure image. When a double-exposure image is irradiated by a collimated beam of laser light, these fringes are produced in the diffracted image of the light passing through a lens and can be observed at the focal plane of the lens. The fringes include information on the velocity of the pattern irradiated by the laser beam and the direction of the normal of the fringes shows the direction of the velocity vector. If the spacing between fringes is d, the wavelength of the irradiating light is λ, the focal length of the lens is f, the magnification at which the double-exposure image is photographed is M and the interval between light pulses is T, then the velocity v of a fluid will satisfy the following. v = (fλ)/(MdT). The flow velocity can be obtained by optically processing the double-exposure image and analyzing the fringes. Using manual methods to find the spacing and direction of the fringes derived from each point of the double-exposure image is extremely time-consuming and inefficient. One recent solution is to display the fringes on a screen, use a TV camera to record the fringes on the screen and find the velocity using a method based on image-processing techniques. This is described in "Automatic fringe analysis with a computer image-processing system," by D. W. Robinson, Applied Optics, Vol. 22, No. 14, 2169 (1983). However, image processing increases the complexity of the system, establishing the fluid flow velocity requires considerable time, and overall, the system becomes costly. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and apparatus for measuring the velocity of a fluid which uses the fact that fringes appear as a periodic series of bands in one dimension and which is capable of determining the velocity vector of a fluid, that is, the flow direction and speed of the fluid rapidly and easily. According to the invention, there is provided a method of measuring the velocity of a fluid, in which a double-exposure image of the speckle pattern or particle images of tracer particles in a fluid is irradiated with a collimated beam of coherent light, and is subjected to Fourier transformation through a lens to produce at its focal plane Young's fringes which are analyzed to determine their spacing and direction in order to measure the velocity of the fluid, characterised in that a slit is disposed at the focal plane of the lens and relative rotation is effected between the slit and the Young's fringes to produce changes in the amount of light transmitted through the slit and determine the direction of the fringes from the changes thus produced, and the thus determined direction of the fringes being used to measure the direction of the fluid velocity, disposing a variable-spacing grating filter which is complementary to the image of the fringes and has a transmission distribution function of sin² or cos² at the focal plane of the lens, aligning the variable-spacing grating filter with the direction of the fringes, and changing the grating spacing of the filter to produce changes in the amount of light transmitted by the filter and determine the spacing of the fringes from the changes thus produced, the thus determined spacing of the fringes being used to measure the value of the fluid velocity. The present invention further comprises an apparatus for measuring the velocity of a fluid, in which a double exposure image of the speckle pattern or particle images of tracer particles in a fluid is irradiated with a collimated beam of coherent light, and is subjected to Fourier transformation through a lens to produce at its focal plane Young's fringes which are analyzed to determine their spacing and direction in order to measure the velocity of the fluid, comprising: a transparent medium for recording a double-exposure image; means for irradiating the recording medium with a collimated beam of laser light; a lens for Fourier transformation of the double-exposure image to produce Young's fringes at its focal plane; a slit disposed at the focal plane of the lens; means for generating relative rotation between the slit and the Young's fringes to determine the fringe direction; a variable-spacing grating filter which is disposed at the focal plane of the lens and has a transmission distribution function sin² or cos² which is complementary to the image of the fringes, and means for aligning the variable-spacing grating filter parallel to the Young's fringes; the spacing of the fringes being determined from changes in the amount of light transmitted by the filter relative to changes in the grating spacing of the filter, wherein the measured direction and spacing of the fringes are used to determine the direction and value of the fluid velocity of the fluid. Thus, the invention uses the fact that fringes appear as a periodic series of bands in a one-dimensional direction and enables the velocity vector of a fluid, that is, the flow direction and speed of a fluid to be determined rapidly and easily. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 shows the principle behind the formation of Young's fringes; Figure 2 shows how a double-exposure image is formed; Figure 3 shows an arrangement for measuring the direction of Young's fringes; Figure 4 is a graph showing slit light amount transmission characteristics; Figure 5 shows another arrangement for measuring the direction of Young's fringes; Figure 6 is a graph showing light transmission amounts plotted against filter grating spacing; Figure 7 shows an arrangement for measuring the spacing of Young's fringes; Figure 8 shows another arrangement for measuring the spacing of Young's fringes; Figure 9 shows a further arrangement for measuring the spacing of Young's fringes; Figure 10 shows an arrangement for measuring both the direction and the spacing of Young's fringes; and Figure 11 is a graph showing light transmission amounts plotted against filter grating spacing; DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows the principle behind the formation of Young's fringes. In Figure 1, reference numeral 1 denotes a multi-exposure film (although reference is to double exposure throughout the description, the invention is not limited thereto). A double-exposure image 2 on the film 1 is irradiated by a collimated laser beam 3 using an optical system (not illustrated), and is imaged by a Fourier transformation lens 4 into an image on a screen 5 to produce Young's fringes 6. The screen 5 is disposed at the focal plane of the Fourier transformation lens 4, that is, at a Fourier transformation plane. As shown in Figure 2, the double-exposure image 2 is formed by using a pulsed light to twice irradiate a fluid 7 which includes tracer particles 7a. Light scattered by the particles is focussed via a lens 8 onto the film 1 which receives a double exposure. The double-exposure image is recorded on the film using a pulsed laser beam or a chopped beam from a continuous light source. When a laser beam is used the double-exposure image is produced by movement of a speckle pattern which can be produced by the movement of the particles. When white light is used the double-exposure image is produced by the movement of the particles themselves. The Young's fringes 6 produced at the Fourier transformation plane of the double-exposure image thus formed include velocity information relating to the object irradiated by the laser beam, which in this case is the fluid 7. The arrow X normal to the Young's fringes 6 indicates the direction of the fluid's velocity vector. It is well known that if the spacing between fringes is d, the wavelength of the irradiating light is λ, the focal length of the lens 4 is f, the magnification at which the double-exposure image 2 is photographed is M and the emission timing of the light source is T, then the velocity v of a fluid will be v = (fλ)/(MdT) It is also known that the intensity distribution of Young's fringes 6 assumes a cos² function. Figure 3 shows an embodiment for establishing the direction of the Young's fringes, that is, the direction perpendicular to the direction X of the velocity vector. Here a rotating slit plate 10 with a slit 10a is disposed in the Fourier transformation plane. Figure 4 shows the characteristics obtained as a function of the angle of rotation if the light passing through the slit 10a is focussed by a focussing lens 11 and the amount of transmitted light is measured by a photosensor 12. Here, 90 degrees is when the normal direction X of the Young's fringes 6 coincide with the radial direction of the rotating slit 10a. As shown in Figure 4, the amount of transmitted light is at a maximum when the long dimension S of the slit coincides with the Young's fringes orientation Y. Thus, determining the angle of slit rotation that produces the maximum light transmission amount enables the Y and X (normal) orientations of the fringes to be ascertained. Figure 5 shows a second embodiment of an arrangement for obtaining the direction of Young's fringes. This embodiment uses a fixed slit plate 13 provided with a slit 13a, and a Dove prism 14 is provided between the double-exposure image 2 and the Fourier transformation lens 4, the Dove prism 14 being rotatable about the optical axis. Rotating the Dove prism 14 also rotates the double-exposure image 2. The Dove prism can rotate the image by an amount of twice the angle of its own rotation. Thus, only the half angle is needed to detect the direction of the Young's fringes. In this method, the direction of the fringes can be determined by rotating the slit relative to the fringes. Therefore, as in Figure 4, the direction of the fringes can be determined by establishing the angle of rotation of the Dove prism at which light passing through the slit reaches a maximum. Embodiments for measuring the spacing of the fringes after obtaining the direction of the fringes will now be described with reference to Figures 6 to 9. For the Young's fringes generated as explained above, it is well known that the intensity distribution of the fringes takes a squared cosine (cos²) function. When, therefore, a grating filter having a cos² or sin² function intensity distribution that is complementary to the distribution of the Young's fringes is disposed at the focal plane of the Fourier transformation lens 4, the amount of light transmitted through the filter will reach a minimum when the spacing of the fringes is the same as the spacing of the filter grating. Figure 6 shows numerically-computed light transmission amount characteristics. For calculation, filter transmission characteristics are approximated to a sin² function to perform numerical integration. In Figure 6, the horizontal axis f is f = (spacing of grating filter)/(spacing of Young's fringes). Figure 6 shows that by continuously varying the spacing of the filter grating, it can be inferred that the complementary grating spacing at which the amount of light transmitted reaches a minimum will be the same as the spacing of the Young's fringes. Figure 7 shows an arrangement for implementing this. In this embodiment, a light absorbing grating filter constituted by a transmission type liquid-crystal display element is provided at the Fourier transformation plane. This filter 20 has a plurality of variable-spacing gratings 21, so that by varying the spacing of the gratings 21 while using the photosensor 12 to measure the amount of light that is passing through the grating, the grating spacing at which the light amount is at a minimum can be taken as the spacing of the Young's fringes. In the embodiment shown in Figure 8, the light absorbing filter has been replaced by a combination of a continuously-changing-pitch filter 22 and a slit 23. The spacing of the grating at the slit 23 can be changed by moving the filter 22 in the direction F, which changes the amount of incident light at the photosensor 12. The black spot 24 shown in Figures 7 and 8 is provided to eliminate the focusing beam of the irradiating collimated laser beam. The embodiment shown in Figure 9 uses a Fourier transformation lens 25 with a variable focal length to form the Young's fringes of the double-exposure image on the grating filter 26. In this example the grating filter 26 has a fixed spacing. As in the examples shown in Figures 7 and 8, the system is arranged so that the displacement direction recorded in the double-exposure image 2 takes place in direction A and the focal point of the Fourier transformation lens 25 changes in direction B. The photosensor 12 is used to detect the amount of light passing through the grating filter 26 via the focussing lens 11 and to find the focal length at which the amount of light is at a minimum. The spacing of the Young's fringes can thus be obtained from equation (1). In the examples described above, finding the direction of the Young's fringes is carried out separately from finding the spacing of the fringes. Figure 10, however, shows an embodiment of a single apparatus which can be used to obtain both fringe direction and fringe spacing. It is assumed that the arrangement will be used to measure first the direction, followed by the spacing. In the apparatus shown in Figure 10, the light passes through the double-exposure image 2 and the Fourier transformation lens 4 and is split by a beam splitter 32 so that one part goes to a fringe spacing measurement apparatus 30 and the other part to a fringe direction measurement apparatus 31. Each of the apparatuses 30 and 31 is provided at a Fourier transformation plane. The fringe spacing measurement apparatus 30 is constituted by a line sensor 33 which can be rotated about the optical axis, while the fringe direction measurement apparatus 31 is comprised of a rotating slit plate 34 with a slit 34a focussing lens 35, and a photosensor 36. The focussing lens 35 serves to focus the light onto the photosensor 36. Here, too, the direction of the Young's fringes is determined by rotating the slit 34 to find the angle at which the amount of light passing through the slit 34a reaches a maximum. The axis of the line sensor 33 is then aligned with the direction of the fringes thus obtained. When the line sensor has been positioned, the fringe spacing can be measured. Although in Figure 10 a line sensor is used as the fringe spacing measurement apparatus 30, the spacing may of course be measured using a filter with complementary transmission characteristics to the Young's fringes. In the above method the fringe spacing measurement apparatus 30 is rotated, but it is also possible to use a Dove prism to rotate the fringe images and omit the two rotating mechanisms at the Fourier transformation plane. In the embodiments above described, the transmission distribution of the grating filters has been described as having a sin² or cos² function. It is, however, appreciated that the same effects can be obtained by a grating filter having an exponent of the above function. A necessary condition for the grating filter is, therefore, that its transmission distribution follows a function having a periodicity which are opposite in phase and complementary to the Young's fringes. Taking the detection sensitivity into account, however, it will be recommended to use the grating filter having the periodicity of a sin² or cos² function. It will be noted that the graph as shown in Figure 6 is shown for the Young's fringes which extend infinitely. Actually, the fringes are finite, so that a too great parameter f causes the grating fringes to cover the finite Young's fringes with the amount of transmitted light being reduced. This is shown in Figure 11. Since many pseudo minimums appear experimentally as shown in Figure 6, it is preferable to detect not a local minimum of the amount of light transmitted through the grating filter, but the smallest of the minimums to determine the spacing of the Young's fringes. In Figure 7, the light absorption grating filter 20 is comprised of a liquid crystal display element, whose picture elements are rectangular or rhomboid and arranged two-dimensionally. It is, however, not necessary to cause any change in the amount of transmitted light in the direction of the Young's fringes, so that a one-dimensionally arranged, simple liquid crystal display element can preferably be used. Such a one-dimensional liquid crystal element makes it easy to improve the resolution in the one-dimensional direction, thus providing an improvement in controlling a transmission factor of the liquid crystal display element. It is, on the other hand, necessary to use a two-dimensionally arranged liquid crystal display element having small-sized picture elements with high resolution, when the rotating slit is constituted by a liquid crystal display element. While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention should not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out the invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims: A method of measuring the velocity of a fluid, in which a double-exposure image (2) of the speckle pattern or particle images of tracer particles (7a) in a fluid is irradiated with a collimated beam of coherent light (3), and is subjected to Fourier transformation through a lens (4) to produce at its focal plane Young's fringes (6) which are analyzed to determine their spacing and direction in order to measure the velocity of the fluid, characterized in that a slit (10,13) is disposed at the focal plane of the lens (4) and relative rotation is effected between the slit (10,13) and the Young's fringes to produce changes in the amount of light transmitted through the slit (10,13) and determine the direction (Y) of the fringes (6) from the changes thus produced, the thus determined direction (Y) of the fringes being used to measure the direction of the fluid velocity, disposing a variable-spacing grating filter (20,22) which is complementary to the image of the fringes and has a transmission distribution function of sin² or cos² at the focal plane of the lens, aligning the variable spacing grating filter (20,21) with the direction (Y) of the fringes, and changing the grating spacing of the filter (20,22) to produce changes in the amount of light transmitted by the filter and determine the spacing of the fringes (6) from the changes thus produced, the thus determined spacing of the fringes being used to measure the value of the fluid velocity. A method according to claim 1, characterised in that the slit (10) is constituted by a liquid-crystal display type element. A method according to claim 1 or 2, characterized in that the Young's fringes are fixed and the slit (10) is rotated. A method according to claim 1 or 2, characterized in that the slit (13) is fixed and the Young's fringes are rotated by a Dove prism (14). A method according to any preceding claim in that the variable-spacing grating filter (20) is constituted by a transmission type liquid-crystal display element. A method according to any of claims 1-4, characterized in that a filter (22) is used which has a grating spacing that is continuously variable along an axis perpendicular to the orientation (F) of the grating, and part of the filter is superimposed on the fringes to produce changes in the amount of transmitted light and determine a complementary fringe pitch from the changes thus produced. An apparatus for measuring the velocity of a fluid, in which a double-exposure image (2) of the speckle pattern or particle images of tracer particles (7a) in a fluid is irradiated with a collimated beam of coherent light (3), and is subjected to Fourier transformation through a lens (4) to produce at its focal plane Young's fringes (6) which are analyzed to determine their spacing and direction in order to measure the velocity of the fluid, comprising: a transparent medium (1) for recording a double-exposure image (2); means for irradiating the recording medium (1) with a collimated beam (3) of laser light; a lens (4) for Fourier transformation of the double-exposure image to produce Young's fringes (6) at its focal plane; a slit (10) disposed at the focal plane of the lens (4); means (31) for generating relative rotation between the slit (10) and the Young's fringes to determine the fringe direction; a variable-spacing grating filter (30) which is disposed at the focal plane of the lens (4) and has a transmission distribution function sin² or cos² which is complementary to the image of the fringes, and means for aligning the variable-spacing grating filter (30) parallel to the Young's fringes; the spacing of the fringes (6) being determined from changes in the amount of light transmitted by the filter relative to changes in the grating spacing of the filter, wherein the measured direction and spacing of the fringes (6) are used to determine the direction of value of the fluid velocity of the fluid. An apparatus according to claim 7, characterized in that light passing through the lens (4) is split by a beam splitter (32), the determination of the direction and spacing of the Young's fringes being effected at different positions. An apparatus according to claim 8, characterized in that the means for generating relative rotation between the slit and the Young's fringes comprise means for rotating the slit (10). An apparatus according to claim 8, characterized in that the slit is a fixed slit (13) and a Dove prism is provided to rotate the Young's fringes. An apparatus according to any of claims 7-10 characterized in that the variable spacing grating filter is replaced by a line sensor (33) provided for measuring the spacing of the Young's fringes, the line sensor (33) being disposed at the focal plane of the lens and provided with a rotation mechanism to enable alignment of the axis of the sensor with the determined direction of the fringes, the spacing of the fringes being measured by measuring the intensity distribution of fringes.

Inventors: ISHIKAWA, MUNEHARU, TANIJI, AYAFUMI
Applicants: KOWA COMPANY LTD.
Application year: 1990
Main classification: G01P3
IPC codes: G01P5