Method and device for measuring flow velocities using in-line particle image velocimetry

Abstract

Method for measuring flow velocities in a spatial measurement area (3) using in-line Particle Image Velocimetry, - wherein at least one illumination optic (6) directs light (4) from at least one light source (5) in a mean illumination direction (15) into the measuring area (3), - wherein the measuring area (3) is imaged onto an image sensor (9) in the middle illumination direction (15) using an imaging optic (8) and - wherein at least one beam trap (7) for the light (4) from the at least one light source (5) is arranged such that the image sensor (9) records dark-field images of the measuring area (3), characterized in that the measuring area (3), into which the light (4) from the at least one light source (5) is directed with the at least one illumination optic (6) and which is imaged onto the image sensor (9) with the imaging optic (8), has a larger cross-sectional area (14) perpendicular to the mean illumination direction (15) than an exit pupil (16) of the at least one illumination optic (6).

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a method for measuring flow velocities in a spatial measurement area using in-line particle image velocimetry. Furthermore, the invention relates to a device for carrying out such a method. More specifically, the invention relates to a method having the features of the preamble of independent patent claim 1 and a device having the features of the preamble of independent patent claim 10. STATE OF THE ART

[0002] EP 2 989 471 B1 discloses a device and a method for optically detecting flow movements in liquid and gaseous media. A measuring volume is defined in the respective medium, and at least one measuring plane is defined within the measuring volume. Particles moving along the measuring plane are optically detected by illuminating and illuminating the measuring plane in at least one detection area from one side of the measuring plane, i.e. from an illumination side, using a light beam comprising parallel light beams from an illumination unit, and by generating scattered light from particles located within the detection area of ​​the measuring plane. The light beam is directed through the detection area of ​​the measuring plane, and its transmitted light component is directed past at least one light-guiding element.Scattered light emitted as pre-light scattering to a pre-light scattering side opposite the illumination side is at least partially guided by the at least one light-guiding element to the image recording plane of a camera. The light-guiding element has a scattered light inlet end facing the detection area of ​​the measuring plane, which is provided with a window element transparent to the scattered light. The illumination unit has a light beam outlet end facing the detection area of ​​the measuring plane and assigned to the scattered light inlet end. The camera records a sequence of images of the detection area, and based on the image sequence, a motion vector is determined for at least one particle in the detection area of ​​the measuring plane by computer-assisted image analysis.

[0003] Regarding the state of the art there, EP 2 989 471 B1 states that a method called a "dark field probe" for measuring a radial groundwater flow velocity in well gauges uses natural suspended solids contained in the groundwater as markers. Since the markers are carried along with the flow movement, their drift maps the flow movement. The suspended solids are illuminated and optically recorded by a CCD camera. The flow movement is calculated computer-aided using PIV (Particle Image Velocimetry) via the drift of the suspended solids. When using the "dark field probe", the horizontal flow direction of the groundwater is measured using a camera according to the dark field principle. In the dark field technique, the light emitted by a light source towards the camera lens is blocked out by so-called beam traps so that ideally no light reaches the lens.Light is scattered and diffracted by the finest particles in the groundwater, whose size is far below the optical resolution of the camera, thus mapping their position in the camera image. The images are recorded on video using a deep-stationary measuring probe, and the direction of particle movement is statistically evaluated and visualized using special software. The "dark-field probe" consists of a conventional well inspection camera with a different lens and a light source mounted below the probe, pointing toward the camera.

[0004] From US 2006 / 0 175 561 A1, a method for measuring flow velocity called “particle shadow velocimetry” is known. An illuminated spatial measuring region is defined between at least one LED and an image sensor, with an optical element arranged between the measuring region and the image sensor. The spatial measuring region is arranged optically inline between the light source and the image sensor. A flow comprising particles passes through the spatial measuring region. The spatial measuring region is illuminated with the light source in order to induce light extinction by the particles, i.e. to create shadow marks of the positions of the particles in the flow. The image sensor is used to detect the shadow marks created by the particles and to record a displacement of the particles as a function of time, which corresponds to the movement of the flow.The light source can have multiple LEDs that emit different colors.

[0005] US 2008 / 0 068 605 A1 discloses a device and method for simultaneously measuring particle velocity and particle position in three dimensions. Using a structured laser beam, a spatial light pattern is projected into an aerosol flow. A particle of the aerosol passing transversely through the light pattern scatters light elastically onto a photodetector. The photodetector outputs data to a processor to record the scattered light as a time-dependent waveform. Position and velocity can be determined from the waveform using only a laser light source and a photodetector. OBJECT OF THE INVENTION

[0006] The invention is based on the object of demonstrating a method and a device for measuring flow velocities by means of in-line particle image velocimetry, which are also suitable for higher flow velocities and / or for covering larger spatial measuring ranges. SOLUTION

[0007] The object of the invention is achieved by a method having the features of patent claim 1 and a device having the features of patent claim 10. The further patent claims are directed to preferred embodiments of the method and device according to the invention. DESCRIPTION OF THE INVENTION

[0008] In a method according to the invention for measuring flow velocities in a spatial measuring area by means of in-line particle image velocimetry (in-line PIV), in which light from at least one light source is directed into the measuring area in a central illumination direction using at least one illumination optical unit, the measuring area is imaged onto an image sensor in the central illumination direction using an imaging optical unit, and at least one beam trap for the light from the at least one light source is arranged such that the image sensor records dark-field images of the measuring area, the measuring area into which the light from the at least one light source is directed with the at least one illumination optical unit and which is imaged onto the image sensor with the imaging optical unit has a larger cross-sectional area transverse to the central illumination direction than an exit pupil of the at least one illumination optical unit.

[0009] In a device according to the invention for measuring flow velocities by means of in-line PIV according to the method according to the invention, which has at least one light source, at least one illumination optics which directs light from the at least one light source in a central illumination direction into a spatial measuring area, an image sensor, an imaging optics which images the measuring area in the central illumination direction onto an image sensor, and at least one beam trap for the light from the at least one light source, which is arranged such that the image sensor records dark field images of the measuring area, the measuring area into which the at least one illumination optics directs the light from the at least one light source and which the imaging optics images onto the image sensor has a larger cross-sectional area transverse to the central illumination device than an exit pupil of the at least one illumination optics.

[0010] In the present invention, the measuring area into which the light from the at least one light source is directed by the at least one illumination optics and which is imaged on the image sensor by the imaging optics is enlarged with respect to its cross-sectional area transverse to the central illumination device compared to the exit pupil of the illumination optics. Preferably, the measuring area has a cross-sectional area transverse to the central illumination direction that is at least 100%, and even more preferably at least 300%, larger than the exit pupil of the at least one illumination optics.

[0011] Specifically, it depends on the exit pupil of the at least one illumination optics facing the measuring area, ie the measuring area-side image of an aperture diaphragm of the at least one illumination optics.

[0012] Due to the spatial measuring range which is enlarged in cross-section compared to the exit pupil of at least one illumination optic, on the one hand the flow velocities can be measured in an enlarged spatial area and on the other hand higher flow velocities can also be measured, since the particles carried by the respective flow do not leave the larger measuring range too quickly, even at higher flow velocities, in order to be able to measure their speed.

[0013] In order to implement the enlargement of the spatial measuring area relative to the exit pupil of the at least one illumination optics, the light from an array of at least 2 x 2 and preferably at least 3 x 3 light sources can be directed into the measuring area using the at least one illumination optics. In this implementation, the light from the individual light sources of the array is directed into the measuring area using one illumination optics, but it does not originate from a single light source, but rather from several light sources arranged at laterally spaced intervals from one another and can therefore illuminate the measuring area whose cross-section extends beyond the exit pupil of the one illumination optics. In a modification of this implementation of the method according to the invention, the light is directed into the measuring area using an array of at least 2 x 2 and preferably at least 3 x 3 illumination optics and light sources.To illuminate the measurement area, both multiple light sources and multiple illumination optics are used to achieve illumination of the measurement area across the cross-section of the exit pupil of each of the illumination optics. In these two implementations of the method according to the invention, the light from each of the light sources is preferably focused by the at least one illumination optic onto a beam trap arranged behind the measurement area and in front of the imaging optics. Accordingly, the beam traps for the light from the individual light sources of the array of at least 2 x 2 and preferably of at least 3 x 3 light sources are arranged in an array of at least 2 x 2 or at least 3 x 3 beam traps.

[0014] In another implementation of the method according to the invention, which can, however, in principle be combined with the implementation based on an array of at least light sources, the light from the at least one light source is directed into the measuring area as a divergent beam by the at least one illumination optics. In this implementation, for example, a conical diaphragm or a so-called Toepler edge or cutting edge can be arranged behind the imaging optics as a beam trap. Alternatively, in this implementation, the beam trap can be arranged in a parallel beam path of the at least one illumination optics and imaged onto the entrance pupil of the imaging optics with the at least one illumination optics in such a way that its image completely covers the entrance pupil. This alternative corresponds to projecting an image of a black circle onto the entrance pupil of the imaging optics using a slide projector.This means that no light from the light source reaches the entrance pupil of the imaging optics directly, but only scattered light scattered by particles in the measurement area. In both alternatives explained here, the beam trap lies outside the area along the illumination direction, which is delimited by the front lenses of the illumination optics and the imaging optics facing the measurement area.

[0015] The dark-field images of the measurement area recorded using the method or device according to the invention exhibit bright spots attributable to individual particles in the measurement area, but with a high background light intensity of the scattered light in between. This high background light intensity is attributable to scattered light that has been scattered multiple times in the measurement area and / or by particles outside the depth of field of the imaging optics. However, this background light intensity can be easily removed by high-pass filtering of the dark-field images, preferably digitally. The high-pass filtered dark-field images then only show the positions of the particles in the measurement area within the depth of field of the imaging optics. As a result, the position of the particles along the illumination direction is also at least approximately known.

[0016] However, when evaluating dark-field images using correlation techniques, prior high-pass filtering is usually unnecessary because, for example, the background light intensities of two consecutive dark-field images, unlike the scattered light originating from the particles in the measurement area, are not correlated and therefore do not provide a signal when calculating a cross-correlation.

[0017] If the dark field images in the invention are recorded in an event-based manner, for example with a so-called event camera, the effective frame rate, i.e. the temporal resolution of the image of the measuring area and thus the detectable maximum flow velocity can be effectively increased.

[0018] In fact, the invention can be successfully implemented even with light sources of relatively low light output. Specifically, LEDs are sufficient, although an array of LEDs is particularly preferred for implementing an array of light sources.

[0019] Advantageous further developments of the invention emerge from the patent claims, the description and the drawings.

[0020] The advantages of features and combinations of several features mentioned in the description are merely exemplary and can be effective alternatively or cumulatively, without the advantages necessarily having to be achieved by embodiments according to the invention.

[0021] With regard to the disclosure content – ​​not the scope of protection – of the original application documents and the patent, the following applies: Further features can be found in the drawings – in particular the illustrated geometries and the relative dimensions of several components to one another, as well as their relative arrangement and operative connection. The combination of features of different embodiments of the invention or features of different patent claims is also possible, deviating from the chosen references of the patent claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features of different patent claims.Likewise, features listed in the patent claims may be omitted for further embodiments of the invention, but this does not apply to the independent patent claims of the granted patent.

[0022] The number of features mentioned in the patent claims and the description is to be understood as meaning that exactly this number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least." Thus, for example, if reference is made to an image sensor, this is to be understood as meaning that exactly one image sensor, two image sensors, or more image sensors are present. The features mentioned in the patent claims may be supplemented by further features or may be the only features present in the subject matter of the respective patent claim.

[0023] The reference signs contained in the patent claims do not represent a limitation of the scope of the subject-matter protected by the patent claims. They serve solely to make the patent claims easier to understand. BRIEF DESCRIPTION OF THE CHARACTERS

[0024] In the following, the invention is further explained and described with reference to preferred embodiments shown in the figures. Fig. 1 is a schematic representation of a first embodiment of the device according to the invention and the method according to the invention with several light sources and illumination optics. Fig. 2 is a schematic representation of a second embodiment of the device according to the invention and of the method according to the invention with a light source and an illumination optics which directs light from the light source as a divergent beam into a measuring area and in which a beam trap is arranged behind an imaging optics which images the measuring area onto an image sensor. Fig. 3 is a schematic representation of a third embodiment of the device and method according to the invention, again with an illumination optics that directs light from a light source as a divergent beam into a measuring area, this time the beam trap being arranged in a parallel beam path of the illumination optics. Fig. Figure 4 illustrates the light intensity distribution of scattered light according to the Mie theory when scattering incident light of a wavelength of 532 nm from a 1 µm glass particle in air. Fig. Figure 5 shows a double-layered dark-field image of a flow inoculated with 3 µm particles through the measuring area, recorded with a device according to the invention. Fig. 6 shows the image according to Fig. 5 after high-pass filtering. FIGURE DESCRIPTION

[0025] The Fig. The device 1 shown in Figure 1 is used to measure flow velocities of a flow 2 in a spatial measuring area 3. The spatial measuring area 3 is illuminated with light 4 from several light sources 5, each of which is assigned an illumination optics 6. The light sources 5 can be LEDs or laser diodes. Alternatively, the light sources 5 can be formed by exit cross-sections of optical fibers, into which, for example, laser light from a laser is coupled. Each of the illumination optics 6 focuses the light 4 from one of the light sources 5 onto a beam trap 7, which is arranged in Fig. 1 is arranged on the side of the measuring area 3 opposite the illumination optics 6 in front of an imaging optics 8. The imaging optics 8 images the measuring area 3 onto an image sensor 9. The image of the image area 3 on the image sensor 9 is a dark field image because no light 4 from the light sources 5 reaches the image sensor 9 directly. All convergent beams 10 of the light 4 emerging from the illumination optics 6 from the light sources 5 end in the beam traps 7. However, scattered light 12 scattered by particles 11 in the measuring area 3 can reach the image sensor 9 and result in images 13 of the particles. As a result, the images 13 become visible against a very high background light intensity, which, without the beam traps 7, would result from the light 4 coming directly from the light sources 5 on the image sensor 9.Even without the direct incidence of light 4 from the light sources 5, a high background light intensity occurs in the dark field images recorded with the image sensor 9. This high background light intensity results from multiple scattering by several particles 11, of which . Fig. 1 only a single one is shown, and from scattering on particles 11 outside the measuring area 3, which corresponds to the depth of field of the imaging optics 8, insofar as it is illuminated with the light 4 from the light sources 5. The measuring area 3 is illuminated via a cross-sectional area 14 transverse to a central illumination direction 15, which is larger than an exit pupil 16 of each of the illumination optics 6. This is achieved here by the multiple light sources 5 and the multiple illumination optics 6, which can specifically be arranged in a 3 x 3 array that also extends perpendicular to the plane of the drawing.

[0026] In the device 1 according to Fig. 2, the measuring area 3 is illuminated with light 4 from only one light source 5, which is directed by the illumination optics 6 in the form of a divergent beam 17 onto the measuring area 3. The light 4 directed onto an entrance pupil 18 of the imaging optics 8 is blocked by the beam trap 7 arranged behind the imaging optics 8 in the form of a conical diaphragm or Toepler edge or cutting edge, while the scattered light 12 scattered by the particles 11 in the measuring area 3 reaches the image sensor 9 and results there in the images 13. The device 1 according to Fig. 2 corresponds to a Focault Schlieren structure with focusing of the light 4 coming from the light source 5 and entering the entrance pupil 18 into the beam trap 7 behind the imaging optics 8. Here too, the cross-sectional area 14 of the measuring area 3 transverse to the central illumination direction 15 is larger than the exit pupil 16 of the illumination optics 6.

[0027] In the device 1 according to Fig. 3, the difference between the cross-sectional area 14 and the exit pupil 16 of the illumination optics 6 is smaller due to the short distance between the measuring area 3 and the illumination optics 6. However, the measuring area 3 can be enlarged by moving the imaging optics 8 in the direction of the central illumination direction 15. In the device 1, the beam trap 7 is arranged in the region of a parallel beam path 19 of the illumination optics 6 such that it is imaged onto the entrance pupil 18 of the imaging optics 8. Thus, no light 4 from the light source 5 reaches the image sensor 9 directly, but only scattered light 12 scattered by the particles 11. Specifically, the beam trap 7 is arranged in the center of a diffusion disk 20.

[0028] The image sensors 9 of the devices 1 according to the Fig. 1 to 3 can be parts of so-called event cameras, which output incoming light intensities only on an event basis, i.e., only for pixels with a change in the incident light intensity above a threshold value. This allows time-resolved measurements up to a frame rate of approximately 1 kHz, which corresponds to a Δt for the repeated imaging of the particles 11 of 1 ms. With pulsed light, the frame rate can even be increased to 10 kHz, corresponding to ΔT = 100 µs. Two or three devices 1 according to the Fig. 1 to 3 with intersecting mean illumination directions 15 can be used to resolve three-dimensional flow fields with respect to all components of the occurring flow velocities.

[0029] The devices according to the invention and the methods according to the invention carried out therewith are particularly advantageous for small particles 11 in the flow 2. Fig. Figure 4 is a plot of the light intensity of the scattered light 12 scattered by a 1 µm glass particle in air according to the Mie theory, plotted against the angle to the illumination direction 15 in which light 4 with a wavelength of 532 nm is incident. The light intensities are plotted on a logarithmic scale, with the light intensities of neighboring circles differing by a factor of 100. In the forward direction at 180°, the light intensity of the scattered light 12 is approximately a factor of 10. 6greater than at 90° or 270°. The scattered light 12 scattered by the 1 µm particle 11 is therefore extremely concentrated in the forward direction, and the image sensor 9 is never shadowed by the particle 11. The scattered light 12 scattered by particles 11 within the measuring range 3 determined by the depth of field of the imaging optics 8 leads to high light intensities on the image sensor 9. By recording dark-field images with the image sensor 9, these light intensities are separated from the even higher light intensities of the light 4 coming directly from the light sources 5. However, the dark-field images contain significant background light intensities due to multiply scattered scattered light 12 and scattered light 12 by particles 11 outside the depth of field of the imaging optics 8.

[0030] The Fig. The image shown in Figure 5, which was taken for particles 11 in the form of micro-soap bubbles with a nominal size of 3 µm, shows this high background light intensity. After high-pass filtering, where the threshold intensity is chosen so that it is only exceeded by scattered light 12 directly scattered by the particles 11 in the measuring area 3, the result is Fig. 6, which shows the paths traveled by the individual particles 11 in the measuring area 3 during the exposure time as lines. LIST OF REFERENCE SYMBOLS 1 device 2 Current 3 Measuring range 4 light 5 Light source 6 Lighting optics 7 Beam trap 8 Imaging optics 9 image sensor 10 convergent beam 11 particles 12 Scattered light 13 Image 14 Cross-sectional area 15 middle lighting direction 16 Exit pupil 17 divergent beam 18 Entrance pupil 19 parallel beam path 20 diffuser

Claims

[1] Method for measuring flow velocities in a spatial measuring area (3) using in-line particle image velocimetry, - wherein light (4) from at least one light source (5) is directed into the measuring area (3) in a central illumination direction (15) by means of at least one illumination optic (6), - wherein the measuring area (3) is imaged onto an image sensor (9) in the central illumination direction (15) by means of an imaging optics (8) and - wherein at least one beam trap (7) for the light (4) from the at least one light source (5) is arranged such that the image sensor (9) records dark field images of the measuring area (3), characterized bythat the measuring area (3) into which the light (4) from the at least one light source (5) is directed with the at least one illumination optics (6) and which is imaged onto the image sensor (9) with the imaging optics (8) has a larger cross-sectional area (14) transverse to the mean illumination direction (15) than an exit pupil (16) of the at least one illumination optics (6). [2] Method according to claim 1, characterized by that the measuring area (3) into which the light (4) from the at least one light source (5) is directed with the at least one illumination optics (6) and which is imaged onto the image sensor (9) with the imaging optics (8) has a cross-sectional area (14) transverse to the mean illumination direction (15) that is at least 100% and preferably at least 300% larger than the exit pupil (16) of the at least one illumination optics (6). [3] Method according to claim 1 or 2, characterized by , - that with the at least one illumination optics (6) the light (4) from an array of at least 2x2 and preferably of at least 3x3 of the light sources (5) is directed into the measuring area (3) or - that the light (4) is directed into the measuring area (3) with an array of at least 2x2 and preferably at least 3x3 of the illumination optics (6) and the light sources (5). [4] Method according to claim 3, characterized by that the light (4) from each of the light sources (5) is focused by the at least one illumination optics (6) onto a beam trap (7) arranged behind the measuring area (3) and in front of the imaging optics (8). [5] Method according to claim 1 or 2, characterized by that the light (4) from the at least one light source (5) with the at least one illumination optics (6) is directed into the measuring area (3) as a divergent beam (17). [6] Method according to claim 5, characterized by , - that a conical diaphragm or Toepler's edge is arranged as the beam trap (7) behind the imaging optics (8) or - that the beam trap (7) is arranged in a parallel beam path of the at least one illumination optics (6) and is imaged with the at least one illumination optics (6) onto an entrance pupil (18) of the imaging optics (8). [7] Method according to one of the preceding claims, characterized by that the light intensities of the dark field images are high-pass filtered. [8] Method according to one of the preceding claims, characterized by that the darkfield images are recorded event-based. [9] Method according to one of claims 1 to 8, characterized by that the at least one light source (5) is an LED. [10] Device for measuring flow velocities by means of in-line particle image velocimetry according to the method according to one of the preceding claims, with - at least one light source (5) - with at least one illumination optics (6) which directs light (4) from the at least one light source (5) in a central illumination direction (15) into a spatial measuring area (3), - an image sensor (9), - an imaging optics (8) which images the measuring area (3) in the central illumination direction (15) onto an image sensor (9), and - at least one beam trap (7) for the light (4) from the at least one light source (5), which is arranged so that the image sensor (9) records dark field images of the measuring area (3), characterized bythat the measuring area (3) into which the at least one illumination optics (6) directs the light (4) from the at least one light source (5) and which the imaging optics (8) images onto the image sensor (9) has a larger cross-sectional area (14) transverse to the mean illumination direction (15) than an exit pupil (16) of the at least one illumination optics (6). [11] Device according to claim 10, characterized by that the measuring area (3) into which the at least one illumination optics (6) directs the light (4) from the at least one light source (5) and which the imaging optics (8) images onto the image sensor (9) has a cross-sectional area (14) transverse to the mean illumination direction (15) that is at least 100% and preferably at least 300% larger than the exit pupil (16) of the at least one illumination optics (6). [12] Device according to claim 10 or 11, characterized by , - that the at least one light source (5) is part of an array of at least 2x2 and preferably of at least 3x3 light sources (5) and that the at least one illumination optics (6) directs the light (4) from the array into the measuring area (3) or - that the at least one light source (5) and that the at least one illumination optics (6) are parts of an array of at least 2x2 and preferably of at least 3x3 light sources (5) and illumination optics (6) and that the array directs the light (4) into the measuring area (3). [13] Device according to claim 12, characterized by that the at least one illumination optics (6) focuses the light (4) from each of the light sources (5) onto a beam trap (7) arranged behind the measuring area (3) and in front of the imaging optics (8). [14] Device according to claim 10 or 11, characterized bythat the at least one illumination optics (6) directs the light (4) from the at least one light source (5) as a divergent beam (17) into the measuring area (3). [15] Device according to claim 14, characterized by , - that a conical diaphragm or Toepler's edge is arranged as the beam trap (7) behind the imaging optics (8) or - that the beam trap (7) is arranged in a parallel beam path of the illumination optics (6) and is imaged by the illumination optics (6) onto an entrance pupil (18) of the imaging optics (8). [16] Device according to one of claims 10 to 15, characterized by that the image sensor (9) is part of an event camera that records the dark field images on an event-based basis. [17] Device according to one of claims 10 to 16, characterized by that the at least one light source (5) is an LED.

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