Apparatus and method for determining density fluctuations in a fluid

Abstract

The invention relates to a method and an apparatus (100) for determining density fluctuations in a fluid (50), at least comprising an optical transmitting device (10) for emitting an optical pattern (24), which device has a light source (12) and an optical pattern generator (14), wherein the optical pattern generator (14) is designed to split light beams (20) that are received from the light source (12), and are intended for forming the optical pattern (24), into a multiplicity of individual rays (22), and also comprising an optical receiving device (30) for receiving an image of the optical pattern (26) altered as it passes through the fluid (50), wherein the fluid (50) is arranged or can be arranged in the beam path (26) between the optical transmitting device (10) and the optical receiving device (30). The receiving device (30) is configured to evaluate a density fluctuation of the fluid (50) from the altered optical pattern (26) received. The invention also relates to a use of such an apparatus and to a computer program for operating, in particular controlling, such an apparatus.

Description

[0001] Description

[0002] title

[0003] Device and method for determining density fluctuations in a fluid

[0004] State of the art

[0005] The invention relates to a device for determining density fluctuations, in particular turbulence, in a fluid. The invention also relates to a method for determining density fluctuations, in particular turbulence, in a fluid and a use of the device.

[0006] Various procedures, methods, and devices are known for determining density fluctuations and / or density gradients in a fluid. Density fluctuations typically occur in turbulence and air currents or air eddies. These arise, for example, around moving objects such as aircraft in air or ship propellers in water. Furthermore, density fluctuations can also arise when different materials are mixed in liquids. The devices typically each have an optical transmitter unit that emits light beams that penetrate the fluid to be measured in a volume region or layer region, and an optical receiver unit in which the received light is imaged.

[0007] A widely used method for determining density fluctuations is known as Schlieren photography and is a photographic technique in which local fluctuations in the refractive index in the fluid, for example in a liquid or a gas, are imaged.

[0008] For example, the simultaneous mixing of different substances with one another, thermal convection movements and / or air and shock waves around flying projectiles can be measured, for example in a wind tunnel. The volume region or layer to be examined is illuminated with parallel light and imaged onto a screen through a lens. The majority of the light hits an aperture arranged after the light source. The density fluctuations of an inhomogeneous fluid in the volume or layer direct some of the light rays past the aperture, whereby the image on the screen is only created by the light that passes directly past the aperture. The brightness fluctuations of the image correspond to the local changes in the refractive index caused by the density fluctuations or mixing.To obtain a high-contrast image of density fluctuations, the light must be sharply focused on the edge of the aperture. This is achieved by using a light source that is as point-shaped as possible and positioning the aperture in the image plane of the light source. In the Schlieren PIV (PIV: Particle Image Velocimetry) method, the speed of a flow is determined based on the speed of vortices or turbulence moving within the flow. The speed is determined based on two photographs and the progress of the vortex's movement. The optics are similar to those of Schlieren photography; the method relies on the presence of vortices or turbulence in the flow.

[0009] In the process known as BOS (Background Oriented Schlieren), it is not the striae that are imaged, but a background with a grid. The background is static. In an image projected onto a screen, for example, the position of grid points is distorted according to the gradients of the refractive index. The process does not require a point light source. In the BOS process, gradients of the refractive index typically lead to a collection or dispersion of the light rays from a (quasi-)parallel light source, with each of the optically denser regions causing an increased luminance on the screen, and conversely, regions with a lower refractive index creating a dark spot and thus a shadow on the screen. For this reason, the BOS process is also referred to as the shadow process.The shadow method is less sensitive than schlieren photography because the differences in light intensity are proportional to the second derivative of the refractive index, whereas the differences in intensity in the schlieren method are proportional to its first derivative. A variation of the BOS method is a method known as speckle-BOS (speckle-background oriented schlieren), in which a so-called speckle pattern is used as the background pattern instead of a real background pattern. The speckle pattern is typically created by illuminating a wall behind the fluid to be measured, usually with laser light. Microscopic irregularities in the illuminated wall create an interference pattern of light and dark dots in the laser light reflected from the wall on a camera sensor. As the laser light passes through the fluid, the intensity minima and intensity maxima of the interference pattern shift.The contrasts are created by self-interference of the light rays after passing through the fluid with the density fluctuations on the camera sensor and / or a screen.

[0010] From US 9,599,497 B1, a so-called speckle BOS method and system are known in which a fluid flow around a moving object is visualized using a speckle pattern instead of a real background pattern.

[0011] Disclosure of the invention

[0012] The object of the invention is to provide an improved device for determining density fluctuations in a fluid.

[0013] A further object of the invention is to provide a method for determining density fluctuations in a fluid using an improved device.

[0014] A further object of the invention is to enable the use of an improved device for determining density fluctuations.

[0015] The objects are achieved by the features of the independent claims. Favorable embodiments and advantages of the invention emerge from the further claims, the description, and the drawings. According to one aspect of the invention, a device for determining density fluctuations, in particular turbulence, in a fluid, in particular in air, is proposed. The device comprises at least one optical transmitting device for emitting an optical pattern, which has a light source and an optical, in particular transmissible, pattern generator. The optical pattern generator is designed to split light beams received from the light source into a plurality of individual beams to form the optical pattern.The device further comprises an optical receiving device for receiving an image of the optical pattern that changes upon passing through the fluid, wherein the fluid is arranged or can be arranged in the beam path between the optical transmitting device and the optical receiving device. The receiving device is configured to evaluate a density fluctuation, in particular turbulence of the fluid, from the received changed optical pattern.

[0016] The expression “in the beam path between the optical transmitting device and the optical receiving device” also includes an arrangement in which the light rays are reflected, scattered or diffracted on a projection surface or a mirror.

[0017] For example, in the case of a bent beam path, as occurs when the optical pattern is projected onto a projection surface and then recorded by the optical receiving device, the fluid can be arranged in the beam path between the optical transmitting device and the projection surface or between the projection surface and the optical receiving device. If the fluid is arranged in front of the projection surface, the fluid creates a changed optical pattern which is projected onto the projection surface and recorded by the optical receiving device. If the fluid is arranged in the beam path behind the projection surface, the original optical pattern is projected onto the projection surface and, when the fluid records this optical pattern projected onto the projection surface, creates a changed optical pattern which is then received by the optical receiving device.If the beam path is deflected by another optical element such as a mirror, the fluid can also be arranged in the beam path in front of or behind the mirror.

[0018] The determination of density fluctuations present or generated in the fluid can be achieved by visualizing the density fluctuations. The fluid can be a liquid, a gas, a plasma, or an aerosol.

[0019] Density fluctuations or fluctuations in the refractive index can also be generated by the mixing of two or more fluids. The visualization can take place in the optical receiving device. The density fluctuation can be a density gradient of the fluid in a volume. The fluid can be arranged in a measuring volume. A density field which has density fluctuations and / or density gradients can be formed in the fluid. The density field can have several regions of different density. The density fluctuations can be or will be generated by the movement of an object in the fluid, for example an aircraft or subcomponents of the aircraft in air. Density fluctuations can also occur in the region of a rotating turbine. Density fluctuations in water can occur, for example, in the region of rotating ship propellers.

[0020] The device can advantageously be used to measure density fluctuations without contact. The device can determine and / or visualize density gradients under different flow conditions. This allows aerodynamic effects to be visualized, and these can be classified and evaluated.

[0021] The optical transmitter device can have a laser as the light source. Lasers whose laser radiation can be detected by the receiver device can be used for the optical transmitter device. This typically covers the range from UV (typically, e.g., 250 nm) to NIR (typically, e.g., 1100 nm). Receiver devices that operate down to a wavelength of 2 pm are also available. The device can be conveniently used even at very low laser powers (e.g., in the range of 1 mW). Therefore, virtually all laser types can be used: fiber lasers, diode lasers, gas lasers, diode-pumped solid-state lasers (DPSS lasers), and dye lasers.

[0022] The power used depends on many factors. For larger areas, lasers with an average power of 1 W can be cost-effective. For very large structures, lasers with even higher average power can be used.

[0023] If the device is to be operated with the least possible risk to the human eye, lasers with a longer wavelength can be used, e.g. with a wavelength of 1550 nm with a suitable receiving system.

[0024] For a pulsed device, laser pulses in the microsecond or nanosecond range can be used. This is typically achieved with fiber lasers, DPSS lasers, and diode lasers.

[0025] In principle, pulse durations and laser types can be selected according to requirements.

[0026] The optical pattern generated by the optical pattern generator can be a homogeneous, ordered and / or pseudo-random and / or random optical pattern, wherein the optical pattern can be generated with a predetermined, in particular freely selectable, spatial frequency. The optical pattern can be generated dynamically. The optical pattern can comprise individual beams. It is an advantage over the prior art methods using static optical patterns to generate the pattern in such a way that the optical pattern moves with moving objects. Using the optical pattern generator, the generated optical pattern can be adapted to the expected magnitude of the density fluctuations. Furthermore, the optical pattern generator can be movable and used dynamically, for example, in an aircraft for visualizing aerodynamic effects, and can be used flexibly.The contrast of the generated optical pattern can be very high, since individual rays are imaged. A further advantage of the device may be that the light intensity for visualizing density fluctuations can be kept low, since no background with a pronounced contrast structure is required. One advantage over a conventional speckle BOS system is that the parameters of the pattern generated in the optical pattern generator, especially a random pattern, are easily adjustable.

[0027] According to an advantageous embodiment of the device, the optical transmitting device can have an optical unit for parallelizing the emitted individual beams. The individual beams can form the optical pattern. The individual beams can be individually deflected in the fluid and form the modified optical pattern, which is received and imaged in the optical receiving device.

[0028] According to an advantageous embodiment of the device, the optical receiving device can have a receiving-side telescope which is designed to image the changed optical pattern onto a camera sensor of the optical receiving device. The changed optical pattern can have geometrically offset individual beams. The size of the geometric offset of the respective individual beams is a measure of the density in the fluid. Thus, the changed optical pattern in comparison to the transmitted optical pattern can be used to draw conclusions about a density distribution, density gradients and thus density fluctuations in the fluid. The offset of the respective individual beam can depend on the local density it encounters in the fluid. A high density can lead to a greater offset of the respective individual beam than a low density.

[0029] According to an advantageous embodiment of the device, a projection surface can be arranged in the beam path of the plurality of individual beams for imaging the optical pattern and / or the modified optical pattern, wherein the optical receiving device is designed to receive the optical pattern imaged onto the projection surface and / or the modified optical pattern.

[0030] The optical pattern and / or the modified optical pattern can be imaged onto a camera, particularly a digital camera. This allows the modified optical pattern to be stored, particularly digitally, as a data set. The stored data set can be used for analysis, allowing conclusions to be drawn about the density fluctuation in the fluid. The stored data set can contain the offset of each individual ray of the optical pattern, which is caused by its passage through the fluid, in the modified optical pattern. The camera can also visualize the modified optical pattern as an image. The camera can advantageously be a high-speed camera.

[0031] According to an advantageous embodiment of the device, the fluid can be arranged in the beam path before or after the projection surface. For example, a pseudo-speckle pattern similar to a speckle pattern from the known speckle-BOS method can be projected onto the projection surface. In contrast to the known speckle-BOS method, however, the pseudo-speckle pattern here is generated by the pattern generator, for example a diffractive optical element (DOE), and not by the projection surface itself. The pseudo-speckle pattern can be a pattern created by the self-interference of a laser beam. Speckle patterns, light granulation, or laser granulation are the terms used to describe the granular interference phenomena that can arise with sufficiently coherent illumination of optically rough object surfaces.

[0032] According to an advantageous embodiment of the device, the optical transmission device can have a transmitting-side optical device, in particular a telescope, for projecting a light beam from the light source onto the optical pattern generator. The pattern generator can thus generate the optical pattern.

[0033] According to an advantageous embodiment of the device, the optical receiving device can have a bandpass filter. The bandpass filter is advantageously an optical bandpass filter.

[0034] According to an advantageous embodiment of the device, the light source can be designed to emit a pulsed light beam. In particular, the optical receiving device can be triggered or triggerable synchronously with the pulsed light beam. A suitable pulsed light beam can be a pulsed laser beam. Pulse lengths and laser powers can be selected as required.

[0035] According to an advantageous embodiment of the device, the optical pattern can be designed as a pseudo-speckle pattern, which can be generated from the plurality of individual beams by the pattern generator, for example, a diffractive optical element (DOE). The diffractive optical element (DOE) can also be a digitally controllable, variable diffractive optical element (DOE).

[0036] According to an advantageous embodiment of the device, the optical pattern generator can comprise at least one of the following components: a diffractive optical element (DOE), an LCD mask, a light modulator, or a digital hologram. This advantageously allows for the generation of a homogeneously ordered, pseudo-random, or random optical pattern. The contrast is very high, since the optical path consists of individual light rays.

[0037] The diffractive optical element (DOE) is typically a glass substrate onto which microstructures can be applied by photolithography or stamping. In microstructures, phase modulation can occur due to different optical path lengths of the partial beams, creating interference patterns. In addition, the amplitude can be modulated through constructive and destructive superposition. DOEs can perform two tasks: they can shape the laser beam (beam shaping) or split it into several partial beams (beam splitting). The microstructure in the DOE can shape the beam through the refractive index or through height modulation. Good components achieve efficiencies of 80-99% and transmittances of 95-99%. This allows the DOE to generate an optical pattern from individual beams without loss. The light modulator can be, for example, a liquid crystal light modulator (SLM) (spatial light modulator).The hologram can be a three-dimensional image created using holographic techniques.

[0038] According to a further aspect of the invention, a method for determining density fluctuations, in particular turbulence, in a fluid, in particular in air, using a device as described above is proposed, which comprises at least the steps of: emitting a plurality of individual beams arranged in the form of an optical pattern by an optical transmitting device; receiving an image of the optical pattern changed upon passing through the fluid by an optical receiving device, wherein the fluid is arranged in the beam path between the optical transmitting device and the optical receiving device for irradiation by means of the plurality of individual beams. In this case, a density fluctuation, in particular turbulence of the fluid, is evaluated from the received changed optical pattern.

[0039] The modified optical pattern may show the individual rays after passing through the fluid.

[0040] The modified optical pattern can exhibit an offset of the individual beams, in particular individually offset individual beams. From the respective beam offset, conclusions can be drawn about a density field, density fluctuations in the fluid. Suitable evaluations can be applied for this purpose. Common PIV algorithms (OpenPIV) can be used advantageously. These are essentially based on a cross-correlation of individual, gradually shifted image sections. This allows a point pattern to be localized rather than the displacement of individual points. The randomness of the pseudo-speckle patterns is advantageous because no periodic self-similarity occurs here. However, tracking the position of individual points, essentially tracking each individual point, is also conceivable.

[0041] Advantageously, the density fluctuation or a density field can be determined from the offset of the individual rays in the changed pattern.

[0042] According to an advantageous embodiment of the method, the plurality of individual beams can be emitted as parallel individual beams. The parallel individual beams can form the optical pattern.

[0043] According to an advantageous embodiment of the method, the optical pattern or the modified optical pattern can be imaged onto a projection surface arranged in the beam path of the plurality of individual beams, wherein the optical pattern imaged onto the projection surface is received by the optical receiving device as an optical pattern modified upon passing through the fluid or as the modified optical pattern imaged onto the projection surface. According to an advantageous embodiment of the method, the fluid can be arranged upstream or downstream of the projection surface in the beam path.

[0044] According to an advantageous embodiment of the method, the received optical pattern can be filtered by a bandpass filter of the optical receiving device. The optical bandpass filter can comprise a shortpass filter and a longpass filter. This allows the light beam, in particular a laser beam, to be transmitted in a frequency band of the bandpass filter. Thus, ambient light can be significantly suppressed, and the signal-to-noise ratio of the measurement signal can be correspondingly improved.

[0045] According to an advantageous embodiment of the method, the light source can emit pulsed light beams, with the optical receiving device being triggered synchronously with the pulsed light beams. This can achieve significant signal noise suppression and / or contrast improvement.

[0046] According to an advantageous embodiment of the method, a pseudo-speckle pattern generated from the plurality of individual beams can be received as a modified optical pattern by the optical receiving device. It can be advantageous to generate the pseudo-speckle pattern because it is a high-contrast interference phenomenon. The pseudo-speckle pattern can be generated by the pattern generator, for example, by a diffractive optical element, and not, as in the speckle-BOS method, by the projection surface. The pseudo-speckle pattern can advantageously have a random character, since the scattering centers can typically be randomly distributed in the pattern generator.

[0047] According to a further aspect of the invention, the use of a device for determining density fluctuations, in particular turbulence, in a fluid, in particular in air, is proposed. A plurality of individual beams arranged in the form of an optical pattern are emitted by means of an optical transmitting device. The fluid is arranged in a beam path of the plurality of individual beams. A modified optical pattern generated in the fluid is received by an optical receiving device. A density fluctuation, in particular turbulence, of the fluid is evaluated from the received modified optical pattern.

[0048] According to an advantageous embodiment, the optical transmitting device and / or the optical receiving device can be arranged on a mobile system, in particular an airworthy system.

[0049] According to an advantageous embodiment, the device can be used for schlieren measurements in a fluid, in particular for background-oriented schlieren measurements and / or similarly for speckle-based background-oriented schlieren measurements. This allows schlieren measurements to be performed with a dynamic background pattern.

[0050] According to a further aspect of the invention, a computer program is proposed for operating, in particular controlling, a device for determining density fluctuations, in particular turbulence, in a fluid, in particular in air, wherein the device carries out a method for determining density fluctuations, in particular turbulence, in the fluid, in particular in air. The computer program can control at least one optical transmitting device for emitting an optical pattern, which has a light source and an optical, in particular transmissible, pattern generator. Light beams received by the light source of the pattern generator can be split into a plurality of individual beams by means of the computer program to form the optical pattern.Furthermore, an optical receiving device of the device, which is designed to receive an image of the optical pattern changed upon passing through the fluid, can be operated and controlled by the computer program, wherein the fluid is arranged or can be arranged in the beam path between the optical transmitting device and the optical receiving device. Devices comprise a computer unit on which a computer program is executed. The computer program controls, for example, the optical pattern generator of the optical transmitting device. The computer program reads the camera sensor of the optical receiving device. This is advantageous if the camera sensor is installed in a high-speed camera, the data from which can be quickly read out using the computer program.

[0051] The computer program can carry out the method using a device described above, wherein at least the following steps are controlled by the computer program: emitting a plurality of individual beams arranged in the form of an optical pattern by an optical transmitting device; receiving an image of the optical pattern changed upon passage through the fluid by an optical receiving device, wherein the fluid is arranged in the beam path between the optical transmitting device and the optical receiving device for irradiation by the plurality of individual beams. The changed optical pattern can comprise the individual beams after passage through the fluid. The received measurement data can be stored on a storage unit, for example a computer, by means of the computer program.Advantageously, a camera sensor, in particular a so-called high-speed camera sensor, of the optical receiving device can be quickly read out by means of the computer program.

[0052] According to an advantageous embodiment of the computer program, instructions can be included which, when the program is executed by the computer, cause the computer to carry out the method described above.

[0053] The computer program can control the device such that, in the method, the plurality of individual beams are emitted as parallel individual beams. The parallel individual beams can form the optical pattern. Using the computer program, the optical pattern or the modified optical pattern can be projected onto a projection surface arranged in the beam path of the plurality of individual beams. The optical pattern projected onto the projection surface is received by the optical receiving device as an optical pattern modified upon passing through the fluid or as the modified optical pattern projected onto the projection surface.

[0054] The fluid can be arranged in the beam path before or after the projection surface.

[0055] Using the computer program, the received optical pattern can be filtered through a bandpass filter of the optical receiving device. The optical bandpass filter can comprise a shortpass filter and a longpass filter. This allows the light beam, in particular a laser beam, to be transmitted in a frequency band of the bandpass filter. This allows ambient light to be significantly suppressed, and the signal-to-noise ratio of the measurement signal can be correspondingly improved.

[0056] The computer program can control the light source such that pulsed light beams are emitted in the process, whereby the optical receiving device can be triggered synchronously with the pulsed light beams by means of the computer program. This can achieve significant signal noise reduction and / or contrast improvement.

[0057] According to an advantageous embodiment, the computer program can be configured such that a pseudo-speckle pattern generated from the plurality of individual beams is received as a modified optical pattern by the optical receiving device. It can be advantageous to generate the pseudo-speckle pattern because it is a high-contrast interference phenomenon. The pseudo-speckle pattern can be generated by the pattern generator, for example, by a diffractive optical element, and not, as in the speckle-BOS method, by the projection surface. The pseudo-speckle pattern can advantageously have a random character, since the scattering centers can typically be randomly distributed in the pattern generator. According to an advantageous embodiment of the computer program, an optical pattern generator of the optical transmitting device can be controlled. In particular, a digitally controllable, variable diffractive optical element (DOE) can be controlled in this way.

[0058] According to an advantageous embodiment of the computer program, measurement data from a camera sensor of the optical receiving device can be read out. Measurement data from the camera sensor can thus be conveniently stored on a memory device of the computer unit.

[0059] According to an advantageous embodiment of the computer program, a density fluctuation, in particular turbulence of the fluid, can be evaluated from a received changed optical pattern.

[0060] drawing

[0061] Further advantages will become apparent from the following description of the drawings. The figures illustrate exemplary embodiments of the invention. The figures, the description, and the claims contain numerous features in combination. Those skilled in the art will also expediently consider the features individually and combine them into useful further combinations.

[0062] Examples include:

[0063] Fig. 1 is a schematic representation of a device for determining density fluctuations in a fluid according to an embodiment of the invention;

[0064] Fig. 2 is a schematic representation of the device for determining density fluctuations in the fluid according to a further embodiment of the invention;

[0065] Fig. 3 is a schematic representation of another embodiment of the device for determining density fluctuations in the fluid; Fig. 4 is a representation of a method for determining density fluctuations in the fluid.

[0066] Embodiments of the invention

[0067] In the figures, components of the same type or function similarly are designated by the same reference numerals. The figures are merely examples and are not to be construed as limiting.

[0068] Before describing the invention in detail, it should be noted that it is not limited to the specific components of the device and the specific method steps, as these components and methods may vary. The terms used herein are intended only to describe particular embodiments and are not intended to be limiting.

[0069] Furthermore, where the description or claims use the singular or indefinite articles, this also refers to the plural of these elements, unless the overall context clearly indicates otherwise.

[0070] The directional terminology used below, including terms such as "left," "right," "top," "bottom," "before," "behind," "after," and the like, is intended solely to enhance understanding of the figures and is in no way intended to limit the scope of the invention. The components and elements depicted, as well as their design and use, may vary according to the considerations of a person skilled in the art and may be adapted to specific applications.

[0071] Figure 1 shows a schematic side view of a device 100 for determining a density fluctuation in a fluid 50. The determination of the density fluctuation typically includes visualizing the density fluctuation. The device 100 has an optical transmitting device 10 and an optical receiving device 30. The optical transmitting device 10 and the optical receiving device 30 are arranged along an optical axis 5 shown in dash-dotted lines.

[0072] The optical transmission device 10 comprises a light source 12 that emits a light beam 20. The light source 12 is, for example, a laser. Instead of a laser, a projector could also be used, which could generate an image with "randomly" bright spots, thus achieving a similar effect. However, this is less luminous than a laser, which would nevertheless be sufficient for the arrangement shown in Figure 1.

[0073] An optical pattern generator 14 is arranged downstream of the light source 12. The optical pattern generator 14 can be, for example, a diffractive optical element (DOE), a liquid crystal display (LCD), a spatial light modulator (SLM), or a digital hologram.

[0074] The optical pattern generator 14 can be controlled by means of a computer program.

[0075] The optical pattern generator 14 can essentially generate individual beams 21, which subsequently propagate as a diverging beam bundle. In contrast to the individual beams 21, which diverge less or hardly at all, the beam bundle is divergent.

[0076] A transmitting-side optical device 18, such as a telescope, can be arranged between the light source 12 and the optical pattern generator 14 for imaging the light beam 20 onto the pattern generator 14. The transmitting-side optical device 18 can, in particular, be a transmitting-side optical telescope. The transmitting-side optical device 18 can typically have at least one lens 19.

[0077] The optical unit 16, by means of which the individual beams 22 are parallelized, is arranged between the pattern generator 14 and the fluid 50 to be examined. During operation of the device 100, the optical pattern 24 is generated from the light beam 20 by means of the pattern generator 14 and the optical unit 16. The divergent individual beams 21 generated in the pattern generator 14, in particular in the diffractive optical element 14, are parallelized by the optical unit 16 and impinge on the fluid 50 as essentially parallel individual beams 22 of the optical pattern 24. The optical pattern 24 can be formed as a homogeneous, ordered, pseudo-random, or random pattern with a predetermined spatial frequency. If the light source 12 is a laser, a high contrast in the optical pattern 24 advantageously results.

[0078] In the fluid 50, the parallel individual beams 22 are each individually deflected due to the density fluctuations, density gradients and / or turbulence in the fluid 50 and generate a changed optical pattern 26 with offset individual beams 26. The path along the optical axis 5 after the optical transmitting device 10, along which the individual beams 22 propagate, is referred to as the beam path 28.

[0079] The optical receiving device 30 for receiving the modified pattern 26 is arranged downstream of the fluid 50 along the optical axis 5. The optical receiving device 30 has a receiving-side telescope 34, which focuses the modified optical pattern 26 onto a camera sensor 32.

[0080] The receiving-side optical telescope 34 can in particular have at least one optical lens 35.

[0081] The individual beams 22 of the modified optical pattern 26 are directed onto the camera sensor 32 by means of the telescope 34.

[0082] The detection and visualization of the changed optical pattern 26 in the fluid 50 takes place in the camera sensor 32, on whose surface the position of the rays of the changed optical pattern 26 is imaged. By measuring the beam offset of each individual beam 22, conclusions can be drawn about the density field within the measurement volume of the fluid 50.

[0083] The optical receiving device 30 can, for example, have a bandpass filter. The optical transmitting device 10 can be a pulsed optical light source 12 for emitting a pulsed light beam 20. This allows the optical receiving device 30 to be advantageously triggered synchronously with the pulsed light beam 20. This can achieve significant signal noise suppression and / or contrast improvement.

[0084] Figure 2 shows a schematic representation of a specific embodiment of the device 100. The optical transmitting device 10 corresponds to that shown in Figure 1. The optical receiving device 30 is a camera 30. A projection surface 40 is arranged in the beam path 28. The fluid 50 to be irradiated is arranged between the optical transmitting device 10 and the projection surface 40, onto which the optical pattern 26, which changes as the individual beams 22 pass through the fluid 50, is projected. The projection surface 40 reflects, scatters, or diffracts the individual beams 22 of the changed optical pattern 26 that impinge upon it.

[0085] The camera 30 of the optical receiving device 30 records the changed optical pattern 26 projected onto the projection surface 40.

[0086] Figure 3 shows a schematic representation of a further embodiment of the device 100, wherein the fluid 50 is arranged between the projection surface 40 and the camera 30.

[0087] In this exemplary embodiment, the original optical pattern 24 generated by the pattern generator 14 is projected onto the projection surface 40. The camera 30 of the optical receiving device 30 records the optical pattern 24 projected onto the projection surface 40 through the fluid 50 as a modified optical pattern 26. Due to density fluctuations within the measurement volume of the fluid 50, the individual beams 22 are individually deflected. By measuring the beam offset of each individual beam 22, conclusions can be drawn about the density field within the measurement volume.

[0088] The two embodiments of the device 100 shown in Figures 2 and 3 do not have a receiving-side telescope 34. The devices described in Figures 1, 2, and 3 comprise a computer unit (not shown) on which a computer program is executed. The computer program can operate, in particular control, the transmitting device 10 and the receiving device 30. Furthermore, the computer program is configured to acquire measurement data from the receiving device 30 and store it on the computer unit. The computer program controls, for example, the optical pattern generator 14 of the optical receiving unit 10. The computer program reads the camera sensor 32. This is particularly advantageous when the camera sensor 32 is installed in a high-speed camera.

[0089] Figure 4 shows a method for determining density fluctuations in a fluid 50, in particular in air. The method is particularly suitable for determining turbulence in the fluid 50, in particular in air. The method is configured to visualize density fluctuations, in particular turbulence, in the fluid 50, in particular in air. The method is performed using the device 100 and comprises at least steps S210, S220, S230, and S240.

[0090] In method step S210, a plurality of individual beams 22 arranged in the form of an optical pattern 24 are emitted by the optical transmitting device 10.

[0091] This involves generating a light beam 20 with a light source 12, wherein the light beam 20 is typically a laser beam. Furthermore, the imaging of the light beam 20 onto the pattern generator 14 is included, wherein the optical pattern 24 is generated from the individual beams 22 by means of the pattern generator 14.

[0092] The fluid 50 is arranged for irradiation by the plurality of individual beams 22 in the beam path 28 between the optical transmitting device 10 and the optical receiving device 30. In method step S220, the optical pattern 24 composed of individual beams 22 penetrates the fluid 50, and the density fluctuations in the fluid 50 cause a deflection of the individual beams 22, whereby the modified optical pattern 26 composed of deflected individual beams is formed. The modified optical pattern 26 is received by the optical receiving device 30 in method step S230. An image of the optical pattern 26, which has changed upon passing through the fluid 50, is received by the optical receiving device 30.

[0093] In a process step S240, the density fluctuations can be evaluated and visualized.

[0094] The plurality of individual beams 22 are emitted as parallel individual beams 22. Depending on the embodiment of the device 100, the fluid 50 to be irradiated can, for example, be arranged in the beam path 28 before or after a projection surface 40.

[0095] In method step S230, the modified optical pattern 26 generated by the fluid 50 can thus be imaged onto the projection surface 40 arranged in the beam path 28 of the plurality of individual beams 22, wherein the modified optical pattern 26 imaged onto the projection surface 40 is received by the optical receiving device 30.

[0096] Alternatively, the original optical pattern 24 can be imaged onto the projection surface 40, wherein the optical pattern 24 imaged onto the projection surface 40 is received by the optical receiving device 30 as an optical pattern 26 modified upon passing through the fluid 50.

[0097] The received modified optical pattern 26 can be filtered, for example, by a bandpass filter of the optical receiving device 30. The bandpass filter can be implemented by optical elements arranged in the beam path 28.

[0098] The light source 12 can, for example, emit pulsed light beams 20. This allows the optical receiving device 30 to be triggered synchronously with the pulsed light beams 20. Depending on the embodiment, a pseudo-speckle pattern generated from the plurality of individual beams 22 can be received as an optical pattern 24 or as a modified optical pattern 26, depending on whether the fluid 50 is arranged before or after the projection surface 40.

[0099] Following method step S230, an evaluation step S240 can be provided for visualizing the density fluctuations and / or turbulences in the fluid 50. Common PIV algorithms (OpenPIV) can be advantageously used for this purpose. These are essentially based on a cross-correlation of individual, gradually shifted image sections. This allows a point pattern to be localized instead of the displacement of individual points. The randomness of the pseudo-speckle patterns is advantageous because no periodic self-similarity occurs.

[0100] It is also conceivable to track the position of individual points, essentially tracking each individual point. Advantageously, the PIV algorithms can also be based on a fast Fourier transform. Artificial intelligence methods (AI) can also be advantageously employed.

[0101] The device 100 is used to determine density fluctuations, in particular turbulence, in the fluid 50, in particular in air. The plurality of individual beams 22 arranged in the form of the optical pattern 24 are emitted by the optical transmitting device 10. The fluid 50 is arranged in the beam path 28. The modified optical pattern 26 is received by the optical receiving device 30. Thus, a density fluctuation, in particular turbulence, of the fluid 50 can advantageously be evaluated from the received optical pattern 26.

[0102] When using the device 100, the optical transmitting device 10 and / or the optical receiving device 30 can be arranged on a mobile system, in particular an airworthy system.

[0103] The device 100 is suitable for schlieren measurements in the fluid 50. In particular, the device 100 can be used for background-oriented schlieren measurements and / or similarly to speckle-based background-oriented schlieren measurements. The method is carried out using the computer program, which is executed on the computer unit. Density fluctuations, in particular turbulences, in the fluid 50, in particular in air, are determined using the computer program. In particular, the method steps S210, S220, 230, and S240 are operated, in particular controlled, using the computer program. For example, the optical transmission unit 10 with the light source 12 is controlled by the computer program. Conveniently, measurement data from the reception device 30 with the camera sensor 32 are acquired using the computer program. The evaluation of the measurement data is typically carried out using the computer program.The density fluctuations, in particular turbulence of the fluid 50, are determined from the changed optical pattern 26 by means of the computer program.

[0104] Reference symbol

[0105] 5 optical axis

[0106] 10 optical transmitting device

[0107] 12 Light source

[0108] 14 optical pattern generator

[0109] 16 Optical unit

[0110] 18 transmitting-side optical device

[0111] 19 optical lens

[0112] 21 single beam

[0113] 22 single beam

[0114] 24 optical pattern

[0115] 26 modified optical pattern

[0116] 28 Beam path

[0117] 30 optical receiving device

[0118] 32 camera sensor

[0119] 34 receiving-side telescope

[0120] 35 optical lens

[0121] 40 projection screen

[0122] 50 Fluid

[0123] 100 device

[0124] S200 Method for determining density variations

[0125] S210 Emission of a plurality of individual beams arranged in the form of an optical pattern by an optical transmission device

[0126] S220 Radiation of the fluid

[0127] S230 Receiving an image of a changed optical pattern generated in the fluid by an optical receiving device

[0128] S240 Analysis of the modified pattern recorded by the optical receiving device

Claims

Claims 1. A device (100) for determining density fluctuations, in particular turbulence, in a fluid (50), in particular in air, at least comprising an optical transmitting device (10) for emitting an optical pattern (24), which has a light source (12) and an optical, in particular transmissible, pattern generator (14), wherein the optical pattern generator (14) is designed to split light beams (20) received by the light source (12) into a plurality of individual beams (22) to form the optical pattern (24), and an optical receiving device (30) for receiving an image of the optical pattern (26) changed upon passing through the fluid (50), wherein the fluid (50) is arranged or can be arranged in the beam path (26) between the optical transmitting device (10) and the optical receiving device (30), wherein the receiving device is designed to detect a density fluctuation, in particular turbulence, of the fluid (50),from the received modified optical pattern (26)., 2. Device according to claim 1, wherein the optical transmitting device (10) has an optical unit (16) for parallelizing the emitted individual beams (22).

3. Device according to claim 1 or 2, wherein the optical receiving device (30) has a receiving-side telescope (34) which is designed to image the changed optical pattern (26) onto a camera sensor (32) of the optical receiving device (30).

4. Device according to one of the preceding claims, wherein a projection surface (40) is arranged in the beam path (28) of the plurality of individual beams (22) for imaging the optical pattern (24) and / or the modified optical pattern (26), wherein the optical receiving device (30) is designed to receive the optical pattern (24) and / or modified optical pattern (26) imaged onto the projection surface (40).

5. Device according to claim 4, wherein the fluid (50) is arranged in the beam path (26) before or after the projection surface (40).

6. Device according to one of the preceding claims, wherein the optical transmitting device (10) has a transmitting-side optical device (18), in particular a telescope, for imaging a light beam (20) of the light source (12) onto the optical pattern generator (14).

7. Device according to one of the preceding claims, wherein the optical receiving device (30) comprises a bandpass filter.

8. Device according to one of the preceding claims, wherein the light source (12) is designed to emit a pulsed light beam (20), in particular wherein the optical receiving device (30) is triggered or triggerable synchronously with the pulsed light beam (20).

9. Device according to one of the preceding claims, wherein the optical pattern (24) is designed as a pseudo-speckle pattern which can be generated by the pattern generator (14) from the plurality of individual beams (22).

10. Device according to one of the preceding claims, wherein the optical pattern generator (14) comprises at least one of the following components: diffractive optical element, LCD mask, light modulator, digital hologram.

11. Method for determining density fluctuations, in particular turbulences, in a fluid (50), in particular in air, with a device (100) according to one of the preceding claims, at least comprising Emitting a plurality of individual beams (22) arranged in the form of an optical pattern (24) by an optical transmitting device (10); Receiving an image of the optical pattern (26) changed upon passage through the fluid (50) by an optical receiving device (30), wherein the fluid (50) is arranged for irradiation by means of the plurality of individual beams (22) in the beam path (28) between the optical Transmitting device (10) and the optical receiving device (30), wherein a density fluctuation, in particular turbulence of the fluid (50), is evaluated from the received changed optical pattern (26).

12. The method according to claim 11, wherein the changed optical pattern (26) has an offset of the individual beams (22).

13. Method according to claim 11 or 12, wherein the density fluctuation or a density field is determined from the offset of the individual beams (22) in the changed pattern (26).

14. The method according to any one of claims 11 to 13, wherein the plurality of individual beams (22) are emitted as parallel individual beams (22).

15. The method according to any one of claims 11 to 14, wherein the optical pattern (24) or the changed optical pattern (26) is imaged onto a projection surface (40) arranged in the beam path (26) of the plurality of individual beams (22), wherein the optical pattern (24) imaged onto the projection surface (40) is received by the optical receiving device (30) as an optical pattern (26) changed upon passing through the fluid (50) or as the changed optical pattern (26) imaged onto the projection surface (40).

16. The method according to claim 15, wherein the fluid (50) is arranged in the beam path (28) before or after the projection surface (40).

17. The method according to any one of claims 11 to 16, wherein the received modified optical pattern (26) is filtered by a bandpass filter of the optical receiving device (30).

18. Method according to one of claims 11 to 17, wherein the light source (12) emits pulsed light beams (20), wherein the optical receiving device (30) is triggered synchronously with the pulsed light beams (20).

19. The method according to any one of claims 11 to 18, wherein a pseudo-speckle pattern generated from the plurality of individual beams (22) is received as a modified optical pattern (26) by the optical receiving device (30).

20. Use of a device (100) for determining density fluctuations, in particular turbulence, in a fluid (50), in particular in air, according to one of claims 1 to 10, wherein a plurality of individual beams (22) arranged in the form of an optical pattern (24) are emitted by means of an optical transmitting device (10), wherein the fluid (50) is arranged in a beam path (26) of the plurality of individual beams (22), wherein a changed optical pattern (26) is received by means of an optical receiving device (30), wherein a density fluctuation, in particular turbulence of the fluid (50) is evaluated from the received changed optical pattern (26).

21. Use of the device (100) according to claim 20, wherein the optical transmitting device (10) and / or the optical receiving device (30) are arranged on a mobile system, in particular an airworthy system.

22. Use of the device (100) according to claim 20 or 21 for Schlieren measurements in a fluid (50), in particular background-oriented Schlieren measurements and / or speckle-based background-oriented Schlieren measurements 23. Computer program for operating, in particular controlling, a device (100) according to one of claims 1 to 10 with a method according to one of claims 11 to 19.

24. A computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of claims 11 to 19.

25. Computer program according to claim 23 or 24, wherein an optical pattern generator (14) of the optical transmitting device (10) is controlled.

26. Computer program according to one of claims 23 to 25, wherein measurement data of a camera sensor (32) of the optical receiving device (30) are read out.

27. Computer program according to one of claims 23 to 26, wherein a density fluctuation, in particular turbulence of the fluid (50) is evaluated from a received changed optical pattern (26).

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