High-spatial-resolution background schlieren measurement system and method based on Fresnel lens

Abstract

The invention discloses a high-spatial-resolution background schlieren measurement system and method based on a Fresnel lens. The system comprises a point light source, a concave mirror, the Fresnel lens, a camera, an image acquisition and processing system and a computer. The background schlieren technology is adopted, based on the principle that light deflects when propagating in a non-uniform gas refractive index environment, a point light source serves as a light source and is arranged at the focal length of a concave mirror, the light is reflected into parallel light through the concave mirror, and the parallel light irradiates a Fresnel lens sprayed with random scattered points serving as background patterns and then enters a test area. The light is deflected due to the change of the refractive index of the flow field in the test area and is finally imaged in the camera, and the computer analyzes the shot image to obtain refractive index data. According to the invention, the problems of large flow field coverage range and low spatial resolution of each scattering point scattering conical light beam in the traditional background schlieren scheme are solved, and high spatial resolution measurement of the flow refractive index field is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical measurement of flow fields, and particularly relates to a high-spatial-resolution background schlieren measurement system and method based on a Fresnel lens, which are used to achieve high-spatial-resolution measurement of the refractive index field of complex flow structures and accurately capture the detailed information of complex flow structures. Background Art

[0002] Background Oriented Schlieren (BOS) is an advanced non-contact optical measurement technology mainly used for quantitative measurement of the refractive index field of variable-density flows. This technology has wide applications in complex flow scenarios such as supersonic flows, combustion, and plasmas. Compared with traditional schlieren technologies such as blade-type and rainbow-type schlieren measurement technologies, it has significant advantages such as simple optical path setup and convenient calibration.

[0003] The basic principle of the background schlieren technology lies in the deflection of light when it propagates in a non-uniform refractive index environment. When light passes through a medium with a density gradient, the change in refractive index causes the light to deflect. In a background schlieren experiment, two images are acquired: one is a reference image not affected by the density gradient, and the other is an image captured in the presence of flow disturbances. By applying a displacement prediction algorithm to analyze these two images, we can obtain the displacement information of background points, and then use this displacement field data to quantitatively reconstruct the refractive index distribution of the flow field.

[0004] However, in a background schlieren experiment, the camera usually focuses on the background plate pattern. The conical light beam reflected by the background plate pattern converges on the sensor to form an image. These conical light beams form a blur circle in the area of the flow field to be measured, affecting the spatial resolution of the measurement. To improve the above problems, a Chinese patent with the patent number CN202311772662.6 discloses a high-spatial-resolution background schlieren system. However, the single-mirror off-axis optical path adopted in this invention allows the light to pass through the refractive index field twice, and the light distribution is uneven after the first pass through the refractive index field, greatly affecting the measurement accuracy.

[0005] Therefore, it is urgent to propose a technical solution to eliminate the influence of the blur circle and allow the light to enter the refractive index field uniformly and reach the camera sensor, so as to achieve high-spatial-resolution measurement of the refractive index field of the flow field to be measured. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a high-spatial-resolution background schlieren measurement system and method based on a Fresnel lens in view of the above-mentioned existing deficiencies. The present invention can effectively eliminate the influence of the blur circle on the spatial resolution of the measurement of the flow refractive index field and significantly improve the measurement accuracy.

[0007] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows: A high-spatial-resolution background schlieren measurement system based on a Fresnel lens, comprising a point light source, a concave mirror, a Fresnel lens, a camera, an image acquisition and processing system, and a computer. The point light source is placed at the focal point of the concave mirror, the camera is placed on one side of the concave surface of the concave mirror, the Fresnel lens is placed between the concave mirror and the camera, the camera is located at the focal point of the Fresnel lens, a background pattern is sprayed on the surface of the Fresnel lens, the flow field to be measured is placed between the Fresnel lens and the camera, the camera can observe the flow field to be measured with the background pattern on the surface of the Fresnel lens as the background, the concave mirror reflects the light of the point light source to the Fresnel lens, the Fresnel lens transmits the light to the camera, the camera is connected to the image acquisition and processing system, and the image acquisition and processing system is connected to the computer.

[0008] To optimize the above technical solution, the specific measures taken also include: The above point light source is an LED point light source.

[0009] The electric power of the above point light source ranges from 3 to 200 watts, the point light source is a monochromatic light source, and the wavelength range of the point light source is from 350 to 650 nanometers.

[0010] The above concave mirror is a parabolic reflector or a spherical reflector, the diameter range of the concave mirror is from 100 to 500 millimeters, and the surface of the concave mirror is coated with an aluminum film and a protective film.

[0011] The background pattern sprayed on the surface of the above Fresnel lens is random scattered dots, the diameter of the random scattered dots is less than 1 millimeter, and the diameter of the Fresnel lens is equal to or greater than the diameter of the concave mirror.

[0012] The above concave mirror and Fresnel lens are both slidably mounted on an adjustable mirror base, and the concave mirror and Fresnel lens can be adjusted in position by sliding on the adjustable mirror base.

[0013] The above camera is a high-speed industrial camera, and the size range of the sensor of the camera is from 1 / 3 to 1 inch.

[0014] The above image acquisition and processing system is connected to the shutter signal of the camera, the image acquisition and processing system can receive the signal from the computer, control the shutter of the camera, and send the image taken by the camera to the computer.

[0015] The processor of the above computer is a multi-core CPU and the computer has a GPU acceleration function.

[0016] A method for measuring a flow field using the above high-spatial-resolution background schlieren measurement system based on a Fresnel lens, comprising the following steps: Step S1: Install a point light source, a concave mirror, a Fresnel lens, a camera, an image acquisition and processing system, and a computer. Place the point light source at the focal point of the concave mirror, spray a background pattern on the surface of the Fresnel lens, set the flow field to be measured between the Fresnel lens and the camera, connect the camera to the image acquisition and processing system, and connect the image acquisition and processing system to the computer; Step S2: Adjust the positions of the point light source, the concave mirror, and the Fresnel lens to ensure that the light is reflected by the concave mirror into parallel light and uniformly irradiates the area of the flow field to be measured through the Fresnel lens; Step S3: Set the flow field experimental conditions in the area of the flow field to be measured between the Fresnel lens and the camera. The flow field experimental conditions include flow velocity and temperature parameters; Step S4: Set the shooting parameters in the computer. The computer transmits the shooting signal to the image acquisition and processing system. The image acquisition and processing system controls the camera to take pictures and transmits the patterns taken by the camera back to the computer; Step S5: After the computer receives the image, obtain the displacement data of the background pattern through the displacement prediction algorithm; Step S6: Combine the displacement data of the background pattern with the optical path setting data to calculate and obtain the refractive index field data of the flow field to be measured.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Improve spatial resolution: By adopting the combination of a concave mirror and a Fresnel lens, the present invention overcomes the disadvantage of the large coverage range of the conical beam of the scattered point light source in the traditional background schlieren technology, thus significantly improving the spatial resolution of the measurement system. It can capture the detailed information in complex flow structures more clearly.

[0018] 2. Automatic data processing and analysis: Combining the image acquisition and processing system and computer analysis, the present invention not only realizes the rapid acquisition of images, but also can automatically process the captured images, extract the key refractive index information of the flow field, reduce manual intervention, and improve the measurement efficiency and analysis accuracy.

[0019] 3. Wide application prospects: This technology has wide application prospects in the fields of aerodynamics, fluid mechanics, gas dynamics, etc. Especially in complex flow environments such as aerospace, combustion engineering, and gas injection, it can provide accurate flow field measurement data for scientific research personnel. Description of the Drawings

[0020] Figure 1 It is a schematic structural diagram of the high-spatial-resolution background schlieren measurement system based on the Fresnel lens of the present invention; Figure 2 It is a schematic diagram of the spatial resolution of the traditional background schlieren technology; Figure 3Spatial resolution schematic diagram of the high-spatial-resolution background schlieren measurement system based on Fresnel lens of the present invention; Figure 4 Flow chart of the high-spatial-resolution background schlieren measurement method based on Fresnel lens of the present invention; The reference signs in the figure are: 1 is a point light source, 2 is a concave mirror, 3 is a Fresnel lens, 4 is a camera, 5 is an image acquisition and processing system, 6 is a computer, and 7 is a flow field to be measured. Detailed implementation manners

[0021] In order to make the objectives, technical solutions and advantages of this application clearer, the following describes and explains this application with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit this application. Based on the embodiments provided in this application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of this application.

[0022] Obviously, the accompanying drawings in the following description are only some examples or embodiments of this application. For those of ordinary skill in the art, without making creative efforts, this application can also be applied to other similar scenarios based on these drawings. In addition, it can also be understood that although the efforts made in this development process may be complex and lengthy, for those of ordinary skill in the art related to the content disclosed in this application, some design, manufacturing or production changes based on the technical content disclosed in this application are only conventional technical means and should not be understood as the content disclosed in this application being insufficient.

[0023] When "embodiment" is mentioned in this application, it means that the specific features, structures or characteristics described in combination with the embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. It is explicitly and implicitly understood by those of ordinary skill in the art that the embodiments described in this application can be combined with other embodiments without conflict.

[0024] Unless otherwise defined, the technical terms or scientific terms involved in this application shall have the ordinary meanings understood by those with ordinary skills in the technical field to which this application belongs. The words such as "a", "an", "one kind", "the" and the like involved in this application do not indicate a quantity limitation and may represent a singular or plural number. The terms "include", "comprise", "have" and any variations thereof involved in this application are intended to cover non-exclusive inclusion; for example, a process, method, system, product or device that includes a series of steps or units (units) is not limited to the listed steps or units, but may further include steps or units not listed, or may further include other steps or units inherent to these processes, methods, products or devices. The similar words such as "connect", "be connected", "couple" and the like involved in this application are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. The "multiple" / "several" involved in this application refers to two or more. "And / or" describes the association relationship of associated objects and indicates that three relationships may exist. For example, "A and / or B" may represent: A exists alone, A and B exist simultaneously, and B exists alone. The character " / " generally represents an "or" relationship between the front and rear associated objects. The terms "first", "second", "third" and the like involved in this application are only used to distinguish similar objects and do not represent a specific order for the objects.

[0025] As Figure 1 shown, this embodiment provides a high-spatial-resolution background schlieren measurement system based on a Fresnel lens, including the following components: 1 is a point light source, 2 is a concave mirror, 3 is a Fresnel lens, 4 is a camera, 5 is an image acquisition and processing system, 6 is a computer, and 7 is a flow field to be measured. The point light source 1 is placed at the focal point of the concave mirror 2. The concave mirror 2 and the Fresnel lens 3 are placed on an adjustable mirror mount. The surface of the Fresnel lens 3 is sprayed with a background pattern. A flow field 7 to be measured is provided between the Fresnel lens 3 and the camera 4. The camera 4 is connected to the image acquisition and processing system 5, and the image acquisition and processing system 5 is connected to the computer 6. Each component has the following specific characteristics: The point light source 1 uses an LED point light source and is placed at the focal point of the concave mirror 2. The LED point light source has an adjustable power characteristic, with a power range of 3 to 200 watts and a wavelength range of 350 to 650 nanometers, ensuring sufficient light intensity under different measurement conditions.

[0026] The concave mirror 2 uses a high-quality parabolic reflector. The surface is aluminized to improve the reflection efficiency of the mirror surface, and a protective film is plated on the surface to improve the corrosion resistance and durability of the mirror surface. The main function of the concave mirror 2 is to turn the light emitted by the point light source 1 into parallel light, ensuring that the light can evenly illuminate the Fresnel lens 3, and its diameter range is 100 to 500 millimeters.

[0027] The purpose of the combination of the point light source 1 and the concave mirror 2 is to generate parallel light. In this embodiment, a reflective placement solution is adopted. In a specific implementation, a projection placement solution can also be adopted.

[0028] The diameter of the Fresnel lens 3 is equal to or greater than the diameter of the concave mirror 2. A high-precision background pattern is sprayed on the surface of the Fresnel lens 3, which is generally sprayed on the plane side of the Fresnel lens 3, which is technically easier to achieve. The background pattern is a random scattered point pattern, and the maximum scattered point diameter is less than 1 mm. The density and size of the background pattern are optimized according to the experimental requirements and the flow field size. If a plano-convex lens is used to replace the Fresnel lens 3, the same effect can be achieved. However, the processing technology of a large-diameter plano-convex lens is very complicated. The Fresnel lens 3 is the preferred choice considering practical use and cost.

[0029] The camera 4 uses a high-speed industrial camera and is mounted on a tripod to ensure stability during the experiment. The camera 4 is connected to the image acquisition and processing system 5 through an electrical connection or a communication connection.

[0030] The image acquisition and processing system 5 is connected to the camera 4 and is controlled by the computer 6. The system can acquire images taken by the camera 4 in real time and transmit them to the computer 6.

[0031] Computer 6 is a data processing core equipped with multi-core CPU and GPU acceleration function. Computer 6 is connected to image acquisition and processing system 5, receives image data and analyzes it through displacement estimation method. During data analysis, computer 6 can automatically process images, extract key flow field refractive index information, reduce manual intervention, and improve measurement accuracy and efficiency.

[0032] Schematic diagram of the spatial resolution of traditional background schlieren technology Figure 2 As shown in the figure, any point of the background pattern is diffusely reflected and reflects light in all directions. However, due to the limitation of the camera lens aperture, only one cone beam can pass through the flow field to be measured and reach the lens, and finally form an image on the sensor. The cross section of the cone beam at the flow field to be measured is the diffusion circle. The displacement of the background pattern on the sensor is caused by the deflection of light within the diffusion circle. Therefore, the diameter of the diffusion circle is the spatial resolution of the background Schlieren system. The spatial resolution expression of the traditional background Schlieren technology is:

[0033] in, is the spatial resolution of the background schlieren system, is the diameter of the lens aperture (i.e. the focal length of the lens With aperture ratio), is the ratio of the distance from the measured flow field to the lens to the distance from the background plate to the lens. Considering the spatial arrangement of common background schlieren systems, such as the focal length of the lens ranges from 50 millimeters to 140 millimeters, and the aperture ranges from 2.8 to 16, ranges from 0.4 to 0.6, and the spatial resolution of the traditional background schlieren technique ranges from 1.25 millimeters to 30 millimeters.

[0034] The high-spatial-resolution background schlieren system provided by the present invention has a schematic diagram of spatial resolution as shown in Figure 3 Figure . It utilizes the principle that parallel light is focused at a point after passing through the Fresnel lens 3. Only one ray is transmitted through any point on the Fresnel lens 3. Therefore, the influence of the dispersion circle generated by the traditional background schlieren technique at the flow field to be measured on the spatial resolution is eliminated, and the spatial resolution is improved. Its spatial resolution is the diameter of the random scatter points sprayed on the Fresnel lens, usually less than 1 millimeter. Therefore, the spatial resolution of the background schlieren system is greatly improved.

[0035] As shown in Figure 4 Figure , this embodiment provides a high-spatial-resolution background schlieren measurement method based on the Fresnel lens 3, which specifically includes the following steps: Step S1, preliminary setup of the background schlieren system: Ensure that the point light source 1, the concave mirror 2, the Fresnel lens 3, the camera 4, the image acquisition and processing system 5, and the computer 6 are all correctly installed and connected. The point light source 1 is placed at the focus of the concave mirror 2. The concave mirror 2 and the Fresnel lens 3 are placed on an adjustable mirror mount. The surface of the Fresnel lens 3 is sprayed with a background pattern. There is a flow field 7 to be measured between the Fresnel lens 3 and the camera 4. The image acquisition and processing system 5 is connected to the camera 4, and the computer 6 is connected to the image acquisition and processing system 5.

[0036] Step S2, system calibration: Adjust the positions of the point light source 1, the concave mirror 2, and the Fresnel lens 3 to ensure that the light is reflected as parallel light by the concave mirror 2 and uniformly irradiates the test area through the Fresnel lens 3.

[0037] Step S3, flow field setup: Set the flow field experimental conditions, including parameters such as flow velocity and temperature, in the flow field area to be measured between the Fresnel lens 3 and the camera 4.

[0038] Step S4, shooting and data acquisition: Set appropriate shooting parameters on the computer 6. The computer 6 transmits the shooting signal to the image acquisition and processing system 5. The image acquisition and processing system 5 controls the camera 4 to take pictures and acquires the patterns taken by the camera 4 and transmits them back to the computer 6.

[0039] Step S5, image processing: After the computer 6 receives the image, it obtains the displacement data of the background pattern through a displacement prediction algorithm.

[0040] Step S6, data analysis: Combine the displacement data of the background pattern with the optical path setup data to calculate and obtain the refractive index field data of the flow field 7 to be measured.

[0041] Taking the two-dimensional case as an example, under the assumption of the near optical axis, the thickness of the flow field 7 to be measured is , and the refractive index of the flow field 7 to be measured and the displacement of the background pattern are related as follows: .

[0042] Among them, is the environmental refractive index, is the distance from the flow field 7 to be measured to the camera 4, and are the displacements of the background pattern in the direction and the direction respectively. The above equation is usually solved by the Poisson equation in the least squares sense, and the expression is: .

[0043] By solving the above Poisson equation, the refractive index field distribution of the flow field 7 to be measured can be obtained.

[0044] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the described embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A high spatial resolution background schlieren measurement system based on Fresnel lens, characterized in that: The invention comprises a point light source (1), a concave mirror (2), a Fresnel lens (3), a camera (4), an image acquisition processing system (5) and a computer (6). The point light source (1) is placed at the focus of the concave mirror (2), the camera (4) is placed on one side of the concave surface of the concave mirror (2), the Fresnel lens (3) is placed between the concave mirror (2) and the camera (4), the camera (4) is located at the focus of the Fresnel lens (3), a background pattern is sprayed on the surface of the Fresnel lens (3), a flow field (7) to be measured is placed between the Fresnel lens (3) and the camera (4), the camera (4) can observe the flow field (7) to be measured with the background pattern on the surface of the Fresnel lens (3) as the background, the concave mirror (2) reflects the light of the point light source (1) to the Fresnel lens (3), the Fresnel lens (3) transmits the light to the camera (4), the camera (4) is connected to the image acquisition processing system (5), and the image acquisition processing system (5) is connected to the computer (6).

2. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The point light source (1) is an LED point light source.

3. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The electric power of the point light source (1) is in the range of 3 to 200 watts, the point light source (1) is a monochromatic light source, and the wavelength of the point light source (1) is in the range of 350 to 650 nanometers.

4. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The concave mirror (2) is a parabolic reflector or a spherical reflector, the diameter of the concave mirror (2) ranges from 100 to 500 mm, and the surface of the concave mirror (2) is coated with an aluminum film and a protective film.

5. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The background pattern sprayed on the surface of the Fresnel lens (3) is random scattered points, the diameter of the random scattered points is less than 1 mm, and the diameter of the Fresnel lens (3) is equal to or greater than the diameter of the concave mirror (2).

6. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The concave mirror (2) and the Fresnel lens (3) are both slidably mounted on an adjustable mirror seat, and the positions of the concave mirror (2) and the Fresnel lens (3) can be adjusted by sliding on the adjustable mirror seat.

7. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The camera (4) is a high-speed industrial camera, and the size of the sensor of the camera (4) ranges from 1 / 3 to 1 inch.

8. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The image acquisition and processing system (5) is connected to the shutter signal of the camera (4). The image acquisition and processing system (5) can receive the signal of the computer (6), control the shutter of the camera (4), and send the image taken by the camera (4) to the computer (6).

9. The high spatial resolution background schlieren measurement system based on Fresnel lens according to claim 1, characterized in that: The processor of the computer (6) is a multi-core CPU and the computer (6) has a GPU acceleration function.

10. A method for flow field measurement using the high spatial resolution background schlieren measurement system based on Fresnel lens according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step S1, installing a point light source (1), a concave mirror (2), a Fresnel lens (3), a camera (4), an image acquisition and processing system (5) and a computer (6), wherein the point light source (1) is placed at the focus of the concave mirror (2), a background pattern is sprayed on the surface of the Fresnel lens (3), a flow field to be measured (7) is set between the Fresnel lens (3) and the camera (4), the camera (4) is connected to the image acquisition and processing system (5), and the image acquisition and processing system (5) is connected to the computer (6); Step S2, adjusting the positions of the point light source (1), the concave mirror (2) and the Fresnel lens (3) to ensure that the light is reflected by the concave mirror (2) as parallel light and is evenly irradiated to the flow field area to be measured through the Fresnel lens (3); Step S3, setting flow field experimental conditions in the flow field area to be measured between the Fresnel lens (3) and the camera (4), wherein the flow field experimental conditions include flow velocity and temperature parameters; Step S4, setting shooting parameters in the computer (6), the computer (6) transmits the shooting signal to the image acquisition and processing system (5), the image acquisition and processing system (5) controls the camera (4) to shoot and collects the pattern shot by the camera (4) and transmits it back to the computer (6); Step S5, after receiving the image, the computer (6) obtains the background pattern displacement data through a displacement estimation algorithm; Step S6, combining the background pattern displacement data and the light path setting data, and calculating and obtaining the refractive index field data of the flow field (7) to be measured.

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