High Spatial Resolution Background Schlieren Measurement System and Method Based on Fresnel Lens

Abstract

The present invention discloses a high-spatial-resolution background schlieren measurement system and method based on a Fresnel lens. The system includes a point light source, a concave mirror, a Fresnel lens, a camera, an image acquisition and processing system, and a computer. The present invention adopts the background schlieren technique, based on the principle that light is deflected when propagating in a non-uniform gas refractive index environment. Using a point light source as the light source, it is placed at the focal length of the concave mirror. The light is reflected by the concave mirror into parallel light. The parallel light irradiates the Fresnel lens sprayed with random scatter points as the background pattern and then enters the test area. The light is deflected due to the change in the refractive index of the flow field in the test area and finally forms an image on the camera. The computer analyzes the captured image to obtain refractive index data. The present invention solves the problem that in the traditional background schlieren scheme, the scattering cone beam of each scatter point covers a large range of the flow field and the spatial resolution is low, and realizes the high-spatial-resolution measurement of the flow refractive index field.

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 a wide range of 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 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 estimation 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 is usually focused 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 address 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:

[0008] 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. The surface of the Fresnel lens is sprayed with a background pattern. 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. The image acquisition and processing system is connected to the computer.

[0009] To optimize the above technical solution, the specific measures taken also include:

[0010] The above-mentioned point light source is an LED point light source.

[0011] The electric power of the above-mentioned point light source ranges from 3 to 200 watts. The point light source is a monochromatic light source. The wavelength range of the point light source is from 350 to 650 nanometers.

[0012] The above-mentioned concave mirror is a parabolic reflector or a spherical reflector. The diameter range of the concave mirror is from 100 to 500 millimeters. The surface of the concave mirror is coated with an aluminum film and a protective film.

[0013] The background pattern sprayed on the surface of the above-mentioned Fresnel lens is random scatter points. The diameter of the random scatter points is less than 1 millimeter. The diameter of the Fresnel lens is equal to or greater than the diameter of the concave mirror.

[0014] The above-mentioned concave mirror and Fresnel lens are both slidably mounted on an adjustable mirror mount. The concave mirror and Fresnel lens can be adjusted in position by sliding on the adjustable mirror mount.

[0015] The above-mentioned camera is a high-speed industrial camera. The size range of the sensor of the camera is from 1 / 3 to 1 inch.

[0016] The above-mentioned 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 captured by the camera to the computer.

[0017] The processor of the above-mentioned computer is a multi-core CPU and the computer has a GPU acceleration function.

[0018] A method for measuring a flow field using the above-mentioned high-spatial-resolution background schlieren measurement system based on a Fresnel lens, comprising the following steps:

[0019] 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;

[0020] 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;

[0021] 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;

[0022] 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 collects the patterns taken by the camera and transmits them back to the computer;

[0023] Step S5: After the computer receives the image, obtain the displacement data of the background pattern through the displacement prediction algorithm;

[0024] 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.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] 1. Improve spatial resolution: By adopting the combination of a concave mirror and a Fresnel lens, the present invention overcomes the disadvantage that the conical beam of the scattered point light source in the traditional background schlieren technology covers a large range of the flow field, thereby significantly improving the spatial resolution of the measurement system. It can capture the detailed information in complex flow structures more clearly.

[0027] 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.

[0028] 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 researchers. Brief Description of the Drawings

[0029] 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;

[0030] Figure 2 Spatial resolution schematic diagram of the traditional background schlieren technology;

[0031] Figure 3 Spatial resolution schematic diagram of the high-spatial-resolution background schlieren measurement system based on Fresnel lens of the present invention;

[0032] Figure 4 Flowchart of the high-spatial-resolution background schlieren measurement method based on Fresnel lens of the present invention;

[0033] Reference numerals 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. Specific implementation manners

[0034] In order to make the objectives, technical solutions and advantages of the present application clearer and more understandable, the present application will be described and illustrated below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application. Based on the embodiments provided in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present application.

[0035] Obviously, the accompanying drawings in the following description are only some examples or embodiments of the present application. For those of ordinary skill in the art, without creative efforts, the present 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 the present application, some design, manufacturing or production changes based on the technical content disclosed in the present application are only conventional technical means and should not be understood as the content disclosed in the present application being insufficient.

[0036] When "embodiment" is mentioned in the present application, it means that the specific features, structures or characteristics described in conjunction with the embodiment can be included in at least one embodiment of the present 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. Those of ordinary skill in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments without conflict.

[0037] 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 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 back 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.

[0038] 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:

[0039] 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.

[0040] 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 irradiate the Fresnel lens 3, and its diameter range is 100 to 500 millimeters.

[0041] The purpose of combining the point light source 1 and the concave mirror 2 is to generate parallel light. In this embodiment, a reflective placement scheme is adopted. In specific implementation, a projection placement scheme can also be used.

[0042] 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, generally on the planar side of the Fresnel lens 3, which is easier to implement technically. The background pattern is a random scatter pattern, and the maximum scatter diameter is less than 1 mm. The density and size of the background pattern are optimized according to experimental requirements and the size of the flow field. If a plano-convex lens is selected to replace the Fresnel lens 3, the same effect can also be achieved. However, the processing technology of a large-diameter plano-convex lens is very complex, and the Fresnel lens 3 is the preferred choice considering practical use and cost.

[0043] The camera 4 uses a high-speed industrial camera. The camera 4 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 electrical connection or communication connection.

[0044] The image acquisition and processing system 5 is connected to the camera 4 and is controlled by the computer 6. This system can real-time collect the images captured by the camera 4 and transmit them to the computer 6.

[0045] The computer 6 serves as the data processing core and is equipped with a multi-core CPU and GPU acceleration function. The computer 6 is connected to the image acquisition and processing system 5, receives the image data and analyzes it through the displacement prediction algorithm. During the data analysis process, the computer 6 can automatically process the images, extract the key flow field refractive index information, reduce manual intervention, and improve the measurement accuracy and efficiency.

[0046] The schematic diagram of the spatial resolution of the traditional background schlieren technique is as Figure 2 shown. Any point on the background plate pattern is diffusely reflected and reflects light in all directions. Due to the limitation of the camera lens aperture, only a conical beam of light 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 this conical beam at the flow field to be measured is the blur circle. The displacement of the background pattern on the sensor is caused by the deflection of the light within the blur circle. Therefore, the diameter of the blur circle is the spatial resolution of the background schlieren system. The expression for the spatial resolution of the traditional background schlieren technique is:

[0047] Among them, is the spatial resolution of the background schlieren system, is the diameter of the lens aperture (i.e., the ratio of the lens focal length to the aperture ), 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 The range is 50mm to 140mm, the aperture The range is 2.8 to 16, The range of is 0.4 to 0.6, and the spatial resolution of traditional background schlieren technology ranges from 1.25 mm to 30 mm.

[0048] The high spatial resolution background schlieren system provided by the present invention has a schematic diagram of spatial resolution as shown in FIG. Figure 3 As shown, the principle of parallel light focusing on one point after passing through the Fresnel lens 3 is used. Only one light ray is transmitted at any point on the Fresnel lens 3, thereby eliminating the influence of the diffusion circle generated by the traditional background schlieren technology at the flow field to be measured on the spatial resolution, and improving the spatial resolution. Its spatial resolution is the diameter of the random scattered points sprayed on the Fresnel lens, which is usually less than 1 mm, thus greatly improving the spatial resolution of the background schlieren system.

[0049] like Figure 4 As shown, this embodiment provides a high spatial resolution background schlieren measurement method based on Fresnel lens 3, which specifically includes the following steps:

[0050] Step S1, preliminary construction of the background schlieren system: ensure that the point light source 1, concave mirror 2, Fresnel lens 3, camera 4, image acquisition and processing system 5 and 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, a flow field 7 to be measured is provided 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.

[0051] 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 by the concave mirror 2 as parallel light and is evenly irradiated to the test area through the Fresnel lens 3.

[0052] Step S3, flow field setting: setting flow field experimental conditions in the flow field area to be measured between the Fresnel lens 3 and the camera 4, including parameters such as flow velocity and temperature.

[0053] Step S4, shooting and data collection: appropriate shooting parameters are set on the computer 6, and 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 captured by the camera 4 and transmits it back to the computer 6.

[0054] Step S5, image processing: after receiving the image, the computer 6 obtains the background pattern displacement data through the displacement estimation algorithm.

[0055] Step S6, data analysis: Combining the background pattern displacement data and the optical path setting data, calculate to obtain the refractive index field data of the flow field 7 to be measured.

[0056] 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 , the refractive index of the flow field to be measured and the displacement of the background pattern The relationship is:

[0057] .

[0058] Among them, is the environmental refractive index, is the distance from the flow field 7 to be measured to the camera 4, and are respectively the displacements of the background pattern in the direction and the direction. The above equation is usually solved by the Poisson equation in the least squares sense, and the expression is: .

[0059] Solving the above Poisson equation can obtain the refractive index field distribution of the flow field 7 to be measured.

[0060] 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 a Fresnel lens, characterized in that It includes 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). The point light source (1) is placed at the focal point 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 focal point of the Fresnel lens (3). The surface of the Fresnel lens (3) is sprayed with a background pattern. The flow field to be measured (7) is placed between the Fresnel lens (3) and the camera (4). The camera (4) can observe the flow field to be measured (7) 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 and processing system (5), and the image acquisition and processing system (5) is connected to the computer (6).

2. The high-spatial-resolution background schlieren measurement system based on a Fresnel lens according to claim 1, wherein The said point light source (1) is an LED point light source.

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

4. The high-spatial-resolution background schlieren measurement system based on a Fresnel lens according to claim 1, wherein The said concave mirror (2) is a parabolic reflector or a spherical reflector. The diameter range of the concave mirror (2) is from 100 to 500 millimeters. 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 a Fresnel lens according to claim 1, wherein The background pattern sprayed on the surface of the said Fresnel lens (3) is random scatter points. The diameter of the random scatter points is less than 1 millimeter. 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 a Fresnel lens according to claim 1, wherein Both the said concave mirror (2) and the Fresnel lens (3) are slidably mounted on an adjustable mirror base. The said concave mirror (2) and the Fresnel lens (3) can be adjusted in position by sliding on the adjustable mirror base.

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

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

10. A method for measuring a flow field using the Fresnel lens-based high-spatial-resolution background schlieren measurement system according to any one of claims 1 to 9, characterized in that, It includes the following steps: Step S1: Install 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). Place the point light source (1) at the focal point of the concave mirror (2). Spray the background pattern on the surface of the Fresnel lens (3). Set the flow field to be measured (7) between the Fresnel lens (3) and the camera (4). Connect the camera (4) to the image acquisition and processing system (5), and connect the image acquisition and processing system (5) to the computer (6). Step S2: 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 by the concave mirror (2) into parallel light and evenly irradiates the area of the flow field to be measured through the Fresnel lens (3). Step S3: Set the flow field experimental conditions in the flow field area to be measured between the Fresnel lens (3) and the camera (4). The flow field experimental conditions include flow velocity and temperature parameters. Step S4: Set the 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 take pictures and acquires the patterns taken by the camera (4) and transmits them back to the computer (6). Step S5: After the computer (6) receives the images, obtain the background pattern displacement data through the displacement prediction algorithm. Step S6: Combine the background pattern displacement data with the optical path setting data to calculate and obtain the refractive index field data of the flow field to be measured (7).

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