Grating-free field-lens-free large-view-field focusing schlieren system

Abstract

The invention belongs to the technical field of wind tunnel experiments, and discloses a grid-free field-lens-free large-view-field focusing schlieren system. The grid-free field-lens-free large-view-field focusing schlieren system comprises a light source, a small hole, a collimating convex lens, a spectroscope, a model, a focusing convex lens, a knife edge and a scientific camera. Light emitted by the light source passes through the small hole, reaches the collimating convex lens and becomes a quasi-parallel light beam after passing through the collimating convex lens, the light beam passes through the spectroscope, reaches the model, returns to the spectroscope after being reflected by the model and reaches the focusing convex lens after being reflected by the spectroscope, and the focusing convex lens converges the light beam to the knife edge; and finally, focusing the to-be-measured flow field area by the scientific camera and then acquiring a schlieren image. According to the grating-free field-lens-free large-view-field focusing schlieren system, flow field density gradient distribution with small optical path integral effect under the large-view-field condition is obtained in a large wind tunnel, and the grating-free field-lens-free large-view-field focusing schlieren system has engineering practical value.

Description

Technical Field

[0001] The invention belongs to the technical field of wind tunnel experiments, and in particular relates to a gridless and field-mirror-free large-field-of-view focusing schlieren system. Background Art

[0002] In the field of high-speed and hypersonic wind tunnel experimental technology, schlieren is a widely used non-contact measurement experimental technology, which uses the property that the refractive index of light in fluid media of different densities to obtain the density gradient distribution of the flow field to be measured.

[0003] The Schlieren results obtained by conventional parallel light non-focused Schlieren have an integral effect along the optical path. If the wind tunnel test model adopts a conventional layout, that is, the normal direction of the model surface is upward, and the span direction is horizontal and parallel to the measurement optical path, then the Schlieren results obtained are the results of span-wise integration, which is mainly used for the measurement of two-dimensional flow with no span-wise change. In order to measure the three-dimensional flow structure with span-wise flow, the model can be rotated 90°, that is, the normal direction of the model surface is parallel to the measurement optical path, and the span direction of the model surface is perpendicular to the measurement optical path. If the model is made of transparent material or the surface coating becomes part of the optical path, the span-wise integration problem can be avoided. However, the use of translucent materials will inevitably introduce errors caused by the incomplete symmetry of the flow on both sides of the model. Although the reflective schlieren coated on the surface of the model can solve this problem, the schlieren results obtained are the results of integration along the normal direction of the model surface. If there are additional disturbances along the normal direction of the model, errors will also occur. For example, for supersonic wind tunnels that do not use a soft wall integrated design for the nozzle and the test section, and the most common jet-type hypersonic wind tunnel, the disturbance at the nozzle outlet will inevitably develop downstream, causing the flow field density to change, thereby interfering with the final schlieren results. In addition, traditional parallel light non-focused schlieren usually uses a concave mirror with a focal length greater than 3000mm to generate parallel light and converge parallel light. A scientific camera can only focus to near infinity to observe a clear image. If it is forced to focus to a close distance, the camera can only observe a very bright point instead of a clear image area due to the long focal length of the concave mirror.

[0004] The use of focused schlieren technology can reduce the integral effect along the optical path to a certain extent, thereby reducing the interference caused by the nozzle outlet disturbance. The traditional focused schlieren system includes an imaging lens (also known as a field lens) and a pair of source grids and knife-edge grids. The source grid and the knife-edge grid are required to ensure strict optical conjugation, otherwise the uniformity and sensitivity of the schlieren results will be greatly adversely affected. In practice, in order to ensure the optical conjugation relationship between the source grid and the knife-edge grid, the experimenter needs to spend a lot of time and energy to fine-tune the optical path. The Chinese Patent Literature Library discloses an invention named a focused schlieren system and method based on adaptive generation of knife-edge grids (CN202010678170). The method proposed in the application for adaptively generating knife-edge grids through transparent liquid crystal display screens can theoretically reduce the difficulty of adjustment, but it still does not get rid of the traditional idea of ​​using source grids plus field lenses and knife-edge grids, and the high-precision transparent liquid crystal display screens used are expensive, which is not conducive to popularization and use.

[0005] Fresnel lenses are often used in traditional focusing schlieren systems to simply converge the divergent light of the light source. The measurement area is located between the source grid and the field lens, which is usually a finished commercial lens. In large wind tunnel applications, the field lens is mostly a telephoto lens with a focal length of more than 200mm and an aperture of F4 or even F5.6. The divergent light source not only makes the light spot uneven, bright in the middle and dark around, affecting the result interpretation, but also is limited by the size of the field lens. Only a small part of the light emitted by the divergent light source passes through the field lens into the scientific camera, and the light intensity of the light source is not fully utilized, resulting in a dark field of view and low temporal resolution of the entire system. On the other hand, in traditional focused schlieren, the size of the observation field of the measurement area depends on the size of the source grid, the distance from the grid to the measurement area, and the distance from the measurement area to the field mirror. After the sizes of the source grid and the knife-edge grid are determined, the closer the measurement area is to the source grid, the larger the observation field of view. However, the distance from the core flow field of a large wind tunnel to the source grid and scientific cameras outside the test section is usually on the order of several meters. At this time, it is not only difficult to fully utilize the size of the light source spot (the light intensity is low), but the field of view of the measurement area is also small. After the size ratio of the source grid and the knife-edge grid is determined, the relative position relationship of each component is basically determined, so it cannot be moved back and forth to adapt to the test requirements of different scenarios. Such many limitations make focused schlieren difficult to be effectively applied in large wind tunnels.

[0006] In order to obtain the flow field density gradient distribution with small optical path integral effect under large field of view conditions in large wind tunnels, it is urgently necessary to develop a gridless and field mirror-free large field of view focusing schlieren system. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a gridless and field-mirror-free large-field-of-view focusing schlieren system.

[0008] The gridless and field-mirror-free large-viewing-field focusing schlieren system of the present invention comprises a light source, a pinhole, a collimating convex lens, a beam splitter, a model, a focusing convex lens, a knife edge and a scientific camera; The light emitted by the light source passes through the small hole and reaches the collimating convex lens. After passing through the collimating convex lens, it becomes a quasi-parallel light beam. The light beam penetrates the beam splitter and reaches the model. After being reflected by the model, it returns to the beam splitter. After being reflected by the beam splitter, it reaches the focusing convex lens. The focusing convex lens converges the light beam to the knife edge. Finally, the scientific camera focuses on the flow field area to be measured and then collects the schlieren image.

[0009] Furthermore, the surface of the model is coated.

[0010] Furthermore, the shape of the small hole is a rectangle, the length of the long side of the rectangle is in the range of 0.5mm~2mm, and the length of the short side of the rectangle is in the range of 0.2mm~1mm.

[0011] Furthermore, the collimating convex lens is a plano-convex lens with a diameter not less than 100 mm and a focal length not greater than 500 mm.

[0012] Furthermore, the focusing convex lens is a plano-convex lens with a diameter not less than 100 mm and a focal length not greater than 500 mm.

[0013] Furthermore, the collimating convex lens collimates the divergent light into a quasi-parallel light beam; the collimating convex lens and the focusing convex lens perform imaging together; when installed, the light beam is dispersed by reducing the focal length of the collimating convex lens and the focusing convex lens, thereby reducing the depth of field and improving the focus.

[0014] Furthermore, the optical path distance from the collimating convex lens to the focusing convex lens 6 is adjusted to match wind tunnels of different sizes.

[0015] Furthermore, the scientific camera gradually focuses from the nearest point to the farthest point until the image of the area to be measured is the clearest.

[0016] The gridless and field-lens-free large-field focusing Schlieren system of the present invention does not include components such as source grids, knife-edge grids, and field lenses in traditional focusing Schlieren, greatly simplifies the optical composition, and greatly reduces the difficulty of adjusting the optical path. Compared with traditional focusing Schlieren, two convex lenses replace field lenses for imaging, and the collimating convex lens makes the light more concentrated, the light intensity is stronger, the exposure time can be shorter, and the time resolution is higher.

[0017] The collimating convex lens in the gridless and field-mirror-free large-field-of-view focusing schlieren system of the present invention not only plays the role of collimating the divergent light into a quasi-parallel light beam, but also plays the role of imaging similar to a field lens together with the focusing convex lens. By reducing the focal length of the collimating convex lens and the focusing convex lens, the light beam is made more dispersed, thereby achieving the effect of reducing the depth of field and improving the focusing. Therefore, although the optical path of the gridless and field-mirror-free large-field-of-view focusing schlieren system of the present invention is similar to the traditional parallel light non-focusing schlieren, it produces the effect of focusing schlieren.

[0018] The scientific camera of the present invention is different from the traditional parallel light non-focused schlieren that focuses to the vicinity of infinity when focusing, but focuses from the nearest point to the far distance gradually until the image of the area to be measured is the clearest. The traditional parallel light non-focused schlieren has a long focal length of the concave mirror and a high degree of light beam parallelism. When the camera focuses, it can only focus to the vicinity of infinity to obtain a clear image. The gridless and field-mirror-free large-field focusing schlieren system of the present invention adopts a collimating convex lens and a focusing convex lens with a relatively short focal length, making it possible to focus at close range.

[0019] In summary, the gridless and field-lens-free large-field-of-view focusing schlieren system of the present invention eliminates the source grid, knife-edge grid, and field lens in the traditional focusing schlieren, greatly simplifies the optical path structure, and reduces the difficulty of optical path adjustment; by adopting a small hole and a large-diameter collimating convex lens to form a large-sized uniform light spot, and then using it together with a focusing convex lens instead of a field lens for imaging, the light intensity utilization rate is higher, and the observation field range is also increased, solving the problems of low light intensity, uneven light spot, and small observation field range in the traditional focusing schlieren; by changing the distance from the collimating convex lens to the focusing convex lens, it can be adapted to wind tunnels of different sizes, solving the problem that the traditional focusing schlieren is difficult to apply to large wind tunnels; by reducing the focal length of the collimating convex lens and the focusing convex lens, the depth of field is reduced, and the integral effect of the schlieren along the optical path is weakened, thereby enhancing the focusing effect, reducing the focal length of the collimating convex lens and the focusing convex lens, and at the same time making it possible for the scientific camera to focus to a close distance to see a clear area to be measured, solving the problem that the traditional non-focusing schlieren has no focusing effect. The flow field density gradient distribution with small optical path integral effect under large field of view conditions was achieved in a large wind tunnel, which has practical engineering value. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the overall structure of the gridless and field-mirror-less large-field focusing schlieren system of the present invention (stereoscopic diagram); Figure 2 This is a typical schlieren test result of the gridless and field-mirror-less large-field-of-view focusing schlieren system of the present invention when it is focused on section 1; Figure 3 This is a typical schlieren test result of the gridless and field-mirror-less large-field-of-view focusing schlieren system of the present invention, which is focused on section 2.

[0021] In the picture, 1. light source; 2. pinhole; 3. collimating convex lens; 4. beam splitter; 5. model; 6. focusing convex lens; 7. knife edge; 8. scientific camera. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0023] Example 1: Figure 1 As shown, the gridless and field-mirror-free large-field focusing schlieren system of this embodiment includes a light source 1, a pinhole 2, a collimating convex lens 3, a beam splitter 4, a model 5, a focusing convex lens 6, a knife edge 7 and a scientific camera 8; The light emitted by the light source 1 passes through the pinhole 2 and reaches the collimating convex lens 3. After passing through the collimating convex lens 3, it becomes a quasi-parallel light beam. The light beam penetrates the beam splitter 4 and reaches the model 5. After being reflected by the model 5, it returns to the beam splitter 4. After being reflected by the beam splitter 4, it reaches the focusing convex lens 6. The focusing convex lens 6 converges the light beam to the knife edge 7. Finally, the scientific camera 8 focuses on the flow field area to be measured and then collects the schlieren image.

[0024] Furthermore, the surface of the model 5 is coated.

[0025] Furthermore, the small hole 2 is in the shape of a rectangle, the length of the long side of the rectangle is in the range of 0.5 mm to 2 mm, and the length of the short side of the rectangle is in the range of 0.2 mm to 1 mm.

[0026] Furthermore, the collimating convex lens 3 is a plano-convex lens with a diameter not less than 100 mm and a focal length not greater than 500 mm.

[0027] Furthermore, the focusing convex lens 6 is a plano-convex lens with a diameter not less than 100 mm and a focal length not greater than 500 mm.

[0028] Furthermore, the collimating convex lens 3 collimates the divergent light into a quasi-parallel light beam; the collimating convex lens 3 and the focusing convex lens 6 perform imaging together; during installation, the light beam is dispersed by reducing the focal length of the collimating convex lens 3 and the focusing convex lens 6, thereby reducing the depth of field and improving the focus.

[0029] Furthermore, the optical path distance from the collimating convex lens 3 to the focusing convex lens 6 is adjusted to match wind tunnels of different sizes, thereby solving the problem that traditional focusing schlieren are difficult to apply to large wind tunnels.

[0030] Furthermore, the scientific camera 8 gradually focuses from the nearest point to the farthest point until the image of the area to be measured is the clearest.

[0031] Figure 2 This is a typical schlieren test result of the gridless and field-mirror-less large-field focusing schlieren system of the present invention, focusing on section 1. Figure 3 This is a typical Schlieren test result of the gridless and field-mirrorless large-field focusing Schlieren system of the present invention, which is focused on Section 2. Section 1 and Section 2 are two sections about 1.5 meters apart on the optical path, where Section 1 is closer to the scientific camera 8 and Section 2 is farther from the scientific camera 8. There is an image of flow field disturbance on each of these two sections. Figure 2 As shown, when the scientific camera 8 focuses on section 1, a clear image of the flow field disturbance of section 1 can be observed, while section 2 is out of focus at this time, and its image of the flow field disturbance is obviously blurred, and the whole system focuses on section 1. Figure 3 As shown, when the scientific camera 8 is focused on Section 2, a clear image of the flow field disturbance of Section 2 can be observed, while Section 1 is out of focus at this time, and its image of the flow field disturbance is obviously blurred, and the effect of the entire system focusing on Section 2 is obvious.

[0032] The gridless and field-lens-free large-field focusing schlieren system of the present embodiment greatly simplifies the optical path structure and reduces the difficulty of adjusting the optical path by eliminating the source grid, knife-edge grid and field lens in the traditional focusing schlieren. A large-sized uniform light spot is formed by using a small hole 2 and a large-diameter collimating convex lens 3, and then used together with a focusing convex lens 6 to replace the field lens for imaging, thereby improving the utilization rate of light intensity and increasing the observation field range, solving the problems of low light intensity, uneven light spot and small observation field range in the traditional focusing schlieren. By changing the distance from the collimating convex lens 3 to the focusing convex lens 6, it can be adapted to wind tunnels of different sizes, solving the problem that the traditional focusing schlieren is difficult to apply to large wind tunnels. By reducing the focal length of the collimating convex lens 3 and the focusing convex lens 6, the depth of field is reduced, and the integral effect of the schlieren along the optical path is weakened, thereby enhancing the focusing effect of the entire system. Reducing the focal length of the collimating convex lens 3 and the focusing convex lens 6 can also make it possible for the scientific camera 8 to focus to a close distance to see a clear area to be measured, solving the problem that the traditional non-focusing schlieren has no focusing effect.

[0033] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and the embodiments, and can be fully applied to various fields suitable for the present invention. For those familiar with the art, the present invention is not limited to the specific details and the illustrations shown and described here without departing from the general concept defined by the claims and their equivalents.

Claims

1. A gridless and field-mirror-free large-field-of-view focusing schlieren system, characterized in that: The gridless and field-mirror-free large-field focusing schlieren system comprises a light source (1), a pinhole (2), a collimating convex lens (3), a beam splitter (4), a model (5), a focusing convex lens (6), a knife edge (7) and a scientific camera (8); Light emitted by a light source (1) passes through a pinhole (2) and reaches a collimating convex lens (3). After passing through the collimating convex lens (3), the light beam becomes a quasi-parallel light beam. The light beam passes through a beam splitter (4) and reaches a model (5). After being reflected by the model (5), the light beam returns to the beam splitter (4). After being reflected by the beam splitter (4), the light beam reaches a focusing convex lens (6). The focusing convex lens (6) converges the light beam to a knife edge (7). Finally, a scientific camera (8) focuses on the flow field area to be measured and collects a schlieren image.

2. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The model (5) is surface coated.

3. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The small hole (2) is in the shape of a rectangle, the length of the long side of the rectangle is in the range of 0.5 mm to 2 mm, and the length of the short side of the rectangle is in the range of 0.2 mm to 1 mm.

4. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The collimating convex lens (3) is a plano-convex lens with a diameter of not less than 100 mm and a focal length of not more than 500 mm.

5. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The focusing convex lens (6) is a plano-convex lens with a diameter of not less than 100 mm and a focal length of not more than 500 mm.

6. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The collimating convex lens (3) collimates the divergent light into a quasi-parallel light beam; the collimating convex lens (3) and the focusing convex lens (6) form an image together; during installation, the focal lengths of the collimating convex lens (3) and the focusing convex lens 6 are reduced, so that the light beam is dispersed, thereby reducing the depth of field and improving the focus.

7. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The optical path distance from the collimating convex lens (3) to the focusing convex lens (6) is adjusted to match wind tunnels of different sizes.

8. The gridless and field-mirror-less large-field-of-view focusing schlieren system according to claim 1, characterized in that: The scientific camera (8) starts to focus from the nearest point and gradually moves towards the far point until the image of the area to be measured is the clearest.

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