A gridless and field-mirror-free large-field focusing schlieren system

Abstract

The present invention belongs to the technical field of wind tunnel experiments, and discloses a gridless and field mirror-less large-field-of-view focusing schlieren system. The gridless and field mirror-less large-field-of-view focusing schlieren system 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 pinhole and reaches the collimating convex lens, and becomes a quasi-parallel light beam after passing through the collimating convex lens. The light beam penetrates the beam splitter and reaches the model, and then returns to the beam splitter after being reflected by the model, and then reaches the focusing convex lens after being reflected by the beam splitter. The focusing convex lens converges the light beam to the knife edge, and finally the scientific camera focuses on the flow field area to be measured and then collects the schlieren image. The gridless and field mirror-less large-field-of-view focusing schlieren system achieves the acquisition of a flow field density gradient distribution with a small optical path integral effect under large field-of-view conditions in a large wind tunnel, and has practical engineering 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. It uses the property that the refractive index of light in fluid media of different densities is different to obtain the density gradient distribution of the flow field to be measured.

[0003] The schlieren results obtained from traditional collimated light unfocused schlieren have an integration 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, the schlieren results obtained are the result of span-wise integration. This is mainly used to measure two-dimensional flows with no span-wise changes. To measure three-dimensional flow structures 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 inevitably introduces errors due to the asymmetry of the flow on both sides of the model. Although reflective schlieren applied by coating the model surface can solve this problem, the obtained schlieren results are the result of integration along the normal direction of the model surface. If there are additional disturbances along the model normal, errors will also be introduced. For example, in supersonic wind tunnels that do not use a flexible wall integrated design for the nozzle and test section, as well as the most common jet-type hypersonic wind tunnels, the disturbance at the nozzle outlet will inevitably propagate downstream, causing changes in the flow field density, thereby interfering with the final schlieren results. Moreover, traditional parallel light unfocused schlieren usually uses a concave mirror with a focal length greater than 3000mm to generate and converge parallel light. A scientific camera can only observe a clear image by focusing near infinity. If the focus is forced to be close, the long focal length of the concave mirror will result in the camera only observing a very bright point rather than a clear image area.

[0004] Focused schlieren technology can, to a certain extent, reduce the integration effect along the optical path, thereby alleviating the aforementioned interference caused by nozzle exit disturbances. Traditional focused schlieren systems consist of an imaging lens (also known as a field lens) and a pair of source and knife-edge gratings. These two elements must maintain strict optical conjugation, otherwise the uniformity and sensitivity of the schlieren results will be significantly adversely affected. In practice, to ensure the optical conjugation between the source and knife-edge gratings, experimenters must expend considerable time and effort fine-tuning the optical path. The China Patent Literature Library discloses an invention entitled "A Focused Schlieren System and Method Based on Adaptive Knife-Edge Generation" (CN202010678170). This application proposes a method for adaptively generating a knife-edge grating using a transparent liquid crystal display (LCD) screen, which theoretically reduces the difficulty of adjustment. However, this approach still follows the traditional approach of combining a source grating, a field lens, and a knife-edge grating. Furthermore, the high-precision transparent LCD screen employed is expensive, making it difficult to scale up.

[0005] Traditional focusing schlieren systems often use a Fresnel lens to simply converge the divergent light from the light source. The measurement area is located between the source grid and the field lens, which is typically a commercially available lens. In large wind tunnel applications, the field lens is often a telephoto lens with a focal length of 200mm or longer, and an aperture of F4 or even F5.6 or higher. The divergent light source not only results in an uneven light spot, with a bright center and dark edges, which affects the interpretation of the results, but also, due to the limited aperture of the field lens, only a small portion of the light from the divergent light source passes through the field lens and enters the scientific camera. 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 view 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 lens. 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 devices such as 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 relatively low), the field of view of the measurement area is also relatively small, and 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 currently urgent 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-less large-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;

[0009] 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 beam. The beam passes through 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 beam to the knife edge. Finally, the scientific camera focuses on the flow field area to be measured and then collects the schlieren image.

[0010] Furthermore, the surface of the model is coated.

[0011] 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.

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

[0013] Furthermore, the focusing convex lens is a plano-convex lens with a diameter of not less than 100 mm and a focal length of not more than 500 mm.

[0014] Furthermore, the collimating convex lens collimates the divergent light into a quasi-parallel beam; the collimating convex lens and the focusing convex lens form an image together; when installed, the focal length of the collimating convex lens and the focusing convex lens is reduced to disperse the light beam, thereby reducing the depth of field and improving the focus.

[0015] 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.

[0016] 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.

[0017] The gridless, field-lens-free, large-field-of-view focusing schlieren system of this invention does not include the source grid, knife-edge grid, field lens, and other components found in traditional focusing schlieren. This significantly simplifies the optical structure and reduces the difficulty of optical path adjustment. Compared to traditional focusing schlieren, two convex lenses replace the field lens for imaging. The collimating convex lens allows for more concentrated light, higher light intensity, shorter exposure times, and higher temporal resolution.

[0018] The collimating convex lens in the gridless, field-lens-free, large-field-of-view focusing schlieren system of the present invention not only collimates divergent light into a quasi-parallel beam, but also, together with the focusing convex lens, performs an imaging function similar to a field lens. By reducing the focal lengths of the collimating and focusing convex lenses, the light beam becomes more dispersed, thereby achieving the effect of reducing depth of field and improving focus. Therefore, although the optical path of the gridless, field-lens-free, large-field-of-view focusing schlieren system of the present invention is similar to that of conventional parallel light, unfocused schlieren, it produces the effect of focused schlieren.

[0019] Unlike traditional parallel light, unfocused schlieren, which focuses to near infinity, the scientific camera of the present invention focuses from the closest point to the target area, gradually moving toward the distance until the image of the target area is clearest. Traditional parallel light, unfocused schlieren, requires only focusing to near infinity to achieve a clear image due to the long focal length of the concave mirror and the high parallelism of the beam. The gridless, field-mirror-less, large-field-of-view focused schlieren system of the present invention utilizes a collimating convex lens and a focusing convex lens with relatively short focal lengths, making close focus possible.

[0020] In summary, the gridless and field-lens-free large-field focusing schlieren system of the present invention eliminates the source grid, knife-edge grid, and field lens in 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, which is then used in conjunction 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 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 traditional focusing schlieren are 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 can also make it possible for the scientific camera to focus to a close distance to see a clear area to be measured, solving the problem that traditional non-focused schlieren have 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

[0021] Figure 1 This is a schematic diagram (stereoscopic diagram) of the overall structure of the gridless and field-mirror-less large-field focusing schlieren system of the present invention;

[0022] Figure 2 This is a typical schlieren test result of the gridless and field mirrorless large field focusing schlieren system of the present invention, focused on section 1;

[0023] Figure 3This is a typical schlieren test result of the gridless and field-mirror-less large-field-of-view focusing schlieren system of the present invention, focused on section 2.

[0024] In the figure, 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

[0025] The present invention will be further described below with reference to the accompanying drawings and examples.

[0026] Example 1: Figure 1 As shown, the gridless and field-mirror-less 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;

[0027] 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 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.

[0028] Furthermore, the surface of the model 5 is coated.

[0029] Furthermore, the shape of the small hole 2 is a rectangle, the length of the long side of the rectangle is in the range of 0.5mm to 2mm, and the length of the short side of the rectangle is in the range of 0.2mm to 1mm.

[0030] Furthermore, 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.

[0031] Furthermore, 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.

[0032] 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 form an image together; during installation, the focal length of the collimating convex lens 3 and the focusing convex lens 6 is reduced to disperse the light beam, thereby reducing the depth of field and improving the focus.

[0033] 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, solving the problem that traditional focusing schlieren is difficult to apply to large wind tunnels.

[0034] 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.

[0035] Figure 2 This is the typical Schlieren test result of the gridless and field mirrorless large field focusing Schlieren system of the present invention, which is focused on section 1. Figure 3 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 2. Sections 1 and 2 are two sections about 1.5 meters apart on the optical path, with Section 1 closer to the scientific camera 8 and Section 2 farther away from the scientific camera 8. There is an image of flow field disturbance on each of these two sections. Figure 2 As shown in FIG, 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. The effect of the entire system focusing on section 1 is obvious. Figure 3 As shown, when the scientific camera 8 focuses 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.

[0036] The gridless and field-lens-free large-field focusing schlieren system of this embodiment greatly simplifies the optical path structure and reduces the difficulty of optical path adjustment by eliminating the source grid, knife-edge grid and field lens in traditional focusing schlieren. By adopting the small hole 2 and the large-diameter collimating convex lens 3 to form a large-sized uniform light spot, which is then used together with the focusing convex lens 6 to replace the field lens for imaging, the light intensity utilization rate is improved while also increasing the observation field range, solving the problems of low light intensity, uneven light spot and small observation field range in 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 traditional focusing schlieren are difficult to apply to large wind tunnels. By reducing the focal lengths 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 lengths of the collimating convex lens 3 and the focusing convex lens 6 also makes it possible for the scientific camera 8 to focus on a close distance to see a clear area to be measured, solving the problem that traditional non-focused schlieren have no focusing effect.

[0037] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and embodiments, and can be applied to various fields suitable for the present invention. It will be understood by those skilled in the art that the present invention is not limited to the specific details and illustrations shown and described herein without departing from the general concept defined by the claims and their equivalents.

Claims

1. A gridless and field-mirror-less large-field-of-view focusing schlieren system, characterized in that: The gridless and field-mirror-less 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); 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; when installed, the focal length of the collimating convex lens (3) and the focusing convex lens (6) is reduced, so that the light beam is dispersed, thereby reducing the depth of field and improving the focus; 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 passes through 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. 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.

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 shape of the small hole (2) is a rectangle, the length of the long side of the rectangle is in the range of 0.5mm to 2mm, and the length of the short side of the rectangle is in the range of 0.2mm to 1mm.

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 optical path distance from the collimating convex lens (3) to the focusing convex lens (6) is adjusted to match wind tunnels of different sizes.

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