Schlieren measurement device and method for three-dimensional flow near the wall of a hypersonic wind tunnel model

Abstract

The present invention belongs to the technical field of hypersonic wind tunnel experiments and discloses a device and method for measuring the three-dimensional flow near the wall of a hypersonic wind tunnel model. The device includes a light source, a spectrometer, a concave mirror, a model, a knife edge, and a scientific camera. The light emitted by the light source passes through the spectrometer and reaches the concave mirror. The light reflected by the concave mirror is parallel light. The parallel light passes through the flow field area to be measured and reaches the surface of the model. The surface of the model is coated with a reflective film. The parallel light is reflected by the model and returns to the concave lens. It is then reflected by the spectrometer and reaches the knife edge. Finally, the scientific camera captures the schlieren image. The design method includes determining a technical solution; determining a model structure; and determining an optical path arrangement. The device and design method of the schlieren measurement make the model itself part of the measurement optical path by coating the surface of the model. By imposing quality constraints on the design, processing, assembly, and other processes of the model, the accuracy of the schlieren measurement experiment is improved, and the device has practical engineering value.

Description

Technical Field

[0001] The present invention belongs to the technical field of hypersonic wind tunnel experiments, and in particular relates to a schlieren measurement device and method for three-dimensional flow near the wall of a hypersonic wind tunnel model. Background Art

[0002] As a non-contact measurement technology, Schlieren uses the different refractive indices of light in fluid media of different densities to measure the density gradient distribution of the flow field. It is widely used in high-speed and hypersonic wind tunnel experiments.

[0003] In traditional schlieren experiments, the experimental model is typically opaque, so only two-dimensional flows without spanwise flow can be measured. If spanwise flow is present, the resulting schlieren results cannot distinguish flows at different spanwise locations, and the integration effect along the optical path introduces significant errors. The grayscale value of the same point in the schlieren result contains information about the flow field density gradient at different spanwise locations. Therefore, traditional schlieren cannot be used to measure three-dimensional flow structures.

[0004] In order to measure the three-dimensional flow structure near the wall of the model, penetrating schlieren can be used. Specifically, the experimental model is processed with translucent materials and placed vertically. Although this method will inevitably be subject to errors caused by integration along the optical path, the density gradient in the near-wall area that researchers are most concerned about is more significant than that in the outer flow field, and even if there are areas with large density gradients such as shock waves in the outer flow, the high-frequency unsteady flow of the flow structure in the near-wall area can usually be distinguished from the quasi-steady structure of the outer flow field. Therefore, this method can obtain a certain degree of effective information. The biggest difficulty and challenge it faces is that the light path passes through the near-wall areas on the left and right sides of the model at the same time. Although the flows in these two areas are consistent in theory, it is impossible to ensure complete symmetry in actual experiments. Therefore, the schlieren results will have ghosting or even be completely unusable. In order to solve the problem of ghosting, it is necessary to add an isolation device on one side of the model to block the development of the near-wall boundary layer and only retain the information of the near-wall flow on the other side. However, the isolation device itself will bring additional errors. For example, cavity flow and the newly generated boundary layer of the isolation device will affect the schlieren measurement results. Especially when the frequency characteristics of these additional errors are similar to those of the boundary layer flow of concern, the effective signal and interference will become difficult to distinguish, which will bring great difficulties to scientific research and even misleading.

[0005] In order to more accurately measure the three-dimensional flow structure near the wall of the model, there is an urgent need to develop a schlieren measurement device and method for the three-dimensional flow near the wall of a hypersonic wind tunnel model. Summary of the Invention

[0006] One technical problem to be solved by the present invention is to provide a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model. Another technical problem to be solved by the present invention is to provide a design method for a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model.

[0007] The present invention provides a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model, comprising a light source, a spectrometer, a concave mirror, a model, a knife edge, and a scientific camera arranged on the same side of a wind tunnel test section; the model is vertically fixed in the wind tunnel test section, and a reflective film is coated on the surface of the model;

[0008] The light emitted by the light source passes through the beam splitter and reaches the concave mirror. The light reflected by the concave mirror is parallel light. The parallel light passes through the flow field area to be measured and reaches the surface of the model. The parallel light is reflected by the model and returns to the concave mirror. It is then reflected by the beam splitter and reaches the knife edge. Finally, the scientific camera collects the schlieren image.

[0009] Furthermore, the model includes a model body, a model component to be tested and a model pressing block. The model body is fixed on the middle bracket of the hypersonic wind tunnel through a supporting device. An installation groove is opened on the model body, and the model component to be tested is inserted into the installation groove and fixed by the model pressing block.

[0010] Furthermore, the surface of the model component to be tested is coated, and the flatness of the coated surface of the model component to be tested is less than 1 / 4 wavelength; the installation of the model component to be tested and the mounting groove of the model body adopts clearance fit, and the fit tolerance grade is higher than F9 / f9.

[0011] Furthermore, the minimum exposure time of a single frame of the scientific camera is no more than 2 microseconds, and the maximum number of pictures taken per second is no less than 400,000.

[0012] The method for designing a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention comprises the following steps:

[0013] S10. Determine the technical solution;

[0014] The purpose of the experiment was to measure the three-dimensional flow structure near the wall of the model. Given that penetrating schlieren cannot avoid interference from the flow field on the other side, the model was not processed with a translucent material. Instead, the model component to be tested was mounted vertically and coated on its surface. The model component to be tested was then arranged as part of the measurement light path. The light was reflected by the model component to avoid interference from the flow field on the other side in the penetrating schlieren. The light also passed through the same flow field area to be tested twice, doubling the effective signal and improving the signal-to-noise ratio.

[0015] S20. Determine the model structure;

[0016] The flatness of the coating surface of the model component to be tested is less than 1 / 4 wavelength;

[0017] To avoid stress concentration and local deformation of the model part to be tested caused by drilling and clamping on the left and right sides, ensure optical reflection performance, and improve the accuracy of schlieren measurement, the model part to be tested is not drilled, but slotted at the tail and fixed to the model body via a model clamping block. The model part to be tested and the model body are clearance-fitted, with a tolerance grade higher than F9 / f9, to avoid deformation of the model part to be tested due to extrusion, and to prevent the model part from shaking during the experiment, which may increase the error.

[0018] S30. Determine the optical path layout;

[0019] A light source, a beam splitter, a concave mirror, a knife-edge, and a scientific camera are arranged on the same side of the wind tunnel test section. The light emitted by the light source passes through the beam splitter and reaches the concave mirror. The light reflected by the concave mirror is parallel light. The parallel light passes through the flow field area to be measured and reaches the surface of the model. After being reflected by the model, the parallel light returns to the concave mirror, and is then reflected by the beam splitter to reach the knife-edge. Finally, the scientific camera collects the schlieren image. Among them, the parallel light passes through the flow field area to be measured twice, doubling the effective signal.

[0020] The device and method for measuring the three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention make the model itself part of the measurement light path by coating the surface of the model, thereby solving the problem of large errors in the three-dimensional flow measurement near the wall due to the asymmetry of the flow on the left and right sides of the transparent model. By constraining the design, processing, assembly and other processes of the model, the problems of easy deformation of the model's test parts during processing and easy shaking during the experiment are solved, thereby improving the experimental accuracy and having practical engineering value. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram (stereoscopic view) of the overall structure of the schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention;

[0022] Figure 2 A schematic diagram (stereoscopic diagram) of a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention;

[0023] Figure 3 A schematic diagram of the main body of the model of the schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention (top view);

[0024] Figure 4 A schematic diagram (front view) of a model component to be measured of a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention;

[0025] Figure 5 A schematic diagram (front view) of a model compression block of a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention;

[0026] Figure 6 These are typical Schlieren test results of the Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention.

[0027] In the figure, 1. Light source; 2. Spectrometer; 3. Concave mirror; 4. Model; 5. Wind tunnel test section; 6. Knife edge; 7. Scientific camera;

[0028] 401. Model body; 402. Model component to be tested; 403. Model pressing block. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0030] Example: Figures 1 to 5 As shown, the schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model of this embodiment includes a light source 1, a spectrometer 2, a concave mirror 3, a model 4, a knife edge 6 and a scientific camera 7 arranged on the same side of a wind tunnel test section 5. The light emitted by the light source 1 penetrates the spectrometer 2 and reaches the concave mirror 3. The light reflected by the concave mirror 3 is parallel light. The parallel light passes through the flow field area to be measured and reaches the surface of the model 4. The surface of the model 4 is coated with a reflective film. The parallel light is reflected by the model 4 and returns to the concave lens 3. It is then reflected by the spectrometer 2 and reaches the knife edge 6. Finally, the schlieren image is captured by the scientific camera 7.

[0031] Model 4 includes a model body 401, a model component to be tested 402, a model clamping block 403 and other components. The model body 401 is connected to the wind tunnel through supporting devices such as support rods. The model component to be tested 402 is arranged in the installation slot of the model body 401 and is connected to the model body 401 through the model clamping block 403. The model clamping block 403 is connected to the model body 401 by screws to fix the model component to be tested 402.

[0032] The surface coating of the model component to be tested 402 becomes part of the measurement optical path and acts as a reflector. To ensure the quality of schlieren imaging, the flatness of the coated surface of the model component to be tested 402 must be less than 1 / 4 wavelength. The model component to be tested 402 and the mounting groove of the model body 401 are installed using a clearance fit, and the fit tolerance grade is higher than F9 / f9 (see the national standard GB / T 1800.1-2020 for details).

[0033] Scientific camera 7 is a high-speed scientific camera with a minimum single-frame exposure time of no more than 2 microseconds, which can ensure that the flow structure in the hypersonic flow field is frozen. The maximum number of frames per second is no less than 400,000, so that the time evolution process of the three-dimensional flow structure can be obtained.

[0034] The design method of the schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model of this embodiment includes the following contents:

[0035] A surface-coated model component 402 is used, arranged vertically as part of the measurement optical path. Light reflected from component 402 avoids interference from the flow field on the other side of the penetrating schlieren. This also allows the light to pass through the same flow field twice, doubling the effective signal and improving the signal-to-noise ratio. The schlieren results obtained using this method contain information about the near-wall flow structure at different spanwise locations on the model, making them useful for studying three-dimensional near-wall flow structures.

[0036] The design of the coating surface of the model component to be measured 402 requires a flatness less than 1 / 4 wavelength to achieve higher measurement accuracy.

[0037] The model component to be tested 402 has no openings, only a slot at the tail, and is fixedly connected to the model body by the model clamping block 403, which can effectively avoid the stress concentration and local deformation caused by the punching process and the clamping installation on the left and right sides, thereby ensuring its optical reflection performance and improving the accuracy of the schlieren measurement.

[0038] The model test part 402 and the model body 401 are clearance-fitted, and the fit tolerance grade is higher than F9 / f9, which can avoid deformation of the model test part due to extrusion and prevent the model test part from shaking during the experiment, which may cause an increase in error.

[0039] This embodiment obtains Figure 6 Typical Schlieren test results shown, from Figure 6 It can be seen that the three-dimensional flow structure near the wall is clearly visible, meeting the requirements of scientific research.

[0040] The schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model in this embodiment coats the surface of the model component to be measured 402, making the model a part of the measurement optical path, and can measure the three-dimensional flow structure near the wall of the model. This solves the problem that traditional schlieren cannot measure the three-dimensional flow structure near the wall, and also solves the problem of large interference when penetrating schlieren measures the three-dimensional flow structure.

[0041] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the description and implementation methods. For those familiar with the art, all features disclosed in the present invention, or all steps in the disclosed methods or processes, except for mutually exclusive features and / or steps, can be combined in any way without departing from the principles of the present invention. The present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model, characterized by: The schlieren measurement device comprises a light source (1), a spectroscope (2), a concave mirror (3), a model (4), a knife edge (6) and a scientific camera (7) arranged on the same side of a wind tunnel test section (5); the model (4) is vertically fixed to the wind tunnel test section (5), and a reflective film is coated on the surface of the model (4); The model (4) includes a model body (401), a model component to be tested (402), and a model pressing block (403), wherein the model body (401) is fixed on a middle bracket of the hypersonic wind tunnel via a supporting device, and a mounting groove is provided on the model body (401), and the model component to be tested (402) is inserted into the mounting groove and fixed via the model pressing block (403); The light emitted by the light source (1) passes through the beam splitter (2) and reaches the concave mirror (3). The light reflected by the concave mirror (3) is parallel light. The parallel light passes through the flow field area to be measured and reaches the surface of the model (4). The parallel light is reflected by the model (4) and returns to the concave mirror (3). It is then reflected by the beam splitter (2) and reaches the knife edge (6). Finally, the scientific camera (7) collects the schlieren image.

2. The Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to claim 1, characterized in that: The surface of the model component to be tested (402) of the model (4) is coated, and the flatness of the coated surface of the model component to be tested (402) is less than 1 / 4 wavelength; the installation of the model component to be tested (402) and the installation groove of the model body (401) adopts clearance fit, and the fit tolerance grade is higher than F9 / f9.

3. The Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to claim 1, characterized in that: The minimum exposure time of a single frame of the scientific camera (7) is no more than 2 microseconds, and the maximum number of pictures taken per second is no less than 400,000.

4. A method for designing a Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model, the method being used to design a Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: S10. Determine the technical solution; The purpose of the experiment is to measure the three-dimensional flow structure near the wall of the model. In view of the fact that the penetrating schlieren is difficult to avoid the interference of the flow field on the other side, the model (4) is not processed with a transparent material, and the model component to be measured (402) is retained to be installed vertically. A film is coated on the surface of the model component to be measured (402), and the model component to be measured (402) is arranged to become a part of the measurement light path. The light is reflected by the model component to be measured (402), avoiding the interference of the flow field on the other side in the penetrating schlieren. The light is allowed to pass through the same flow field area to be measured twice, so that the effective signal is doubled and the signal-to-noise ratio is improved. S20. Determine the structure of model (4); The flatness of the coating surface of the model component to be tested (402) is less than 1 / 4 wavelength; In order to avoid stress concentration and local deformation of the model component to be tested (402) caused by punching and clamping installation on the left and right sides, ensure optical reflection performance, and improve the accuracy of schlieren measurement, it is determined that the model component to be tested (402) does not have a hole, but is slotted at the tail, and is fixedly connected to the model body (401) through a model clamping block (403); the model component to be tested (402) and the model body (401) are clearance-fitted, and the fit tolerance grade is higher than F9 / f9, so as to avoid deformation of the model component to be tested (402) caused by extrusion, and at the same time prevent the model component to be tested (402) from shaking during the experiment, which may cause an increase in error; S30. Determine the optical path layout; A light source (1), a beam splitter (2), a concave mirror (3), a knife edge (6) and a scientific camera (7) are arranged on the same side of a wind tunnel test section (5); light emitted by the light source (1) passes through the beam splitter (2) and reaches the concave mirror (3); the light reflected by the concave mirror (3) is parallel light, which passes through the flow field area to be measured and reaches the surface of the model (4); the parallel light is reflected by the model (4) and returns to the concave mirror (3), and then is reflected by the beam splitter (2) and reaches the knife edge (6); finally, the scientific camera (7) collects a schlieren image; wherein, the parallel light passes through the flow field area to be measured twice, thereby doubling the effective signal.

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