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

Abstract

The invention belongs to the technical field of hypersonic wind tunnel experiments, and discloses a schlieren measurement device and method for near-wall three-dimensional flow of a hypersonic wind tunnel model. The schlieren measuring device comprises a light source, a spectroscope, a concave mirror, a model, a knife edge and a scientific camera, light emitted by the light source penetrates through the spectroscope and then reaches the concave mirror, the light reflected by the concave mirror is parallel light, the parallel light penetrates through a to-be-measured flow field area and then reaches the surface of the model, the surface of the model is plated with a reflecting film, and the parallel light is reflected by the model and then returns to the concave mirror. And finally, a schlieren image is acquired by a scientific camera. The design method comprises the steps of determining a technical scheme; determining a model structure; and determining a light path arrangement scheme. According to the schlieren measurement device and the design method, the model becomes a part of a measurement light path in a model surface coating mode, quality constraint is carried out on the processes of designing, processing, assembling and the like of the model, the schlieren measurement experiment accuracy is improved, and the schlieren measurement device and the design method have engineering practical 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 usually opaque, so only two-dimensional flow without spanwise flow can be measured. If spanwise flow exists, the Schlieren results obtained cannot distinguish the flow at different spanwise positions, and the integration effect along the optical path will cause large errors. The grayscale value of the same point in the Schlieren result contains the flow field density gradient information at different spanwise positions. 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 transparent material and placed vertically. Although this method will inevitably be subject to errors caused by integration along the optical path, the density gradient of the near-wall area that researchers are most concerned about is more significant than that of 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 both 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 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 invention discloses a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model, comprising a light source, a spectroscope, 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; 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 schlieren image is collected by the scientific camera.

[0008] Furthermore, the model includes a model body, a model component to be tested and a model clamping 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. The model component to be tested is inserted into the installation groove and fixed by the model clamping block.

[0009] 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.

[0010] 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.

[0011] The design method of the schlieren measurement device of the three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention comprises the following steps: 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 difficulty of avoiding the interference of the flow field on the other side in the penetrating schlieren, the model is processed using transparent materials instead of being processed by them. The model parts to be tested are installed vertically, and the surface of the model parts to be tested is coated. The model parts to be tested are arranged as part of the measurement light path. The light is reflected by the model parts to be tested, 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 tested twice, which doubles the effective signal and improves the signal-to-noise ratio. S20. Determine the model structure; The flatness of the coating surface of the model component to be tested is less than 1 / 4 wavelength; In order to avoid stress concentration and local deformation of the model parts to be tested caused by drilling 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 parts to be tested do not have holes, but are slotted at the tail, and are fixedly connected to the model body through the model clamping block; the model parts to be tested and the model body are clearance-fitted, and the fit tolerance grade is higher than F9 / f9 to avoid deformation of the model parts to be tested due to extrusion, and to prevent the model parts to be tested from shaking during the experiment, which may cause an increase in error; S30. Determine the optical path arrangement plan; 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 penetrates the beam splitter and reaches the concave mirror, and 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, and then reflects through the beam splitter to reach the knife edge, and finally the scientific camera collects the schlieren image; the parallel light passes through the flow field area to be measured twice, so the effective signal is doubled.

[0012] The schlieren measurement device and method for three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention make the model itself a part of the measurement light path by coating the surface of the model, thereby solving the problem of large errors in three-dimensional flow measurement near the wall due to the asymmetry of the flow on the left and right sides of the light-transmitting model. The problems of easy deformation of the model's test parts during processing and easy shaking during the experiment are solved by constraining the design, processing, assembly and other processes of the model, thereby improving the experimental accuracy and having practical engineering value. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the overall structure of the schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention (stereoscopic diagram); Figure 2 A schematic diagram (stereoscopic diagram) of a model of a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention; Figure 3 A schematic diagram of the model body (top view) of the schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model of the present invention; Figure 4 A schematic diagram 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 (front view); Figure 5 A schematic diagram (front view) of a model pressing block of a schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model according to the present invention; 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.

[0014] In the figure, 1. light source; 2. spectroscope; 3. concave mirror; 4. model; 5. wind tunnel test section; 6. knife edge; 7. scientific camera; 401. Model body; 402. Model component to be tested; 403. Model pressing block. DETAILED DESCRIPTION

[0015] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0016] Example: Figure 1~Figure 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 beam splitter 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 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 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 beam splitter 2 and reaches the knife edge 6. Finally, the schlieren image is collected by the scientific camera 7.

[0017] 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 groove of the model body 401 and connected to the model body 401 through the model clamping block 403. The model clamping block 403 is connected to the model body 401 through screws to fix the model component to be tested 402.

[0018] The surface coating of the model component to be tested 402 becomes part of the measuring 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 installation of the model component to be tested 402 and the mounting groove of the model body 401 adopts clearance fit, and the fit tolerance grade is higher than F9 / f9 (see national standard GB / T 1800.1-2020 for details).

[0019] The scientific camera 7 is a high-speed scientific camera with a minimum exposure time of no more than 2 microseconds for a single frame, thereby ensuring that the flow structure in the hypersonic flow field is frozen. The maximum number of pictures taken per second is no less than 400,000, thereby obtaining the time evolution process of the three-dimensional flow structure.

[0020] 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: A model component 402 with a surface coating is used. The model component 402 is arranged vertically to become part of the measurement optical path. The light is reflected back by the component 402 to avoid the interference of the flow field on the other side in the penetrating schlieren. The light passes through the same flow field area to be measured twice, which doubles the effective signal and improves the signal-to-noise ratio. The schlieren results obtained by this method contain the near-wall flow structure information at different spanwise positions of the model, which can be used to study the three-dimensional flow structure near the wall.

[0021] The design of the coating surface of the model component 402 to be tested requires a flatness less than 1 / 4 wavelength to achieve higher measurement accuracy.

[0022] The model component to be tested 402 has no openings, only a groove at the tail, and is fixedly connected to the model body by a model clamping block 403, which can effectively avoid stress concentration and local deformation caused by drilling processing and clamping installation on the left and right sides, thereby ensuring its optical reflection performance and improving the accuracy of schlieren measurement.

[0023] The model test part 402 and the model main 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, resulting in increased errors.

[0024] 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.

[0025] The schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model in this embodiment, by coating the surface of the model component 402 to be measured, makes the model a part of the measurement light path, and can measure the three-dimensional flow structure near the wall of the model, which not only solves the problem that traditional schlieren cannot measure the three-dimensional flow structure near the wall, but also solves the problem of large interference when penetrating schlieren measures the three-dimensional flow structure.

[0026] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and implementation modes. For those familiar with the art, all features disclosed in the present invention, or steps in all methods or processes disclosed, 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 in that: The schlieren measurement device comprises a light source (1), a beam splitter (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 on the wind tunnel test section (5), and a reflective film is coated on the surface of the model (4); Light emitted by a light source (1) passes through a beam splitter (2) and reaches a 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 a 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 a knife edge (6). Finally, a schlieren image is collected by a scientific camera (7).

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 model (4) comprises a model body (401), a model component to be tested (402) and a model pressing block (403); the model body (401) is fixed to a middle bracket of a hypersonic wind tunnel via a supporting device; a mounting groove is provided on the model body (401); the model component to be tested (402) is inserted into the mounting groove and fixed via the model pressing block (403).

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 model component to be tested (402) of the model (4) is coated on its surface, and the flatness of the coated surface of the model component to be tested (402) is less than 1 / 4 wavelength; the model component to be tested (402) and the mounting groove of the model body (401) are installed using clearance fit, and the fit tolerance grade is higher than F9 / f9.

4. 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 scientific camera (7) has a minimum exposure time of no more than 2 microseconds for a single frame and a maximum number of pictures taken per second of no less than 400,000.

5. A method for designing a Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model, which is used to design a Schlieren measurement device for three-dimensional flow near the wall of a hypersonic wind tunnel model as described in any one of claims 1 to 4, 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 difficulty of avoiding the interference of the flow field on the other side in the penetrating schlieren, the model (4) is not processed with a light-transmitting material, and the model component to be tested (402) is retained to be installed vertically, and a film is coated on the surface of the model component to be tested (402). The model component to be tested (402) is arranged to become a part of the measurement light path. The light is reflected by the model component to be tested (402) to avoid 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 tested twice, so that the effective signal is doubled and the signal-to-noise ratio is improved. S20. Determine the model (4) structure; 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 arrangement plan; 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; 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); and finally a schlieren image is collected by the scientific camera (7); wherein the parallel light passes through the flow field area to be measured twice, thereby doubling the effective signal.

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