A method of flow field surface pressure measurement

Abstract

This invention discloses a method for measuring surface pressure in a flow field, belonging to the field of non-contact pressure measurement in wind tunnel experiments. The method includes a measurement system comprising a subwavelength metamaterial structure, a spectrometer, a reflector, an experimental cavity, and a detector. Before measurement, the sample is fixed on the model surface. Probe light emitted from the spectrometer is reflected by the sample and enters the detector. The detector measures the shift in the reflected spectrum, thus achieving direct pressure measurement. This invention employs a flexible material to design the subwavelength metamaterial structure. The flexible layer deforms under pressure, causing changes in the plasma resonance frequency and intensity on the structure's surface, thereby modulating the reflected spectrum and controlling the device's color. The change in device color allows for the measurement of surface pressure in a high-speed flow field, enabling efficient non-contact pressure measurement in high-speed wind tunnel experiments.

Description

Technical Field

[0001] This invention relates to the field of non-contact pressure measurement in wind tunnel experiments, and particularly to a method for measuring surface pressure in a flow field. Background Technology

[0002] With the development of aerospace engineering, high-speed flight simulation tests are becoming increasingly important in the development of advanced aircraft. Pressure measurement test data is an important test data in the test, and the accuracy of its measurement results is directly related to the structural design and performance optimization of the aircraft. At present, the main technologies for measuring pressure on the model surface in high-speed aircraft tests can be divided into: pressure hole method, pressure sensor method and pressure-sensitive paint method. However, the existing methods all have their own defects, such as: (1) The pressure hole method involves drilling holes on the model surface and connecting the holes and pressure sensors (such as piezoelectric and strain gauge types) through pipes to measure the flow field pressure. This method will damage the model surface structure and affect the local airflow in high-speed airflow, resulting in deviation of the measurement results. In addition, since the pressure signal needs to be transmitted through pipes, the signal has serious hysteresis and attenuation, which cannot meet the requirements for accurate pressure measurement in high-speed unsteady flow fields. (2) The pressure sensor method involves pasting or integrating a micro MEMS pressure sensor array onto the model surface to measure pressure. Since a single MEMS sensor is expensive, large-scale arrays are not economical, and there are also problems such as low measurement spatial resolution and electromagnetic interference. (3) The pressure-sensitive paint method measures pressure by spraying a coating containing fluorescent molecules onto the model surface. This method suffers from oxygen dependence. In high-speed flow fields, strong adverse pressure gradients exist on the surface, making it difficult for oxygen to diffuse uniformly and effectively, leading to measurement errors. Furthermore, the composition and coating thickness affect the measurement results, resulting in large deviations and poor measurement stability. The coating is prone to failure after long-term storage, and the accuracy of long-term and repeatable measurements is poor. Existing measurement methods cannot meet the requirements of strong three-dimensional and unsteady flow field experiments in high-speed flow fields, limiting the understanding of this type of flow characteristics. There is an urgent need to introduce new technologies to conduct further in-depth research. In view of the shortcomings and deficiencies of existing technologies, this technology proposes to use a subwavelength metamaterial structure. Through the pressure-sensitive mechanism of the metamaterial structure, the surface plasmon resonance mode of the metal patch antenna changes with pressure, and the pressure is shifted by the absorption spectrum frequency to achieve real-time measurement of the pressure in high-speed flow fields. Summary of the Invention

[0003] The purpose of this invention is to provide a method for measuring surface pressure in a flow field to solve the problems mentioned in the background art. This solution uses a subwavelength metamaterial structure. Through the pressure-sensitive mechanism of the metamaterial structure, the surface plasmon resonance mode of the metal patch antenna changes with pressure, and the absorption spectrum frequency shifts to achieve real-time measurement of the pressure in a high-speed flow field.

[0004] This invention is achieved through the following scheme: A method for measuring surface pressure in a flow field is applied to a measurement system comprising a subwavelength metamaterial structure, a spectrometer, a mirror, an experimental cavity, and a detector. The subwavelength metamaterial structure is placed in the experimental cavity, and a sample is placed on the surface of the subwavelength metamaterial structure. The mirror is positioned between the spectrometer and the subwavelength metamaterial structure to reflect probe light, and the detector is positioned in the reflected light path of the subwavelength metamaterial structure. Before measurement, the sample is fixed on the model surface. The probe light emitted from the spectrometer is reflected by the sample and enters the detector. The detector measures the shift in the reflected spectrum, thereby achieving direct pressure measurement.

[0005] The subwavelength metamaterial structure includes a bottom metal layer, a waveguide layer, and a grating metal layer. Each waveguide layer is disposed on the bottom metal layer, and the grating metal layer is disposed on the waveguide layer. The grating metal layer includes multiple metal blocks, which are discretely disposed on the waveguide layer.

[0006] The waveguide layer is made of flexible PDMS material with a thickness of 35nm; the bottom metal is a gold reflective layer with a thickness of 100nm; and the grating metal layer is made of gold cube with a thickness of 20nm.

[0007] PDMS has a Young's modulus of 750 kPa and a Poisson's ratio of 0.49.

[0008] Since the thickness of the gold reflective layer is greater than the skin depth of the incident electromagnetic wave, the transmitted light intensity is assumed to be zero; the period of the structure in both the x and y directions is L=765nm, and the bandwidth of the gold cube in both the x and y directions is W=200nm.

[0009] The fabrication method of subwavelength metamaterial structures includes the following steps: Step S1: First, a gold reflective layer is prepared on a Si substrate using electron beam sputtering. Step S2: Mix the PDMS component and the curing agent in a predetermined ratio to obtain the PDMS prepolymer. Evenly drop the PDMS prepolymer onto the metal reflective layer, spin coat it evenly using a spin coater, and then heat and cure it on a constant temperature table at a preset temperature. Step S3: Prepare a metasurface pattern mask on the PDMS thin film using electron beam lithography, and then prepare a metasurface metal pattern on the PDMS thin film by sequentially using plasma etching, electron beam sputtering and metal lift-off methods. Step S4: Finally, the device is fabricated using dicing technology.

[0010] In step S2, the PDMS component and the curing agent are mixed in a ratio of 10:1.

[0011] In step S2, the product is heated and cured on a constant temperature table at 80°C.

[0012] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: (1) The subwavelength metamaterial structure of this scheme enhances the interaction between light and matter through subwavelength periodic array units, thereby manipulating physical quantities such as electromagnetic wave amplitude, phase, and polarization. This invention uses flexible materials to design subwavelength metamaterial structure devices. The flexible layer material deforms under pressure, thereby causing changes in the plasma resonance frequency and intensity of the structure surface. The reflection spectrum is measured by a spectrometer. The pressure on the model surface in a high-speed flow field is measured by shifting the reflection spectrum. It can be efficiently applied to non-contact pressure measurement in high-speed wind tunnel experiments. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a typical structure of the metamaterial structure device in this invention; Figure 2 This is a schematic diagram of the single-cycle structure in this invention; Figure 3 This is a schematic diagram illustrating the change of absorption spectrum with pressure in this invention; Figure 4 This is a schematic diagram illustrating the variation of resonant wavelength and absorption with pressure in this invention; Figure 5 This is a schematic diagram of the fabrication process of the pressure-sensitive metasurface structure device in this invention; Figure 6 This is a schematic diagram of the flow field surface pressure measurement system in this invention; Figure 7 This is a schematic diagram of the calibration system in this invention; In the figure: 1. Bottom metal layer; 2. Waveguide layer; 3. Grating metal layer; 4. Spectrometer; 5. Mirror; 6. Experimental cavity; 7. Detector; 8. Vacuum pump; 9. Fiber optic spectrometer; 10. Calibration constant pressure cavity; 11. Integrating sphere; 12. Sample device. Detailed Implementation

[0014] All features disclosed in this specification, or all steps in all disclosed methods or processes, may be combined in any way, except for mutually exclusive features and / or steps.

[0015] Any feature disclosed in this specification (including any appended claims and abstract) may be replaced by other equivalent or similar features, unless specifically stated otherwise. That is, unless specifically stated otherwise, each feature is merely one example of a series of equivalent or similar features.

[0016] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0017] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature.

[0018] Example 1 like Figures 1-4 As shown, the present invention provides a technical solution: A subwavelength metamaterial structure includes a bottom metal 1, a waveguide layer 2, and a grating metal layer 3. Each waveguide layer 2 is disposed on the bottom metal 1, and the grating metal layer 3 is disposed on the waveguide layer 2. The grating metal layer 3 includes multiple metal blocks, which are discretely disposed on the waveguide layer 2.

[0019] Based on the above structure, the subwavelength metamaterial in this scheme is an artificially designed material or structure with a characteristic dimension much smaller than the operating wavelength of electromagnetic waves. These devices, through a special periodic arrangement, can modulate the behavior of electromagnetic waves at a microscale, exhibiting unique properties not found in natural materials. They have broad application potential in fields such as communication technology, optical devices, and energy utilization. The single-period structure (a basic unit structure) in this scheme is as follows: Figure 2 As shown.

[0020] This invention employs flexible materials to design subwavelength metamaterial structures. By deforming the flexible layer material under pressure, changes in the plasma resonance frequency and intensity of the structural surface are caused. A spectrometer is used to measure the reflection spectrum, and the pressure on the model surface in a high-speed flow field is measured by shifting the reflection spectrum. This invention can be efficiently applied to non-contact pressure measurement in high-speed wind tunnel experiments.

[0021] The invented subwavelength metamaterial structure pressure measurement device consists of a "metal-elastic medium-metal antenna" sandwich structure, with a typical device structure as follows: Figure 1 As shown. In Figure 1 In the design, the bottom metal layer serves as a reflective layer, the middle medium as a waveguide layer, and the top metal layer as a grating metal layer. Here, θ” is the angle of incidence, and x, y, z” correspond to the x, y, and z axes of the coordinate system; in the attached diagram, “E” represents the electric field, “H” the magnetic field, and “k” the wave vector; Figure 3In this context, d2 represents the thickness of the waveguide layer, and the incident light enters through the incident surface. The incident light couples through a subwavelength metallic grating and a surface plasmon polariton (SPP) mode; this process can be described as wave vector matching.

[0022] In the formula For surface plasma wave vector, Let be the incident light wave vector. For the diffraction order in the x-direction, Let x be the grating reciprocal basis vector in the x-direction. and It is the projection of the surface plasma and the incident light wave vector onto the horizontal plane. The reciprocal lattice basis vector of the grating in the x-direction; where The unit vector representing the x-direction. The incident light wavelength, and These are the dielectric constants of the dielectric layer and the metal layer as a function of wavelength, respectively. This indicates the size of the metal grating in the x-direction.

[0023] right After simplification, we get:

[0024] After wave vector matching, the incident light excites surface plasmon waves at the metal-dielectric interface. Because the skin depth of these surface plasmon waves in the medium is relatively large, when the wavelength of the surface plasmon wave in the medium equals the Fabry-Perot resonance wavelength of the dielectric cavity, it further constrains the distribution of the light field, weakening the radiated light field. This is reflected in the reflection spectrum as a dark band in a certain frequency range. Therefore, the reflection spectrum can be modulated by designing nanostructured surface grating structures and dielectric layer Fabry-Perot cavity structures.

[0025] As an example, the waveguide layer is made of flexible PDMS material with a thickness of 35nm; the bottom metal can be a gold reflective layer with a thickness of 100nm; the grating metal layer is made of gold cubes with a thickness of 20nm.

[0026] Based on the above structure, this invention designs a metamaterial structure operating in the near-infrared band through the surface plasmon resonance mode of a subwavelength nano-optical antenna. Combined with the flexible material PDMS (polydimethylsiloxane), the surface plasmon resonance mode is altered by the elastic deformation of the PDMS structure induced by pressure, thereby enabling pressure measurement. PDMS is a highly elastic and transparent rubber material with an operating temperature between -50 and 200°C. It has a low Young's modulus and is commonly used as a stretchable substrate material. Furthermore, PDMS not only possesses good tensile properties but also exhibits skin-friendliness and non-toxicity, thus making electronic devices and wearable devices using PDMS films as flexible materials promising for widespread applications. PDMS has a Young's modulus of 750 kPa and a Poisson's ratio of 0.49.

[0027] As an example, since the thickness of the gold reflective layer is greater than the skin depth of the incident electromagnetic wave, the transmitted light intensity is assumed to be zero; the period of the structure in both the x and y directions is L=765nm, and the bandwidth of the gold cube in both the x and y directions is W=200nm.

[0028] This scheme uses the RCWA algorithm to calculate the reflection spectra of different structures, obtaining the reflection spectra of the structures under different pressures, such as... Figure 3 As shown, when the ambient pressure increases, the plasma resonance wavelength of the structure redshifts, and the absorption rate decreases. This can be understood as the structure period increasing, the plasma resonance frequency decreasing, and the resonance peak redshifting when the pressure increases. At the same time, due to the decrease in the coupling strength between the incident light and the surface plasma, the antenna's ability to attract incident electromagnetic waves decreases, which in turn leads to a decrease in the overall absorption rate of the structure.

[0029] The structural resonance wavelength and corresponding peak absorptivity vary with ambient pressure as follows: Figure 4 As shown in the figure, as the pressure increases from 0 kPa to 28 kPa, the wavelength of the plasma resonance absorption peak of the patch antenna structure redshifts from 0.83 μm to 1.345 μm, a shift of 515 nm. Simultaneously with the change in the resonance absorption peak, the absorption rate of the antenna structure also decreases from 99% at 0 kPa to 41.7% at 28 kPa, a decrease of 57.9%.

[0030] In summary, this scheme provides a three-layer pressure-sensitive optical antenna structure. This structure utilizes the elastic deformation of the flexible material PDMS under pressure to generate a shift in the absorption spectrum. Studies have shown that as the ambient pressure increases from 0 kPa to 28 kPa, the corresponding absorption peak shifts from 0.83 μm to 1.345 μm, and the resonant wavelength shifts by 515 nm. Simultaneously, the absorption peak percentage decreases from 99% to 41.7%, and the absorptivity decreases by 57.9%. This structure exhibits excellent pressure-sensitive characteristics and, with further optimization, can be applied to non-contact pressure measurement in high-speed wind tunnel experiments.

[0031] Example 2 like Figure 5 As shown, the present invention provides a technical solution: A method for fabricating a subwavelength metamaterial structure, specifically comprising the following steps: Step S1: First, a Ti / Au metal reflective layer is prepared on a Si substrate using electron beam sputtering (Ebeam-evaporator). Step S2: Mix the PDMS component and the curing agent at a ratio of 10:1 to obtain the PDMS prepolymer. Evenly drop the PDMS prepolymer onto the metal reflective layer, spin coat it evenly using a spin coater, and then heat and cure it on a constant temperature table at 80°C. Step S3: A metasurface pattern mask is prepared on the PDMS thin film using electron beam lithography. Then, a metasurface metal pattern is prepared on the PDMS thin film by sequentially using reactive ion etching, electron beam sputtering, and metal lift-off methods. Step S4: Finally, the device is fabricated using dicing technology.

[0032] This approach employs standard cleanroom semiconductor micro / nano fabrication technology, enabling efficient fabrication of the designed pressure-sensitive subwavelength metamaterial structure.

[0033] Example 3 like Figure 6 As shown, the present invention provides a technical solution: An experimental measurement system for a subwavelength metamaterial structure includes a subwavelength metamaterial structure, a spectrometer 4, a reflector 5, an experimental cavity 6, and a detector 7. The subwavelength metamaterial structure is placed in the experimental cavity 6, a sample (not shown) is placed on the surface of the subwavelength metamaterial structure, the reflector 5 is placed between the spectrometer 4 and the subwavelength metamaterial structure to reflect the probe light, and the detector 7 is placed in the reflected light path of the subwavelength metamaterial structure.

[0034] Based on the above structure, before measurement, the sample (not shown) is fixed on the surface of the subwavelength metamaterial structure. The probe light emitted from the spectrometer 4 is reflected by the sample and enters the detector 7. The shift in the reflected spectrum is measured by the detector 7, thus achieving pressure measurement. Specifically, a calibration lookup table method is used for direct measurement. This scheme innovatively proposes using a subwavelength metamaterial structure device for high-speed flow field surface pressure measurement. This scheme also has advantages such as low interference with the flow field, low environmental electromagnetic interference, simple device, high measurement accuracy, and low maintenance cost.

[0035] Pressure calibration system for subwavelength metamaterial structures: To obtain the mapping relationship between the resonant wavelength and pressure of the subwavelength metamaterial structure, the subwavelength metamaterial device needs to be calibrated before pressure measurement. A schematic diagram of the calibration system is shown below. Figure 7 As shown.

[0036] It includes a vacuum pump 8, a fiber optic spectrometer 9, a calibration constant pressure chamber 10, and an integrating sphere 11. During calibration, a laser source is used to irradiate the sample device 12, and the reflected light spectrum is measured using the fiber optic spectrometer and the integrating sphere. To measure the change of the sample's reflectance spectrum with pressure, the sample is placed in the calibration constant pressure chamber during the test. Different pressure adjustments are set via the vacuum pump using a pressure regulation system, and then the reflectance spectrum is measured using the spectrometer to obtain a pressure-resonance wavelength mapping table for subsequent pressure measurements.

[0037] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of flow field surface pressure measurement, the method comprising: The application is applied to a measuring system, which comprises a sub-wavelength metamaterial structure, a spectrometer (4), a mirror (5), an experimental cavity (6) and a detector (7); the sub-wavelength metamaterial structure is arranged in the experimental cavity (6), a sample is arranged on the surface of the sub-wavelength metamaterial structure, the mirror (5) is arranged between the spectrometer (4) and the sub-wavelength metamaterial structure to reflect probe light, and the detector (7) is arranged on the reflection light path of the sub-wavelength metamaterial structure; before measurement, the sample is fixed on the surface of the model, probe light is reflected into the detector (7) after being emitted from the spectrometer (4) through the sample, and the movement of the reflected spectrum is measured by the detector (7), so that direct measurement of pressure is realized.

2. A method of flow field surface pressure measurement as claimed in claim 1, wherein: The sub-wavelength metamaterial structure comprises a bottom metal layer (1), a waveguide layer (2) and a grating metal layer (3), the waveguide layer (2) is arranged on the bottom metal layer (1), and the grating metal layer (3) is arranged on the waveguide layer (2); the grating metal layer (3) comprises a plurality of metal blocks which are discretely arranged on the waveguide layer (2).

3. A method of flow field surface pressure measurement as claimed in claim 2, wherein: The waveguide layer (2) is made of flexible material PDMS and has a thickness of 35 nm; the bottom metal layer (1) is a gold reflection layer and has a thickness of 100 nm; and the grating metal layer (3) is made of gold square blocks and has a thickness of 20 nm.

4. A method of flow field surface pressure measurement as defined in claim 3, wherein: The Young's modulus of the PDMS is 750 kPa, and the Poisson's ratio is 0.

49.

5. A method of flow field surface pressure measurement as defined in claim 4, wherein: Since the thickness of the gold reflection layer is greater than the skin depth of the incident electromagnetic wave, the transmission light intensity is considered to be zero; the period of the structure in the x and y directions is L=765 nm, and the bandwidth of the gold square block in the x and y directions is W=200 nm.

6. A method of flow field surface pressure measurement as defined in claim 5, wherein: The preparation method of the sub-wavelength metamaterial structure comprises the following steps: Step S1: a gold reflection layer is first prepared on a Si substrate by an electron beam sputtering method; Step S2: PDMS components and a curing agent are mixed in a predetermined ratio to obtain a PDMS prepolymer, the PDMS prepolymer is dropped on the metal reflection layer, uniform spin coating is performed by using a uniform coating machine, and then heating and curing are performed on a constant temperature table at a preset temperature; Step S3: a super surface pattern mask is prepared on the PDMS film by an electron beam lithography method, and then a super surface metal pattern is prepared on the PDMS film by using plasma etching, electron beam sputtering and metal stripping methods in sequence; Step S4: finally, the device is prepared by using a dicing technique.

7. A method of flow field surface pressure measurement as defined in claim 6, wherein: In step S2, the PDMS components and the curing agent are mixed in a ratio of 10:

1.

8. A method of flow field surface pressure measurement as defined in claim 7, wherein: In step S2, heating and curing are performed on the constant temperature table at a temperature of 80℃.

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