Visualization device for measuring sound field in low and medium frequency standing wave tube

Abstract

The invention discloses a visualization device for measuring a sound field in a low and medium frequency standing wave tube, which comprises a sound cavity, one end of the sound cavity is provided with a baffle plate, and the other end of the sound cavity is sealed by a pore plate; the at least five sound pressure sensors are arranged on the outer wall of the sound cavity at equal intervals; a sound wave outlet pipe of the horn is provided with a sound wave heating net, the sound wave heating net is connected with a heating device, the power of the heating device is 100W-300W, and the heating temperature is 50-100 DEG C; the sound wave outlet pipe is communicated with a through hole in the pore plate; the schlieren system comprises two concave mirrors, and the focal length of the two concave mirrors is larger than or equal to 3m; the two concave mirrors are located on the two sides of the sound cavity respectively, and a light path between the two concave mirrors can penetrate through any observation window according to needs. Wherein the sound wave heating net is used for heating sound waves output from the loudspeaker; the schlieren system is used for recording a density gradient field generated by the gas medium under the action of sound waves in the form of stripe images. The problem that in the prior art, a low and medium frequency standing wave sound field cannot be visualized is solved.

Description

Technical field

[0001] The present invention belongs to the technical field of intersection of optical measurement and sound field analysis, and in particular relates to a visualization device for measuring the sound field in a medium- and low-frequency standing wave tube. [Background Technology]

[0002] As engines rapidly develop toward high packing densities, ultra-high combustion pressures, and large aspect ratios, combustion instabilities have become increasingly prominent and a key bottleneck restricting engine reliability. In-depth research has revealed a strong coupling between periodic pressure oscillations within the combustion chamber (i.e., oscillatory combustion) and the inherent pressure standing wave modes of the acoustic cavity. This acoustic-combustion coupling can significantly amplify combustion instabilities, even leading to catastrophic consequences. Current research on engine acoustic instabilities primarily involves establishing a stable pressure standing wave field within the acoustic cavity to observe the combustion response characteristics of the propellant at specific acoustic frequencies. However, existing research methods have significant limitations: conventional acoustic field measurements rely solely on localized data from a limited number of wall pressure sensors, and then indirectly infer the acoustic field distribution of the entire acoustic cavity through theoretical calculations. This approach cannot fully visualize the acoustic field, resulting in significant uncertainty in understanding the actual pressure fluctuation amplitude and spatial distribution characteristics experienced by the propellant surface. To further unravel the acoustic-combustion coupling mechanism and overcome the limitations of existing research methods, it is urgently necessary to develop new acoustic field visualization measurement techniques that can directly observe and accurately characterize the spatial distribution characteristics of the acoustic field within the combustion chamber. This will provide key experimental support for establishing more accurate acoustic instability prediction models and optimizing engine combustion stability design.

[0003] Schlieren optical imaging technology is an effective detection method for the distribution of acoustic fields in gases. When sound waves propagate in a gas medium, changes in sound pressure will cause changes in the density of the medium, thereby affecting its refractive index distribution. Schlieren technology converts the refractive index distribution into a light intensity distribution, indirectly reflecting the density changes in the medium, thereby characterizing the distribution of the acoustic field. Existing studies have shown that this technology has been successfully applied in high-frequency (above kHz level) and ultra-high-frequency (MHz level) sound field measurements. For example, it has shown significant advantages in open-field acoustic research such as ultrasonic wavelength measurement and sound velocity calibration.

[0004] However, when low- and medium-frequency sound waves (especially in the 100-1000 Hz range) propagate in a closed acoustic cavity, the magnitude of the refractive index gradient caused by their smaller sound pressure amplitude is significantly reduced (typically on the order of 10^-6-10^-7). This poses a serious challenge to the measurement sensitivity of traditional schlieren systems. Specifically, these challenges are: 1) the weak light deflection signal is easily overwhelmed by the noise of the optical system, severely degrading the signal-to-noise ratio; 2) random thermal disturbances (ΔT ~ 0.1K) caused by the thermal boundary layer on the acoustic cavity wall produce refractive index fluctuations comparable to the acoustic signal; and 3) the complex spatial distribution characteristics of low-frequency standing wave modes require higher spatial resolution.

[0005] Therefore, there is an urgent need to develop a high-sensitivity schlieren measurement system for medium and low frequency sound fields to achieve quantitative characterization of medium and low frequency standing wave sound fields in closed acoustic cavities, and provide crucial experimental observation data for the study of engine combustion instability. [Summary of the invention]

[0006] The purpose of the present invention is to provide a visualization device for measuring the sound field in a medium and low frequency standing wave tube, so as to solve the problem in the prior art that the medium and low frequency standing wave sound field cannot be visualized.

[0007] The present invention adopts the following technical solution: a visualization device for measuring the sound field in a medium and low frequency standing wave tube, comprising:

[0008] A long, tubular, hollow acoustic cavity, one end of which is open, and within which is disposed a baffle that can reciprocate along the axial direction of the acoustic cavity; the other end of the acoustic cavity is closed by a perforated plate having a through hole in the center; a porous filter plate is disposed parallel to and spaced apart from the inner side of the perforated plate, the porous filter plate being a flat plate structure with a plurality of through holes evenly distributed thereon;

[0009] At least five sound pressure sensors are arranged at equal intervals on the outer wall of the sound cavity;

[0010] A horn, wherein the acoustic wave outlet pipe is provided with an acoustic wave heating net, the acoustic wave heating net is connected to a heating device, the heating device has a power of 100W to 300W, and a heating temperature of 50°C to 100°C; the acoustic wave outlet pipe is connected to a through hole on the orifice plate;

[0011] An acoustic wave generating system, including a signal generator and a power amplifier; the signal generator is used to set the frequency of pressure fluctuations, the frequency being 10 to 1000 Hz;

[0012] a nitrogen cylinder, the outlet of which is connected to the air inlet on the acoustic cavity; used to input gas with a predetermined pressure into the acoustic cavity to adjust the pressure in the acoustic cavity;

[0013] Three observation windows are respectively opened at the position where the baffle is provided, the middle position and any position of the sound cavity;

[0014] A schlieren system comprising two concave mirrors with a focal length of ≥3 m; the two concave mirrors are located on either side of the acoustic cavity, and the light path between the two concave mirrors can pass through any observation window as needed;

[0015] Among them, the acoustic wave heating network is used to heat the sound waves output from the speaker; the schlieren system is used to record the density gradient field generated by the gas medium under the action of the sound waves in the form of fringe images.

[0016] Furthermore, the baffle can move back and forth along its axis in the acoustic cavity to adjust the natural frequency of the acoustic cavity to match the operating frequency set in the experiment.

[0017] Furthermore, the aperture of the one-way through hole is smaller than that of the through hole.

[0018] Furthermore, the schlieren system includes a light source, two large focal length concave mirrors, a reflector and a high-speed camera arranged in sequence along the direction of light path.

[0019] Furthermore, the acoustic wave heating net is composed of a resistance heating wire wound around an asbestos support plate.

[0020] The second technical solution adopted by the present invention is a method for using a visualization device for measuring the sound field in a medium- and low-frequency standing wave tube, comprising the following contents:

[0021] According to the working frequency, open the nitrogen bottle and introduce a certain amount of gas into the acoustic cavity to make the acoustic cavity reach the specified pressure;

[0022] Set the speaker frequency through the signal generator and then turn on the speaker;

[0023] Turn on the heating device to heat the sound waves emitted by the speaker;

[0024] Use each sound pressure sensor to detect the boost amplitude at each position to determine whether there is a standing wave in the sound cavity, that is, whether the sound cavity frequency is consistent with the speaker frequency; if not, adjust the speaker frequency through the signal generator; if so, proceed to the next step;

[0025] Turn on the light source and use the Schlieren system to measure and record the image data of the density gradient of the gas medium in the measured area under the action of the sound wave.

[0026] The beneficial effects of the present invention are:

[0027] This invention achieves visual capture of low- and medium-frequency sound waves through the optical path selection of the schlieren system, the optimized design of the acoustic cavity heating device, and the spatiotemporal synchronization of multimodal data acquisition. It is particularly suitable for dynamic monitoring of low- and medium-frequency sound fields in closed acoustic cavity environments such as engine combustion chambers, and achieves full-range dynamic imaging of the spatial distribution of the sound field within a closed acoustic cavity under complex airflow conditions. It can measure the sound field distribution within a closed acoustic cavity under conditions of flow rates of 0-1 m / s and the formation of standing waves of 10-1000 Hz. It is suitable for closed acoustic cavity environments with gas flow, overcoming the limitation of existing devices that can only be used for static or open sound field measurements.

[0028] The present invention increases the temperature of the acoustic wave by providing a heating device, effectively raising the temperature of the gas medium within the acoustic cavity. This reduces gas density, increases the refractive index, and enhances the sensitivity of the schlieren imaging system, allowing precise capture of minute disturbances in the acoustic field. Furthermore, the use of a long-focal-length concave mirror reduces beam divergence, increasing the sensitivity of the schlieren imaging system and enabling clearer observation of minute-scale acoustic field disturbances. Furthermore, the high temperature reduces the random effects of thermal disturbances on the density of the airflow within the acoustic cavity, further improving imaging and overcoming the limitation of existing devices that only allow for measurement of higher-energy ultrasonic fields.

[0029] The present invention can also be combined with the acoustic-optical coupling data fusion method to effectively solve the dynamic measurement problem of low-frequency sound field distribution in complex airflow environments such as high-temperature combustion and turbulence, and provide visual analysis means and technical support for revealing the combustion coupling mechanism under the action of acoustic oscillation.

Brief Description of the Drawings

[0030] Figure 1 A schematic structural diagram of a visualization device for measuring the sound field in a medium- and low-frequency standing wave tube according to the present invention;

[0031] Figure 2 Schematic diagram of the structure of the orifice plate and the porous filter plate of the present invention;

[0032] Figure 3 An image of one acoustic wave cycle acquired by the schlieren system in the embodiment;

[0033] Figure 4 Graph showing the acoustic vibration signal data measured by the sound pressure sensor closest to the baffle 2 in the embodiment.

[0034] Among them: 1. Acoustic cavity; 2. Baffle; 3. Speaker; 4. Porous filter plate; 5. Sound wave generating system; 6. Sound wave heating network; 7. Air inlet; 8. Spatial filter; 9. Nitrogen bottle; 10. Sound pressure sensor; 11. Sound pressure acquisition system; 12. Reflector; 13. Light source; 14. Concave mirror; 15. High-speed camera; 16. High-speed acquisition computer; 17. Observation window; 18. Orifice plate. [Specific implementation method]

[0035] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The present invention provides a visualization device for measuring the sound field in a medium and low frequency standing wave tube, such as Figure 1 Shown, including:

[0037] A long, hollow, transparent acoustic cavity 1 is constructed of polycarbonate (transmittance ≥ 92%), offering high light transmittance and excellent mechanical strength, ensuring clarity and experimental safety during optical measurements. One end of the acoustic cavity 1 is open, and within this open end is a baffle 2 that can reciprocate along the axial direction of the acoustic cavity 1. The cross-sectional shape of baffle 2 is identical to that of the acoustic cavity 1, and moving baffle 2 can change the length of the internal passageway of the acoustic cavity 1, thereby changing the natural frequency of the acoustic cavity 1. The other end of the acoustic cavity 1 is sealed with a perforated plate 18, which has a through-hole at its center. A porous filter plate 4 is disposed parallel to and spaced apart on the inner side of the perforated plate 18. The porous filter plate 4 is a flat plate structure with multiple through-holes evenly distributed. The porous filter plate 4 is disposed at the inlet of the acoustic cavity 1 to ensure laminar flow within the cavity, while a rigid baffle 2 is disposed at the outlet to create hard acoustic boundary conditions at both ends, which helps form ideal standing waves and reduces interference from external noise on the internal gas vibration state.

[0038] At least five sound pressure sensors 10 are arranged at equal intervals on the outer wall of the sound cavity 1; for example, they are arranged at both ends of the sound cavity 1, as well as in the middle, 1 / 4 and 3 / 4 of the sound cavity 1; such an arrangement allows each sound pressure sensor 10 to be located at the antinode or node of the sound wave corresponding to the sound cavity 1, which is a typical waveform position, facilitating the precise collection of sound wave pressure data, and used to more accurately determine whether a standing wave state has been reached in the sound cavity 1. In the present invention, when the pressure fluctuation frequency generated by the speaker is equal to the natural frequency of the sound cavity, a standing wave will be formed in the sound cavity, resulting in the generation of periodic pressure fluctuations, causing periodic changes in the density of the medium, which is manifested as periodic changes in light and dark stripes in the image through the schlieren system, and can intuitively reflect the distribution of the sound field.

[0039] A speaker 3, whose sound wave outlet pipe is provided with a sound wave heating net 6, which is connected to a heating device with a power of 100W to 300W and a heating temperature of 50°C to 100°C; the sound wave outlet pipe is connected to a through hole on the orifice plate 18;

[0040] The heating device can realize real-time monitoring and adjustment of temperature to meet the needs of different experimental working conditions. The acoustic heating network 6 is used to heat the acoustic wave medium. According to the ideal gas state equation (P=ρRT), under the condition of rising temperature, the same sound pressure disturbance will cause a more significant relative density change. Furthermore, according to the Gladstone-Dale relationship n-1=Kρ, the increase in density change directly leads to an increase in the refractive index gradient in the medium, thereby exacerbating the spatial difference in the refractive index of light. Through the above-mentioned coupling relationship between temperature-density-refractive index, the heating effect significantly amplifies the refractive index change caused by the acoustic density fluctuation, effectively enhancing the detectability of weak acoustic wave signals in the optical imaging system. At the same time, the heating of the gas medium can also significantly suppress the random noise caused by background thermal disturbances, improve the signal-to-noise ratio of the signal in the measurement system, thereby enhancing the ability of the schlieren system to obtain effective information and improving the stability of the system.

[0041] An acoustic wave generating system 5 includes a signal generator and a power amplifier; the signal generator is used to set the frequency of the pressure fluctuation, the frequency is 10 to 1000 Hz, and the power amplifier is used to adjust the amplitude of the pressure fluctuation.

[0042] A nitrogen cylinder 9, the outlet of which is connected to the air inlet 7 located on the acoustic cavity 1; is used to input gas with a predetermined pressure into the acoustic cavity 1 to adjust the pressure in the acoustic cavity 1. The predetermined pressure is selected according to the experimental requirements, and the adjustment can be made to the working conditions under different pressures. The gas supply system used in the present invention includes a high-pressure gas source nitrogen cylinder 9, a pressure reducing valve and a solenoid valve. The gas supply system is controlled by the solenoid valve, and the gas source pressure is adjusted using a needle valve. The gas supply system is used to accurately control the gas pressure in the acoustic cavity 1 to meet the needs under different experimental conditions.

[0043] Three observation windows 17 can be made of organic glass and are respectively opened at the position where the baffle is provided, the middle position and any position of the sound cavity 1;

[0044] A Schlieren system, comprising two large focal length concave mirrors 14, with a focal length of ≥3m; the two concave mirrors 14 are respectively located on either side of the acoustic cavity 1, and the light path between the two concave mirrors 14 can pass through any observation window 17 as needed;

[0045] Among them, the acoustic wave heating network 6 is used to heat the acoustic waves output from the speaker 3 to increase the acoustic energy density, enhance the detectability of weak acoustic wave signals in the optical imaging system, and reduce thermal disturbances to improve the system's ability to obtain effective information; the schlieren system is used to record the density gradient field generated by the gas medium under the action of acoustic waves in the form of fringe images.

[0046] The large focal length concave mirror 14 is used to improve the sensitivity of the schlieren system: the large focal length concave mirror component achieves sensitivity improvement through the direct proportional function relationship between its focal length parameter and the system sensitivity. The sensitivity is defined as the ratio of the light offset Δx to the medium refractive index gradient. The quotient value of its physical model satisfies Δx=f·θ and Where f represents the focal length parameter of the concave mirror, and θ is the deflection angle obtained by integrating the refractive index gradient along the light propagation direction z. Based on this mathematical model, the system sensitivity S∝f is derived. By adopting a concave mirror component with a larger focal length, the detectable refractive index gradient threshold is lowered, and the corresponding medium density fluctuation detection resolution is improved, thereby achieving high-precision measurement of tiny density fields.

[0047] In some embodiments, baffle 2 can reciprocate along its axis within acoustic cavity 1 to adjust the natural frequency of acoustic cavity 1 to match the operating frequency set in the experiment. The movement of baffle 2 can adjust the internal length of acoustic cavity 1, thereby adjusting its natural frequency.

[0048] In some embodiments, as Figure 2 As shown, the aperture of the one-way through hole is smaller than the through hole. The reflection surface of the porous filter plate 4 becomes larger, so that more sound waves are reflected back to the sound cavity by the porous filter plate 4 and will not be lost from the hole plate 18, reducing the influence of the internal volume of the speaker on the natural frequency of the sound cavity.

[0049] In some embodiments, a schlieren system includes a light source 13, two large-focal-length concave mirrors 14, a reflector 12, and a high-speed camera 15, arranged in sequence along the optical path. The longer-focal-length concave mirrors are used to form a Z-shaped optical path, extending the optical path by 2-3 times that of traditional schlieren. This significantly amplifies the refractive effect of minute density gradients, such as weak shock waves and low-speed flow fields. Light source 13 is typically a laser, and a spatial filter 8 may be positioned at its exit.

[0050] In some embodiments, the acoustic heating mesh 6 is composed of a resistive heating wire wrapped around an asbestos support plate. When energized, the resistive heating wire generates Joule heat, which heats the acoustic waves through heat conduction, raising the temperature of the gas medium within the acoustic cavity. This temperature-rise effect increases the refractive index gradient of the gas, enhancing the schlieren system's sensitivity to minute pressure fluctuations. Furthermore, the thermal turbulence caused by uneven temperature distribution is suppressed, reducing its optical interference with the schlieren imaging system.

[0051] The present invention also provides a method for using a visualization device for measuring the sound field in a medium- and low-frequency standing wave tube, which is characterized by comprising the following contents:

[0052] According to the working frequency, open the nitrogen bottle 9 and introduce a certain amount of gas into the acoustic cavity 1 to make the acoustic cavity 1 reach the specified pressure;

[0053] Set the frequency of speaker 3 through signal generator 5, and then turn on speaker 3;

[0054] Turning on the heating device to heat the sound waves emitted by the speaker 3;

[0055] Each sound pressure sensor 10 is used to detect the pressure increase amplitude at each position to determine whether there is a standing wave in the sound cavity 1, that is, whether the sound cavity frequency is consistent with the speaker frequency; if not, the signal generator 5 is used to adjust the frequency of the speaker 3; if so, the next step is entered;

[0056] The light source 13 is turned on, and the image data of the density gradient of the gas medium in the measured area under the action of the sound wave is measured and recorded by the Schlieren system.

[0057] Example

[0058] A visualization device for measuring the sound field in a medium- and low-frequency standing wave tube is constructed according to the present invention. The optical path of the Schlieren system is adjusted by controlling the angle between the line connecting the light source and the concave lens and the line connecting the two concave lenses to be less than 15°. When the size of the light spot presented by the light reflected by the concave mirror does not change with the position of the imaging plane and is always consistent with the area of ​​the concave mirror, the angle of the optical path is adjusted so that the light from the concave mirror is completely irradiated on the other concave mirror. At this time, the light between the two concave mirrors is parallel light. The position of the small reflector at the front end of the blade is adjusted so that all optical fibers enter the channel where the blade is located. The focus position of the optical path is found. The blade is moved so that the blade is at the lead plane where the focus is located. The amount of cutting by the blade is adjusted. The light spot on the screen is observed to change in brightness. If the light spot changes in brightness evenly, the optical path debugging is completed. This embodiment uses a split vertical Schlieren instrument purchased from Jinzhou Optoelectronic Equipment Co., Ltd. Its concave mirror has the characteristics of a large focal length (3m) and a small diameter (0.3m), and the overall Schlieren system is relatively small.

[0059] Five sound pressure sensors 10 are installed at equal intervals on the sound cavity 1 , and each sound pressure sensor 10 is connected to a sound pressure acquisition system to display pressure fluctuation data in real time.

[0060] This working condition requires the pressure inside the acoustic cavity to be 0.1 MPa, so the nitrogen bottle is not opened, and baffle 2 is placed at one end of the open end of the acoustic cavity 1;

[0061] Turn on the ultrasonic heating device to heat the ultrasonic wave; the heating device has a power of 300W, and heats the ultrasonic wave and stabilizes it to 100°C;

[0062] Turn on the sound wave generating system, use the signal generator, set the frequency of pressure fluctuation to 50Hz, and use the power amplifier to adjust the amplitude of pressure fluctuation to about 1000Pa, so that the speaker generates pressure fluctuation and thus sound waves;

[0063] The Schlieren system's optical path utilizes a 532nm laser source with a narrowband filter and a 100mm diameter biconcave reflector. Its focal length extends to the order of 3m, and its adjustable knife-edge sensitivity range reaches 0.1-10μrad, enabling precise control of light deflection. The present invention utilizes a manufacturer and model that provides small-aperture, high-focal-length Schlieren systems, achieving a spatial resolution of 0.5mm for low-frequency sound fields at 100Hz. The optical path between the two concave mirrors 14 of the Schlieren system passes through an observation window 17 located on baffle 2.

[0064] High-speed camera 15 was turned on and its parameters were adjusted to a frame rate of 11,000 and an exposure time of 20.01 μs. The position of camera 15 was adjusted to align with the optical path. High-speed camera 15 was a Phantom VEO 3610 monochrome high-speed camera 15, which has a maximum capture capacity of 290,000 fps, a dynamic resolution of 1.1 μs, and a dynamic measurement range of 10-1000 Hz. It can accurately record transient dynamic changes in the sound field and is suitable for complex standing waves and high-frequency acoustic oscillation environments.

[0065] After determining that a stable standing wave has formed in the acoustic cavity 1 through the pressure data collected by the acoustic pressure sensor, the pressure fluctuation data is further collected through the acoustic pressure sensor, and the image is collected through the schlieren system;

[0066] After the acquisition is completed, the image data is saved to the high-speed acquisition computer 16, and finally the high-speed camera 15 and the high-speed acquisition computer 16 are turned off; the sound wave generation system and the sound pressure acquisition system are turned off.

[0067] Analysis of experimental results:

[0068] 1. The image data collected by the Schlieren system in Example 1 is shown in the figure. Through the Schlieren system, the movement of the fluid inside the acoustic cavity under the action of sound waves can be clearly captured through the observation window 17 under low-frequency working conditions, which confirms the feasibility of using the visualization device of the present invention to use the Schlieren system to capture the sound field.

[0069] According to the existing technology, if no heating device is used, it is impossible to observe the light and dark changes of the bright lines in the acoustic cavity at this location through the schlieren system alone. Figure 3 It can be seen that with the change of time, there is an obvious density change in the red frame area, and the bright stripes vibrate periodically with time. Figure 3 The change of one cycle of the sound wave is given in , the total duration of one cycle is 0.02s, so the vibration frequency is 1 / 0.02=50Hz, which is consistent with the excitation frequency of the sound wave generating system input. Using the Michelson contrast definition, Figure 3The contrast of the obtained schlieren image is 0.57. It can be seen that heating the sound wave to 100°C can greatly improve the contrast of the schlieren image of the 50Hz sound wave, solving the problem that conventional means cannot visualize the medium and low frequency standing wave sound field.

[0070] 2. Figure 4 The data diagram of the acoustic vibration signal measured by the sound pressure sensor located closest to the baffle 2 is shown in the figure. Figure 4 As can be seen, the frequency measured by the sound pressure sensor is 50 Hz, which is consistent with the excitation frequency input by the acoustic wave generating system and the frequency of the bright streaks captured by the schlieren system. This indicates that the bright streaks captured by the present invention are indeed density gradient changes caused by acoustic waves near the wall, thus verifying the feasibility of using the laser schlieren instrument to observe the near-wall acoustic field distribution.

[0071] The innovation of the present invention is:

[0072] 1. The present invention uses a concave mirror with a focal length of at least 3m to improve the sensitivity of the schlieren system, making it more suitable for detecting medium and low frequency standing waves:

[0073] The Schlieren system in the present invention is a technology for visual measurement of flow field density distribution. It adopts a Z-type Schlieren optical path design, and its specific working principle is as follows: the light emitted by the light source is reflected by a concave mirror to form a parallel light beam, and the parallel light beam passes through the area to be measured (such as the sound field distribution area in a transparent acoustic cavity). Due to the density gradient in the area to be measured, the light is refracted and deflected in the medium. The refracted light is converged by a second concave mirror and passes through a knife edge perpendicular to the density gradient. Part of the deflected light is blocked by the knife edge, thereby forming a stripe image with light and dark contrast on the screen of the high-speed acquisition computer 16. The stripe image is recorded by a high-speed camera, and finally the density distribution in the measurement area is visualized.

[0074] In the Schlieren system of the present invention, fringe offset is directly related to system sensitivity. Sensitivity reflects the system's ability to detect changes in refractive index, and fringe offset is a direct manifestation of this change. The sensitivity of the Schlieren system stems from the deflection of light when it passes through a medium with a refractive index gradient. The fringe offset on the screen is proportional to the refractive index gradient of the medium. The key relationship is as follows:

[0075] Refractive index gradient and light deflection angle:

[0076]

[0077] in, is the refractive index gradient of the medium, and Z is the propagation distance of light in the refractive index gradient area.

[0078] Light deflection angle and fringe offset:

[0079] After refraction, the relationship between the light offset, the deflection angle, and the focal length of the optical system is:

[0080] Δx=f·θ,

[0081] in:

[0082] Δx: Fringe offset measured by the Schlieren system,

[0083] f: focal length of the concave mirror,

[0084] θ: The angle of light segregation, which is proportional to the refractive index gradient and the propagation path length.

[0085] The sensitivity S of a Schlieren system refers to the smallest refractive index change that the system can detect and is defined as:

[0086]

[0087] therefore:

[0088] S=f·dz.

[0089] That is, the sensitivity of the Schlieren system is proportional to the focal length of the concave mirror of the Schlieren system. Therefore, when the present invention uses a concave mirror with a larger focal length, the sensitivity of the Schlieren system is better, so that it can measure smaller refractive index gradient changes, that is, smaller density fluctuations.

[0090] 2. The present invention adds an acoustic wave heating network 6 to heat the acoustic wave. When the acoustic wave temperature rises, the following technical effects are achieved:

[0091] (1) Reducing the reverse effect of temperature gradient: The increase in temperature reduces the background density of the gas medium in the acoustic cavity, reducing the negative impact of the temperature gradient on the density gradient, thereby enhancing the dominance of the pressure gradient.

[0092] (2) Improve system sensitivity: The reduction of gas medium background density makes the refraction of light more significant, increases the angle of light deflection, and ultimately improves the Schlieren system's ability to detect tiny density changes.

[0093] (3) Reduce the impact of environmental thermal disturbances: When the gas temperature rises, the impact of random thermal disturbances is significantly reduced, and the acoustic field signal is more stable, thereby improving the measurement accuracy of the system.

[0094] The density gradient in the acoustic field is mainly composed of the following two parts: one is the co-absorption of the pressure gradient, and the other is the contribution of the temperature gradient.

[0095]

[0096] The presence of a temperature gradient causes an uneven distribution of density changes, thereby weakening the contribution of the pressure gradient to the overall density gradient. In this invention, heating the airflow reduces the opposing effect of the temperature gradient on the density gradient, thereby enhancing the dominance of the pressure gradient. This design significantly improves the sensitivity of the schlieren system, enabling it to more accurately capture density disturbances caused by tiny pressure fluctuations within the acoustic cavity.

[0097] 3. The present invention solves the problem of visualization of medium and low frequency standing waves:

[0098] In current research, many existing technologies have also achieved sound wave visualization, such as a real-time visualization and control system for ultrasonic fields (CN 118882807 A), an acoustic wave ripple suspension device (CN 218631101U), a sound velocity measurement device based on sound field visualization (CN 215865468 U), and a visualization device for measuring ultrasonic wavelength and its operating method (CN 116412898 A). All of these technologies use schlieren technology to visualize ultrasonic waves. Because ultrasonic waves have high energy and strong directionality, they create large density gradients, making sound wave visualization easier using schlieren systems. However, the energy of medium and low-frequency standing waves is relatively low, and the density gradients generated by their waves are relatively small. Conventional schlieren systems are unable to observe images of the density gradients of gas media under the influence of acoustic waves. However, the present invention uses a large-focal-length concave mirror, combined with a heating method for the acoustic waves, to successfully visualize medium and low-frequency sound waves.

[0099] 4. The present invention can use pressure data and optical data fusion measurement technology:

[0100] The data acquisition system used is a sound pressure acquisition system for pressure data and a high-speed acquisition computer for image data. Synchronous triggering technology enables simultaneous data fusion measurement. The data acquisition system includes a synchronous trigger module, a PCB sound pressure sensor, a Dewesoft data acquisition card, and corresponding data acquisition and processing software, achieving spatiotemporal alignment of pressure fluctuation data and schlieren images. Data fusion algorithms enable high-precision modeling, dynamic visualization, and multi-dimensional analysis of the sound field distribution.

Claims

1. A visualization device for measuring the sound field in a medium and low frequency standing wave tube, characterized in that: include: A long tubular hollow acoustic cavity (1), one end of the acoustic cavity (1) being an open end, and a baffle (2) being arranged inside the open end and being movable back and forth along the axis of the acoustic cavity (1); the other end of the acoustic cavity (1) being closed by a perforated plate (18), the center of the perforated plate (18) being provided with a through hole; a porous filter plate (4) being arranged parallel to and spaced apart from the inner side of the perforated plate (18), the porous filter plate (4) being a flat plate structure with a plurality of through holes evenly distributed thereon; At least five sound pressure sensors (10) are arranged at equal intervals on the outer wall of the sound cavity (1); A speaker (3) is provided with a sound wave heating net (6) on its sound wave outlet pipe, the sound wave heating net (6) is connected to a heating device, the power of the heating device is 100W to 300W, and the heating temperature is 50°C to 100°C; the sound wave outlet pipe is connected to the through hole on the orifice plate (18); An acoustic wave generating system (5) includes a signal generator and a power amplifier; the signal generator is used to set the frequency of the pressure fluctuation, the frequency being 10 to 1000 Hz; a nitrogen bottle (9), the outlet of which is connected to the gas inlet (7) located on the acoustic cavity (1); used for inputting gas with a predetermined pressure into the acoustic cavity (1) to adjust the pressure in the acoustic cavity (1); Three observation windows (17) are respectively provided at a position where a baffle is provided, a middle position and an arbitrary position of the sound cavity (1); A schlieren system comprises two concave mirrors (14) with a focal length of ≥3 m; the two concave mirrors (14) are respectively located on both sides of the acoustic cavity (1), and the light path between the two concave mirrors (14) can pass through any observation window (17) as needed; The acoustic wave heating network (6) is used to heat the acoustic waves output from the speaker (3); and the schlieren system is used to record the density gradient field generated by the gas medium under the action of the acoustic waves in the form of fringe images.

2. A visualization device for measuring the sound field in a medium and low frequency standing wave tube according to claim 1, characterized in that: The baffle (2) can reciprocate along its axis in the acoustic cavity (1) to adjust the natural frequency of the acoustic cavity (1) to match the operating frequency set in the experiment.

3. A visualization device for measuring the sound field in a medium and low frequency standing wave tube according to claim 1 or 2, characterized in that: The aperture of the one-way through hole is smaller than that of the through hole.

4. A visualization device for measuring the sound field in a medium and low frequency standing wave tube according to claim 3, characterized in that: The schlieren system comprises a light source (13), two large-focal-length concave mirrors (14), a reflecting mirror (12), and a high-speed camera (15) sequentially arranged along the direction of travel of the light path.

5. The visualization device for measuring the sound field in a medium and low frequency standing wave tube according to claim 3, characterized in that: The acoustic wave heating net (6) is composed of a resistance heating wire wound around an asbestos support plate.

6. A method for using a visualization device for measuring the sound field in a medium and low frequency standing wave tube, characterized in that: Includes the following: According to the working frequency, the nitrogen bottle (9) is opened to introduce a certain amount of gas into the acoustic cavity (1) so that the acoustic cavity (1) reaches a specified pressure; Setting the frequency of the speaker (3) by means of the signal generator (5), and then turning on the speaker (3); turning on the heating device to heat the sound waves emitted by the speaker (3); Using each sound pressure sensor (10) to detect the boost amplitude at each position, to determine whether there is a standing wave in the sound cavity (1), that is, whether the sound cavity frequency is consistent with the speaker frequency; if not, the frequency of the speaker (3) is adjusted by the signal generator (5); if yes, proceed to the next step; The light source (13) is turned on, and the image data of the density gradient of the gas medium in the measured area under the action of the sound wave is measured and recorded by the schlieren system.

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