A three-dimensional non-invasive method for extracting acoustic impedance of acoustic lining

Abstract

The present application provides a three-dimensional non-invasive method for extracting the acoustic impedance of an acoustic liner, including: constructing a coupled field test environment of flow, acoustic, and temperature fields for the acoustic liner to be tested; utilizing a three-dimensional LDV to measure the fluid velocity in the measurement area above the surface of the acoustic liner; utilizing the coherence function method to obtain the acoustic particle velocity, and further calculating the acoustic pressure; and finally obtaining the Ingard-Myers boundary impedance through forward calculation. The advantages of this application are: the test environment is closer to the actual operating conditions of the acoustic liner; the use of non-contact measurement avoids interference with the internal flow and acoustic fields, resulting in more accurate test results; and because this method utilizes local scanning testing, it is also applicable to measurements under conditions of acoustic liners with complex geometric parameters, complex flow fields, complex temperature fields, and acoustic fields, including: surface acoustic impedance distribution measurement of non-uniform acoustic liners, surface acoustic impedance measurement of curved surface acoustic liners, swirl conditions, non-uniform temperature fields, and high-frequency acoustic impedance measurement above the pipe cutoff frequency.

Description

Technical Field

[0001] The present application belongs to the technical field of acoustic characteristics measurement of acoustic pads, and in particular to the measurement of acoustic impedance parameters of acoustic pads under conditions of tangential flow, high temperature and high sound intensity coupling, and specifically to a non-invasive acoustic pad acoustic impedance extraction method. Background Art

[0002] Aircraft engine noise accounts for a large proportion of aircraft noise. For aircraft engines, installing acoustic pads (hereinafter referred to as acoustic pads) in the engine anechoic nacelle is an important technical approach to reducing engine radiated noise. Acoustic impedance is a key parameter that characterizes the acoustic performance of acoustic pads. The extraction of acoustic impedance mainly includes microphone-based measurement methods and optical non-contact measurement methods according to the measurement method. The microphone-based acoustic impedance extraction method was developed earlier and the technology is relatively mature; the acoustic impedance extraction methods based on optical non-contact measurement mainly include the acoustic impedance extraction method based on LDV (Laser Doppler Velocimetry) and the acoustic impedance extraction method based on PIV (Particle Image Velocimetry).

[0003] However, the acoustic lining of the anechoic nacelle often operates in complex working conditions with high sound pressure level, high flow rate, and high temperature coupling. Existing acoustic impedance extraction methods cannot fully cover the actual application environment conditions, or are limited by flow rate or temperature. Some test methods are even destructive to the test samples. For example, the impedance tube method is now quite mature, but this method does not measure flow rate and can only measure the normal surface impedance of samples of fixed size; in the microphone-based method, the measurement means is the microphone, and ordinary measurement microphones are not resistant to high temperatures, and the measurement accuracy of the microphone is relatively sensitive to temperature. The accuracy of sound pressure measurement in high temperature environments is difficult to guarantee; the direct measurement method requires drilling a hole at the sample measurement position, which damages the sample and has a great impact on the measurement accuracy.

[0004] Reference (1) A. Minotti, F. Simon, F. Gantié, Characterization of an acoustic liner by means of Laser Doppler Velocimetry in a subsonic flow, Aerospace Science and Technology, Volume 12, Issue 5, 2008, Pages 398-407, ISSN 1270-9638, first achieved the measurement of acoustic characteristics of an acoustic liner using a two-dimensional LDV under flow conditions. The research object in this paper is room temperature conditions, and high temperature conditions are not considered. In addition, the LDV used is two-dimensional. Compared with a three-dimensional LDV, it lacks one velocity component, namely the velocity component in the transverse direction of the flow tube (i.e., the horizontal direction of the flow tube cross section). Therefore, the influence of the wall boundary along the horizontal direction cannot be considered. At the same time, it only proposes a method for extracting the standard acoustic impedance. This impedance form cannot reflect the physical characteristics of the acoustic liner itself under the corresponding test conditions and is therefore not suitable.

[0005] Reference (2) Victor Lafont, Fabien Méry, and Frank Simon. Liner Multiphysics Coupling Between Grazing Flow, Thermal Gradients, and Sound Pressure Levels. AIAA Journal 2022 60: 8, 4754-4763. The acoustic transmission loss before and after the sample and the acoustic impedance of the sample were studied under the conditions of acoustic, flow, and thermal coupling. This paper uses two heating modes: one is airflow heating and the other is heating the bottom of the sample. The two different heating methods are used to observe the changes in the acoustic lining results under different temperature gradients. The acoustic measurement method is a microphone. Because the microphone is not resistant to high temperatures, the highest airflow temperature in the paper is 120°C. At the same time, this method uses an inverse method based on the velocity of the sound particle as the objective function in impedance extraction, which has the disadvantages of large computational complexity and the possibility of multi-valued solutions.

[0006] The defects of the existing methods and patents for extracting acoustic lining impedance mainly include:

[0007] The impact of high temperatures has not been considered: Both the devices described in domestic patents and foreign articles on laser sound measurement are designed for normal temperature conditions and cannot be directly used for high-temperature testing. The actual operating conditions of the engine anechoic nacelle, in addition to flow and high sound intensity, also have the characteristics of high inlet airflow temperature, generally reaching 200°C. Therefore, the actual working conditions of the acoustic lining are a complex working condition with coupled flow, sound, and heat. In air, the speed of sound increases by approximately 0.6 m / s for every degree Celsius increase in medium temperature. For anechoic nacelles with temperatures exceeding 100 degrees Celsius or with significant temperature gradients, the speed of sound of the surrounding medium changes significantly. The acoustic changes caused by high temperatures will be more obvious and should not be ignored.

[0008] Microphone-based acoustic lining impedance testing methods require drilling holes in the test section's walls, which inevitably impacts the localized flow and acoustic fields, particularly at high frequencies. Furthermore, the accuracy of existing microphones is highly sensitive to temperature and is generally not heat-resistant, rarely operating above 150°C. Some high-temperature-resistant probe microphones, due to their operating principles, have relatively low measurement accuracy, making them difficult to meet high-precision measurement requirements.

[0009] Under high-temperature test conditions, the temperature distribution of the fluid in the test section is often uneven due to the limitations of actual heating equipment. Existing forward impedance extraction methods based on microphone arrays, due to the assumption of a uniform temperature field in their testing principles, fail to extract acoustic impedance under conditions of uneven temperature fields. Existing inverse impedance extraction methods based on microphone arrays and LDV-based acoustic particle velocity measurements require obtaining the fluid temperature distribution throughout the entire acoustic liner test section. However, due to the limitations of thermal sensors, it is difficult to accurately obtain the three-dimensional spatial temperature distribution throughout the test section in actual testing, making these methods no longer applicable. Summary of the Invention

[0010] The purpose of the present invention is to address the problems existing in the prior art and solve the problems of damage to the test sample, limited measurement area, poor measurement position accuracy, unsuitability of the measurement method for high temperature, failure to consider temperature non-uniformity, large amount of inversion calculation and multi-valuedness in the inverse method during the acoustic impedance test of the acoustic lining material. Based on three-dimensional non-invasive measurement technology and signal processing technology, the surface acoustic impedance parameters of the acoustic lining sample can be forward extracted, thereby providing an accurate test method for the acoustic impedance test of the acoustic lining surface.

[0011] To achieve the above objectives, this application proposes a three-dimensional non-invasive acoustic lining acoustic impedance extraction method, comprising:

[0012] Construct a coupled field test environment including flow field, acoustic field and temperature field for the acoustic liner to be tested;

[0013] The three-dimensional velocity of the fluid is measured in a designated measurement area above the acoustic liner surface using a three-dimensional laser Doppler velocimeter;

[0014] The signal processing method based on the coherence function method is used to obtain the sound particle velocity result;

[0015] Determine the uniform flow condition and shear flow condition, and calculate the sound pressure in the measurement area;

[0016] The standard acoustic impedance and Ingard-Myers boundary impedance are obtained by forward calculation from the sound pressure and sound particle velocity;

[0017] The surface impedance distribution of the acoustic liner was obtained through three-dimensional laser Doppler velocimeter scanning test.

[0018] As an improvement to the above method, the sound particle velocity result includes:

[0019]

[0020]

[0021]

[0022] Among them, x, y, z represent the three coordinate components of the Cartesian coordinate system; u′ j represents the j-direction component of the velocity of the sound particle; u j represents the j-direction component of the air velocity; s represents the sound source signal; G represents the cross power spectrum density between the j-direction component of the air velocity and the sound source signal; s Represents the autopower spectral density of the sound source signal; Represents the autopower spectral density of the j-direction component of the sound particle velocity; It represents the phase difference between the j-component of the sound particle velocity and the sound source signal; Im(·) represents the imaginary part of the complex number; Re(·) represents the real part of the complex number.

[0023] As an improvement to the above method, the determination of uniform flow conditions and shear flow conditions includes:

[0024] When the actual average flow velocity gradient in the measurement area is less than or equal to the set threshold, it is defined as a uniform flow condition; when the actual average flow velocity gradient is greater than the set threshold, it is defined as a shear flow condition.

[0025] As an improvement to the above method, obtaining the sound pressure in the measurement area includes:

[0026] When the measurement area is in uniform flow conditions:

[0027]

[0028] Where p′ represents the sound pressure; ρ0 and c0 represent the density and sound velocity of the mean flow, respectively, which are corrected by the temperature at the measuring point; M0 represents the flow Mach number; x, y, and z represent the three coordinate components of the Cartesian coordinate system, where x is the flow direction, y is the direction perpendicular to the flow direction on the horizontal plane, and z is the vertical direction; u′ x represents the x-component of the velocity of the sound particle; u′ y Represents the y-component of the velocity of the sound particle; u′ z represents the z-component of the velocity of the sound particle; ω represents the angular frequency; i represents the imaginary unit;

[0029] When the measurement area is under shear flow conditions in the z direction:

[0030]

[0031] When the measurement area is in shear flow conditions in both the z and y directions:

[0032]

[0033] The partial derivatives of the sound particle velocity components in the above formula are obtained by performing spatial discretization calculations on the corresponding sound particle velocity components in the measurement area using the finite difference method.

[0034] As an improvement to the above method, the standard acoustic impedance obtained from the sound pressure and the sound particle velocity is:

[0035]

[0036] Among them, Z i Indicates standard acoustic impedance; represents the velocity vector of the sound particle; Represents the unit normal vector of the acoustic lining surface.

[0037] As an improvement to the above method, the Ingard-Myers boundary impedance is expressed as:

[0038]

[0039] Among them, Z n represents the Ingard-Myers boundary impedance; Z i represents standard acoustic impedance; p′ represents sound pressure; represents the velocity vector of the sound particle; Indicates that multiplication with a vector yields the normal component of the vector; represents the gradient operator; represents the average flow velocity; and It is obtained by discrete calculation using the finite difference method.

[0040] The average flow velocity in the measurement area obtained by the three-dimensional laser Doppler velocimeter Sound particle velocity Sound pressure p′ and standard acoustic impedance Z i Substitute the above formula into the Ingard-Myers boundary impedance of the sound lining surface.

[0041] As an improvement to the above method, the method further includes:

[0042] Before obtaining the sound particle velocity results, the measurement data of the three-dimensional laser Doppler velocimeter is preprocessed;

[0043] The pretreatment includes:

[0044] The data is linearly interpolated to obtain uniformly distributed data;

[0045] Convert each velocity component into a Cartesian coordinate system according to the laser angle;

[0046] Perform time domain windowing function processing on the data.

[0047] As an improvement to the above method, a coupled field test environment including flow field, acoustic field and temperature field is constructed for the acoustic liner to be tested, and the device used includes a flow tube device;

[0048] The acoustic impedance flow tube device includes an air source section, a heating section, an upstream muffler section, a stabilization section, a contraction section, a sound source section, a test section, a diffusion section, a downstream muffler section and an outlet, which are connected in sequence.

[0049] The air source section uses a centrifugal fan to provide power to accelerate the air and then blow it into the heating section;

[0050] The heating section includes an electric heating tube, which is used to heat the airflow blown out of the origin section with the electric heating tube, and then heat it to a set temperature and output it to the upstream muffler section;

[0051] The upstream muffler section is a pipe with a straight interior and a constant cross-section, and is filled with sound-absorbing material to reduce the noise of the airflow and output it to the stabilizing section;

[0052] The stabilizing section is an internal straight pipe with a square cross section, which is equipped with a set of honeycombs and multi-layer damping mesh to reduce the turbulence of the airflow and output it to the contracting section;

[0053] The contraction section is a pipe whose cross section gradually decreases along the airflow direction, which increases the velocity of the airflow and then introduces it into the sound source section;

[0054] The sound source section has speakers installed on its left and right sides, and the speakers generate sound signals of a set type;

[0055] The test section has high light transmittance materials on both sides and a sound liner sample installation device at the bottom;

[0056] The cross section of the pipe in the diffusion section increases continuously along the flow direction;

[0057] The downstream silencer section has a cross section that increases continuously along the flow direction and is surrounded by silencer material;

[0058] The outlet is trumpet-shaped.

[0059] As an improvement to the above method, the measurement area includes a measurement point array with L×M×N measurement points, where L represents the number of flow direction measurement points, M represents the number of vertical measurement points, and N represents the number of horizontal lateral measurement points; the spacing between adjacent measurement points in each direction of the point array is equal; the distance between the bottom layer of the measurement array points and the upper surface of the acoustic liner is the position closest to the acoustic liner surface that can be reached by the laser optical path of the three-dimensional laser Doppler velocimeter; and the measurement area is located in the area directly above the acoustic liner surface.

[0060] Compared with the prior art, the advantages of this application are:

[0061] 1. The three-dimensional LDV system used in this application can obtain information on the transverse velocity component of the flow tube cross section by measuring. The actual measurement result can be 1mm away from the surface of the acoustic lining, that is, closer to the surface of the acoustic lining, and therefore more accurate;

[0062] 2. A coupled field test environment, encompassing flow, acoustic, and temperature fields, was constructed. Compared to existing microphone array test methods, this method is more suitable for high-temperature testing, particularly for measuring the surface acoustic impedance of acoustic liners used in aircraft engine anechoic nacelles operating in high-temperature environments.

[0063] 3. This application proposes a three-dimensional LDV test method, which can calculate the sound pressure at the corresponding measuring point from the sound particle velocity obtained by the test, and further directly calculate the Ingard-Myers boundary impedance of the sound lining surface. The test and calculation process of this method is simple. On the one hand, it can avoid the shortcomings of some existing impedance extraction inverse methods based on sound pressure or sound particle velocity tests, such as large amount of calculation, multi-valuedness, and large uncertainty; on the other hand, compared with the existing two-dimensional LDV method for extracting standard impedance, this method takes into account the three-dimensional effect, and can obtain the sound lining impedance distribution results in the horizontal lateral area close to the side wall, and the measurement area is wider. The most important advantage of this method is that the Ingard-Myers boundary impedance form can be obtained according to this method, which removes the influence of the flow field information implicit in the standard acoustic impedance and has a clearer physical meaning. Therefore, it is more suitable for the characterization and performance evaluation of the sound lining surface impedance, including the consistency comparison of the sound lining surface impedance extraction results under corresponding conditions on different test platforms;

[0064] 4. Since this method only needs to measure the velocity of sound particles in a local area above the surface of the acoustic lining to obtain the local surface impedance of the acoustic lining without introducing other assumptions and boundary conditions, this method is also applicable to measurements under conditions of acoustic liners with complex geometric parameters, complex flow fields, complex temperature fields, and complex acoustic fields. Specifically, this method includes: surface acoustic impedance distribution measurement of non-uniform acoustic liners, surface acoustic impedance distribution measurement of curved surface acoustic liners, surface impedance measurement of acoustic liners under complex flow field conditions (non-average flow, such as three-dimensional vortex flow) and complex temperature field conditions (non-uniform temperature field), high-frequency acoustic impedance measurement above the pipe cutoff frequency, and nonlinear impedance measurement of acoustic liners under different sound pressure levels.

[0065] In summary, compared with the existing acoustic lining surface impedance extraction method, this method has a relatively simple measurement process and high test accuracy. It effectively overcomes some limitations of the existing acoustic lining surface acoustic impedance extraction method and greatly expands the applicable measurement range. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 The figure shows a schematic diagram of the three-dimensional LDV scanning test of the fluid velocity in the test section measurement area.

[0067] Figure 2 The figure shows a flow chart of a three-dimensional non-invasive acoustic liner acoustic impedance extraction method;

[0068] Figure 3 Shown is a schematic diagram of the basic principle of LDV speed measurement;

[0069] Figure 4 The following is a working diagram of the control system;

[0070] Figure 5 Shown is a structural diagram of the flow tube equipment;

[0071] Figure 6 Shown is a schematic diagram of the structure of the heating section of the flow tube equipment;

[0072] Figure 7 Shown is a contraction curve diagram of the contraction section of the flow tube device;

[0073] Figure 8 Shown is a plan view of the measurement point layout in the measurement area. DETAILED DESCRIPTION

[0074] The technical solution of this application is described in detail below with reference to the accompanying drawings.

[0075] The present invention provides a non-invasive acoustic impedance extraction method for acoustic linings. This method takes into account the influence of tangential flow and high temperature environment. First, a coupled field test environment including flow field, acoustic field and temperature field is constructed for the acoustic lining to be tested. After the flow field, acoustic field and temperature field are stabilized, a three-dimensional LDV system is used to test the flow, acoustic and temperature coupled field measurement area of ​​the device test section, such as Figure 1 As shown, the three-dimensional velocity results of the target area are measured; then the signal processing method based on the coherence function method is used to calculate the sound particle velocity results:

[0076]

[0077]

[0078]

[0079] Where x, y, and z represent the three coordinate components of the Cartesian coordinate system, where x is the flow direction, y is the direction perpendicular to the flow direction on the horizontal plane (abbreviated as horizontal), and z is the vertical direction; u′ j represents the j-direction component of the velocity of the sound particle; u j represents the j-direction component of the air velocity; s represents the sound source signal; G represents the cross power spectrum density between the j-direction component of the air velocity and the sound source signal; s Represents the autopower spectral density of the sound source signal; Represents the autopower spectral density of the j-direction component of the sound particle velocity; represents the phase difference between the j-direction component of the sound particle velocity and the sound source signal; Im(·) represents the imaginary part of the complex number; R e (·) means taking the real part of a complex number;

[0080] Different from the method of the French Space Agency (ONERA) in reference (1) that indirectly obtains the sound pressure value by using the relationship between the sound particle velocity and the sound displacement, this method can obtain the relationship between the sound particle velocity and the sound pressure under different flow conditions based on the linearized Euler equation. When the velocity gradient in the LDV measurement area is not large, that is, the velocity values ​​at each measuring point are nearly equal, it is treated as uniform flow; if the velocity gradient in the LDV test area close to the acoustic liner surface is large along the vertical z or horizontal y direction, it is treated as shear flow. This is equivalent to configuring a threshold for the average flow velocity gradient. When the velocity gradient in the LDV test area along the vertical z and horizontal y direction is less than or equal to the threshold, it is defined as a uniform flow condition, and when the velocity gradient is greater than the threshold, it is defined as a shear flow condition.

[0081] When the measurement area is in uniform flow conditions:

[0082]

[0083] Where ρ0, c0 represent the density and speed of sound of the mean flow, respectively; M0 represents the flow Mach number; p′ represents the sound pressure; ω represents the angular frequency; and i represents the imaginary unit.

[0084] When the measurement area is in shear flow conditions in the vertical z direction:

[0085]

[0086] When the measurement area is in shear flow conditions in both the vertical z direction and the horizontal y direction:

[0087]

[0088] By using the finite difference method to discretize equations (4), (5) and (6), the sound pressure results at the corresponding measuring points are calculated, and then according to the definition of standard acoustic impedance The standard acoustic impedance result can be directly calculated. It should be noted that the acoustic impedance used in the French Space Agency (ONERA) in the literature (1) is consistent with the standard acoustic impedance definition in acoustics. As can be seen from its definition, the standard acoustic impedance is only defined by the sound pressure and the sound particle velocity. It is usually used for boundary conditions under no-flow conditions. For flow conditions, the standard acoustic impedance not only includes the characteristics of the acoustic liner itself, but is also related to flow field quantities such as tangential flow velocity and sound pressure gradient, that is, it includes information about the local flow field. In actual tests, due to the limitation of the laser light path, the lowest position of the LDV test can only reach the area 1-3mm above the upper surface of the acoustic liner, and the actual flow velocity at this position is not 0. Therefore, this standard acoustic impedance form cannot independently reflect the characteristics of the acoustic liner itself and is not suitable for the characterization and evaluation of the acoustic liner characteristics. For acoustic propagation under tangential flow, this application employs the Ingard-Myers boundary condition, considering that the average velocity at the boundary of the LDV measurement area closest to the acoustic liner surface is non-zero. This condition is based on the thin boundary layer assumption, assuming that the effect of the boundary layer on the acoustic field is negligible. Its unique characteristic is that it considers both the impedance of the acoustic liner itself and the effect of slip flow on the boundary condition, resulting in a clear physical meaning. When the average velocity at the boundary is non-zero, the relationship between the standard acoustic impedance and the Ingard-Myers boundary impedance condition can be converted to each other using the following equation: Therefore, after obtaining the standard acoustic impedance result, further calculations can be used to determine the impedance form of the Ingard-Myers boundary condition.

[0089] The relationship between the impedance in the Ingard-Myers boundary condition and the sound pressure, sound particle velocity and average flow velocity at the corresponding boundary position can be expressed as:

[0090]

[0091] in, represents the gradient operator; represents the unit normal vector of the sound lining surface; represents the velocity vector of the sound particle; represents the mean flow velocity vector.

[0092] The standard acoustic impedance can be expressed as:

[0093]

[0094] The relationship between the Ingard-Myers boundary impedance and the standard acoustic impedance is as follows:

[0095]

[0096] During the calculation, the gradient and derivative in the above formula are spatially discretized using the central format of the finite difference method. In addition, due to the measurement characteristics of the LDV, the sound particle information at a position 1 mm on the surface of the sound lining can be measured. The measurement area can be very narrow. When the temperature gradient in this area is not very large, the temperature change in this narrow area can be basically ignored. The temperature at this position is measured by a temperature sensor, and then the sound velocity and air density after considering the corresponding temperature correction are substituted into formulas (4)-(6) to calculate the sound pressure value at the corresponding position. Finally, it is substituted into formula (9). After spatial discretization, the Ingard-Myers boundary impedance at the measurement position on the surface of the sound lining at the corresponding temperature is calculated. Through LDV scanning testing, this method can not only test the surface acoustic impedance of a local area of ​​a uniform acoustic lining, but also directly test the surface acoustic impedance distribution of a non-uniform acoustic lining. It can also be used to test the local acoustic impedance changes caused by the nonlinearity generated by high sound pressure levels and the acoustic impedance non-uniformly distributed along the surface of the acoustic lining. At the same time, LDV testing is a local test and can be applied to surface scanning tests of curved acoustic linings. Therefore, this method can also directly test the surface acoustic impedance of curved acoustic linings. In addition, since this method adopts the basic relationship of the fluid acoustic differential equation and does not assume the form of the internal sound field solution, it is also suitable for high-frequency acoustic impedance measurement above the cutoff frequency of the pipeline.

[0097] The above three-dimensional non-invasive acoustic lining acoustic impedance extraction method process is as follows: Figure 2 shown.

[0098] Schematic diagram of the basic principle of LDV laser speed measurement Figure 3As shown, the device primarily consists of six components: a laser, a beam splitter and coupler, a signal processor, a mobile measurement stand, a laser probe, and control software. An argon ion laser generates laser light and transmits it to the beam splitter and coupler. The beam splitter separates the laser light, producing laser beams of corresponding wavelengths. The laser light travels through an optical fiber to the laser probe, where it is focused by the probe lens and emitted into the measurement area. The two laser beams, angled at an angle, form an ellipsoidal focal point, known as the measurement volume. The size of the measurement volume is determined by the angle, wavelength, and diameter of the laser beams. Tracer particles in the fluid can track the fluid's motion. When these tracer particles pass through the measurement volume, they emit scattered light under the incident laser light. This scattered light is received by the laser's receiving probe. The optical signal from the receiving probe is transmitted to a photomultiplier tube, converted into an electrical signal, and then sent to the signal processor for processing. Utilizing the laser Doppler principle, data processing yields the flow velocity at the measurement point in the corresponding direction.

[0099] In the three-dimensional LDV velocity measurement system, the laser is separated by a beam splitter to produce three lasers with different wavelengths: green light (wavelength 514.4nm), blue light (wavelength 488nm), and violet light (wavelength 476.5nm). These three lasers are focused at different angles onto the same measuring point. Based on the above-mentioned velocity measurement principle, the velocity components of the same measuring point in three directions can be obtained. Then, through coordinate transformation, the three-dimensional orthogonal velocity components in the Cartesian coordinate system of a given flow tube can be obtained.

[0100] The LDV-based acoustic impedance extraction method proposed in reference (1) uses a two-dimensional LDV system. This LDV system can only measure vertical and axial velocities and cannot obtain information on the horizontal lateral velocity component of the flow tube cross section. In particular, when there is a large horizontal velocity gradient near the transparent glass wall, the horizontal lateral velocity component of the test area cannot be obtained, which will cause a large error in calculating the sound pressure using formula (5). This method can only test the acoustic impedance of the acoustic lining surface near the center of the axis, and the measurement area is greatly limited. In addition, due to the limitation of the probe optical path angle, this LDV system can only measure the velocity information at a position 3 mm and above the acoustic lining surface. The acoustic impedance result obtained at this position is often quite different from the actual acoustic impedance of the acoustic lining surface. This application proposes the use of a 3D LDV system, which can obtain information on the transverse velocity component of the flow tube cross section by measuring, thereby expanding the test area. Even under complex flow conditions such as non-uniform flow, such as the three-dimensional vortex in which the actual acoustic liner is located, the acoustic impedance of the acoustic liner surface under corresponding conditions can be obtained. In addition, by configuring the optical path of the laser probe as described above, the 3D LDV system in this application can measure the result of the lowest 1mm position on the acoustic liner surface, which is closer to the actual conditions and therefore more accurate.

[0101] The measurement device used in the above-mentioned three-dimensional non-invasive acoustic liner acoustic impedance extraction method may include a variable wind speed and variable temperature test control system (software) and a high-temperature and high-flow rate acoustic impedance flow tube device (hardware), wherein:

[0102] The wind speed and variable temperature test control system can provide a human-machine interactive interface, monitor the test process according to the input system parameters, and output the test results. The system parameters include the target temperature and target speed of the test; secondly, it can control the heating section device and the air source section device according to the input system parameters to provide the acoustic lining sample with the required temperature and fluid velocity simulation environment. The control system workflow is as follows: Figure 4 shown.

[0103] like Figure 5 As shown, the high-temperature and high-flow-rate acoustic impedance flow tube device may include a gas source section 1, a heating section 2, an upstream muffler section 3, a stabilization section 4, a contraction section 5, a sound source section 6, a test section 7, a diffusion section 8, a downstream muffler section 9, and an outlet 10 connected in sequence.

[0104] In the air source section 1, a centrifugal fan provides power to accelerate the air. The fan draws in external air, pressurizes the air, and blows it into the flow pipe.

[0105] Heating section 2, which includes an electric heating tube, can heat the airflow in the origin section 1 to a specified temperature after heat exchange with the electric heating tube. The device is equipped with three temperature measurement and control points, one at the outlet of heating section 2, and the other two at one point before and after the device. The temperature control is set and adjusted by the human-machine interface system, which can realize the temperature control within the measuring section. Different from the resistance wire heating used in the literature (2), this device uses batch heating tubes (such as Figure 6 As shown in the figure, the air flow is uniformly heated as a whole to ensure the uniformity of the air flow temperature in the test section.

[0106] The upstream silencer section 3 has a straight internal pipe with no change in cross section. The silencer section is filled with sound-absorbing material (existing sound-absorbing cotton, etc. or heat-resistant sound-absorbing material specially optimized for the background sound field under flow conditions in the pipe) to reduce the noise of the test section 7.

[0107] Stable section 4: The internal pipe in this section is straight, the cross-section of the pipe is square, and it is equipped with a set of honeycombs and four layers of damping nets (the distribution of sound transmission direction is from upstream to downstream; the honeycombs and rectifier nets are existing technologies). After passing through this section, the turbulence of the airflow will be greatly reduced, thereby improving the airflow quality.

[0108] The cross section of the contraction section 5 gradually decreases along the airflow direction. The wide diameter can be a square with a side length of 250mm, the contraction diameter can be a square with a side length of 50mm, the length can be 350mm, and the contraction ratio can be 25. When the inlet flow rate is 10m / s, the outlet flow rate can be increased to 25m / s. The contraction section 5 can convert part of the potential energy in the airflow into kinetic energy, thereby increasing the flow rate in the tube. Figure 7 As shown, the contraction curve of the contraction section adopts the classic Vickers curve. After calculation, the contraction section 5 basically has no negative impact on the airflow turbulence and has little effect on the airflow quality.

[0109] Sound source section 6 has interfaces for installing speakers on the left and right sides. Through the signal generator and power amplifier, the speakers can be driven to generate signals of specified types (single-frequency sine wave / swept frequency / added noise) to meet the sound field requirements of test section 7.

[0110] Test Section 7: Highly transparent white glass replaces metal on both sides (which has little impact on sound propagation and flow fields) to meet laser measurement requirements. The upper section has a reserved mounting position for a microphone array to meet microphone measurement requirements. The lower section houses a sample mounting device with a built-in screw that adjusts the sample tray height to accommodate testing of samples of varying heights. It can also be used to create a cavity at the bottom of the sample to meet various experimental requirements. The microphone array can be used to measure the sound pressure of the test section. Based on this sound pressure, the acoustic impedance of the acoustic lining can be calculated using traditional methods. This can be used to verify the accuracy of the proposed method and to measure the acoustic impedance of the acoustic lining using only microphones.

[0111] The diffusion section 8 is a section where the cross section of the pipe continuously increases. The function of this section is opposite to that of the contraction section 5. It can convert part of the kinetic energy of the fluid into potential energy, reduce the fluid velocity, and ensure that the exhaust air flow velocity is not too fast.

[0112] The downstream silencer section 9 has an increasing cross-section and is surrounded by silencer materials. This can further reduce the airflow velocity and reduce external noise from entering the measuring section of the equipment, thereby improving the background noise index of the measuring section.

[0113] The outlet 10 is trumpet-shaped, which can further reduce the airflow velocity on the one hand, and on the other hand, the change in the trumpet-shaped cross-section will cause the impedance to change continuously, thereby achieving the purpose of noise reduction and reflection reduction.

[0114] like Figure 2 and Figure 4 As shown, the non-invasive acoustic impedance extraction method provided by the present application includes the following steps:

[0115] The test sample is a sound lining, the length of which can be 300-500 mm, preferably 310 mm, the width of which can be 50-75 mm, preferably 50 mm, the height of which can be freely adjusted within the range of 0-90 mm, and the upper and lower surfaces of which are flat.

[0116] Determine the measurement area, consider the reflection effect of the device outlet, and arrange the measurement point at 1 / 4 to 1 / 3 of the length of the acoustic liner from the leading edge of the sample close to the air flow side to reduce the influence of the reflected sound at the end of the flow tube. The measurement area consists of multiple measurement array points at different heights (measurement points are evenly distributed) (such as Figure 8 As shown in the figure, the minimum distance between the array point and the upper surface of the sample is 1 mm (the position closest to the acoustic lining surface that the laser light path can reach), and Δl is the minimum moving distance of the mobile measurement frame of the LDV system, which is generally more than 10 microns. The smaller the value, the better.

[0117] Set the target speed (0-0.3 Ma) and temperature (normal temperature-200°C) of test section 7 in the control system, and wait for the speed and temperature of the test section to stabilize. The speed and temperature values ​​are determined by the test plan.

[0118] The acoustic signal is generated by a signal generator driving a loudspeaker through a power amplifier. The sound generated by the loudspeaker enters the device through the sound source section 6, generating a specified sound field in the test section 7. A single-frequency sinusoidal signal is generally used, with a frequency range of 500Hz to 3400Hz, typically with a frequency interval of 100Hz. The sound pressure level in the tube is generally 130dB. These parameters may vary depending on the test plan.

[0119] After the acoustic field, flow field, and temperature field are stabilized, tracer particles are spread into the device through the tracer particle generator of the 3D LDV system. Then, the LDV is used to perform velocity measurement. During measurement, the measurement area is scanned point by point according to the distribution of measurement points.

[0120] After the test, record and save the LDV measurement data and acoustic signal data (the data needs to be tested synchronously).

[0121] The process of processing the saved data includes:

[0122] Step 1: Uniformly distributed signals are easier to process and analyze. However, the data obtained from LDV measurements is randomly distributed (non-uniform). Therefore, the data must be reconstructed to obtain uniformly distributed data. During reconstruction, the minimum time interval between adjacent data points is used as the interpolation interval. The data is linearly interpolated at this interval to obtain uniformly distributed data.

[0123] Step 2: During LDV measurement, the velocities are not strictly orthogonal, and there is a fixed angle between the lasers. The velocity components can be converted to a specified Cartesian coordinate system based on the laser angle.

[0124] Step 3: Use a suitable window function (such as a flat-top window function) on the data obtained in step 2 to reduce the truncation error and energy leakage generated during frequency analysis. Combined with the sound source signal (reference signal), the amplitude and phase results of the sound particle velocity can be obtained by equations (1)-(3).

[0125] Step 4: Calculate the sound pressure result based on equations (4)-(6).

[0126] Step 5: Calculate the standard impedance result using the standard impedance definition formula (8); and calculate the Ingard-Myers boundary impedance result using the finite difference method based on the standard impedance and the Ingard-Myers boundary impedance relationship formula (9).

[0127] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of this application and are not intended to limit the scope of the present invention. Although this application has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions to the technical solutions of this application do not depart from the spirit and scope of the technical solutions of this application and should be encompassed by the claims of this application.

Claims

1. A three-dimensional non-invasive acoustic liner acoustic impedance extraction method comprising: Construct a coupled field test environment including flow field, acoustic field and temperature field for the acoustic liner to be tested; The three-dimensional velocity of the fluid is measured in a designated measurement area above the acoustic liner surface using a three-dimensional laser Doppler velocimeter; The signal processing method based on the coherence function method is used to obtain the sound particle velocity result; Determine the uniform flow condition and shear flow condition, and calculate the sound pressure in the measurement area; The standard acoustic impedance and Ingard-Myers boundary impedance are obtained by forward calculation from the sound pressure and sound particle velocity; The surface impedance distribution of the acoustic lining is obtained through three-dimensional laser Doppler velocimeter scanning test; The determination of uniform flow conditions and shear flow conditions includes: When the actual average flow velocity gradient in the measurement area is less than or equal to the set threshold, it is defined as a uniform flow condition; when the actual average flow velocity gradient is greater than the set threshold, it is defined as a shear flow condition; The obtaining of the sound pressure in the measurement area includes: When the measurement area is in uniform flow conditions: Where p' represents the sound pressure; ρ0 and c0 represent the density and sound velocity of the mean flow, respectively, which are corrected by the temperature at the measuring point; M0 represents the flow Mach number; x, y, and z represent the three coordinate components of the Cartesian coordinate system, where x is the flow direction, y is the direction perpendicular to the flow direction on the horizontal plane, and z is the vertical direction; u' x represents the x-component of the velocity of the sound particle; u' y Represents the y-component of the sound particle velocity; u' z represents the z-component of the velocity of the sound particle; ω represents the angular frequency; i represents the imaginary unit; When the measurement area is under shear flow conditions in the z direction: When the measurement area is in shear flow conditions in both the z and y directions: The partial derivatives of the sound particle velocity components in the above formula are obtained by using the finite difference method to perform spatial discretization calculations on the corresponding sound particle velocity components in the measurement area; The Ingard-Myers boundary impedance is expressed as: Among them, Z n represents the Ingard-Myers boundary impedance; Z i represents standard acoustic impedance; p' represents sound pressure; represents the velocity vector of the sound particle; represents the unit normal vector of the sound lining surface; represents the gradient operator; represents the average flow velocity; and It is obtained by discrete calculation using the finite difference method; The average flow velocity in the measurement area obtained by the three-dimensional laser Doppler velocimeter Sound particle velocity Sound pressure p' and standard acoustic impedance Z i Substitute the above formula to calculate the Ingard-Myers boundary impedance of the acoustic lining surface; The measurement area includes a measurement point array of L×M×N measurement points, where L represents the number of flow-direction measurement points, M represents the number of vertical measurement points, and N represents the number of horizontal measurement points. Adjacent measurement points in each direction of the array are spaced equally apart. The distance between the bottom layer of the measurement points and the upper surface of the acoustic liner is the closest position to the acoustic liner surface that can be reached by the laser light path of the three-dimensional laser Doppler velocimeter. The measurement area is located directly above the acoustic liner surface. The minimum distance between the bottom layer of the array points and the upper surface of the sound liner is 1 mm.

2. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: The sound particle velocity results include: Among them, x, y, z represent the three coordinate components of the Cartesian coordinate system; u′ j represents the j-direction component of the velocity of the sound particle; u j represents the j-direction component of the air velocity; s represents the sound source signal; G represents the cross power spectrum density between the j-direction component of the air velocity and the sound source signal; s Represents the autopower spectral density of the sound source signal; Represents the autopower spectral density of the j-direction component of the sound particle velocity; It represents the phase difference between the j-component of the sound particle velocity and the sound source signal; Im(·) represents the imaginary part of the complex number; Re(·) represents the real part of the complex number.

3. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: The standard acoustic impedance obtained from the sound pressure and the sound particle velocity is: Among them, Z i Indicates standard acoustic impedance; represents the velocity vector of the sound particle; Represents the unit normal vector of the acoustic lining surface.

4. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: Also includes: Before obtaining the sound particle velocity results, the measurement data of the three-dimensional laser Doppler velocimeter is preprocessed; The pretreatment includes: The measured data is linearly interpolated to obtain uniformly distributed data; Convert each velocity component into a Cartesian coordinate system according to the laser angle; Perform time domain windowing function processing on the data.

5. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: The acoustic lining to be tested is constructed into a coupled field test environment including flow field, acoustic field and temperature field, and the device used includes an acoustic impedance flow tube device; The acoustic impedance flow tube device includes an air source section, a heating section, an upstream muffler section, a stabilization section, a contraction section, a sound source section, a test section, a diffusion section, a downstream muffler section and an outlet, which are connected in sequence. The air source section uses a centrifugal fan to provide power to accelerate the air and then blow it into the heating section; The heating section includes an electric heating tube, which is used to heat the airflow blown out of the air source section with the electric heating tube, and then heat it to a set temperature and output it to the upstream muffler section; The upstream muffler section is a pipe with a straight interior and a constant cross-section, and is filled with sound-absorbing material to reduce the noise of the airflow and output it to the stabilizing section; The stabilizing section is an internal straight pipe with a square cross section, which is equipped with a set of honeycombs and multi-layer damping mesh to reduce the turbulence of the airflow and output it to the contracting section; The contraction section is a pipe whose cross section gradually decreases along the airflow direction, which increases the velocity of the airflow and then introduces it into the sound source section; The sound source section has speakers installed on its left and right sides, and the speakers generate sound signals of a set type; The test section has high light transmittance materials on both sides and a sound liner sample installation device at the bottom; The cross section of the pipe in the diffusion section increases continuously along the flow direction; The downstream silencer section has a cross section that increases continuously along the flow direction and is surrounded by silencer material; The outlet is trumpet-shaped.

Images