Three-dimensional non-invasive acoustic liner acoustic impedance extraction method

Abstract

The invention provides a three-dimensional non-invasive acoustic liner acoustic impedance extraction method. The method comprises the following steps: constructing a coupled field test environment of a flow field, an acoustic field and a temperature field for an acoustic liner to be tested; a three-dimensional LDV is used for testing the fluid velocity in a measurement area above the surface of the acoustic liner; a coherence function method is adopted to obtain sound particle speed, and sound pressure is further obtained through calculation; and then the Ingard-Myers boundary impedance is obtained through forward calculation. The method has the advantages that the testing environment is closer to the actual working condition of the acoustic liner; non-contact measurement is adopted, so that interference to an internal flow field and a sound field is avoided, and a test result is more accurate; due to the fact that local scanning testing is adopted, the method is also suitable for measurement under the conditions of an acoustic liner containing complex geometric parameters, a complex flow field, a complex temperature field and a sound field. Comprising the steps of surface acoustic impedance distribution measurement of a non-uniform acoustic liner, surface acoustic impedance measurement of a curved surface acoustic liner, measurement of high-frequency acoustic impedance under a rotational flow condition, a non-uniform temperature field condition and a pipeline cut-off frequency or above, and the like.

Description

Technical Field

[0001] The present application belongs to the technical field of acoustic property 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 nacelle is an important technical approach to reduce engine radiation noise. Acoustic impedance is a key parameter to characterize 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 LDV (Laser Doppler Velocimetry)-based acoustic impedance extraction methods and PIV (Particle Image Velocimetry)-based acoustic impedance extraction methods.

[0003] However, the acoustic lining of the anechoic nacelle often works in complex working conditions with high sound pressure level, high flow rate and high temperature coupling. The 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 sensitive to temperature, and the sound pressure measurement accuracy in high temperature environments is difficult to guarantee; the direct measurement method requires drilling holes at the sample measurement position, which damages the sample and has a great impact on the measurement accuracy.

[0004] References (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, for the first time, realized the use of two-dimensional LDV to measure the acoustic characteristics of acoustic liners 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, which measures one less velocity component than the three-dimensional LDV. It lacks the velocity component in the transverse direction of the flow tube (i.e., the horizontal direction of the flow tube cross section), so the influence of the wall boundary in 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, so it is 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 202260:8, 4754-4763. The acoustic transmission loss before and after the sample and the acoustic impedance results of the sample under the conditions of acoustic, flow and thermal coupling were studied. This paper uses two heating modes, one is airflow heating and the other is sample bottom heating. 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. Since 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 calculation amount and possible multi-value solutions.

[0006] The existing methods for extracting acoustic lining impedance and related patent defects mainly include:

[0007] The influence of high temperature is not considered: both the device described in the domestic patent and the foreign articles on laser sound measurement are for normal temperature conditions and cannot be directly used for high temperature testing. For the actual working conditions of the engine anechoic nacelle, in addition to flow and high sound intensity, the airflow temperature in the inlet duct is also high, generally up to 200°C. Therefore, the actual working conditions of the acoustic liner are a complex working condition of flow, sound and heat coupling. In the air, the speed of sound will increase by about 0.6m / s for every degree Celsius increase in the medium temperature. For the anechoic nacelle with a temperature of hundreds of degrees Celsius or a significant temperature gradient, the sound speed of the surrounding medium changes significantly. The acoustic changes caused by high temperature will be more obvious and should not be ignored.

[0008] The corresponding microphone-based acoustic liner impedance test method requires drilling holes in the wall of the test section, which will inevitably have some impact on the internal local flow field and sound field, especially the high frequency impact. In addition, the working accuracy of existing microphones is very sensitive to temperature, and they are generally not resistant to high temperatures and can hardly work at temperatures above 150°C. Some high-temperature resistant probe microphones have relatively low measurement accuracy due to their working principles, and it is difficult to meet high-precision measurement requirements.

[0009] Under high temperature test conditions, due to the limitations of the actual heating device, the temperature distribution of the fluid in the test section is often uneven. The existing forward extraction method of acoustic liner impedance based on microphone arrays will fail for acoustic field impedance extraction under uneven temperature field conditions because its test principle includes the premise of uniform temperature field. The existing inverse impedance extraction method based on microphone arrays and LDV test sound particle velocity requires obtaining the fluid temperature distribution of the entire acoustic liner test section. Due to the limitations of the thermal sensor, it is difficult to accurately obtain the three-dimensional spatial distribution of temperature in the entire test section in actual testing, and these methods are no longer applicable. Summary of the invention

[0010] The purpose of the present invention is to solve the problems existing in the prior art, such as damage to the test sample, limited measurement area, poor measurement position accuracy, measurement method not applicable to high temperature, failure to consider temperature non-uniformity, large amount of inversion calculation and multi-value inversion of the inverse method, etc. in 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, so as to provide an accurate test method for the acoustic impedance test of the acoustic lining surface.

[0011] In order to achieve the above objectives, the present application proposes a three-dimensional non-invasive acoustic liner 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 the 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 acoustic pressure and acoustic 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 of 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 The j-direction component of the velocity of the sound particle; u j represents the j-direction component of air velocity; s represents the sound source signal; G represents the cross power spectral density between the j-direction component of the air velocity and the sound source signal; s Represents the auto-power 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 of 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 of the above method, obtaining the sound pressure of the measurement area includes:

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

[0027]

[0028] Where p′ represents the sound pressure; ρ0 and c0 represent the density and sound velocity of the average flow, respectively, and 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 spatially discretizing the corresponding sound particle velocity components in the measurement area using the finite difference method.

[0034] As an improvement of 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 liner surface.

[0037] As an improvement of 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 after multiplying a vector, the normal component of the vector is obtained; 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 of the above method, it also includes:

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

[0043] The pre-processing comprises:

[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 of 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 comprises 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, wherein:

[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 straight pipe with a constant cross section, which is filled with sound-absorbing material to reduce the noise of the airflow and output it to the stable section;

[0052] The stabilizing section is an internal straight pipe with a square cross section, and a set of honeycombs and multi-layer damping nets are arranged inside the pipe to reduce the turbulence of the airflow and output it to the contraction section;

[0053] The contraction section is a pipe whose cross section becomes smaller and smaller along the airflow direction, and increases the velocity of the airflow before introducing 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-transmitting materials on its left and right sides, and a sound liner sample installation device at the bottom;

[0056] The diffusion section has a cross section that increases continuously along the flow direction;

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

[0058] The outlet is trumpet-shaped.

[0059] As an improvement of the above method, the measurement area includes a measurement point array with L×M×N measurement points, L represents the number of flow measurement points, M represents the number of vertical measurement points, and N represents the number of horizontal lateral measurement points; the distance 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 sound liner is the position closest to the sound 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 sound 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 1 mm away from the surface of the acoustic liner, which is closer to the surface of the acoustic liner and therefore more accurate.

[0062] 2. A coupled field test environment including flow field, acoustic field and temperature field was constructed. Compared with the existing microphone array test method, it is more suitable for high temperature testing, especially for the surface acoustic impedance measurement of acoustic pads working in high temperature environment of aircraft engine anechoic nacelles;

[0063] 3. The present 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 acoustic 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 acoustic 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 acoustic lining surface impedance, including the consistency comparison of the acoustic lining surface impedance extraction results under corresponding conditions on different test platforms;

[0064] 4. Since this method only needs to test the velocity of sound particles in a local range above the surface of the sound lining to obtain the local surface impedance of the sound lining, without introducing other assumptions and boundary conditions, this method is also suitable for measurements under conditions of sound lining with complex geometric parameters, complex flow fields, complex temperature fields and complex sound fields, including: surface acoustic impedance distribution measurement of non-uniform sound lining, surface acoustic impedance distribution measurement of curved surface sound lining, surface impedance measurement of sound lining 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 cutoff frequency of the pipeline, nonlinear impedance measurement of sound lining under different sound pressure levels, etc.

[0065] In summary, compared with the existing acoustic liner surface impedance extraction method, the measurement process of this method is relatively simple and the test accuracy is high. It effectively overcomes some limitations of the existing acoustic liner surface acoustic impedance extraction 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 Shown is 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 the working flow chart 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 device;

[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 the present 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 lining. The 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 stable, 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 acoustic particle velocity results:

[0076]

[0077]

[0078]

[0079] In the formula, 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 The j-direction component of the velocity of the sound particle; u j represents the j-direction component of air velocity; s represents the sound source signal; G represents the cross power spectral density between the j-direction component of the air velocity and the sound source signal; s Represents the auto-power 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 French Space Agency (ONERA) in the literature (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 processed as uniform flow; if the velocity gradient in the LDV test area close to the sound liner surface is large along the vertical z or horizontal lateral y, it is processed 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 lateral y 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 under uniform flow conditions:

[0082]

[0083] Where ρ0, c0 represent the density and sound speed of the mean flow respectively; M0 represents the flow direction Mach number; p′ represents the sound pressure; ω represents the angular frequency; 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 lateral 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 reference (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 characteristics of the acoustic liner. For the sound propagation problem under tangential flow, considering that the average flow velocity at the boundary of the LDV measurement area closest to the surface of the acoustic liner is not 0, this application adopts the Ingard-Myers boundary condition. The Ingard-Myers boundary condition is based on the thin boundary layer assumption, that is, it is assumed that the influence of the boundary layer on the sound field can be ignored. Its characteristics are that it considers the influence of the acoustic liner itself and the slip flow at the boundary on the boundary condition at the same time, and its physical meaning is clear. When the average flow velocity at the boundary is not 0, the relationship between the standard acoustic impedance and the Ingard-Myers boundary impedance condition can be converted to each other by the following formula. Therefore, after obtaining the standard acoustic impedance result, the impedance form in the Ingard-Myers boundary condition can be obtained by further calculation.

[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 acoustic liner 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 calculation, the gradient and derivative in the above formula are spatially discretized using the finite difference method center format. In addition, due to the measurement characteristics of LDV, the sound particle information at the 1mm position on the surface of the sound lining can be measured, and 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 tested by a temperature sensor, and then the sound velocity and air density after the corresponding temperature correction are substituted into formulas (4)-(6) to calculate the sound pressure value at the corresponding position, and finally 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 the LDV scanning test, this method can not only test the surface acoustic impedance of the local area of ​​the uniform sound lining, but also directly test the surface acoustic impedance distribution of the non-uniform sound 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 sound lining; at the same time, the LDV test is a local test and can be applied to the surface scanning test of the curved surface sound lining. Therefore, this method can also directly test the surface acoustic impedance of the curved surface sound lining; 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 liner 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 in the figure, the device mainly includes six parts: laser, spectrometer and coupler, signal processor, mobile measuring frame, laser probe and control software. The argon ion laser generates laser and transmits it to the spectrometer and coupler. The laser is separated in the spectrometer to generate laser of corresponding wavelength. The laser reaches the laser probe through the optical fiber and is focused by the probe lens, and finally emitted into the measurement area. Two laser beams with a certain angle form an ellipsoidal focus, which is called the measurement body. The size of the measurement body is related to the laser angle, wavelength, laser diameter, etc. The tracer particles in the fluid can move well with the fluid. When these tracer particles pass through the measurement body, they will emit scattered light under the irradiation of the incident laser. The scattered light is received by the laser receiving probe. The optical signal of the receiving probe is transmitted to the photomultiplier tube and converted into an electrical signal. The electrical signal is transmitted to the signal processor for processing. Using the laser Doppler principle, the flow velocity at the measuring point in the corresponding direction is obtained after data processing.

[0099] In the three-dimensional LDV velocity measurement system, the laser is separated by a spectrometer to generate three lasers with different wavelengths (green light (wavelength 514.4nm), blue light (wavelength 488nm), and purple light (wavelength 476.5nm)), which are focused on the same measuring point at different angles. According to the above velocity measurement principle, the velocity components of the same measuring point in three directions can be obtained, and then the three-dimensional orthogonal velocity components in the Cartesian coordinate system in a given flow tube can be obtained through coordinate transformation.

[0100] In the LDV-based acoustic impedance extraction method proposed in reference (1), a two-dimensional LDV system is used. 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 produce 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, the LDV system can only measure the velocity information at a position of 3 mm or more on 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. The present 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 measurement, that is, the test area is larger, and 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 performing the above-mentioned setting on the optical path of the laser probe, the three-dimensional LDV system in the present 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 measuring device used in the above 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 and output the test results according to the input system parameters. 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 speed 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 stabilizing 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. Among them,

[0104] In the air source section 1, a centrifugal fan provides power to accelerate the air. The fan inhales the outside air, pressurizes the air and blows it into the pipeline of the flow tube.

[0105] Heating section 2 includes an electric heating tube, which can heat the airflow in the source 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 located at the outlet of heating section 2, and the other two located before and after the device. The temperature control is set and adjusted by the human-machine interface system, which can realize the temperature control in the measurement section. Different from the resistance wire heating used in the literature (2), this device uses batch heating tubes (such as Figure 6 The airflow is uniformly heated as a whole to ensure the uniformity of the airflow 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] The stable section 4 has a straight internal pipe with a square cross section and is equipped with a set of honeycombs and 4 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 quality of the airflow.

[0108] The cross section of the contraction section 5 becomes smaller 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 is used on both sides of this section to replace metal (which has little effect on sound propagation and flow field) to meet the requirements of laser measurement. The upper part is reserved for the installation position of the microphone array to meet the requirements of microphone measurement. The lower part is a sample installation device with a built-in screw that can be used to adjust the height of the sample tray to meet the test requirements of samples of different heights. It can also be used to generate a cavity at the bottom of the sample to meet different experimental requirements. The microphone array can be used to measure the sound pressure results of the test section. Based on the sound pressure results, the acoustic impedance of the sound lining can be calculated by traditional methods. On the one hand, it can be used to verify the accuracy of the method of this application, and on the other hand, it can be used to measure the acoustic impedance of the sound lining using only microphones.

[0111] The diffusion section 8 is a section where the cross section of the pipe is continuously increasing. 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, the cross-section of this section of the pipeline is also constantly increasing, and silencer materials are installed around it. On the one hand, it can further reduce the air flow velocity, and on the other hand, it can reduce the external noise entering the measuring section of the equipment and improve the background noise index of the measuring section.

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

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

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

[0116] Determine the measurement area, consider the reflection effect at the outlet of the device, 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 reflection effect of the end of the flow tube. The measurement area consists of multiple measurement array points at different heights (measurement points are equidistantly distributed) (e.g. 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 measuring 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 sound signal is generated by the signal generator driving the speaker through the power amplifier. The sound generated by the speaker enters the device through the device sound source section 6, and generates a specified sound field in the test section 7. Generally, a single-frequency sine signal is used, the frequency range of the signal is 500Hz to 3400Hz, the frequency interval is generally 100Hz, and the sound pressure level in the tube is generally 130dB. The above parameters can be changed according to the test plan.

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

[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 more convenient for signal processing and analysis, but the data distribution obtained by LDV measurement is random (non-uniform), so the data needs to be reconstructed first to obtain uniformly distributed data results. During reconstruction, the minimum value of the time interval between adjacent data is used as the interpolation time interval, and the data is linearly interpolated at this time 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); Calculate the Ingard-Myers boundary impedance result using the finite difference method using the standard impedance and the Ingard-Myers boundary impedance relationship formula (9).

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application is described in detail with reference to the embodiments, a person skilled in the art should understand that any modification or equivalent replacement of the technical solution of the present application does not depart from the spirit and scope of the technical solution of the present application and should be included in the scope of the claims of the present 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 velocity result of the sound particle; Determine the uniform flow condition and the 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 acoustic pressure and acoustic particle velocity; The surface impedance distribution of the acoustic liner was obtained through three-dimensional laser Doppler velocimeter scanning test.

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 The j-direction component of the velocity of the sound particle; u j represents the j-direction component of air velocity; s represents the sound source signal; G represents the cross power spectral density between the j-direction component of the air velocity and the sound source signal; s Represents the auto-power 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 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.

4. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: The obtaining of the sound pressure in the measurement area comprises: When the measurement area is under uniform flow conditions: Where p′ represents the sound pressure; ρ0 and c0 represent the density and sound velocity of the average flow, respectively, and 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; When the measurement area is in 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 spatially discretizing the corresponding sound particle velocity components in the measurement area using the finite difference method.

5. 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 liner surface.

6. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: 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 into the Ingard-Myers boundary impedance of the sound lining surface.

7. 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 result, the measurement data of the three-dimensional laser Doppler velocimeter is preprocessed; The pre-processing comprises: The 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.

8. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: The acoustic liner to be tested is constructed to have a coupled field test environment including a flow field, an acoustic field and a temperature field, and the device used includes a flow tube device; The acoustic impedance flow tube device comprises 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, wherein: 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 origin 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 straight pipe with a constant cross section, which is filled with sound-absorbing material to reduce the noise of the airflow and output it to the stable section; The stabilizing section is an internal straight pipe with a square cross section, and a set of honeycombs and multi-layer damping nets are arranged inside the pipe to reduce the turbulence of the airflow and output it to the contraction section; The contraction section is a pipe whose cross section becomes smaller and smaller along the airflow direction, and increases the velocity of the airflow before introducing 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-transmitting materials on its left and right sides, and a sound liner sample installation device at the bottom; The diffusion section has a cross section that increases continuously along the flow direction; The downstream silencer section has a pipe section whose cross section increases continuously along the flow direction and is surrounded by silencer materials; The outlet is trumpet-shaped.

9. The three-dimensional non-invasive acoustic liner acoustic impedance extraction method according to claim 1, characterized in that: The measurement area includes a measurement point matrix with L×M×N measurement points, L represents the number of flow measurement points, M represents the number of vertical measurement points, and N represents the number of horizontal lateral measurement points; the distance between adjacent measurement points in each direction of the point matrix is ​​equal; the distance between the bottom layer of the measurement array points and the upper surface of the sound liner is the position closest to the sound liner surface that can be reached by the laser light path of the three-dimensional laser Doppler velocimeter; the measurement area is located in the area directly above the sound liner surface.

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