Optical Measurement Method for Aircraft Engine Thrust Using Background Schlieren Technology

Abstract

This invention relates to an optical measurement method for aero-engine thrust using background schlieren technology. The method utilizes the principle that the refractive index gradient of light in the measured flow field is proportional to the airflow density, transforming changes in the density gradient in the flow field into changes in relative light intensity on a recording plane. An LCD display screen and a high-speed camera for acquiring background schlieren images are respectively arranged on both sides of the observation section of the outlet flow field to be measured. When the density of the observation area changes, light rays from the background pattern are refracted through the areas of density change. The background image is processed using a cross-correlation calculation method to calculate the displacement of points on the background image between every two images, converting it into a digital image field for calculation to obtain the velocity and density fields. These are then combined with the thrust calculation formula to obtain the engine thrust. This invention does not affect the internal structure of the outlet flow field to be measured, is easy to operate, has a large measurement range, fast response speed, high measurement accuracy, and lower cost.

Description

Technical Field

[0001] This invention relates to the field of aero-engine performance testing technology, and in particular to a method for testing aero-engine thrust. Background Technology

[0002] Currently, the thrust of aero-engines is mainly measured through cross-testing on indoor and outdoor test rigs. The advantage of this method is that indoor test rigs can simulate the thrust of aero-engines under different operating conditions by adding aerodynamic drag corrections. The disadvantages are that indoor test rigs are not accurate enough in measuring flow field distribution, and outdoor benchmark test rigs are more expensive.

[0003] In terms of exit velocity field measurement, the test bench thrust measurement method involves arranging as many measurement points as possible in the radial and circumferential directions of the measurement section, acquiring test parameters, and then calculating the parameter distribution of the measurement section. This method has the following disadvantages: the arrangement of a large number of measurement points can cause flow section blockage and even disrupt the flow field; a large number of measurement points require a matching number of acquisition channels and modules, and while the engine is undergoing temperature and velocity field measurement tests, it is also necessary to detect a large amount of other test data, which puts considerable pressure on the acquisition capacity of the test bench; and it cannot simultaneously implement multiple measurement schemes, that is, it cannot simultaneously measure the exit velocity field and temperature field of the tail nozzle.

[0004] From the perspective of aero-engine thrust calculation principles, the flow field at the exhaust nozzle is a crucial indicator of aero-engine thrust. However, test bench thrust measurement skips the exhaust nozzle flow field and directly measures the reaction force generated after the entire aero-engine starts. While this method is simple in principle, its operation is complex and it cannot provide in-depth analysis of the exhaust nozzle exit flow field.

[0005] Background-oriented schlieren is a technique that uses quantitative deflection caused by changes in the refractive index of light rays in a flow field to determine the density information of the flow field. When light passes through the measurement area, its propagation direction is deflected accordingly due to the change in refractive index within the test area, causing the corresponding points on the background image to shift on the imaging plane. By cross-correlation calculations between two images—one with a refractive index change and one without—the ray shift corresponding to each point on the background screen can be obtained, allowing for analysis of the light deflection information. This method has at least the following advantages: it does not interfere with the flow field under test, it is relatively easy to operate, it allows for rapid and repeated measurements, it does not require expensive and precise optical components compared to other optical measurement methods, the size and orientation of the background screen can be flexibly adjusted according to measurement needs, and it is relatively inexpensive.

[0006] However, there is no precedent for using background schlieren technology in optical measurement of aero-engine thrust. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide an optical measurement method for aero-engine thrust using background schlieren technology. This method is based on the principle that the refractive index gradient of light in the flow field is proportional to the airflow density of the flow field, and transforms the change of density gradient in the flow field into the change of relative light intensity on the recording plane.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: an optical measurement method for aero-engine thrust using background schlieren technology, comprising the following steps:

[0009] Step 1: Place the LCD screen and the high-speed camera coaxially on both sides of the engine exhaust nozzle outlet along the airflow direction via guide rails. The optical axis of the high-speed camera is perpendicular to the center of the LCD screen. The size of the LCD screen covers the exhaust flow field of the engine exhaust nozzle and is larger than the field of view of the high-speed camera.

[0010] At the same time, the center of the engine exhaust nozzle's outlet section, the center of the LCD screen, and the center of the high-speed camera's lens plane are on the same horizontal plane.

[0011] Furthermore, taking the projection of the center of the engine exhaust nozzle's outlet cross-section onto the ground as the origin, and the direction of the airflow at the engine exhaust nozzle's outlet as... The positive direction of the axis, with the origin pointing towards the LCD screen as... Positive axis direction, vertical Axial upward Establish a rectangular coordinate system for the deflection space of the engine exhaust nozzle in the positive direction of the axis;

[0012] Step 2: Measure the radius of the engine exhaust nozzle when the engine is not running. Atmospheric static pressure And measure the actual distance between the LCD screen and the lens of the high-speed camera. The outlet flow field of the engine exhaust nozzle Maximum width in the positive direction of the axis Furthermore, a high-speed camera was used to capture a reference exit flow field image of the engine exhaust nozzle. I 1;

[0013] Step 3: Measure the engine rotor speed after starting the engine. Fuel flow airflow When the fluctuation values ​​do not exceed 0.2%-0.5% of the values ​​corresponding to the engine's maximum state, the obtained static pressure at the engine exhaust nozzle outlet is... Under the condition that the engine is not running, the radius of the engine exhaust nozzle is used. Displacement of actuator cylinder The angle between the deflection angle of the adjusting plate and the displacement of the actuator cylinder Calculate the exit cross-sectional area parameters of the engine exhaust nozzle. At least 50 images of the outlet flow field are continuously captured, and at least 10 consecutive images are selected as the outlet flow field images to be tested. I 2;

[0014] Step 4: In the particle image velocimetry software, based on the outlet flow field image to be measured... I 2. Establish a query network, and then, based on the reference outlet flow field image. I 1 and the outlet flow field image to be measured I 2. Using the cross-correlation algorithm, the displacement field of the outlet flow field of the engine tail nozzle is obtained, and then the velocity field is obtained by combining the time interval.

[0015] Step 5: In the rectangular coordinate system of the deflection space of the outlet flow field, after calculating the refractive index field based on the displacement field of the outlet flow field of the engine tail nozzle, solve the density field of the outlet flow field of the engine tail nozzle using the Gladstone-Dale formula.

[0016] Step 6: Combine the velocity field and density field with the thrust calculation formula, and the exit cross-sectional area of ​​the engine exhaust nozzle. Static pressure at the engine exhaust nozzle outlet Atmospheric static pressure Density field of the outlet flow field of the tail nozzle and velocity field In query grid value at , Grid area Calculate the thrust;

[0017] Step 7: Traverse the outlet flow field image to be tested, repeat the calculation process from Step 4 to Step 6, check for errors, and finally determine the engine thrust.

[0018] Furthermore, in step one, in the rectangular coordinate system of the outlet flow field deflection space, the coordinates of the center of the mirror plane of the high-speed camera are ( , , The center coordinates of the LCD screen are ( ). , , Then we have:

[0019] ,

[0020] ,

[0021] In the formula, The diameter of the engine's exhaust nozzle;

[0022] The atmospheric static pressure The actual distance between the measured LCD screen and the lens of the high-speed camera. The outlet flow field of the engine exhaust nozzle Maximum width in the positive direction of the axis The value satisfies the requirement of the exhaust nozzle outlet rear... Within the range, static pressure pulsation values ​​at at least five randomly selected locations. .

[0023] Furthermore, in step two, the reference outlet flow field image I The outlet flow field of the engine exhaust nozzle in 1 satisfies:

[0024] Quiet temperature Static temperature fluctuation Static pressure pulsation Reynolds number Dryness .

[0025] Furthermore, in step three, the outlet cross-sectional area of ​​the engine exhaust nozzle... The calculation formula is:

[0026] ,

[0027] In the formula, The radius of the engine exhaust nozzle; For the displacement of the actuator cylinder; To adjust the angle between the deflection angle of the plate and the displacement of the actuator cylinder;

[0028] At this time, take images of the exhaust flow field of the engine tail nozzle at intervals of 10s to 15s, and take at least 50 images continuously.

[0029] Then, by subtracting the gray values ​​of two adjacent outlet flow field images pairwise, the image with the largest gray value difference not exceeding 5 is selected as the preferred outlet flow field image to be tested. I 2.

[0030] Furthermore, step four specifically involves:

[0031] Step 41: Select the preferred outlet flow field image to be tested. I 2. Import the particle image velocimetry software, establish a query network, and define the parameters of the query window. Based on the expected maximum displacement of the outlet flow field as (Δx_max, Δy_max), determine the size of the query window as (W, H) according to the Nyquist criterion. Then:

[0032] W≥4Δx_max, H≥4Δy_max;

[0033] Set the window movement step size so that the window overlap rate is between 50% and 75% after each movement, in order to improve the spatial resolution of the displacement field;

[0034] Step 42: Through the query window, traverse the reference outlet flow field images respectively. I 1 and the outlet flow field image to be measured I 2; For a single traversal, the cross-correlation algorithm is used to calculate the reference outlet flow field image. I 1. Image of the outlet flow field to be measured I 2. Cross-correlation function for querying pixel grayscale values ​​in the window region for:

[0035] ,

[0036] In the formula, This represents the coordinate position under the query window during traversal. The pixel grayscale value at that location, Represents pixel displacement; multiple cross-correlation functions are calculated under different displacement values. , find Maximum displacement This refers to the displacement of the query window.

[0037] When referencing the outlet flow field image I The pixel size of 1 is denoted as When, then the displacement satisfy 0.25M 0.25N;

[0038] Step 43: Using the Fast Fourier Transform, the cross-correlation calculation is simplified. According to the convolution theorem, performing cross-correlation in the time domain is equivalent to performing dot product in the frequency domain, thus obtaining:

[0039] ,

[0040] In the formula, This represents the Fourier transform, which converts an image from the time domain to the frequency domain. represents the complex conjugate of the Fourier transform, and represents the inverse Fourier transform;

[0041] Based on the principle of simplifying cross-correlation calculations, the reference image... Image of the outlet flow field disturbance after translation Perform Fourier transform and complex conjugate operations, followed by dot product in the frequency domain to obtain... Finally, an inverse Fourier transform is performed to obtain the cross-correlation function in the time domain. ;

[0042] Similarly, multiple cross-correlation functions were calculated for different displacements. , find Maximum displacement This refers to the displacement of the query window;

[0043] Cross-correlation calculations are performed on all query windows to generate the outlet flow field image to be measured. I The displacement vector corresponding to 2, that is, the displacement field of the outlet flow field of the engine exhaust nozzle, is: ;

[0044] Step 44: Using the displacement field of the engine exhaust nozzle outlet flow field combined with the time interval, divide the average displacement vector of the background dots captured in the query window by the time interval between the current calculation frame and the starting frame to obtain the average velocity. Perform the same operation on all query windows to generate the velocity field of the engine exhaust nozzle outlet flow field. .

[0045] Furthermore, step five specifically includes:

[0046] In the aforementioned outlet flow field deflection space rectangular coordinate system, the velocity field Decomposed into velocity components in the x-direction and the velocity component in the y direction The velocity component in the y-direction is obtained based on the geometric relationship of high-speed camera imaging. for:

[0047] ,

[0048] In the formula, The outlet flow field of the engine exhaust nozzle is Maximum width in the positive direction of the axis This is the actual distance between the LCD screen and the lens of the high-speed camera. It is the focal length of a high-speed camera. It is the refractive index of air. The outlet flow field to be measured is The refractive index distribution function in the direction;

[0049] Similarly, based on the velocity component in the x-direction The flow field at the outlet of the test was calculated. Refractive index distribution function in the direction ;

[0050] Finally, the air refractive index and the refractive index distribution function in the y direction are... Refractive index distribution function in the x-direction By adding them together, we obtain the refractive index parameters of the outlet flow field to be measured. ;

[0051] For each displacement in the displacement field of the engine exhaust nozzle outlet flow field, the corresponding refractive index parameter is calculated, and then combined to obtain the refractive index field of the outlet flow field to be measured. ;

[0052] Continuing, the Gladstone-Dale formula used is:

[0053] ,

[0054] In the formula, It is the Gladstone-Dale constant, a fixed value for a specific gas; combined with the refractive index field The density field of the exhaust flow field at the engine tail nozzle after startup was calculated. .

[0055] Furthermore, step six specifically includes the following steps:

[0056] During ground testing, the thrust calculation formula for an aircraft engine is as follows:

[0057] ,

[0058] ,

[0059] In the formula, For the thrust of aircraft engines; This is the exit cross-sectional area of ​​the engine's tail nozzle; The static pressure at the engine exhaust nozzle outlet; Atmospheric static pressure; This refers to the mass flow rate at the engine exhaust nozzle outlet. The outlet velocity of the engine exhaust nozzle; The radial volume density of the exit particles; For the mass of exported particles; The particles are distributed radially with their velocity centered on the center of the exit section of the engine exhaust nozzle. The radius of the engine exhaust nozzle; The volume density radial distribution of particles with the center of the exit section of the engine tail nozzle as the center; For the definite integral variable, For variables The differential, , These are the density fields of the flow field at the engine exhaust nozzle exit. and velocity field In query grid The value at that location, It queries the area of ​​the grid.

[0060] The momentum term was calculated. Then, use the outlet static pressure of the tail nozzle. Environmental pressure The difference multiplied by the cross-sectional area of ​​the engine exhaust nozzle. The corresponding pressure term is calculated. Then, by adding the momentum and pressure terms together, we obtain the flow field image at the outlet to be measured. I The thrust corresponding to 2.

[0061] Furthermore, step seven specifically includes:

[0062] Repeat steps four to six for the output flow field image to be tested, and compare the thrust calculation results. If at least 10 sets of thrust calculation results meet the condition that the standard deviation is not greater than 5KN, it indicates that the calculation results of this round are relatively accurate. Take the average value of at least 10 sets of thrust calculation results as the final determined engine thrust.

[0063] If the standard deviation is greater than 5 kN, it indicates that the outlet flow field to be tested is not yet stable. Select at least 10 consecutive outlet flow field images that are later in the time sequence and repeat the calculations from step four to step seven until at least 10 sets of thrust calculation results meet the condition that the standard deviation is not greater than 5 kN. Then, take the average value of at least 10 sets of thrust calculation results as the final determined engine thrust.

[0064] Furthermore, the resolution of the LCD display screen is not less than 1280×1024.

[0065] The beneficial effects of this invention are: This invention provides an optical measurement method for aero-engine thrust using background schlieren technology. It innovatively combines background schlieren technology with aero-engines, analyzes the physical quantities required for aero-engine thrust measurement layer by layer, and combines the unique advantage of background schlieren technology in obtaining information through light deflection to construct a simplified and effective optical measurement method.

[0066] This method uses background schlieren technology to achieve non-contact measurement. It leverages the measurement advantages of background schlieren technology in velocity and density fields to transform the exit velocity and temperature fields of the tail nozzle, which are the focus of traditional test bench thrust measurement, into velocity and density fields. It transforms the mass flow rate of the exit flow field into the radial distribution of the volume density of the exit flow field and the velocity field of the exit flow field, and then into cross-correlation calculations. Furthermore, it analyzes the object-image relationship between the exit flow field and the high-speed camera and embeds it into the cross-correlation calculations.

[0067] This method can record relevant parameters of the flow field during transient changes, with high resolution and signal-to-noise ratio, ensuring measurement accuracy without interfering with the flow field under test. The measurement cost is relatively low, making it suitable for scenarios that require frequent monitoring of engine operating status. It has advantages such as convenient operation, no impact on the internal structure of the flow field, large measurement range, fast response speed, high measurement accuracy, and lower cost. Attached Figure Description

[0068] Figure 1 This is a schematic diagram of the installation location of the test system of the present invention;

[0069] Figure 2 This is a schematic diagram of the background shading technology in this invention;

[0070] Figure 3 This is a schematic diagram of a schlieren image during the implementation of the present invention;

[0071] Figure 4 This is a schematic diagram of the displacement field of the outlet flow field of the engine tail nozzle during the implementation of this invention.

[0072] In the diagram, 1. LCD display screen, 2. Exit flow field of engine tail nozzle, 3. Lens, 4. Mirror plane, 5. High-speed camera, 6. Engine tail nozzle, 7. Guide rail. Detailed Implementation

[0073] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0074] To achieve the above objectives, the present invention provides the following specific embodiments:

[0075] Example 1: As Figure 1 , Figure 2 As shown, an optical measurement method for aero-engine thrust using background schlieren technology includes the following steps:

[0076] S01. The LCD display screen 1 and the high-speed camera 5 are coaxially positioned on both sides of the outlet of the engine tail nozzle 6 along the airflow direction via the guide rail 7. The optical axis of the high-speed camera 5 is perpendicular to the center of the screen of the LCD display screen 1. The size of the LCD display screen 1 covers the outlet flow field 2 of the engine tail nozzle and is larger than the field of view of the high-speed camera 5. The resolution of the LCD display screen 1 is not less than 1280×1024.

[0077] At the same time, the center of the exit section of the engine tail nozzle 6, the center of the LCD display screen 1, and the center of the plane of the high-speed camera lens 3 are on the same horizontal plane.

[0078] Furthermore, taking the projection of the center of the outlet section of the engine exhaust nozzle 6 onto the ground as the origin, and the direction of the airflow at the outlet of the engine exhaust nozzle 6 as... The positive direction of the axis, with the origin pointing towards the LCD screen as... Positive axis direction, vertical Axial upward In the positive direction of the axis, establish a deflection space rectangular coordinate system for the outlet flow field of the engine tail nozzle.

[0079] S02, in the rectangular coordinate system of the outlet flow field deflection space, the center coordinates of the mirror plane 4 of the high-speed camera are ( , , The center coordinates of the LCD display screen 1 are ( , , Then we have:

[0080] ,

[0081] ,

[0082] In the formula, The diameter of the engine exhaust nozzle 6;

[0083] The atmospheric static pressure The actual distance between the measured LCD display 1 and the lens 3 of the high-speed camera. The outlet flow field of the engine exhaust nozzle 2 Maximum width in the positive direction of the axis The value satisfies the requirement of the outlet of the engine exhaust nozzle 6. Within the range, static pressure pulsation values ​​at at least five randomly selected locations. .

[0084] S03. Measure the radius of the engine exhaust nozzle 6 when the engine is not started. Atmospheric static pressure And measure the actual distance between the LCD display 1 and the lens 3 of the high-speed camera. The outlet flow field of the engine exhaust nozzle 2 Maximum width in the positive direction of the axis Furthermore, a reference exit flow field image of the engine exhaust nozzle 6 is captured using a high-speed camera 5. I 1. And refer to the outlet flow field image. I The outlet flow field 2 of the engine exhaust nozzle in 1 satisfies:

[0085] Quiet temperature Static temperature fluctuation Static pressure pulsation Reynolds number Dryness .

[0086] S04. Measure the engine rotor speed after starting the engine. Fuel flow airflow When the fluctuation values ​​do not exceed 0.2%-0.5% of the values ​​corresponding to the engine's maximum state, the obtained static pressure at the outlet of the engine exhaust nozzle 6 is... Under the condition that the engine is not started, the radius of the engine exhaust nozzle 6 is used. Displacement of actuator cylinder The angle between the deflection angle of the adjusting plate and the displacement of the actuator cylinder The exit cross-sectional area parameters of engine tail nozzle 6 were calculated. ,

[0087] Exit cross-sectional area of ​​engine tail nozzle 6 The calculation formula is:

[0088] ,

[0089] In the formula, The radius of the engine exhaust nozzle 6; For the displacement of the actuator cylinder; To adjust the angle between the deflection angle of the plate and the displacement of the actuator cylinder;

[0090] At this time, take images of the outlet flow field 2 of the engine tail nozzle at intervals of 10s to 15s, and take at least 50 images continuously.

[0091] Then, by subtracting the gray values ​​of two adjacent outlet flow field images pairwise, the image with the largest gray value difference not exceeding 5 is selected as the preferred outlet flow field image to be tested. I 2.

[0092] S05, Select the preferred outlet flow field image to be tested I 2. Import the particle image velocimetry software, establish a query network, and define the parameters of the query window. Based on the expected maximum displacement of the outlet flow field as (Δx_max, Δy_max), determine the size of the query window as (W, H) according to the Nyquist criterion. Then:

[0093] W≥4Δx_max, H≥4Δy_max;

[0094] Set the window movement step size so that the window overlap rate is between 50% and 75% after each movement, in order to improve the spatial resolution of the displacement field.

[0095] S06. Through the query window, traverse the reference outlet flow field images respectively. I 1 and the outlet flow field image to be measuredI 2; For a single traversal, the cross-correlation algorithm is used to calculate the reference outlet flow field image. I 1. Image of the outlet flow field to be measured I 2. Cross-correlation function for querying pixel grayscale values ​​in the window region for:

[0096] ,

[0097] In the formula, This represents the coordinate position under the query window during traversal. The pixel grayscale value at that location, Represents pixel displacement; multiple cross-correlation functions are calculated under different displacement values. , find Maximum displacement This refers to the displacement of the query window.

[0098] When referencing the outlet flow field image I The pixel size of 1 is denoted as When, then the displacement satisfy 0.25M 0.25N.

[0099] S07. Using the Fast Fourier Transform (FFT), the cross-correlation calculation is simplified. According to the convolution theorem, performing cross-correlation in the time domain is equivalent to performing dot product in the frequency domain, thus obtaining:

[0100] ,

[0101] In the formula, This represents the Fourier transform, which converts an image from the time domain to the frequency domain. represents the complex conjugate of the Fourier transform, and represents the inverse Fourier transform;

[0102] Based on the principle of simplifying cross-correlation calculations, the reference image... Image of the outlet flow field disturbance after translation Perform Fourier transform and complex conjugate operations, followed by dot product in the frequency domain to obtain... Finally, an inverse Fourier transform is performed to obtain the cross-correlation function in the time domain. ;

[0103] Similarly, multiple cross-correlation functions were calculated for different displacements. , find Maximum displacement This refers to the displacement of the query window;

[0104] Cross-correlation calculations are performed on all query windows to generate the outlet flow field image to be measured. I The displacement vector corresponding to 2, that is, the displacement field of the outlet flow field 2 of the engine tail nozzle, is .

[0105] S08. Using the displacement field of the engine exhaust nozzle outlet flow field 2 combined with the time interval, divide the average displacement vector of the background dots captured in the query window by the time interval between the current calculation frame and the starting frame to obtain the average velocity. Perform the same operation on all query windows to generate the velocity field of the engine exhaust nozzle outlet flow field 2. .

[0106] S09. In the aforementioned outlet flow field deflection space rectangular coordinate system, the velocity field Decomposed into velocity components in the x-direction and the velocity component in the y direction The velocity component in the y-direction is obtained based on the geometric relationship of the high-speed camera 5 imaging. for:

[0107] ,

[0108] In the formula, The outlet flow field 2 of the engine exhaust nozzle is Maximum width in the positive direction of the axis This is the actual distance between the LCD display 1 and the lens 3 of the high-speed camera. It is the focal length of the high-speed camera 5. It is the refractive index of air. The outlet flow field to be measured is The refractive index distribution function in the direction;

[0109] Similarly, based on the velocity component in the x-direction The flow field at the outlet of the test was calculated. Refractive index distribution function in the direction ;

[0110] Finally, the air refractive index and the refractive index distribution function in the y direction are... Refractive index distribution function in the x-direction By adding them together, we obtain the refractive index parameters of the outlet flow field to be measured. ;

[0111] For each displacement in the displacement field of the outlet flow field 2 of the engine tail nozzle, the corresponding refractive index parameter is calculated, and then combined to obtain the refractive index field of the outlet flow field to be measured. .

[0112] S10. The Gladstone-Dale formula used is:

[0113] ,

[0114] In the formula, It is the Gladstone-Dale constant, a fixed value for a specific gas; combined with the refractive index field The density field of the exhaust flow field 2 at the engine tail nozzle after startup was calculated. .

[0115] S11. During ground testing, the formula for calculating the thrust of an aero-engine is as follows:

[0116] ,

[0117] ,

[0118] In the formula, For the thrust of aircraft engines; This is the exit cross-sectional area of ​​the engine tail nozzle 6; The static pressure at the outlet of the engine tail nozzle 6; Atmospheric static pressure; The mass flow rate at the outlet of the engine exhaust nozzle 6; The outlet velocity of the engine tail nozzle 6; The radial volume density of the exit particles; For the mass of exported particles; The particles are radially distributed with their velocity centered on the center of the exit section of the engine tail nozzle 6; The radius of the engine exhaust nozzle 6; The particle density is distributed radially with the center of the exit section of the engine tail nozzle 6 as the center. For the definite integral variable, For variables The differential, , These are the density fields of the flow field at the engine exhaust nozzle exit. and velocity field In query grid The value at that location, It queries the area of ​​the grid.

[0119] The momentum term was calculated. Then, use the static pressure at the outlet of nozzle 6. Environmental pressure The difference multiplied by the exit cross-sectional area of ​​the engine exhaust nozzle 6 The corresponding pressure term is calculated. Then, by adding the momentum and pressure terms together, we obtain the flow field image at the outlet to be measured. I The thrust corresponding to 2.

[0120] S12. Traverse the outlet flow field image to be measured, repeat the calculation process from S05 to S11, and compare the thrust calculation results. If at least 10 sets of thrust calculation results meet the condition that the standard deviation is not greater than 5KN, it indicates that the calculation results of this round are relatively accurate. Take the average value of at least 10 sets of thrust calculation results as the final determined engine thrust.

[0121] If the standard deviation is greater than 5 kN, it indicates that the outlet flow field to be tested is not yet stable. Select at least 10 consecutive outlet flow field images that are later in the time sequence and repeat the calculations from step four to step seven until at least 10 sets of thrust calculation results meet the condition that the standard deviation is not greater than 5 kN. Then, take the average value of at least 10 sets of thrust calculation results as the final determined engine thrust.

[0122] like Figure 1-4 As shown, to further illustrate the technical solution and effects of the present invention, a specific example of the present invention is also provided: an optical measurement method for the thrust of an aero-engine using background schlieren technology, specifically including the following steps:

[0123] Step 1: Establish the outlet flow field of the engine exhaust nozzle in a deflection space rectangular coordinate system and install the test system as follows: Figure 1 As shown, the radius of the engine exhaust nozzle 6 is measured when the engine is not running. Atmospheric static pressure The distance between the LCD display 1 and the lens 3 of the high-speed camera The maximum width of the engine exhaust nozzle 6 in the direction of the outlet flow field ;

[0124] Among them, LCD display 1 is Liquid Cryo LCD display screen;

[0125] Step one, establishing the deflection space rectangular coordinate system of the engine exhaust nozzle outlet flow field and installing the test system, includes:

[0126] Taking the projection of the center of the outlet section of the engine exhaust nozzle 6 onto the ground as the origin, the direction of airflow towards the outlet is... In the positive direction of the axis, the LCD display 1 and the high-speed camera 5 are respectively positioned on both sides of the outlet of the engine tail nozzle 6 along the airflow direction via guide rails 7, with the direction from the origin towards the LCD display 1 as... The positive direction of the axis is vertically upward. In the positive direction of the axis, establish a spatial rectangular coordinate system for the deflection of the engine exhaust nozzle outlet flow field 2. This spatial rectangular coordinate system is used to describe the deflection of the engine exhaust nozzle outlet flow field 2.

[0127] like Figure 2As shown, the LCD display 1 and the high-speed camera 5 are respectively mounted on the guide rail 7. The resolution of the LCD display 1 is not less than 1280×1024. The optical axis of the high-speed camera 5 is perpendicular to the center of the screen of the LCD display 1. The size of the LCD display 1 needs to cover the flow field 2 at the outlet of the engine exhaust nozzle and be larger than the corresponding field of view of the high-speed camera. The center of the outlet section of the engine exhaust nozzle 6, the center of the screen of the spliced ​​LCD display 1, and the center of the mirror plane 4 of the high-speed camera are on the same horizontal plane.

[0128] The engine exhaust nozzle 6 is located on both sides of a rectangular coordinate system according to the deflection space of the outlet flow field, with the center of the mirror plane 4 of the high-speed camera ( , , ), the center of the LCD display screen 1 ( , , )satisfy: , , The diameter of the engine exhaust nozzle is 6.

[0129] The distance between the LCD display 1 and the lens 3 of the high-speed camera is measured. 2. Engine tailpipe outlet flow field Maximum width in direction Atmospheric static pressure Atmospheric static pressure The value must meet the requirement of the engine exhaust nozzle outlet 6 rear Take any five static pressure pulsation values ​​within the range .

[0130] Step 2: Turn on LCD display 1 and high-speed camera 5 to capture a reference outlet flow field image. I 1;

[0131] Furthermore, in step two, a reference outlet flow field image is captured. I 1 refers to the flow field at the engine exhaust nozzle outlet before starting; 2 satisfies: static temperature. Static temperature fluctuation Static pressure pulsation Reynolds number Dryness At this time, an image of the engine exhaust nozzle outlet flow field 2 when the engine is not running is captured as a reference outlet flow field image. I 1. Denote the pixel dimensions of the image as... ,like Figure 3 As shown.

[0132] Step 3: Measure the static pressure at the outlet of the engine exhaust nozzle 6 after engine start-up. Utilizing the radius of the engine exhaust nozzle 6 Displacement of actuator cylinder The angle between the deflection angle of the adjusting plate and the displacement of the actuator cylinder The exit cross-sectional area parameters of engine tail nozzle 6 were calculated. Fifty consecutive images of the outlet flow field to be measured were captured, and ten consecutive images were selected for thrust calculation. These ten selected images were then used to calculate the thrust. I 2 - I 11 express;

[0133] Furthermore, in step three, the static pressure at the outlet of the engine exhaust nozzle 6 is measured after the engine is started. This refers to the rotor speed of the engine. Fuel flow airflow When the fluctuation values ​​do not exceed 0.2%-0.5% of the engine's maximum value, measure the static pressure at the engine exhaust nozzle 6. Then, through the radius of the engine exhaust nozzle 6 when the engine is not started. Displacement of actuator cylinder The angle between the deflection angle of the adjusting plate and the displacement of the actuator cylinder Calculate the exit cross-sectional area of ​​engine exhaust nozzle 6. ;

[0134] ,

[0135] At this time, images of the engine exhaust nozzle outlet flow field 2 are captured at intervals of 15s-30s, for a total of 50 images of the outlet flow field to be tested. The grayscale values ​​of adjacent images are subtracted pairwise; images with a maximum grayscale value difference of no more than 5 are selected as preferred images, and 10 images are chosen for use. I 2 - I 11 express.

[0136] Step 4: Use the cross-correlation algorithm to analyze the outlet flow field image captured in Step 3. I 2. After processing, the velocity field is finally obtained; such as Figure 4 The image shown is of the outlet flow field. I 2;

[0137] Furthermore, in step four, the image of the flow field 2 at the engine exhaust nozzle exit is processed using a cross-correlation algorithm to finally obtain the velocity field, specifically including:

[0138] (1) Define query window parameters. Based on the expected maximum displacement (Δx_max, Δy_max) of the outlet flow field, determine the window size (W, H) according to the Nyquist criterion: W≥4Δx_max, H≥4Δy_max; set the window movement step size so that the window overlap rate is between 50% and 75% after each movement, so as to improve the spatial resolution of the displacement field.

[0139] (2) First, using the reference outlet flow field image I 1 and the outlet flow field image to be measured I 2. Calculate the thrust by iterating through the reference outlet flow field images via the query window. I 1. Flow field image at the outlet to be measured I 2; For a single traversal, the cross-correlation algorithm is used to calculate the cross-correlation function of the pixel gray values ​​in the query window region of the two images. :

[0140] ,

[0141] This represents the coordinate position within this window. The pixel grayscale value at that location, Represents pixel displacement; multiple cross-correlation functions are calculated under different displacement values. Find the displacement that maximizes its value. This is the displacement of the window, where the displacement... Must meet 0.25M 0.25N; Performing the above cross-correlation calculations on all windows will generate the outlet flow field image to be measured. I The displacement vector corresponding to 2 is the displacement field of the outlet flow field 2 of the engine exhaust nozzle. ;

[0142] (3) The cross-correlation calculation is simplified by using the Fast Fourier Transform. According to the convolution theorem, performing cross-correlation in the time domain is equivalent to performing dot product in the frequency domain:

[0143] ,

[0144] In the formula, This represents the Fourier transform, which converts an image from the time domain to the frequency domain. Let denote the complex conjugate of the Fourier transform, and denote the inverse Fourier transform; based on the above principle, for the reference image... Image of the outlet flow field disturbance after translation Perform Fourier transform and complex conjugate operations, followed by dot product in the frequency domain to obtain... Finally, an inverse Fourier transform is performed to obtain the cross-correlation function in the time domain. Similarly, multiple cross-correlation functions were calculated for different displacements. Find the displacement that maximizes its value. This represents the displacement of the window; by performing the above cross-correlation calculation on all windows, the outlet flow field disturbance image can be generated. I The displacement vector corresponding to 2 is the displacement field of the outlet flow field 2 of the engine tail nozzle;

[0145] (4) Calculate the velocity field of the engine exhaust nozzle exit flow field 2 by combining the displacement field of the engine exhaust nozzle exit flow field 2 with the time interval. Divide the average displacement vector of the background dots captured in the query window by the time interval between the current calculation frame and the starting frame to obtain the average velocity. Perform the same operation on all windows to generate the velocity field. .

[0146] Step 5: Calculate the refractive index field from the displacement field of the flow field 2 at the engine tail nozzle exit;

[0147] Furthermore, in step five, the refractive index field is calculated from the displacement field of the flow field 2 at the engine exhaust nozzle exit. The specific process includes:

[0148] velocity field Decomposed into velocity components in the x-direction and the velocity component in the y direction Based on the geometric relationship of camera imaging, the velocity component in the y-direction can be obtained. for:

[0149] ,

[0150] Combined with the measured distance between LCD display 1 and high-speed camera lens 3 And known parameters: the focal length of high-speed camera 5 air refractive index Based on the velocity component in the y-direction The refractive index distribution function of the outlet flow field under test in the y direction is obtained. ;

[0151] Similarly, based on the velocity component in the x-direction The outlet flow field of the test object can be calculated. Refractive index distribution function in the direction Finally, the air refractive index and the refractive index distribution function in the y direction are... Refractive index distribution function in the x-direction By adding them together, we obtain the refractive index parameters of the outlet flow field to be measured. For each displacement in the displacement field of the outlet flow field 2 of the engine tail nozzle, the corresponding refractive index parameter can be calculated, and the refractive index field of the outlet flow field to be measured can be obtained by combining them. .

[0152] Step 6: Solve for the density field of the flow field 2 at the engine tail nozzle exit using the Gladstone-Dale formula and the refractive index field;

[0153] Furthermore, the specific process of solving the density field of the engine exhaust nozzle outlet flow field 2 in step six includes:

[0154] The Gladstone-Dale formula is as follows: ,in The Gladstone-Dale constant is a fixed value for a specific gas; combined with the refractive index field of the engine exhaust nozzle outlet flow field 2 obtained in step five. The density field of the flow field 2 at the engine exhaust nozzle outlet after engine start-up was calculated. .

[0155] Step 7: Calculate the thrust by combining the velocity field and density field of the flow field 2 at the engine tail nozzle exit with the thrust calculation formula and other parameters;

[0156] Furthermore, in step seven, the velocity field of the flow field 2 at the engine exhaust nozzle outlet... and density field The thrust is obtained by combining other parameters and the error is verified, specifically including:

[0157] The formula for calculating the thrust of an aircraft engine during ground testing is as follows:

[0158] ,

[0159] ,

[0160] in, For the thrust of aircraft engines, Let be the exit cross-sectional area of ​​the engine exhaust nozzle 6. The outlet static pressure of the engine exhaust nozzle 6. Atmospheric static pressure The mass flow rate at the outlet of the engine exhaust nozzle 6 is [value missing]. The outlet velocity of the engine exhaust nozzle 6. The radial volume density of the exit particles. For the mass of exported particles, The particles are distributed radially with their velocities centered on the center of the exit section of the engine's exhaust nozzle. Let be the radius of the engine's exhaust nozzle. The particle density is distributed radially with the center of the exit section of the engine exhaust nozzle as the center. , These are the density fields of the flow field 2 at the engine exhaust nozzle exit. and velocity field In query grid The value at that location, It queries the area of ​​the grid.

[0161] The momentum term was calculated. Then, use the static pressure at the outlet of the engine tail nozzle 6. Environmental pressure The difference multiplied by the exit cross-sectional area of ​​the engine exhaust nozzle 6 The corresponding pressure term is calculated. Adding the momentum and pressure terms together yields the flow field image at the outlet. I The thrust corresponding to 2.

[0162] Step 8: For the remaining nine outlet flow field images to be tested... I 3 - I 11 Repeat steps four through seven above, check for errors, and finally determine the engine thrust.

[0163] Furthermore, the final determination of engine thrust described in step eight specifically includes,

[0164] Image of the outlet flow field to be measured I 3 - I 11 Repeat steps four through seven above and compare the thrust calculation results. If the standard deviation of the 10 sets of thrust calculation results is no greater than 5 kN, it means that the calculation results of this round are relatively accurate. Take the average value as the final engine thrust calculation result. If the standard deviation between the 10 sets of results is large, it means that the outlet flow field to be measured has not yet stabilized. Select ten consecutive outlet flow field images to be measured that are later in the time sequence and repeat the calculations of steps four through eight above until the standard deviation of the 10 sets of thrust calculation results is no greater than 5 kN. Take the average value of the 10 sets of thrust calculation results as the final engine thrust calculation result.

[0165] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for optical measurement of aero-engine thrust using background schlieren technology, characterized in that, Includes the following steps: Step 1: Place the LCD screen and the high-speed camera coaxially on both sides of the engine exhaust nozzle outlet along the airflow direction via guide rails. The optical axis of the high-speed camera is perpendicular to the center of the LCD screen. The size of the LCD screen covers the exhaust flow field of the engine exhaust nozzle and is larger than the field of view of the high-speed camera. At the same time, the center of the engine exhaust nozzle's outlet section, the center of the LCD screen, and the center of the high-speed camera's lens plane are on the same horizontal plane. Furthermore, taking the projection of the center of the engine exhaust nozzle's outlet cross-section onto the ground as the origin, and the direction of the airflow at the engine exhaust nozzle's outlet as... The positive direction of the axis, with the origin pointing towards the LCD screen as... Positive axis direction, vertical Axial upward Establish a rectangular coordinate system for the deflection space of the engine exhaust nozzle in the positive direction of the axis; Step 2: Measure the radius of the engine exhaust nozzle when the engine is not running. Atmospheric static pressure And measure the actual distance between the LCD screen and the lens of the high-speed camera. The outlet flow field of the engine exhaust nozzle Maximum width in the positive direction of the axis Furthermore, a high-speed camera was used to capture a reference exit flow field image of the engine exhaust nozzle. I 1; Step 3: Measure the engine rotor speed after starting the engine. Fuel flow airflow When the fluctuation values ​​do not exceed 0.2%-0.5% of the values ​​corresponding to the engine's maximum state, the obtained static pressure at the engine exhaust nozzle outlet is... Under the condition that the engine is not running, the radius of the engine exhaust nozzle is used. Displacement of actuator cylinder The angle between the deflection angle of the adjusting plate and the displacement of the actuator cylinder Calculate the exit cross-sectional area parameters of the engine exhaust nozzle. At least 50 images of the outlet flow field are continuously captured, and at least 10 consecutive images are selected as the outlet flow field images to be tested. I 2; Step 4: In the particle image velocimetry software, based on the outlet flow field image to be measured... I 2. Establish a query network, and then, based on the reference outlet flow field image. I 1 and the outlet flow field image to be measured I 2. Using the cross-correlation algorithm, the displacement field of the outlet flow field of the engine tail nozzle is obtained, and then the velocity field is obtained by combining the time interval. Step 5: In the rectangular coordinate system of the deflection space of the outlet flow field, after calculating the refractive index field based on the displacement field of the outlet flow field of the engine tail nozzle, solve the density field of the outlet flow field of the engine tail nozzle using the Gladstone-Dale formula. Step 6: Combine the velocity field and density field with the thrust calculation formula, and the exit cross-sectional area of ​​the engine exhaust nozzle. Static pressure at the engine exhaust nozzle outlet Atmospheric static pressure Density field of the outlet flow field of the tail nozzle and velocity field In query grid value at , Grid area Calculate the thrust; Step 7: Traverse the outlet flow field image to be measured, repeat the calculation process from Step 4 to Step 6, check for errors, and finally determine the engine thrust.

2. The optical measurement method for aero-engine thrust using background schlieren technology as described in claim 1, characterized in that, In step one, in the rectangular coordinate system of the outlet flow field deflection space, the coordinates of the center of the mirror plane of the high-speed camera are ( , , The center coordinates of the LCD screen are ( ). , , Then we have: ， ， In the formula, The diameter of the engine's exhaust nozzle; The atmospheric static pressure The actual distance between the measured LCD screen and the lens of the high-speed camera. The outlet flow field of the engine exhaust nozzle Maximum width in the positive direction of the axis The value satisfies the requirement of the exhaust nozzle outlet rear... Within the range, static pressure pulsation values ​​at at least five randomly selected locations. .

3. The optical measurement method for aero-engine thrust using background schlieren technology as described in claim 1, characterized in that: In step two, the reference outlet flow field image I The outlet flow field of the engine exhaust nozzle in 1 satisfies: Quiet temperature Static temperature fluctuation Static pressure pulsation Reynolds number Dryness .

4. The optical measurement method for aero-engine thrust using background schlieren technology as described in claim 1, characterized in that, The outlet cross-sectional area of ​​the engine tail nozzle in step three The calculation formula is: ， In the formula, The radius of the engine exhaust nozzle; For the displacement of the actuator cylinder; To adjust the angle between the deflection angle of the plate and the displacement of the actuator cylinder; At this time, take images of the exhaust flow field of the engine tail nozzle at intervals of 10s to 15s, and take at least 50 images continuously. Then, by subtracting the gray values ​​of two adjacent outlet flow field images pairwise, the image with the largest gray value difference not exceeding 5 is selected as the preferred outlet flow field image to be tested. I 2.

5. The optical measurement method for aero-engine thrust using background schlieren technology as described in claim 1, characterized in that, Step four specifically involves: Step 41: Select the preferred outlet flow field image to be tested. I 2. Import the particle image velocimetry software, establish a query network, and define the parameters of the query window. Based on the expected maximum displacement of the outlet flow field as (Δx_max, Δy_max), determine the size of the query window as (W, H) according to the Nyquist criterion. Then: W≥4Δx_max, H≥4Δy_max; Set the window movement step size so that the window overlap rate is between 50% and 75% after each movement, in order to improve the spatial resolution of the displacement field; Step 42: Through the query window, traverse the reference outlet flow field images respectively. I 1 and the outlet flow field image to be measured I 2; For In a single pass, using a cross-correlation algorithm, the reference outlet flow field image is calculated. I 1. Image of the outlet flow field to be measured I 2. Cross-correlation function for querying pixel grayscale values ​​in the window region for: ， In the formula, This represents the coordinate position under the query window during traversal. The pixel grayscale value at that location, Represents pixel displacement; Multiple cross-correlation functions were calculated under different displacement values. , find Maximum displacement This refers to the displacement of the query window. When referencing the outlet flow field image I The pixel size of 1 is denoted as When, then the displacement satisfy 0.25M 0.25N; Step 43: Using the Fast Fourier Transform, the cross-correlation calculation is simplified. According to the convolution theorem, performing cross-correlation in the time domain is equivalent to performing dot product in the frequency domain, thus obtaining: ， In the formula, This represents the Fourier transform, which converts an image from the time domain to the frequency domain. represents the complex conjugate of the Fourier transform, and represents the inverse Fourier transform; Based on the principle of simplifying cross-correlation calculations, the reference image... Image of the outlet flow field disturbance after translation Perform Fourier transform and complex conjugate operations, followed by dot product in the frequency domain to obtain... Finally, an inverse Fourier transform is performed to obtain the cross-correlation function in the time domain. ; Similarly, multiple cross-correlation functions were calculated for different displacement amounts. , find Maximum displacement This refers to the displacement of the query window; Cross-correlation calculations are performed on all query windows to generate the outlet flow field image to be measured. I The displacement vector corresponding to 2, that is, the displacement field of the outlet flow field of the engine exhaust nozzle, is: ; Step 44: Using the displacement field of the engine exhaust nozzle outlet flow field combined with the time interval, divide the average displacement vector of the background dots captured in the query window by the time interval between the current calculation frame and the starting frame to obtain the average velocity. Perform the same operation on all query windows to generate the velocity field of the engine exhaust nozzle outlet flow field. .

6. The optical measurement method for aero-engine thrust using background schlieren technology as described in claim 1, characterized in that, Step five specifically involves: In the aforementioned outlet flow field deflection space rectangular coordinate system, the velocity field Decomposed into velocity components in the x-direction and the velocity component in the y direction The velocity component in the y-direction is obtained based on the geometric relationship of high-speed camera imaging. for: ， In the formula, The outlet flow field of the engine exhaust nozzle is Maximum width in the positive direction of the axis This is the actual distance between the LCD screen and the lens of the high-speed camera. It is the focal length of a high-speed camera. It is the refractive index of air. The outlet flow field to be measured is The refractive index distribution function in the direction; Similarly, based on the velocity component in the x-direction The flow field at the outlet of the test was calculated. Refractive index distribution function in the direction ; Finally, the air refractive index and the refractive index distribution function in the y direction are... Refractive index distribution function in the x-direction By adding them together, we obtain the refractive index parameters of the outlet flow field to be measured. ; For each displacement in the displacement field of the engine exhaust nozzle outlet flow field, the corresponding refractive index parameter is calculated, and then combined to obtain the refractive index field of the outlet flow field to be measured. ; Continuing, the Gladstone-Dale formula used is: ， In the formula, It is the Gladstone-Dale constant, a fixed value for a specific gas; combined with the refractive index field The density field of the exhaust flow field at the engine tail nozzle after startup was calculated. .

7. The optical measurement method for aero-engine thrust using background schlieren technology as described in claim 1, characterized in that, Step six specifically includes the following steps: During ground testing, the thrust calculation formula for an aircraft engine is as follows: ， ， In the formula, For the thrust of aircraft engines; This is the exit cross-sectional area of ​​the engine's tail nozzle; The static pressure at the engine exhaust nozzle outlet; Atmospheric static pressure; This refers to the mass flow rate at the engine exhaust nozzle outlet. The outlet velocity of the engine exhaust nozzle; The radial volume density of the exit particles; For the mass of exported particles; The particles are distributed radially with their velocity centered on the center of the exit section of the engine exhaust nozzle. The radius of the engine exhaust nozzle; The volume density radial distribution of particles with the center of the exit section of the engine tail nozzle as the center; For the definite integral variable, For variables The differential, , These are the density fields of the flow field at the engine exhaust nozzle exit. and velocity field In query grid The value at that location, It queries the area of ​​the grid. The momentum term was calculated. Then, use the outlet static pressure of the tail nozzle. Environmental pressure The difference multiplied by the cross-sectional area of ​​the engine exhaust nozzle. The corresponding pressure term is calculated. Then, by adding the momentum and pressure terms together, we obtain the flow field image at the outlet to be measured. I The thrust corresponding to 2.

8. The optical measurement method for aero-engine thrust using background schlieren technology as described in any one of claims 1-7, characterized in that, Step seven specifically refers to: Repeat steps four to six for the output flow field image to be tested, and compare the thrust calculation results. If at least 10 sets of thrust calculation results meet the condition that the standard deviation is not greater than 5KN, it indicates that the calculation results of this round are relatively accurate. Take the average value of at least 10 sets of thrust calculation results as the final determined engine thrust. If the standard deviation is greater than 5 kN, it indicates that the outlet flow field to be tested is not yet stable. Select at least 10 consecutive outlet flow field images that are later in the time sequence and repeat the calculations from step four to step seven until at least 10 sets of thrust calculation results meet the condition that the standard deviation is not greater than 5 kN. Then, take the average value of at least 10 sets of thrust calculation results as the final determined engine thrust.

9. The optical measurement method for aero-engine thrust using background schlieren technology as described in any one of claims 1-7, characterized in that, The resolution of the LCD display screen is not less than 1280×1024.

Images