Speed measurement method based on single-frame multiple exposure

Abstract

The invention discloses a speed measurement method based on single-frame multiple exposure, and belongs to the field of hypersonic velocity field measurement. The implementation method comprises the following steps: recording equal time interval change of molecular marking in a flow field along with movement of the flow field through single-frame multi-time exposure imaging of an image enhancement camera, and realizing continuous and rapid acquisition of speed information within a short time after single-time marking. The morphological position after the time interval is determined according to the molecular marker, and the velocity field with spatial resolution in the flow field is extracted by mapping the marker before and after displacement, so that the velocity measurement precision is improved. Compared with other two-dimensional velocity field acquisition methods based on femtosecond laser molecular marker tracing velocity measurement, the method provided by the invention does not need a complex focusing lens combination, can obtain spatial positions of molecular markers at different moments only through isochronous continuous triggering exposure of a camera, thereby obtaining velocity flow field displacement information. And corresponding acceleration information can be further deduced according to the obtained velocity field. The method is especially suitable for hypersonic velocity field measurement.

Description

Technical Field

[0001] The present invention relates to a speed measurement method based on single-frame multiple exposures, and belongs to the field of hypersonic speed field measurement. Background Art

[0002] At present, with the development of technologies such as aircraft design and engines, the flight speed has reached the supersonic field. However, due to the extremely complex aerodynamic problems involved in the supersonic field and a large number of unknown physical phenomena, ground tests are still an important means to obtain the dynamic characteristics of aircraft and assist aircraft design at present. Among them, speed, acceleration, etc. are all extremely important physical quantities in the analysis of the airflow field flow process. It is of great significance to obtain spatially resolved speed information in a high-speed wind tunnel.

[0003] However, there are limited methods in current measurement means to obtain speed field and acceleration field information with spatially resolved information. Such technologies mainly include Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV). LDA and PIV technologies have been commercially applied at present and are widely used in the speed measurement of the internal flow field of construction machinery. However, in the speed measurement process of these two technologies, solid tracer particles need to be dispersed into the flow field to be measured. The followability of the tracer particles and their stability in extreme environments limit the application of these two technologies in high-turbulence, hypersonic flow fields / combustion fields.

[0004] The speed measurement method based on femtosecond laser molecular tagging velocimetry is not limited by particle followability, but still faces the following two problems: First, the femtosecond laser molecular tagging velocimetry focuses on generating a one-dimensional spatially resolved line in the flow field. Only the one-dimensional spatially resolved speed field information on this line can be obtained in one measurement. Scanning or large-scale tagging is required to obtain the speed information of the two-dimensional spatial plane. Second, most of the algorithms stay at speed calculation and do not further obtain other physical quantities such as acceleration. Summary of the Invention

[0005] The object of the present invention is to provide a speed measurement method based on single-frame multiple exposures. Based on the single-frame multiple exposure method, through the continuous pulse trigger of the synchronization system, isochronous continuous multiple exposures are performed on a single-frame picture to achieve continuous and rapid acquisition of speed information in a short time after a single tagging, and then obtain the information of the speed field with spatial resolution, and extract the airflow field acceleration information based on the speed field. Compared with other two-dimensional speed field acquisition methods based on femtosecond laser molecular tagging velocimetry, the present invention does not require a complex focusing lens combination. Only by continuously triggering and exposing the camera isochronously can the spatial positions of molecular tags at different times be obtained, so as to obtain the displacement information of the speed flow field, and the corresponding acceleration information can be further deduced based on the obtained speed field.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] The present invention discloses a speed measurement method based on single-frame multiple exposure, comprising the following measurement steps:

[0008] Step 1: Through multiple exposures of a single frame of the camera, the spatial position information of the same molecular marker that is continuously recorded N times at equal time intervals is obtained on a single picture.

[0009] Step 2: Based on the velocity image obtained in step 1 and the analysis of molecular marker deformation and corresponding displacement, velocity field information is extracted by mapping the marker before and after displacement to achieve velocity measurement based on single-frame multiple exposures.

[0010] Establish coordinates in the flow field, and fit the shape of the displacement of the marker line at each moment to get the fitting function:

[0011] y t1 =f t1 (x) (1)

[0012] y t2 =f t2 (x) (2)

[0013] y t3 =f t3 (x) (3)

[0014] Among them: each function corresponds to a marking line at a moment, and each marking line is derived to obtain the extreme points on each marking line, and group them according to order and category, and y t1 =f t1 The first point p in (x) t1，1 (x t1,1 ,y t1,1 ), y t2 =f t2 The first extreme point p in (x) t2,1 (x t2,1 ,y t2,1 ), y t3 =f t3 The first extreme point p in (x) t3,1 (x t3,1 ,y t3,1 )… and so on. The first extreme point is divided into a group and fitted to obtain the equation G1(x,y), which corresponds to the flow state of this part of the flow field. The flow state in similar areas of the flow field is obtained by analogy. G1(x,y) is solved together with the initial state function of the flow field to obtain the initial position corresponding to each extreme point, y t2 =f t2 (x) The first extreme point pt1,1 (x t1,1 ,y t1,1 ) The position at the next moment is p t2,1 (x t2,1 ,y t2,1 ). y t2 = f t2 (x), and the second extreme point p t1,2 (x t1,2 ,y t1,2 ) at the next moment is p t2,2 (x t2,2 ,y t2,2 ). Further, the last time point in time can be set, and the Nth time point is t n ;

[0015] The method for calculating the speed of any point between the two extreme points is as follows:

[0016] Define the target coordinate at time t1 as p t1,i (x t1,i ,y t1,i ).

[0017] Define any point between the two extreme points after movement as p t2,i (x t2,i ,y t2,i ). The line length between the two points is s. The calculation formula for the line length l is as follows:

[0018]

[0019] Then there is p t2,1 (x t2,1 ,y t2,1 ), p t2,2 (x t2,2 ,y t2,2 ), and the marked line length between the two points is p t1,1 (x t1,1 ,y t1,1 ), p t1,2 (x t1,2 ,y t1,2 ), and the distance between the two points is p t2,1 (x t2,1 ,y t2,1 ), p t2,i (x t2,i ,y t2,i ), and the marked distance between the two points is p t1,1 (x t1,1 ,y t1,1 ), p t1,i (x t1,i ,y t1,i)The distance between two points is If the molecular marker flows and diffuses relatively uniformly between the poles, then there is the following corresponding relationship:

[0020]

[0021] According to equations (4) to (5), p is obtained t1,i (x t1,i , y t1,i ) After moving, the coordinate is then p t2,i (x t2,i , y t2,i ), and thus the coordinates of any point between any two poles within the marked flow field before and after movement can be obtained. The corresponding displacement s is given by the following formula:

[0022]

[0023] At the same time, since the time interval of the pulse train provided by the synchronization system is a fixed value Δt, the time difference corresponding to this displacement can be obtained:

[0024] t2 - t1 = Δt (7)

[0025] The velocity is obtained as

[0026]

[0027] Generalizing to all moments corresponding to the pulse trigger of the synchronization system gives

[0028] t j+1 - t j = Δt (9)

[0029] Generalizing to displacement gives

[0030]

[0031] Then, the absolute value of the velocity of any point i on the molecular marker line from time j to j + 1 is:

[0032]

[0033] At the same time, knowing the starting point of the marker and the position of the end point after movement, the velocity direction can also be obtained:

[0034]

[0035] It further includes Step 3: According to the velocity field information extracted in Step 2, the acceleration is derived according to equation (13), and then the acceleration field information is obtained.

[0036]

[0037] A velocity measurement system based on single-frame multiple exposures disclosed by the present invention is used to implement a velocity measurement method based on single-frame multiple exposures. A velocity measurement system based on single-frame multiple exposures includes a femtosecond laser light source, a focusing system, a synchronous control device, and an image intensifier camera.

[0038] The femtosecond laser is used to generate high-energy femtosecond laser pulses, induce nitrogen molecules to emit light, and achieve particle image velocimetry (PIV) in the flow field based on long-lived nitrogen fluorescence molecule tagging. Through single-frame multiple-exposure imaging of the image intensifier camera, the changes in the equal time intervals of the molecular tags in the flow field moving with the flow field over time are recorded. According to the molecular tags, the morphological positions after a certain time are determined, and the information of the molecular tags in the plane is extracted through equations (11) - (12) to obtain the velocity field information. The synchronous control device controls the precise synchronization of the wind tunnel operation time, the laser pulse incidence time, and the camera shutter opening time, and presets a time difference to generate continuous pulses to trigger the image intensifier camera, ensuring that the time difference between the image acquisition time and the laser incidence time is stable enough. The image intensifier camera is used to obtain single-frame multiple-exposure image information.

[0039] Beneficial effects:

[0040] 1. A velocity measurement method based on single-frame multiple exposures disclosed by the present invention, based on the single-frame multiple-exposure method, through the continuous pulses triggered by the synchronization system, records the changes in the equal time intervals of the molecular tags in the flow field moving with the flow field over time through single-frame multiple-exposure imaging of the image intensifier camera. According to the molecular tags, the morphological positions after a certain time are determined, and the spatially resolved velocity field in the flow field is extracted by mapping the tags before and after displacement, improving the velocity measurement accuracy. The present invention can also extract the air flow field acceleration information based on the velocity field.

[0041] 2. A velocity measurement method based on single-frame multiple exposures disclosed by the present invention uses femtosecond laser to generate molecular fluorescence tags to mark the flow field, and adopts long-lived nitrogen fluorescence molecule tagging to achieve PIV in the flow field, which is more suitable for the requirements of single-frame multiple-exposure velocity measurement.

[0042] 3. Compared with other two-dimensional velocity field acquisition methods based on femtosecond laser molecular tagging velocimetry, a velocity measurement method based on single-frame multiple exposures disclosed by the present invention, on the basis of achieving beneficial effects 1 and 2, compared with other molecular tagging devices based on complex lens arrays, the present invention only requires a set of beam expanding and focusing lens groups, and can obtain the spatial positions of molecular tags at different times through isochronous continuous triggering and exposure of the camera, so as to obtain the velocity flow field displacement information.

[0043] 4. The present invention is a measurement method based on molecular tagging velocimetry technology, and the tracer particles are gas molecules in the flow field, without the problem of following the flow, and are naturally suitable for hypersonic flow fields. Description of the Drawings

[0044] Figure 1 It is a structural diagram of a velocity measurement system based on single-frame multiple exposures of the present invention;

[0045] Wherein: 1 - femtosecond laser light source, 2 - beam expanding and focusing system, 3 - synchronous control device, 4 - image intensifier camera;

[0046] Figure 2 It is a schematic diagram of the beam expanding and focusing system of the present invention;

[0047] Figure 3 It is a flow chart of a velocity measurement method based on single-frame multiple exposures of the present invention;

[0048] Figure 4 It is a schematic diagram of the algorithm of the present invention, Figure 4 (a) is the result of single-frame continuous exposure, Figure 4 (b) is the schematic diagram of the algorithm. Specific embodiments

[0049] In order to better illustrate the purpose and advantages of the present invention, the following further describes the content of the invention with reference to the accompanying drawings and examples.

[0050] Example 1:

[0051] As Figure 3 shown, a velocity measurement method based on single-frame multiple exposures disclosed in this embodiment is as follows:

[0052] Step 1: Through single-frame multiple exposures of the camera, obtain the spatial position information of the same molecular marker continuously recorded at equal time intervals N times on a single image.

[0053] Step 2: Based on the velocity image obtained in Step 1, on the basis of analyzing the deformation and corresponding displacement of the molecular marker, extract the velocity field information by mapping before and after the displacement of the marker, and realize the velocity measurement based on single-frame multiple exposures.

[0054] Establish coordinates in the flow field, as Figure 3 shown, and fit the morphology of the marker line after displacement at each moment to obtain the fitting function:

[0055] y t1 = f t1 (x) (1)

[0056] y t2 = f t2 (x) (2)

[0057] y t3 = f t3 (x) (3)

[0058] Among them: each function corresponds to a marking line at a moment, and each marking line is derived to obtain the extreme points on each marking line, and group them according to order and category, such as Figure 4 As shown in (b), y t1 =f t1 The first point p in (x) t1，1 (x t1,1 ,y t1,1 ), y t2 =f t2 The first extreme point p in (x) t2,1 (x t2,1 ,y t2,1 ), y t3 =f t3 The first extreme point p in (x) t3,1 (x t3,1 ,y t3,1 ) ... and so on, the first extreme point is divided into a group and fitted to obtain the equation G1 (x, y), which corresponds to the flow state of this part of the flow field. Similarly, the flow state in similar areas of the flow field is obtained, such as Figure 4 (b) As shown in Figure 2, G1(x, y) is solved together with the initial state function of the flow field to obtain the initial position corresponding to each extreme point, y t2 =f t2 (x) The first extreme point p t1,1 (x t1,1 ,y t1,1 ) The position at the next moment is p t2,1 (x t2,1 ,y t2,1 ). t2 =f t2 The second extreme point p in (x) t1,2 (x t1,2 ,y t1,2 ) is the next moment position p t2,2 (x t2,2 ,y t2,2 ). You can further set the last time point in time, and the Nth time point is t n ;

[0059] The velocity at any point between the two poles is calculated as follows:

[0060] Define the target coordinates at time t1 as p t1,i (x t1,i ,y t1,i )

[0061] like Figure 4 As shown in (b), first define any point between the two poles after the movement as p t2,i (x t2,i, y t2,i ). The line length between two points is s. The calculation formula for the line length l is as follows:

[0062]

[0063] Then there is p t2,1 (x t2,1 , y t2,1 ), p t2,2 (x t2,2 , y t2,2 ). The marked line length between two points p p t1,1 (x t1,1 , y t1,1 ), p t1,2 (x t1,2 , y t1,2 ). The distance between two points p p t2,1 (x t2,1 , y t2,1 ), p t2,i (x t2,i , y t2,i ). The marked distance between two points p p t1,1 (x t1,1 , y t1,1 ), p t1,i (x t1,i , y t1,i ). The distance between two points p The molecular marker flow diffusion between the poles is relatively uniform, and there is the following corresponding relationship:

[0064]

[0065] According to equations (4) - (5), p t1,i (x t1,i , y t1,i ). After moving, the coordinates are p t2,i (x t2,i , y t2,i ). Furthermore, the coordinates of any point before and after the movement between any two poles within the marked flow field can be obtained. The corresponding displacement s is calculated using the following formula:

[0066]

[0067] At the same time, since the time interval of the pulse train provided by the synchronization system is a fixed value Δt, the time difference corresponding to this displacement can be obtained:

[0068] t2 - t1 = Δt (7)

[0069] The velocity is obtained as

[0070]

[0071] Generalize to all moments corresponding to the pulse trigger of the synchronization system to obtain

[0072] t j+1 -t j = Δt (9)

[0073] Generalize to displacement, then there is

[0074]

[0075] Then, for any point i on the molecular marker line, the absolute value of the velocity from time j to j + 1 is

[0076]

[0077] At the same time, knowing the starting point of the marker and the end position after movement can also obtain the velocity direction

[0078]

[0079] It also includes Step 3: According to the velocity field information extracted in Step 2, derive the acceleration according to Equation (13), and then obtain the acceleration field information

[0080]

[0081] Such as Figure 1 As shown, a velocity measurement system based on single-frame multiple exposures disclosed in this embodiment is used to implement a velocity measurement method based on single-frame multiple exposures. A velocity measurement system based on single-frame multiple exposures includes a femtosecond laser light source, a focusing system, a synchronization control device, and an image intensifier camera

[0082] The femtosecond laser is used to generate high-energy femtosecond laser pulses, induce nitrogen molecules to emit light, and realize marked velocity measurement in the flow field based on long-lived nitrogen fluorescence molecular markers. The single-frame multiple-exposure imaging of the image intensifier camera records the equal-time interval changes of the molecular markers in the flow field moving with the flow field over time. According to the molecular markers, the morphological positions after the moments are determined, and the information extraction of the molecular markers in the plane is realized through Equations (11) - (12), and then the velocity field information is obtained. The synchronization control device controls the precise synchronization of the wind tunnel operation time, the laser pulse incidence time, and the camera shutter opening time, and preset a time difference to generate continuous pulses to trigger the image intensifier camera, ensuring that the time difference between the image acquisition time and the laser incidence time is stable enough. The image intensifier camera is used to obtain single-frame multiple-exposure image information

[0083] The specific description above further elaborates on the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above is only a specific embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A speed measurement method based on single-frame multiple exposures, characterized in that: The measurement steps include: Step 1: Through multiple exposures of a single frame of the camera, the spatial position information of the same molecular marker continuously recorded N times at equal time intervals is obtained on a single picture; Step 2: Based on the velocity image obtained in step 1 and the deformation and corresponding displacement analysis of the molecular marker, the velocity field information is extracted by mapping the marker before and after displacement, thereby realizing velocity measurement based on single-frame multiple exposures; Establish coordinates in the flow field, and fit the shape of the displacement of the marker line at each moment to get the fitting function: y t1 =f t1 (x) (1) y t2 =f t2 (x) (2) y t3 =f t3 (x) (3) Among them: each function corresponds to a marking line at a moment, and each marking line is derived to obtain the extreme points on each marking line, and group them according to order and category, and y t1 =f t1 The first point p in (x) t1，1 (x t1,1 ,y t1,1 ), y t2 =f t2 The first extreme point p in (x) t2,1 (x t2,1 ,y t2,1 ), y t3 =f t3 The first extreme point p in (x) t3,1 (x t3,1 ,y t3,1 ) ... and so on, the first extreme point is divided into a group and fitted to obtain the equation G1 (x, y), which corresponds to the flow state of this part of the flow field; the flow state in a similar area of ​​the flow field is obtained by analogy, as shown in Figure 3 (b); G1 (x, y) is solved together with the initial state function of the flow field to obtain the initial position corresponding to each extreme point, y t2 =f t2 (x) The first extreme point p t1,1 (x t1,1 ,y t1,1 ) The position at the next moment is p t2,1 (x t2,1 ,y t2,1 );y t2 =f t2 The second extreme point p in (x) t1,2 (x t1,2 ,y t1,2 ) is the next moment position p t2,2 (x t2,2 ,y t2,2 ); then the last time point can be set, and the Nth time point is t n ; The velocity at any point between the two poles is calculated as follows: Define the target coordinates at time t1 as p t1,i (x t1,i ,y t1,i ); Define any point between the two poles after the movement as p t2,i (x t2,i ,y t2,i ); the length of the line between the two points is s; the calculation formula of the line length l is as follows: Then p t2,1 (x t2,1 ,y t2,1 ), p t2,2 (x t2,2 ,y t2,2 ) The length of the marking line between the two points is p t1,1 (x t1,1 ,y t1,1 ), p t1,2 (x t1,2 ,y t1,2 ) The distance between the two points is p t2,1 (x t2,1 ,y t2,1 ), p t2,i (x t2,i ,y t2,i ) The distance between the two marks is p t1,1 (x t1,1 ,y t1,1 ), p t1,i (x t1,i ,y t1,i ) The distance between the two points is The molecular marker flow and diffusion between the poles are relatively uniform, and the corresponding relationship is as follows: According to formula (4) to (5), p t1,i (x t1,i ,y t1,i ) The coordinate after moving is p t2,i (x t2,i ,y t2,i ), and then the coordinates of any point between any two poles in the marked flow field before and after the movement can be obtained; the corresponding displacement s is expressed by the following formula: According to the pulse train time interval provided by the synchronization system, which is a fixed value △t, the time difference corresponding to the displacement can be obtained: t2-t1=△t (7) The speed is Extended to all the moments corresponding to the pulse triggering of the synchronous system, we get t j+1 -t j =△t (9) Extending to displacement, we have Then the absolute value of the velocity of any point i on the molecular marker line from time j to j+1 is: At the same time, the starting point and the end point of the marker after the move are known, and the speed direction can also be obtained:

2. The speed measurement method based on single-frame multiple exposures according to claim 1, characterized in that: The method further includes step 3, according to the velocity field information extracted in step 2, deriving the acceleration according to formula (13), and then obtaining the acceleration field information; 3. A speed measurement system based on single-frame multiple exposures, used to implement a speed measurement method based on single-frame multiple exposures as claimed in claim 1 or 2, characterized in that: It includes a femtosecond laser light source, a focusing system, a synchronization control device, and an image intensification camera; The femtosecond laser is used to generate high-energy femtosecond laser pulses to induce nitrogen molecules to emit light, and based on the long-life nitrogen fluorescent molecular markers, the marker velocity measurement in the flow field is realized; the molecular markers in the flow field are recorded by single-frame multiple exposure imaging of the image intensification camera at equal time intervals as they move along the flow field, and the morphological position after the time interval is determined according to the molecular markers. The information of the molecular markers in the plane is extracted through equations (11) to (12) to obtain the velocity field information; the synchronous control device controls the precise synchronization of the wind tunnel operation time with the laser pulse incident time and the camera shutter opening time, and presets the time difference to generate continuous pulses to trigger the image intensification camera, ensuring that the time difference between the image acquisition time and the laser incident time is sufficiently stable; the image intensification camera is used to obtain single-frame multiple exposure image information.

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