CN110864870B - Method for realizing flow field state observation in closed cavity by using grid silk thread and feedback tracking algorithm - Google Patents
Method for realizing flow field state observation in closed cavity by using grid silk thread and feedback tracking algorithm Download PDFInfo
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Abstract
The invention discloses a method for realizing the observation of a flow field state in a closed cavity by utilizing a grid silk thread and a feedback tracking algorithm, which comprises the steps of selecting silk threads with proper materials, lengths and thicknesses, and fixing the silk threads on the surface of a measured object according to a designed network; preparing positive electron nuclide with certain activity, marking the positive electron nuclide on a carrier solvent, and uniformly coating the positive electron nuclide on a silk thread; fixing a measured object in the closed cavity, and installing the gamma photon detector around the closed cavity, so as to ensure that the measured object is positioned in the effective visual field of the gamma photon detector and ensure that the silk thread can really receive airflow; the air flow flows through the object to be measured, the silk thread on the object to be measured falls down along with the air flow, and the gamma photon detector records the gamma photon coincidence event data; and processing the gamma photon coincidence event data by using a feedback tracking algorithm to obtain a three-dimensional image of the flow field state in the closed cavity. The invention can provide a reliable non-contact flow state observation method for wind tunnel experiments and the like, and has important significance for the development of aircrafts.
Description
Technical Field
The invention belongs to the field of flow field detection, and particularly relates to a method for observing the state of an airflow flow field in a closed cavity by using a grid silk thread and a feedback tracking algorithm.
Background
Fluid mechanics has evolved in the human fight against nature and in production practice. The flow field is an important component in fluid mechanics, is mainly used for qualitatively observing the flow form of a surface or a space, thereby realizing the fine measurement of flow field information, has important significance for the analysis of the flow mechanism in the fields of current aerospace, internal combustion engines, combustion, chemical engineering, atmospheric physics and the like, and is an important means for researching the fluid mechanics. The current commonly used methods for observing the flow field are divided into tracing methods and optical methods, wherein the tracing methods comprise a silk thread method, an oil flow method, a sublimation method, a smoke flow method, a steam screen method and the like, and the optical methods comprise a shadow meter, a schlieren meter, an interferometer, a holographic device and the like, but the methods all have a common defect that the detection cannot be carried out in a closed pipeline with a harsh opaque internal state.
Positron annihilation is a three-dimensional imaging technique that utilizes annihilation of positrons and electrons generated by decay of nuclides to obtain gamma photon pairs and detects the gamma photon pairs. Because gamma photons are extremely strong in penetrability and only attenuate 10% for aluminum alloy with the thickness of 5mm, the image inside the opaque container can be acquired by using positron annihilation technology. With the rapid development of mathematical physics and modern computer technology, positron annihilation technology is applied in more and more industrial fields, such as emission computed tomography (PET) in the medical field, research on the internal microstructure of materials in the material field, nondestructive detection in the industrial field, and the like.
With the progress of science and technology and the development of economy, the demand of high-performance fluid machinery in many fields (particularly petrochemical industry, aviation and the like) is more and more urgent. In order to design a high-performance fluid machine, the traditional design method cannot meet the requirement, and modern design theory and method must be adopted. For example, disturbance of an object model in a wind tunnel to airflow is observed, a traditional method is that silk threads are pasted on the surface of an object, then a high-speed CCD camera is used for shooting and observing, but when the pressure in the wind tunnel is too high, the glass tube of a wind tunnel test section is in danger of being broken, and at the moment, the high-speed CCD camera cannot be used for shooting from the test section; for example, when an airflow field on the inner wall surface of a fluid machine such as a fan or an engine is observed, the state of the yarn inside the fluid machine cannot be observed simply by using a yarn method.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the defects of the prior art, the invention provides a method for realizing the observation of the state of a flow field in a closed cavity by using grid silk threads and a feedback tracking algorithm, which improves a silk thread method by using a positron annihilation technology and can realize the observation of the state of the flow field in the interior of an object in an opaque cavity such as a wind tunnel.
The technical scheme is as follows: in order to realize the purpose, the invention adopts the following technical scheme:
a method for realizing the observation of the flow field state in a closed cavity by using a grid silk thread and a feedback tracking algorithm comprises the following steps:
(1) selecting silk threads with proper materials, lengths and thicknesses, and fixing the silk threads on the surface of a measured object according to a designed network;
(2) preparing a positron nuclide with certain activity, marking the positron nuclide on a carrier solvent, and uniformly coating the positron nuclide on a silk thread;
(3) fixing a measured object in the closed cavity, and installing the gamma photon detector around the closed cavity, so as to ensure that the measured object is positioned in the effective visual field of the gamma photon detector and ensure that the silk thread can really receive the flowing of the airflow;
(4) the airflow flows through the object to be measured, the silk threads on the surface of the object to be measured are laid down along with the airflow, and the gamma photon detector records gamma photon coincidence event data;
(5) and processing the gamma photon coincidence event data recorded by the gamma photon detector by using a feedback tracking algorithm to obtain a three-dimensional image of the surface flow field state of the measured object, namely the three-dimensional image of the flow field state in the closed cavity.
Furthermore, in the step (1), a grid is defined on the surface of the measured object, and the size of the grid is determined by the volume of the measured object, preferably 10-20 mm; selecting silk threads with the length smaller than the length of the grid and the diameter smaller than 0.1mm, softening and antistatic the silk threads, fixing one end of the silk threads at the intersection of the grid, and fixing the other end of the silk threads as a free end without fixation.
Further, in the step (1), glue is used to directly stick one end of the silk thread to the intersection of the surface grid lines of the object to be measured, or small holes are drilled on the intersection of the surface grid lines of the object to be measured, and then the silk thread is passed through the small holes and fixed by fixing pins.
Further, the positron nuclide activity in the step (2) is determined according to the density of the surface threads of the measured object, and the nuclide activity on the threads of the measured object in unit volume is not less than 2.5 mu Ci/mm3。
Further, in the step (3), the measured object with the silk thread fixed on the surface is fixed in the sealed cavity, then the gamma photon detector is arranged at the periphery of the sealed cavity, the measured object is ensured to be positioned in the effective visual field of the gamma photon detector, and meanwhile, the silk thread is ensured to be capable of really receiving the airflow in the sealed cavity.
Further, in the step (4), the airflow flows through the object to be measured, the silk threads on the surface of the object to be measured are laid down along with the airflow, the laying direction of the silk threads is consistent with the direction of the airflow, and a transient and stable laying state is maintained along with the flow of the airflow, at the moment, positrons on the silk threads collide with surrounding electrons and annihilate to generate a pair of gamma photons with the energy of 511KeV and the directions of 180 degrees, and the gamma photon detector records the gamma photon information meeting the judgment standard to form a count, so that the gamma photon coincidence event data is recorded.
Further, the step (5) of processing the gamma photon coincidence event data by using a feedback tracking algorithm to obtain a three-dimensional image comprises the following steps:
(51) according to gamma photon coincidence event data recorded by a gamma photon detector, positioning the occurrence position of the gamma photon coincidence event;
(52) carrying out linking and smoothing treatment on the gamma photon coincidence event occurrence positions in each axial slice frame to obtain an original silk thread three-dimensional image;
(53) and taking the grid design parameters and the design length of the silk thread as correction data for image processing, and correcting the original silk thread three-dimensional image to obtain a processed silk thread three-dimensional image, so as to obtain a three-dimensional image of the surface flow field state of the object to be measured, namely the three-dimensional image of the flow field state in the closed cavity.
Further, the step (51) is specifically: firstly, dividing data in each axial slice frame into K clusters by using a K-means clustering algorithm, wherein the center of each cluster is the position where a gamma photon coincidence event occurs, and the value of K is determined by the number of grid intersections.
Further, the step (52) is specifically: linking the position data by using a method of tracking and searching frame by frame and reducing the distance sum of gamma photon coincidence events between adjacent frames; the gamma photon detector needs a certain time for recording gamma photon coincidence event data, so that the time dimension in the gamma photon coincidence event data is not continuous data, the central position of the gamma photon coincidence event in the same track obtained by processing is also discrete data, and sawtooth-shaped jump can occur during drawing of the whole track.
Further, the step (53) is specifically: and correcting the obtained original silk thread three-dimensional image by using the initial positioning point of the silk thread at the grid intersection and the length of the silk thread, eliminating redundant bifurcation points, dividing the breakpoint and the extension line, and finally obtaining an accurate silk thread track, namely a processed silk thread three-dimensional image, so as to obtain a three-dimensional image of the surface flow field state of the measured object, namely the three-dimensional image of the flow field state in the closed cavity.
Has the advantages that: compared with the prior art, the invention has the following advantages:
(1) and observing the surface flow state of the object in the sealed opaque cavity. The traditional silk thread method utilizes a CCD camera to record the position and the direction of a silk thread for observing the flow state, and the precondition is that a transparent area for observing the silk thread must exist, thereby causing the limitation and the inconvenience of the use of the silk thread method; the invention improves the silk thread method by utilizing the strong penetrability of gamma photons, so that the flow state observation in opaque cavities such as the surface of a model to be measured in a wind tunnel, the internal wall surface of a fluid machine and the like becomes possible, and the application range of the silk thread method is greatly expanded.
(2) And (5) three-dimensional observation. The traditional silk thread method uses a CCD camera to record the state of the silk thread, but pictures shot by the CCD camera can only observe the flow state from a two-dimensional angle, so that much information is lost and the pictures are not intuitive enough; by using the improved wire method, as long as the measured object is in the gamma photon detector, a three-dimensional image of the shape and the position of the wire can be restored by using a Feedback and Tracking (FT) algorithm according to the time information and the position information of the data detected by the detector, and the airflow flow state on the surface of the measured object can be observed more intuitively.
Drawings
FIG. 1 is a schematic flow chart of the observation method of the present invention;
FIG. 2 is a schematic view of an air intake model; wherein (a) is a top view and (b) is a front view;
FIG. 3 is a thread pasting diagram of an air inlet model in an embodiment of the invention; wherein (a) is a top plate thread pasting picture, and (b) is a side plate thread pasting picture;
FIG. 4 is a schematic diagram of a positron annihilation event;
FIG. 5 is a schematic structural diagram of a simple measuring device according to an embodiment of the present invention; wherein (a) is a perspective view; (b) is a front view;
FIG. 6 is a schematic view of a small low-speed wind tunnel;
FIG. 7 is a flow chart of three-dimensional image restoration of a filament;
in the figure: 1-electron shell, 2-nucleus, 3-annihilation site, 4-electron, 5-gamma photon pair, 6-positron; 7-small low-speed wind tunnel, 8-gamma photon detector; 9-a honeycomb device, 10-a double-layer gauze, 11-a measured object, 12-a stable section, 13-a contraction section, 14-a test section, 15-a diffusion section and 16-a fan; 17-top plate, 18-side plate, 19-lip.
Detailed Description
The technical solution and the advantages of the present invention will be described in detail with reference to the accompanying drawings.
When the measured object is positioned in the closed cavity, at the moment, the surface silk thread of the measured object is positioned in the flow field in the closed cavity, and the CCD camera cannot be used for observation, so the technical scheme combining the grid silk thread and the feedback tracking algorithm provided by the invention can realize non-contact observation of the surface airflow flow field of the object in the closed opaque cavity such as a wind tunnel, and has important significance for the development of aircrafts.
As shown in fig. 1, a method for observing a flow field state in a sealed cavity by using a mesh silk thread and a feedback tracking algorithm includes the following steps:
a grid is defined on the surface of a measured object, one end of a silk thread is fixed at the intersection of the grid, and the other end of the silk thread is a free end and is not fixed; the length of the grid is determined according to the volume of the object to be measured, preferably 10-20 mm. Selecting silk threads with proper length, thickness and material, and softening and antistatic treatment are carried out on the silk threads. The silk thread can adopt metal platinum silk also can adopt steel wire, blue and green copper wire or other filamentous materials that have toughness to can accurately reflect the state of flow field after handling. The diameters of the silk threads can be different according to different air volumes of air flows in the flow field, but the diameters of the silk threads are guaranteed to be smaller than 0.1mm, the purpose is to reduce the influence of the silk threads on the air flows on the premise of guaranteeing toughness, and the lengths of the silk threads are smaller than the lengths of the grids, so that the silk threads are prevented from being wound, overlapped and the like under the action of the air flows, which are not beneficial to observing the state of the flow field. Some wires are hard after being cut short because the elastic modulus of the material is large, so that the wires need to be softened to have better lodging property along with airflow.
The fixing mode of the silk thread is divided into two types: one is to use glue to stick one end of the silk thread on the cross point of the grid line on the surface of the measured object, the other is to drill small holes on the cross point of the grid line on the surface of the measured object, then the silk thread is passed through the small holes and fixed by fixing pins.
In the embodiment of the invention, the measured object is an air inlet channel model, and the specific structure is shown in fig. 2, wherein (a) is a top view and (b) is a front view; in the figure, 17 is a top plate, 18 is a side plate, and 19 is a lip; therefore, comprehensively considering, the grid lines with the side length of 12mm are defined on the surface of the air inlet channel model, the wires are 0.1mm of platinum wires with the length of 10mm, and the fixing is performed by using fixing pins, and the fixing effect is shown in fig. 3, wherein (a) is a top plate wire sticking diagram, and (b) is a side plate wire sticking diagram.
different carrier solvents are selected according to different silk thread materials, and the carrier solvents which are easy to be smeared on the silk threads are generally selected according to the materials of the silk threads. Thoroughly mixing a carrier solvent with the positively charged electron nuclide with activity; and then fully soaking the silk thread in the silk thread to obtain the silk thread marked by the nuclide, wherein the positive electronic nuclide is prepared by a medical cyclotron.
The species of the positive electronic nuclide can be selected according to specific conditions, the activity of the positive electronic nuclide is determined according to the density of the surface threads of the object to be measured, and the nuclide activity on the threads of the unit volume of the object to be measured is not less than 2.5 mu Ci/mm3。
In the embodiment, a metal platinum wire is selected as a wire fixed on the surface of a measured object, a carrier solvent is a heat-resistant quick-drying solvent, a positive electron nuclide is marked on the quick-drying solvent and then is uniformly coated on the metal platinum wire, and then the metal platinum wire is kept stand for a short time to ensure that the solvent on the surface of the metal platinum wire is dried.
As shown in fig. 4, the process of positron annihilation is described, in which 1 is an electron shell, 2 is an atomic nucleus, 3 is an annihilation site, 4 is an electron, 5 is a gamma photon pair, and 6 is a positron; beta generation of nuclides+Decay generates positrons, the positrons are difficult to independently exist in the nature for a long time as an anti-substance, and the survival life of the positrons on the earth is only 10 in the natural state-12In about seconds, annihilation reaction occurs with nearby electrons. According to the einstein mass-energy conversion equation: e ═ mc2It is known that the mass of the positron and the electron in the annihilation process all converts energy, i.e., releases a pair of gamma photon pairs with 180 ° and 511KeV energy, which reflects the position information of the annihilation point.
Fixing the measured object in the closed cavity, reasonably installing the gamma photon detector around the closed cavity, ensuring that the measured object is positioned in the effective visual field of the gamma photon detector, and ensuring that the silk thread can really receive the flowing of the airflow;
as shown in fig. 5, wherein (a) is a perspective view; (b) is a front view; in the embodiment, the closed cavity is a small low-speed wind tunnel, and the measured object is an air inlet channel model to measure the state of the flow field in the wind tunnel. The air inlet channel model with the surface fixed with the metal platinum wire is fixed in the small-sized low-speed wind tunnel, then the gamma photon detector 8 is arranged on the periphery of the small-sized low-speed wind tunnel 7, the air inlet channel model is ensured to be positioned in the effective visual field of the gamma photon detector, and meanwhile, the metal platinum wire is ensured to be capable of really receiving stable and uniform linear airflow provided by the small-sized low-speed wind tunnel.
As shown in fig. 6, the small low-speed wind tunnel is divided into four sections: a stabilizing section 12, a contracting section 13, a testing section 14 and a diverging section 15. The stable section is a constant-diameter pipeline which enables the airflow to be kept uniform and stable, so that a honeycomb device 9 and a double-layer gauze 10 are arranged in the stable section, the airflow is mainly straightened, large vortexes are broken, and the turbulence and the unevenness of the airflow are reduced; the contraction section is a contraction pipeline which enables the airflow to be accelerated uniformly, and when the airflow flows along the contraction section, the phenomenon of separation on the wall of the hole is ensured not to occur; placing an air inlet channel model to be tested (namely a tested object 11) in the test section; the diffuser section is fitted with a fan 16 which blows air to the right end to cause air to enter the stabilising section of the tunnel from the environment at the left end. During testing, the two ends of the wind tunnel are kept smooth, the fan starts blowing air, and the silk threads on the air inlet channel model are laid down along with the airflow field.
The gamma photon detector has various types, and can be selected according to actual measurement requirements, and a flat plate type and a circular ring type are commonly used. In the embodiment, the gamma photon detector is in a ring shape, and 24 gamma photon detectors surround the gamma photon detector to form a ring shape and are used for receiving gamma photon coincidence event data. The annular gamma photon detector is sleeved on the periphery of the small-sized low-speed wind tunnel test section.
when the air flow passes through the object to be measured(i.e. the air inlet channel model), the surface-fixed silk threads can be fallen along with the air flow, the falling direction of the silk threads is consistent with the direction of the air flow, and the silk threads can maintain a transient and relatively stable fluttering state along with the flow of the air flow. In the process, the nuclide marked on the silk thread generates beta+Positron generated by decay continuously generates annihilation reaction with nearby electrons, and emits gamma photon pairs forming 180 degrees to the periphery, the position of the annihilation reaction continuously changes along with the lodging of the silk thread, and meanwhile, a gamma photon detector records gamma photon information meeting the judgment standard to form a count, so that gamma photon coincidence event data capable of reflecting the occurrence time and position of the annihilation reaction is recorded.
And 5, processing the gamma photon coincidence event data recorded by the gamma photon detector by using a Feedback Tracking (FT) algorithm to obtain a three-dimensional image of the surface flow field state of the measured object, namely the three-dimensional image of the flow field state in the closed cavity.
The flow field state is a dynamic object, and the wires have short stabilization time along with the airflow lodging, so that rapid imaging is required. The gamma photon data collected in a short time are subjected to image reconstruction, the quality of the obtained image is poor, the contrast is weak, and the problems of noise, difficulty in positioning of a starting point and an end point, disconnection, bifurcation, blurring and the like of the silk image can occur. The invention adopts a Feedback Tracking (FT) algorithm to process gamma photon coincidence event data to obtain the occurrence position of the gamma photon coincidence event in each axial slice frame, and carries out linking and smoothing processing on the gamma photon coincidence event data to obtain an original silk thread three-dimensional image, then uses grid design parameters and the design length of the silk thread as correction data to determine the optimal track of the silk thread in the image, eliminates redundant bifurcation points, bifurcation points and extension lines, finally obtains an accurate silk thread track by a vector tracking algorithm on a fuzzy part of the line segment, and further obtains a three-dimensional image of the surface flow field state of a measured object, namely the three-dimensional image of the flow field state in a closed cavity.
As shown in fig. 7, the method specifically includes the following steps:
(51) positioning the occurrence position of the gamma photon coincidence event according to the gamma photon coincidence event data recorded by the gamma photon detector;
firstly, dividing data in each axial slice frame into K clusters by using a K-means clustering algorithm, wherein the center of each cluster is the position where a gamma photon coincidence event occurs, and the value of K is determined by the number of grid intersections.
(52) Performing linking and smoothing processing on the silk thread motion track image to obtain an original silk thread three-dimensional image;
and (3) linking the motion tracks of the silk threads by utilizing a Kuhn-Munkres (KM) algorithm through a method of tracking and searching frame by frame and reducing the sum of the distance of the coincident events between adjacent frames. The gamma photon detector needs a certain time for recording gamma photon coincidence event data, so that the time dimension in the gamma photon coincidence event data is not continuous data, the position of the gamma photon coincidence event in the same track obtained by using a KM algorithm is also discrete data, and sawtooth jumping occurs during drawing of the whole track.
(53) Correcting the original silk thread three-dimensional image to obtain a processed silk thread three-dimensional image, and further obtaining a three-dimensional image of the surface flow field state of the object to be measured, namely the three-dimensional image of the flow field state in the closed cavity;
and correcting the obtained original silk thread three-dimensional image by using the grid design parameters and the design length of the silk thread, eliminating redundant bifurcation points, dividing points and extension lines, and finally obtaining an accurate silk thread track, namely the processed silk thread three-dimensional image, so as to obtain a three-dimensional image of the surface (namely the closed cavity) of the measured object in the flow field state.
The mesh intersection point position is known to be determined information when the mesh is designed, and the silk thread is fixed at the mesh intersection point, so that the initial position of the silk thread is the position of the mesh intersection point, and if the initial position of the silk thread in the image is not overlapped with the mesh intersection point position, the initial position of the silk thread can be corrected by using the mesh intersection point position information.
In addition, the length of the silk thread is also known information after being selected, and if the length of the silk thread in the image is not equal to the selected length, the length information of the silk thread can be used for correcting the length information; the silk thread is a continuous smooth line, and correction such as smoothing, removing, supplementing and the like is carried out on a bifurcation point and a disjunction point in the image according to the continuous smooth line;
to sum up, the silk threads of the embodiment of the invention are positioned in a small low-speed wind tunnel, and cannot be observed by using a CCD camera, but the three-dimensional state of the silk threads can be observed by using the technical scheme of combining the grid silk threads and the feedback tracking algorithm, which shows that the silk threads can be applied to the observation of the flow state in any opaque closed cavity, such as wind tunnel experiments, the internal wall surface of fluid machinery and the like.
Claims (9)
1. A method for realizing the observation of the flow field state in a closed cavity by using a grid silk thread and a feedback tracking algorithm is characterized by comprising the following steps:
(1) selecting silk threads with proper materials, lengths and thicknesses, and fixing the silk threads on the surface of a measured object according to a designed network;
(2) preparing a positron nuclide with certain activity, marking the positron nuclide on a carrier solvent, and uniformly coating the positron nuclide on a silk thread;
(3) fixing a measured object in the closed cavity, and installing the gamma photon detector around the closed cavity, so as to ensure that the measured object is positioned in the effective visual field of the gamma photon detector and ensure that the silk thread can really receive the flowing of the airflow;
(4) the airflow flows through the object to be measured, the silk threads on the surface of the object to be measured are laid down along with the airflow, and the gamma photon detector records gamma photon coincidence event data;
(5) processing gamma photon coincidence event data recorded by the gamma photon detector by using a feedback tracking algorithm to obtain a three-dimensional image of the surface flow field state of the measured object, namely the three-dimensional image of the flow field state in the closed cavity;
the method for processing gamma photon coincidence event data by using a feedback tracking algorithm to obtain a three-dimensional image comprises the following steps:
(51) according to gamma photon coincidence event data recorded by a gamma photon detector, positioning the occurrence position of the gamma photon coincidence event;
(52) carrying out linking and smoothing treatment on the gamma photon coincidence event occurrence positions in each axial slice frame to obtain an original silk thread three-dimensional image;
(53) and taking the grid design parameters and the design length of the silk thread as correction data for image processing, and correcting the original silk thread three-dimensional image to obtain a processed silk thread three-dimensional image, so as to obtain a three-dimensional image of the surface flow field state of the object to be measured, namely the three-dimensional image of the flow field state in the closed cavity.
2. The method for realizing the observation of the flow field state in the closed cavity by using the grid silk thread and the feedback tracking algorithm as claimed in claim 1, wherein the grid is defined on the surface of the measured object in the step (1), and the size of the grid is determined by the volume of the measured object and is 10-20 mm; selecting silk threads with the length smaller than the length of the grid and the diameter smaller than 0.1mm, softening and antistatic the silk threads, fixing one end of the silk threads at the intersection of the grid, and fixing the other end of the silk threads as a free end without fixation.
3. The method for observing the state of the flow field in the closed cavity by using the grid wires and the feedback tracking algorithm as claimed in claim 1, wherein glue is used in the step (1) to directly stick one end of each wire to the intersection of the grid lines on the surface of the object to be measured, or small holes are drilled in the intersection of the grid lines on the surface of the object to be measured, and then the wires are passed through the small holes and fixed by fixing pins.
4. The method for observing the state of the flow field in the closed cavity by using the mesh wires and the feedback tracking algorithm as claimed in claim 1, wherein the activity of the positron nuclide in the step (2) is determined according to the density of the wires on the surface of the object to be measured, and the nuclide activity on the wires per unit volume of the object to be measured is not less than 2.5 μ Ci/mm3。
5. The method for observing the flow field state in the closed cavity by using the grid silk thread and the feedback tracking algorithm as claimed in claim 1, wherein the measured object with the silk thread fixed on the surface is fixed in the closed cavity in the step (3), and then the gamma photon detector is arranged at the periphery of the closed cavity, so as to ensure that the measured object is positioned in the effective visual field of the gamma photon detector and ensure that the silk thread can really receive the air flow in the closed cavity.
6. The method for observing the state of the flow field in the sealed cavity by using the grid wires and the feedback tracking algorithm according to claim 1, wherein the airflow flows through the object to be measured in the step (4), the wires on the surface of the object to be measured fall along with the airflow, the falling direction of the wires is consistent with the direction of the airflow, the wires keep a transient stable falling state along with the flow of the airflow, at the moment, positrons on the wires collide with surrounding electrons and annihilate to generate a pair of gamma photons with energy of 511KeV and directions of 180 degrees, and the gamma photon detector records the gamma photon information meeting the judgment standard to form a count, so that the data of a gamma photon meeting the event is recorded.
7. The method for observing the state of the flow field in the closed cavity by using the mesh wire and the feedback tracking algorithm according to claim 1, wherein the step (51) is specifically as follows: firstly, dividing data in each axial slice frame into K clusters by using a K-means clustering algorithm, wherein the center of each cluster is the position where a gamma photon coincidence event occurs, and the value of K is determined by the number of grid intersections.
8. The method for observing the state of the flow field in the closed cavity by using the mesh wire and the feedback tracking algorithm according to claim 1, wherein the step (52) is specifically as follows: linking the position data by using a method of tracking and searching frame by frame and reducing the distance sum of gamma photon coincidence events between adjacent frames; the gamma photon detector needs a certain time for recording gamma photon coincidence event data, so that the time dimension in the gamma photon coincidence event data is not continuous data, the central position of the gamma photon coincidence event in the same track obtained by processing is also discrete data, and sawtooth-shaped jump can occur during drawing of the whole track.
9. The method for observing the flow field state in the closed cavity by using the grid wire and feedback tracking algorithm as claimed in claim 1, wherein the step (53) is specifically as follows: and correcting the obtained original silk thread three-dimensional image by using the initial positioning point of the silk thread at the grid intersection and the length of the silk thread, eliminating redundant bifurcation points, dividing the breakpoint and the extension line, and finally obtaining an accurate silk thread track, namely a processed silk thread three-dimensional image, so as to obtain a three-dimensional image of the surface flow field state of the measured object, namely the three-dimensional image of the flow field state in the closed cavity.
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