CN113588208A - Liutex integral quantitative flow field measurement method based on image method - Google Patents
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Abstract
The invention relates to a Liutex integral quantitative flow field measurement method based on an image method, which realizes Liutex integral calculation by experimental data obtained by measurement based on the image method, wherein the integral absolute quantity and the integral area are respectively used for representing the rotation intensity and the size of a vortex core of a vortex structure in a flow field. The method can quantitatively measure the vortex rotation strength and the size of the vortex core, and researches the motion and evolution process of each vortex structure in the flow field by a more efficient and accurate means.
Description
Technical Field
The invention relates to a flow field measurement technology, in particular to a Liutex integral quantitative flow field measurement method based on an image method.
Background
The flow field measurement technology is widely applied to the fields of aerospace, ships, engines and the like. The representative and widely applied flow field measurement technologies mainly include a pitot tube technology, a hot wire wind speed, a particle track velocity measurement technology, a particle image velocity measurement technology and a laser doppler velocity measurement technology. The single-frame long exposure image method based on the particle track velocity measurement technology has the advantages of wide applicability, accurate measurement, high speed, no contact and the like. However, there are many methods for capturing and acquiring information of important structures in a flow field, and at present, many methods such as fluid motion particle swarm trajectories, average velocity, instantaneous velocity field, acceleration, velocity gradient tensor, vorticity field, invariant based on velocity gradient tensor, and the like are used.
Although many scholars realize the display and capture of important structures in the flow field through scalar, vector and tensor fields based on the Euler idea or tracks, particle swarms and the like based on the Lagrange idea, or research the time-space development process of the flow field based on various display methods and combination forms, most scholars are based on qualitative or statistical research. For example, from a velocity field, the location of vortices can be estimated by velocity shear, i.e. upward-blowing downward-sweeping; the vortex region can also be predicted to some extent from the pressure field or the density field; most researches judge the area of vortex generation and the strength of rotation through a vorticity field. However, since the vorticity cannot distinguish between the rotation amount and the asymmetric shear amount, the vorticity region may be misjudged in some cases, and it is difficult to distinguish the vortex core boundary. The Liutex method is proposed by the Liu super group project group at the university of Arlington, Tex, USA, and can accurately describe the rigid rotation strength and the rotation axis direction of the local area fluid. Liutex is a vector, calculated from the velocity field, whose direction is defined as the velocity gradient tensor
Feature vector of
The size is defined as
(
Is the vector of vorticity, λciThe imaginary part of the conjugate complex eigenvalue of the velocity gradient tensor) (Liu C, Gao Y, Tian S,&Dong X.Rortex a new vortex vector definition and vorticity tensor and vector decompositions.Physics of Fluids,2018,30(3):035103.)，(Wang Y,Gao Y,Liu J&Liu C.Explicit formula for the Liutex vector and physical meaning of vorticity based on the Liutex-shear decomposition.Journal of Hydrodynamics,2019,31(3):464-474)。
the method for the quantitative measurement of the Liutex based on the turbulent boundary layer numerical simulation data with high-order precision is provided by the Liutex, the calculation of the Liutex is based on a velocity gradient field, and if the Liutex is applied to a flow field measurement experiment, the accuracy requirement on data measured by the experiment is extremely high, and a Liutex quantitative measurement method with higher robustness needs to be found.
Disclosure of Invention
Aiming at the ubiquitous problem of characterization of quantification of vortexes in flow field measurement, the method for measuring the flow field by the aid of the Liutex integral quantification based on the image method is provided, experimental data obtained by measurement based on the image method is subjected to Liutex integral calculation, and the integral absolute quantity and the integral area of the Liutex integral quantification are respectively used for characterizing the rotation intensity and the size of a vortex core of a vortex structure in the flow field.
The technical scheme of the invention is as follows: a Liutex integral quantitative flow field measurement method based on an image method specifically comprises the following steps:
1) acquiring flow field image information by adopting a measurement system through an experiment, and acquiring a velocity field after processing the flow field image information by a non-contact image method;
2) according to the Liutex vector
By calculation of the velocity field obtained in step 1)
Wherein R is the local rigid rotation strength of the space point,
the direction of the local rigid rotating shaft;
3) volume Liutex is proposed: rIntIt is defined as follows:
for three-dimensional experimental data: rInt＝∫ΔVRdV, the rotation intensity of the vortex is determined by the integral value R of R at each space point in the vortex coreIntIs represented by, wherein Δ V is represented by R>A volume area of 0 indicating the size of the vortex core volume surrounded by the vortex boundary;
for two-dimensional experimental data: rInt＝∫ΔSRdS, the intensity of vortex rotation is determined by the integral value R of R at each point in the vortex coreIntIs represented by, wherein Δ S is represented by R>An area region of 0 indicating the size of the area of the vortex core surrounded by the vortex boundary;
4) r is to beIntAnd delta V or delta S, recording the rotation intensity of the vortex and the change of the volume or area of the vortex core along with time, and finishing the quantitative measurement of the rotation intensity of the vortex and the size of the vortex core.
Further, the flow field encompasses flow fields within the categories of laminar and turbulent flow, internal and external flow, low reynolds number and high reynolds number, subsonic and supersonic velocity, two-dimensional or three-dimensional.
Further, the flow field is a structure of the evolution of the flow with different scales along with time-space, and comprises a laminar or turbulent boundary layer, a jet flow, a step flow structure, a pseudo-sequence structure and a strip structure which appear in the process of the internal flow or the bypass flow of the pipeline flow and the gap flow.
Further, the non-contact image method in step 1) includes a moving or non-moving particle image velocimetry method, a moving or non-moving particle tracking velocimetry method, and a moving or non-moving single-frame long-exposure image method, a moving or non-moving multi-frame multi-exposure image method.
Further, the method for obtaining the average velocity field by the moving or non-moving single-frame long exposure method comprises the following specific steps: adding trace particles into the flow field, shooting the obtained particle track image by continuous laser, long exposure, motion or non-motion, and calculating the length S of the particle track, the known particle diameter D, the exposure time delta t and the magnification M
And (4) calculating by a formula to obtain the average particle velocity, and interpolating to a space point according to the particle velocity distribution so as to obtain a space point average velocity field.
Further, the specific method for obtaining the velocity field by the moving or non-moving particle image velocimetry method is as follows: adding tracer particles into the flow field, acquiring tracer particle instantaneous information by using motion or non-motion shooting, inputting two continuous frames of tracer particle images into PIVview2CDemo software for image processing, and acquiring instantaneous velocity field time-space distribution data with pseudo vectors removed.
The invention has the beneficial effects that: the Liutex integral quantitative flow field measurement method based on the image method can be used for quantitatively measuring the vortex rotation strength and the vortex nucleus size, and researching the motion and evolution process of each vortex structure in the flow field by a more efficient and accurate means.
Drawings
FIG. 1 is a schematic diagram of a three-dimensional turbulent boundary layer measurement system apparatus according to an embodiment of the present invention;
FIG. 2 shows the R in the vortex generation and evolution process of the three-dimensional turbulent boundary layer measurement experiment in the embodiment of the present inventionIntAnd a time-dependent trend plot of Δ S.
Detailed Description
The invention is suitable for quantitative measurement of important structures in two-dimensional or three-dimensional flow field image method measurement experiments, and the flow field covers flow fields in the ranges of laminar flow and turbulent flow, inner flow and outer flow, low Reynolds number and high Reynolds number, subsonic speed and supersonic speed, and two-dimensional or three-dimensional. Flow field importance includes the common laminar or turbulent boundary layer, jet, step flow structure, pseudo-sequence structure, stripe structure occurring in the process of internal flow or bypass flow of pipe flow, gap flow, and structure of the evolution of flow with time-space of different scales.
Non-contact image methods in the flow field measurement method include particle image velocimetry, particle tracking velocimetry, single-frame long exposure, multi-frame multi-exposure image methods and other non-contact image methods.
Orientation of Liutex
And the size R is obtained by calculating a post-processing velocity field and a velocity gradient field of the flow field obtained by image method measurement. If two-dimensional velocity field information is obtained from the image, the value in the third direction is set to a constant value, for example, 1, when calculating Liutex. The Liutex is obtained by calculating the eigenvalue and the eigenvector of the velocity gradient tensor based on the velocity field, the calculation accuracy of the Liutex depends on the velocity field, and the data accuracy of the velocity field obtained through experiments cannot meet the calculation accuracy requirement of the Liutex under the normal condition. In order to more effectively apply the Liutex to the flow field measurement experiment of the image method, the invention provides the volume Liutex: rIntIt is defined as follows:
for three-dimensional experimental data: rInt＝∫ΔVRdV (1)
Wherein: r is the rigid rotation intensity, and if R is not equal to 0, the vortex exists in the local area; and R is 0, which indicates that no vortex exists. Therefore, in the above formula (1), the rotation intensity of the vortex is determined by the integral value R of R at each spatial point in the vortex coreIntIs represented by, wherein Δ V is represented by R>The volume region of 0 indicates the size of the vortex core volume surrounded by the vortex boundary.
For two-dimensional experimental data: rInt＝∫ΔSRdS (2)
As above, in the above formula (2), the intensity of vortex rotation is determined by the integral value R of R at each point in the vortex coreIntIs represented by, wherein Δ S is represented by R>The area region 0 indicates the size of the area of the vortex core surrounded by the vortex boundary.
The invention also relates to: r is to beIntAnd delta V (or delta S) carries out quantitative statistics, and the rotation intensity of the vortex and the volume (or area) of the vortex core are recorded at any timeThe trend of the change between the vortex systems is further explored to further explore the evolution mechanism, the vortex system coherence mechanism, the flow control technology and other vortex applications and researches.
The experimental procedures used in the following examples are conventional unless otherwise specified.
The apparatus, materials, reagents, image processing software, and the like used in the following examples are commercially available unless otherwise specified.
The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.
Example 1:
the method comprises the steps of obtaining motion information of a vortex structure in a flat turbulent flow boundary layer by adopting a motion single-frame long exposure image measuring method, and realizing quantitative measurement of the rotation intensity and the size of a vortex core of the vortex structure in turbulent flow by combining a volume Liutex, or Liutex integration method.
The schematic diagram of the three-dimensional turbulent boundary layer measurement system device of this embodiment is shown in fig. 1, and the system mainly includes a flat plate flow channel 1 connected to a constant level water tank 6, tracer particles 2 in the flat plate flow channel 1, a laser 3 irradiating the tracer particles 2, a camera and a lens 4 capturing the tracer particles 2, a stepping motor guide rail 5 driving the camera to move, the constant level water tank 6, a flow regulating valve 7 at the front end of the flat plate flow channel 1, and a computer 8 connected to the camera.
The measuring method comprises the following steps:
1. according to the figure 1, trace particles 2 (with the particle size of 1-2 mu m) are added into a channel 1 of the organic glass flat pipe section filled with water, and the concentration of the trace particles is low. Wherein the water injection is provided by a constant level water tank 6 and the flow rate is controlled by a flow regulating valve 7. The image measuring system consists of a camera with resolution, a telecentric lens 4 and a continuous semiconductor laser 3, the image measuring system is arranged on a stepping motor guide rail 5 which is arranged in parallel along the flow direction, and the stepping motor controls the image measuring system on the guide rail to move at a constant speed so as to capture a vortex structure with a speed which is close to the movement speed in a fluid boundary layer at the bottom of the rectangular pipeline.
The camera exposure time adopted in the method for measuring the moving single-frame long-exposure image is 100ms, the frame rate is 9.995fps, the inter-frame interval is 50 mus, and continuous measurement can be basically realized. The trace image of the tracer particle 2 captured by the camera and lens 4 is sent to the computer 8 for analysis.
2. The images taken in the experiment were processed to obtain an average velocity field.
The velocity of the movement of the tracer particle 2 can be obtained from equation (3):
in the formula: v represents the moving average speed of the tracer particles, S represents the moving track length of the tracer particles in the image, D represents the particle diameter, M is the magnification of the camera lens, and delta t is the exposure time of the camera.
The method for acquiring the flow field velocity information from the image measured by adopting the moving single-frame long exposure image method mainly comprises two parts of particle track identification and velocity calculation. The particle track recognition image processing mainly comprises four parts of denoising and sharpening, self-adaptive threshold segmentation, small particle removal and skeleton extraction. The calculation of the track length based on the accurate identification of the particle track can enable the calculation of the speed to be more accurate, and the slight difference of the speed direction at each pixel point can be more distinguished for the bending track, and meanwhile, the morphological characteristics of the image can be kept. After the track length is obtained, the flow field speed is calculated by a formula (3), and the speed direction is obtained by analyzing two continuous frames of particle images.
3. This example performs the Liutex integral calculation based on the velocity field obtained in step 2.
Compared with the vorticity, the Liutex can accurately describe the rigid rotation strength and the rotation axis direction of the local area fluid. Liutex is calculated from a velocity field, and the vector direction of the Liutex is expressed as a velocity gradient tensor
Feature vector of
A size of
The calculation of the Liutex vector is not within the scope of the present invention.
In the embodiment, the two-dimensional velocity field information is obtained from the image, and the value in the third direction is set to be a fixed value, for example, 1 when calculating the Liutex. The Liutex is obtained by calculating the eigenvalue and the eigenvector of the velocity gradient tensor based on the velocity field, the calculation accuracy of the Liutex depends on the velocity field, and the data accuracy of the velocity field obtained through experiments cannot meet the calculation accuracy requirement of the Liutex under the normal condition. Therefore, integral statistics calculation is performed on the Liutex, that is, the volume Liutex is calculated by using equation (2): rIntThe rotation intensity of the vortex is determined by the integral value R of R at each point in the vortex coreIntIs represented by, wherein Δ S is represented by R>The area 0 indicates the size of the area of the vortex core surrounded by the vortex boundary.
4. R is to beIntAnd Δ S, carrying out quantitative statistics, and recording the change trend of the rotation intensity of the vortex and the volume (or area) size of the vortex core along with time.
Example 2:
the method comprises the steps of obtaining instantaneous Euler field information of a vortex structure in a flat turbulent flow boundary layer by adopting a moving particle image velocimetry (M-PIV), and realizing quantitative measurement of the rotation intensity and the size of a vortex core of the vortex structure in turbulent flow by combining a Liutex integration method.
The measuring method comprises the following steps:
1. the measuring system device of the embodiment is the same as that of the embodiment 1. According to the figure 1, tracer particles 2 (with the particle size of 1-2 mu m) are added into a channel 1 of the organic glass flat pipe section filled with water, and the concentration of the particles is higher. Wherein the water injection is provided by a constant level water tank 6 and the flow rate is controlled by a flow regulating valve 7. The image measuring system consists of a camera with resolution, a telecentric lens 4 and a continuous semiconductor laser 3, the image measuring system is arranged on a stepping motor guide rail 5 which is arranged in parallel along the flow direction, and the stepping motor controls the image measuring system on the guide rail to move at a constant speed so as to capture a vortex structure with a speed which is close to the movement speed in a fluid boundary layer at the bottom of the rectangular pipeline. The exposure time of a camera adopted in the M-PIV measurement experiment is 5ms, the frame rate is 172.1fps, the inter-frame interval is 810 mus, and continuous measurement can be realized.
2. Two continuous frames of tracer particle images shot by the measurement system in constant-speed motion are input into PIVview2CDemo software for image processing, and instantaneous speed field time-space distribution data with pseudo vectors removed are obtained. Wherein the processing window is 32 × 32pixels and the step size is 16 pixels. And introducing the speed field data as an input end into a Liutex calculation program to perform Liutex vector calculation.
3. This example performs the Liutex integral calculation based on the velocity field obtained in step 2. The specific procedure was the same as in step 3 of example 1.
4. R is to beIntAnd Δ S to perform quantitative statistics, and record the variation trend of the rotation intensity and the volume (or area) size of the vortex core with time, as shown in fig. 2, so as to further explore the evolution mechanism, the vortex system coherence mechanism, the flow control technology and other applications and researches of the vortex. The specific procedure was the same as in step 4 of example 1.
Example 3:
the method comprises the steps of obtaining instantaneous Euler field information of a vortex structure in a flat turbulent flow boundary layer by adopting a Particle Image Velocimetry (PIV), and combining a Liutex integral method to realize quantitative measurement of the rotation intensity and the size of a vortex core of the vortex structure in turbulent flow.
The measuring method comprises the following steps:
1. the measuring system device of the embodiment is the same as the embodiments 1 and 2. In the measurement process, an image measurement system consisting of the telecentric lens 4 and the continuous semiconductor laser 3 is arranged in an experimental measurement section and is kept still. The rest of the setup was the same as in step 1 of example 2.
2. Inputting two continuous frames of tracer particle images which are fixedly shot by a measuring system into PIVview2CDemo software for image processing, and obtaining instantaneous velocity field time-space distribution data with pseudo vectors removed. The specific procedure was the same as in step 2 of example 2.
3. This example performs the Liutex integral calculation based on the velocity field obtained in step 2. The specific procedure was the same as in step 3 of examples 1 and 2.
4. R is to beIntAnd Δ S to develop quantitative statistics. The specific procedure was the same as in step 4 of examples 1 and 2.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (6)
1. A Liutex integral quantitative flow field measurement method based on an image method is characterized by comprising the following steps:
1) acquiring flow field image information by adopting a measurement system through an experiment, and acquiring a velocity field after processing the flow field image information by a non-contact image method;
2) according to the Liutex vector
By calculation of the velocity field obtained in step 1)
3) Volume Liutex is proposed: rIntIt is defined as follows:
for three-dimensional experimental data: rInt＝∫ΔVRdV，
The rotation intensity of the vortex is determined by the integral value R of R at each space point in the vortex coreIntIs represented by, wherein Δ V is represented by R>A volume area of 0 indicating the size of the vortex core volume surrounded by the vortex boundary;
for two-dimensional experimental data: rInt＝∫ΔSRdS，
The rotation intensity of the vortex is determined by the integral value R of R at each point in the vortex coreIntIs represented by, wherein Δ S is represented byR>An area region of 0 indicating the size of the area of the vortex core surrounded by the vortex boundary;
4) r is to beIntAnd delta V or delta S, recording the rotation intensity of the vortex and the change of the volume or area of the vortex core along with time, and finishing the quantitative measurement of the rotation intensity of the vortex and the size of the vortex core.
2. The image-method-based Liutex integrated quantitative flow field measurement method of claim 1, wherein the flow field encompasses flow fields in the laminar and turbulent flow, inner and outer flow, low and high Reynolds numbers, subsonic and supersonic, two-dimensional or three-dimensional categories.
3. The method for measuring the Liutex integral quantitative flow field based on the image method according to claim 1 or 2, wherein the flow field is a structure of the evolution of flows with time-space of different scales, and the structure comprises a laminar or turbulent boundary layer, a jet flow, a step flow structure, a quasi-sequence structure and a stripe structure which occur in the process of the internal flow or the bypass flow of a pipeline flow and a gap flow.
4. The method for measuring the Liutex integral quantitative flow field based on the image method as claimed in claim 3, wherein the non-contact image method in step 1) includes moving or non-moving particle image velocimetry, moving or non-moving particle tracking velocimetry, and moving or non-moving single-frame long-exposure image method, moving or non-moving multi-frame multi-exposure image method.
5. The method for measuring the Liutex integral quantitative flow field based on the image method as claimed in claim 4, wherein the method for obtaining the average velocity field by the moving or non-moving single-frame long exposure method comprises the following specific steps: adding trace particles into the flow field, shooting the obtained particle track image by continuous laser, long exposure, motion or non-motion, and calculating the length S of the particle track, the known particle diameter D, the exposure time delta t and the magnification M
And (4) calculating by a formula to obtain the average particle velocity, and interpolating to a space point according to the particle velocity distribution so as to obtain a space point average velocity field.
6. The method for measuring the Liutex integral quantitative flow field based on the image method as claimed in claim 4, wherein the moving or non-moving particle image velocimetry method for obtaining the velocity field is as follows: adding tracer particles into the flow field, acquiring tracer particle instantaneous information by using motion or non-motion shooting, inputting two continuous frames of tracer particle images into PIVview2CDemo software for image processing, and acquiring instantaneous velocity field time-space distribution data with pseudo vectors removed.
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| CN116500580A (en) * | 2023-06-28 | 2023-07-28 | 中国民航大学 | Vortex core parameter calculation method and system based on Doppler radar detection |
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