CN110260945B - Total reflection type gas-liquid interface flow display method and gas-liquid interface position identification method - Google Patents
Total reflection type gas-liquid interface flow display method and gas-liquid interface position identification method Download PDFInfo
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Abstract
The invention discloses a total reflection type gas-liquid interface flow display method and a gas-liquid interface position identification method. According to the total reflection type gas-liquid interface flow display method, the white screen illuminated by the light source is arranged on one side of the liquid to serve as the surface light source for imaging, light rays emitted by diffuse reflection of the white screen are irradiated to the gas-liquid interface at an angle larger than or equal to a total reflection critical angle, incident light can be totally reflected on the gas-liquid interface, the light rays cannot penetrate through the gas-liquid interface to be refracted to the gas side, the intensity of the reflected light is greatly enhanced, on the other hand, the light rays on the gas side cannot be refracted to enter one side of the liquid and are transmitted along the direction of the reflected light, and invalid information is reduced. And then the shooting direction of the camera is just opposite to the direction of the reflected light, so that a gas-liquid interface image with high contrast is shot, invalid information is basically not contained in the image, and the position of the gas-liquid interface can be quickly and accurately identified on the basis of the obtained high-contrast image. In addition, the total reflection type gas-liquid interface flow display method can still obtain the detailed information of all interface positions when water splash or spray exists.
Description
Technical Field
The invention relates to the field of fluid mechanics experiments, in particular to a total reflection type gas-liquid interface flow display method, and further relates to a gas-liquid interface position identification method adopting the total reflection type gas-liquid interface flow display method.
Background
Particle Image Velocimetry (PIV) is a measurement method frequently used in fluid mechanics experiments, and can measure the instantaneous velocity field of fluid in a certain area. The most important calculation method is to divide the picture into a plurality of small blocks (query windows), calculate the correlation coefficient of the photo brightness between the small block at the earlier moment and the small block at the later moment, and the position of the peak value of the correlation coefficient represents the speed of the fluid micro-cluster near the fluid corresponding query window. The particle image velocimetry has the advantages of small interference, capability of measuring all position velocities of a certain area at the same time and the like.
The identification of the gas-liquid interface is a crucial link in particle image velocity measurement, and the accuracy of the identification result of the gas-liquid interface directly influences the accuracy of the particle image velocity measurement. The imaging technology commonly adopted in the process of identifying the gas-liquid interface at present is as follows: natural light or surface light source illumination is used, and a camera is directly used for shooting the shape of an interface above a gas-liquid interface. However, the difference of the intensity of the reflected light at each position of the interface is small, so that the contrast of the obtained interface image is poor, and meanwhile, the light emitted from the position below the liquid level can be refracted to the position above the interface through the interface and is recorded by the camera, so that invalid information is increased, and the position of the gas-liquid interface cannot be accurately identified. Therefore, the image obtained by the flow display method adopted in the existing gas-liquid interface identification process has poor contrast and contains more invalid information, so that the position of the gas-liquid interface cannot be accurately identified.
Disclosure of Invention
The invention provides a total reflection type gas-liquid interface flow display method and a gas-liquid interface position identification method, and aims to solve the technical problems that images obtained by the flow display method adopted in the existing gas-liquid interface identification process are poor in contrast and contain more invalid information, so that the gas-liquid interface position cannot be accurately identified.
According to an aspect of the present invention, there is provided a total reflection type gas-liquid interface flow display method, including the steps of:
step S100: arranging a white screen illuminated by a light source on one side of the liquid as an imaging surface light source, wherein light rays emitted by diffuse reflection of the white screen irradiate to a gas-liquid interface at an angle larger than or equal to a total reflection critical angle;
step S200: a camera is arranged on one side of the liquid and used for shooting a gas-liquid interface, and the shooting direction of the camera is opposite to the direction of the reflected light.
Further, the total reflection type gas-liquid interface flow display method further comprises the following steps:
step S300: perspective distortion of an image captured by a camera is corrected.
Furthermore, the surface of the white screen is a curved surface, the light source is arranged near the focus of the white screen, and the light from the white screen to the liquid level cannot be shielded by the light source.
Furthermore, the camera adopts a shift lens or a shift adapter ring or a reflector.
Further, the step S3 is specifically:
a latticed reference object is placed in a near area below an interface before the interface is shot to serve as a perspective correction reference object, images of the reference object before and after illumination are recorded by a camera, a first-order or high-order coordinate transformation relation is calculated from the shot images of the reference object before and after illumination, and then the original image shot by the camera is corrected by using the coordinate transformation relation.
The invention also provides a gas-liquid interface position identification method, which is suitable for the condition that trace particles exist on both sides of the gas-liquid interface,
the method comprises the following steps:
step S1: respectively scattering tracer particles in gas and liquid, and then arranging a sheet light source for illumination, wherein a white screen illuminated by the light source is arranged on one side of the liquid and serves as an imaging surface light source, and light rays emitted by diffuse reflection of the white screen irradiate a gas-liquid interface at an angle larger than or equal to a total reflection critical angle;
step S2: shooting tracer particle images by using two high-speed cameras on two sides of a gas-liquid interface respectively, exposing at equal time intervals, and calibrating the cameras on the two sides of the interface respectively, wherein the shooting directions of the two high-speed cameras on one side of liquid are opposite to the direction of reflected light;
step S3: identifying the interface position at the initial moment in images obtained by two cameras shooting an area above the interface;
step S4: preprocessing images obtained by two cameras in an area above a shooting interface at the same time;
step S5: identifying the gas-liquid interface position in each subsequent frame of image;
step S6: performing time-space smoothing operation on the gas-liquid interface position to obtain the gas-liquid interface position in the image obtained by two cameras for shooting the area above the interface;
step S7: the gas-liquid interface position in the image obtained by the two cameras in the area below the shooting interface is obtained according to the gas-liquid interface position in the image obtained by the two cameras in the area above the shooting interface.
Further, the step S3 specifically includes the following steps:
step S31: if the gas movement speed is far greater than the liquid movement speed, the brightness of each pixel is the minimum of the brightness of the corresponding position in the plurality of pictures; otherwise, skipping the step;
step S32: carrying out gray scale morphological opening operation on the picture for a plurality of times;
step S33: setting a brightness threshold to identify particles in the liquid to obtain a binary image;
step S34: opening the binary image for a plurality of times, and then closing the binary image for a plurality of times;
step S35: setting a height value y0 according to the height of the interface in the image, wherein the height position of the interface is below a straight line with the height being the value, and the part with the height being above y0 is set as 0;
step S36: recording the highest position among all non-zero values of each column of pixels in the binary image, and recording the position of the last pixel of a column to obtain a sequence if no non-zero value exists in the column;
step S37: performing median filtering and Gaussian filtering processing on the sequence to obtain the position of an interface;
step S38: calculating the average value of the interface positions in the pictures obtained by the two cameras;
step S39: an offset is set and then the initial interface position is moved upward according to the offset.
Further, the step S5 specifically includes the following steps:
step S51: setting a brightness threshold value for the preprocessed image to identify particles in the liquid so as to obtain a binary image;
step S52: opening the binary image for a plurality of times, and then closing the binary image for a plurality of times;
step S53: translating the interface of the previous frame of image upwards by 15 pixels to serve as a boundary, and setting the binary image numerical value of an area above the boundary to be 0;
step S54: recording the highest position among all non-zero values of each column of pixels in the binary image, and if no non-zero value exists in a certain column, recording the smaller value of the position of the last pixel of the column and the same position of the interface of the previous frame, thereby obtaining a sequence;
step S55: performing median filtering and Gaussian filtering on the sequence to obtain the position of an interface;
step S56: the position of the interface is moved upward according to the offset.
Further, the step S6 specifically includes the following steps:
step S61: calculating the cross-correlation value of the interface position between every two frames every several frames;
step S62: adding the multiple cross-correlation values to find a peak position;
step S63: repeating the steps S61 and S62 for all calculable moments to obtain a time sequence of the interface speed;
step S64: performing phase-locking average calculation on the interface position based on the obtained time sequence of the interface speed;
step S65: and performing time-space smoothing operation on the boundary positions at all the moments to obtain a smoothed boundary position time-space sequence.
Further, the step S7 specifically includes the following steps:
step S71: transforming images obtained by two cameras in an area below a shooting interface from original images to images in a physical space coordinate system according to a coordinate transformation relation obtained by calibration;
step S72: checking whether the positions of interfaces in the images obtained by the two cameras after coordinate transformation are overlapped, and if not, performing self-calibration on the two cameras in the area below the shooting interface;
step S73: zooming and interpolating the time-space sequence of the boundary position to adapt to images obtained by two cameras in an area below a shooting interface after coordinate transformation;
step S74: temporarily setting the initial offset of the interface position, displaying images of the two cameras after spatial transformation and the temporary position of the interface after offset on one picture, wherein the images of the two cameras respectively occupy different color channels, and outputting videos consisting of the pictures at different moments;
step S75: observing the difference between the tentative interface position in the video and the interface position reflected by the brightness of the particles in the picture, and modifying the offset of the interface position according to the difference;
step S76: and repeatedly executing the step S74 and the step S75 until the temporary position of the interface is coincided with the position of the interface reflected by the brightness of the particles in the picture.
The invention has the following beneficial effects:
according to the total reflection type gas-liquid interface flow display method, the white screen illuminated by the light source is arranged on one side of the liquid to serve as the surface light source for imaging, light rays emitted by diffuse reflection of the white screen are irradiated to the gas-liquid interface at an angle larger than or equal to a total reflection critical angle, incident light can be totally reflected on the gas-liquid interface, the light rays cannot penetrate through the gas-liquid interface to be refracted to the gas side, the intensity of the reflected light is greatly enhanced, on the other hand, the light rays on the gas side cannot be refracted to enter one side of the liquid and are transmitted along the direction of the reflected light, and invalid information is reduced. And then a camera is arranged on one side of the liquid for shooting a gas-liquid interface, the shooting direction of the camera is over against the direction of the reflected light, so that a gas-liquid interface image with high contrast is shot, and the image basically does not contain invalid information, so that the position of the gas-liquid interface can be quickly and accurately identified on the basis of the obtained high-contrast image. In addition, the total reflection type gas-liquid interface flow display method can still obtain the detailed information of all interface positions when water splash or spray exists.
In addition, the gas-liquid interface position identification method of the invention firstly scatters trace particles in gas and liquid respectively, then two high-speed cameras are respectively used for shooting tracer particle images at two sides of a gas-liquid interface, then identifying the interface position at the initial moment in the images obtained by the two cameras for shooting the area above the interface, then all images at the same time are preprocessed, the gas-liquid interface position in each subsequent frame of image is identified, the obtained gas-liquid interface position is subjected to time-space smoothing, and finally, the gas-liquid interface position in the image obtained by shooting the two cameras in the area below the shooting interface is obtained according to the gas-liquid interface position in the image obtained by shooting the two cameras in the area above the shooting interface. The gas-liquid interface position identification method can accurately identify the position of the gas-liquid interface under the condition that particles exist on both sides of the gas-liquid interface, has high identification accuracy, and can be well applied to the near-wall measurement process in particle image velocimetry. In addition, the method for identifying the gas-liquid interface position of the present invention employs the total reflection type gas-liquid interface flow display method in the first embodiment when capturing an image of a region below the gas-liquid interface, and the captured image of the region below the gas-liquid interface has a higher contrast, better definition, and contains less invalid information, so that the gas-liquid interface position in the image of the region below the gas-liquid interface can be identified quickly and accurately.
In addition to the objects, features and advantages described above, other objects, features and advantages of the present invention are also provided. The present invention will be described in further detail below with reference to the drawings.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:
fig. 1 is a schematic flow chart of a total reflection type gas-liquid interface flow display method according to a first embodiment of the present invention.
Fig. 2 is a schematic view of a light source and a camera provided in the total reflection type gas-liquid interface flow display method according to the first embodiment of the present invention.
Fig. 3 is a schematic view of an experimental apparatus for a total reflection type gas-liquid interface flow display method according to a first embodiment of the present invention.
Fig. 4 is a gas-liquid interface image captured by the total reflection gas-liquid interface flow display method according to the first embodiment of the present invention.
Fig. 5 is a gas-liquid interface image captured by the conventional flow display method according to the first embodiment of the present invention.
Fig. 6 is a flowchart illustrating a gas-liquid interface position identifying method according to a second embodiment of the present invention.
Fig. 7 is a schematic diagram illustrating calibration of cameras on both sides of the interface in step S2 in fig. 6 according to the second embodiment of the present invention.
Fig. 8 is a sub-flowchart of step S3 in fig. 6 according to the second embodiment of the present invention.
Fig. 9 is a sub-flowchart of step S4 in fig. 6 according to the second embodiment of the present invention.
Fig. 10 is a sub-flowchart of step S5 in fig. 6 according to the second embodiment of the present invention.
Fig. 11 is a sub-flowchart of step S6 in fig. 6 according to the second embodiment of the present invention.
Fig. 12 is a sub-flowchart of step S7 in fig. 6 according to the second embodiment of the present invention.
Detailed Description
The embodiments of the invention will be described in detail below with reference to the accompanying drawings, but the invention can be embodied in many different forms, which are defined and covered by the following description.
As shown in fig. 1, 2 and 3, a first embodiment of the present invention provides a total reflection type gas-liquid interface flow display method, which can provide a high-contrast image in the process of identifying the gas-liquid interface position, and the obtained image almost only includes an image formed by reflecting light from the liquid side and excluding an image formed by refracting light from the gas side, and the image has little invalid information, so that the gas-liquid interface position can be quickly and accurately identified from the high-contrast image. The total reflection type gas-liquid interface flow display method comprises the following steps:
step S100: arranging a white screen illuminated by a light source on one side of the liquid as an imaging surface light source, wherein light rays emitted by diffuse reflection of the white screen irradiate to a gas-liquid interface at an angle larger than or equal to a total reflection critical angle;
step S200: a camera is arranged on one side of the liquid and used for shooting a gas-liquid interface, and the shooting direction of the camera is opposite to the direction of the reflected light.
It can be understood that, in the step S100, the light source may be a laser, light emitted by the laser illuminates the white screen, so that diffuse reflection occurs on the white screen, when light generated by the diffuse reflection is irradiated to the gas-liquid interface at an angle greater than or equal to the critical angle of total reflection, total reflection occurs at the gas-liquid interface at this time because the incident angle of the light is greater than or equal to the critical angle of total reflection, and the light will not pass through the gas-liquid interface and be refracted to the gas side, so that the intensity of the reflected light is greatly enhanced. Preferably, the best illumination mode is that the light generated after the white screen is subjected to diffuse reflection is parallel light, and the intensity of the reflected light can be further enhanced, specifically: a white screen with a curved surface, such as a spherical screen, is used and the light source is then placed near the focus of the white screen while ensuring that the light source does not block the light from the white screen to the liquid surface.
It can be understood that, in the step S200, since the shooting direction of the camera is directly opposite to the direction of the reflected light, that is, the included angle between the photosensitive chip of the camera and the liquid level is equal to the incident angle of the light generated by the diffuse reflection of the white screen irradiating the gas-liquid interface, the light generated by the total reflection can directly enter the camera, and the camera can shoot a clear interface image because the intensity of the total reflected light is very high. In addition, as an optimization, in order to further improve the definition of an image, the camera adopts a shift lens or a shift adapter ring or a reflector, so that imaging under the Scheimpflug (Scheimpflug's law) condition can be met, and the obtained image is more clean.
It can be understood that, as preferable, the total reflection type gas-liquid interface flow display method further includes the steps of:
step S300: perspective distortion of an image captured by a camera is corrected.
It can be understood that, in the step S300, due to the perspective relationship, the coordinate system on the original image recorded by the camera is no longer a rectangular coordinate system in the physical space coordinates, so that the perspective distortion of the image captured by the camera needs to be corrected. The specific process is as follows: a latticed reference object is placed in a near area below an interface before the interface is shot to serve as a perspective correction reference object, images of the reference object before and after illumination are recorded by a camera, a first-order or high-order coordinate transformation relation is calculated from the shot images of the reference object before and after illumination, and then the original image shot by the camera is corrected by using the coordinate transformation relation.
In addition, preferably, when the area of the liquid region is large, the light source and the camera need to be placed in the liquid after being installed in the streamline waterproof housing. In addition, ground glass can be arranged at the light source to enable the illumination light to be softer and the imaging effect to be better.
The inventors of the present invention have also attempted to image using several methods: 1. the method can directly shoot the image formed by locking the screen, namely a shadow method, or can add a knife edge on the light path, filter out the light with smaller deflection rate and then image, namely a schlieren method; 2. background schlieren method: placing a background plate with a pattern on one side of the interface, generally using synthetic particles as clusters, shooting the pattern of the background plate on the other side of the interface, and reversely pushing out the shape of the interface through the deformation of the pattern of the background plate; 3. x light scattering method. However, the shadow method, the schlieren method and the background schlieren method are all limited by the fluctuation degree of the interface, if the fluctuation degree of the interface is too large, the screen or the background plate needs to be close to the interface so as to be capable of clearly imaging, but in the actual use process, the screen or the background plate is not allowed to be too close to the interface so as to prevent the influence on the experimental conditions; the X-ray scattering method is limited by experimental equipment, so that the cost is high, and the X-ray scattering method is difficult to popularize and apply widely.
The invention discloses a total reflection type gas-liquid interface flow display method, which is characterized in that a white screen illuminated by a light source is arranged on one side of liquid to serve as a surface light source for imaging, light emitted by diffuse reflection of the white screen is irradiated to a gas-liquid interface at an angle larger than or equal to a total reflection critical angle, incident light can be totally reflected on the gas-liquid interface, the light cannot pass through the gas-liquid interface to be refracted to one side of gas, the intensity of the reflected light is greatly enhanced, then a camera is arranged on one side of the liquid to shoot the gas-liquid interface, the shooting direction of the camera is opposite to the direction of the reflected light, so that a gas-liquid interface image with high contrast is shot, invalid information is basically not contained in the image, and the position of the gas-liquid interface can be rapidly. In addition, the total reflection type gas-liquid interface flow display method can still obtain the detailed information of all interface positions when water splash or spray exists. Fig. 4 is an interface image obtained by the total reflection type gas-liquid interface flow display method of the present invention, and fig. 5 is an interface image obtained by the prior art, and it can be seen from the comparison between the two, that the interface image obtained by the present invention has higher contrast and is clearer, and contains less invalid information, so that the position of the gas-liquid interface can be identified more quickly and accurately.
In addition, as shown in fig. 6, a second embodiment of the present invention further provides a gas-liquid interface position identification method, which preferably adopts the total reflection type gas-liquid interface flow display method as described in the first embodiment, obtains a high-contrast image by the total reflection type gas-liquid interface flow display method of the first embodiment, can identify the position of the gas-liquid interface in the process of measuring the velocity fields at both sides of the gas-liquid interface, can adapt to the situation that there are trace particles at both sides of the interface, and has high accuracy of interface position identification. The gas-liquid interface position identification method comprises the following steps:
step S1: respectively scattering tracer particles in gas and liquid, and then arranging a sheet light source for illumination;
step S2: shooting tracer particle images by using two high-speed cameras on two sides of a gas-liquid interface respectively, exposing at equal time intervals, and calibrating the cameras on the two sides of the interface respectively;
step S3: identifying the interface position at the initial moment in images obtained by two cameras shooting an area above the interface;
step S4: preprocessing images obtained by two cameras in an area above a shooting interface at the same time;
step S5: identifying the gas-liquid interface position in each subsequent frame of image;
step S6: performing time-space smoothing operation on the gas-liquid interface position to obtain the gas-liquid interface position in the image obtained by two cameras for shooting the area above the interface;
step S7: the gas-liquid interface position in the image obtained by the two cameras in the area below the shooting interface is obtained according to the gas-liquid interface position in the image obtained by the two cameras in the area above the shooting interface.
It will be appreciated that in said step S1, particles of sufficiently small diameter are required to ensure good particle following properties, taking into account that the properties of the particle following fluid are related to the stokes number, but on the other hand, in the image recorded by the camera, the brightness of the particles is related to the square of the particle diameter, which requires that the particle radius is not too small. Therefore, in this embodiment, particles having a small particle size are used for scattering in a gas, particles having a large particle size are used for scattering in a liquid, and the particles are repeatedly scattered in the liquid in the vicinity of the gas-liquid interface so that the particle density in this region is sufficiently large, thereby ensuring sufficient followability of the particles and at the same time, the brightness and the particle size of the particles having two particle sizes are different from each other in an image. Preferably, the gas is injected with a fan using smoke having a diameter of about 1 μm as tracer particles, and the liquid is filled with hollow organic glass fine particles having a diameter of about 50 μm as tracer particles, wherein a part of the particles in the gas is injected from a funnel into a relatively deep position, and the other part of the particles in the liquid is gently poured onto the liquid surface. The sheet light sources are arranged in the measurement area for illumination, and the specific arrangement manner may be that one sheet light source is respectively arranged on two sides of the gas-liquid interface for illumination, the illumination direction of the surface light source on the gas side is vertical downward, and the arrangement manner of the surface light source on the liquid side adopts the light source arrangement manner in the first embodiment, which is specifically described in step S100 in the first embodiment and is not described herein again. Wherein the two light sources may be different in color so as to be easily distinguished by the filter.
It can be understood that, in the step S2, the preferred embodiment employs a time-series stereo particle image velocimetry method to measure the velocity fields of the regions on both sides of the cross section simultaneously, specifically, two high-speed cameras are used to capture trace particle images on both sides of the interface, that is, two cameras capture the region above the gas-liquid interface, two cameras capture the region below the gas-liquid interface, and then the equal-time-interval exposure is performed, that is, a series of pictures are captured at uniform time intervals, and calibration is performed on both sides of the gas-liquid interface. As shown in fig. 7, the specific calibration method is as follows: the reference plane calibrated on two sides of the gas-liquid interface is required to coincide with the plane irradiated by the laser, the bottom edge of the calibration plate is horizontally placed, and the distance can be reserved between the origin of coordinates, namely, in two physical space coordinate systems for calibration, the positive directions of the y axes of the two physical space coordinate systems are vertical upwards, the z axis is vertical to the front surface of the calibration plate, the z is equal to 0 plane coincidence, and the x, y and z directions are the same. Preferably, in order to avoid the interface before the plane where the measurement is performed from blocking the interface on the plane where the measurement is performed during the photographing, the camera in the region above the photographing interface needs to be inclined with respect to the horizontal plane, and the inclination angle is about 10 °. It can be understood that when the camera forms an included angle of about 10 degrees with the horizontal plane, two images obtained by two cameras for shooting the area above the gas-liquid interface mainly have two contents: particles in gas are directly imaged, and particles in liquid are imaged after interface refraction. The arrangement of the two cameras in the area below the shooting interface is as in step S200 in the first embodiment, and therefore, the detailed description thereof is omitted here. The images obtained by two cameras for shooting the following areas of the gas-liquid interface mainly have two contents: the scattered light of the tracer particles in the area below the gas-liquid interface is directly imaged in the camera, and the scattered light of the tracer particles in the area below the gas-liquid interface is reflected by the interface and then imaged in the camera. It is considered that the deformation and decoking phenomena of the reflected scattered light are more serious as the distance from the gas-liquid interface is increased, the edge is more fuzzy, and the relationship between the light intensity and the scattering angle exists in the Mie scattering, and the scattering light intensity is higher when the included angle between the scattered light and the irradiation light is smaller, so that the brightness of the particles in the area above the gas-liquid interface is higher than that of the particles in the area below the interface, and the particles are more fuzzy as the distance from the interface is farther. Therefore, the interface position is identified by adopting the images obtained by the two cameras for shooting the area above the gas-liquid interface, and the identification result is more accurate.
It can be understood that, in step S3, identifying the positions of the interfaces in the images obtained by the two cameras capturing the area above the interfaces requires transforming the images to physical space coordinates according to the calibrated coordinate mapping relationship, and therefore, it is necessary to check whether the calibrated coordinate mapping relationship is accurate. Preferably, the gas-liquid interface position identifying method further includes, between step S2 and step S3, step S23,
step S23: the brightness of the pictures of the two cameras at the same time after coordinate transformation is converted into the numerical value of a red/green channel, then the numerical values are combined into one picture to be displayed, and whether the interfaces of the two pictures are overlapped or not is judged. If the coordinate mapping relation obtained by calibration is accurate, the positions of the interfaces in the two pictures are overlapped; if the coordinate mapping relationship obtained by calibration is not accurate and the positions of the interfaces of the two cameras do not coincide after the coordinates of the two images are changed, the two cameras need to be calibrated, namely, the initial offset, namely the distances of the corresponding points in the x direction and the y direction, can be set according to the distance between the positions of the relative points of the interfaces in the two images, and then the overall offset is determined according to the offsets, wherein the overall offset is the average value of the offset values of the points. To ensure accuracy, multiple pairs of pictures of two cameras may be used for self-calibration, e.g., 100 pairs of pictures.
It is understood that the following steps are also executed between the step S2 and the step S3:
and horizontally turning the images obtained by the two cameras for shooting the area above the interface or horizontally turning the images obtained by the two cameras for shooting the area below the interface so as to compare the images of the area below the interface with the images of the area below the interface.
It is understood that, in the step S3, there are two methods for identifying the interface position at the initial moment, the first method is manual input after the interface position is manually identified, and the second method is identification using an algorithm. In order to ensure the accuracy of identification, the second identification method is preferably adopted in the present embodiment. Specifically, as shown in fig. 8, the step S3 includes the following steps:
step S31: if the gas movement speed is far greater than the liquid movement speed, under a proper sampling frequency, the brightness of each pixel is the minimum of the brightness of the corresponding position in a plurality of pictures; if the gas movement speed is not far greater than the liquid movement speed, skipping the step;
step S32: carrying out gray scale morphological opening operation on the picture for a plurality of times, eliminating particles in gas in the picture and reserving particles in liquid;
step S33: setting a brightness threshold to identify particles in the liquid to obtain a binary image;
step S34: opening the binary image for a plurality of times, and then closing the binary image for a plurality of times;
step S35: setting a height value y0 according to the height of the interface in the image, wherein the height position of the interface is below a straight line with the height being the value, and the part with the height being above y0 is set as 0;
step S36: recording the highest position among all non-zero values of each column of pixels in the binary image, and recording the position of the last pixel of a column to obtain a sequence if no non-zero value exists in the column;
step S37: performing median filtering and Gaussian filtering processing on the sequence to obtain the position of an interface;
step S38: the average of the interface positions in the pictures taken by the two cameras is calculated.
It can be understood that, in the step S31, if the moving speed of the gas is much greater than the moving speed of the liquid, that is, the average moving speed of the gas is greater than the average moving speed of the liquid by more than one order of magnitude, and at a suitable sampling frequency, the tracer particles in the gas in the pictures have significant movement, that is, the movement of each frame is greater than 1 pixel, and the tracer particles in the liquid have no significant movement, that is, the movement of each frame is less than 1 pixel, then the sliding minimum of 3 or 5 pictures in the time series, that is, the brightness of each pixel is the minimum of the brightness of the corresponding position of 3 or 5 pictures, may be used, and the subsequent steps are performed. Otherwise, skip this step.
It is understood that in step S32, the gray scale morphology opening operation refers to performing gray scale morphology etching on the picture, and then performing gray scale morphology expansion, so as to eliminate the particles in the gas in the picture and retain the particles in the liquid.
It is understood that in the step S33, since the brightness of the particles is related to the square of the diameter thereof, the diameter of the particles in the gas is about 1 μm, and the diameter of the particles in the liquid is about 50 μm, a brightness threshold can be set to identify the particles in the liquid, for example, 1 indicates that the local brightness is higher than the threshold, and 0 indicates that the local brightness is lower than the threshold. And, can set up the overall threshold value, namely the brightness threshold value of the same size of every place use in the space, can set up the local threshold value of gradual change, namely there are different brightness threshold values of different positions in the space, it depends on every imaging parameter, if use long focal lens and large-scale photosensitive chip, the whole luminance of the marginal area of picture may be smaller than the central area, can choose to reduce the marginal area threshold value at this moment.
It can be understood that, in the step S34, the binary image obtained in the step S33 is subjected to gray scale morphological erosion first and then to gray scale morphological dilation several times, and then to a closing operation several times, that is, the binary image is subjected to gray scale morphological erosion first and then to gray scale morphological erosion several times.
It is understood that, in the step S35, a height value y0 is set according to the height of the interface in the binary image so that the height position of the interface is below the line having a height y0 and the portion having a height above y0 is set to 0.
It is to be understood that in the step S36, the position of the highest pixel among all the non-zero values of each column of pixels in the binary image is recorded, and if no non-zero value exists in a column of pixels, that is, the height positions of the column of pixels are all above the straight line with the height of y0, the position of the last pixel of the column of pixels is recorded, so as to obtain a position sequence.
It is understood that, in the step S38, in order to ensure the accuracy of the recognition, an average value of the interface positions in the pictures obtained by the two cameras is calculated as the final initial interface position.
It can be understood that, in the algorithm, the purpose of identifying the interface is achieved by identifying and smoothing the particles in the liquid, and the tracer particles in the liquid are always below the liquid level of the gas and the liquid, so that an offset needs to be set, and then the initial interface position obtained in the step S38 is moved upwards according to the offset so as to enable the identified interface position to coincide with the actual interface position, thereby further improving the accuracy of identification. The offset is obtained by a plurality of experiments of actual pictures. Therefore, the step S3 further includes, after the step S38, a step S39:
step S39: an offset is set and then the initial interface position is moved upward according to the offset.
It can be understood that, as shown in fig. 9, the step S4 specifically includes the following steps:
step S41: if the gas movement speed is far greater than the liquid movement speed, under a proper sampling frequency, the brightness of each pixel is the minimum of the brightness of the corresponding position in a plurality of pictures; if the gas movement speed is not far greater than the liquid movement speed, skipping the step;
step S42: synthesizing the simultaneous photos obtained by the two cameras into a photo;
step S43: the grey scale morphological opening operation is performed on the synthesized picture for several times, and the particles in the gas in the image are eliminated while the particles in the liquid are retained.
It is understood that the step S41 is identical to the step S31, and therefore, the description thereof is omitted here.
It can be understood that step S42 specifically includes: translating the interface of the previous frame of image upwards by 10 pixels to serve as a boundary, comparing the brightness of the two photos at the same position in an area above the boundary, and taking a relatively lower value as the brightness of a synthesized photo; then, translating the interface of the previous frame of image downwards by 10 pixels to be used as a boundary, comparing the brightness of the two photos at the same position in an area below the boundary, and taking a relatively high value as the brightness of the synthesized photo; the middle area of the two boundaries may be gradually changed from a lower value to an average value and then to a higher value, or may be an average value of local brightness.
It is understood that the step S43 is identical to the step S32, and therefore, the description thereof is omitted here.
It can be understood that, as shown in fig. 10, the step S5 specifically includes the following steps:
step S51: setting a brightness threshold value for the preprocessed image to identify particles in the liquid so as to obtain a binary image;
step S52: opening the binary image for a plurality of times, and then closing the binary image for a plurality of times;
step S53: translating the interface of the previous frame of image upwards by 15 pixels to serve as a boundary, and setting the binary image numerical value of an area above the boundary to be 0;
step S54: recording the highest position among all non-zero values of each column of pixels in the binary image, and if no non-zero value exists in a certain column, recording the smaller value of the position of the last pixel of the column and the same position of the interface of the previous frame, thereby obtaining a sequence;
step S55: performing median filtering and Gaussian filtering on the sequence to obtain the position of an interface;
step S56: the position of the interface is moved upward according to the offset.
It is understood that the step S51 is identical to the step S33, and therefore, the description thereof is omitted here, wherein the brightness threshold is set according to the brightness distribution of the two cameras in the area above the set of test shooting interfaces.
It is understood that the step S52 is identical to the step S34, and therefore, the description thereof is omitted here.
It will be appreciated that in said step 54, when no non-zero value exists in a column of pixels, the smaller value, i.e. the higher of the two, of the position of the last pixel in the column and the same position of the previous frame boundary is recorded.
It is understood that step S55 is identical to step S37, and therefore will not be described herein.
It is understood that, in the step S56, the offset is set in the step S39.
It can be understood that, as shown in fig. 11, the step S6 specifically includes the following steps:
step S61: calculating the cross-correlation value of the interface position between every two frames every several frames;
step S62: adding the multiple cross-correlation values to find a peak position;
step S63: repeating the steps S61 and S62 for all calculable moments to obtain a time sequence of the interface speed;
step S64: performing phase-locking average calculation on the interface position based on the obtained time sequence of the interface speed;
step S65: and performing time-space smoothing operation on the boundary positions at all the moments to obtain a smoothed boundary position time-space sequence.
It is to be understood that, in the step S61, for example, the frame a and the frame b are selected according to the chronological order, where the interface position of the frame b is fixed, and the interface of the frame a moves along the x direction, so as to obtain the relationship between the cross-correlation value and the moving distance of the interface of the frame a, and then the moving distance corresponding to the cross-correlation peak is found. The purpose of calculating the cross-correlation value several frames apart is to increase the time interval to improve the precision in consideration of the slow moving speed of the interface.
It is to be understood that in the step S62, in order to further improve the accuracy, a plurality of cross correlation values may be selected to be added to find the peak position. For example, a first frame, a second frame, a third frame, a second frame and a third frame are selected, wherein the time interval between the first frame and the second frame is the same as the time interval between the first frame and the second frame and the time interval between the first frame and the second frame, the relationship between the three sets of cross-correlation values of the interface positions of the two sets of data along with the moving distance is respectively calculated, then the three sets of cross-correlation values are added to obtain a new relationship between the cross-correlation values of the interface positions along with the moving distance, and the moving distance corresponding to the corresponding cross-correlation peak value is searched. In addition, the number of groups to be calculated by superposition may be selected as needed, and is not particularly limited herein.
It is understood that, in the step S63, the interface moving distance per unit time is the interface speed, and the interface moving distance is obtained according to the positions of the cross-correlation peaks in the steps S61 and S62. The steps S61 and S62 are repeated for all the moments that can be calculated, resulting in a time series of interface speeds. Preferably, in step S63, the time series of interface velocities may be further processed by using a smoothing filter to reduce the time series pulsation amount, where the smoothing filter may be a median filter, a gaussian filter, a butterworth filter, or the like.
It can be understood that step S64 specifically includes: note that the interface position at time t0 is < s (< x >, t0) >, where < s > represents the interface height, < x > represents the abscissa sequence of the interface, < > represents a vector, and the interface position < s (< x >, t0) > represents the component corresponding to the interface height < s > of the column in which a certain point x0 is located at time t0 and < x >. Note that the time interval for calculating the cross correlation value in step S61 is Δ t, and the moving distance of the interface within Δ t at time t0 is obtained by smoothing (t 0). Then, with N as the temporal-spatial smoothing radius, calculate < s (< x > -N < Δ x (t0) >, t0-N Δ t) >, < s (< x > - (N-1) < Δ x (t0) >, t0- (N-1) Δ t) >, …, < s (< x >, t0) >, < s (< x > + (N-1) < Δ x (t0) >, t0+ (N-1) Δ t) >, < s (< x > + N < Δ x (t0) >, t0+ N Δ t) > as an average over < xe > < s1(< xe >, t0) >, where < xe > represents the new abscissa sequence obtained after extending < x > to both sides. In addition, one-dimensional interpolation is required in the step S64, and the average calculation uses only the interpolated values and not the extrapolated values. Where N is set to 30 so that the interface position extends to both sides by a sufficient length.
It is understood that in the step S65, N may be reduced when the temporal-spatial smoothing operation is performed at the edge. In addition, in the obtained smoothed boundary position time-space sequence, a portion lacking data may obtain data using interpolation. Up to this point, the interface position in the images obtained by the two cameras taking the region above the gas-liquid interface has been obtained.
It can be understood that the interface positions in the images obtained by the two cameras for shooting the area above the interface are obtained through the above steps, and the interface positions in the images obtained by the two cameras for shooting the area below the interface are determined according to the positions of the obtained interfaces in the physical space. As shown in fig. 12, the step S7 specifically includes the following steps:
step S71: transforming images obtained by two cameras in an area below a shooting interface from original images to images in a physical space coordinate system according to a coordinate transformation relation obtained by calibration;
step S72: checking whether the positions of interfaces in the images obtained by the two cameras after coordinate transformation are overlapped, and if not, performing self-calibration on the two cameras in the area below the shooting interface;
step S73: zooming and interpolating the time-space sequence of the boundary position to adapt to images obtained by two cameras in an area below a shooting interface after coordinate transformation;
step S74: temporarily setting the initial offset of the interface position, displaying images of the two cameras after spatial transformation and the temporary position of the interface after offset on one picture, wherein the images of the two cameras respectively occupy different color channels, and outputting videos consisting of the pictures at different moments;
step S75: observing the difference between the tentative interface position in the video and the interface position reflected by the brightness of the particles in the picture, and modifying the offset of the interface position according to the difference;
step S76: and repeatedly executing the step S74 and the step S75 until the temporary position of the interface is coincided with the position of the interface reflected by the brightness of the particles in the picture.
It is understood that, in the step S71, the images obtained by the two cameras capturing the area below the interface are transformed from the original images to the images in the physical space coordinate system according to the coordinate transformation relationship obtained by the calibration in the step S2. In addition, because the total reflection type gas-liquid interface flow display method in the first embodiment is adopted when the image of the area below the interface is shot, the obtained image has high contrast and high definition, and contains little invalid information, so that the gas-liquid interface can be identified quickly and accurately.
It can be understood that, in the step S72, whether the interface positions in the images obtained by using the two cameras for shooting the area below the interface coincide with each other is checked by using the naked eye, the interface positions can be roughly judged by the brightness of the particles in the images, if the interface positions of the two cameras do not coincide with each other, the two cameras for shooting the area below the interface need to be self-calibrated, and the self-calibration process is consistent with the self-calibration process in the step S23, and therefore, the description is omitted here.
It can be understood that, in the step S73, since the calibration of the two cameras in the area above the shooting interface and the calibration of the two cameras in the area below the shooting interface are performed separately, the number of pixels per millimeter in the two sets of calibration relations is not necessarily the same, and therefore, the obtained time-space sequence of the boundary position needs to be scaled and interpolated to adapt to the images obtained by the two cameras in the area below the shooting interface after the coordinate transformation.
It is understood that in the step S74, an initial offset amount of the interface position is temporarily given, then the spatially transformed images of the two cameras and the temporary position of the interface after the offset are displayed on one picture, the images of the two cameras respectively occupy different color channels for distinction, and finally, a video composed of pictures at different time instants is output.
It can be understood that, in step S75, since the luminance of the particles in the region above the interface in the picture is relatively high and the luminance of the particles in the region below the interface is relatively low, the difference between the tentative interface position in the video and the interface position reflected by the luminance of the particles in the picture can be clearly observed.
It is understood that, in step S76, the position of the gas-liquid interface in the images obtained by the two cameras capturing the area below the interface is obtained when the temporary position of the interface coincides with the interface position reflected by the particle brightness in the picture. The gas-liquid interface position in the images obtained by all the cameras has been obtained so far.
The inventors of the present application have also attempted to identify the gas-liquid interface position by using three methods: 1. identifying characteristics of light reflection at the interface; 2. adding a fluorescent substance to the liquid to identify areas where the fluorescent substance is present; 3. a region where the tracer particle is present is identified. However, in the first method, the shooting direction of the camera needs to be flush with the interface to promote capturing of interface reflection, but because the gas-liquid interface is actually up-and-down, the problem that the sight line is blocked exists, and the blocking is particularly serious when a stereoscopic particle image is used for speed measurement; in the second method, two light sources with different colors and three groups of cameras for shooting different color contents are needed, the whole system is complex, the cost is high, and a common fluorescent dye (rhodamine B) is toxic and harmful to human health; the third method is not at all applicable to the case where particles are present on both sides of the interface because the particles in the gas are not easily distinguishable from the particles in the liquid.
Accordingly, the inventors of the present application have proposed a gas-liquid interface position identification method of the preferred embodiment of the present application, which first scatters trace particles in a gas and a liquid respectively, then two high-speed cameras are respectively used for shooting tracer particle images at two sides of a gas-liquid interface, then identifying the interface position at the initial moment in the images obtained by the two cameras for shooting the area above the interface, then all images at the same time are preprocessed, the gas-liquid interface position in each subsequent frame of image is identified, the obtained gas-liquid interface position is subjected to time-space smoothing, and finally, the gas-liquid interface position in the image obtained by shooting the two cameras in the area below the shooting interface is obtained according to the gas-liquid interface position in the image obtained by shooting the two cameras in the area above the shooting interface. The gas-liquid interface position identification method can accurately identify the position of the gas-liquid interface under the condition that particles exist on both sides of the gas-liquid interface, has high identification accuracy, and can be well applied to the near-wall measurement process in particle image velocimetry. In addition, the method for identifying the gas-liquid interface position of the present invention employs the total reflection type gas-liquid interface flow display method in the first embodiment when capturing an image of a region below the gas-liquid interface, and the captured image of the region below the gas-liquid interface has a higher contrast, better definition, and contains less invalid information, so that the gas-liquid interface position in the image of the region below the gas-liquid interface can be identified quickly and accurately.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A total reflection type gas-liquid interface flow display method is characterized in that,
the method comprises the following steps:
step S100: arranging a white screen illuminated by a light source on one side of the liquid as an imaging surface light source, wherein light rays emitted by diffuse reflection of the white screen irradiate to a gas-liquid interface at an angle larger than or equal to a total reflection critical angle, the surface of the white screen is a curved surface, the light source is arranged near the focus of the white screen, and the light rays from the white screen to the liquid surface cannot be shielded by the light source;
step S200: a camera is arranged on one side of the liquid and used for shooting a gas-liquid interface, and the shooting direction of the camera is opposite to the direction of the reflected light.
2. The total reflection type gas-liquid interface flow display method according to claim 1,
the total reflection type gas-liquid interface flow display method further comprises the following steps:
step S300: perspective distortion of an image captured by a camera is corrected.
3. The total reflection type gas-liquid interface flow display method according to claim 1,
the camera adopts a shift lens or a shift adapter ring or a reflector.
4. The total reflection type gas-liquid interface flow display method according to claim 2,
the step S300 specifically includes:
a latticed reference object is placed in a near area below an interface before the interface is shot to serve as a perspective correction reference object, images of the reference object before and after illumination are recorded by a camera, a first-order or high-order coordinate transformation relation is calculated from the shot images of the reference object before and after illumination, and then the original image shot by the camera is corrected by using the coordinate transformation relation.
5. A gas-liquid interface position identification method, which is suitable for the condition that trace particles exist on both sides of a gas-liquid interface, adopts the total reflection type gas-liquid interface flow display method as claimed in any one of claims 1 to 4,
the method comprises the following steps:
step S1: respectively scattering tracer particles in gas and liquid, and then arranging a sheet light source for illumination, wherein a white screen illuminated by the light source is arranged on one side of the liquid and serves as an imaging surface light source, light rays emitted by diffuse reflection of the white screen irradiate to a gas-liquid interface at an angle larger than or equal to a total reflection critical angle, the surface of the white screen is a curved surface, the light source is arranged near the focus of the white screen, and the light rays from the white screen to the liquid surface cannot be shielded by the light source;
step S2: shooting tracer particle images by using two high-speed cameras on two sides of a gas-liquid interface respectively, exposing at equal time intervals, and calibrating the cameras on the two sides of the interface respectively, wherein the shooting directions of the two high-speed cameras on one side of liquid are opposite to the direction of reflected light;
step S3: identifying the interface position at the initial moment in images obtained by two cameras shooting an area above the interface;
step S4: preprocessing images obtained by two cameras in an area above a shooting interface at the same time;
step S5: identifying the gas-liquid interface position in each subsequent frame of image;
step S6: performing time-space smoothing operation on the gas-liquid interface position to obtain the gas-liquid interface position in the image obtained by two cameras for shooting the area above the interface;
step S7: the gas-liquid interface position in the image obtained by the two cameras in the area below the shooting interface is obtained according to the gas-liquid interface position in the image obtained by the two cameras in the area above the shooting interface.
6. The gas-liquid interface position identification method according to claim 5,
the step S3 specifically includes the following steps:
step S31: if the gas movement speed is far greater than the liquid movement speed, the brightness of each pixel is the minimum of the brightness of the corresponding position in the plurality of pictures; otherwise, skipping the step;
step S32: carrying out gray scale morphological opening operation on the picture for a plurality of times;
step S33: setting a brightness threshold to identify particles in the liquid to obtain a binary image;
step S34: opening the binary image for a plurality of times, and then closing the binary image for a plurality of times;
step S35: setting a height value y0 according to the height of the interface in the image, wherein the height position of the interface is below a straight line with the height being the value, and the part with the height being above y0 is set as 0;
step S36: recording the highest position among all non-zero values of each column of pixels in the binary image, and recording the position of the last pixel of a column to obtain a sequence if no non-zero value exists in the column;
step S37: performing median filtering and Gaussian filtering processing on the sequence to obtain the position of an interface;
step S38: calculating the average value of the interface positions in the pictures obtained by the two cameras;
step S39: an offset is set and then the initial interface position is moved upward according to the offset.
7. The gas-liquid interface position identification method according to claim 6,
the step S5 specifically includes the following steps:
step S51: setting a brightness threshold value for the preprocessed image to identify particles in the liquid so as to obtain a binary image;
step S52: opening the binary image for a plurality of times, and then closing the binary image for a plurality of times;
step S53: translating the interface of the previous frame of image upwards by 15 pixels to serve as a boundary, and setting the binary image numerical value of an area above the boundary to be 0;
step S54: recording the highest position among all non-zero values of each column of pixels in the binary image, and if no non-zero value exists in a certain column, recording the smaller value of the position of the last pixel of the column and the same position of the interface of the previous frame, thereby obtaining a sequence;
step S55: performing median filtering and Gaussian filtering on the sequence to obtain the position of an interface;
step S56: the position of the interface is moved upward according to the offset.
8. The gas-liquid interface position identification method according to claim 7,
the step S6 specifically includes the following steps:
step S61: calculating the cross-correlation value of the interface position between every two frames every several frames;
step S62: adding the multiple cross-correlation values to find a peak position;
step S63: repeating the steps S61 and S62 for all calculable moments to obtain a time sequence of the interface speed;
step S64: performing phase-locking average calculation on the interface position based on the obtained time sequence of the interface speed;
step S65: and performing time-space smoothing operation on the boundary positions at all the moments to obtain a smoothed boundary position time-space sequence.
9. The gas-liquid interface position identification method according to claim 8,
the step S7 specifically includes the following steps:
step S71: transforming images obtained by two cameras in an area below a shooting interface from original images to images in a physical space coordinate system according to a coordinate transformation relation obtained by calibration;
step S72: checking whether the positions of interfaces in the images obtained by the two cameras after coordinate transformation are overlapped, and if not, performing self-calibration on the two cameras in the area below the shooting interface;
step S73: zooming and interpolating the time-space sequence of the boundary position to adapt to images obtained by two cameras in an area below a shooting interface after coordinate transformation;
step S74: temporarily setting the initial offset of the interface position, displaying images of the two cameras after spatial transformation and the temporary position of the interface after offset on one picture, wherein the images of the two cameras respectively occupy different color channels, and outputting videos consisting of the pictures at different moments;
step S75: observing the difference between the tentative interface position in the video and the interface position reflected by the brightness of the particles in the picture, and modifying the offset of the interface position according to the difference;
step S76: and repeatedly executing the step S74 and the step S75 until the temporary position of the interface is coincided with the position of the interface reflected by the brightness of the particles in the picture.
Priority Applications (1)
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