JP7281396B2 - Particle image imaging method and particle velocity distribution creation method - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies Publisher name: Acoustical Society of Japan, Publication name: Acoustical Society of Japan 2019 Autumn Research Presentation Abstract, Publication date: August 21, 2019 Meeting name: Nihon Onkyo Academic Conference 2019 Autumn Research Presentation, Ritsumeikan University, Biwako-Kusatsu Campus, Date: September 4, 2019

The present invention relates to a particle image capturing method and a particle velocity distribution creating method for visualizing a sound field.

Many attempts have been made to visualize sound fields when investigating unknown acoustic phenomena. Among them, PIV (particle image velocimetry) is a method of photographing air vibrations using a camera, which has the great advantage of being able to measure the spatial distribution of particle velocities without contact (e.g., non-patent literature 1).

Shigeto Takeoka, 3 others, "Sound field recording by PIV method using high-speed camera", Proceedings of Acoustical Society of Japan, March 2010, p. 1451-1452

However, sound-induced vibrations are short-period vibrations that cannot be captured by ordinary cameras. Therefore, at least a high-speed camera is required to use conventional PIV in the sound field.

Therefore, an object of the present invention is to provide a particle image capturing method and a particle velocity distribution generating method for visualizing a sound field by PIV without using a high-speed camera.

The present invention has been made to solve at least part of the above problems, and can be implemented as the following aspects or application examples.

[1] One aspect of the particle imaging method according to the present invention is
a step of irradiating tracer particles dispersed in a stationary sound field in which a pure tone of frequency f _S is dominant with a light sheet blinking at frequency f _L from a light source;
a step of capturing an image of the tracer particles irradiated with the light sheet by an imaging means to obtain a plurality of particle images including a vibration component of frequency f _A obtained by the following formula (1);
including
The imaging means is characterized in that the frame rate (fps) is at least twice the frequency _fA .

［２］上記粒子画像撮像方法の一態様において、
前記フレームレートは、前記周波数ｆ_Ｌよりも低い値とすることができる。

[2] In one aspect of the particle image capturing method,
The frame rate may be lower than the frequency _fL .

[3] One aspect of the method for creating a particle velocity distribution according to the present invention is
a step of obtaining a flow velocity distribution by a particle image velocimetry using the particle images obtained at two times in one aspect of the particle image imaging method;
a step of Fourier transforming the flow velocity distribution on the time axis to remove the direct current component due to the airflow to obtain a pseudo-particle velocity distribution composed of the oscillation component of the frequency f _A ;
obtaining a particle velocity distribution by multiplying the pseudo-particle velocity in the pseudo-particle velocity distribution by f _S /f _A ;
characterized by comprising

According to one aspect of the particle image capturing method according to the present invention, a particle image for visualizing a sound field can be obtained by PIV without using a high-speed camera. Moreover, according to one aspect of the method for creating a particle velocity distribution according to the present invention, it is possible to obtain a particle velocity distribution in which a sound field is visualized by PIV without using a high-speed camera.

1 is a flow chart of a particle image capturing method and a particle velocity distribution creating method according to an embodiment of the present invention; 1 is a schematic diagram of a particle imaging device; FIG. It is a schematic diagram explaining the relationship between the frequency _fS and the frequency _fA . It is an example of a particle image. FIG. 10 is a diagram showing the flow velocity distribution with the speaker turned off; It is a figure which shows particle velocity distribution when a phase is set to 0 (rad). It is a figure which shows particle velocity distribution when phase is set to (pi)/2 (rad). It is a figure which shows particle velocity distribution when phase is set to (pi) (rad).

Preferred embodiments of the present invention will be described in detail below with reference to the drawings. It should be noted that the embodiments described below do not unduly limit the scope of the invention described in the claims. Moreover, not all the configurations described below are essential constituent elements of the present invention.

1. Overview of Particle Image Imaging Method and Flow Velocity Distribution Creation Method An embodiment of a particle image imaging method and a particle velocity distribution creation method will be described with reference to FIGS.

FIG. 1 is a flow chart of a particle image capturing method (S10) and a particle velocity distribution creating method (S20) according to one embodiment of the present invention. As shown in FIG. 1, the particle velocity distribution generating method (S20) is executed after the particle image capturing method (S10). The particle image capturing method (S10) includes a step of irradiating (S12) and a step of obtaining a particle image (S14). The particle image capturing method (S10) may be a separate process from the particle velocity distribution creating method (S20).

1.1. Particle Imaging Apparatus First, the particle imaging apparatus 1 used in the particle imaging method (S10) will be described with reference to FIG. FIG. 2 is a schematic diagram showing an overview of the particle imaging device 1. As shown in FIG.

As shown in FIG. 2, the particle imaging device 1 includes a sound source unit 10, a light source unit 20, an acoustic tube 30, a video camera 40, and a computing section .

The sound source unit 10 generates a stationary sound field in which pure tones of frequency f _S predominate. The sound source unit 10 includes a speaker 12 attached to one end of an acoustic tube 30 and an amplifier 14 and an oscillator 16 electrically connected to the speaker 12 . The speaker 12 excites the primary mode in the acoustic tube 30 by outputting a pure tone.

The acoustic tube 30 is a rectangular parallelepiped made of transparent walls. The acoustic tube 30 has an internal space that is sealed so as not to be affected by air currents from the outside. The speaker 12 is attached to one end of the acoustic tube 30 in the longitudinal direction (the direction along the Z-axis in FIG. 2), and the sound level meter 32 is attached to the other end. The wall of the acoustic tube 30 may be at least partially transparent to the extent that a particle image can be picked up, and the transparent wall is composed of, for example, an acrylic plate or a glass plate. Inside the acoustic tube 30, the output from the speaker 12 excites the primary mode in the longitudinal direction of the acoustic tube 30, forming a stationary sound field in which the pure tone of the frequency _fS is dominant. A sound level meter 32 measures the sound pressure level inside the acoustic tube 30 .

Inside the acoustic tube 30, tracer particles are dispersed in a gas such as air or an inert gas. As the tracer particles, known gas-phase tracer particles used in PIV can be employed, such as cigarette smoke, incense smoke, oil mist, and the like.

Here, the steady sound field in which the pure tone of frequency f _S is dominant is provided as the acoustic tube 30, but the present invention is not limited to this, and a space with little influence of air currents capable of capturing particle images with the video camera 40 can be adopted. .

The light source unit 20 irradiates a light sheet 28 flickering at a frequency _fL to the tracer particles dispersed in a stationary sound field in which the pure tone of the frequency _fS is dominant. The light source unit 20 includes an irradiation device 22 that is a light source that irradiates a light sheet 28 , a motor 24 , and a disk 26 that rotates by the motor 24 . The irradiation device 22 can irradiate the light sheet 28 with, for example, a YVO ₄ laser, a YAG laser, or the like. Slits are formed in the disk 26 at predetermined intervals. Rotating a disk 26 by a motor 24 in front of the illumination device 22 causes the light sheet 28 to flash at a frequency _fL . The configuration of the light source unit 20 is not limited to this, and a known light source used for PIV can be adopted, for example, a laser irradiation device capable of pulse emission may be used.

Light sheet 28 is a thin light sheet extending along the YZ direction in FIG. 2 that is transmitted through the transparent walls of acoustic tube 30 .

A video camera 40 images the scattered light from the tracer particles illuminated by the light sheet 28 inside the acoustic tube 30 . The video camera 40 can capture images at a predetermined frame rate (fps: frames per second), and the frame rate can be lower than that of a high-speed camera used for visualizing a sound field by conventional PIV. A commercially available digital video camera can be adopted as the video camera 40 .

Image data picked up by the video camera 40 is image-analyzed by PIV by the calculation unit 42, and further Fourier-transformed to create a desired flow velocity distribution. The computing unit 42 can employ a personal computer, and includes, for example, a CPU (Central Processing Unit), HDD (Hard Disk Drive), SSD (Solid State Drive), ROM (Read-Only Memory), RAM (Random Access Memory). ), an input device such as a keyboard, a mouse, and a touch pad, and a display device such as a liquid crystal display and an organic EL (Electro Luminescence) display.

1.2. Particle Image Imaging Method Next, an embodiment of the particle image imaging method (S10) will be described with reference to FIGS. 1 to 3. FIG. FIG. 3 is a schematic diagram for explaining the relationship between the frequency f _S and the frequency f _A , where the horizontal axis indicates time and the vertical axis indicates the displacement of the tracer particles.

As shown in FIG. 1, the particle image capturing method (S10) includes a step of irradiating (S12) and a step of obtaining a particle image (S14).

The irradiating step (S12) is a step of irradiating the light sheet 28 flickering at the frequency _fL from the light source to the tracer particles dispersed in the stationary sound field in which the pure tone of the frequency _fS is dominant. In the particle imaging apparatus 1 shown in FIG. 2, the sound tube 30, in which tracer particles such as incense smoke or the like are dispersed in the air, becomes a stationary sound field in which the pure tone of the frequency _fS is dominant due to the output from the speaker 12. , a light sheet 28 flashing at a frequency _fL from the light source unit 20 is irradiated onto the tracer particles.

In FIG. 3, the short-cycle sine wave indicated by the dashed line indicates the pure tone of frequency _fS output from the speaker 12 . Although the frequency _fS is an arbitrary frequency, it is explained here as a pure tone of 121 Hz, for example.

Also, in FIG. 3, the light sheet 28 blinking at the frequency _fL is irradiated from the light source unit 20 in the non-shaded area (the position indicated by the arrow). This flashing of the light sheet 28 causes the tracer particles in the acoustic tube 30 to glow at the frequency _fL . The frequency _fL is determined by the frame rate of the video camera 40 and the frequency _fS , but here, for example, it will be described as flickering at 120 Hz.

The step of obtaining a particle image (S14) is a step of obtaining a plurality of particle images containing a vibration component of frequency f _A obtained by the following formula (1) by imaging the tracer particles irradiated by the light sheet 28 with an imaging means. is. In the particle image capturing apparatus 1 shown in FIG. 2, the video camera 40 captures images of the tracer particles shining at the frequency _fL , and a plurality of particle images are stored in the storage device inside the video camera 40, for example. The stroboscopic effect caused by the blinking of the light source unit 20 causes the tracer particles to oscillate apparently at least at the frequency f _A , as shown in FIG. Therefore, from these multiple particle images, it is possible to observe the apparent movement of the tracer particles including the vibration component of the frequency f _A and the movement due to the airflow inside the acoustic tube 30 .

For example, when the airflow in the acoustic tube 30 is so small that the movement of the tracer particles due to the airflow can be ignored, the flow velocity is measured using PIV on these multiple particle images, and the sound field is set in advance by using a plurality of It can be visualized as a velocity vector at the observation point.

The frame rate (fps) of the imaging means used in the step of obtaining the particle image (S14) is at least twice the frequency _fA . According to the above equation (1), the particle image capturing apparatus 1 in this example has a frequency f _A of 1 Hz (=121 Hz-120 Hz), so the frame rate of the video camera 40 is twice that, which is 2 (fps) or more. Become. In the conventional visualization of the sound field by PIV, a camera with a frame rate of 242 (fps) or more was required to image the vibration of 121 Hz, but according to this embodiment, the frame rate is 2 (fps) or more. Since it is sufficient, for example, a general video camera 40 of 30 (fps) to 60 (fps) can be used to obtain a particle image for visualizing a sound field by PIV.

Also, the frame rate of the video camera 40 is preferably set to a value lower than the frequency _fL . If the frame rate of the video camera 40 is a value lower than the frequency _fL , the video camera 40 can image the blinking tracer particles.

For example, if the video camera 40 to be used in the particle image capturing apparatus 1 is determined, the frequency _fS and the frequency _fL are adjusted based on the above equation (1) so that the frequency _fA is a value equal to or less than half the frame rate of the video camera 40. may be set.

1.3. Step of Obtaining Flow Velocity Distribution Next, a step of obtaining a flow velocity distribution other than the particle velocity distribution generating method (S20) in FIG. 1, which will be described later, will be described. In the step of obtaining the flow velocity distribution, the particle velocity distribution can be obtained using a known PIV using the two-time particle images obtained by the particle image capturing method (S10) of FIG.

As described above, if there is no airflow in the acoustic tube 30, it is possible to obtain the particle velocity distribution by PIV using the particle images taken at two times. The image is actually at a lower frequency than the frequency of the particles vibrating with the sound. Therefore, the particle image obtained by the particle image capturing method (S10) is likely to be affected by air currents.

2. Particle Velocity Distribution Creation Method One mode of the particle velocity distribution creation method (S20) will be described with reference to FIGS. 1 and 4 to 8. FIG. FIG. 4 is an example of a particle image, FIG. 5 is a diagram showing the flow velocity distribution with the speaker 12 stopped, and FIG. 6 is a diagram showing the particle velocity distribution when the phase is 0 (rad). FIG. 7 is a diagram showing the particle velocity distribution when the phase is π/2 (rad), and FIG. 8 is a diagram showing the particle velocity distribution when the phase is π (rad). The images of FIGS. 4-8 are all shown with the images rotated 90 degrees to the right (the Z-axis extends left and right in the figures), so to the left of each image is the speaker 12 in the acoustic tube 30 of FIG. is arranged, and the light source unit 20 is arranged on the lower side.

In FIG. 4, in addition to the shading due to the thickness of the light sheet 28, a striped pattern generated during the diffusion process of the tracer particles can be seen.

FIG. 5 is a diagram showing the flow velocity distribution when the output of the speaker 12 is stopped. Specifically, the flow velocity distribution was obtained by PIV from particle images taken at two times one hour or more after the smoke was sealed in the acoustic tube 30 while the output of the speaker 12 of the particle image capturing device 1 was stopped. is the flow velocity distribution (instantaneous value). Arrows in FIG. 5 indicate flow velocity vectors, and it can be seen from FIG. When the particle image is affected by the airflow in this way, the particle velocity distribution creating method (S20) described below is suitable.

As shown in FIG. 1, the particle velocity distribution creation method (S20) according to one embodiment of the present invention comprises a step of obtaining a flow velocity distribution (S22), a step of obtaining a pseudo particle velocity distribution (S24), and a particle velocity distribution and a step of obtaining (S26).

2.1. Step of Obtaining Flow Velocity Distribution The step of obtaining the flow velocity distribution (S22) is to obtain the flow velocity distribution by the particle image velocimetry (hereinafter referred to as “PIV”) using the particle images obtained at two times in the particle image imaging method (S10). It is a process of obtaining It is desirable that the two times are as short a time interval as possible. It is preferable to obtain the flow velocity distribution by PIV using the two time particle images.

Here, the "flow velocity" does not simply mean the flow velocity due to the air current, but is explained as including the movement velocity of the particles due to sound. Since the tracer particles are affected by the convection inside the acoustic tube 30, the flow velocity distribution includes a vibration component due to sound in addition to the direct current component due to the airflow. Also, the vibration component in the flow velocity distribution is a pseudo vibration component in which the vibration is apparently slowed down by the strobe effect. The pseudo-particle velocity distribution, which will be described later, is a distribution showing the pseudo-particle velocity of only the vibration component due to the pseudo sound in which the vibration is apparently slowed down by the strobe effect by removing the direct-current component due to the airflow from the flow velocity distribution.

PIV can use known PIV analysis software stored in the calculation unit 42 . PIV uses image correlation to quantify the moving distance of a group of particles. For PIV, a direct correlation method or FFT can be used to calculate the correlation value, for example cross-correlation PIV, which analyzes two consecutive frames of exposed particle images, can be employed. Examples of PIV analysis software include Python OpenPIV Library v.1. 0.20.9, ZVECTOR, VisiVector DP2D/3D, etc. can be employed.

2.2. Step of Obtaining Pseudo-Particle Velocity Distribution The step of obtaining the pseudo-particle velocity distribution (S24) includes Fourier transforming the flow velocity distribution on the time axis to remove the direct-current component due to the airflow, thereby obtaining a pseudo-particle velocity distribution consisting of an oscillation component with a frequency _fA. is the process of obtaining

Since the flow velocity distribution obtained by the step of obtaining the flow velocity distribution (S22) described above contains a direct-current component due to the airflow as described above using FIG. The DC component due to the airflow can be removed by Fourier transform. The pseudo-particle velocity distribution thus obtained is a distribution of pseudo-particle velocity vectors composed of vibration components of frequency f _A at the observation point. The Fourier transform can be performed by the calculator 42 .

2.3. Step of Obtaining Particle Velocity Distribution The step of obtaining the particle velocity distribution (S26) obtains the particle velocity distribution by multiplying the pseudo-particle velocity in the pseudo-particle velocity distribution by f _S /f _A .

Since the pseudo-particle velocity distribution obtained by the step of obtaining the above-described pseudo-particle velocity distribution (S24) is a distribution of apparent velocity vectors f _A /f _S times the actual particle velocity due to the strobe effect, The actual particle velocity distribution can be obtained by multiplying each pseudo-particle velocity by f _S /f _A . That is, in this step, the actual particle velocity is obtained by back-calculating the amount by which the vibration is slowed down by the strobe effect.

6 to 8 show the phases of the tube axis direction (Z-axis direction) component at the center of the image for the 1 Hz component (frequency f _A component) at each point in the flow velocity distribution, respectively, 0 (rad) and π/2 Particle velocity distribution obtained by multiplying the pseudo-particle velocity at each point by f _S /f _A is an example. 6 to 8, the left side of the drawing is the speaker side. In this manner, the particle velocity near the center of the acoustic tube 30 in the primary mode occurring within the acoustic tube 30 can be accurately measured.

According to one aspect of the flow velocity distribution creating method according to the present invention, it is possible to obtain a particle velocity distribution in which a sound field is visualized by PIV without using a conventional high-speed camera. By increasing the degree of freedom in camera selection, it is possible to adopt a camera that is inexpensive, highly sensitive, and has a large number of pixels.

The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same function, method, and result, or configurations that have the same purpose and effect). Moreover, the present invention includes configurations in which non-essential portions of the configurations described in the embodiments are replaced. In addition, the present invention includes a configuration that achieves the same effects or achieves the same purpose as the configurations described in the embodiments. In addition, the present invention includes configurations obtained by adding known techniques to the configurations described in the embodiments.

DESCRIPTION OF SYMBOLS 1... Particle imaging device 10... Sound source unit 12... Speaker 14... Amplifier 16... Oscillator 20... Light source unit 22... Irradiation device 24... Motor 26... Disc 28... Optical sheet 30... Acoustic tube 32 Sound level meter 40 Video camera 42 Operation unit

Claims (3)

a step of irradiating tracer particles dispersed in a stationary sound field in which a pure tone of frequency f _S is dominant with a light sheet blinking at frequency f _L from a light source;
a step of capturing an image of the tracer particles irradiated with the light sheet by an imaging means to obtain a plurality of particle images including a vibration component of frequency f _A obtained by the following formula (1);
including
The particle image capturing method, wherein the image capturing means has a frame rate (fps) that is at least twice the frequency _fA .

In claim 1,
The particle image capturing method, wherein the frame rate is a value lower than the frequency _fL . a step of obtaining a flow velocity distribution by a particle image velocimetry using the two-time particle images obtained in claim 1 or claim 2;
a step of Fourier transforming the flow velocity distribution on the time axis to remove the direct current component due to the airflow to obtain a pseudo-particle velocity distribution composed of the oscillation component of the frequency f _A ;
obtaining a particle velocity distribution by multiplying the pseudo-particle velocity in the pseudo-particle velocity distribution by f _S /f _A ;
A method for creating a particle velocity distribution, comprising:

Images