**2. Experiment Instrument and cavitation bubble generator**

The experimental setup for the flow field measurement of cavitation bubble collapse is shown in Figure 1. This device is consisted of an insulated optical platform, a motor, a rotatable U-shape platform, a transparent cylindrical tube, a set of light sources, a shock wave pressure generator, a high speed camera and a pressure sensor. The DC brushless motor is capable to produce a maximum controlled rotational velocity up to 2,000 RPM, to supply a maximum power up to two horsepower, and to stop the rotational motion in a relatively short period of time.

Fig. 1. Schematic diagram of the experiment setup.

The U-shape platform was made up of an acrylic platform of 20 mm in thickness. Centered at the rotational axis of the motor, its rotatable arm has a radius of 250 mm, which resulted in a total horizontal length of 500 mm. Two vertical forearms each of 150 mm in height are fixed to the edge of the platform. On the platform of the horizontal rotatable level arm sites the transparent cylindrical tube of 200 mm in length, with its internal and external diameter of 5mm and 8mm respectively. A soft PVC tube with an internal diameter of 5mm is fixed to the vertical forearm in order to conveniently exchange the experimental equipment. At one end, this tube is connected to the shock wave pressure generator with a piston while it is extended to connect the transparent cylindrical tube at the other end. At the extremity of the transparent cylindrical tube, a rigid boundary with a 1 mm drilled hole is set up to connect

cavitation bubble collapse flow are clearly manifested. Improvement in the further used the PIV method that can be clear revealed velocity flow field feature during the bubble collapse. The present study focuses on the investigation of the formation of the liquid jet and the counter jet, at different stand-off distances to the boundary, and their consequent influences

The experimental setup for the flow field measurement of cavitation bubble collapse is shown in Figure 1. This device is consisted of an insulated optical platform, a motor, a rotatable U-shape platform, a transparent cylindrical tube, a set of light sources, a shock wave pressure generator, a high speed camera and a pressure sensor. The DC brushless motor is capable to produce a maximum controlled rotational velocity up to 2,000 RPM, to supply a maximum power up to two horsepower, and to stop the rotational motion in a

The U-shape platform was made up of an acrylic platform of 20 mm in thickness. Centered at the rotational axis of the motor, its rotatable arm has a radius of 250 mm, which resulted in a total horizontal length of 500 mm. Two vertical forearms each of 150 mm in height are fixed to the edge of the platform. On the platform of the horizontal rotatable level arm sites the transparent cylindrical tube of 200 mm in length, with its internal and external diameter of 5mm and 8mm respectively. A soft PVC tube with an internal diameter of 5mm is fixed to the vertical forearm in order to conveniently exchange the experimental equipment. At one end, this tube is connected to the shock wave pressure generator with a piston while it is extended to connect the transparent cylindrical tube at the other end. At the extremity of the transparent cylindrical tube, a rigid boundary with a 1 mm drilled hole is set up to connect

**2. Experiment Instrument and cavitation bubble generator** 

on the bubble collapse flow.

relatively short period of time.

Fig. 1. Schematic diagram of the experiment setup.

the highly sensitive pressure sensor that measures the shock wave pressure at different strengths during the process of the single bubble collapse (shown at the upper part of Figure 1). On the other hand, the cavitation bubble generation takes place at the site on the platform of the rotational axis where the pressure is at the lowest. Therefore, the transparent cylindrical tube must be located across the center of the rotational axis for easier cavitation bubble generation.

During the experiment of generating a single cavitation bubble, the transparent cylindrical tube on the U-shape platform is filled with tap water shown in Figure 2. The surface of the fluid at the part of the vertical forearm tube is in touch with air with one atmosphere pressure. Therefore, the center location of the L tube at initial condition has a hydrostatic pressure of p0

$$\mathbf{p}\_0 \mathbf{=} \mathbf{p}\_{\text{atm}} \mathbf{+} \mathbf{p} \mathbf{g} \Delta \mathbf{h},\tag{2}$$

where patm is the atmosphere pressure, g is the acceleration of gravity, and h is the water depth difference.

Fig. 2. The pressure distribution for a rotating U-shape platform.

When the U-shape platform is rotated by the motor, the fluid is subjected to a centrifugal force resulting in a parabolic fluid pressure distribution shown as the solid line in Figure 2 at different radius. At the vertical forearm, although the h is slightly increased, the hydrostatic water pressure is still kept at one atmospheric pressure because the surface interface is still in touch with the air. Therefore, the pressure difference between the free surface atmospheric pressure and the pressure at the center of rotation is pc

$$P\_c = \text{pg}\Delta\text{h} \cdot 1/2\text{pr}^2\text{o}\Delta\text{h} \tag{3}$$

where r is the rotational radius and is the rotational velocity. When is gradually increased, the pressure at the center of the rotation in the transparent cylindrical tube is gradually decreased to a saturated vapor pressure at local present water temperature. At this condition, a single cavitation bubble at the rotational center can be generated. The

Experimental Study on Generation

from the first to the third rows of Figure 3.

**3.1 Flow field measurement of bubble collapse atγ≈ 7** 

of Single Cavitation Bubble Collapse Behavior by a High Speed Camera Record 469

Under this condition, the distance between the center of the cavitation bubble and the solid boundary is nearly seven time of its radius. The flow field of the process of cavitation bubble collapse is not affected by the solid boundary. Therefore the solid boundary is assumed to be insignificant to the process of bubble collapse. This process of the cavitation bubble being pressurized followed by its final collapse is shown in Figure 3. The pressure wave is sent from the left side of the bubble surface, impacting the bubble with peak strength up to 155kPa. The pressure wave caused a concaved deformation of the bubble shown in images

Fig. 3. Top view of images of the process of bubble collapse at γ≈ 7. From 1st row to 3rd row: image of the inward dent process; 4th and 6th rows: images of the Kelvin-Helmholtz vortex process (the Kelvin-Helmholtz vortex is indicated by a dotted line with an arrow). The peak strength of the pressure wave is 155 kPa. Image interval time is 1/4000 second. The size of

When sufficient energy is accumulated by the liquid jet during its continuous motion to the right side of the bubble, the overlaid surface is squeezed and subsequently spouted into a jet flow. When the jet flow extended to the static fluid at the right side of the bubble, rapid variation in the flow velocity is created which leaded to a Kelvin-Helmholtz vortex shown in images listed in images from the fourth to the sixth rows of Figure 3. Jaw et al. (2007) clearly described the Kelvin-Helmholtz vortex, indicating that the interaction between the pressure

The bubble collapse process is a complicated and three dimension flow structures. Using the 2D PIV analysis method was lacked a vertical direction motion measurement. In other word,

each individual frame is 10.8 mm 3.1 mm. The bubble Rmax is 2.5 mm.

and the velocity variation is the main cause of this phenomenon.

rotational speed needed for generating a cavitation bubble is related to the h . Greater h means a greater rotational velocity required for the production of cavitation bubble. If h is kept constant, an increasing rotational velocity would result in a greater size of cavitation bubble. Therefore by controlling the rotational velocity of the U-shape platform, a desirable size of a single cavitaiton bubble could be generated.

After the cavitation bubble is generated, the U-shape platform is stopped to restore the pressure back to the hydrostatic pressure instantly. This pressure difference alone is not enough to break the cavitation bubble. Therefore, in order to observe the flow field of the collapse of the cavitation bubble, this study uses a pulse setup to hit the piston of the PVC soft tube in contact with the free water surface and instantly generates a shock wave pressure sending an impact to cause the collapse of the cavitation bubble. The signal to propel the pulse setup impacting the piston device is triggered while the image data and the pressure profile are recorded and stored by the computer through the high speed camera and the pressure sensor respectively. This experimental setup allows the real-time recording of the time-series relationships between the flow field image data and the pressure change profile with their subsequent analysis.

A Fastec high speed camera is used to extract and record the experimental images. The speed of image extraction is determined by the size of the image. For example, an image extraction speed of 4,000 frame /second is used for an image size of 1280×128 pixels. A Kulite XTL-190 pressure sensor incorporating with the NI-6221 Analog I/O device are used for the measurement of the pressure profile. The NI-6221 Analog I/O device can send a 10 V signal to drive the pressure sensor and receive a 0 – 0.5 V pressure signal to record data which enables itself for the analysis of the pressure change profile in the transparent cylindrical tube.

On the other hand, PIV method is used Argun laser pass a transparent cylindrical glass to form a light sheet and in the liquid arranged TSI glass bead-hollow particle (8-12μm) to assist the camera catch the particle image during the cavitation bubble collapse process, as shown in upper right schematic diagram in Figure 1. The light sheet thickness is 1.5 mm pass the bubble location and the camera catch the bubble collapse image process then record a cinematograph file. After this file is transfer to several sequence particle image data. Using the particle images and the PIV analysis method can obtain the velocity flow field feature during the bubble collapse process. Therefore, a single cavitation bubble and the subsequent bubble collapse flows induced by pressure waves are easily generated by the experimental setup proposed in this study. Cinematographic analysis of the cavitation bubble collapse flows at different stand-off distances are performed and discussed in the following.
