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

468 Mechanical Engineering

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

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

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

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.

It is found that the presence of the solid boundary has distinct influence on the flow field of a pressurized cavitation bubble and its final collapse. A distance parameter γ=d/RMAX (where RMAX is the maximum radius and *d* is the distance between the bubble center and the solid boundary) is assigned to represent the distance from the center of the bubble to the solid boundary. When γ is in the range of 1 3 , counter jet could be observed. However, no counter jet is generated under the condition of γ > 3. The results are described below:

**3. Flow field measurement of the collapse of cavitation bubbles** 

size of a single cavitaiton bubble could be generated.

profile with their subsequent analysis.

cylindrical tube.

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 from the first to the third rows of Figure 3.

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

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

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,

Experimental Study on Generation

of Single Cavitation Bubble Collapse Behavior by a High Speed Camera Record 471

Fig. 5. Upper Part: The process of bubble collapse at γ≈ 2 (the Kelvin-Helmholtz vortex is indicated by a dotted line with an arrow, the counter jet indicated by a solid line with an arrow). The peak strength of the pressure wave is 250kPa. The image time interval is 1/4000 second. The size of each individual frame is 9.4mm 3.1 mm. Rmax is 2.5mm. (Lower Part:

sketch of Kelvin-Helmholtz vortex forming the counter jet.

during the bubble surface was pressured to touch the solid boundary, the pressure is uniformly distributed across the tube area, the bubble deformation was approximate a symmetrical development condition. Under this condition, using a high speed camera and 2D PIV method could be obtained flow field. Figure 4 show the velocity flow field of the Kelvin-Helmholtz vortex formation process that used the PIV method to obtain flow field variation during the liquid jet to form vortex formation. The jet flow instantaneously spouted into the static fluid that cause between the jet flow and static fluid shear force difference increased, then the Kelvin-Helmholtz vortex formation is generated, as shown in Figure 4. From these series of images, the features of the cavitation bubble collapse without solid boundary effect are clearly manifested.

Fig. 4. Exhibit the PIV measurement results at 7 . (The velocity flow field of the Kelvin-Helmholtz vortex formation process) The peak strength of the pressure wave is 155 kPa. Image interval time is 1/4000 second. The size of each individual frame is 11.0 mm 3.1 mm. The bubble Rmax is 2. 3 mm.
