**3.3 Flow field measurement of bubble collapse at γ≈ 3**

The generation of the counter jet needs to satisfy the condition of 1 < γ ≈ 3. A critical value of γ ≈ 3 is found to be a decisive value for the generation of a counter jet. In this study, three different strengths of pressure waves were used to trigger the breakdown the cavitation bubble. Flow field observation for this process of the collapse of cavitation bubble is carried out under this critical condition.

The images located at the first and second row of Figure 9 revealed the flow field of bubble collapse under a pressure wave of 200 kPa in strength. A liquid jet was formed followed by penetrating the bubble surface to produce the jet flow and the Kelvin-Helmholtz vortex. The bubble was divided into two small bubbles because the Kelvin-Helmholtz vortex did not touch the solid boundary. This process of collapse was similar to the case at γ ≈ 7 where the counter jet was not generated.

Fig. 9. Images of the process of bubble collapse at γ ≈ 3. The peak strength of the pressure wave is 200 kPa. The size of each individual frame for is 10.5 mm 3.1 mm, Rmax : 2.55 mm. The image time interval is 1/4000 second. (The Kelvin-Helmholtz vortex is indicated by a dotted line with an arrow).

The process of the collapse of the bubble, with the strength of pressure wave increased to 300 kPa, is shown from the third to the sixth rows in Figure 10. Unlike the semi-hemispheric form of the Kelvin-Helmoholtz vortex shown in the third row of Figure 9, the vortex shown here was clearly influenced by the solid boundary when the liquid jet penetrated the bubble surface. The right side of the head of the vortex touched the solid boundary and turned into a planiform shape before splashing and spreading outwardly along its surrounding interface. On the other hand, before the head of the vortex touched the solid boundary, the outer ring of the vortex had already touched the tube wall and started spreading outwardly shown in the images at the first row in Figure 10. This spreading vortex kept

near the solid boundary. The counter jet formation is located at between two stagnation rings. The above-mentioned of the PIV calculation results reveal that the stagnation ring and

The generation of the counter jet needs to satisfy the condition of 1 < γ ≈ 3. A critical value of γ ≈ 3 is found to be a decisive value for the generation of a counter jet. In this study, three different strengths of pressure waves were used to trigger the breakdown the cavitation bubble. Flow field observation for this process of the collapse of cavitation bubble is carried

The images located at the first and second row of Figure 9 revealed the flow field of bubble collapse under a pressure wave of 200 kPa in strength. A liquid jet was formed followed by penetrating the bubble surface to produce the jet flow and the Kelvin-Helmholtz vortex. The bubble was divided into two small bubbles because the Kelvin-Helmholtz vortex did not touch the solid boundary. This process of collapse was similar to the case at γ ≈ 7 where the

Fig. 9. Images of the process of bubble collapse at γ ≈ 3. The peak strength of the pressure wave is 200 kPa. The size of each individual frame for is 10.5 mm 3.1 mm, Rmax : 2.55 mm. The image time interval is 1/4000 second. (The Kelvin-Helmholtz vortex is indicated by a

The process of the collapse of the bubble, with the strength of pressure wave increased to 300 kPa, is shown from the third to the sixth rows in Figure 10. Unlike the semi-hemispheric form of the Kelvin-Helmoholtz vortex shown in the third row of Figure 9, the vortex shown here was clearly influenced by the solid boundary when the liquid jet penetrated the bubble surface. The right side of the head of the vortex touched the solid boundary and turned into a planiform shape before splashing and spreading outwardly along its surrounding interface. On the other hand, before the head of the vortex touched the solid boundary, the outer ring of the vortex had already touched the tube wall and started spreading outwardly shown in the images at the first row in Figure 10. This spreading vortex kept

the counter jet formation identically with Figure 5 lower part schematic diagram.

**3.3 Flow field measurement of bubble collapse at γ≈ 3** 

out under this critical condition.

counter jet was not generated.

dotted line with an arrow).

Fig. 10. Images of the process of bubble collapse at γ ≈ 3. The peak strength of the pressure wave is 300kPa. The size of each individual frame is 9.6 mm 3.1 mm. Rmax : 2.35 mm. The image time interval is 1/4000 second. (The Kelvin -Helmholtz vortex is indicated by a dotted line with an arrow).

moving towards the right side until it touched the solid boundary and generated a subsequent shock wave which rebounded to produce the phenomenon of Richtmyer-Meshkov instability shown near the solid boundary in every image from the second to the third rows of Figure 10. Although the Kelvin-Helmholtz vortex could be generated under this strength of pressure wave, the vortex had already splashed and touched the surrounding solid boundary, disabling the vortex from forming the stagnation ring and the counter jet. At the end of this process, the bubble was divided by the liquid jet and the root of the vortex into two smaller bubbles shown in the images from the fifth to seventh rows in Figure 10.

If the strength of the pressure wave is increased to a peak value of 365 kPa, the Kelvin-Helmholtz vortex would touch the solid boundary before the formation of the stagnation ring and the counter jet. This process is shown in the image listed at the 5th and 6th rows of Figure 11.

Experimental Study on Generation

stagnation ring and the counter jet formation.

of Single Cavitation Bubble Collapse Behavior by a High Speed Camera Record 477

Fig. 12. Images of the process of bubble collapse at γ = 1; the peak strength of the pressure wave is 325 kPa; the image time interval is 1/4000 second. The size of each individual frame is 8.3mm 3.1 mm. Rmax is 2.4 mm. (The counter jet is indicated by a solid line with an arrow).

the three cases mentioned before shown in Figure 12. After the liquid jet penetrated the bubble surface, there is not enough space to form a complete Kelvin-Helmholtz vortex. However, the space between the bubble surface and the solid boundary would still exist a gap allow the formation of stagnation ring after the liquid jet touches the solid boundary. This is followed by an outward splash along the radial direction while the inward stagnation ring was squeezed along the central direction to form the counter jet. Finally the bubble was divided into two smaller bubbles by the counter jet shown in the image of Figure 12 and diagram in Figure 13. In the further, using PIV calculation results shown in Figure 14. This result are clear revealed that the liquid jet direct touch the solid boundary and then form the

Fig. 11. Images of the process of bubble collapse at γ ≈ 3. The peak strength of the pressure wave is 365 kPa. The size of each individual frame is 2.25 mm. The image time interval is 1/4000 second. (The Kelvin-Helmholtz vortex is indicated by a dotted line with an arrow; the counter jet is indicated by a solid line with an arrow).
