**3.4 Flow field measurement of bubble collapse at γ ≈ 1 and γ =1**

The other critical value for the formation of the counter jet occurs at γ ≈ 1 where the bubble is tightly close to the solid boundary. In this study, in order to understand the characteristics of the flow fields under this critical condition, measurements of flow fields at both locations where γ is slightly greater than and equal to one were carried out.

1. When the bubble is located at γ slightly greater than 1, there would be a small distance between the bubble surface and the rigid boundary. When the bubble was pressurized and concaved inward, the bubble became more planiform in shape for this deformation was caused by the solid boundary. The area of inward concaved bubble is larger than

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 other critical value for the formation of the counter jet occurs at γ ≈ 1 where the bubble is tightly close to the solid boundary. In this study, in order to understand the characteristics of the flow fields under this critical condition, measurements of flow fields at both locations

1. When the bubble is located at γ slightly greater than 1, there would be a small distance between the bubble surface and the rigid boundary. When the bubble was pressurized and concaved inward, the bubble became more planiform in shape for this deformation was caused by the solid boundary. The area of inward concaved bubble is larger than

the counter jet is indicated by a solid line with an arrow).

**3.4 Flow field measurement of bubble collapse at γ ≈ 1 and γ =1** 

where γ is slightly greater than and equal to one were carried out.

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 stagnation ring and the counter jet formation.

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).

Experimental Study on Generation

images and diagrams in Figure 15.

boundary condition).

of Single Cavitation Bubble Collapse Behavior by a High Speed Camera Record 479

2. Under the condition of γ = 1, the bubble interface was pressurized to form an inward concaved bubble. It was followed by the overlaying of the bubble interfaces on the solid boundary without any spaces left for the fluid. After the liquid jet impacted the solid boundary, it just moved outwardly as a splash along the radial direction. The bubble collapses subsequently on the radial trajectory without forming of the stagnation ring and the inwardly squeezed counter jet. This process of bubble collapse is shown in the

the upper right diagram near solid boundary enlarged the result that can reveal the stagnation ring location and the counter jet formation. ( Note: the velocity vectors near the solid boundary are the particle motion results, not from the solid boundary extra velocity

Fig. 13. sketch of the liquid jet position. (Note: Left diagram: the solid line is the bubble surface and the dotted line with an arrow is the splashing.).

Fig. 14. Upper Part: exhibit the PIV measurement results at 1 . The peak strength of the pressure wave is 320 kPa. Image interval time is 1/4000 second. The size of each individual frame is 8.8.0 mm 3.1 mm. The bubble Rmax is 2. 4 mm. Lower part: exhibit

Fig. 13. sketch of the liquid jet position. (Note: Left diagram: the solid line is the bubble

Fig. 14. Upper Part: exhibit the PIV measurement results at 1 . The peak strength of the

individual frame is 8.8.0 mm 3.1 mm. The bubble Rmax is 2. 4 mm. Lower part: exhibit

pressure wave is 320 kPa. Image interval time is 1/4000 second. The size of each

surface and the dotted line with an arrow is the splashing.).

the upper right diagram near solid boundary enlarged the result that can reveal the stagnation ring location and the counter jet formation. ( Note: the velocity vectors near the solid boundary are the particle motion results, not from the solid boundary extra velocity boundary condition).

2. Under the condition of γ = 1, the bubble interface was pressurized to form an inward concaved bubble. It was followed by the overlaying of the bubble interfaces on the solid boundary without any spaces left for the fluid. After the liquid jet impacted the solid boundary, it just moved outwardly as a splash along the radial direction. The bubble collapses subsequently on the radial trajectory without forming of the stagnation ring and the inwardly squeezed counter jet. This process of bubble collapse is shown in the images and diagrams in Figure 15.

Experimental Study on Generation

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Fig. 15. Images of the process of bubble collapse at γ = 1; the peak strength of the pressure wave is 520 kPa; the image time interval is 1/4000 second. The size of each individual frame is 6.2 mm 3.1 mm. Rmax is 2.25 mm. Lower Part: sketch of the liquid jet position.
