Oscillations, Waves, and Interactions - GWDG

154 W. Lauterborn et al.
P koll [kbar]
30
20
10
0
0 0.5 1 1.5 2 2.5 3
R max [mm]
3.3 Laser-bubble erosion
Figure 16. Collapse pressure at
minimum bubble radius from a
laser-induced bubble versus maximum
attained radius. The collapse
pressures are extrapolated
from pressure-distance measurements
taken at the corresponding
maximum bubble radii, Rmax.
The shaded region depicts the error
margins of the extrapolation.
It is long known that cavitation bubbles can damage solid surfaces. Mainly two
characteristic effects are believed to be responsible for the destructive action of cavitation
bubbles: the emission of shock waves upon collapse of the bubble and the
generation of a high-speed liquid jet directed towards the solid boundary. The main
results obtained so far are the following [22]. Damage is observed when the bubble
is generated at a distance less than twice its maximum radius from a solid boundary
(γ ≤ 2). The impact of the jet contributes to the damage only at small normalized
distances (γ ≤ 0.7). The largest erosive force is caused by the collapse of a bubble in
direct contact with the boundary, where pressures of up to several GPa act on the
material surface. Bubbles in the ranges γ ≤ 0.3 and γ = 1.2 to 1.4 cause the largest
damage.
Figure 17 gives an example from the numerous measurements done. It shows
the collapse of a bubble immediately touching the surface (γ =0.5) with high time
Figure 17. First collapse of a laser-induced bubble near a solid wall with γ = 0.5 taken
at one million frames per second. The maximum radius of the bubble was 1.45 mm. Upper
sequence: side view; lower sequence: bottom view trough the transparent solid wall. Both
views together demonstrate the transformation of the spherical bubble into a toroidal bubble
after the jet has formed.