Full text loading...
Sketch of the geometry of the experiment to correlate the wall shear stress with the bubble dynamics. The bubble is generated at the laser focus at -position . The center of the probe is located at .
Simultaneous measurement of the bubble shape, the acoustic emission, and the shear stress exerted on the boundary. The selected frames in the top row present the bubble pulsation close to a boundary. The bottom part of each frame is a reflection from the boundary and helps to locate the position of the wall. The bubble collapses aspherically with the formation of a re-entrant jet and deforms during re-expansion into a pancake shape. Acoustic emission (third row) is recorded during the bubble creation and bubble collapses; yet, intense shearing of the liquid close to the boundary (second row, measured at the marked position in the first frame) is only observed after the first collapse. Thus maximum wall shear stress does not occur during bubble collapse but occurs when the radial spreading jet flows across the sensor. The maximum bubble radius is 0.75 mm and the bubble is created 0.8 mm away from the boundary. The dashed vertical lines indicate the time of the picture taken and depicted in the top row.
In the upper part of the figure the maximum of the wall shear stress as a function of the distance for a stand-off distance of is plotted. At the three marked positions a, b, and c, the time resolved signals are displayed below. The inset of the upper plot depicts the wall shear stress double logarithmically and is compared to the Glauert solution (Ref. 14) with a similarity exponent of (● for and ◇ for ).
Article metrics loading...