Kl De Graaf

Bubble breakup in a turbulent shear layer

The breakup of a millimetre size buoyantly rising bubble encountering a horizontal plane turbulent jet is experimentally investigated using high-speed shadowgraphy and acoustic techniques. The bubble diameter to jet height ratio is 0.75 and the jet height based Reynolds number is 4000. The high-speed imaging was recorded at 7 kHz simultaneous with hydrophone output at 100 kHz. Bubble breakup events were seen to produce simple binary divisions into products of similar size as well as three products where at least one was much smaller than the other products. Coalescence of products was also observed. In almost all cases time-frequency analysis of the acoustic emissions enabled the products to be identified and sized.

Cavitation due to an impacting sphere

Cavitation associated with the impact of a sphere on a flat surface is investigated using high-speed photography. The sphere, of diameter 15 or 45 mm and made from Ertacetal® or stainless steel, was fully submerged and accelerated using a spring-loaded mechanism to achieve Reynolds numbers based on impact velocity and sphere radius of up to 7.2×104. The static pressure and impact velocity were varied to achieve cavitation numbers ranging from 8.9 to 120.9. High-speed photography of the impacting sphere and induced cavitation bubble was filmed at 105-140 kHz. A log law relationship was found between the non-dimensional maximum bubble radius and the cavitation number. The relationship was modulated by the material properties. Interaction between the sphere and the bubble was also noted.