Ultrasonic Cavitation Structures in a Conical Vessel
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During the acoustic cavitation of non-Newtonian liquids, the bubbles that emerge upon “isothermal rupture” of a liquid do not disperse homogenously. The ultrasonic method provides well-defined pressure fields, and its combination of interfacial motion, vortices, and liquid streams generate self-organized structure ensembles in the form of clusters, filamentary patterns extending to webs of bubbles, and dense clouds in the bulk liquid. This behavior is not clearly identified by a simple inspection of the dynamic models. Therefore, it is essential to obtain insight into the collective and bulk behavior of these fluid structures from the experiments. In particular, we observe the geometrical characteristics of the recurrent structures in a cavitating flow and analyze their fluctuations as a function of driving frequency from 20 kHz to 200 kHz, using a viscoelastic liquid inside a conical glass vessel. We implemented a variable ultrasonic frequency generator with a bandwidth of 2 MHz and 50 W power that replaced the amplifier of a commercial acoustic horn system, and used as an ultrasonic source. This, along with a high-definition digital camera and acoustic emission measurements were used to characterize the behavior of the cavitation structures as a function of driving frequency. This system induces cavitation in the viscoelastic liquid confined in an axisymmetric conical glass vessel with a volume of 1000 mL. We used degassed 1,2-propanediol with an initial temperature between 5–9°C. Experimental runs showed unexpected self-organization as the frequency increased, with bubble accumulations, an acoustic pressure field, and liquid viscosity. We observed that outside of the strong acoustic field, small increases in both the frequency and liquid temperature produced a significant change in the type of fluidic structure observed, with a bubble cloud transforming into multiple bubble rings.