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Физика
Acoustic streaming (AS) is a bulk flow caused by attenuation of an acoustic wave propagating in the medium. When cavitating bubbles are present in the fluid, they actively absorb and re-radiate acoustic waves, generating acoustic streaming. Therefore, measurements of acoustic streaming can provide information on the cavitation field. The prevalent methods used in studies of cavitation are optical and acoustical. The shielding effect of surrounding bubbles makes it difficult to obtain information about the central zones of the cavitating cloud. In this work, we applied magnetic resonance imaging (MRI) methods to studies of AS in the cavitating fluid. MRI methods are very flexible because many physical factors such as viscosity, velocity, diffusion, etc. may modulate the MRI signal. The addition of magnetic field gradients allows the resolution of spatial or dynamical information in three dimensions non-invasively and from an arbitrarily opaque medium. Our experiments were performed with a 31 kHz ultrasonic transducer, which was immersed in a cylindrical vessel aligned with the direction of the main magnetic field. Transient cavitation was studied. We obtained velocity and dispersion maps of cavitating water with a temporal resolution of 20 s, a velocity resolution of 0.56 mm/s and a spatial resolution of 1 mm along the axis. The measurements were performed for air-saturated water, degassed water, water with surfactant (SDS) and water with SDS and NaCl. The results demonstrated markedly different streaming patterns. Whereas an initially strong streaming in saturated water was damped quickly, streaming in water with SDS was present for longer. The addition of NaCl to water with SDS decreased the AS. The dispersion maps also indicate different dynamic behaviours of the fluids. The results can be explained by the modifying effects of surfactants on the coalescence of cavitating bubbles.
Примечание. Тезисы докладов публикуются в авторской редакции
Дата последней модификации: 06-Jul-2012 (11:52:45)