PubMed · 713763
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M A Ronkin, B N Viktorov, A P Shibulkin. [Multichannel rheograph with automatic calibration].. https://pubmed.ncbi.nlm.nih.gov/713763/
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The thixotropic behavior of sewage sludge is analysed by varying several parameters such as the time of rest, the slope of the shear rate ramp or the data sampling. We observed that only the initial stress overshoot is time-dependent but always appears, for a given material, at the same critical angle which indicates it is linked to elastic effects: during shear, the material has a short memory. The visualisation of the velocity profile within the sheared material confirms this assumption. We also demonstrate that the hysteresis area is highly dependant on the data sampling and cannot be considered as a material characteristic. Finally, we pointed out that shear rate cycles are not the most powerful method to characterize thixotropy.
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Particle image velocimetry (PIV) is applied for the first time to study Eckart streaming induced by a medical ultrasonic transducer operating at a frequency of 3.3 MHz and effective acoustic intensities of 0.25 and 3 Wcm(-2). A temporal series of velocities in a two-dimensional plane were recorded resulting in an experimental set comprising over half a million velocity data points. These enabled average and fluctuating properties to be determined and clearly indicated the quasi-steady nature of the flow. The average large scale velocity fluctuations along the axis caused by this quasi-steady property were calculated to be 2 and 20 m ms(-1) at effective intensities of 0.25 and 3 Wcm(-2) respectively corresponding to approximately 25% of the peak flow velocity in both cases. Furthermore averaged shear rates were calculated with peak values of 1 and 8 s(-1) for the low and high intensities respectively. The present investigation indicates the usefulness of PIV for such studies and serves as a prelude to investigations of streaming in biological type fluids.