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Particle vibration at the boundary in turbulent shear flow

University of British Columbia

Observations were made of the vibratory motion of individual gravel particles near the threshold of motion in a flume. Since it is not known what flow-boundary parameters modify the pressure and velocity fluctuations, a phenomenological approach was used. The study focuses on the processes and conditions that result in vibration and on the factors that modify the vibration frequency. Four hypotheses that may provide an explanation for the vibration were investigated: a) mechanical instability of the particles; b) self excitation arising from wake shedding; c) wake interaction or vorticity amplification leading to vibration; d) excitation arising from turbulent bursting. Individual particles were observed to exhibit irregular vibratory motion. Measurements of the vibration period of gravel in water were found to conform to the scaling relationship proposed by Rao, Narasimha and Badri Narayanan (1971) for the period of turbulent bursts in air and water. Measurements taken by Vanoni (1964) and Sutherland (1967) for the motion of sand in water and by Lyles (1970) for sand in air are shown also to conform to this scaling relationship. As flow parameters approach the threshold condition for a particle, the non-dimensional vibration period consistently decreases towards a value of approximately five. This possibly may provide an objective criterion to determine the threshold of motion. On this criterion, there appears to be no basis for differentiating entrainment mechanisms for coarse sand and gravel - at least for normally loose boundaries. The present work supports the modification of Sutherland's entrainment mechanism by Sumer and Oguz (1978): Rather than a transverse vortex whose lower most portion rotates in the same direction as the mean flow, they propose that the vortex rotates counter to the mean flow. This would be consistent with observations obtained in flow visualization studies (Offen and Kline, 1975) and the correspondence found between particle vibration frequency and the burst periodicity found in this work. Particle vibration and entrainment are considered to result from local, temporarily adverse pressure gradients imposed on the wall by high speed fluid sweeps that form transverse vortices as part of the turbulent burst sequence.

Text

2010-03-25

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http://hdl.handle.net/2429/22500

Geography

University of British Columbia

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