Small amplitude kinematic wave propagation in two-component media

Abstract

The speed and attenuation of small amplitude kinematic waves were measured in vertical bubbly and particulate flows in a continuous medium of water. This was done by evaluating the time delay and phase lag of coherent random fluctuations in the volume fraction signal at two measuring locations. The volume fraction was monitored using two closely spaced Impedance Volume Fraction Meters (Kytomaa (1986)). Using the broad-band volume fraction perturbations yields the dependence of the kinematic speed and attenuation of wave number from a single experiment for one set of conditions. The kinematic waves were found to be non-dispersive. Bubbly flows are observed to undergo a change in flow regime at an approximate volume fraction of 45%. Prior to onset of churn-turbulence, a sharp drop in kenematic wave attenuation is observed above volume fractions of 40%. When further increase in volume fraction is attempted, the homogeneous dispersion suddenly becomes unstable. The particulate flows remain uniformly dispersed for all volume fractions, but above a value of ~55%, the mixture flows like a solid plug. The volume fraction fluctuations become incresingly persistent as the volume fraction approaches the solidification value, but no instability is observed. It is argued that the inability of air-water flows to withstand bubble-bubble forces without break-up may account for the differences between the bubbly and particulate flow results.

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