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The effect of bubble size and rise velocity on the efficiency of a foam flotation microalgae harvesting unit was determined. Three sparger and input airflow combinations were used: (1) limewood sparger with constant airflow, (2) ceramic flat plate sparger with constant airflow and (3) ceramic flat plate sparger with an oscillating airflow. The ceramic sparger with oscillating flow generated the smallest bubbles within the liquid pool and the largest bubbles within the foam phase. This delivered the highest levels of biomass recovery due to enhanced bubble-algae collision and attachment efficiencies. The smaller bubbles generated by the ceramic sparger under constant or oscillating airflow had significantly faster rise velocities when compared to the larger bubbles produced by the limewood spargers. The faster velocities of the smaller bubbles were due to momentum transfer to the liquid phase. Analyses of the harvest economics revealed that the ceramic flat plate sparger with an oscillating airflow delivered the best overall cost-benefit relationship.

Passive tracer dispersion in oscillating Poiseuille liquid flows of zero net velocity is studied experimentally in a Hele-Shaw cell and numerically by 2D simulations: this study is particularly focused on the time dependence and local properties of the dispersion. The dispersion mechanism is found to be controlled by the ratio τm/T of the molecular diffusion time across the gap and the oscillation period (when molecular diffusion parallel to the flow is negligible). The 2D numerical simulations complement the experiments by providing the local concentration c(x,z,t) at a given distance z from the cell walls (instead of only the average over z). Above a time lapse scaling like τm, the variation of c with the distance x along the flow becomes a Gaussian of width constant with z while the mean distance ¯x may depend both on z and t. For τm/T . 2, the front spreads through Taylor-like dispersion and the normalized dispersivity scales as τm/T. The front oscillates parallel to the flow with an amplitude constant across the gap; its width increases monotonically at a rate modulated at twice the flow frequency, due to variations of the instantaneous dispersivity. For τm/T & 20, the molecular diffusion distance during a period of the flow is smaller than the gap and the normalized dispersivity scales as (τm/T)−1 . The oscillations of the different points of the front follow the local fluid velocity: this produces a reversible modulation of the global front width at twice the flow frequency and in quadrature with that in the Taylor-like regime. Fil: Roht, Yanina Lucrecia. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Auradou, H.. Université Paris Sud; Francia Fil: Hulin, J. P.. Université Paris Sud; Francia Fil: Salin, D.. Université Paris Sud; Francia Fil: Chertcoff, Ricardo Héctor. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina Fil: Ippolito, Irene Paula. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Grupo de Medios Porosos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The fundamentals of oscillating flow in a reservoir-pipe-orifice system are revisited in a theoretical study related to acoustic resonance experiments carried out in a large-scale pipeline. Four different types of system excitation are considered: forcing velocity, forcing pressure, linear oscillating resistance and nonlinear oscillating resistance. Analytical solutions are given for the periodic responses to the first three excitations. Analytical and numerical results for the large-scale pipeline are presented and some peculiarities are discussed. Key words: Hydraulic transients; Acoustic resonanceImpedance; Orifice; Rotating valve; Analytical solution.

An experimental study has been carried out of the two phase counter-current gas-liquid flow in a packed bed column operated in the proximity of the flooding point under periodic variations of inlet velocity of gas or liquid. Additional experiments have been focused on evaluating axial dispersion characteristics in the proximity of the flooding line in both liquid and gas phase using inert tracers. The transient flow experiments have revealed hysteretic behavior of liquid holdup and gas pressure in the bed. The tracer RTD experiments have shown that no deterioration of axial dispersion in both gas and liquid place takes place unless the flooding phenomenon has already prevailed. In fact, axial dispersion in the gas phase lessens with increasing gas velocity and so does axial dispersion in liquid phase at higher liquid loads.

A direct analogy is drawn between the streamwise vorticity generated in an asymmetric swirling flow and that developed in secondary flow in a curved duct.

The flow velocity fluctuation generated behind a circular cylinder which is placed in a unidirectional stream has been measured by means of laser velocimetry. The variance of the fluctuation is proportional to the square of the mean flow rate and is found to reduce with going down the stream.

We study the initial-value problem posed by the small-amplitude (linearized) free oscillations of free drops, gas bubbles, and drops in a host liquid when viscous effects cannot be neglected. It is found that the motion consists of modulated damped oscillations, with the damping parameter and frequency approaching only asymptotically the results of the normal-mode analysis. The connexion with the normal-mode method is demonstrated explicitly and the experimental relevance of our results is discussed.

Oscillating flow in a tapered channel was studied experimentally by observing the deformation of a dye streak within a Plexiglas channel during piston induced, volume-cycled oscillations. The observed steady streaming was compared to theoretical calculations. Both the theoretical and experimental results show bi-directional drift for all frequencies, with characteristics dependent upon the value of the Womersley parameter, α = b(ω/ν)½. For large values of α, e.g. α > 10, the fluid near the walls and along the centreline drifted towards the narrow end of the channel, while fluid in an intermediate region drifted towards the wide end of the channel. For small values of α, e.g. α < 5, the fluid drifted towards the narrow end at the wall and the wide end of the channel along the centreline. Applications to mass transport in the lung are discussed.

An experiment was designed to measure the fluid oscillations in planar isochronous containers. Measurements of the resonant fluid response to forced oscillations provide direct experimental confirmation of the existence of these special containers.

The occurrence of oscillatory solutions to the Benard problem in the presence of horizontal temperature gradients is investigated as a function of Prandtl number and of the ratio of the applied vertical and horizontal gradients using a linear, mean field approximation. It is shown that in low Prandtl number fluids, even small horizontal gradients readily generate oscillatory modes.

A study of the Benard problem under the influence of a random gravitational field. It is shown that under these conditions the fluid may be slightly destabilized.

The viscous damping of the capillary oscillations of a simple drop in rotation in air is evaluated in the small viscosity limit.

The change by an order of magnitude of the oscillating properties of the flow past a circular cylinder in the Reynolds number range 2*103 to 5*104 is demonstrated experimentally. It is shown that in this range these properties are highly susceptible to small disturbances of the frequency of the transition waves which precede turbulence in the shear layers just downstream of the cylinder. It is suggested that this susceptibility is responsible for the different lift coefficient values measured by various workers.The effect of disturbances on the mean flow properties is also described.The frequency-determining mechanism and the characteristic length of the oscillating flow are discussed.