E. John List

Investigations of round vertical turbulent buoyant jets

The axial and radial velocity components w and u, and the concentration c of a Rhodamine 6G dye were measured simultaneously in a turbulent buoyant jet, using laser-Doppler anemometry combined with a recently developed laser-induced-fluorescence concentration measurement technique. These non-intrusive techniques enable measurements in a region of plume motion where conventional probe-based techniques have had difficulties. The results of the study show that the asymptotic decay laws for velocity and concentration of a tracer transported by the flow are verified experimentally in both jets and plumes. The momentum and volume fluxes and the mean dilution factor are determined in dimensionless form as a function of the normalized distance from the flow source. Contradictory results from earlier experimental plume investigations concerning the decay laws of w and c and the plume width ratio b_c/b_w are discussed. The turbulence properties and the transition from momentum-driven jets to buoyancy-driven plumes are presented. The turbulence is found to scale with the mean flow as predicted by dimensional analysis and self-similarity. Buoyancy-produced turbulence is found to transport twice as much tracer as jet turbulence. Although velocity statistics in jets and plumes are found to be highly self-similar there is a strong disparity in the distribution of tracer concentration in the two flows. This occurs in the time-average mean flows as well as the r.m.s. turbulent quantities. Instantaneous concentration fluctuations are found to exceed time averages by as much as a factor of 3. The experimental results should provide a reasonable basis for validation of computer models of axisymmetric plumes.

Turbulence structure near a sharp density interface

The effects of a sharp density interface and a rigid flat plate on oscillating-grid induced shear-free turbulence were investigated experimentally. A two-component laser-Doppler velocimeter was used to measure turbulence intensities in and above the density interface (with matched refractive indices) and near the rigid flat plate. Energy spectra, velocity correlations, and kinetic energy fluxes were also measured. Amplification of the horizontal turbulent velocity, coupled with a sharp reduction in the vertical turbulent velocity, was observed near both the density interface and the flat plate. These findings are in agreement with some previous results pertaining to shear-free turbulence near rigid walls (Hunt & Graham 1978) and near density interfaces (Long 1978). The results imply that, near the density interface, the turbulent kinetic energy in the vertical velocity component is only a small fraction of the total turbulent kinetic energy and indicate that the effects of the anisotropy created by the density interface or the flat plate are confined to the large turbulence scales.

LARGE-SCALE STRUCTURE IN THE FAR FIELD OF BUOYANT JETS

The flow structure and entrainment mechanisms in the far field of a round vertical buoyant jet have been studied experimentally by use of an optical technique based on laser-induced fluorescence (LIF). A large number of essentially instantaneous tracer concentration profiles were recorded for each experimental run by combining LIF with linear photodiode array imaging and high-speed digital data acquisition. Analysis of the resulting high-resolution flow images indicates that the far-field region is dominated by the periodic passage of structures spanning the entire radial flow extent. Ambient fluid is entrained by vortical motions and is transported to regions deep into the flow interior. Correlation analysis discloses that the passage frequency of the structures scales with the local mean velocity and flow width. Conditional averaging of the data indicates that the downstream frontal region of the structure is well mixed and at higher concentration level than the back and side regions where ambient fluid is intermittently present. This results in an axial concentration gradient within the structure, analogous to the ramp-like pattern previously observed in heated air jets. In comparison to the momentum-driven flow the ambient fluid presence in the flow interior is greatly increased when body forces are the driving mechanism. This appears to result from the influence of buoyancy forces in the production of turbulent vortices at the integral scale. An important feature of both the momentum-driven and buoyancy-driven flows investigated is the strongly intermittent character of the concentration field. This raises the issue of the appropriateness of gradient-diffusion theories for the description of such flows.

Plane turbulent buoyant jets. Part 1. Integral properties

An integral technique suggested for the analysis of turbulent jets by Corrsin & Uberoi (1950) and Morton, Taylor & Turner (1956) is re-examined in an attempt to improve the description of the entrainment. It is determined that the hypothesis of Priestley & Ball (1955), that the entrainment coefficient is a linear function of the jet Richardson number, is reasonable, and that two empirically determined plume parameters are sufficient to describe the transition of buoyant jets to plumes. The results of a series of experiments in which both time-averaged velocity and time-averaged temperature profiles were recorded in a substantial number of plane turbulent buoyant jets of varying initial Richardson numbers are used to verify the basic ideas. In addition, measurements of the mean tracer flux in a series of buoyant jets indicate that as much as 40% of the transport in plumes is by the turbulent flux.

Turbulent mixing at a shear-free density interface

The interaction of a sharp density interface with oscillating-grid-induced shear-free turbulence was experimentally investigated. A linear photodiode array was used in conjunction with laser-induced fluorescence to measure the concentration of dye that was initially only in the less dense layer. A laser-Doppler velocimeter was used to measure the vertical velocity in and above the density interface at a point where the dye concentration was also measured. Potential refractive-index-fluctuation problems were avoided using solutes that provided a homogeneous optical environment across the density interface. Internal wave spectra, amplitudes and velocities, as well as the vertical mass flux were measured. The results indicate that mixing occurs in intermittent bursts and that the gradient (local) Richardson number remains constant for a certain range of the overall Richardson number R_j, defined in terms of an integral lengthscale, buoyancy jump and turbulence intensity. The spectra of the internal waves decay as f^(−3) at frequencies below the maximum Brunt-Vaisala frequency. These findings give support to a model for oceanic mixing proposed by Phillips (1977) in which the internal waves are limited in their spectral density by sporadic local instabilities and breakdown to turbulence. The results also indicate that, for a certain R_j range, the thickness of the interfacial layer (normalized by the integral lengthscale of the turbulence) is a decreasing function of R_j. At sufficiently high R_j the interfacial thickness becomes limited by diffusive effects. Finally, we discuss a simple model for entrainment at a density interface in the presence of shear-free turbulence.

Kinetic analysis of virus adsorption and inactivation in batch experiments

The mobility and ecology of viruses in natural environments is strongly influenced by the adsorption of virus particles to sand or soil surfaces. This binding process is frequently studied by conducting batch experiments in which fluid suspensions of virus particles are contacted with the adsorbent of interest. In this report, a simple first-order kinetic theory is presented which accounts for many of the complicated interactions that can occur when viruses contact an adsorbent in a batch system. Closed-form solutions and numerical simulations of the model indicate that four classes of virus-surface interactions can be identified, including quasi-equilibrium adsorption, quasi-equilibrium adsorption with surface sinks, quasi-equilibrium adsorption with reduced inactivation, and direct irreversible adsorption. Based on these results, a new experimental approach for studying virus-surface interactions is proposed and tested using a model system consisting of bacteriophage lambda and Ottawa sand. Fluid samples were collected from sand-containing and sand-free virus suspensions over the course of 5–6 days and analyzed for plaque forming units (PFU). These experiments were repeated using three different pH values and six different electrolyte compositions. Nondimensionalization of the PFU data from the sand-free suspension collapsed all of the data onto a single line, as predicted by the kinetic model. When plotted in a nondimensional format, data from the sandcontaining suspensions exhibited behavior which tould readily be interpreted within the context of the kinetic model. These results suggest that the proposed approach offers a powerful alternative to conventional methods for studying virus adsorption at the solid-liquid interface, and for predicting the potential mobility and fate of viruses in porous media.

Statistical and spectral properties of tracer concentration in round buoyant jets

Abstract Spatial growth, temperature decay, and turbulence structure have been studied experimentally in axially-symmetric buoyant jets in transition to plumes and in fully developed plume flows. Temperature records obtained with fast response thermistors located in these flows form the basis for the study. The results obtained support previous asymptotic arguments pertaining to the rate of decay of mean temperature and r.m.s. temperature fluctuations. Spectral energy distributions in jets and plumes show that the evolution from buoyant jets to plumes is characterized by a significant shift in the wave number distribution of temperature variance density.

Collision efficiencies of diffusing spherical particles: hydrodynamic, van der Waals and electrostatic forces

A practical limitation of the application of Smoluchowskis classical estimate for the collisions probability of two diffusing spherical particles in Brownian motion is the non-consideration of interparticle forcves. For suspended particles in water such forces can arise from the disturbance the particle causes in the fluid (hydrodynamic forces), from the cloud of ions which surround an electrically charged particle (double layer forces) or they can be of molecular origin (van der Waals forces). In this paper corrections to Smoluckhowskis collision probability are computed when such forces operate Scoluchowskis collision probability are computed when such forces operate between two approaching particles of various sizes. Results for several values of the van der Waals energy of attraction and the ionic strength of the electrolyte are presented in a way convenient for particle collision modeling.

Numerical Simulation of a Sedimentation Basin. 1. Model Development

A method for the numerical simulation of a rectangular sedimentation basin operating under steady or unsteady conditions is described. The computer model follows the spatial and temporal development of the influent particle size distribution toward the outlet of the tank. It is based on the fundamental mechanisms which govern particle motion and growth. The model accounts for the variability of the flow field and the particle size distribution in the tank and, from the local development of the particle size spectrum, predicts the overall performance of the settling basin.

On mixing and transport at a sheared density interface

Mixing and transport of a stratifying scalar are investigated at a density interface imbedded in a turbulent shear flow. Steady-state interfacial shear flows are generated in a laboratory water channel for layer Richardson numbers, Ri, between about 1 and 10. The flow field is made optically homogeneous, enabling the use of laser-induced fluorescence with photodiode array imaging to measure the concentration field at high resolution. False-colour images of the concentration field provide valuable insight into interfacial dynamics: when the local mean shear Richardson number, Ri_s, is less than about 0.40–0.45, interfacial mixing appears to be dominated by Kelvin–Helmholtz (K–H) instabilities; when Ri_s is somewhat larger than this, interfacial mixing appears to be dominated by shear-driven wave breaking. In both cases, vertical transport of mixed fluid from the interfacial region into adjacent turbulent layers is accomplished by large-scale turbulent eddies which impinge on the interface and scour fluid from its outer edges. Motivated by the experimental findings, a model for interfacial mixing and entrainment is developed. A local equilibrium is assumed in which the rate of loss of interfacial fluid by eddy scouring is balanced by the rate of production (local mixing) by interfacial instabilities and molecular diffusion. When a single layer is turbulent and entraining, the model results are as follows: in the molecular-diffusion-dominated regime, δ/h ~ Pe^(−1/2) and E ~ Ri^(−1)Pe^(−1/2); in the wave-breaking-dominated regime, δ/h ~ Ri^(−1/2) and E ~ Ri^(−3/2); and in the K–H-dominated regime, δ/h ~ Ri^(−1) and E ~ Ri^(−2), where δ is the interface thickness, h is the boundary-layer thickness, Pe is the Peclet number, and E is the normalized entrainment velocity. In all three regimes, the maximum concentration anomaly, Γ_m ~ Ri^(−1). When both layers are turbulent and entraining, E and δ depend on combinations of parameters from both layers.