Turbulent Shear Rate Research Articles

Laboratory experiments of fine-scale mixing and mass transport within a coral canopy

Laboratory experiments obtained fine scale measurements of turbulent shear stresses and rates of mixing and mass transfer over a nonliving bed of the coral, Porites compressa, the dominant species found in Kaneohe Bay, Hawaii. A reef canopy was placed in a recirculating wave-current flume and flow was generated that simulated the flow characteristics of the reef flat of Kaneohe Bay. Turbulence and velocity structure under both unidirectional and wave-dominated currents were measured using a two-dimensional laser Doppler anemometer. Mass transport measurements were made using a planar laser-induced fluorescence technique in which the scalar transport of Rhodamine 6G dye, fluxed from the surfaces of the coral, was quantified. Results show that the action of surface waves, interacting with the structure of the reef, can increase instantaneous shear and mixing up to six times compared to that of unidirectional currents. Maximum shear and mass transport events coincided with flow separation within the wave-current boundary layer and the ejection of vortices into the flow. Wave action also acted to increase the vertical flux of water from within the coral structure. The combined effects of increased turbulent stress and fluid exchange from the interior of the canopy increased mass flux due to wave action 2.3±0.5 times that measured for comparable unidirectional currents.

Role of turbulent shear rate distribution in aggregation and breakage processes

AbstractThe effect of spatial flow heterogeneity on shear‐induced aggregation and breakage of fine particles in turbulent flow conditions is investigated using computational fluid dynamics (CFD) and population balance modeling. The quadrature method of moments (QMOM), particularly suitable for implementation in commercial CFD codes, has been used to solve the corresponding population balance equation. QMOM is first tested and compared with alternative numerical methods (sectional/fixed‐pivot methods) for a specific set of realistic operating conditions. Then QMOM is implemented in a CFD code and the effect of spatial heterogeneities on the cluster mass distribution in a Taylor–Couette vessel is investigated. Simplified models have been derived based on the separation of the timescales of mixing on one hand and of aggregation and breakage on the other and compared with the full CFD model. Guidelines for use and limitations of such models and the identification of the underlying aggregation and breakage kernels are discussed. © 2005 American Institute of Chemical Engineers AIChE J, 2006

Characteristics of the chemical plume behind a sinking particle in a turbulent water column

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 283:55-71 (2004) - doi:10.3354/meps283055 Characteristics of the chemical plume behind a sinking particle in a turbulent water column André W. Visser1,*, George A. Jackson2 1Danish Institute for Fisheries Research, Kavalergården 6, 2920 Charlottenlund, Denmark 2Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA *Email: awv@dfu.min.dk ABSTRACT: Many aquatic organisms, from bacteria to crustaceans, use chemical plumes released by sinking particulate organic material either directly as a food source or as a signal to find potential food items (marine snow aggregates, fecal pellets). This work examines how the relevant metrics of the plume (length, volume and cross sectional area) relate to encounter rates and how they vary with turbulence. Total plume length appears to be invariant to turbulent shear rate, although plume volume and cross section do decrease strongly with increased turbulence. Turbulence can also break a plume into multiple segments. The length of the segment connected to the particle (the first unbroken segment) tends to be about 50% of the total plume length, as well as containing about half of the total plume volume and cross sectional area. Despite the complexity of the processes involved, these relationships can be described by relatively simple functions that are revealed both empirically through modelling and through theoretical analysis. The critical parameter is γT0*, the product of the average turbulent shear rate and the diffusive time scale—the length of time a plume would last in calm water. The results highlight the ecological differences associated with different turbulence intensities as a function of depth in the water column. KEY WORDS: Turbulence · Marine snow · Fecal pellet · Dissolved organic matter · Chemical trail · Encounter rate Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 283. Online publication date: November 30, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Application of in situ adaptive tabulation to CFD simulation of nano-particle formation by reactive precipitation

Reactive precipitation involves four fundamental processes: mixing-limited reaction, nucleation, growth, and aggregation. A novel algorithm, in situ adaptive tabulation (ISAT), has been implemented in a code for micromixing simulations, which is often applied together with computational fluid dynamics (CFD), using full probability density function (PDF) methods to incorporate these fundamental processes in the formation of nano-particles by reactive precipitation in a plug-flow reactor. The quadrature method of moments is applied to solve population balance equations for turbulent aggregation of the growing particles. The various performance issues (error control, accuracy, number of records, speed-up) for ISAT are discussed. Based on a large number of simulations, an error tolerance of 10 −4–10 −5 is found to be satisfactory for carrying out time-evolving full PDF simulations of nano-particle formation by reactive precipitation. Our results show that CFD simulation of reactive precipitation requires a much smaller computational effort when the ISAT algorithm is implemented than when direct integration is used. Finally, the effects of initial species concentrations, micromixing time, and turbulent shear rate on the reactive precipitation of barium sulfate are studied.

The shear sensitivity of activated sludge: an evaluation of the possibility for a standardised floc strength test

The presence of micron-sized primary particles is critical in many solid/liquid separation processes, and characterisation of stability with respect to the degree of dispersion is therefore relevant. Characterisation of floc strength so far has mainly dealt with the size of flocs and not addressed quantification with respect to primary particles. In the present work floc strength was quantified in terms of the shear sensitivity ( k SS) as a standardised parameter based on the recent adhesion–erosion model (AE-model). The shear sensitivity quantifies the degree of dispersion for low total solids contents and intermediate turbulent shear rates. It reflects the affinity of adhesion/erosion of primary particles to and from flocs and thus provides a means of characterising the interaction energy between the sludge colloids.

Quantification of the bond energy of bacteria attached to activated sludge floc surfaces

The great majority of activated sludge bacteria exist incorporated in flocs. The increase in dispersed bacteria when exposed to increasing turbulent shear rates has been successfully modelled by a model assuming that the adhesion and erosion of cells may be considered in analogy to ordinary chemical phase transitions. By this adhesion-erosion model (AE-model), an "enthalpy" of cell adhesion can be estimated, and this value in turn determines the range of shear rates in which erosion of cells predominates. Application of the model has indicated that only a mass fraction less than ca. 6-17% may be released from activated sludge, even when exposed to a severe turbulent environment, i.e. only a small fraction of the flocs is dispersible by means of erosion by turbulence. The shear sensitivity and the dispersible floc fraction were found to depend on the floc composition. A net decrease in the floc EPS content during anaerobic sludge stabilisation causes a dramatic increase in the dispersed fraction, indicating the important role of EPS for the floc strength. It was found also that activated sludge cells do not reflocculate completely after exposure to high shear rates. This may be an indication that the cohesion energy of bacteria growing in colonies is greater than the energy of the more stochastic adhesion of dispersed cells to floc surfaces. It could also be another indicator of the importance of entanglement forces, which do not reform instantly, once broken. When the bond strength of cell attachment to sludge surfaces is altered by changes in the chemical environment, this may cause a change in the dispersible floc fraction as well as a change in the shear range of erosion. When the shear sensitivity constant kss is adopted for sludge characterisation, an increased degree of dispersion under standard test conditions will result in increased shear sensitivity estimates. The shear sensitivity may be used for the estimation of en equivalent change in the Gibb's energy of cell adhesion (delta (delta Gad/RT)).

Aggregation kinetics of small particles in agitated vessels

Rapid coagulation by turbulence in stirred tanks was studied for particles and aggregates smaller than the Kolmogorov microscale. The coagulation kinetics are determined by the floc structure and by the hydrodynamic and colloidal interactions between the colliding particles. The collision efficiency for doublet formation in the heterogeneous shear field of a stirred tank follows from particle trajectory analysis of solid particles in simple shear flow, provided the simple shear rate is made to correspond to the residence time weighted turbulent shear rate. Experimentally, the resulting aggregates proved to be fractal-like with their porosity increasing with aggregate size. Porosity within the aggregates results in penetration of the floc surface by the fluid flow, giving rise to enhanced collision efficiencies compared to solid particles. The collision efficiencies between porous flocs may be estimated by a model that pictures a porous floc as consisting of an impermeable core and a completely permeable shell. With the collision efficiencies from this shell-core model the aggregate growth could be described adequately.

On the condition of streak formation in a bounded turbulent flow

Flow between two surfaces at which no-slip and free-slip conditions can be imposed has been investigated numerically with a Fourier–Chebyshev pseudospectral method. Different mean shear rates have been applied to each boundary to study the effect of shear and boundary condition on the streaky structures that have been observed near walls in many previous investigations. In addition to the streaks found near the no-slip wall, the computations also reveal streaky structures when the free-slip surface is under a sufficiently high shear. The low-speed streaks observed near the free-slip surface, although appearing somewhat more pronounced, have much the same characteristics as the wall-layer streaks—e.g., the average spanwise spacing between the streaks both near the wall and the free surface is about 100 when normalized by the kinematic viscosity and the appropriate shear velocity (at the wall or at the free-slip surface). The results show that shear is much more important than the nature of the boundary in determining the dominant flow structure, rather unexpected since vortex lines can attach at a free-slip boundary whereas they cannot at a no-slip one. The formation of streaks appears to be governed by a local nondimensional shear parameter defined as S̃≡S‖〈u′w′〉‖/ε, where S is the mean shear rate, and 〈u′w′〉 and ε are the kinematic turbulent shear stress and rate of dissipation of turbulent kinetic energy, respectively. It is found that S̃≂1.0 is the condition at which the elongated low-speed streaky regions first appear.

Kinetics of floc breakage and aggregation in agitated liquid suspensions

A complete population balance equation governing the floc size distribution interpreting both breakage and aggregation mechanisms was obtained in this work. The present work deals with floc formation and breakage in the presence of polymer coagulants. In order to obtain all the parameters involved in the proposed governing equation, a nonlinear least-squares curve fitting procedure was applied. The aggregation collision efficiency was found to be 0.33 for our clay-polymer flocculation system under optimum dosage. The floc splitting frequency was found to be proportional to the product of turbulent shear rate and parent floc volume. The rate of decrease of parent floc volume due to erosion was verified to be proportional to the product of turbulent shear rate and the mean particle volume of erosion fines. The “maximum stable floc size” in a given turbulence field was found to exist in our system.

Floc breakage in agitated suspensions: Effect of agitation rate

A population balance equation that governs the floc size distribution in turbulent flow, incorporating both the splitting and erosion mechanisms of floc breakage has been presented in previous studies[6,7]. Experiments have been conducted in which transient floc size distributions of dilute suspensions of Kaolin-hydrous ferric-oxide flocs undergoing breakage in agitated batch system are obtained over turbulent microscale shear rates ranging from 60 to 400 sec −1. Floc size distributions over a range of floc diameters from about 1 to 100 μ m are measured by overlapping techniques, including computerized optical scanning of photographs and rapid electronic sizing of individual flocs using a light blockage transducer. The experimentally determined size distributions are then fit to those computed from the population balance equation, using constrained nonlinear least squares. The splitting frequency of parent flocs is found to vary as the 0.33 power of the parent floc volume and 0.71 power of the shear rate. The average number of daughter fragments produced upon splitting of individual flocs is found to be about 2.5. The mean and standard deviation of the daughter fragments produced upon splitting are found to be in fixed ratio to one another, independent of the parent floc size and shear rate. An appropriately nondimensionalized coefficient characterizing floc erosion has been found to be independent of shear rate.