Mass transfer from a cylinder to an air stream in axisymmetrical flow (an experimental study)

Mass transfer from wetted surfaces on one-inch cylinders with unwetted approach sections was studied experimentally by means of the evaporation of n-octane and n-heptane into an air stream in axisymmetrical flow, for Reynolds numbers from 5,000 to 310,000. A transition from the laminar to the turbulent boundary layer was observed to occur at Reynolds numbers from 10,000 to 15,000. The results were expressed in terms of the Sherwood number as a function of the Reynolds number, the Schmidt number, and the ratio of the unwetted approach length to the total length. Empirical formulas were obtained for both laminar and turbulent regimes. The rates of mass transfer obtained were higher than theoretical and experimental results obtained by previous investigators for mass and heat transfer from flat plates.

Que paraíso é esse?: a turismização da Ilha Grande

A Ilha Grande (Angra dos Reis, RJ) apresenta dois acontecimentos que marcam a história recente do lugar: a instituição de leis ambientais com a criação de quatro unidades de conservação da natureza (entre 1971 e 1990); e a intensificação do fluxo turístico após a implosão do Instituto Penal Cândido Mendes - IPCM (1994), que marcou o fim de cerca de 100 anos de um sistema penitenciário. A Ilha, sempre associada às belezas naturais e à ideia de paraíso, a partir desse momento, deixa de ser referida como o paraíso-inferno e passa a ser reconhecida predominantemente pelas potencialidades paisagísticas, voltando-se para o turismo, que se torna um componente central da vida no lugar. Reconstituindo a trajetória do turismo na Ilha, e considerando o quadro atual das atividades turísticas (a partir da minha participação em fóruns para discussão e resolução dos problemas da Ilha, de pesquisa em jornais, e de entrevistas com moradores e participantes desses fóruns), mostro os significados e visões sobre o paraíso Ilha Grande e o turismo o que se tem e o se quer , tanto no nível dos discursos e das representações, quanto no nível das práticas e das relações sociais instituídas. Revela-se aí um campo de disputa entre diferentes atores sociais com interesses diversos para salvar e explorar a Ilha Grande; onde a natureza e o turismo se apresentam como dois elementos utilizados como estratégias para sobrevivência de grande parte da população e como estratégia política do poder público; um paraíso marcado pelo turismo e um paraíso para o turismo. Faço uma leitura da ocorrência e das implicações do turismo na Ilha Grande através da ideia de turismização, em analogia com o pensamento de Norbert Elias sobre processo civilizador e de J. S. Leite Lopes sobre ambientalização, e reconhecendo o diálogo e os embates entre o processo turismizador e o processo ambientalizador na Ilha Grande.

On the transport properties of fluid-particle flow

The hydrodynamic forces acting on a solid particle in a viscous, incompressible fluid medium at low Reynolds number flow is investigated mathematically as a prerequisite to the understanding of transport processes in two-phase flow involving solid particles and fluid. Viscous interaction between a small number of spherical particles and continuous solid boundaries as well as fluid interface are analyzed under a “point-force” approximation. Non-spherical and elastic spherical particles in a simple shear flow area are then considered. Non-steady motion of a spherical particle is briefly touched upon to illustrate the transient effect of particle motion.

A macroscopic continuum description of particle-fluid flow is formulated in terms of spatial averages yielding a set of particle continuum and bulk fluid equations. Phenomenological formulas describing the transport processes in a fluid medium are extended to cases where the volume concentration of solid particles is sufficiently high to exert an important influence. Hydrodynamic forces acting on a spherical solid particle in such a system, e.g. drag, torque, rotational coupling force, and viscous collision force between streams of different sized particles moving relative to each other are obtained. Phenomenological constants, such as the shear viscosity coefficient, and the diffusion coefficient of the bulk fluid, are found as a function of the material properties of the constituents of the two-phase system and the volume concentration of solid. For transient heat conduction phenomena, it is found that the introduction of a complex conductivity for the bulk fluid permits a simple mathematical description of this otherwise complicated process. The rate of heat transfer between particle continuum and bulk fluid is also investigated by means of an Oseen-type approximation to the energy equation.

The phytoplankton of an enriched, soft-water lakel subject to intermittent hydraulic flushing (Grasmere, English Lake District)

This paper describes some characteristic features of the phytoplankton of Grasmere, one of the smaller of the principal lakes of the English Lake District, and attempts to relate these to distinctive physical and chemical properties of the lake. Quantitative data presented herein are derived from 5-m vertical column samples, collected with a flexible polyethylene hose close to the deepest point of Grasmere, generally at intervals of 14 days ( 7 days from 1972 to 1978, inclusive). The study concludes that although Grasmere has been subject to increased phosphorus-loading and has quickly developed many features associated with eutrophication, the composition of its plankton has retained the characteristics of a mesotrophic, soft-water lake: a vernal diatom maximum, generally dominated by Asterionella, is followed by summer growths of nanoplanktonic species, of various colonial Chlorophyceae, before a substantial return to Asterionella-dominance in the autumn.

Biological environments of larger UK rivers: progress report 1987

This interim progress report for the 9 months from January 1987 to September 1987 aims to provide insights into the mechanisms by which populations of particles (in this instance, phytoplankton) behave in relation to fluvial flow and, thus, to better model the dispersive properties of rivers and the ecological principles governing the distribution of potamoplankton generally. The author has been able to show with dye-tracers that significant water retention in pool reaches occurs within the range of (low) discharges obtaining, in accord with the Aggregated Dead Zone model and to an extent comparable with streams and small rivers investigated previously.

Theoretical investigation of turbulent boundary layer over a wavy surface

The important features of the two-dimensional incompressible turbulent flow over a wavy surface of wavelength comparable with the boundary layer thickness are analyzed.

A turbulent field method using model equation for turbulent shear stress similar to the scheme of Bradshaw, Ferriss and Atwell (1967) is employed with suitable modification to cover the viscous sublayer. The governing differential equations are linearized based on the small but finite amplitude to wavelength ratio. An orthogonal wavy coordinate system, accurate to the second order in the amplitude ratio, is adopted to avoid the severe restriction to the validity of linearization due to the large mean velocity gradient near the wall. Analytic solution up to the second order is obtained by using the method of matched-asymptotic-expansion based on the large Reynolds number and hence the small skin friction coefficient.

In the outer part of the layer, the perturbed flow is practically "inviscid." Solutions for the velocity, Reynolds stress and also the wall pressure distributions agree well with the experimental measurement. In the wall region where the perturbed Reynolds stress plays an important role in the process of momentum transport, only a qualitative agreement is obtained. The results also show that the nonlinear second-order effect is negligible for amplitude ratio of 0.03. The discrepancies in the detailed structure of the velocity, shear stress, and skin friction distributions near the wall suggest modifications to the model are required to describe the present problem.

The ecology of phytoplankton in Shropshire and Cheshire meres

This review summarizes the findings of 5 years' research (June 1970-June 1975) on the meres of the Shropshire-Cheshire Plain. A mere is a small, shallow lake; supplied principally by ground water, whose chemical composition is infkuenced by the glacial frift through which it is percolating. The seasonal periodicity of the phytoplankton in the meres involved work mainly in the Grose Mere. Here diatoms were typically dominant in Feb & March, green algae in April & May, blue-green algae in early summer and dinoflagellates in late summer. This pattern is broadly similar from year to year, and has been suggested to be representative of a 'regional type'; it is also similar to that described for many of the world's mildly eutrophic temperate lakes. Vertical distribution of phytoplankton is influenced by their buoyancy (or lack of it) of by their ability to swim. A stylized depth-time distribution of 4 major phytoplankton components in Crose Mere is given diagrammatically.

Particle fluid mechanics in shear flows, acoustic waves, and shock waves

Three different categories of flow problems of a fluid containing small particles are being considered here. They are: (i) a fluid containing small, non-reacting particles (Parts I and II); (ii) a fluid containing reacting particles (Parts III and IV); and (iii) a fluid containing particles of two distinct sizes with collisions between two groups of particles (Part V).

Part I

A numerical solution is obtained for a fluid containing small particles flowing over an infinite disc rotating at a constant angular velocity. It is a boundary layer type flow, and the boundary layer thickness for the mixture is estimated. For large Reynolds number, the solution suggests the boundary layer approximation of a fluid-particle mixture by assuming W = W_p. The error introduced is consistent with the Prandtl’s boundary layer approximation. Outside the boundary layer, the flow field has to satisfy the “inviscid equation” in which the viscous stress terms are absent while the drag force between the particle cloud and the fluid is still important. Increase of particle concentration reduces the boundary layer thickness and the amount of mixture being transported outwardly is reduced. A new parameter, β = 1/Ω τ_v, is introduced which is also proportional to μ. The secondary flow of the particle cloud depends very much on β. For small values of β, the particle cloud velocity attains its maximum value on the surface of the disc, and for infinitely large values of β, both the radial and axial particle velocity components vanish on the surface of the disc.

Part II

The “inviscid” equation for a gas-particle mixture is linearized to describe the flow over a wavy wall. Corresponding to the Prandtl-Glauert equation for pure gas, a fourth order partial differential equation in terms of the velocity potential ϕ is obtained for the mixture. The solution is obtained for the flow over a periodic wavy wall. For equilibrium flows where λ_v and λ_T approach zero and frozen flows in which λ_v and λ_T become infinitely large, the flow problem is basically similar to that obtained by Ackeret for a pure gas. For finite values of λ_v and λ_T, all quantities except v are not in phase with the wavy wall. Thus the drag coefficient C_D is present even in the subsonic case, and similarly, all quantities decay exponentially for supersonic flows. The phase shift and the attenuation factor increase for increasing particle concentration.

Part III

Using the boundary layer approximation, the initial development of the combustion zone between the laminar mixing of two parallel streams of oxidizing agent and small, solid, combustible particles suspended in an inert gas is investigated. For the special case when the two streams are moving at the same speed, a Green’s function exists for the differential equations describing first order gas temperature and oxidizer concentration. Solutions in terms of error functions and exponential integrals are obtained. Reactions occur within a relatively thin region of the order of λ_D. Thus, it seems advantageous in the general study of two-dimensional laminar flame problems to introduce a chemical boundary layer of thickness λ_D within which reactions take place. Outside this chemical boundary layer, the flow field corresponds to the ordinary fluid dynamics without chemical reaction.

Part IV

The shock wave structure in a condensing medium of small liquid droplets suspended in a homogeneous gas-vapor mixture consists of the conventional compressive wave followed by a relaxation region in which the particle cloud and gas mixture attain momentum and thermal equilibrium. Immediately following the compressive wave, the partial pressure corresponding to the vapor concentration in the gas mixture is higher than the vapor pressure of the liquid droplets and condensation sets in. Farther downstream of the shock, evaporation appears when the particle temperature is raised by the hot surrounding gas mixture. The thickness of the condensation region depends very much on the latent heat. For relatively high latent heat, the condensation zone is small compared with Ʌ_D.

For solid particles suspended initially in an inert gas, the relaxation zone immediately following the compression wave consists of a region where the particle temperature is first being raised to its melting point. When the particles are totally melted as the particle temperature is further increased, evaporation of the particles also plays a role.

The equilibrium condition downstream of the shock can be calculated and is independent of the model of the particle-gas mixture interaction.

Part V

For a gas containing particles of two distinct sizes and satisfying certain conditions, momentum transfer due to collisions between the two groups of particles can be taken into consideration using the classical elastic spherical ball model. Both in the relatively simple problem of normal shock wave and the perturbation solutions for the nozzle flow, the transfer of momentum due to collisions which decreases the velocity difference between the two groups of particles is clearly demonstrated. The difference in temperature as compared with the collisionless case is quite negligible.

Phytoplankton periodicity: its motivation, mechanisms and manipulation

This review summarizes some recent work to find a generalized explanation of phytoplankton periodicity in lakes. Much of the observational and experimental evidence is drawn from work centred on the large enclosures (Lund Tubes) installed in Blelham Tarn, English Lake District. Observations on the phytoplankton in the tubes are related to the periodic changes that occur in natural lakes and it is suggested that such changes have common patterns, that they are due to common causes, that they are affected by similar processes and that they are therefore predictable and, potentially, manipulable.

Advancements in jet turbulence and noise modeling: accurate one-way solutions and empirical evaluation of the nonlinear forcing of wavepackets

Jet noise reduction is an important goal within both commercial and military aviation. Although large-scale numerical simulations are now able to simultaneously compute turbulent jets and their radiated sound, lost-cost, physically-motivated models are needed to guide noise-reduction efforts. A particularly promising modeling approach centers around certain large-scale coherent structures, called wavepackets, that are observed in jets and their radiated sound. The typical approach to modeling wavepackets is to approximate them as linear modal solutions of the Euler or Navier-Stokes equations linearized about the long-time mean of the turbulent flow field. The near-field wavepackets obtained from these models show compelling agreement with those educed from experimental and simulation data for both subsonic and supersonic jets, but the acoustic radiation is severely under-predicted in the subsonic case. This thesis contributes to two aspects of these models. First, two new solution methods are developed that can be used to efficiently compute wavepackets and their acoustic radiation, reducing the computational cost of the model by more than an order of magnitude. The new techniques are spatial integration methods and constitute a well-posed, convergent alternative to the frequently used parabolized stability equations. Using concepts related to well-posed boundary conditions, the methods are formulated for general hyperbolic equations and thus have potential applications in many fields of physics and engineering. Second, the nonlinear and stochastic forcing of wavepackets is investigated with the goal of identifying and characterizing the missing dynamics responsible for the under-prediction of acoustic radiation by linear wavepacket models for subsonic jets. Specifically, we use ensembles of large-eddy-simulation flow and force data along with two data decomposition techniques to educe the actual nonlinear forcing experienced by wavepackets in a Mach 0.9 turbulent jet. Modes with high energy are extracted using proper orthogonal decomposition, while high gain modes are identified using a novel technique called empirical resolvent-mode decomposition. In contrast to the flow and acoustic fields, the forcing field is characterized by a lack of energetic coherent structures. Furthermore, the structures that do exist are largely uncorrelated with the acoustic field. Instead, the forces that most efficiently excite an acoustic response appear to take the form of random turbulent fluctuations, implying that direct feedback from nonlinear interactions amongst wavepackets is not an essential noise source mechanism. This suggests that the essential ingredients of sound generation in high Reynolds number jets are contained within the linearized Navier-Stokes operator rather than in the nonlinear forcing terms, a conclusion that has important implications for jet noise modeling.

Observação do transporte de sedimentos em suspensão ao longo do Canal Norte do rio Amazonas durante condições de baixa descarga (outubro 2008)

The Amazon river, located in northernBrazil, discharges between 80,000 and 250,000 m3s-1 of water onto the adjacent shelf, creating a plume of brackish water that extends hundreds of kilometers away from the river mouth. This river also carries a large amount of fine sediments to the ocean where fluid mud has been found in the topset and upper foreset layers of the subaqueous delta formed on the mid-shelf. One of the main goals of this dissertation is to describe how turbulence and suspended sediment concentration vary along the Northern Channel of the Amazon river. Water column measurements were carried out in October 2008 at six anchor stations (P1, P3, P5, P6, P8 e P9) located seaward of the river mouth; P1 and P9 were 125 km apart. Each station was occupied during 13 hours during which current speed and direction were continuously sampled with a 600 kHz Teledyne-RDI ADCP; hourly profiles of temperature, salinity, turbidity and depth were also obtained. Water samples were collected for determination of Suspended Particulate Matter (SPM) concentration and calibration of the turbidity sensor. Current speed reached values above 1.5 m s1 in the along-channel direction (NE-SW); a remarkable ebb-flood asymmetry was observed and flows were strongly ebb-dominated. Throughout the water column, SPM concentration at stations P1 and P3 varied between 100 and 300 mg L1 in association with the presence of freshwater. In contrast, a strong salinity gradient was observed between stations P6 and P9, coinciding with the occurrence of concentrations of SPM above 10 g L-1 (fluid mud). At stations P3, P5 and P6, interface between freshwater from the Amazon river and salt water from the continental shelf, shear stresses wereestimated through four diferents methods: Reynolds, Turbulent Kinetic Energy (TKE), modified TKE and Quadratic Law; in the nearbed region (3 mab) the computed values varied between 0 and 3 Pa. At the three stations (P3, P5 and P6) the lowest and the highest shear stress values were obtained through, respectively, the Reynolds and the TKE methods. Over the whole water column turbulence intensity was estimated through the standard deviation of the turbulent component of the along-channel current velocity (root-mean square of u); from these values, it was estimated the turbulent dissipation of energy (G), whose values at 3 mab varied between zero and 20 s1.

A general assessment of environmental and biological features of Windermere and their susceptibility to change

Bassenthwaite (Lake) is one of the larger Cumbrian lakes, certainly one of the most distinctive, and of considerable conservation and amenity value. Although its shores lack sizeable settlements, its main inflow receives sewage effluent from a major tourist centre (Keswick) and is subject to episodic floods. These influences, the growing development of leisure activities at the lake (e.g. sailing, time-share units), and recent road-construction, have led to past appraisals of ecological impacts and lake management. The lake has not been the subject of intense and long-term ecological study, but much scattered information exists that is relevant to future management decisions. In the present Report, commissioned by North West Water, such information - published and unpublished - is surveyed. Especial attention is given to evidence bearing on susceptibility to change, affecting the lake environment and its biota or species of conservation interest. Extensive use has been made of the results of a recent (1986-7) seasonal survey by the FBA.

Modelling algal blooms

This is the final report on the research project to develop predictive models to quantify algal blooms in relation to environmental variables. The project's objectives were to develop models simulating the impact of vertical structure and mass transfer upon the dynamics of planktonic algae, including cyanobacteria, in lakes and reservoirs, to assess the potential of sedimentary phosphorus to sustain algal growth following reduction in external loading and to expand and enhance formulations to predict behaviour of blue-green algal populations and to incorporate these into a model software package. As part of the project a strategy for the production of a user-friendly packaging for the modelling software PROTEC-2 adaptable to particular sites was developed.

Processes controlling the quantities of biogenic materials in lakes and reservoirs subject to cultural eutrophication

The processes which control the growth, composition, succession and loss from suspension of phytoplankton algae are briefly reviewed, with special reference to function in eutrophic reservoir systems. The ecology of larger algal biomasses supported by high nutrient loading rates are more likely to be subject to physical (wash-out, underwater light penetration, thermal stability and mixing) than to chemical constraints. Sudden changes in the interactions between physical factors temporarily impair the growth of dominant algal species, and advance the succession. Certain algae may be cropped heavily, but selectively, by zooplankton feeding, but they are rarely the species which cause problems in waterworks practice. Grazing, however, does influence succession. A deeper understanding of the operation of loss control mechanism is urgently required. Potentially, manipulation of the physical environment provides an important means of alleviating day-to-day algal problems in eutrophic reservoirs; in terms of cost effectiveness these may prove to be more attractive than reducing nutrient loads at source.

Small scale turbulence in high Karlovitz number premixed flames

The purpose of this thesis is to characterize the behavior of the smallest turbulent scales in high Karlovitz number (Ka) premixed flames. These scales are particularly important in the two-way coupling between turbulence and chemistry and better understanding of these scales will support future modeling efforts using large eddy simulations (LES). The smallest turbulent scales are studied by considering the vorticity vector, ω, and its transport equation.

Due to the complexity of turbulent combustion introduced by the wide range of length and time scales, the two-dimensional vortex-flame interaction is first studied as a simplified test case. Numerical and analytical techniques are used to discern the dominate transport terms and their effects on vorticity based on the initial size and strength of the vortex. This description of the effects of the flame on a vortex provides a foundation for investigating vorticity in turbulent combustion.

Subsequently, enstrophy, ω² = ω • ω, and its transport equation are investigated in premixed turbulent combustion. For this purpose, a series of direct numerical simulations (DNS) of premixed n-heptane/air flames are performed, the conditions of which span a wide range of unburnt Karlovitz numbers and turbulent Reynolds numbers. Theoretical scaling analysis along with the DNS results support that, at high Karlovitz number, enstrophy transport is controlled by the viscous dissipation and vortex stretching/production terms. As a result, vorticity scales throughout the flame with the inverse of the Kolmogorov time scale, τ_η, just as in homogeneous isotropic turbulence. As τ_η is only a function of the viscosity and dissipation rate, this supports the validity of Kolmogorov’s first similarity hypothesis for sufficiently high Ka numbers (Ka ≳ 100). These conclusions are in contrast to low Karlovitz number behavior, where dilatation and baroclinic torque have a significant impact on vorticity within the flame. Results are unaffected by the transport model, chemical model, turbulent Reynolds number, and lastly the physical configuration.

Next, the isotropy of vorticity is assessed. It is found that given a sufficiently large value of the Karlovitz number (Ka ≳ 100) the vorticity is isotropic. At lower Karlovitz numbers, anisotropy develops due to the effects of the flame on the vortex stretching/production term. In this case, the local dynamics of vorticity in the strain-rate tensor, S, eigenframe are altered by the flame. At sufficiently high Karlovitz numbers, the dynamics of vorticity in this eigenframe resemble that of homogeneous isotropic turbulence.

Combined, the results of this thesis support that both the magnitude and orientation of vorticity resemble the behavior of homogeneous isotropic turbulence, given a sufficiently high Karlovitz number (Ka ≳ 100). This supports the validity of Kolmogorov’s first similarity hypothesis and the hypothesis of local isotropy under these condition. However, dramatically different behavior is found at lower Karlovitz numbers. These conclusions provides/suggests directions for modeling high Karlovitz number premixed flames using LES. With more accurate models, the design of aircraft combustors and other combustion based devices may better mitigate the detrimental effects of combustion, from reducing CO₂ and soot production to increasing engine efficiency.