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- Universidade Federal do Rio Grande do Sul
- Taylor & Francis Ltd
- AIP Publishing LLC
- Universidade Federal do Rio Grande
- Elsevier
- Academia Brasileira de Ciências
- Quens University
- Biblioteca Digitais de Teses e Dissertações da USP
- Universidade Cornell
- Cambridge University Press
- University of Cambridge; Department of Engineering; Clare College
- California Institute of Technology
- Mais Publicadores...

## Análise numérica de escoamentos turbulentos não reativos com transferência de calor por convecção e radiação térmica em meios participantes; Numerical analysis of non-reactive turbulent flows with convection and thermal radiation heat transfer in participanting media

Fonte: Universidade Federal do Rio Grande do Sul
Publicador: Universidade Federal do Rio Grande do Sul

Tipo: Tese de Doutorado
Formato: application/pdf

POR

Relevância na Pesquisa

46.6%

#Transferencia de calor#Turbulence-radiation interactions#Escoamento turbulento#LES#Análise numérica#RANS#Participating media

O presente trabalho apresenta um estudo numérico sobre escoamentos turbulentos combinando os mecanismos de transferência de calor por convecção e radiação térmica em meios participantes. Os principais propósitos são obter um melhor entendimento a respeito da relevância das interações Turbulência-Radiação (TRI) em escoamentos turbulentos não reativos, bem como, investigar o efeito da radiação térmica sobre o comportamento transiente, médio e estatístico dos campos térmicos. Para investigar a relevância das interações TRI em escoamentos turbulentos internos, realiza-se uma comparação entre os fluxos temporais médios por convecção e radiação térmica obtidos através da simulação de grandes escalas (LES) e da modelagem clássica da turbulência (RANS) para escoamentos no regime permanente com as seguintes espessuras ópticas: τ0 = 0.01, 0.10, 1.0, 10.0 e 100.0, que representam desde meios opticamente muito finos até meios muito espessos. Para todos os casos, o número de Reynolds baseado na velocidade de fricção e o número de Prandtl são mantidos fixos: Reτ = 180 e Pr = 0.71. A abordagem da turbulência é realizada a partir dos modelos submalha dinâmico de Smagorinsky (DSSGS) e k – ε padrão no âmbito de LES e RANS...

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## A Reynolds stress model for turbulent flows of viscoelastic fluids

Fonte: Taylor & Francis Ltd
Publicador: Taylor & Francis Ltd

Tipo: Artigo de Revista Científica
Formato: 1-36

ENG

Relevância na Pesquisa

46.49%

A second-order closure is developed for predicting turbulent flows of viscoelastic fluids described by a modified generalised Newtonian fluid model incorporating a nonlinear viscosity that depends on a strain-hardening Trouton ratio as a means to handle some of the effects of viscoelasticity upon turbulent flows. Its performance is assessed by comparing its predictions for fully developed turbulent pipe flow with experimental data for four different dilute polymeric solutions and also with two sets of direct numerical simulation data for fluids theoretically described by the finitely extensible nonlinear elastic - Peterlin model. The model is based on a Newtonian Reynolds stress closure to predict Newtonian fluid flows, which incorporates low Reynolds number damping functions to properly deal with wall effects and to provide the capability to handle fluid viscoelasticity more effectively. This new turbulence model was able to capture well the drag reduction of various viscoelastic fluids over a wide range of Reynolds numbers and performed better than previously developed models for the same type of constitutive equation, even if the streamwise and wall-normal turbulence intensities were underpredicted.

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## Turbulent dispersion via fan-generated flows

Fonte: AIP Publishing LLC
Publicador: AIP Publishing LLC

Tipo: Artigo de Revista Científica

EN

Relevância na Pesquisa

46.52%

Turbulent dispersion of passive scalar quantities has been extensively studied in wind tunnel settings, where the flow is carefully conditioned using flow straighteners and grids. Much less is known about turbulent dispersion in the “unconditioned” flows generated by fans that are ubiquitous in indoor environments, despite the importance of these flows to pathogen and contaminant transport. Here, we demonstrate that a point source of scalars released into an airflow generated by an axial fan yields a plume whose width is invariant with respect to the fan speed. The results point toward a useful simplification in modeling of disease and pollution spread via fan-generated flows.

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## Numerical investigation of turbulent forced convective flows over a pair of circular cylinders

Fonte: Universidade Federal do Rio Grande
Publicador: Universidade Federal do Rio Grande

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

56.38%

The present study presents large eddy simulation (LES) of forced convective heat transfer in transient, two-dimensional, incompressible turbulent flows over a pair of cylinders with two different arrangements: 1) with two circular cylinders in tandem (both cylinders are in line with the streamwise direction of the flow, β = 0º) and 2) two side-by-side circular
cylinders (where both cylinders are placed transversally to the streamwise direction of the flow, β = 90º). The dynamic Smagorinsky model is employed for the sub-grid treatment. The simulations are based on the finite volume method solution for the conservation equations of mass, momentum and energy. Both simulations are performed with Reynolds and Prandtl numbers of ReD = 22000 and Pr = 0.71, respectively. The results showed that the transient fluid dynamic and thermal patterns are strongly affected
by the configuration of circular cylinders. The kind of arrangement led to a difference of nearly 20 % for time-averaged Nusselt number (NuD).

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## A numerical study of combined convective and radiative heat transfer in non-reactive turbulent channel flows with several optical thicknesses: a comparison between LES and RANS

Fonte: Universidade Federal do Rio Grande
Publicador: Universidade Federal do Rio Grande

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.52%

This work presents a numerical study of turbulent channel flows with combined convective and radiative heat transfer in participating media by means of large Eddy simulation and Reynolds averaged Navier–Stokes (RANS) with k–ε model. The main purpose is to evaluate the employment of the traditional modeling (RANS without TRI) for the prediction of time-averaged parameters of turbulent non-reactive flows with convective and radiative heat transfer. All cases are investigated with Re τ = 180, Pr = 0.71 and for various optical thicknesses: τ 0 = 0.01, 0.1, 1.0, 10 and 100. In spite of the consideration of non-relevance of turbulence–radiation interactions for non-reactive turbulent flows, the results shown that for τ 0 ≥10 the employment of RANS with k–ε model led to considerable deviations for the prediction of time-averaged radiative fluxes and divergence of the radiative fluxes. For τ 0 = 100, both models also led to different results for the time-averaged convective fluxes.

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## Large eddy simulation of turbulent forced convective flows over a pair of circular cylinders with different arrangements

Fonte: Universidade Federal do Rio Grande
Publicador: Universidade Federal do Rio Grande

Tipo: Conferência ou Objeto de Conferência

ENG

Relevância na Pesquisa

56.38%

The present study presents large eddy simulation (LES) of forced convective heat transfer in transient, twodimensional, incompressible turbulent flows over a pair of cylinders with two different arrangements: 1) with two circular cylinders in tandem (both cylinders are in line with the streamwise direction of the flow, α = 0º) and 2) two side-by-side circular cylinders (where both cylinders are placed transversally to the streamwise direction of the flow, α = 90º). The dynamic Smagorinsky model is employed for the sub-grid treatment. The simulations are based on the finite volume method solution for the conservation equations of mass, momentum and energy. Both simulations are performed with Reynolds and Prandtl numbers of ReD = 22000 and Pr = 0.71, respectively. The results showed that the transient fluid dynamic and thermal patterns are strongly affected by the configuration of circular cylinders. The kind of arrangement led to a difference of nearly 20 % for time-averaged Nusselt number (NuD).

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## Finite element analysis of laminar and turbulent flows using LES and subgrid-scale models

Fonte: Universidade Federal do Rio Grande
Publicador: Universidade Federal do Rio Grande

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

66.52%

Numerical simulations of laminar and turbulent flows in a lid driven cavity and over a backward-facing step are presented in this work. The main objectives of this research are to know more about the structure of turbulent flows, to identify their three-dimensional characteristic and to study physical effects due to heat transfer. The filtered Navier–Stokes equations are used to simulate large scales, however they are supplemented by subgrid-scale (SGS) models to simulate the energy transfer from large scales toward subgridscales,
where this energy will be dissipated by molecular viscosity. Two SGS models are applied: the classical Smagorinsky s model and the Dynamic model for large eddy simulation (LES). Both models are implemented in a three-dimensional finite element code using linear tetrahedral elements. Qualitative and quantitative aspects of two and three-dimensional flows in a lid-driven cavity and over a backward-facing step, using LES, are analyzed comparing numerical and experimental results obtained by other authors.

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## Dynamics of large turbulent structures in a steady breaker

Fonte: Elsevier
Publicador: Elsevier

Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/article
Formato: application/pdf; text/plain

Publicado em 02/02/2011
ENG

Relevância na Pesquisa

56.36%

The flow near the leading edge of a steady breaker has been studied experimentally using Bubble Image Velocimetry (BIV) with the aim of characterizing the dynamics of the large eddies responsible for air entrainment. It is well reported in the literature, and confirmed by our measurements of the instantaneous velocity field, that this flow shares some important features with the turbulent shear-layer formed between two parallel semi-infinite streams with different velocities. Namely, the formation of a periodic array of coherent vortices, the constant convective velocity of those vortices, the linear relation between their size and their downstream position and the self-similar structure of both mean velocity profiles and Reynolds shear stresses. Nonetheless, important differences exists between the dynamics of the large eddies in a steady breaker and those in a free shear-layer. Particularly, the convective velocity of these large structures is slower in a steady breaker and, consistent with this, their growth rates are larger. A physical interpretation of these differences is provided together with a discussion of their implications. To support our measurements and conclusions, we present a careful analysis of the accuracy of the BIV technique in turbulent flows with large bubbles; The authors wish to thank Professor Emil J. Hopfinger for his valuable suggestions on the interpretation of the experimental data. This work was supported by the ONR through Grant N00014-05-1-0121 and by the Spanish Ministry of Science (MICINN) through Grant DPI2008-06369

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## Mathematical and Numerical Modeling of Turbulent Flows

Fonte: Academia Brasileira de Ciências
Publicador: Academia Brasileira de Ciências

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/06/2015
EN

Relevância na Pesquisa

66.56%

#Turbulent flows#Large Eddy Simulation#Immersed Boundary Method#Low-Mach Number Flows#Industrial Flows

The present work is devoted to the development and implementation of a computational framework to perform numerical simulations of low Mach number turbulent flows over complex geometries. The algorithm under consideration is based on a classical predictor-corrector time integration scheme that employs a projection method for the momentum equations. The domain decomposition strategy is adopted for distributed computing, displaying very satisfactory levels of speed-up and efficiency. The Immersed Boundary Methodology is used to characterize the presence of a complex geometry. Such method demands two separate grids: An Eulerian, where the transport equations are solved with a Finite Volume, second order discretization and a Lagrangian domain, represented by a non-structured shell grid representing the immersed geometry. The in-house code developed was fully verified by the Method of Manufactured Solu- tions, in both Eulerian and Lagrangian domains. The capabilities of the resulting computational framework are illustrated on four distinct cases: a turbulent jet, the Poiseuille flow, as a matter of validation of the implemented Immersed Boundary methodology, the flow over a sphere covering a wide range of Reynolds numbers, and finally, with the intention of demonstrating the applicability of Large Eddy Simulations - LES - in an industrial problem...

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## Direct Numerical Simulation of Compressible and Incompressible Wall Bounded Turbulent Flows with Pressure Gradients

Fonte: Quens University
Publicador: Quens University

Tipo: Tese de Doutorado
Formato: 3555645 bytes; application/pdf

EN; EN

Relevância na Pesquisa

56.56%

#Direct numerical simulation#Discontinuous Galerkin method#Wall bounded Turbulent flow#Adverse pressure gradient#Favourable pressure gradient#Compressible channel flow#Pressure gradient vorticity flux correlation

This thesis is focused on direct numerical simulation (DNS) of compressible and
incompressible fully developed and developing turbulent flows between isothermal
walls using a discontinuous Galerkin method (DGM).
Three cases (Ma = 0.2, 0.7 and 1.5) of DNS of turbulent channel flows between
isothermal walls with Re ~ 2800, based on bulk velocity and half channel width,
have been carried out. It is found that a power law seems to scale mean streamwise
velocity with Ma slightly better than the more usual log-law. Inner and outer scaling
of second-order and higher-order statistics have been analyzed. The linkage between
the pressure gradient and vorticity flux on the wall has been theoretically derived
and confirmed and they are highly correlated very close to the wall. The correlation
coefficients are influenced by Ma, and viscosity when Ma is high. The near-wall
spanwise streak spacing increases with Ma. Isosurfaces of the second invariant of the
velocity gradient tensor are more sparsely distributed and elongated as Ma increases.
DNS of turbulent isothermal-wall bounded flow subjected to favourable and adverse
pressure gradient (FPG, APG) at Ma ~ 0.2 and Reref ~ 428000, based on the
inlet bulk velocity and the streamwise length of the bottom wall...

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## Validation of the Lattice Boltzmann Method for Direct Numerical Simulation of Wall-Bounded Turbulent Flows

Fonte: Quens University
Publicador: Quens University

Tipo: Tese de Doutorado

EN; EN

Relevância na Pesquisa

46.56%

In this work, the lattice Boltzmann method (LBM) was validated for direct numerical simulation (DNS) of wall-bounded turbulent flows. The LBM is a discrete-particle-based method that numerically solves the Boltzmann equation as opposed to conventional DNS methods that are based on the Navier-Stokes (NS) equations. The advantages of the LBM are its simple implementation, its ability to handle complex geometries, and its scalability on modern high-performance computers.
An LBM code was developed and used to simulate fully-developed turbulent channel flow. In order to validate the results, the turbulence statistics were compared to those calculated from a conventional NS-based finite difference (FD) simulation. In the present study, special care was taken to make sure the computational domains for LBM and FD simulations were the same. Similar validation studies in the literature have used LBM simulations with smaller computational domains in order to reduce the computational cost. However, reducing the size of the computational domain affects the turbulence statistics and confounds the results of the validation.
The turbulence statistics calculated from the LBM and FD simulations were found to agree qualitatively; however, there were several significant deviations...

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## Um esquema \"upwind\" para leis de conservação e sua aplicação na simulação de escoamentos incompressíveis 2D e 3D laminares e turbulentos com superfícies livres; The \"upwind\" scheme to the conservation laws and their application in simulation of 2D and 3D incompressible laminar and turbulent flows with free surfaces

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Tese de Doutorado
Formato: application/pdf

Publicado em 26/02/2009
PT

Relevância na Pesquisa

56.34%

#Averaged Navier-Stokes equations#Conservation laws#Equações de Navier-Stokes#Equações médias de Reynolds#Escoamentos com superfícies livres#Esquema "upwind" de alta resolução#Finite difference method#Free surface flows#High-resolution upwind scheme#Método de diferenças finitas#Modelagem 'capa'- ' epsilon' da turbulência

Apesar de as EDPS que modelam leis de conservação e problemas em dinâmica dos fluídos serem bem estabelecidas, suas soluções numéricas continuam ainda desafiadoras. Em particular, há dois desafios associados à computação e ao entendimento desses problemas: um deles é a formação de descontinuidades (choques) e o outro é o fenômeno turbulência. Ambos os desafios podem ser atribuídos ao tratamento dos termos advectivos não lineares nessas equações de transporte. Dentro deste canário, esta tese apresenta o estudo do desenvolvimento de um novo esquema \"upwind\" de alta resolução e sua associação com modelagem da turbulência. O desempenho do esquema é investigado nas soluções da equação de advecção 1D com dados iniciais descontínuos e de problemas de Riemann 1D para as equações de Burgers, Euler e águas rasas. Além disso, são apresentados resultados numéricos de escoamentos incompressíveis 2D e 3D no regime laminar a altos números de Reynolds. O novo esquema é então associado à modelagem \'capa\' - \'epsilon\' da turbulência para a simulação numérica de escoamentos incompressíveis turbulentos 2D e 3D com superfícies livres móveis. Aplicação, verificação e validação dos métodos numéricos são também fornecidas; Althought the PDEs that model conservation laws and fluid dynamics problems are well established...

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## Feasible domain of Walker's unsteady wall-layer model for the velocity profile in turbulent flows

Fonte: Academia Brasileira de Ciências
Publicador: Academia Brasileira de Ciências

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/12/2014
EN

Relevância na Pesquisa

56.3%

The present work studies, in detail, the unsteady wall-layer model of Walker et al. (1989, AIAA J., 27, 140 – 149) for the velocity profile in turbulent flows. Two new terms are included in the transcendental non-linear system of equations that is used to determine the three main model parameters. The mathematical and physical feasible domains of the model are determined as a function of the non-dimensional pressure gradient parameter (p+). An explicit parameterization is presented for the average period between bursts (), the origin of time () and the integration constant of the time dependent equation (A0) in terms of p+. In the present procedure, all working systems of differential equations are transformed, resulting in a very fast computational procedure that can be used to develop real-time flow simulators.

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## Irreversibility and small-scale generation in 3D turbulent flows

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/06/2015

Relevância na Pesquisa

46.56%

In three-dimensional turbulent flows energy is supplied at large scales and
cascades down to the smallest scales where viscosity dominates. The flux of
energy through scales implies the generation of small scales from larger ones,
which is the fundamental reason for the irreversibility of the dynamics of
turbulent flows. As we showed recently, this irreversibility manifests itself
by an asymmetry of the probability distribution of the instantaneous power $p$
of the forces acting on fluid elements. In particular, the third moment of $p$
was found to be negative. Yet, a physical connection between the
irreversibility manifested in the distribution of $p$ and the energy flux or
small-scale generation in turbulence has not been established. Here, with
analytical calculations and support from numerical simulations of fully
developed turbulence, we connect the asymmetry in the power distribution, {\it
i.e.}, the negative value of $\langle p^3 \rangle$, to the generation of small
scales, or more precisely, to the amplification (stretching) of vorticity in
turbulent flows. Our result is the first step towards a quantitative
understanding of the origin of the irreversibility observed at the level of
individual Lagrangian trajectories in turbulent flows.; Comment: 13 pages...

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## Macroscopic effects of the spectral structure in turbulent flows

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/09/2009

Relevância na Pesquisa

46.67%

Two aspects of turbulent flows have been the subject of extensive, split
research efforts: macroscopic properties, such as the frictional drag
experienced by a flow past a wall, and the turbulent spectrum. The turbulent
spectrum may be said to represent the fabric of a turbulent state; in practice
it is a power law of exponent \alpha (the "spectral exponent") that gives the
revolving velocity of a turbulent fluctuation (or "eddy") of size s as a
function of s. The link, if any, between macroscopic properties and the
turbulent spectrum remains missing. Might it be found by contrasting the
frictional drag in flows with differing types of spectra? Here we perform
unprecedented measurements of the frictional drag in soap-film flows, where the
spectral exponent \alpha = 3 and compare the results with the frictional drag
in pipe flows, where the spectral exponent \alpha = 5/3. For moderate values of
the Reynolds number Re (a measure of the strength of the turbulence), we find
that in soap-film flows the frictional drag scales as Re^{-1/2}, whereas in
pipe flows the frictional drag scales as Re^{-1/4} . Each of these scalings may
be predicted from the attendant value of \alpha by using a new theory, in which
the frictional drag is explicitly linked to the turbulent spectrum. Our work
indicates that in turbulence...

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## A connection between the Camassa-Holm equations and turbulent flows in channels and pipes

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/03/1999

Relevância na Pesquisa

46.59%

In this paper we discuss recent progress in using the Camassa-Holm equations
to model turbulent flows. The Camassa-Holm equations, given their special
geometric and physical properties, appear particularly well suited for studying
turbulent flows. We identify the steady solution of the Camassa-Holm equation
with the mean flow of the Reynolds equation and compare the results with
empirical data for turbulent flows in channels and pipes. The data suggests
that the constant $\alpha$ version of the Camassa-Holm equations, derived under
the assumptions that the fluctuation statistics are isotropic and homogeneous,
holds to order $\alpha$ distance from the boundaries. Near a boundary, these
assumptions are no longer valid and the length scale $\alpha$ is seen to depend
on the distance to the nearest wall. Thus, a turbulent flow is divided into two
regions: the constant $\alpha$ region away from boundaries, and the near wall
region. In the near wall region, Reynolds number scaling conditions imply that
$\alpha$ decreases as Reynolds number increases. Away from boundaries, these
scaling conditions imply $\alpha$ is independent of Reynolds number. Given the
agreement with empirical and numerical data, our current work indicates that
the Camassa-Holm equations provide a promising theoretical framework from which
to understand some turbulent flows.; Comment: tex file...

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## On the meaning of mixing efficiency for buoyancy driven mixing in stratified turbulent flows

Fonte: Cambridge University Press
Publicador: Cambridge University Press

Tipo: Article; accepted version

EN

Relevância na Pesquisa

56.38%

This is the author accepted manuscript. The final version is available from Cambridge University Press via http://dx.doi.org/10.1017/jfm.2015.462; This article is Crown Copyright.; The concept of a mixing efficiency is widely used to relate the amount of irreversible diabatic mixing in a stratified flow to the amount of energy available to support mixing. This common measure of mixing in a flow is based on the change in the background potential energy, which is the minimum gravitational potential energy of the fluid that can be achieved by an adiabatic rearrangement of the instantaneous density field. However, this paper highlights examples of mixing that is primarily ?buoyancy-driven? (i.e. energy is released to the flow predominantly from a source of available potential energy) to demonstrate that the mixing efficiency depends not only on the specific characteristics of the turbulence in the region of the flow that is mixing, but also on the density profile in regions remote from where mixing physically occurs. We show that this behaviour is due to the irreversible and direct conversion of available potential energy into background potential energy in those remote regions (a mechanism not previously described). This process (here termed ?relabelling?) occurs without requiring either a local flow or local mixing...

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## On the full lagrangian approach and thermophoretic deposition in gas-particle flows

Fonte: University of Cambridge; Department of Engineering; Clare College
Publicador: University of Cambridge; Department of Engineering; Clare College

Tipo: Thesis; doctoral; PhD

EN

Relevância na Pesquisa

46.51%

Theoretical and experimental studies of particle deposition in turbulent pipe flow have been carried out for over forty years, but some of the most important transport mechanisms are still not well understood. The first part of this thesis is concerned with the calculation of particle density when using Lagrangian methods to predict inertial particle transport in two-dimensional laminar fluid flows. Traditionally, Lagrangian calculations involve integrating the particle equations of motion along particle pathlines, and the particle density is obtained by applying a statistical averaging procedure to those pathlines which intersect a particular computational grid cell. Unfortunately, extremely large numbers of particles are required to reduce the statistical errors to acceptable levels, and this makes the method computationally expensive.
Recently, the Full Lagrangian approach has been developed, which allows the direct calculation of the particle density along particle pathlines. This method had previously been applied only to simple analytical flow fields. The application of the method to CFD generated fluid velocity fields was shown to be possible, and the results obtained using the Full Lagrangian approach were compared to those from a traditional Lagrangian approach. It was found that better quality solutions could be obtained with the use of far fewer particle pathlines. An analysis of the manner in which the Full Lagrangian approach deals with particles whose paths cross each other (and the resulting discontinuity in particle density) was also undertaken...

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## Entrainment of fine sediments by turbulent flows

Fonte: California Institute of Technology
Publicador: California Institute of Technology

Tipo: Report or Paper; PeerReviewed
Formato: application/pdf

Publicado em /06/1966

Relevância na Pesquisa

46.55%

A study was made of the means by which turbulent flows entrain sediment grains from alluvial stream beds. Entrainment was considered to include both the initiation of sediment motion and the suspension of grains by the flow. Observations of grain motion induced by turbulent flows led to the formulation of an entrainment hypothesis. It was based on the concept of turbulent eddies disrupting the viscous sublayer and impinging directly onto the grain surface. It is suggested that entrainment results from the interaction between fluid elements within an eddy and the sediment grains.
A pulsating jet was used to simulate the flow conditions in a turbulent boundary layer. Evidence is presented to establish the validity of this representation. Experiments were made to determine the dependence of jet strength, defined below, upon sediment and fluid properties. For a given sediment and fluid, and fixed jet geometry there were two critical values of jet strength: one at which grains started to roll across the bed, and one at which grains were projected up from the bed. The jet strength, K, is a function of the pulse frequency, [omega] , and the pulse amplitude, A, defined by
K=A[omega]^-s
where s is the slope of a plot of log A against log [omega]. Pulse amplitude is equal to the volume of fluid ejected at each pulse divided by the cross sectional area of the jet tube.
Dimensional analysis was used to determine the parameters by which the data from the experiments could be correlated. Based on this...

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## Whole-Field Measurements in Gas-Phase Turbulent Flows

Fonte: California Institute of Technology
Publicador: California Institute of Technology

Tipo: Report or Paper; PeerReviewed
Formato: application/pdf

Publicado em 13/03/1995

Relevância na Pesquisa

46.53%

The purpose of this experimental, nine-month effort was to investigate the scalar field in fully-developed, gas-phase, turbulent flows, using planar index-of-refraction
imaging at elevated pressures (p ≃ 10 atm).
The motivation behind this work is to further our understanding of phenomena that rely on the behavior of scalar gradients, such as aero-optic effects, laser propagation through, and scattering by, gas-phase turbulent flows, as well as turbulent mixing and combustion.
In this effort, we have used planar laser-Rayleigh scattering to image simultaneously the index-of-refraction field of a turbulent jet and the optical degradation of
the planar laser probe beam caused by the turbulent flow-field. From these results, we have demonstrated that conducting these experiments at elevated pressure increases
the index-of-refraction gradients and improves the signal-to-noise ratio over measurements conducted at atmospheric conditions. The optical degradation occurs
in the jet-fluid region and manifests itself as a spatial amplitude modulation (streaks) in the laser sheet. This optical degradation illustrates the same loss of coherence
undergone by laser beams and by coherent information when propagating through the turbulent atmosphere.

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