Download or read book Examining the Effects of a Controled Turbulent Velocity Field on a Passive Scalar written by Don O'Regan and published by . This book was released on 2005 with total page 79 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Distortion of Turbulent Velocity and Passive Scalar Fields written by Hamid Reza Rahai and published by . This book was released on 1988 with total page 426 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Theoretical Investigation of a Passive Scalar Such as Temperature in Isotropic Turbulence written by Robert McDougall Kerr and published by . This book was released on 1981 with total page 296 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Effect of Length Scales on the Mixing of Passive Scalars in Grid Turbulence written by Abdullah MohammadFarid N. Alkudsi and published by . This book was released on 2012 with total page 184 pages. Available in PDF, EPUB and Kindle. Book excerpt: The study of passive scalars in turbulence research is a technologically important topic. This study seeks to investigate the effect of the variation of integral length scale, and scalar injector size and separation on turbulent mixing. The study is motivated by devices such as the simple gas turbine pre-mixer where these three relevant length scales are present. Heated wakes of different sizes and separations are produced by heating various sets of thin wires in a horizontal wire grid. The horizontal wire screen is placed downstream of biplane grids of different mesh sizes. Time-resolved velocity and temperature are measured by a dual hot and cold wire probe. Assessments of the effect of the array of thin wires used as the scalar generator on the velocity and scalar fields behind the turbulence grid are presented. The evolution of the profiles of the first four statistical moments of temperature fluctuations across the wake with downstream distance is studied for various scalar injector size and separation for several integral length scales. Specifically, the downstream evolution of a single thermal wake is studied for source sizes that range from less than the Kolmogorov to larger than the integral length scale. Also included in the study the downstream evolution and interaction of two thermal wakes is also presented. For the single wake, the wake growth, sufficiently far from the generator, for all source sizes is found to be consistent with the assumption that the effective diffusivity is constant across the wake. The evolution of the effective diffusivity with downstream distance is found to be independent of the source size and identical to the evolution of the effective diffusivity of a line source in the same velocity field. The effects of changing the velocity integral scale are governed by the effect of the integral scale on the evolution of the effective turbulent diffusivity. Those effects are found to be similar in character to the effect on a line source. The mean scalar of two sources is found to be the sum of the mean of the individual sources. Higher statistical moments show more complex relationships.

Download or read book Mixing of a Passive Scalar in Isotropic and Sheared Homogeneous Turbulence written by E. Shirani and published by . This book was released on 1981 with total page 192 pages. Available in PDF, EPUB and Kindle. Book excerpt: In order to calculate the velocity and passive scalar fields in homogeneous turbulent flows, the three-dimensional, time-dependent equations of motion and the diffusion equation have been solved numerically.

Download or read book Effect of Background Turbulence on the Scalar Field of a Turbulent Jet written by Alejandro Perez Alvarado and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "The effect of background turbulence on the scalar field of an axisymmetric turbulent jet is investigated experimentally. The present investigation builds on the work of Gaskin et al. (2004), who studied the concentration and velocity fields of a plane jet in a shallow coflow with different turbulence levels and Khorsandi et al. (2013), who studied the velocity field of an axisymmetric turbulent jet emitted into a turbulent background. Different driving algorithms for a large RJA were tested and the statistics of the turbulence generated downstream of the RJA were compared to characterize the algorithms' performance. Variations in the spatial configuration of jets operating at any given instant, as well as in the statistics of their on/off times were studied. The algorithm identified as RANDOM generated the closest approximation of zero-mean-flow homogeneous isotropic turbulence. The flow generated by the RANDOM algorithm had a relatively high turbulent Reynolds number (ReT = uTl/[nu] = 2360, where uT is a characteristic RMS velocity, l is the integral length scale of the flow, [nu] is the kinematic viscosity of the water) and the integral length scale (l = 11.6 cm) is the largest reported to date. Thus, RANDOM algorithm was used to generate the background turbulence for the investigation of scalar mixing within a turbulent jet.The effect of background turbulence on the mixing of a passive scalar within a turbulent jet at different Reynolds numbers was investigated. To this end, planar laser-induced fluorescence was employed to obtain concentration measurements of dye (disodium fluorescein, Schmidt number = 2000) within the jet. Two jet Reynolds numbers (Re=UjD/[nu], where Uj is the jet exit velocity, D is the nozzle diameter and [nu] is the kinematic viscosity of the jet fluid, water) were studied: 10600 and 5800. The resulting statistics of the scalar fields showed that the mean concentrations of jets emitted into turbulent backgrounds were lower than those of jets emitted into a quiescent background near the centerline. However, near the edges of the jet (r/x>0.15), the concentrations were higher for the jets issued into turbulent surroundings. The RMS concentrations of the jet emitted into a turbulent background significantly increased. Examination of the probability density functions of concentration revealed a higher degree of intermittency of the scalar field. The probability of low concentrations increased in the presence of background turbulence although the maximum concentrations were comparable to those of the jet emitted into a quiescent background. Flow visualizations revealed meandering of the jet issued into background turbulence, which is associated with the increased probability of lower concentrations and higher intermittency. Additionally, the widths of the jets emitted into a turbulent background were increased. For the lower jet Reynolds number, the described effects were more evident and the jet structure was destroyed by the background turbulence within the measurement region, resulting in flat radial profiles of both the mean and RMS concentrations. Comparison of the results of the scalar field with those of the hydrodynamic jet of Khorsandi et al. (2013) revealed a similar behavior of the two fields. However, the most significant difference was the larger radial extent of the profiles of mean and RMS concentrations, which resulted from the meandering of the jet and increased transport of scalar by turbulent diffusion. The flow visualizations suggest that the entrainment and mixing in the jet in a turbulent background changes with the destruction of jet structure, from jet driven entrainment to become potentially dominated by i) increased lateral advection of the jet by large scales of the background turbulence during the meandering of the jet, which is subsequently mixed by its smaller scales, and ii) turbulent diffusion that is significantly enhanced by the turbulent background." --

Download or read book The Structure of the Velocity and Passive Scalar Fields in High Reynolds number and High Peclet number Grid Turbulence written by Laurent Bernard Mydlarski and published by . This book was released on 1998 with total page 404 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Assessment of Two Turbulent Flow Passive Scalar Transport Models written by Matthew Eugene Thomas and published by . This book was released on 1984 with total page 216 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Simultaneous Measurements of Velocity and Temperature in the Heated Wake of a Cylinder with Applications to the Modeling of Turbulent Passive Scalars written by Arpi Berajeklian and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Priori Study of Subgrid scale Flux of a Passive Scalar in Isotropic Homogeneous Turbulence written by and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: We perform a direct numerical simulation (DNS) of forced homogeneous isotropic turbulence with a passive scalar that is forced by mean gradient. The DNS data are used to study the properties of subgrid-scale flux of a passive scalar in the framework of large eddy simulation (LES), such as alignment trends between the flux, resolved, and subgrid-scale flow structures. It is shown that the direction of the flux is strongly coupled with the subgrid-scale stress axes rather than the resolved flow quantities such as strain, vorticity, or scalar gradient. We derive an approximate transport equation for the subgrid-scale flux of a scalar and look at the relative importance of the terms in the transport equation. A particular form of LES tensor-viscosity model for the scalar flux is investigated, which includes the subgrid-scale stress. Effect of different models for the subgrid-scale stress on the model for the subgrid-scale flux is studied.

Download or read book The Small scale Structure of Passive Scalar Mixing in Turbulent Boundary Layers written by Lakshmi P. Dasi and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective is to contribute to several issues regarding the traditional view of the local structure of passive scalar fields: (1) probability density function (PDF) of the scalar concentration and scalar gradient, (2) the scalar power spectrum, (3) the structure functions, and (4) correlation functions and multi-point correlators. In addition, the research provides a geometric description of two-dimensional transects of the passive scalar iso-surfaces using the tools of fractal geometry. The local structure is analyzed as a function of large-scale anisotropy, intermittency factor, Reynolds number, and initial condition of the scalar injection. Experiments were performed in the bed boundary layer produced by a uniform depth open channel flow of water in a tilting flume for Re [subscript lambda] = 63, 94, and 120. A small nozzle iso-kinetically delivers a passive scalar of high Schmidt number (Sc = 1000) at mid-depth to generate the turbulent scalar field. Three nozzle diameters are used to study the effects of the injection length scale. High-resolution planar laser induced fluorescence (PLIF) technique is used to measure the scalar field. The local structure far from isotropic and is influenced even at the smallest scales by large-scale anisotropy, initial injection length scale and the Reynolds number of the flow. The PDF of the scalar fluctuations is non-Gaussian and dependent on large-scale anisotropy. The PDF of scalar gradients show the influence of large-scale anisotropy on the structure at the smallest scales. The spectrum of the scalar field deviates from the k [superscript -5/3] prediction in the inertial convection regime and is dependent on large-scale anisotropy, external intermittency, and low Reynolds number. There is no evidence of Batchelors k [superscript -1] scaling law. The scaling exponents of the even-ordered structure functions appear to be inversely correlated with the kurtosis of the scalar fluctuations. The fractal geometry of the two dimensional transects of passive scalar iso-surfaces is scale dependent. The fractal dimension is 1.0 at the smallest length scale and increases in a universal manner in the viscous-convective regime. The coverage length underestimate reflects this universal behavior with practical significance. The lacunarity function shows that the instantaneous scalar field is most in-homogenous around the Kolmogorov scale.

Download or read book Toward a Fundamental Understanding of Scalar Iso surface Kinematics in Turbulent Flows written by Brandon Blakeley and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding the behavior of scalar iso-surfaces in turbulent flows is of particular interest for many problems in turbulent mixing that contain sharp interfaces between regions of the flow. Common examples include combustion, where the chemical reactions occur in thin regions within the flow, and the turbulent/non-turbulent interface in shear flows, where a thin region separates the rotational, turbulent motions from the irrotational, non-turbulent background. These interfaces can be described by an iso-surface of a scalar field, e.g., the flame surface is defined by an iso-value of a progress variable (usually temperature or species concentration) in premixed combustion, the stoichiometric value of the mixture fraction in non-premixed combustion, and the turbulent/non-turbulent interface is defined by a suitably `small' value of the vorticity magnitude. Experimental and computational limitations have previously made investigations of iso-surface kinematics difficult to quantify, but recent advances in computing power and numerical algorithms allow for in-depth diagnostics of iso-surfaces interacting with three-dimensional, turbulent velocity fields. This dissertation is comprised of a series of studies focused on gaining a deeper, fundamental understanding of the kinematics of iso-surfaces, with the intent that the results can be applied and extended to a wide range of turbulent mixing problems in the field. First, direct numerical simulations of two differently initialized scalar fields evolving in homogeneous, isotropic turbulence are discussed, in which the iso-surface area density [Sigma] and the terms in the evolution equation of [Sigma] are evaluated as a function of time for a wide range of iso-values. In particular, the terms governing production of [Sigma], related to the fluid strain-rate, and destruction, related to the surface curvature and molecular diffusion, are considered. It is found that, because the fluid strain-rate is constant for both scalar fields, the production term behaves similarly for the two different scalar fields, but the behavior of the destruction term depends significantly on the initial condition of the scalar field. In the following chapters, the iso-surface area density and the terms in its evolution equation are considered for a passive scalar field in the direct numerical simulation of a turbulent, temporally developing mixing layer. A novel approach has been taken to calculate surface averages as a function of the cross-stream direction, which allows for a more nuanced understanding of how iso-surface area is produced, destroyed, and transported (via turbulent and molecular diffusion) within the turbulent flow. A notable finding is that the profiles of [Sigma] develop in a self-similar manner when scaled by the Taylor scale of the scalar field, [lambda phi] Remarkably, the scaling appears to hold for a wide range of iso-values. A rough scaling argument based on the formal definition of [Sigma] and properties of a temporal mixing layer is presented which also exposes a dependence on [lambda phi]. Based on these results, a possible scaling for the iso-surface area is presented as Aiso/A0 ~ (ReSc)1/2, where Re and Sc are local Reynolds and Schmidt numbers, respectively. The terms in the evolution equation for [Sigma] are also found to evolve in a self-similar manner, although there are significant differences in the scalings between several of the terms. Based on data from the direct numerical simulations, it is suggested that the rate of change of [Sigma] and the turbulent flux terms scale with h[lambda phi Delta U], where h is the width of the mixing layer and [Delta] U is the velocity difference across the mixing layer. In contrast, the production and destruction terms are found to scale with an additional factor of (Re Sc)1/2 compared to [partial Sigma partial t] which is easily observed in previous studies as a significant discrepancy in the magnitude of the production and destruction terms compared to [partial Sigma partial t]. Importantly, the net effect of the production and destruction terms, sometimes referred to as net surface "stretch" is found to scale with [partial Sigma partial t] .The molecular diffusion term is found to scale with a factor of (Re Sc)-1/2 compared to [partial Sigma partial t], which suggests that molecular diffusion is negligible for ReSc ”1.

Download or read book Extraction of Velocity Fields from Measurements of Passive Or Reactive Scalars written by Bonnie N. Carpenter and published by . This book was released on 1994 with total page 84 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Data driven and Nonlocal Approaches in Modeling Analysis and Simulation of Turbulent Mixing Phenomena written by Ali Akhavan Safaei and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The overreaching goal of this study is utilizing data-driven methods and sophisticated mathematical tools for modeling and simulation of turbulent transport of passive scalars. We focus on embedding the intrinsic nonlocal nature of the turbulence into our models. We study the nonlocal dynamics in the context of (i) subgrid-scale (SGS) modeling for largeeddy simulation (LES), and (ii) the turbulent cascade under large-scale anisotropic sources. Moreover, we implement stochastic modeling methodologies to systematically investigate the contributing mechanisms leading a high-speed hydrodynamic transport system into instability and chaos, as well as discovering the anomalies in the featured characteristics of the transport.First, we present a computational-statistical framework to obtain high-fidelity data for homogeneous isotropic turbulent (HIT) flow and passive scalar transport. A parallel implementation of the well-known pseudo-spectral method in addition to the comprehensive record of the statistical and small-scale quantities of the turbulent transport are offered for executing on distributed memory CPU-based supercomputers.Afterwards, we investigate the inherent nonlocal behavior of the SGS passive scalar flux through studying its two-point statistics obtained from the filtered direct numerical simulation (DNS) data for passive scalar transport in HIT flow. We propose a statistical model for microscopic SGS motions by considering the filtered Boltzmann transport equation (FBTE) for passive scalar. In FBTE, we approximate the filtered equilibrium distribution with an Îł-stable Levy distribution that incorporates a power-law behavior to resemble the observed nonlocal statistics of SGS scalar flux. Through generic ensemble-averaging of FBTE, we formulate a continuum-level closure model for the SGS scalar flux appearing in terms of a fractional-order Laplacian that is a nonlocal operator.Moreover, we revisit the spectral transfer model for the turbulent intensity in the passive scalar transport (under large-scale anisotropic forcing), and a subsequent modification to the scaling of scalar variance cascade is presented. Accordingly, we obtain a revised scalar transport model using fractional-order Laplacian operator that facilitates the robust inclusion of the nonlocal effects originated from large-scale anisotropy transferred across the multitude of scales in the turbulent cascade. We provide an a priori estimate for the nonlocal model, and examine the model through a new DNS. We conduct a detailed analysis on the evolution of the scalar variance, high-order statistics of scalar gradient, and two-point statistical metrics of the turbulent transport to compare the developed nonlocal model and its standard version.In another study, a deep learning surrogate model in the form of fully connected feedforward neural networks is developed to predict the SGS scalar flux in the context of large eddy simulation of turbulent transport. The deep neural network (DNN) model is trained and validated using filtered DNS dataset at P eλ = 240, Sc = 1 that includes the filtered scalar and velocity gradients as input features. Using the transfer learning concept, we generalize the performance of this trained model to turbulent scalar transport regimes with higher P eλ and Sc numbers with a relatively low amount of data and computations.Finally, in stochastic modeling of hydrodynamic transport, we study the flow dynamics inside a high-speed rotating cylinder after introducing strong symmetry-breaking disturbance factors at cylinder wall motion. We perform a statistical analysis on the fluctuating fields characterizing the fingerprints and measures of intense and rapidly evolving non-Gaussian behavior through space and time. Such non-Gaussian statistics essentially emerge and evolve due to an intensified presence of coherent vortical motions initially triggered by the flow instability due to symmetry-breaking rotation of the cylinder. We show that this mechanism causes significant memory effects in the flow so that noticeable anomaly in the time-scaling of enstrophy record is observed in the long run apart from the onset of instability.

Download or read book Cascades Spectra Real Space Structure Inhomogeneous Mixing and Transport in Active Scalar Turbulence written by Xiang Fan and published by . This book was released on 2019 with total page 104 pages. Available in PDF, EPUB and Kindle. Book excerpt: An active scalar system refers to a system with a scalar field that is coupled to the fluid dynamics and gives feedback to the velocity field through local forces. Active scalar turbulence systems are ubiquitous, and the study of these systems is a central focus of research in theoretical plasma physics. As examples, the 2D Cahn-Hilliard Navier-Stokes (CHNS) system and 2D Magnetohydrodynamics (MHD) system are studied in this dissertation. The similarities and differences between 2D CHNS and 2D MHD are discussed. These are both elastic (i.e., self-restoring) systems, and display a memory, governed by freezing-in laws. The CHNS system supports an elastic wave, which is analogous to Alfven wave in MHD. Cascades and spectra in 2D CHNS are investigated, with focus on the interaction between inverse and forward cascades. The inverse cascade of mean square concentration $\langle\psi^2\rangle$, which is closely related to the real space dynamics of blob formation and merger, is found to be the dominant nonlinear transfer process. The spectrum of $\langle\psi^2\rangle_k$ exhibits a scaling law of $\sim k^{-7/3}$, and this exponent is the same as the corresponding one in 2D MHD. On the other hand, the kinetic energy spectrum follows $E_k\sim k^{-3}$. This exponent is closer to that for 2D Navier-Stokes, instead of that for 2D MHD. We suggest this is because the restoring force is significant only in the interfacial regions. The packing fraction of interfacial regions is small because of the formation and merger of blobs. This suggests that the inverse cascade of $\langle \psi^2 \rangle$--related to blob coalescence--modifies the forward cascade in 2D CHNS. The evolution of the concentration field of the Cahn-Hilliard system in the background of a single eddy is studied. This is analogous to the flux expulsion phenomenon in 2D MHD. Though the system is simple, complex evolution is observed. 3 stages are observed: the “jelly roll” pattern stage, the stage of topological evolution, and the “target” pattern stage. The target pattern is metastable, as the bands gradually merge with time. We also study turbulent transport in active scalar systems. We intended to first explore the classic problem of the suppression of turbulent transport in 2D MHD as an exercise in code verification, and then move to 2D CHNS. However, novel blob-and-barrier real space structures were observed with higher magnetic Reynolds number $\mathrm{Rm}$ in 2D MHD. We argue that the conventional approach of mean field theory is not applicable for the case without an external large scale magnetic field. The magnetic energy is observed to be concentrated in the intermittent, thin transport barrier regions, which located in the interstices between blobs of magnetic potential. The turbulent transport is quenched primarily because of these barriers. Barrier formation is linked to the inverse cascade of mean square magnetic potential $\langle A^2\rangle$ and negative turbulent resistivity. For small scale forcing, spontaneous formation of layering occurs. More generally, we demonstrate that synergistic studies of related but different systems--2D CHNS and 2D MHD--can lead to improved understanding. These studies can provide insights for all active scalar turbulence systems, since these systems share important common properties such as memory, elastic waves, and conservation laws.

Download or read book Dynamical System Prediction of the Scalar Field in a Turbulent Channel Flow written by and published by . This book was released on 1994 with total page 18 pages. Available in PDF, EPUB and Kindle. Book excerpt: The importance of large-scale (or coherent) structure to optical propagation through turbulent shear flow has been demonstrated. Direct simulations of low-Reynolds-number flows which include a passive scalar as well as experimental data have been examined. A passive scalar in the simulations is related to refractive-index fluctuations, while a heated jet was used in the experiment. Large fluctuations associated with large-scale turbulent structure produce a majority of the optical phase error. A low-order dynamical model for the near-wall region of a turbulent channel flow was developed. These predictions illustrate the importance of the dynamics of the turbulent shear flow to optical phase error. Techniques to use limited data to estimate the effect of large-scale structure upon optical propagation were developed. A round turbulent jet was also studied using a large eddy simulation as well as experimental data. Temperature at several locations and jitter in an optical beam propagated through the flow can be measured simultaneously in an experimental facility constructed at the Air Force Phillips Laboratory. A low- dimensional dynamical model for the round jet with passive scalar to be developed in subsequent work will be compared with this experimental data. Turbulence, Dynamical systems, Aero-optics.

Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1995 with total page 704 pages. Available in PDF, EPUB and Kindle. Book excerpt: