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 Experimental Studies of Homogeneous Passive Scalars in Grid generated Turbulence written by Anuvat Sirivat and published by . This book was released on 1983 with total page 554 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 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 Turbulent Premixed Flames written by Nedunchezhian Swaminathan and published by Cambridge University Press. This book was released on 2011-04-25 with total page 447 pages. Available in PDF, EPUB and Kindle. Book excerpt: A work on turbulent premixed combustion is important because of increased concern about the environmental impact of combustion and the search for new combustion concepts and technologies. An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts. Lean premixed flames have the potential to offer ultra-low emission levels, but they are notoriously susceptible to combustion oscillations. Thus, sophisticated control measures are inevitably required. The editors' intent is to set out the modeling aspects in the field of turbulent premixed combustion. Good progress has been made on this topic, and this cohesive volume contains contributions from international experts on various subtopics of the lean premixed flame problem.

Download or read book Investigation of Dynamic Subgrid scale and Wall Models for Turbulent Boundary Layers written by Hyun Ji Bae and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Most turbulent flows cannot be calculated by direct numerical simulation (DNS) of the Navier-Stokes equations because the range of scales of motions is so large that the computational cost becomes prohibitive. In large-eddy simulation (LES), only the large eddies are resolved and the effect of the small scales on the larger ones is modeled through a subgrid-scale (SGS) model. Given that accurate representation and prediction of turbulence is needed in many engineering and scientific applications, development of accurate yet computationally efficient SGS models is an important task. Additionally, wall models are necessary to overcome the prohibitive near-wall resolution requirements for the large scales in high-Reynolds-number turbulent flows. This study investigates a new SGS model, the anisotropic minimum-dissipation (AMD) model, which is constructed to provide the minimum eddy viscosity required to avoid energy pile-up in the smallest resolved scales. The AMD model is successfully applied in simulations of decaying grid turbulence for isotropic grids, and temporal mixing layer and turbulent channel flow for anisotropic grids. This model is more cost-effective than the dynamic Smagorinsky model (DSM) and appropriately switches off in laminar and transitional flows. The formulation of the AMD model is extended to the transport equation for scalar concentration to model the subfilter scalar flux. The performance of the model is tested in the simulation of high-Reynolds-number rough-wall boundary-layer flow with a constant and uniform surface scalar flux. The simulation results obtained from the scalar model show good agreement with well-established empirical correlations and theoretical predictions of the resolved flow statistics. The accuracy of the SGS models is tested by studying the convergence properties in the outer region of a channel flow at moderate to high Reynolds numbers. As LES requires scale separation of the resolved and subgrid scales, the convergence study must be conducted in high-Reynolds-number flows. However, the analysis shows that the errors from the near-wall region are dominant for SGS models in usual LES grid resolutions, where the grid is not refined in the wall-parallel directions. For evaluation of SGS models, in order to overcome the grid requirements imposed by the near-wall turbulent eddies as well as the errors accumulated near the wall, a possible solution is to isolate the outer region of wall-bounded flows. This is made possible by one of two ways: suppressing the near-wall dynamics through a modified wall, or supplying the correct mean stress at the wall with a wall model. Theoretical analysis of the error scaling of SGS models for the mean velocity profile, turbulence intensities, and energy spectra is performed. The numerical convergence studies of the DSM and AMD models show that both models are first-order accurate in terms of the mean velocity profile, which is consistent with the theoretical assessments. Lastly, a new dynamic wall model based on the slip boundary condition is proposed. The use of the slip boundary condition for wall-modeled LES is motivated through theoretical analysis and a priori study of DNS data. The effect of the slip boundary condition on the one-point statistics of the flow is investigated in LES of turbulent channel and flat-plate turbulent boundary layer. The slip boundary condition provides a framework to compensate for the deficit or excess of mean momentum at the wall. The requirements for the slip lengths to be used in conjunction with wall models are discussed, and the equation that connects the slip boundary condition with the stress at the wall is derived. A dynamic procedure based on the invariance of wall stress under test filtering is formulated for the slip condition, providing a dynamic slip wall model free of any a priori specified coefficients. The performance of the proposed dynamic wall model is tested in a series of LES of turbulent channel flow at varying Reynolds numbers, non-equilibrium three-dimensional transient channel flow, and zero-pressure-gradient flat-plate turbulent boundary layer. The results show that the dynamic wall model is able to accurately predict mean and turbulence intensities for various flow configurations, Reynolds numbers, and grid resolutions.

Download or read book On the Subgrid scale Modeling of Compressible Turbulence written by C. G. Speziale and published by . This book was released on 1987 with total page 20 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Mixing of a Passive Scalar in Isotropic and Sheared Homogeneous Turbulence written by Ebrahim Shirani-Chaharsughi and published by . This book was released on 1981 with total page 157 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 A Multifractal Subgrid scale Model for Large eddy Simulation of Turbulent Flows written by Gregory Charles Burton and published by . This book was released on 2003 with total page 568 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book The Structure and Dynamics of the Vorticity and Passive Scalar Fields at Small Scales in Homogeneous Isotropic Turbulence written by Gregory R. Ruetsch and published by . This book was released on 1992 with total page 220 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Large Eddy Simulations of Turbulence written by M. Lesieur and published by Cambridge University Press. This book was released on 2005-08-22 with total page 240 pages. Available in PDF, EPUB and Kindle. Book excerpt: Large-Eddy Simulations of Turbulence is a reference for LES, direct numerical simulation and Reynolds-averaged Navier-Stokes simulation.

Download or read book Advanced Turbulent Combustion Physics and Applications written by N. Swaminathan and published by Cambridge University Press. This book was released on 2022-01-06 with total page 485 pages. Available in PDF, EPUB and Kindle. Book excerpt: Explore a thorough overview of the current knowledge, developments and outstanding challenges in turbulent combustion and application.

Download or read book Subgrid scale Modeling and Wavelet Analysis for Preferential Concentration of Inertial Point Particles in Turbulent Flows written by Maxime Bassenne and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A striking feature of particle-laden turbulent flows is the presence of particle clouds that result from the tendency of inertial particles to preferentially sample specific regions of the flow field. This phenomenon is central to a number of important physical processes. However, computational predictions of preferential concentration at high Reynolds numbers are challenging, since the numerical resolution of the participating scales is typically unaffordable. This dissertation contributes both to the analysis of the preferential concentration phenomenon and the development of subgrid-scale models for the prediction of preferential concentration in large-eddy simulations of particle-laden turbulence. First, direct numerical simulations of incompressible homogeneous-isotropic turbulence laden with a dilute suspension of inertial point particles are performed in conjunction with a wavelet multi-resolution analysis of the results. The use of spatially localized wavelet basis functions enables the simultaneous consideration of physical and scale spaces in the spectral characterization of the flow field of the carrier phase and the concentration field of the disperse phase. The multi-resolution analysis of the disperse phase provides statistical information about the spatial variability of a scale-dependent coarse-grained number density field and the local energy spectra of its fluctuations, characterizing the sensitivities of those quantities to variations in scale and Stokes number. In particular, the spatial variabilities of the wavelet energy spectrum of the particle concentration fluctuations are observed to be maximum in regimes where the particles preferentially concentrate. The results highlight the scale-dependent inhomogeneities of the structures in the concentration field generated by preferential concentration, and the existence of characteristic scales of interaction between the disperse and carrier phases. Additionally, an inter-phase multi-resolution analysis is performed that indicates the occurrence of a spatial anti-correlation between the enstrophy and kinetic-energy spectra of the carrier phase and the particle concentration at small scales in regimes where preferential concentration is important. This anti-correlation vanishes as the scale is increased, and is largely suppressed when the preferential-concentration effect is negligible. Secondly, a wavelet-based method for extraction of clusters of inertial particles in turbulent flows is presented that is based on decomposing Eulerian particle number-density fields into the sum of a coherent (organized) and an incoherent (disorganized) components. The coherent component is associated with the clusters and is extracted by filtering the wavelet-transformed particle number-density field based on an energy threshold. The analysis shows that in regimes where the preferential concentration is important, the coherent component representing the clusters can be described by just 1.6% of the total number of wavelet coefficients, thereby illustrating the sparsity of the particle number-density field. On the other hand, the incoherent portion is visually structureless and much less correlated that the coherent one. An application of the method is illustrated in the form of a grid-adaptation algorithm that results in non-uniform meshes with fine and coarse elements near and away from particle clusters, respectively. In regimes where preferential concentration in clusters is important, the grid adaptation leads to a reduction of the number of control volumes by one to two orders of magnitude. Thirdly, two dynamic models for turbulent velocity fluctuations are proposed for large-eddy simulations of dispersed multiphase flows. The first model is simple, involves no significant computational overhead, contains no adjustable parameters, and is flexible enough to be deployed in any type of flow solvers and grids, including unstructured setups. The approach is based on the use of elliptic differential filters to model the subgrid-scale velocity. The only model parameter, which is related to the nominal filter width, is determined dynamically by imposing consistency constraints on the estimated subgrid energetics. The second model constructs a velocity that contains scales smaller than the coarse-grid resolution, thereby enabling the prediction of small-scale phenomena such as the preferential concentration of particles in high-strain regions. The construction of the spectrally enriched velocity field in physical space is made dynamically, and is based on 1) modeling the smallest resolved eddies of sizes comparable to the grid size via approximate deconvolution, and 2) reconstructing the subgrid-scale fluctuations via non-linear generation of small-scale turbulence. The model does not contain tunable parameters, can be deployed in non-uniform grids, and is applicable to inhomogeneous flows subject to arbitrary boundary conditions. The performance of both models is tested in large-eddy simulations of homogeneous-isotropic turbulence laden with particles, where improved agreement with direct numerical simulation results is obtained for the statistics of preferential concentration. Lastly, application to wall-modeled large-eddy simulations of particle-laden channel flow is presented. Results of the application of existing wall models to particle-laden turbulent channel flows are described, and prospective pathways for improving their performance are suggested. The focus is on the prediction of the spatial distribution statistics of the disperse phase. It is observed that wall-modeled large-eddy simulations without particular treatment for the particles in the wall-adjacent cells overpredict the near-wall accumulation of particles. The choice of the continuous representation of the velocity field between the first grid point and the wall is shown to be of primary importance. A wall-modeling strategy is explored that performs well at large Stokes numbers. It relies on using interpolation kernels near the wall that mimic the law of the wall for the wall-parallel velocity, and direct numerical simulation profiles of the fluctuations for the wall-perpendicular velocity. Applications of the two developed subgrid-scale models are shown to improve the prediction of preferential concentration, but have no effect on the mean concentration profile.

Download or read book Development of a Nonlinear Model for Subgrid Scale Turbulence and It s Applications written by Shanti Bhushan and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The present work addresses the fundamental question involving the modeling of subgrid-scale turbulence as a function of resolved field. A new-nonlinear model has been developed from the constitutive equation of subgrid stresses extending the Reynolds stress model proposed by Warsi. The time scale is expressed in terms of subgrid scale kinetic energy as opposed to strain rate tensor. Effort has been made to identify the terms appearing in the modeled subgrid stresses with "Reynolds term", "Leonard's term" and "cross term". The physical nature of these terms can be best understood from the triadic interactions in wave number space. Understanding these three terms leads to decouple the complex nature of the subgrid stresses. Modeling of these terms separately helps to capture the physics of the problem accurately. The turbulent field is assumed to be isotropic and Kolmogrov's hypothesis is used. The model coefficients are expressed as universal constants for Gaussian filter so as to satisfy the dissipation criteria in inertial subrange. Further dissipation term is assumed to be isotropic and equilibrium condition is used. Although the definition of the subgrid stress terms becomes less clear and separate for smooth filter, an attempt has been made to compare the stress terms with the exact definition obtained for sharp cut-off filter. An estimate of the backscatter of energy can be obtained from the Eddy-Damped Quasi Normal Markovian (EDQNM) theory. The model coefficients thus obtained are tested with results of plain homogeneous shear layer. The model results have been compared with the mixed-nonlinear model and Smagorinsky model. A priori test shows that new-nonlinear model has a good correlation with Smagorinsky model, which in turn has good correlation with experimental results, and has the behavior of the mixed-nonlinear model. The above model has been used for solving two-dimensional flow over backward facing step as a test case. The numerical model solves the vertically hydrostatic boundary layer equation. The top boundary is assumed to be a free surface. Terrain following coordinate system has been used. Because of the non-negativity of the subgrid scale dissipation term i.e. backscatter of energy, the nature of the solution is stochastic. The deterministic solution is obtained by clipping the dissipation term. The results are compared with the experimental data of Kim et al. Good agreement with the experimental data is obtained for the velocity profile and SGS kinetic energy. The reattachment point obtained is at 5.2h (h is the step height), which is less compared to 6h as suggested by other authors. This discrepancy may be due to the assumptions involved in the equations, which is being solved. The model is further extended for the diffusion of scalar variables and to include the buoyancy effect. It is implemented to explore the hydrostatic flow over three dimensional elliptical mountain ridges, where Boussinesq approximation is used for variable density. The flow characteristics have been studied for the various aspect ratios of the mountain and Froude's number (Nh/U) based on Brunt-Vaisala frequency (N). The phenomenon of upstream blocking and Lee-vortices generation has been studied.

Download or read book Evaluation of Simple Subgrid scale Models for the Numerical Simulation of Homogeneous Isotropic and Anisotropic Turbulence written by Khoa Dang and published by . This book was released on 1983 with total page 8 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Numerical Experiments in Homogeneous Turbulence written by Robert Sugden Rogallo and published by . This book was released on 1981 with total page 98 pages. Available in PDF, EPUB and Kindle. Book excerpt: