1,721,007 research outputs found
Turbulent entrainment in jets and plumes
No full textWe perform direct simulation of a statistically steady jets and plume and present the value of the entrainment coefficient decomposed into 1) turbulence production; 2) buoyancy effects; and 3) deviations from self-similarity. We explain theoretically how the two cases are linked and present a generalisation valid for forced and lazy plumes
Dispersion in unsteady jets and plumes
No full textWe investigate the transport of both passive and active scalars in fully developed turbulent axisymmetric jets and plumes using data from direct numerical simulation. In both cases we simulate the response of the flow to an instantaneous increase in the scalar flux at the source and our focus is on the determination of the rate at which the resulting disturbance propagates and spreads in the longitudinal direction. We apply Taylor's theory of shear-flow dispersion to free-shear flows and therefore model the way in which departures from self-similarity result in the longitudinal mixing of integral quantities. The resulting integral models exhibit a good agreement with the simulation data and, in the case of passive scalar transport, admit an analytical similarity solution. For the case of active scalar transport we examine the buoyancy flux in an unsteady plume and show that the momentum-energy framework, rather than the classical volume-momentum framework, provides the natural setting from which to view the effects of dispersion. Consequently, we demonstrate the effect that dispersion has on turbulent entrainment and the way in which a plume responds to source perturbations in its buoyancy flux
Entrainment of a turbulent patch in a stratified fluid
No full textTurbulent patches are localized events of turbulence, typically characterized by sharp differences between the flow characteristics across their interfaces. These localized events might add to the global mixing, heat exchange and mass transfer, playing a non-negligible role in the total energy balance in lakes or the ocean. This study takes a detailed look at the inner structure of a localized, mechanically forced patch in a linearly stratified ambient using laboratory experiments utilizing synchronized PIV and PLIF. The results point out that the role of the turbulent/non-turbulent interface at the edge of the patch could be significant in determining the growth rate and the maximum size of the patch
Structure of turbulence in temporal planar jets
Get PDFA detailed analysis of the structure of turbulence in a temporal planar turbulent jet is reported. Instantaneous snapshots of the flow and three-dimensional spatial correlation functions are considered. It is found that the flow is characterized by large-scale spanwise vortices whose motion is felt in the entire flow field. Superimposed to this large-scale motion, a hierarchy of turbulent structures is present. The most coherent ones take the form of quasi-streamwise vortices and high and low streamwise velocity streaks. The topology of these interacting structures is analyzed by quantitatively addressing their shape and size in the different flow regions. Such information is recognized to be relevant for a structural description of the otherwise disorganized motion in turbulent free-shear flows and can be used for the assessment of models based on coherent structure assumptions. Finally, the resulting scenario provides a phenomenological description of the elementary processes at the basis of turbulence in free-shear flows
Spatially evolving cascades in temporal planar jets
Get PDFStarting from an alternative decomposition of the turbulent field, a multi-dimensional statistical formalism for the description and understanding of turbulence in free-shear flows is proposed and applied to the symmetries of planar temporal jets. The theoretical framework is based on the exact equation for the second-order moment of the two-point velocity increment and allows us to trace, for the first time, the spatially evolving cascade processes at the basis of turbulence mixing and entrainment. Fascinating reverse energy cascade mechanisms are found to be responsible for the generation of long and wide structures in the interface region. Analogously to two-dimensional turbulence, the energy provided by these spatially ascending reverse cascades is found to be eventually dissipated by viscosity at large scales through friction shearing processes involving a thin cross-flow layer of these large-scale structures. Finally, the external non-turbulent region of the jet is also found to be active from an energetic point of view. It is found that pressure-mediated non-local phenomena of displacement of almost quiescent fluid give rise to non-turbulent fluctuations that in time, through transitional mechanisms, would contribute to the growth of the turbulent jet. Overall, the unexpected paths taken by the scale-energy flux in the combined physical/scale space, which are a substantial novelty with respect to known descriptions of turbulent mixing and entrainment, may have major repercussions on our theoretical understanding and modelling, as anticipated here by reduced equations capable of giving a simple scale-dependent description of the rich dynamics of the flow
Countergradient turbulent transport in a plume with a crossflow
Get PDFDirect numerical simulation of a turbulent forced buoyant plume in a crossflow is performed at a source Reynolds number Re0=1000\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}, Richardson number Ri0=1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}, Prandtl number Pr=1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document} and source-to-crossflow velocity ratio R0=1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}. The instantaneous and temporally averaged flow fields are assessed in detail, providing an overview of the flow dynamics. The velocity, temperature and pressure fields are used together with enstrophy fields to describe qualitatively the evolution of the plume as it is swept downstream by the crossflow, and the mechanisms involved in its evolution are outlined. The plume trajectory is determined quantitatively in a number of ways, and it is shown that the central streamline and the centre of buoyancy of the plume differ significantly-as with jets in crossflow, the central streamline is seen to follow the top of the plume, whereas the centre of buoyancy, by definition, describes the plume as a whole. We then investigate the turbulence properties inside the plume; in particular the eddy viscosity and diffusivity are presented, which are significant parameters in turbulence modelling. Assessment of turbulence production demonstrates the presence of regions where turbulence kinetic energy is redistributed to the kinetic energy of the mean flow, implying a negative eddy viscosity within certain regions of the domain. Similarly, the observation that the buoyancy flux and buoyancy gradient are anti-parallel in specific regions of the flow implies a negative eddy diffusivity in said regions, which must be realised in models of such flows in order to capture the countergradient transport of thermal properties. A characteristic eddy viscosity and diffusivity are presented, and shown to be approximately constant in the fully developed regime, resulting in a constant characteristic turbulent Prandtl number, in turn signifying self-similarity
A Turbulent Plume in Crossflow
No full textThe behaviour of a turbulent forced buoyant plume subjected to uniform crossflow is investigated utilising Direct Numerical Simulation (DNS) employing a fourth-order finite difference scheme and third-order Adams-Bashforth temporal integration. The flow features are assessed phenomenologically in the statistically steady state obtained by averaging 1,200 instantaneous 3D fields. Preliminary results on the structure of turbulent production and dissipation are shown in view of future discussions on LES modeling assumptions
Direct simulation and regularization modeling of turbulent thermal convection
No full textThis dissertation focuses on turbulent thermal convection, which occurs in a wide range of geophysical and engineering situations, such as the atmosphere, the sun, the earth's mantle, indoor climates etc. The first part of the thesis comprises a fundamental study of Rayleigh-BÉnard convection in a domain with aspect-ratio four. An ensemble of independent realizations, obtained by direct simulation, is processed with symmetry-accounted ensemble-averaging in order to distinguish the 'wind' from the fluctuations. Using this decomposition, the role of the wind and the behavior of the boundary layers is studied in detail. The second part is a study of the Leray-alpha model for wall-bounded flows. It is shown that the Leray-alpha model does not guarantee the filtered velocity field to be divergence-free in the presence of no-slip boundary conditions. Then, the Leray-alpha model is validated for three generic wall-bounded flows, i.e. plane channel flow, Rayleigh-BÉnard convection and the side-heated vertical channel. In the last part, the effects of truncation errors of finite difference approximations are included as low-pass filter operations in the Navier-Stokes equations. The resulting modified equations of the "numerical" fluid are studied to quantify the effect of discretizations on the turbulent energy spectrum.Applied Science
Efficient simulation of non-hydrostatic free-surface flow
No full textA numerical non-hydrostatic 2DV free surface flow model has been developed. The equations are based on the Navier-Stokes equations without friction, and are solved in the co-ordinate system. In the model, the number of pressure layers can be chosen independently of the number of horizontal velocity layers. This extra degree of freedom allows, at minimal computational effort, for a high vertical resolution of the horizontal velocities, which is needed for the simulation of density currents or transport problems. With splines, a continuous pressure function is constructed, using the discrete pressure in the vertical. This pressure function is used to estimate the pressure in-between the pressure layers, such as dictated by multiple velocity layers. The model is applied for simulation of short wave propagation. The behavior of a standing wave in a closed basin is simulated, as well as wave propagation over a trapezoidal bar. It is shown that the model allows for a significant reduction of computational effort, while maintaining a high resolution.Civil Engineering and Geoscience
Transition from shear-dominated to Rayleigh-Taylor turbulence
Get PDFTurbulent mixing layers in nature are often characterised by the presence of a mean shear and an unstable buoyancy gradient between two streams of different velocities. Depending on the relative strength of shear versus buoyancy, either the former or the latter may dominate the turbulence and mixing between the two streams. In this paper, we present a phenomenological theory that leads to the identification of two distinct turbulent regimes: an early regime, dominated by mean shear, and a later regime dominated by buoyancy. The main theoretical result consists of the identification of a cross-over timescale that distinguishes between the shear- and the buoyancy-dominated turbulence. This cross-over time depends on three large-scale constants of the flow, namely, the buoyancy difference, the velocity difference between the two streams and the gravitational acceleration. We validate our theory against direct numerical simulations of a temporal turbulent mixing layer compounded with an unstable stratification. We observe that the cross-over time correctly predicts the transition from shear- to buoyancy-driven turbulence, in terms of turbulent kinetic energy production, energy spectra scaling and mixing layer thickness
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