Turbulent Transport Phenomena Research Articles

Contribution of direct numerical simulation to understanding and modelling turbulent transport

With the advances in large scale computers, reliable numerical methods and efficient post-processing environment, direct numerical simulation (DNS) has become a valuable and indispensable resource for fundamental turbulence research, although DNS is possible only when the turbulent Reynolds (or Peclet) number remains small to moderate. This paper reviews the contribution that various DNSS have made to understanding and modelling turbulent transport phenomena. After general remarks are made on the grid requirements and numerical methods of DNS, its novelties as a numerical experiment are summarized and some of them are demonstrated by introducing typical DNS results at the University of Tokyo. Emphasis is laid upon new findings on the turbulence statistics, their budgets and quasi-coherent eddy structures revealed by the simulations of the fully developed channel flow with heat transport at different Prandtl numbers, and also a recent modelling attempt to take into account the new knowledge extracted from these DNSS, i. e. a remarkable change of the destruction mechanism of turbulent scalar flux with the Prandtl number and a low Reynolds number effect on the redistribution process of the Reynolds stress.

Turbulent transport in flames

A review is presented of turbulent transport phenomena in flames. Both large-scale turbulent transport and small-scale mixing processes are considered and various mechanisms of interaction between combustion and turbulent flow are identified. Flame-surface density descriptions of turbulent combustion at high Damköhler numbers are discussed in detail and some topics are identified which require further attention. Emphasis is placed on problems of premixed turbulent combustion.

Turbulent transport phenomena in a channel with periodic rib turbulators

Periodic fully developed turbulent flow in a 2D channel with rib turbulators on two opposite walls has been studied numerically and experimentally. In numerical predictions, an algebraic Reynolds stress turbulence model is adopted, and a smoothed hybrid central/skew upstream difference scheme is developed. In experiments, the laser-Doppler velocimetry and laser holographic interferometry are employed to measure the local flow and heat transfer characteristics. The results are obtained with the ratio of pitch to rib height 5, 10, 15, and 20, for Reynolds number of 3.3 x 10 exp 4 and are presented in terms of the reattachment length, mean velocity and turbulent kinetic energy profiles, isotherm patterns, and distributions of local pressure recovery and Nusselt number. A detailed comparison with experimental data shows that the present calculations have an improvement over the previous work in the prediction of periodic ribbed-wall flow and heat transfer. In addition, regions susceptible to hot spots are identified by examining the distributions of the local Nusselt number. Furthermore, the enhancement of mean Nusselt number is documented in terms of relative contributions of the increased turbulence intensity and surface area provided by the ribs. 32 refs.

Turbulent Transport Phenomena across the Stable Thermal Stratified Layer Formed in a Circular Pipe.

The behavior of a thermally stratified mixing layer formed in a circular pipe has been experimentally investigated in order to estimate the thermal stress of the horizontal pipe wall caused by the emergency cooling of power plants. Two component velocities and the temperature of fluid are simultaneously measured by fiber LDVs and a hot film thermometer. Experiments were performed at the bulk Reynolds number of 7350 where the overall Richardson numbers ranging up to 2.46. Flow visualization at the cross section of the pipe and measurements of the turbulence characteristics in the fields of velocity and temperature showed the difference of flow structure between isothermal and thermally stratified conditions. Upper and lower layers mixed well in the isothermal condition, while in the thermally stratified conditions, there obviously existed a thermally stratified mixing layer where turbulent mixing was suppressed by the buoyancy effect. This thermally stratified mixing layer spread in the horizontal direction with increasing Ri number and generated secondary flow between the wall and this mixing layer.

Well-Ordered Motions and Relevant Heat Transfer in Wall Turbulence.

In wall turbulence, the well-ordered motions determine the essential features of turbulent transport phenomena. In the present study, we have refined a previously developed detection algorithm for well-ordered motions (i.e., trajectory analysis method based on the (u, v)-quadrant splitting technique) and have investigated the relationship between well-ordered motions and heat transfer from structural points of view, i.e., trajectory analysis of the VITA heat transfer events, extraction of basic flow modules and the relevant heat transport, and the simulation of temperature fluctuations by means of an autoregressive (AR) model. As a result, it is shown that phase relationship of fluctuating velocity components dominates the essential characteristics of the transport processes of heat and momentum in wall turbulence and there exist the distinct differences in individual correspondence between well-ordered motions and heat transport processes which cannot be revealed by the widely used VITA technique.

Atmospheric Motions and Turbulent Transport Phenomena under Stratified Conditions

Atmospheric Motions and Turbulent Transport Phenomena under Stratified Conditions

Turbulent transport phenomena in three-dimensional side-dump ramjet combustors

Numerical analysis has been performed for exploring transport phenomena of three-dimensional (3-D) turbulent flows with or without chemical reaction in ramjet combustors. The turbulence effects on flow property transport for 2-D and 3-D isothermal flow fields are simulated by two two-equation turbulence and the variant turbulence models (i.e. k- ε and k- kl), and results are then compared with the LDV measured data. It is found that the 3-D results predicted by the k- ε turbulence model are both qualitatively and quantitatively consistent with the experimental data. In addition, the turbulent flow with chemical reaction, assumed to be single-phase, diffusion-controlled combustion with negligible radiation heat transfer, is studied. The theoretical results of this reacting flow such as the velocity, temperature fields and surface stream function will be valuable for the hydraulic and thermal design of 3-D side-dump ramjet combustors.

Momentum transport in a turbulent mixing layer

AbstractThe turbulent momentum transport phenomena in a two‐dimensional mixing layer are investigated numerically by a discrete vortex method. The numerical model and calculations are verified through a comparison with existing numerical simulations and experimental measurements. The main emphasis is placed on the exploration of the detailed time‐dependent instantaneous local momentum fluctuations and on the comparison of numerical results with available experimental measurements. The current simulations confirm qualitatively the various trends in the turbulent momentum flux and fluctuating components of the velocity in the mixing layer found with several experimental results. The study shows that similarity exists in turbulent momentum quantities along the axial direction of the mixing layer. The calculations also show a definite correlation between the passage of a large‐scale structure and a burst in the turbulent momentum flux. The probability density functions of the fluctuating quantities are shown to be mostly Gaussian‐like, with only a few exceptions.

Turbulent characteristics of thermal stratified flow with increasing interfacial level in a cylindrical vessel.

Water tests in a cylindrical vessel have been performed to investigate the turbulent transport phenomena in the mixing shear layer which moves upwardly and affects the structural integrity of the vessel and internal structures during a transient condition of a fast breeder reactor. The turbulent characteristics of the coolant adjacent to a thermally stable stratified interface have been clarified by using the technique combined with 3 beam LDV and thermocouple probe. The correlations among two-component volocities and temperature are obtained simultaneously. From the experimental results, it was found that the maximum temperature gradient can be realized in the almost laminar-like condition in which the turbulent heat fluxes in the upper and lower layers are both damped.

The parameterization of regional evaporation — Some directions and strategies

The success of currently available methods, for predicting runoff and other facets of watershed hydrology, depends critically on a knowledge of the water “losses” through evaporation. Also, general circulation models in climate dynamics appear to be quite sensitive to the statement of land surface processes in general, and of evaporation in particular. Several approaches dealing with this problem have resulted from advances in the description of turbulent transport phenomena in the atmospheric boundary layer. Issues under investigation are the dissimilarity between sensible and latent heat transport, the simple slab concept for budget calculations and the development of bulk transfer equations. A number of large-scale experiments, some just completed, others about to start or in the planning stage will improve present understanding and parameterization techniques for surface fluxes of water vapor and related land surface processes.

Numerical analysis of turbulent plane Couette flow at low Reynolds number by Large Eddy Simulation.

Large Eddy Simulation, which is one of the most promising approaches to the analysis of turbulent transport phenomena, is applied to turbulence plane Couette flow. The number of computational grid points is 30×30×25, and the Reynolds number, based on the wall speed and the wall distance, is 1.5×104. Mean velocity profiles and turbulence statistics are calculated and compared with available experimental results. Furthermore, the effects of the Smagorinsky constant on the micro-structures of trurbulence and spanwise spacings of the streaks are discussed.

Water Vapor Contribution to the Erosion of Steel by High Temperature Flows

Experimental results on the role of water vapor in the mass loss mechanism of steel by hot, high pressure flows are described and interpreted in terms of a surface reaction model. Situations in which highly transient heat transfer and chemical interactions at the gas-metal interface produce erosion of the surface occur in a variety of systems, e.g., gun barrels, gas turbines, vents, nozzles, and furnaces. This research is directed at conditions in which the metal surface temperature remains below the melting point and mass loss is the result of chemical attack. The rate of reaction depends on the unsteady pressure and surface temperature. The study reveals that in high shearing flows, when the build-up of oxide protective layer is limited, surface chemical attack of the steel by water vapor can be the main erosion source in combustion gas atmospheres, and the process is controlled mainly by turbulent transport phenomena.

Predictions in Environmental Hydrodynamics Using the Finite Element Method I. Theoretical Development

Differential equation descriptions for specific three-dimensional environmental hydrodynamical flowfields are established. The parabolic Navier-Stokes system is applicable to predominant direction steady flowfields. An integral transform formulation of the transient Navier-Stokes system is applicable to recirculating flowfields in lakes, rivers, and tide water regions. A finite element numerical solution algorithm is established for each of these developed systems including complete nonlinearity and tensor turbulent transport phenomena. The algorithm directly utilizes nonuniform computational meshes and nonregular solution domain closures, upon which boundary condition constraints may be readily applied.

Single phase transport within bare rod arrays at laminar, transition and turbulent flow conditions

A theoretical analysis has been performed to study molecular and turbulent transport phenomena between subchannels of infinite bare rod arrays at laminar, transition and turbulent flow conditions. For this investigation, the theoretical approach of Ramm and Johannsen for predicting turbulent momentum and heat transfer in rod bundles has been extended to evaluate three-dimensional temperature fields. Results are presented enabling the prediction of the onset and growth of laminarization in typical subchannels of square and triangular rod arrays. These results are further applied to interpret the characteristic effects of variations in Reynolds number, Prandtl number or geometric spacing on integral exchange parameters as the thermal mixing flow rate and mixing length scale. These results are of particular significance relative to the explanation of recent data from tracer-type mixing experiments and also exhibit the importance of secondary flow effects on turbulent intersubchannel energy transport. In view of these findings, the physical relevance of current correlations derived from integral-type experiments to numerically predict exchange coefficients for use in lumped parameter subchannel analysis codes is discussed.

NSF summer institute

An NSF Summer Institute in Geophysical Fluid Mechanics for teachers in engineering and science will be held, June 23–August 15, 1969, at Colorado State University. Courses may be taken for graduate credit. Interdisciplinary courses will be offered in the mechanics of geophysical fluids, turbulent transport phenomena, meteorology of fluid mechanics, Fourier analysis of geophysical data, and numerical analysis of geophysical flows.