Showing papers on "Turbulence published in 1979"

Bulk Parameterization of Air-Sea Exchanges of Heat and Water Vapor Including the Molecular Constraints at the Interface

Abstract: A model is developed for the marine atmospheric surface layer including the interfacial sublayers on both sides of the air-sea interface where molecular constraints on transports are important. Flux-profile relations which are based on the postulation of intermittent renewal of the surface fluid aye matched to the logarithmic profiles and compared with both field and laboratory measurements. These relations enable numerical determination of air-sea exchanges of momentum, heat and water vapor (or bulk transfer coefficients) employing the bulk parameters of mean wind speed, temperature and humidity at a certain height in the atmospheric surface layer, and the water temperature. With increasing wind speed, the flow goes from smooth to rough and the bulk transfer coefficient for momentum also increases. The increase in roughness is associated with increasing wave height which in the present model results in sheltering at the wave troughs. Due to the decrease in turbulent transports, the transfer coef...

Evaluation of subgrid-scale models using an accurately simulated turbulent flow

Abstract: A calculation of periodic homogeneous isotropic turbulence is used to simulate the experimental decay of grid turbulence. The calculation is found to match the experiment in a number of important aspects and the computed flow field is then treated as a realization of a physical turbulent flow. From this flow, a calculation is conducted of the large eddy field and the various averages of the subgrid-scale turbulence that occur in the large eddy simulation equations. These quantities are compared with the predictions of the models that are usually applied in large eddy simulation. The results show that the terms which involve the large-scale field are accurately modeled but the subgrid-scale Reynolds stresses are only moderately well modeled. It is also possible to use the method to predict the constants of the models without reference to experiment. Attempts to find improved models have not met with success.

Small-scale structure of two-dimensional magnetohydrodynamic turbulence

Abstract: The formation of singularities in two-dimensional magnetohydrodynamic flow is investigated by direct numerical simulation. It is shown that two-dimensional magnetohydrodynamic turbulence is not as singular as three-dimensional hydrodynamic turbulence (in the sense that it has a less highly excited small-scale structure) but that it is more singular than two-dimensional hydrodynamic turbulence.

Inception of sediment transport

Abstract: Sediment transport inception curves corresponding to both laminar and turbulent flows are presented. Using these curves one can determine the critical bed shear stress (corresponding to the beginning of sediment transport) depending on the properties of the fluid and of the cohesionless bed material. The laminar flow curve has been revealed on the basis of laboratory measurements carried out by the writers. The curve corresponding to turbulent flows has been established mainly using the available data of previous researchers. It has been found that the laminar flow curve lies in the Shields' plane higher than the turbulent flow curve and thus it yields larger critical values of the dimensionless bed shear stress. Both curves tend to become indistinguishable from each other when the grain size Reynolds number decreases, i.e., when the influence of viscosity on the turbulent flow at the bed increases. For larger values of this number the turbulent flow curve can be identified with the classical Shields' curve.

Some aspects of turbulence structure through the depth of the convective boundary layer

Abstract: Results from a series of boundary layer measurements carried out at Ashchurch, Worcestershire during July 1976 are combined with those from the 1973 Minnesota experiment. This data set provides a more complete description of the behaviour of some turbulence statistics through the depth of the convective boundary layer and into the stable air of the free atmosphere. Although the two experimental regions differ quite markedly topographically, the two sets of data are found to merge together quite well in the middle of the boundary layer and do not reveal any systematic differences that might be attributable to surface effects. The vertical profiles of turbulence statistics are compared, where possible, with other results from numerical models and laboratory experiments.

Nonlinear behavior and turbulence spectra of drift waves and Rossby waves

Abstract: Spectrum cascade in drift wave turbulence in a magnetized plasma as well as Rossby wave turbulence in an atmospheric pressure system are studied based on a three‐wave decay process derivable from the model equation applicable to both cases. The decay in the three‐way interaction occurs to smaller and larger values of ‖k‖. In a region of large wavenumbers this leads to the dual cascade; the energy spectrum cascades to smaller ‖k‖ and the enstrophy spectrum to larger ‖k‖, similar to the case of two‐dimensional Navier–Stokes turbulence. In a small wavenumber region a resonant three‐wave decay process dominates the cascade process, and an anisotropic spectrum develops. As a consequence of the cascade, zonal flows in the direction perpendicular to the direction of inhomogeneity appear which presents a potential implication for the particle confinement in a turbulent plasma.

Dynamical instabilities and the transition to chaotic Taylor vortex flow

Abstract: We have used the technique of laser-Doppler velocimetry to study the transition to turbulence in a fluid contained between concentric cylinders with the inner cylinder rotating The experiment was designed to test recent proposals for the number and types of dynamical regimes exhibited by a flow before it becomes turbulent For different Reynolds numbers the radial component of the local velocity was recorded as a function of time in a computer, and the records were then Fourier-transformed to obtain velocity power spectra The first two instabilities in the flow, to time-independent Taylor vortex flow and then to time-dependent wavy vortex flow, are well known, but the present experiment provides the first quantitative information on the subsequent regimes that precede turbulent flow Beyond the onset of wavy vortex flow the velocity spectra contain a single sharp frequency component and its harmonics; the flow is strictly periodic As the Reynolds number is increased, a previously unobserved second sharp frequency component appears at R/Rc = 10·1, where Rc is the critical Reynolds number for the Taylor instability The two frequencies appear to be irrationally related; hence this is a quasi-periodic flow A chaotic element appears in the flow at R/Rc ≃ 12, where a weak broadband component is observed in addition to the sharp components; this flow can be described as weakly turbulent As R is increased further, the component that appeared at R/Rc= 10·1 disappears at R/Rc = 19·3, and the remaining sharp component disappears at R/Rc = 21·9, leaving a spectrum with only the broad component and a background continuum The observance of only two discrete frequencies and then chaotic flow is contrary to Landau's picture of an infinite sequence of instabilities, each adding a new frequency to the motion However, recent studies of nonlinear models with a few degrees of freedom show a behaviour similar in most respects to that observed

Behavior of the three fluctuating velocity components in the wall region of a turbulent channel flow

Abstract: Measurements of the three fluctuating components of the velocity and of the two components of the wall shear stress fluctuations have been made in a fully developed turbulent channel flow at Re=7700 using hot‐film probes. These measurements include rms values, skewness and flatness factors, and probability density functions. Although the wall region is emphasized here, information for the whole channel half‐width is also given.

Turbulence in the Evolving Stable Boundary Layer

Abstract: The turbulence structure observed in seven early evening runs of the 1973 Minnesota experiments is presented and discussed. Wind and temperature sensors mounted on a 32 m tower and on the tethering cable of a large balloon spanned the entire depth of the rapidly evolving nocturnal boundary layer. Spectral shapes and the vertical profiles of turbulence variances and covariances, dissipation rates for turbulent kinetic energy and temperature variance, and energy-containing range length scales show remarkable order when plotted in dimensionless coordinates, even though properties varied widely among the runs. Observed dissipation rates and boundary layer depth agree well with predictions of the Brost-Wyngaard (1978) model. It is shown that the slight (0.0014) terrain slope and possibly baroclinity affected the boundary-layer evolution, and that although the turbulence structure was probably in equilibrium with the wind and temperature fields, these were strongly evolving during the runs.

Buoyancy effects in fluids.

Abstract: Preface 1. Introduction and preliminaries 2. Linear internal waves 3. Finite amplitude motions in stably stratified fluids 4. Instability and the production of turbulence 5. Turbulent shear flows in a stratified fluid 6. Buoyant convection from isolated sources 7. Convection from heated surfaces 8. Double-diffusive convection 9. Mixing across density interfaces 10. Internal mixing processes Bibliography and author index Recent publications Subject index.

A Vortex Model of the Darrieus Turbine: An Analytical and Experimental Study

Abstract: A preliminary aerodynamic performance prediction model has been constructed for the Darrieus turbine using a vortex lattice method of analysis. A series of experiments were conducted for the express purpose of validating the analytical model. These experiments were conducted on a series of two dimensional rotor configurations which were towed in a large tank of water. The use of water as a working fluid was intended to facilitate both flow visualization and the ability to measure aerodynamic blade forces while allowing operation at sufficiently high Reynolds numbers. The primary purpose of this research was to allow reasonable predictions of aerodynamic blade forces and moments to be made.

Wakes in Stratified Fluids

Abstract: In this article we review research activities concerning wakes generated by moving bodies in stratified fluids. A wake is defined to be the non­ propagating disturbance produced by a moving body, and thus research activities concerning internal waves generated by a moving body are not included. Wakes have been of interest to many fluid dynamicists and engineers because they are a basic flow phenomenon associated with fluid flowing over an obstacle or with the movement of a natural or man-made body. The wake flow depends strongly on the Reynolds number RD == VoDjv, which is defined as the ratio of the inertia force to the viscous force. Here U 0 is the body speed, D is the characteristic length, e.g. the diameter of a cylinder or a sphere, and v is the kinematic viscosity of the fluid. For small Reynolds numbers, the viscous force dominates the inertia force and the wake is laminar. As the Reynolds number increases, the wake becomes unstable and a regular flow pattern, such as Karman's vortex street in the wake of a cylinder, can be observed. At still higher Reynolds numbers, the flow pattern becomes irregular and a turbulent flow is formed. Books written by Townsend (1956), Hinze (1959), and Schlichting ( 1960) can be referred to for further information about wakes. When the fluid is stratified thermally or with foreign additives, such as salt, a gravitational force, in addition to inertia and viscous forces, is exerted on the flow. A stratified fluid occurs very commonly in the

Detailed Study of Attached and Separated Compression Corner Flowfields in High Reynolds Number Supersonic Flow

Abstract: An experimental study has been carried out to detail the interaction of a compressible turbulent boundary layer with shock waves of varying strengths. The interaction was produced by two-dimension al compression corners of 8, 16, 20, and 24 deg angles. The incoming boundary layer had an edge Mach number of 2.85 and a Reynolds number of 1.7 million based on overall thickness. Detailed mean flow and surface measurements are presented for the four corner angles. The 8 deg corner flow was found to be fully attached, while the 16 deg case was near incipient separation. Both the 20 and 24 deg corners produced significant flow separation regions. In the discussion of these results, emphasis is placed on the development of flowfield properties from attached to separated conditions. Comparisons made with a computational solution of the Navier-Stokes equations show good agreement when the corner flow is not separated. Separated corner flows seem to require a more complex turbulence model in the computational solution.

Evidence far a k−5/3 Law Inertial Range in Mesoscale Two-Dimensional Turbulence

Abstract: The observational evidence for k−5/3 law behavior in the atmospheric kinetic energy spectrum is reviewed. This evidence includes the results of atmospheric wind variability studies and the observed scale dependence of atmospheric dispersion. It is concluded that k−5/3 law behavior for time and space scales greater than those that can be three-dimensionally isotropic is probably a manifestation of the two-dimensional reverse-cascading energy inertial range.

On the theory of unsteady high Reynolds number flow through a circular aperture

Abstract: This paper examines the theory of the unsteady motion caused by fluctuations in the driving pressure of a high Reynolds number mean flow through a circular aperture in a thin rigid plate. A theoretical model is proposed which is amenable to exact analytical treatment, and involves the shedding of vorticity from the rim of the aperture. The theory determines the dependence of the Rayleigh conductivity of the aperture on the Strouhal number, and provides quantitative estimates for the rate of dissipation of large scale ordered structures as a result of the generation of turbulence at the apertures in a perforated liner. The limit of zero Strouhal number yields a description of steady high Reynolds number flow, the contraction ratio of the emerging jet being predicted to be equal to the minimum theoretical value of ½. Application is made to the problem of sound trans­mission through a uniformly perforated screen in the presence of a low Mach number bias flow.

Relaminarization of fluid flows

Abstract: The mechanisms of the relaminarization of turbulent flows are investigated with a view to establishing any general principles that might govern them. Three basic archetypes of reverting flows are considered: the dissipative type, the absorptive type, and the Richardson type exemplified by a turbulent boundary layer subjected to severe acceleration. A number of other different reverting flows are then considered in the light of the analysis of these archetypes, including radial Poiseuille flow, convex boundary layers, flows reverting by rotation, injection, and suction, as well as heated horizontal and vertical gas flows. Magnetohydrodynamic duct flows are also examined. Applications of flow reversion for turbulence control are discussed.

Mixing in stratified fluids

Abstract: Laboratory experiments on mixing in stratified fluids are examined in the light of some simple energy arguments applied to an analysis of mixing at density interfaces. It is shown that as the stability of the flow, as measured by an overall Richardson number Rio , increases, the velocity interface becomes considerably thicker than the density interface. This mismatch in interface thickness provides the extra kinetic energy required for mixing in these strongly stratified flows. The behaviour of the flux Richardson number Rf as a function of Rio is also discussed. It is found that the results from a number of different experiments are quite similar, and that Rf increases from zero as Rio does, reaches a maximum value (0.2 & plusmn; 0.05) and then decreases again with further increase in Rio . Finally, some recent ideas of Posmentier (1977) on the formation of interfaces in a stratified, turbulent flow are compared with observations.

The application of turbulence theory to the formulation of subgrid modelling procedures

Abstract: The problem of subgrid modelling, that is, of representing energy transfers from large to small eddies in terms of the large eddies only, must arise in any large eddy simulation, whether the equations of motion are open or direct (unaveraged) or closed (averaged). Models for closed calculations are derived from classical closures, and these are used to determine the effect of filter shape, grid-scale spectrum and grid-scale anisotropy on the effective eddy viscosity: the Leonard or resolvable-scale stress is calculated separately and is found to account for 14% of the total drain in a typical high Reynolds number case.The validity of using these eddy viscosities in an open calculation is considered. It is concluded that this is not unreasonable, but that the simulation would be much improved if the gross drain could be separated into net drain and backscatter.

Theory of premixed-flame propagation in large-scale turbulence

Abstract: A statistical theory is developed for the structure and propagation velocity of premixed flames in turbulent flows with scales large compared with the laminar flame thickness. The analysis, free of usual closure assumptions, involves a regular perturbation for small values of the ratio of laminar flame thickness to turbulence scale, termed the scale ratio e, and a singular perturbation for large values of the non-dimensional activation temperature β. Any effects of the flame on the flow are considered to be given. In this initial study, molecular coefficients for diffusion of heat and reactants are set equal. The results identify convective-diffusive and reactive-diffusive zones in the flame and predict thickening of the flame by turbulence through streamwise displacement of the reactive-diffusive zone. Profiles for intensities of temperature fluctuations and for streamwise turbulent transport are obtained. A fundamental quantity occurring in the analysis is the longitudinal displacement of the reactive-diffusive zone in an Eulerian frame by turbulent fluctuations, and to first order in the scale ratio this equals the longitudinal displacement of fluid elements in an Eulerian frame by turbulent fluctuations, herein termed simply the Eulerian displacement. To first order in the scale ratio it is found that, if the Eulerian displacement experiences the same type of statistical non-stationarity as the corresponding Lagrangian displacement, then the diffusion approximation is valid for streamwise turbulent transport but the turbulent flame thickens as time increases, while if the Eulerian displacement is statistically stationary then the diffusion approximation necessitates a negative coefficient of diffusion in part of the flame but the flame thickness remains constant. By carrying the analysis to second order in the scale ratio it is shown that the turbulent-flame speed exceeds the laminar-flame speed by an amount proportional to the mean square of the transverse gradient of the Eulerian displacement. This result can be understood from the mechanistic viewpoint of a wrinkled laminar flame in terms of the increase in flame area produced by turbulence. Thus the theory provides a precise statistical quantification of the model of the wrinkled laminar flame for describing structures of turbulent flames.

Characteristics of Tornado-Like Vortices as a Function of Swirl Ratio: A Laboratory Investigation

Abstract: The investigation of tornado vortex dynamics by means of a laboratory simulation is described. Based on observations from nature and an examination of the Navier-Stokes equations, a laboratory simulator of the Ward type has been constructed. This simulator generates various vortex configurations as a function of swirl ratio, radial Reynolds number and aspect ratio. Configurations which are described are 1) a single laminar vortex; 2) a single vortex with breakdown bubble separating the upper turbulent region from the lower laminar region; 3) a fully developed turbulent core, where the breakdown bubble penetrates to the bottom of the experimental chamber; 4) vortex transition to two intertwined helical vortices; and 5) examples of higher order multiple-vortex configurations that form in the core region. Hot-film anemometry measurements of the magnitude of the velocity vector and inflow (swirl) angle have been obtained in a sequence of flows characterized by progressively increasing values of swirl...

Energy Effectiveness of Arbitrary Arrays of Wind Turbines

Abstract: Determination of power degradation due to interference between wind turbines in an array is of importance in the engineering and economic planning of wind farms. A computer model for an arbitrary array of turbines is described. The basic fluid mechanics are treated in a simple but rational way. The turbine wake expands downstream due to ambient turbulence and mechanically generated turbulence (caused by momentum gradients) and entrains momentum and mass. Drag or momentum deficit, though, is conserved. Ground effect is handled by imaging. The effect of ambient turbulence is shown to be much greater than of that due to the momentum deficit generated by the turbine. The basic equations use fundamental fluid mechanical expressions related to drag conservation and wake growth due to turbulent entrainment and a family of self-similar wake profiles derived from experiment. This approach fully defines the wake velocity field. The wake of each turbine is then determined, subject to all upstream interferences. Power outputs of selected arrays as functions of wind direction are presented. The model is very well supported by the limited data available, and has proven effective and easy to implement. Advanced models incorporating nonuniformities in wind and turbulence and tower shadow are also described.

Calculation of strongly curved open channel flow

Abstract: The paper describes the application of a finite-difference calculation procedure to the problem of simulating the three-dimensional, turbulent flow in a strongly curved, open, 180° bend with straight inlet and outlet reaches. The configuration can be considered to represent an element of a model meander, and the work presented here forms an important stage in efforts to simulate the flow in successive reverse-curvature bends. No restrictions other than the absence of flow separation and hydraulic jumps are imposed. Full account is taken of non-linear fluid-inertia and of turbulent diffusion terms. Effects of turbulence are represented by an eddy viscosity related to two parameters— the turbulent kinetic energy k and its rate of dissipation ϵ\N— for which related differential transport equations are solved. Predictions are presented for the transverse surface slope and velocity field in a configuration experimentally examined by Rozovskii. Agreement between predictions and experimental data is judged to be satisfactory on all major flow phenomena.

Turbulent flow in a depth-limited boundary layer

Abstract: Turbulent flow was investigated at a constant flow Reynolds number in a depth-limited boundary layer. Relative roughness was 0.12. Roughness density (ratio of plan area of elements to total bed area) was varied to cover the range of flow types described by Morris [1955]. The effect of roughness density on velocity profile shape factors, energy dissipation, and streamwise kinetic energy confirmed the existence of three types of roughness density-flow interaction, namely, ‘skimming,’ ‘wake interaction,’ and ‘isolated roughness’ flows. The velocity profiles and kinetic energy measures identified three layers in the vertical profile: an outer layer (y/D > 0.35), a wake layer (0.35 > y/D > 0.10), and an inner region (y/D < 0.10). The wake layer is produced by strong turbulent action accompanying vortex shedding at intermediate roughness densities. Spectral measurements and calculations of the turbulent macroscale indicated that the spectral shape is affected by the free surface but that below y/D = 0.5 a parametric model using the turbulent Reynolds number in the spectral energy equation adequately describes the spectral shape for all roughness densities. Implications of these results are discussed for the study of shallow, natural flows with high relative roughness.