Showing papers on "Turbulence published in 1985"

Computational methods for fluid flow

Abstract: Keywords: methodes : numeriques ; programmation ; differences : finies ; methode : integrale ; ecoulement : incompressible ; ecoulement : compressible ; ecoulement : visqueux ; mecanique des : fluides ; viscosite Reference Record created on 2005-11-18, modified on 2016-08-08

Features of a reattaching turbulent shear layer in divergent channel flow

Abstract: Experimental data have been obtained in an incompressible turbulent flow over a rearward-facing step in a diverging channel flow. Mean velocities, Reynolds stresses, and triple products that were measured by a laser Doppler velocimeter are presented for two cases of tunnel wall divergence. Eddy viscosities, production, convection, turbulent diffusion, and dissipation (balance of kinetic energy equation) terms are extracted from the data. These data are compared with various eddy-viscosity turbulence models. Numerical calculations incorporating the k-epsilon and algebraic-stress turbulence models are compared with the data. When determining quantities of engineering interest, the modified algebraic-stress model (ASM) is a significant improvement over the unmodified ASM and the unmodified k-epsilon model; however, like the others, it dramatically overpredicts the experimentally determined dissipation rate.

Higher-order derivative correlations and the alignment of small-scale structures in isotropic numerical turbulence

Abstract: In a three dimensional simulation higher order derivative correlations, including skewness and flatness factors, are calculated for velocity and passive scalar fields and are compared with structures in the flow. The equations are forced to maintain steady state turbulence and collect statistics. It is found that the scalar derivative flatness increases much faster with Reynolds number than the velocity derivative flatness, and the velocity and mixed derivative skewness do not increase with Reynolds number. Separate exponents are found for the various fourth order velocity derivative correlations, with the vorticity flatness exponent the largest. Three dimensional graphics show strong alignment between the vorticity, rate of strain, and scalar-gradient fields. The vorticity is concentrated in tubes with the scalar gradient and the largest principal rate of strain aligned perpendicular to the tubes. Velocity spectra, in Kolmogorov variables, collapse to a single curve and a short minus 5/3 spectral regime is observed.

A Statistically-Derived Subgrid-Scale Kinetic Energy Model for the Large-Eddy Simulation of Turbulent Flows

Abstract: A subgrid-scale (SGS) kinetic energy model for the large-eddy simulation (LES) of turbulent flows is constructed with the aid of the statistical results obtained from the two-scale direct-interaction approximation. In this model, the SGS Reynolds stress is written in terms of a generalized SGS eddy-viscosity representation which is expressed by using the SGS kinetic energy and the characteristic grid width. A model equation for the SGS kinetic energy is combined with the grid-scale Navier-Stokes and continuity equations to lead to an LES model of one-equation type. The usual Smagorinsky model is derived as its special case, and critical comparison is made between them.

The derivation and numerical solution of the equations for zero mach number combustion

Abstract: We present a limiting system of equations to describe combustion processes at low Mach number in either confined or unbounded regions and numerically solve these equations for the case of a flame propagating in a closed vessel. This system allows for large heat release, substantial temperature and density variations, and substantial interaction with the hydrodynamic flow field, including the effects of turbulence. This limiting system is much simpler than the complete system of equations of compressible reacting gas flow since the detailed effects of acoustic waves have been removed. Using a combination of random vortex techniques and flame propagation algorithms specially designed for turbulent combustion, we describe a numerical method to solve these zero Mach number equations. We use this method to analyze the competing effects of viscosity, exothermicity, boundary conditions and pressure on the rate of combustion for a flame propagating in a swirling flow inside a square.

Open Channel Hydraulics

Abstract: Concepts of fluid flow energy principle the momentum principle development of uniform flow concepts computation of uniform flow theory and analysis of gradually and spatially varied flow design of channels flow measurement rapidly varied flow in nonprismatic channels turbulent diffusion and dispersion in steady open-channel flow turbulent, buoyant, surface jets and associated phenomena gradually varied, unsteady flow rapidly varied, unsteady flow hydraulic models.

Measurement of turbulent energy dissipation rates in the middle atmosphere by radar techniques: A review

Abstract: Radars operating in the frequency band between 2 MHz and several hundred megahertz are capable of supplying a large data base of measurements of turbulent energy dissipation rates in the middle atmosphere. So far this has not been achieved; only occasionally have such radars been used to produce estimates of turbulence intensities. In order to encourage a greater emphasis on this aspect of radar studies of the middle atmosphere, this review summarizes the various techniques which can be used to measure turbulent energy dissipation rates. It is shown how absolute measurements of backscatter cross section can be used to measure turbulence intensities. A new theory is presented which shows that the power backscattered from the mesosphere depends on the turbulent energy dissipation rate, the electron density gradient, the neutral density scale height, the total electron density and the temperature gradient. The effects of turbulence on the width of signal spectra received by these radars are discussed, and it is shown how turbulence intensities may be extracted from spectral width measurements. The importance of removing nonturbulent processes which also broaden the width of the power spectra, such as wind shear broadening and beam width broadening, are stressed.

Galilean invariance of subgrid-scale stress models in the large-eddy simulation of turbulence

Abstract: The modelling of the subgrid-scale stresses in the large-eddy simulation of turbulence is examined from a theoretical standpoint. While there are a variety of approaches that have been proposed, it is demonstrated that one of the more recent models gives rise to equations of motion for the large eddies of turbulence which are not Galilean-invariant. Consequently, this model cannot be of any general applicability, since it is inconsistent with the basic physics of the problem, which requires that the description of the turbulence be the same in all inertial frames of reference. Alternative models that have been proposed which are properly invariant are discussed and compared.

Structure of large-scale vortices and unsteady reverse flow in the reattaching zone of a turbulent separation bubble

Abstract: This paper describes experiments concerning the structure of large-scale vortices and the unsteady reverse-flow properties in the reattaching zone of a nominally two-dimensional separation bubble formed at the leading edge of a blunt flat plate with right-angled corners. The experiment was performed in a wind tunnel with a constant Reynolds number 2.6 × 104 (based on the main-flow velocity and the thickness of the plate). Split-film probes, being sensitive to instantaneous reversals of flow direction, were extensively employed. An important feature of this study is a judicious use of surface-pressure fluctuations as a conditioning signal to educe the structure of the large-scale vortices.Distributions of fluctuating-velocity vectors and contour lines of high-frequency turbulent energy in a few space–time domains are presented and discussed. The most economical interpretation of these space-time distributions is that the large-scale vortices in the reattaching zone are hairpin vortices whose configuration is sketched in the text. The unsteady flow in the reattaching zone is mainly governed by two agents; the motion of the large-scale vortices and the low-frequency unsteadiness. The unsteady flow is clarified in terms of the motion (in a space–time domain) of zeros of the longitudinal velocity close to the surface of the plate; the effects of the two agents on this motion are presented separately. On the basis of these results, a mathematical model of the unsteady flow in the reattaching zone is suggested and found to yield good comparison with measured reverse-flow intermittency and frequency of local-flow reversals. It appears that the separation bubble experiences shrinkage and enlargement in connection with the low-frequency unsteadiness and that the speed of shrinkage is much greater than that of enlargement. The strength of the large-scale vortices in the reattaching zone seems to be dependent on the phase of the low-frequency unsteadiness.

Measurements of entrainment and mixing in turbulent jets

Abstract: An experimental investigation of entrainment and mixing in the self-similar far field of an axisymmetric free turbulent jet in water is presented. Length and time scales for the flame length fluctuations of reacting jets are shown to be approximately equal to the local characteristic large scale length and time of the flow. It is also shown that instantaneous radial profiles of concentration across the jet do not resemble the mean concentration profile, indicating that the mean profile is a poor representation of the mixed fluid states within the jet. These instantaneous profiles also show that unmixed ambient fluid is transported throughout the entire extent of the jet, and that the mixed fluid composition within the jet can be fairly uniform in regions extending across a large part of the local jet diameter. Lastly, the amount of unmixed ambient fluid on the jet centerline is found to vary roughly periodically with a period approximately equal to the local characteristic large scale time of the flow. These results suggest that large scale transport mechanisms, displaying a characteristic organization, play an important role in entrainment and mixing in the far filed of turbulent jets.

The electron‐acoustic mode

Abstract: This paper examines electrostatic modes in an unmagnetized, homogeneous, Vlasov plasma with three Maxwellian components: ions, hot electrons, and cool electrons. In such a plasma, the electron‐acoustic mode with frequencies between the ion and electron plasma frequencies may propagate with light damping. The conditions that allow propagation of this mode, which is distinct from the well‐known ion‐acoustic and Langmuir waves, are given in detail; approximate necessary conditions are 10≲Th/Tc and 0

The Taupo eruption, New Zealand. II. The Taupo Ignimbrite

Abstract: The ca. 30 km 3 Taupo ignimbrite was erupted as a climax to the ca. AD 186 Taupo eruption in the central North Island of New Zealand. It was erupted as a single vent-generated flow unit over a time period of ca. 400 s and was emplaced very rapidly (locally at more than 250-300 m s -1 ) and violently. The parent flow reached 8 0 + 10 km from source in all directions, crossed all but one of the mountains within its range and only stopped when it ran out of material. The ignimbrite is divisible into layers 1 and 2, and a distant facies which combines features of both layers. Layer 1 was generated as a result of strong fluidization in the flow head, caused by air ingestion, and consists of two main facies. Layer 1(P) is a pumiceous, mildly to strongly fines-depleted unit, generated by the expulsion of material from the flow front, and termed the jetted deposits. The overlying layer 1 (H) is a thinner, crystal- and lithic-rich, fines-depleted unit, generated by the sedimentation of coarse/dense constituents segregated out by strong fluidization within the flow head and termed the ground layer. Layer 2 consists of two facies with similar compositions but contrasting morphologies; during emplacement, material left behind by the flow body partially drained into depressions to form the valley-ponded ignimbrite, leaving the veneer deposit as a thin, landscape mantling layer on interfluves. The distant facies occurs in some outermost hilly areas of the ignimbrite where the flow velocity remained high but its volume had shrunk through deposition so that air ingestion fluidization affected the whole flow. The ignimbrite shows great lateral variations. Each facies, or variants therein, exhibits systematic degrees of development with varying distances from vent. Near vent, the flow consisted of a series of batches of material which by ca. 25 km had coalesced into a single wavy flow and by ca. 40 km into a single wave. Out to ca. 13 km, the flow was rather dilute and highly turbulent as it deflated from the collapsing eruption column. Beyond this distance it was fairly concentrated, being less than 100% expanded over its non-fluidized compacted state, and had acquired a fluidization-induced stable density stratification, which strongly suppressed turbulence in the flow body. Deflation from the eruption column was largely complete by ca. 13 km but influenced the flow as far as 20-25 km from vent. Grainsize and compositional parameters measured in the ignimbrite show lateral variations which equal or exceed the entire spectrum of published ignimbrite data. The flow had deflated and coalesced from the eruption column by ca. 20 km from vent. Beyond this distance most lateral variations are modelled by considering the flow to be a giant fluidized bed. As the flow moved, material was deposited from its base, and hence predictable vertical variations in the model fluidized bed are comparable with lateral variations in the ignimbrite. The agreement is excellent, and, in particular, discontinuities in the nature of the ignimbrite at 55-60 km from vent suggest that the more distal ignimbrite represents a vast segregation layer generated above the moving flow. Differences between the model and variations of some parameters reflect the influence of kinetic processes, such as shearing and local fluidization, that operated regardless of the bulk flow composition. The strong fluidization in the flow is a result of the high flow velocities (promoting air ingestion), not vice versa as is often accepted. Contrasts in the natures of layers 1 and 2 imply that the first material erupted contained significantly coarser, and a higher content of, lithics than the bulk of the flow. During emplacement, this earlier material was depleted by deposition and diluted by material introduced from the flow body. Systematic regional variations also occur in the ignimbrite: for example, it contains lower crystal: lithic ratios and higher density pumice in a northeasterly sector, and vice versa to the southwest. Ignimbrite found in mountainous areas shows changes consistent with its derivation from the upper, more mobile and pumiceous top of the flow. Fluidization processes generated structures and facies in the ignimbrite on various scales. Individual segregation bodies found at any exposure show features mimicking those of the ground layer, i.e. fines depletion and crystal- and lithic-enrichment. Fluidization-induced grading visible at individual exposures accounts for the great range of grading styles seen in the valley-ponded ignimbrite, and strong fluidization has locally generated an upper fines- and pumice- rich segregation layer (here termed layer 2c). On the largest scale, fluidization was primarily responsible for the generation of the layer 1 deposits, and for the grainsize and compositional zonation within the flow that produced the lateral variations in the ignimbrite. Ingested and heated air is inferred to have been the most important gas source for fluidization within the flow, although several other gas sources were locally dominant. It is clear that the thickness, grainsize and composition of the ignimbrite at any point are not simply related to values of these parameters in either the originally erupted material or the parent flow, and that, except for its density, the dimensions and composition of the parent flow cannot be directly inferred from the ignimbrite.

Aerodynamics of Sports Balls

Abstract: Research data on the aerodynamic behavior of baseballs and cricket and golf balls are summarized. Cricket balls and baseballs are roughly the same size and mass but have different stitch patterns. Both are thrown to follow paths that avoid a batter's swing, paths that can curve if aerodynamic forces on the balls' surfaces are asymmetric. Smoke tracer wind tunnel tests and pressure taps have revealed that the unbalanced side forces are induced by tripping the boundary layer on the seam side and producing turbulence. More particularly, the greater pressures are perpendicular to the seam plane and only appear when the balls travel at velocities high enough so that the roughness length matches the seam heigh. The side forces, once tripped, will increase with spin velocity up to a cut-off point. The enhanced lift coefficient is produced by the Magnus effect. The more complex stitching on a baseball permits greater variations in the flight path curve and, in the case of a knuckleball, the unsteady flow effects. For golf balls, the dimples trip the boundary layer and the high spin rate produces a lift coefficient maximum of 0.5, compared to a baseball's maximum of 0.3. Thus, a golf ball travels far enough for gravitational forces to become important.

Effect of rotation on isotropic turbulence - Computation and modelling

Abstract: This paper uses numerical simulation to analyse the effects of uniform rotation on homogeneous turbulence. Both large-eddy and full simulations were made. The results indicate that the predominant effect of rotation is to decrease the rate of dissipation of the turbulence and increase the lengthscales, especially those along the axis of rotation. These effects are a consequence of the reduction, due to the generation of inertial waves, of the net energy transfer from large eddies to small ones. Experiments are also influenced by a more complicated interaction between the rotation and the wakes of the turbulence-generating grid which modifies the nominal initial conditions in the experiment. The latter effect is accounted for in simulations by modifying the initial conditions. Finally, a two-equation model is proposed that accounts for the effects of rotation and is able to reproduce the experimental decay of the turbulent kinetic energy.

Application of the E -ε turbulence model to the atmospheric boundary layer

Abstract: In the so called E - e turbulence model, an eddy viscosity is evaluated from turbulent kinetic energy E and energy dissipation e. Although still a first-order closure method in its simpler form, the E- e model yields eddy viscosity for complex turbulent flows without a prior prescription of a length scale needed in so-called mixing-length models. The E - e model has been successfully applied to many flow problems in engineering applications for non-rotating boundary layers. In this paper, the E - e method is extended to the atmospheric boundary layer for which a modification of the dissipation equation is found to be necessary in order to give results comparable with observational data.

The structure of the vorticity field in turbulent channel flow. I - Analysis of instantaneous fields and statistical correlations

Abstract: An investigation into the existence of hairpin vortices in turbulent channel flow is conducted using a database generated by the large-eddy simulation technique. It is shown that away from the wall the distribution of the inclination angle of vorticity vector gains its maximum at about 45° to the wall. Two-point correlations of velocity and vorticity fluctuations strongly support a flow model consisting of vortical structures inclined at 45° to the wall. The instantaneous vorticity vectors plotted in planes inclined at 45° show that the flow contains an appreciable number of hairpins. Vortex lines are used to display the three-dimensional structure of hairpins, which are shown to be generated from deformation (or roll-up) of sheets of transverse vorticity.

Sediment Transport Capacity of Overland Flow

Abstract: OVERLAND flow runoff can be either laminar or turbulent depending on the Reynolds number. The rate of soil erosion may be limited by the sediment transport capacity which depends on the type of flow. Sediment transport equations based on velocity were found to give different results than those based on shear stress. Most of the sediment transport equations developed for turbulent streams should not be applied to soil erosion by overland flow. A general relationship supported by dimensional analysis was derived. The recommended sediment transport capacity relationship can be written as a power function of slope and discharge and the range of exponents was defined from empirical relationships.

Further results from a laboratory model of the convective planetary boundary layer

Abstract: The turbulence in a laboratory convective mixed layer is probed more extensively than in the preliminary study of Willis and Deardorff (1974), and results presented. Turbulence intensities, spectra and probability distributions using mixed-layer scaling compare favorably with similarly scaled field measurements not available or plentiful in 1974. However, the velocity spectra in the convection tank exhibit only a short inertial subrange due to the close proximity of the dissipation subrange to the energy-containing range. The turbulence budget suggests that the convergence of the vertical transport of pressure fluctuations is a rather important term. Results on the entrainment rate are also presented, using both mixed-layer scaling and local interfacial scaling.

Lewis number effects on turbulent burning velocity

Abstract: Experimental values of turbulent burning velocities for propane, hydrogen and iso-octane mixtures with air are reported under conditions of high turbulence and high turbulent Reynolds number. The measurements were made by the double kernel method during explosions in a fan-stirred bomb, with four fans, capable of speeds of up to 10,000 rpm. The ratio of turbulent to laminar burning velocity, u t /u l , is correlated primarily with the ratio of r.m.s. turbulent velocity to laminar burning velocity, u′/u l and a Karlovitz stretch factor given by the ratio of a strain rate u′/λ to a flame gradient given by u l /δ l , where λ is the Taylor microscale and δ l the laminar flame thickness. Asymptotic analyses of strained laminar flames, together with the two-eddy theory of turbulent burning, show the additional importance of Lewis number effects. These result in lean hydrocarbon mixtures being quenched more readily than rich ones, with an opposite effect for H 2 mixtures. This was observed in the experiments. However, full quantitative agreement between theory and experiment was not achieved, due to the inherent limitations of the two theories, which are discussed.

Turbulent Transport in Flexible Plant Canopies

Abstract: The inapplicability of flux-gradient models of turbulent transport in plant canopies is demonstrated through formal manipulation of the rate equations for eddy flux and consideration of the nature of the diffusivity that describes release from point sources in homogeneous turbulence. A new set of scalar conservation and flux equations is derived by volume averaging the primitive equations which hold exactly in the air spaces between the plants. Source terms and extra waving terms arise because of the noncommutivity of differentiation and volume averaging in the multiply-connected canopy air space. The implicatons of the equation set are worked out for three cases. In the first, they are used to show the exact relationship between leaf transfer resistances and the averaged canopy resistances of the Penman-Monteith single leaf model. Second, we explore the relationship between the eddy flux produced by the action of the turbulent wind on the mean concentration field and the flux produced by the interaction of scalar concentration on foliage surfaces with the drag force and its consequences for canopy models. Third, the importance of the production of eddy flux by buoyant plumes generated at the foliage surface is deduced by a consideration of flux and buoyancy equations in the free convection limit.

Experimental study of disturbances produced in a pre-transitional laminar boundary layer by weak freestream turbulence

Abstract: The high-amplitude, low-frequency, disturbances generated in a Blasius boundary layer by grid-produced freestream turbulence were studied by means of hot-wire anemometry and by flow visualization. Emphasis was placed on determining the cause and nature of these, and on their effect upon the mean flow profile. It was found that the motion in the layer was intrinsically three-dimensional, according to which the fronts, of Tollmien-Schlichting waves became warped. Evidence of streamwise vorticity was noted. At Reynolds numbers near 1.6 million, the maximum attained, sharp velocity gradients appeared locally. With increase of grid size, there appeared turbulent spots which were wider than the coherence scale of the background motion.

Measurements of Local Skin Friction in a Microbubble Modified Turbulent Boundary Layer

Abstract: Local skin-friction reductions have been measured using an array of flush-mounted hot-film probes in a microbubble-modified, zero-pressure-gradient, turbulent bounddary layer. The results of earlier integrated skin-friction measurements, that showed the reduction to be a function of plate orientation, gas-flow rate and free-stream velocity, have been confirmed both qualitatively and quantitatively. With the measurement plate oriented so that buoyancy keeps the bubbles in boundary layer, it is shown that skin friction is reduced monotonically for all air-flow rates at each of three free-stream velocities between 4 and 17 m/s. For the opposite plate orientation it is possible for increasing gas injection to lead to smaller local skin-friction reduction at the lowest speeds. Drag reduction appears to persist for as much as 60–70 boundary-layer thicknesses downstream of the injection region. It is further shown, using a probe flush-mounted just upstream of the injection section, that there is no apparent upstream interference due to the gas injection. Spectral measurements indicate that microbubbles can cause a reduction of high-frequency shear-stress fluctuations. This suggests a destruction of some of the turbulence in the near-wall region.

The response of turbulent boundary layers to sudden perturbations

Abstract: The processes present in a turbulent boundary layer (BL) which experiences a sudden change in the surface roughness are discussed in terms of numerical models in comparison with empirical data. Consideration is limited to flow perturbations which preserve the BL approximation. Generalized continuity, momentum and turbulent energy transport equations are reviewed. The perturbed BL has two regions: one with large velocity gradients and an outer zone with smaller gradients. Expressions are defined for the velocity and temperature profiles and the roughness length after a step change in surface roughness, which causes the disturbance. A pertrurbation could also arise from a heat flux input of a pressure gradient. The effects of the shape of the perturbing step and the presence of several disturbing edges are explored. Finally, it is shown that closure forms and constants for self-preserving flows are worthy of further development because of their demonstrable abilities to describe simple perturbed flows.