bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geostatistical analysis of regional differences in stem taper form of Pinus densiflora in central Korea
Authors: Lee Woo‐Kyun , Biging Greg S. , Son Yowhan , Byun Woo‐Hyuk , Lee Kyeong‐Hak , Son Yeong‐Mo , Seo Jeong‐Ho 
Published: April 2006

AbstractThis study verified regional differences in the stem form of Pinus densiflora Sieb. et Zucc. (red pine) and identified the relationship between stem form and climatic factors in the central region of the Korean peninsula. Regional differences in stem form index at tree base (butt) and top stem section were found. Compared to the stem form in the eastern uplands, the stem form in the western lowlands could be characterized by a more conical butt section and more cylindrical middle and upper section. Through geostatistical analysis of kriging and spatial regression, several climatic factors proved to exert a meaningful influence on stem taper form. On the stem form at the butt section, the precipitation during the late growing season exerts statistically significant effects. High precipitation during the growing season in the western lowland and coastal region causes the stem form at the butt section to be more tapered. On the stem form at the middle and upper section, temperature and precipitation during the growing season, and wind during the late growing season have statistically meaningful influences. High temperature, precipitation, and wind during the growing season in the western lowland and coastal region jointly influence the stem form at the middle and upper sections which result in more cylindrical profiles. This study can be considered an initial investigation into the factors controlling stem form variability in the central region of the Korean peninsula. The results can be used to develop more accurate regional stem taper models needed for reasonable management of red pine stands in different regions.
Instability of bounded flows with elliptical streamlines
Authors: Gledzer E. B. , Ponomarev V. M. 
Published: April 2006

In connection with the recent investigations of the instability of unbounded elliptical flows, some methods are discussed for the study of the instability of bounded flows. The stability of a ‘basic flow’ which is two-dimensional and rotating, with elliptical streamlines similar to the elliptical section of an experimentally studied cavity, is investigated in the framework of linear theory (for circular rotation, the flow discussed is stable). The regions of instability for three-dimensional disturbances are found in the plane of the parameters defining the geometry of the system (the height of the ellipsoidal cavity and the degree of ellipticity). It is shown that two types of instability exist, characterized by either monotone or oscillatory growth of the amplitudes of small disturbances.The influence of the Coriolis force field on this instability mechanism is also studied. Rotation of the system as a whole changes the regions of instability in parameter space characterizing the geometry of the cavity and the wavenumbers of unstable disturbances. As a result, the Coriolis force may stabilize or destabilize the basic flow for a given geometry.The instability of rotating density-stratified flow with elliptical streamlines is also considered.
Interaction of electrostatic and fluid dynamic fields in wire—plate electrostatic precipitators
Authors: Kallio G. A. , Stock D. E. 
Published: April 2006

Gas flow through wire–plate electrostatic precipitators is influenced by a secondary flow of electrical origin known as electric wind. This phenomenon arises when significant momentum is transferred from corona-generated ions to the gas. Electric wind can produce turbulence and recirculation. This complex flow field was characterized in a simple, three-wire precipitator by flow visualization, electrostatic and fluid dynamic numerical modelling, and laser-Doppler anemometry (LDA).Velocity of seeded smoke was measured by two-component LDA. Coulomb effects were regionally eliminated by performing measurements along symmetry axes where the electric field and streamwise velocity component were mutually perpendicular. Coulomb drift velocities were also estimated from field charging theory to allow interpretation of measured transverse velocities. For a low inlet velocity (0.5 m/s), mean flow recirculation was evident and turbulence intensities as high as 50 % were measured. Higher inlet velocities (1.0, 2.0 m/s) yielded no flow recirculation and lower turbulence levels that were polarity-dependent. Measured profiles of streamwise velocity showed that flow acceleration zones occurred upstream of each wire and also between wires near the collecting plate. The induced turbulence displayed significant inhomogeneity and anisotropy.A combined finite-element, finite-difference electrostatic model was developed to yield ion density and electric field distributions within the precipitator. These predictions were used to incorporate an electric body force into a two-dimensional, turbulent fluid dynamic model based upon the k–ε formulation. The model predicted recirculating mean flow and turbulent diffusivities that were consistent with the smoke flow visualizations and LDA measurements.
Impingement of an axisymmetric jet on unheated and heated flat plates
Authors: Özdemir İ. Bedii , Whitelaw J. H. 
Published: April 2006

The aerodynamic and thermal aspects of the wall jet flow, formed after angled impingement of an axisymmetric jet, have been studied with emphasis on the large-scale transport of the passive scalar by the spatially coherent structures. Time-averaged and instantaneous structures of the turbulent flow were examined by visualization and local measurements of a jet arrangement with an impingement angle between the jet axis and the surface normal of 20°, a nozzle-to-plate distance to nozzle exit diameter ratio of 22, and a nozzle exit Reynolds number of 1.3 × 104.The results show that the oblique impingement introduced vertical velocities so that boundary-layer approximations were inapplicable and led to the distribution of the time-averaged properties of the velocity and temperature field with strong azimuthal dependence, which increased gradually with angle of impingement to 40° where a sudden change of the orientation of the contours of surface pressure and temperature took place. It also led to instantaneous, spatially coherent structures which were most pronounced at an angle of 20°. These structures improved the large-scale transport of the passive scalar but, owing to the extreme regularity of their path, also led to an inactive zone near the vortex centre.The inner region of the decelerating wall jet exhibited a momentum equilibrium layer extending to the point of radial velocity maximum and the intercept of the linear region of the semilogarithmic wall law varied in the local streamwise direction as for turbulent flows over rough walls with adverse pressure gradient. The thermal equilibrium layer had an invariant functional form but extended far beyond the point of maximum velocity.
An unsteady wake-source model for flow past an oscillating circular cylinder and its implications for Morison's equation
Authors: Chew Y. T. , Low H. T. , Wong S. C. , Tan K. T. 
Published: April 2006

A potential-flow modelling of flow past an oscillating circular cylinder with separated wake is developed here based on Parkinson & Jandali's (1970) wake-source model for steady flow. The phase-averaged pressure distributions, the in-line force coefficients, as well as the drag and added-mass coefficients, for an in-line oscillating circular cylinder in a steady free-stream flow are computed using the present ‘unsteady wake-source model’. The results show that Morison's equation is in some cases a satisfactory model in the study of unsteady bluff-body aerodynamics.The two-dimensional incompressible potential-flow model simulates the effect of flow separation in unsteady flow by placing surface sources, with time-dependent strength and angular positions on the rear wetted surface of the body, and downstream sinks to form a closed wake model in the transformed plane. The unsteady Bernoulli equation is used to obtain the time-dependent pressure distributions over the front wetted surface, from which the in-line force coefficients are obtained through integration.The in-line force equation obtained from the present model is shown to be comprised of an uncoupled drag term and inertia terms. The corresponding hydrodynamie coefficients obtained for the case of oscillatory flow are also more realistic than those obtained in a potential-flow calculation without flow separation which gives a drag coefficient of zero and a constant inertia coefficient of 2.0. The in-line force equation is reduced to the familiar Morison's equation with some simplifications and thus provides some support to the much criticized Morison's equation in the study of unsteady separated flow.Another interesting feature of the present model is that it enables the calculation of instantaneous drag and inertia coefficients which have not been successfully obtained previously. In the cases considered here, the variations of drag and inertia coefficients over a cycle are shown to be small and thus the Morison's equation using mean coefficients is shown to predict the in-line forces rather precisely.The present model was compared with experimental measurements obtained by oscillating a 0.1 m diameter circular cylinder along the direction of free-stream flow. The pressure distributions and in-line force coefficients agree well with the experimental measurements for velocity ratio rω/U∞ up to 0.25, reduced velocity U∞T/d down to 50 and Keulegan—Carpenter number 2πr/d up to 17, where r, ω, T, U∞ and d are the amplitude of oscillation, angular frequency, period of oscillation, free-stream velocity and diameter of the cylinder respectively. The computed drag and inertia coefficients also agree well with those obtained experimentally by previous investigators.
Capillary rollers and bores
Authors: Longuet-Higgins Michael S. 
Published: April 2006

At any free surface at which the tangential stress τns vanishes there must be a surface vorticity ω = −2κq, where κ is the curvature and q the tangential velocity. In a surface wave on water, this condition produces a (Stokes) boundary layer with thickness of order δ = (2ν/σ)½, where ν is the kinemetic viscosity and σ the radian frequency of the wave. To first order in the wave steepness parameter ak, the vorticity remains within the boundary layer, but at second order some escapes through the Stokes layer. The mean vorticity $\overline{\omega}$ at the outer edge of the Stokes layer is of order 2(ak)2σ, twice the mean vorticity generated at the free surface.These results are applied to steep capillary waves, particularly the parasitic capillaries often seen on the forward face of short gravity waves. Because of the high value of σ for the capillaries, the vorticity they generate is much larger than that generated by the gravity wave itself. Hence the capillaries contribute significantly to the vortex (roller) often found at the crest of short gravity waves, when capillaries are present. It is argued that the crest roller and the capillaries form a cooperative system, a ‘capillary roller’ in which each supports the other, with the aid of surface tension and viscosity. Energy is supplied by the gravity wave.A capillary roller is one instance of a more general phenomenon: a ‘capillary bore’, which is a noticeable feature of many disturbed water surfaces.
Three-dimensional dynamics and transition to turbulence in the wake of bluff objects
Authors: Karniadakis George Em , Triantafyllou George S. 
Published: April 2006

The wakes of bluff objects and in particular of circular cylinders are known to undergo a ‘fast’ transition, from a laminar two-dimensional state at Reynolds number 200 to a turbulent state at Reynolds number 400. The process has been documented in several experimental investigations, but the underlying physical mechanisms have remained largely unknown so far. In this paper, the transition process is investigated numerically, through direct simulation of the Navier—Stokes equations at representative Reynolds numbers, up to 500. A high-order time-accurate, mixed spectral/spectral element technique is used. It is shown that the wake first becomes three-dimensional, as a result of a secondary instability of the two-dimensional vortex street. This secondary instability appears at a Reynolds number close to 200. For slightly supercritical Reynolds numbers, a harmonic state develops, in which the flow oscillates at its fundamental frequency (Strouhal number) around a spanwise modulated time-average flow. In the near wake the modulation wavelength of the time-average flow is half of the spanwise wavelength of the perturbation flow, consistently with linear instability theory. The vortex filaments have a spanwise wavy shape in the near wake, and form rib-like structures further downstream. At higher Reynolds numbers the three-dimensional flow oscillation undergoes a period-doubling bifurcation, in which the flow alternates between two different states. Phase-space analysis of the flow shows that the basic limit cycle has branched into two connected limit cycles. In physical space the period doubling appears as the shedding of two distinct types of vortex filaments.Further increases of the Reynolds number result in a cascade of period-doubling bifurcations, which create a chaotic state in the flow at a Reynolds number of about 500. The flow is characterized by broadband power spectra, and the appearance of intermittent phenomena. It is concluded that the wake undergoes transition to turbulence following the period-doubling route.
Steady-state nonlinear internal gravity-wave critical layers satisfying an upper radiation condition
Authors: Lamb Kevin G. , Pierrehumbert Raymond T. 
Published: April 2006

We consider the behaviour of an internal gravity wave encountering a critical level in a stratified fluid, assuming the critical-level flow to be dominated by nonlinear effects. The background flow is a shear layer, and the stratification is sufficiently strong to support wave propagation everywhere. Incident and reflected waves are permitted below the critical level, and a radiation condition is imposed far above it. For this geometry we construct, by a combination of asymptotic and numerical means, steady, nonlinear solutions, and discuss the associated transmission coefficients, reflection coefficients, phase shifts, and resonance positions when the system is forced from below.The inviscid solutions we exhibit have continuous density and velocity everywhere, and so do not require the introduction of internal viscous boundary layers. Further, the streamlines bounding the recirculating cat's-eye regions have corners, just as in the unstratified case. For weak stratification, the transmitted wave is nearly as strong as the incident wave, and there is accompanying strong over-reflection. As the stratification increases, the critical level becomes a nearly perfect reflector. The amount of transmission depends on wave amplitude, and the sensitivity increases with increasing stratification.There are regions of parameter space for which steady solutions could not be found. The critical-layer structure appears to break down by unbounded thickening when the stratification becomes too strong, suggesting that in these cases some neglected physical process must intervene to limit growth of the recirculating region.
Bénard convection in binary mixtures with Soret effects and solidification
Authors: Zimmermann G. , Müller U. , Davis S. H. 
Published: April 2006

Bénard convection of a two-component liquid is considered. The liquid displays Soret effects and the boundary temperatures are fixed to span the solidification temperature of the mixture. Near the lower, heated plate the material is liquid and near the upper cooled plate there is a layer of pure solid solvent; all the solute is rejected during freezing. Linear stability theory is used to determine the effects on the critical conditions for Soret convection in the presence of the solidified layer and the interface between solid and liquid.Experiments on mixtures of ethyl alcohol and water are performed using interferometry, photography and thermocouple measurements. The measured onset of instability to travelling waves at negative Soret coefficient compares well with those predicted by our linear theory. In the absence of ice the waves develop at finite amplitude to a fixed-amplitude state. However, when ice is present, these waves fail to persist but evolve to a state of steady finite-amplitude (overturning) convection. These differences are attributed to the presence of the ice and the nonlinear density profile of the basic state, both of which act as sources of non-Boussinesq effects.
Scaling of adverse-pressure-gradient turbulent boundary layers
Authors: Durbin P. A. , Belcher S. E. 
Published: April 2006

An asymptotic analysis is developed for turbulent boundary layers in strong adverse pressure gradients. It is found that the boundary layer divides into three distinguishable regions: these are the wall layer, the wake layer and a transition layer. This structure has two key differences from the zero-pressure-gradient boundary layer: the wall layer is not exponentially thinner than the wake; and the wake has a large velocity deficit, and cannot be linearized. The mean velocity profile has a y½ behaviour in the overlap layer between the wall and transition regions.The analysis is done in the context of eddy viscosity closure modelling. It is found that k-ε-type models are suitable to the wall region, and have a power-law solution in the y½ layer. The outer-region scaling precludes the usual ε-equation. The Clauser, constant-viscosity model is used in that region. An asymptotic expansion of the mean flow and matching between the three regions is carried out in order to determine the relation between skin friction and pressure gradient. Numerical calculations are done for self-similar flow. It is found that the surface shear stress is a double-valued function of the pressure gradient in a small range of pressure gradients.
Stabilization and destabilization of turbulent shear flow in a rotating fluid
Authors: Tritton D. J. 
Published: April 2006

We consider turbulent shear flows in a rotating fluid, with the rotation axis parallel or antiparallel to the mean flow vorticity. It is already known that rotation such that the shear becomes cyclonic is stabilizing (with reference to the non-rotating case), whereas the opposite rotation is destabilizing for low rotation rates and restabilizing for higher. The arguments leading to and quantifying these statement are heuristic. Their status and limitations require clarification. Also, it is useful to formulate them in ways that permit direct comparison of the underlying concepts with experimental data.An extension of a displaced particle analysis, given by Tritton & Davies (1981) indicates changes with the rotation rate of the orientation of the motion directly generated by the shear/Coriolis instability occurring in the destabilized range.The ‘simplified Reynolds stress equations scheme’, proposed by Johnston, Halleen & Lezius (1972), has been reformulated in terms of two angles, representing the orientation of the principal axes of the Reynolds stress tensor (αa) and the orientation of the Reynolds stress generating processes (αb), that are approximately equal according to the scheme. The scheme necessarily fails at large rotation rates because of internal inconsistency, additional to the fact that it is inapplicable to two-dimensional turbulence. However, it has a wide range of potential applicability, which may be tested with experimental data. αaand αbhave been evaluated from numerical data for homogeneous shear flow (Bertoglio 1982) and laboratory data for a wake (Witt & Joubert 1985) and a free shear layer (Bidokhti & Tritton 1992). The trends with varying rotation rate are notably similar for the three cases. There is a significant range of near equality of αaand αb. An extension of the scheme, allowing for evolution of the flow, relates to the observation of energy transfer from the turbulence to the mean flow.
Rapid distortion theory for homogeneous compressed turbulence with application to modelling
Authors: Durbin P. A. , Zeman O. 
Published: April 2006

Compressible rapid distortion theory is used to examine pressure fluctuations and the pressure–dilatation correlation in a field of turbulence subjected to rapid homogeneous compression. It is shown how a one-dimensional compression produces large solenoidal pressure fluctuations. As the dimensionality of the compression increases, the magnitude of these fluctuations decreases — it vanishes for a spherically symmetric compression. By contrast the dilatational, or acoustic, pressure fluctuations depend mainly on the net volumetric compression, and are relatively insensitive to the dimensionality of the compression. These same comments apply to the pressure–dilatation correlation.The pressure–dilatation correlation appears in the compressible turbulent kinetic energy equation and is significant in rapidly evolving flows; Reynolds stress closure models require that it be represented. The continuity equation provides a relation between pressure dilatation and the rate of change of pressure fluctuation variance. This relation is the basis for our RDT analysis. That analysis leads to a proposal for modelling the rapid contribution to pressure dilatation.