Showing papers on "Boundary layer published in 1998"

Simulations of Solar Granulation. I. General Properties

Abstract: Numerical simulations provide information on solar convection not available by direct observation. We present results of simulations of near surface solar convection with realistic physics: an equation of state including ionization and three-dimensional, LTE radiative transfer using a four-bin opacity distribution function. Solar convection is driven by radiative cooling in the surface thermal boundary layer, producing the familiar granulation pattern. In the interior of granules, warm plasma ascends with ≈ 10% ionized hydrogen. As it approaches and passes through the optical surface, the plasma cools, recombines, and loses entropy. It then turns over and converges into the dark intergranular lanes and further into the vertices between granulation cells. These vertices feed turbulent downdrafts below the solar surface, which are the sites of buoyancy work that drives the convection. Only a tiny fraction of the fluid ascending at depth reaches the surface to cool, lose entropy, and form the cores of these downdrafts. Granules evolve by pushing out against and being pushed in by their neighboring granules, and by being split by overlying fluid that cools and is pulled down by gravity. Convective energy transport properties that are closely related to integral constraints such as conservation of energy and mass are exceedingly robust. Other properties, which are less tightly constrained and/or involve higher order moments or derivatives, are found to depend more sensitively on the numerical resolution. At the highest numerical resolution, excellent agreement between simulated convection properties and observations is found. In interpreting observations it is crucial to remember that surfaces of constant optical depth are corrugated. The surface of unit optical depth in the continuum is higher above granules and lower in the intergranular lanes, while the surface of optical depth unity in a spectral line is corrugated in ways that are influenced by both thermal and Doppler effects.

A Soil Moisture–Rainfall Feedback Mechanism: 1. Theory and observations

Abstract: This paper presents a hypothesis regarding the fundamental role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associated with relatively large boundary layer moist static energy, which favors the occurrence of more rainfall. Since soil moisture conditions themselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land surface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net solar radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil moisture conditions would tend to enhance net terrestrial radiation at the surface through cooling of surface temperature, reduction of upwards emissions of terrestrial radiation, and simultaneous increase in atmospheric water vapor content and downwards flux of terrestrial radiation. Thus, under wet soil moisture conditions, both components of net radiation are enhanced, resulting in a larger total flux of heat from the surface into the boundary layer. This total flux represents the sum of the corresponding sensible and latent heat fluxes. Simultaneously, cooling of surface temperature should be associated with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever these processes occur over a large enough area, the enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics of localized convective storms as well as the dynamics of large-scale atmospheric circulations have been shown to be sensitive to the distribution of boundary layer moist static energy by several previous studies. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper (Zheng and Eltahir, this issue) using a simple numerical model.

Boundary Layer Flow Control With a One Atmosphere Uniform Glow Discharge Surface Plasma

Abstract: Low speed wind tunnel data have been acquired for planar panels covered by a uniform, glow-discharge surface plasma in atmospheric pressure air known as the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP). Streamwise and spanwise arrays of flush, plasma-generating surface electrodes have been studied in laminar, transitional, and fully turbulent boundary layer flow. Plasma between symmetric streamwise electrode strips caused large increases in panel drag, whereas asymmetric spanwise electrode configurations produced a significant thrust. Smoke wire flow visualization and mean velocity diagnostics show the primary cause of the phenomena to be a combination of mass transport and vortical structures induced by strong paraelectric electrohydrodynamic (EHD) body forces on the flow.

Vertical distributions of lightning NOx for use in regional and global chemical transport models

Abstract: We have constructed profiles of lightning NOχ mass distribution for use in specifying the effective lightning NOχ source in global and regional chemical transport models. The profiles have been estimated for midlatitude continental, tropical continental, and tropical marine regimes based on profiles computed for individual storms in each regime. In order to construct these profiles we have developed a parameterization for lightning occurrence, lightning type, flash placement, and NOχ production in a cloud-scale tracer transport model using variables computed in the two-dimensional Goddard Cumulus Ensemble (GCE) model. Wind fields from the GCE model are used to redistribute the lightning NOχ throughout the duration of the storm. Our method produces reasonable results in terms of computed flash rates and NOχ mixing ratios compared with observations. The profiles for each storm are computed by integrating the lightning NOχ mass across the cloud model domain for each model layer at the end of the storm. The results for all three regimes show a maximum in the mass profile in the upper troposphere, usually within 2–4 km of the tropopause. Downdrafts appear to be the strongest in the simulated midlatitude continental systems, evidenced by substantial lightning NOχ mass (up to 23%) in the lowest kilometer. Tropical systems, particularly those over marine areas, tended to have a greater fraction of intracloud flashes and weaker downdrafts, causing only minor amounts of NOχ to remain in the boundary layer following a storm. Minima appear in the profiles typically in the 2–5 km layer. Even though a substantial portion of the NOχ is produced by cloud-to-ground flashes in the lowest 6 km, at the end of the storm most of the NOχ is in the upper troposphere (55–75% above 8 km) in agreement with observations. With most of the effective lightning NOχ source in the upper troposphere where the NOχ lifetime is several days, substantial photochemical O3 production is expected in this layer downstream of regions of deep convection containing lightning. We demonstrate that the effect on upper tropospheric NOχ and O3 is substantial when the vertical distribution of the lightning NOχ source in a global model is changed from uniform to being specified by our profiles. Uncertainties in a number of aspects of our parameterization are discussed.

Zero Viscosity Limit for Analytic Solutions of the Navier-Stokes Equation on a Half-Space. II. Construction of the Navier-Stokes Solution

Abstract: This is the second of two papers on the zero-viscosity limit for the incompressible Navier-Stokes equations in a half-space in either 2D or 3D. Under the assumption of analytic initial data, we construct solutions of Navier-Stokes for a short time which is independent of the viscosity. The Navier-Stokes solution is constructed through a composite asymptotic expansion involving the solutions of the Euler and Prandtl equations, which were constructed in the first paper, plus an error term. This shows that the Navier-Stokes solution goes to an Euler solution outside a boundary layer and to a solution of the Prandtl equations within the boundary layer. The error term is written as a sum of first order Euler and Prandtl corrections plus a further error term. The equation for the error term is weakly nonlinear; its linear part is the time dependent Stokes equation. This error equation is solved by inversion of the Stokes equation, through expressing the solution as a regular (Euler-like) part plus a boundary layer (Prandtl-like) part. The main technical tool in this analysis is the Abstract Cauchy-Kowalewski Theorem.

Model of laser-driven mass transport in thin films of dye-functionalized polymers

Abstract: Based on Newtonian fluid dynamic relations, a model is constructed to describe laser-induced mass transport in thin films of polymers containing isomerizable azobenzene chromophores, in which surface profile diffraction gratings can be inscribed with an interference pattern of coherent light. The Navier–Stokes equations for laminar flow of a viscous fluid are developed to relate velocity components in the film to pressure gradients in the polymer film, by definition of boundary layer conditions. This general laminar flow model is applicable to the formation of surface gratings through a variety of mechanisms. Considering the mechanism of an isomerization-driven free volume expansion to produce internal pressure gradients, a specific model is developed to describe polymer flow resulting from laser-induced isomerization of the bulky chromophores. This yields an expression relating the time evolution of the surface gratings to properties which could be varied experimentally, such as those of the irradiating ...

Air-water gas exchange

Abstract: ▪ Abstract The exchange of inert and sparingly soluble gases—including carbon dioxide, methane, and oxygen—between the atmosphere and oceans is controlled by a thin 20- to 200-μm-thick boundary layer at the top of the ocean. The hydrodynamics in this layer are significantly different from boundary layers at rigid walls, since the orbital motion of the waves is of the same order as the velocities in the viscous boundary layer. Laboratory and field measurements show that wind waves significantly increase the gas transfer rate and that it is significantly influenced in this way by surfactants. Because of limited experimental techniques, the mechanisms for this enhancement and the structure of the turbulence in the boundary layer at a wavy water surface are still not known. A number of new imaging techniques are described that give direct insight into the processes and promise to trigger substantial theoretical progress in the near future.

Analyses of slow high-concentration flows of granular materials

Abstract: A theory intended for slow, dense flows of cohesionless granular materials is developed for the case of planar deformations. By considering granular flows on very fine scales, one can conveniently split the individual particle velocities into fluctuating and mean transport components, and employ the notion of granular temperature that plays a central role in rapid granular flows. On somewhat larger scales, one can think of analogous fluctuations in strain rates. Both kinds of fluctuations are utilized in the present paper. Following the standard continuum approach, the conservation equations for mass, momentum and particle translational fluctuation energy are presented. The latter two equations involve constitutive coefficients, whose determination is one of the main concerns of the present paper. We begin with an associated flow rule for the case of a compressible, frictional, plastic continuum. The functional dependence of the flow rule is chosen so that the limiting behaviours of the resulting constitutive relations are consistent with the results of the kinetic theories developed for rapid flow regimes. Following Hibler (1977) and assuming that there are fluctuations in the strain rates that have, for example, a Gaussian distribution function, it is possible to obtain a relationship between the mean stress and the mean strain rate. It turns out, perhaps surprisingly, that this relationship has a viscous-like character. For low shear rates, the constitutive behaviour is similar to that of a liquid in the sense that the effective viscosity decreases with increasing granular temperature, whereas for rapid granular flows, the viscosity increases with increasing granular temperature as in a gas. The rate of energy dissipation can be determined in a manner similar to that used to derive the viscosity coefficients. After assuming that the magnitude of the strain-rate fluctuations can be related to the granular temperature, we obtain a closed system of equations that can be used to solve boundary value problems. The theory is used to consider the case of a simple shear flow. The resulting expressions for the stress components are similar to models previously proposed on a more ad hoc basis in which quasi-static stress contributions were directly patched to rate-dependent stresses. The problem of slow granular flow in rough-walled vertical chutes is then considered and the velocity, concentration and granular temperature profiles are determined. Thin boundary layers next to the vertical sidewalls arise with the concentration boundary layer being thicker than the velocity boundary layer. This kind of behaviour is observed in both laboratory experiments and in granular dynamics simulations of vertical chute flows.

A Herschel–Bulkley model for mud flow down a slope

Abstract: The spreading and sediment deposit of a two-dimensional, unsteady, laminar mud flow from a constant-volume source on a relatively steep slope is studied theoretically and experimentally. The mud under consideration has the rheological properties of a Herschel–Bulkley fluid. The flow is of low-Reynolds-number type and has a well-formed wave front moving a substantial distance downslope. Due to the nonlinear rheological characteristics, a set of nonlinear partial differential equations is needed for this transient problem. Depth-integrated continuity and momentum equations are derived by applying von Karman's momentum integral method. A matched-asymptotic perturbation method is implemented analytically to get asymptotic solutions for both the outer region away from, and the inner region near, the wave front. The outer solution gives accurate results for spreading characteristics, while the inner solution, which is shown to agree well with experimental results of Liu & Mei (1989) for a Bingham fluid, predicts fairly well the free-surface profile near the wave front. A composite solution uniformly valid over the whole spreading length is then achieved through a matching of the inner and outer solutions in an overlapping region. The range of accuracy of the solution and the size of the inner and overlapping regions are quantified by physical scaling analyses. Rheological and dynamic measurements are obtained through laboratory experiments. Theoretical predictions are compared with experimental results, showing reasonable agreement. The impact of shear thinning on the runout characteristics, free-surface profiles and final deposit of the mud flow is examined. A mud flow with shear thinning spreads beyond the runout distance estimated by a Bingham model, and has a long and thin deposit.

A new Strouhal–Reynolds-number relationship for the circular cylinder in the range 47<Re<2×105

Abstract: Based on experiments a new law is proposed for the vortex shedding from a circular cylinder which describes in a consistent way the Strouhal–Reynolds-number dependency as Sr=Sr*+m/Re from the beginning of the vortex shedding at Re=47 up to the laminar–turbulent transition of the cylinder boundary layer at Re>2×105. The various vortex shedding processes, occurring with increasing Reynolds number, are described by different coefficients Sr* and m.

Stratified Atmospheric Boundary Layers and Breakdown of Models

Abstract: The goal of this study is to assess complications in atmospheric stable boundary layers which are not included in numerical models of the stably stratified boundary layer and to provide a formulation of surface fluxes for use in numerical models. Based on an extensive interpretive literature survey and new eddy correlation data for the stable boundary layer, this study defines two prototype stable boundary layers: the weakly stable case and the very stable case. The weakly stable boundary layer is amenable to existing models. The very stable boundary layer eludes modeling attempts due to breakdown of existing formulations of turbulence and due to features found in the atmosphere which are not normally included in models. The latter includes clear-air radiative cooling, low-level jets, surface heterogeneity, gravity waves, meandering motions, and other mesoscale motions which propagate from outside the local domain. While these mechanisms are not essential to understanding idealized or laboratory versions of the stable boundary layer, they complicate comparisons of numerical models and theories with actual atmospheric boundary layers. Statistics which describe various features of the stable boundary layer are offered for future comparison with modeling results.

Direct numerical simulation of a separated turbulent boundary layer

Abstract: A separated turbulent boundary layer over a flat plate was investigated by direct numerical simulation of the incompressible Navier–Stokes equations. A suction-blowing velocity distribution was prescribed along the upper boundary of the computational domain to create an adverse-to-favourable pressure gradient that produces a closed separation bubble. The Reynolds number based on inlet free-stream velocity and momentum thickness is 300. Neither instantaneous detachment nor reattachment points are fixed in space but fluctuate significantly. The mean detachment and reattachment locations determined by three different definitions, i.e. (i) location of 50% forward flow fraction, (ii) mean dividing streamline (ψ=0), (iii) location of zero wall-shear stress (τw=0), are in good agreement. Instantaneous vorticity contours show that the turbulent structures emanating upstream of separation move upwards into the shear layer in the detachment region and then turn around the bubble. The locations of the maximum turbulence intensities as well as Reynolds shear stress occur in the middle of the shear layer. In the detached flow region, Reynolds shear stresses and their gradients are large away from the wall and thus the largest pressure fluctuations are in the middle of the shear layer. Iso-surfaces of negative pressure fluctuations which correspond to the core region of the vortices show that large-scale structures grow in the shear layer and agglomerate. They then impinge on the wall and subsequently convect downstream. The characteristic Strouhal number St=fδ*in/U0 associated with this motion ranges from 0.0025 to 0.01. The kinetic energy budget in the detachment region is very similar to that of a plane mixing layer.

Studying the effects of aerosols on vertical photolysis rate coefficient and temperature profiles over an urban airshed

Abstract: This paper discusses the effects of size- and composition-resolved aerosols on photolysis and temperatures within and above an urban airshed. With respect to photolysis, three-dimensional simulations indicated that (1) in regions of the boundary layer where absorption of ultraviolet (UV) radiation was strong, aerosols reduced photolysis coefficients of UV-absorbing gases; (2) in regions of the boundary layer where UV scattering dominated UV absorption by aerosols, aerosols enhanced photolysis coefficients of UV-absorbing gases; (3) aerosols increased photolysis coefficients for visible-absorbing gases since visible scattering always exceeded visible absorption by aerosols; (4) scattering and weakly absorbing aerosols above the boundary layer increased photolysis coefficients above the boundary layer for all absorbing gases; and (5) increases in aerosol absorption extinction within the boundary layer reduced photolysis coefficients above the boundary layer for all absorbing gases. Photolysis coefficients changes due to aerosols decreased near-surface ozone mixing ratios in Los Angeles by 5–8%. With respect to temperatures, simulations indicated that aerosols increased radiative heating rates at all altitudes but decreased surface solar irradiances during the day. Surface irradiance reductions cooled the ground, reducing mechanical and thermal turbulent heat fluxes back to the boundary layer, cooling near-surface air, and stabilizing the boundary layer. During the night, aerosols decreased boundary-layer heating rates but increased downward infrared irradiances to the ground. Warmer ground temperatures increased mechanical turbulent heat fluxes to the boundary layer, increasing nighttime near-surface temperatures. Thus, aerosols affected temperatures primarily through ground-atmosphere turbulent heat transfer.

Vortices, Generators and Heat Transfer

Abstract: Longitudinal vortices are more efficient for heat transfer enhancement than transverse vortices.A survey is given on triangular and rectangular protrusions from a heat transfer surface which generate mainly longitudinal vortex systems. Wings and winglets are considered in boundary layer and channel flow, either by themselves, or in a single row transverse to the flow direction, or in a two dimensional array. Local and global heat transfer are studied as a function of the major parameters. For channel flows also the pressure losses are given. Winglets are superior to wings, but winglet form is of minor importance. In laminar flow, heat transfer enhancement increases with Reynolds number. Heat transfer enhancement increases for constant winglet aspect ratio with angle of attack up to a maximum angle of attack. But it increases also up to limiting values with winglet height relative to transverse and streamwise winglet spacing and relative to channel height or boundary layer thickness. The nonlinear character of fluid mechanics does not allow simple predictions.

Radiative–convective equilibrium in a three-dimensional cloud-ensemble model

Abstract: A knowledge of radiative convective interactions is key to an understanding of the tropical climate. In an attempt to address this a cloud-resolving model has been run to a radiative-convective equilibrium state in three dimensions. The model includes a three-phase bulk microphysical scheme and a fully interactive two-stream broadband radiative-transfer scheme for both the infrared and solar radiation. The simulation is performed using a fixed sea surface temperature, and cyclic lateral boundary conditions. No ‘large-scale’ convergence, mean wind shear or background vorticity was imposed. The total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables. It is seen that some variables, such as vertical mass flux, adjust quickly to their equilibrium values while others, such as column-integrated water amount, domain-mean temperature and convective available potential energy (CAPE) display variation on a longer 30-day time-scale. The equilibrium state had a column-integrated vapour amount of 42.3 kg m−2, a mean temperature of 258.7 K and a pseudo-adiabatic CAPE value of 1900 J kg−1. The equilibrium-state statistics are consistent with tropical observations. The convection does not remain randomly distributed but instead becomes organized, aligning in a band structure associated with high moisture values in the boundary layer. This organization seems to result from interactions between radiation, convection and surface fluxes. The surface-flux feedback is due to higher boundary-layer winds, associated with convection, increasing surface fluxes of moisture locally. Horizontally inhomogeneous radiation can act to make clouds longer lasting and also increase convergence into cloudy region. Replacing the wind-sensitive surface-flux calculation with a linear relaxation to surface values appeared to largely destroy this organization, as did the use of an imposed horizontally uniform radiative-heating rate.

Turbulent boundary-layer control by means of spanwise-wall oscillation

Abstract: An experimental investigation into the changes in turbulence structure of the boundary layer over a wall oscillating in spanwise direction was carried out in a wind tunnel using hot-wire anemometry and flow visualisation. The main purpose of this investigation is to confirm recent numerical results which seem to indicate that the turbulent skinfriction drag can be reduced by up to 40 percent over the oscillating wall. The results from the present investigation clearly indicate that the logarithmic velocity profiles are shifted upwards and turbulence intensities reduced by the spanwise-wall oscillation, confirming the basic conclusions of recent direct numerical simulation. Also, the skinfriction reductions as much as 45% are observed in the present experiment at an optimum speed of wall oscillation. The flow-visualisation study indicates that the longitudinal vortices in the near-wall region of the boundary layer are twisted towards the direction of spanwise-wall motion with oscillation. As a result, the vortices are realigned into the cross-flow direction, resulting in a reduction of turbulent wall-skin friction by spanwise-wall oscillation. A conceptual model for a turbulent boundary layer over an oscillating wall is proposed to examine the mechanism of turbulent drag reduction by spanwise-wall oscillation.

Use of Piezoelectric Actuators for Airfoil Separation Control

Abstract: Surface-mounted piezoelectric actuators are used to excite the turbulent boundary layer upstream of separation, where the actuators interact directly with the boundary layer. The actuators are rigid and do not attenuate with increased aerodynamic loading up to the maximum tested speed of 30 m/s

Autumnal Mixed-Phase Cloudy Boundary Layers in the Arctic

Abstract: Two mixed-phase cloudy boundary layer events observed over the Arctic ice pack in autumn are extensively analyzed. The local dynamic and thermodynamic structure of the boundary layers is determined from aircraft measurements including analysis of turbulence, longwave radiative transfer, and cloud microphysics. The large-scale forcing is determined from the National Centers for Environmental Prediction reanalysis fields while mesoscale forcing is estimated from 40-km aircraft box patterns. The two cases differed somewhat in their local static stability, surface characteristics, and large-scale forcing. One case was characterized by a stably stratified cloudy boundary layer over a heterogeneous surface containing numerous open leads. The other case occurred over a fairly homogenous surface of multiyear ice and consisted of a surface-based stable layer surmounted by a low-level jet and a cloud-topped mixed layer. An important large-scale factor in the development of low clouds appears to have been w...

Tetrahedral Unstructured Navier-Stokes Method for Turbulent Flows

Abstract: A method is presented for solving the Navier-Stokes equations for turbulent flow problems on three-dimensional unstructured grids. Spatial discretization is accomplished by a cell-centered, finite volume formulation using an accurate linear reconstruction scheme and upwind flux differencing. Time is advanced by an implicit backward Euler time-stepping scheme. Flow turbulence effects are modeled by the Spalart-Allmaras one-equation model, which is coupled with a wall function to reduce the number of cells in the sublayer region of the boundary layer. A systematic assessment of the method is presented to devise guidelines for more strategic application of the technology to complex problems. The assessment includes the accuracy in predictions of the skin-friction coefficient, law-of-the-wall behavior, and surface pressure for a flat-plate turbulent boundary layer and for the ONERA M6 wing under a high-Reynolds-number, transonic, separated flow condition

Particle nucleation in the tropical boundary layer and its coupling to marine sulfur sources

Abstract: New particle formation in a tropical marine boundary layer setting was characterized during NASA's Pacific Exploratory Mission-Tropics A program. It represents the clearest demonstration to date of aerosol nucleation and growth being linked to the natural marine sulfur cycle. This conclusion was based on real-time observations of dimethylsulfide, sulfur dioxide, sulfuric acid (gas), hydroxide, ozone, temperature, relative humidity, aerosol size and number distribution, and total aerosol surface area. Classic binary nucleation theory predicts no nucleation under the observed marine boundary layer conditions.

Comparing the performance of the Mellor‐Yamada and the κ‐ε two‐equation turbulence models

Abstract: The aim of this paper is to systematically compare κ-e and Mellor-Yamada two-equation turbulence models. Both models include prognostic equations for turbulent kinetic energy and a length scale related parameter which are used to calculate eddy viscosities and vertical diffusivities. The results from laboratory experiments, using mixed and stratified flows, are simulated in order to systematically compare and calibrate the models. It is shown that the Monin-Obukhov similarity theory is well represented in both models. The models are used to simulate stratified tidal flow in the Irish Sea, and the results show that the κ-e models generally predict a larger phase lag between currents and turbulent dissipation, in the bottom boundary layer, than the Mellor-Yamada models. The comparison between the model results and field measurements, of the rate of dissipation of turbulent kinetic energy, shows that both models require modification through the inclusion of an internal wave parameterization in order that they are able to correctly predict the observed levels of turbulent dissipation. As the main result, it is shown that the choice of the stability functions, which are used as proportionality factors for calculating the eddy viscosity and diffusivity, has a stronger influence on the performance of the turbulence model than does the choice of length scale related equation.

Mechanisms for High-Frequency Quasi-periodic Oscillations in Neutron Star and Black Hole Binaries

Abstract: We explain the millisecond variability detected by Rossi X-Ray Timing Explorer (RXTE) in the X-ray emission from a number of low-mass X-ray binary systems (Sco X-1, 4U 1728-34, 4U 1608-522, 4U 1636-536, 4U 0614+091, 4U 1735-44, 4U 1820-30, GX 5-1) in terms of dynamics of the centrifugal barrier, a hot boundary region surrounding a neutron star (NS). We demonstrate that this region may experience the relaxation oscillations and that the displacements of a gas element both in radial and vertical directions occur at the same main frequency, of order of the local Keplerian frequency. We show the importance of the effect of a splitting of the main frequency produced by the Coriolis force in a rotating disk for the interpretation of a spacing between the quasi-periodic oscillation (QPO) peaks. We estimate a magnitude of the splitting effect and present a simple formula for the whole spectrum of the split frequencies. It is interesting that the first three lowest order overtones (corresponding to the azimuthal numbers m = 0, -1, and -2) fall in the range of 200-1200 Hz and match the kHz QPO frequencies observed by RXTE. Similar phenomena should also occur in black hole (BH) systems, but, since the QPO frequency is inversely proportional to the mass of a compact object, the frequency of the centrifugal-barrier oscillations in the BH systems should be a factor of 5-10 lower than that for the NS systems. The X-ray spectrum formed in this region is a result of upscattering of a soft radiation (from a disk and an NS surface) off relatively hot electrons in the boundary layer. The typical size of the emission region should be 1-3 km, which is consistent with the time-lag measurements. We also briefly discuss some alternative QPO models, including the possibility of acoustic oscillations in the boundary layer, the proper stellar rotation, and g-mode disk oscillations.

Ultra-Low velocity zones near the core-mantle boundary from broadband PKP precursors

Abstract: Short- and long-period precursors of the PKP phase were used to study an ultra-low velocity zone (ULVZ) near the core-mantle boundary beneath the Western Pacific. Synthetic seismograms were computed from a hybrid method, which handles seismic wave propagation through two-dimensional complex structures. Long-period precursors were explained by Gaussian-shaped ULVZs of 60 to 80 kilometers height with P velocity drops of at least 7 percent over 100 to 300 kilometers. Short-period precursors suggest the presence of smaller scale anomalies accompanying these larger Gaussian-shaped structures. These fine structures may be areas of partial melt caused by vigorous small-scale convection or the instability of a thermal boundary layer at the mantle's base, or both.

Heat transfer in a fluid with variable thermal conductivity over a linearly stretching sheet

Abstract: This paper considers the boundary layer heat transfer in a two-dimensional Newtonian fluid flow caused by a porous and linearly stretching sheet in the presence of blowing/suction. The thermal conductivity is assumed to vary linearly with temperature as is found in liquid metals. The resulting nonlinear energy equation forms a boundary value problem which is solved by a shooting method. A perturbation method is also used to derive a set of uncoupled, linear boundary value problems which are solved by superposition of solutions.

Control of turbulence

Abstract: ▪ Abstract We discuss a few applications of active control of turbulent fluid flow and their implications for the economy and the environment. We outline a conceptual basis for control, sketching sensors, actuators, and the algorithm. The control of turbulence requires an understanding of turbulent flows beyond our present capabilities, but we describe the physical basis for control of the boundary layer: coherent structures and bursts, the connection between burst frequency and friction velocity, the change of burst frequency and drag reduction possible with polymers or active control, and other effects on burst frequency (e.g. streamline curvature, pressure gradients, and extra rates of strain). Given that the state of the flow must be sensed from the surface, and that this information is necessarily incomplete and aliased, sophisticated techniques may be required to interpret the signals. A control strategy, an algorithm, is necessary, and we express the need for a model of the flow as an interpretor a...

Numerical study of hypersonic reacting boundary layer transition on cones

Abstract: Hypersonic gas flow over cones is solved using computational fluid dynamics to obtain accurate boundary layer profiles A linear stability analysis is performed on the profiles to determine the amplification rates of naturally occurring disturbances, and this information is used with the eN method to predict the boundary layer transition location The effects of free-stream total enthalpy and chemical composition on transition location are studied to give a better understanding of recent experimental observations Namely, there is an increase in transition Reynolds number with increasing free-stream total enthalpy, and this increase is greater for gases with lower dissociation energies The results show that linear stability predicts the same trends that were observed in the experiments, but with N=10, it consistently overpredicts the transition Reynolds numbers by about a factor of 2 The results of numerical experiments are presented which show the effect of reaction endo- or exothermicity on disturbanc