Distinct Boundary Layer Research Articles

Radio Jet–Ambient Medium Interactions on Parsec Scales in the Blazar 1055+018

As part of our study of the magnetic fields of active galactic nuclei, we have recently observed a large sample of blazars with the Very Long Baseline Array. Here we report the discovery of a striking two-component jet in the source 1055+018 that consists of an inner spine with a transverse magnetic field and a fragmentary but distinct boundary layer with a longitudinal magnetic field. The polarization distribution in the spine strongly supports shocked-jet models, while that in the boundary layer suggests interaction with the surrounding medium. This behavior suggests a new way to understand the differing polarization properties of strong- and weak-lined blazars.

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths

Experiments dissolving orthopyroxene (En93) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO2 diffuses outward from the boundary layer whereas CaO and Al2O3 diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na2O and K2O is strongly affected by f O2. Under reducing conditions Na2O and K2O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K2O and to a lesser extent Na2O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO2 contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas.

Correction to flow rate — pressure drop relation in coronary angioplasty: steady streaming effect

The changed flow pattern of pulsatile blood flow in a catheterized stenosed artery has been studied through a mathematical model. The study takes into account the effect of the movement of the flexible catheter influenced by the pulsatile nature of the flow. The contribution of the steady streaming effect brings into focus the existence of a non-zero mean pressure drop in addition to the one predicted by the linear theory — a fact overlooked by the previous authors. Thus our results are intended to provide a correction to the mean pressure drop usually calculated by neglecting the non-linear inertia terms. The calculations based on the geometry and the flow conditions representing a real physiological situation as closely as possible suggest that depending upon the values of k (where k is the ratio of the catheter size to vessel size) ranging from 0.2 to 0.5 mean pressure drop increases for any frequency parameter. In addition, it is found that depending upon the material properties, a thin catheter experiencing small oscillations due to the flow conditions is likely to influence in the same way as a thicker catheter which remains fairly stationary inside the artery. The results are sensitive to the shape of the wall geometry and will be different for different wall geometries even if the cross-sectional area reduction at the peak of stenosis is kept the same. Interesting streamline patterns depict distinct boundary layer characteristics both at the artery wall and catheter wall. Finally, the effect of catheterization and its movement on various physiologically important flow characteristics-mean pressure drop, impedance, wall stress is studied for different range of catheter size and frequency parameter.

Thermocapillary flow near a “cold corner”

The velocity field in a neighborhood of the point of contact between the free and solid boundaries is studied numerically for the problem of noncrucible zone melting in a two-dimensional model formulation. A distinct Prandtl boundary layer on the solid boundary and a Marangoni boundary layer on the free boundary and high gradients of the longitudinal velocity along the free boundary in the immediate vicinity of the “cold corner” are observed. It is found for the first time that with distance from the solid boundary, the velocity curve has a maximum, which is not typical of the ordinary flow near the solid boundary.

An Approach to Solving Two Time-Scale Trajectory Optimization Problems

The indirect method of solving optimal control problems requires the solution of a Hamiltonian boundary-value problem (HBVP). Geometric properties of Hamiltonian systems and the notion of a dichotomy guide the development of a solution approach for HBVPs that involve two distinct time scales and boundary layers.

Accretion disc boundary layers around pre-main-sequence stars

One-dimensional time-dependent calculations of geometrically thin accretion disc boundary layers in pre-main-sequence stars are carried out for mass-accretion rates in the range M=5 × 10−7 to 10− M⊙ yr−1, α = 0.005−0.3 and different inner boundary conditions on the temperature. Two kinds of solution are obtained: those with a distinct thermal boundary layer (BL) component and those without a thermal boundary layer. For Ṁ up to ≈ 10−5 M⊙ yr− 1, and for α > αcritic ≈ 0.04, solutions with a thermal BL are obtained. For Ṁ ≈ 10−4 M⊙ yr−1 or for α < αcritic, solutions without a thermal BL are obtained. The inner boundary condition F = σTeff4 leads to hotter solutions and higher threshold values αcritic, while the no-flux boundary condition dT/dr = 0 leads to cooler solutions. For a very low mass-accretion rate (Ṁ≈5× 10−7 M⊙ yr−), the temperature in the disc drops below 104 K and the ionization front is adjacent to the outer edge of the hot thermal boundary layer. In the vicinity of the ionization front, the medium becomes slightly optically thin. For a very high massaccretion rate, advection of energy (ζ=Ladv/Lacc ≈ 0.1−0.2) from the boundary layer into the inner boundary becomes important, and the boundary layer luminosity is only a fraction of its expected value.

Response of maple sapwood to injury and infection

SummaryIn sapwood challenge experiments in Acer rubrum, columns of discolouration initiated by wounding and inoculation with pioneer fungi (Cephalosporium sp., Phialophora sp.) were similar in size to untreated wounds. Inoculation with decay fungi (Pleurotus ostreatus, Trametes versicolor) produced larger columns of wound‐initiated discolouration. The removal of bark around a bore wound caused a significantly larger column to form compared to the sum of the columns inititiated by separate wounds. Stage‐I discoloured wood, not associated with obviously rotted wood, had concentrations of mobile cations and soluble phenols similar to sapwood. Stage‐II discoloured wood, spatially associated with rotted wood, was frequently bounded by a chemically distinct boundary layer and the discoloured wood contained significantly greater concentrations of mobile cations and soluble phenols than stage‐I discoloured wood.

Metal concentrations in wood of sugar maple infected with sapstreak disease

The inoculation of sugar maple (Acersaccharum Marsh.) saplings with Ceratocystisvirescens (Davidson) C. Moreau (=Ceratocystiscoerulescens (Munch) Bakshi), the causal agent of sapstreak disease, resulted in infection and the development of extensive discoloration. A distinct column boundary layer formed between the discolored wood and sapwood in wounded saplings infected with C. virescens and in noninoculated controls. Elemental markers of discoloration and column boundary layer formation were similar for infected and noninoculated control saplings. The extensive damage caused by sapstreak disease is interpreted as resulting from the rapid spread of the pathogen prior to column boundary layer formation.

Boundary layer ion composition at Jupiter during the inbound pass of the Ulysses flyby

During the inbound segment of the Ulysses flyby of Jupiter, there were multiple incursions into the dawnside low-latitude boundary layer, as identified by Bame et al. ( Science 257, 1539–1542, 1992) using plasma electron data. In the present study, ion composition and spectral measurements provide independent collaborative evidence for the existence of distinct boundary layer regions. Measurements are taken in the energy-per-charge range of 0.6–60 keV/e and involve mass as well as mass-per-charge identification by the Ulysses/SWICS experiment. Ion species of Jovian magnetospheric origin (including O +, O 2+, S 2+, S 3+) and sheath origin (including He 2+ and high charge state CNO) have been directly identified for the first time in the Jovian magnetospheric boundary layer. Protons of probably mixed origin and He + of possibly sheath (ultimately interstellar pickup) origin were also observed in the boundary layer. Sheath-like ions are observed throughout the boundary layer; however, the Jovian ions are depleted or absent for portions of two boundary layer cases studied. Ions of solar wind origin are observed within the outer magnetosphere. and ions of magnetospheric origin are found within the sheath, indicating that transport across the magnetopause boundary can work both ways, at least under some conditions. Although their source cannot be uniquely identified, the proton energy spectrum in the boundary layer suggests a sheath origin for the lower energy protons.

Marine Boundary-layer Structure and Circulation in the Region of Offshore Redevelopment of a Cyclone during GALE

Marine boundary-layer structure and circulation is documented for the 24 February 1986 case of offshore redevelopment of a cyclone during the Genesis of Atlantic Lows Experiment (GALE) Intensive Observing Period (IOP) 9. Mesoscale and satellite information emphasize that the onshore cyclone is not well organized as it moves offshore to the cold shelf waters with redevelopment occurring later over the Gulf Stream region. Within hours of redevelopment, low-level aircraft data were obtained in the region. Vertical aircraft profiles down by the National Center for Atmospheric Research (NCAR) King Air in the vicinity of redevelopment over the Gulf Stream, as well as the midshelf front region and cold shelf waters, reveal two distinct boundary layers. Over the Gulf Stream region approximately 50 km south-southwest of the redeveloping cyclone, the near-neutral marine boundary layer (−h/L = 6.6) capped by layered stratocumulus is characterized by a low cloud base (360 m), relatively thick stratocumulus cloud layer (800–1200 m) and strong subcloud-layer winds (8–9 m s-1). Associated with the developing cyclone near the Gulf Stream is shallow cyclonic flow with convergence and subsequent acceleration of the wind near the western edge. Closer to the coast over the cold shelf waters and the midshelf front region, the relatively cloud-free boundary layer (h/L = 44.4) is characterized by a slightly shallower, new-neutral boundary layer (h = 700 to 755 m) with very light and variable winds. Boundary layer flow is strongly divergent west of the midshelf front. Them two regions are approximately 150–200 km west of the Gulf Stream region or redevelopment. Flux profiles agree with results from other marine boundary layers under similar cloud and stability conditions and emphasize the warming and moistening of the subcloud layer from new the western edge of the Gulf Stream eastward. Temperature and moisture turbulence structure appear less well organized. The mean momentum budget emphasizes the strong baroclinicity in the MABL and the importance of horizontal advection near the western edge of the Gulf Stream. Comparison turbulent kinetic energy (TKE) budgets over the Gulf Stream and over the midshelf front show shear production and dissipation to dominate over the Gulf Stream with strong winds. Turbulent transport over the Gulf Stream is a significant term due primarily to the flux of horizontal velocity variance, which is approximately 5 times that of the flux of vertical velocity variance. Over the midshelf front, all normalized terms in the TKE budget are less active in producing, dissipating and transferring TKE for a given heat flux as compared to the Gulf Stream region, where the effects of the developing cyclone are evident.

Infinite Prandtl-number convection in a cube heated from below. Numerical experiment by the pseudospectral method.

Thermal convection of fluids heated from below occurs when the Rayleigh number becomes greater than a critical value. In general, convection gradually evolves to complex, often time-dependent, states as the Rayleigh number increases. The process involves various forms of transition, such as from one convecting pattern to another, steady to time dependent and simply periodic to chaotic oscillation. The present research focuses on an evolution process of infinite Pr convection in a cubical geometry with stress-free boundary conditions. The numerical computations have been carried out up to Ra = 2×105 using an efficient scheme based on the pseudospectral method. With increasing Ra, a convecting pattern starts losing its symmetries at about Ra = 7×104, but does not evolve to time-dependent states. Effects of truncation number on the numerical accuracy are also investigated. For example, a truncation number of N = 12 is adequate at Ra = 104, while N = 16 is needed at Ra = 105. The Nusselt number appears to grow at a rate of Ra1/3 when Ra exceeds 104, suggesting the formation of distinct thermal boundary layers.

Aerodynamic effects on atmospheric composition measurements from rocket vehicles in the thermosphere

Several distinct classes of rocket-borne instrumentation have been used for composition measurements in the upper atmosphere and there is a possibility that some of these have been affected by the aerodynamic flowfield that surrounds the rocket vehicle. The flowfield is near-continuum at the mesopause, with distinct shock waves and boundary layers. These features thicken and merge with increasing altitude and lose their identity as free-molecule flow is approached at the thermospause. The aerodynamic effects are illustrated through reference to existing studies in rarefied gas dynamics. In addition, the direct simulation Monte Carlo method is used to make some new calculations of the flow past a representative rocket vehicle. The implications of the results for the various classes of instrumentation are discussed.

Histochemistry of lignin and suberin deposition in boundary layers formed after wounding in various plant species and organs

Wound responses in a variety of injured plant tissues were assessed using conventional lignin tests and fluorescence techniques for suberin detection in tissues present at the time of wounding. The tissue assessed included twigs of four conifer species, barley and cherry foliage, fern rachis, potato tuber, carrot root, musksmelon cotyledons, and cucumber hypocotyl. Apple leaves infected by a leaf spotting fungus (Botryosphaeria obtusa) were also examined. All tissues, except barley and apple foliage and fern rachis, formed a morphologically distinct lignosuberized boundary layer from cells present at the time of wounding. The boundary layer consisted initially of cells with lignified walls which with time developed suberin lamellae. In the fern rachis, the boundary layer was suberized in the absence of lignin. In the wounded barley and infected apple foliage, neither lignin nor suberin was detected histochemically.

Effect of Boundary Conditions on Numerically Simulated Tornado-like Vortices

Abstract The boundary conditions for Rotunno's numerical model which simulates tornado-like vortices are examined. In particular, the lateral boundary condition for tangential velocity and the upper boundary condition for radial and tangential velocities are considered to determine if they have any significant impact on vortex development. The choice of the lateral boundary condition did not appear to have any real effect on the development of the vortex over the range of swirl ratios studied (0.87–2.61). The upper boundary conditions attempt to simulate both the presence and absence of the flow-straightening baffle. The boundary condition corresponding to the baffle in place produced a distinct boundary layer in the u and v field and very strong upflow and downflow within the vortex core. When this condition is removed, there is both radial and tangential motion throughout the domain and a reduction of the vertical velocity. At small swirl ratio (S = 0.87) this boundary condition has a profound impact on...

Mass and heat transfer by natural convection in a vertical cavity

This paper reports a fundamental study of laminar natural convection in a rectangular enclosure with heat and mass transfer from the side, when the bouyancy effect is due to density variations caused by either temperature or concentration variations. In the first part of the study scale analysis is used to determine the scales of the flow, temperature and concentration fields in boundary layer flow for all values of Prandtl and Lewis numbers. In particular, scale analysis shows that in the extreme case where the flow is driven by bouyancy due to temperature variations, the ratio of mass transfer rate divided by heat transfer rate scales as Le 1 2 only if ( Pr > 1, Le < 1) or ( Pr < 1, Sc < 1), and as Le 1 3 if ( Pr > 1, Le > 1) or ( Pr < 1, Sc > 1). In the second part of the study, the boundary layer scales derived in the first part are used to determine the heat and mass transport characteristics of a vertical slot filled with fluid. Criteria for the existence of distinct thermal and concentration boundary layers in the slot are determined. Numerical solutions for the flow and concentration fields in a slot without distinct thermal boundary layers are reported. These solutions support further the method of scale analysis employed in the first part of the study

Numerical Study of Transient High Rayleigh Number Convection in an Attic-Shaped Porous Layer

Scaling arguments and numerical analysis are used to document the transient and steady-state regimes of natural convection in a triangular porous layer cooled from above (along the sloping wall). The numerical simulations are conducted in the high Rayleigh number domain, Ra = 100, 1000, where Ra is the Darcy-modified Rayleigh number based on height, H. The scale analysis predicts the existence of distinct thermal boundary layers if Ra1/2 (H/L) &gt; 1, where H/L is the height/length geometric ratio of the attic-shaped porous layer. The numerical simulations confirm the scaling results, as well as the prediction that the flow consists primarily of an elongated horizontal counterflow driven by the cold wall. In addition, the numerical solutions show the presence of a Be´nard-type instability at high enough Rayleigh numbers. For example, if H/L = 0.2, the instability is present when Ra &gt; 620; this critical Rayleigh number is found to increase as H/L increases.

Unsteady natural convection in a porous layer

The focus of this paper is on the fundamentals of the transient mechanism responsible for the establishment of natural convection in a two-dimensional porous layer confined between isothermal vertical walls and insulated horizontal walls. In the first part of the paper, pure scaling arguments are used to identify the characteristic time scales of the phenomenon and, based on these scales, the evolution of the porous system to steady state. The type of steady state is shown to depend on the relative order of magnitude of the characteristic time scales. Distinct vertical boundary layers are shown to be possible if (L/H)Ra1/2&amp;gt;1, and distinct horizontal layers if (H/L)1/2 Ra1/4&amp;gt;1. In the second part of the paper, these and other scaling laws of the transient evolution to steady state are successfully tested against a series of transient numerical experiments conducted in the range H/L=0.2–5 and Ra=10–50.

Structure of the Flow within the Coastal Boundary Layer of the Great Lakes

Abstract Properties of the nearshore flow have been observed in some detail in Douglas Point, Lake Huron. Time series flow data obtained from a network of current meters deployed in a coastal chain perpendicular to the local shoreline have been analyzed to resolve the mean flow properties, horizontal turbulence characteristics and the kinetic energy (in the mean flow and fluctuations) within the coastal boundary layer. The variability of these parameters as a function of the distance from shore for an episode during which persistent short-parallel currents prevailed for several days has revealed two distinct boundary layers, an inner boundary layer dominated by bottom and shore friction (FBL—frictional boundary layer) and an outer boundary layer as a consequence of the adjustments of inertial oscillations to the lateral boundary (IBL—inertial boundary layer). If one takes the width of the frictional boundary layer as the distance to the point where the kinetic energy of currents peak, it is ∼2 km at Dougl...

Hydromagnetic flow near an oscillating porous limiting surface

Unsteady hydromagnetic flow near a harmonically oscillating limiting surface (e.g., of a star) is considered in presence of a transverse magnetic field. Exact solutions, for a periodic boundary layer without a ‘mean steady flow’,are obtained when the magnetic Prandtl number is unity and there is a normal velocity of injection imposed at the wall. The results are also presented for the case when the wall is subjected to a normal velocity of suction instead of injection. It is observed that two distinct boundary (or hydromagnetic boundary) layers exist and tend to coalesce into a single layer when the magnetic field parameter approaches zero. The thicknesses of these boundary layers are significantly affected by the injection/suction velocity and the applied magnetic field.

Erratum to: On the unsteady non-Newtonian rotating flow induced by torsional oscillations of a disk

The unsteady boundary-layer flow generated in an incompressible, homogeneous elastico-viscous fluid bounded by an infinite rigid disk is investigated when the whole system is in a state of solid-body rotation with uniform angular velocity and, at some instant of time, the disk executes small-amplitude torsional oscillations with a given frequency. The analysis is carried out to determine various qualitative and quantitative information about the unsteady and steady velocity distributions, the structure of the associated multiple boundary layers, the effects of the elastic parameter and rotation on the flow and the Ekman suction velocity with its physical significance. It has been shown that the steady-state flow field consists of the two distinct boundary layers which are modified by the angular velocity of rotation, the elastic parameter and the frequency of the imposed oscillations. Several known results of physical interest are recovered as special cases.