Showing papers on "Pressure gradient published in 1988"

Reassessment of the scale-determining equation for advanced turbulence models

Abstract: A comprehensive and critical review of closure approximations for two-equation turbulence models has been made. Particular attention has focused on the scale-determining equation in an attempt to find the optimum choice of dependent variable and closure approximations. Using a combination of singular perturbation methods and numerical computations, this paper demonstrates that: 1) conventional A:-e and A>w formulations generally are inaccurate for boundary layers in adverse pressure gradient; 2) using "wall functions'' tends to mask the shortcomings of such models; and 3) a more suitable choice of dependent variables exists that is much more accurate for adverse pressure gradient. Based on the analysis, a two-equation turbulence model is postulated that is shown to be quite accurate for attached boundary layers in adverse pressure gradient, compressible boundary layers, and free shear flows. With no viscous damping of the model's closure coefficients and without the aid of wall functions, the model equations can be integrated through the viscous sublayer. Surface boundary conditions are presented that permit accurate predictions for flow over rough surfaces and for flows with surface mass addition.

Molecular dynamics of Poiseuille flow and moving contact lines.

Abstract: We report on molecular-dynamics simulations of the low--Reynolds-number flow of Lennard-Jones fluids through a channel. Application of a pressure gradient to a single fluid produces Poiseuille flow with a no-slip boundary condition and Taylor-Aris hydrodynamic dispersion. For an immiscible two-fluid system we find a (predictable) static contact angle and, when accelerated, velocity-dependent advancing and receding contact angles. The approximate local velocity field is obtained, in which the no-slip condition appears to break down near the contact line.

The Gasdynamics of Compact Relativistic Jets

Abstract: The gasdynamics of compact relativistic jets is explored by analyzing a specific idealized flow problem using the method of characteristics. The basic flow pattern of the gas and pressure waves within a jet experiencing a drop in external pressure is calculated, with analytic expressions given for many of the important parameters. Scaling laws which relate the intrinsic properties of the jet to the pressure of the surrounding medium are obtained and discussed. The physical properties of the jet depend critically on the value and abruptness of the decrease in external pressure, as well as on the initial Lorentz factor of the flow. A variety of the flow patterns can result, including jets of oscillating cross section, jets with standing shocks, and broad, nearly hollow beams which can break up into multiple jets. These results are discussed in relation to the observed characteristics of superluminal radio sources in general and the superluminal quasar 4C 39.25 in particular.

Dynamics of Seasonal and Intraseasonal Variability in the Eastern Equatorial Pacific

Abstract: Time series measurements from surface moored buoys in the eastern equatorial Pacific are analyzed for the period 1983–86. The data, collected as part of the EPOCS and TROPIC HEAT programs, consist of currents, temperatures, and winds on the equator at 110°, 124.5° and 140°W. The purpose is to examine the dynamics of seasonal and intraseasonal variability in the upper 250 m from a diagnosis of the depth integrated zonal momentum (i.e., transport) equation. The principal conclusions of this paper are that 1) there is an approximate balance between mean zonal wind stress and depth integrated pressure gradient; nonlinear advection is significantly nonzero however and leads to an enhancement of eastward transport along the equator, 2) there is an interannual change in zonal wind stress and pressure gradient in which both approximately double over the record length; 3) at the annual cycle, zonal wind stress and depth integrated pressure gradient tend to balance, though the uncertainties are large and o...

Direct numerical simulation of a three-dimensional turbulent boundary layer

Abstract: The effects of transverse strain on an initially two‐dimensional turbulent boundary layer are studied in a direct numerical simulation of a planar channel flow with impulsively started transverse pressure gradient. Consistent with experiments in three‐dimensional boundary layers, the simulation shows a decrease in the Reynolds shear stress with increasing transverse strain. Also, the directions of the Reynolds shear stress vector and the mean velocity gradient vector were found to differ. In addition, the simulation shows a drop in the turbulent kinetic energy. Terms in the Reynolds stress transport equations were computed. The balances indicate that the decrease in turbulent kinetic energy is a result of a decrease in turbulence production, along with an increase in turbulent dissipation. Intuitive reasoning and current turbulence models would predict an increase in kinetic energy along with increases in production and dissipation rates as a result of increased mean‐flow strain rate. Later in the evolution of the flow, both turbulence production and dissipation increase.

Vapor‐Dominated Zones Within Hydrothermal Systems: Evolution and Natural State

Abstract: Three conceptual models illustrate the range of hydrothermal systems in which vapor-dominated conditions are found. The first model (model I) represents a system with an extensive near-vaporstatic vapor-dominated zone and limited liquid throughflow and is analogous to systems such as The Geysers, California. Such systems can evolve within low-permeability barriers without changes in boundary conditions or rock properties, given an adequate supply of heat. Their scarcity in nature may be due to the need for a long-lived, potent heat source and for a low-permeability aureole that remains intact for significant lengths of time. Models II and III represent systems with significant liquid throughflow and include steam-heated discharge features at higher elevations and high-chloride springs at lower elevations, connected to and fed by a single circulation system at depth. In model II, as in model I, the vapor-dominated zone has a near-vaporstatic vertical pressure gradient and is generally underpressured with respect to local hydrostatic pressure. The vapor-dominated zone in model III is quite different, in that phase separation takes place at pressures close to local hydrostatic and the overall pressure gradient is near hydrostatic. A relatively large number of high-temperature systems in regions of moderate to great topographic relief are similar to either model II or model III; however, in most cases there are insufficient data to establish a single preferred model.

Gyrokinetic particle simulation of ion temperature gradient drift instabilities

Abstract: Ion temperature gradient drift instabilities have been investigated using gyrokinetic particle simulation techniques for the purpose of identifying the mechanisms responsible for their nonlinear saturation as well as the associated anomalous transport. For simplicity, the simulation has been carried out in a shear‐free slab geometry, where the background pressure gradient is held fixed in time to represent quasistatic profiles typical of tokamak discharges. It is found that the nonlinearly generated zero‐frequency responses for the ion parallel momentum and pressure are the dominant mechanisms giving rise to saturation. This is supported by the excellent agreement between the simulation results and those obtained from mode‐coupling calculations, which give the saturation amplitude as ‖eΦ/Te‖ ≂(‖ωl+iγl‖/Ωi)/(k⊥ ρs)2, and the quasilinear thermal diffusivity as χi ≂γl/k2⊥, where ωl and γl are the linear frequency and growth rate, respectively, for the most unstable mode of the system. In the simulation, the time evolution of χi after saturation is characterized by its slow relaxation to a much lower level of thermal conduction. On the other hand, a small amount of electron–ion collisions, which has a negligible effect on the linear stability, can cause significant enhancement of χi in the steady state.

A model for heat and mass transfer in timber structures during fire: a theoretical, numerical and experimental study

Abstract: A model has been formulated for the pyrolysis of wood. This model includes heat and mass transfer. Heat transfer in the material is assumed to take place by thermal conduction and convection. Mass transfer of volatile pyrolysis products and water vapour occurs under the influlence of gradients in to tal pressure. Pyrolysis is assumed to conform to an Arrheniusfunction, and vaporization occurs at a rate which is governed by the assumption that complete saturation in the pore systemis attained so long as there is water left at the point being studied. The boundary conditions are governed by a gas temperature time curve or the energy flow rate. The energy balance at the boundary also includes oxidation of the charcoal layer and the effectof the outward gas flow on the thermal surface resistance. Solution of the problem is studied by solving two non linear partial differential equations: - one which yields the temperature distribution and- one which yields the pressure distribution. The problem has been formulated using the finite element method. The numerical solutions have been compared with experiments. There is good agreement between the experimental and the calculated results. The importance of taking the initial moisture content into accounts evident from both the experiments and the calculations. Owingt to the pressure gradients which arise, there is a gradual and steeprise in moisture content. In those parts of the material where the temperature is above 100° C, vaporization occurs. The pressure gradients give rise to a mass flow directed towards the cooler parts of the material where the water vapour condenses. (Less)

The ionopause transition and boundary layers at comet Halley from Giotto magnetic field observations

Abstract: Giotto magnetic field observations at a time resolution of 28.24 vectors per second in the innermost part of the interaction region of comet Halley with the solar wind have yielded the following results: (1) The outer boundary of the cavity, the ionopause, has a thickness of 25 km only, both inbound and outbound, and magnetic field magnitude drops of 20 nT and 18.3 nT, respectively. Pressure balance arguments lead to an outward plasma pressure decrease of at least 1.5×10−10 Pa. Plasma densities reported by the Giotto mass spectrometers then lead to three alternatives: If no density jump occurs across the ionopause an electron temperature of at least 6000 K just inside the ionopause cannot be avoided. Smaller electron temperatures require an inward discontinuous increase in plasma density across the ionopause. Third, an additional hot ion population of sufficient pressure inside the cavity would make a high electron temperature unnecessary (D. T. Young, personal communication, 1987). (2) The ionopause boundary layers are characterized by (j × B) forces of 5.7×10−16 N m−3 and 7.3×10−16 N m−3 inbound and outbound, respectively. These magnetic forces must be balanced by the sum of the elastic frictional force, the forces due to massloading by photoions, due to ion molecule reactions and due to ionization by hot electrons. An electron pressure gradient may also contribute. (3) A magnetic field bulge of 80-km length is interpreted as a weak, almost perpendicular, subcritical shock propagating radially outward generated by a temporary pressure increase in the cavity presumably due to a transient population of hot ions and electrons in the cavity. (4) The hot ions and particularly electrons and additional energy input suggested by these observations point to the importance of dynamic phenomena in the innermost part of the interaction region, such as magnetic field line merging in the tail.

Dispersion of contaminant in oscillatory flows

Abstract: An extension of the Aris-Barton method of moments is presented for the study of statustucal behaviour of dispersion of contaminant molecules in oscillatory flows inside uniform conduits. The main idea is to replace the constant pressure gradient which causes the flow simply by the time-dependent one, and then investigate the effects of the amplitude and frequency of the pressure pulsations on the dispersion process. The technique incorporates the case of the time-independent flow also, and gives an exact analysis of the central moments of the distribution of the cloud of contaminant, which are valid for all times after the injection. The general theory is applied to oscillatory laminar flows in tubes, and between parallel plates.

Instability and transition of a three‐dimensional boundary layer on a swept flat plate

Abstract: Stability features are studied experimentally for the unstable three dimensional boundary layer flow on a swept‐back flat plate. A pressure gradient on the flat plate is induced by a displacement body. Infinite sweep conditions are approximated by means of contoured endplates. For the measurements, hot‐wire and surface hot‐film anemometry as well as flow visualization techniques are used. In addition to stationary waves, traveling waves are also traced. The cross‐flow Reynolds numbers for the first appearance of either instability mode are of approximately the same magnitude. Wavelength and the direction of stationary vortices, as well as the frequencies of the most amplified traveling waves, are measured for different Reynolds numbers. The data obtained by the measurements are compared with the results of linear stability theory. The location of the final transition on the swept flat plate has proved to be fairly well predicted by the empirical transition criterion of Coustols (These de Docteur Ingenieur, Ecole Nationale Superieur de l’Aeronautique et de l’Espace, Toulouse, 1983).

An lda study of the backward-facing step flow, including the effects of velocity bias

Abstract: The subsonic, backward-facing step flow was studied experimentally to provide structural information and test data for modellers. A single-component laser-Doppler anemometer was the primary measurement instrument. The effects of velocity bias on the LDA results were examined in detail. It was concluded that velocity bias was small (less than 4%) in the uncorrected measurements and nonexistent when a periodic sampling strategy was employed. The measurements show that the pressure gradient at reattachment is lower in the present experiment than in most previous work, due to the thick boundary layer at separation. Turbulence levels in the separated shear layer are also reduced by the thick upstream boundary layer. Scaling of the streamwise coordinate on the reattachment length produced the best agreement with previous data. Agreement of turbulence quantities was particularly good downstream of reattachment.

Investigations of boundary layer transition in an adverse pressure gradient

Abstract: Boundary layer transition was measured on a flat plate for four different turbulence levels. A range of adverse pressure gradients was imposed for one of these. The zero pressure gradient results were in agreement with accepted data for transition inception, length, and turbulent spot formation rate. They were also well represented by Narasimha’s universal intermittency distribution. A surprisingly strong similarity was also exhibited by intermittency distributions under adverse pressure gradients. Dimensionless velocity profiles were reasonable for the zero pressure gradient cases but difficulties with skin-friction prediction were experienced under adverse pressure gradient conditions. For this moderate turbulence level the transition inception Reynolds number remained reasonably constant with pressure gradient. Transition lengths, however, were greatly reduced by the imposition of even a weak adverse pressure gradient. This was associated with a strong increase in turbulent spot formation rate.

Effect of favorable pressure gradients on turbulent boundary layers

Abstract: Large favorable pressure gradients can cause the drag of a turbulent boundary layer on a solid surface to decrease; if large enough, such gradients can cause a return to laminar flow. Scaling relations and a computational model for the viscous wall layer, recently developed in this laboratory are used to interpret this phenomenon.

Pressure half-time does not always predict mitral valve area correctly.

Abstract: A theory is presented elucidating factors that influence the pressure half-time. By combining the Bernoulli and continuity equations and making certain assumptions about the shape of the atrioventricular pressure difference decay, it can be shown that valve area, volume transported across that area, and initial pressure difference influence the pressure half-time according to a formula in which the pressure half-time is related to V/(Ao square root of delta po), where V is the transported volume across the orifice with the area Ao, and delta po is the initial pressure difference across that area. In a subsequent hydraulic model experiment pressure half-time was determined for three different hole areas, with various initial volumes and initial pressure gradients. We did not obtain a unique relation between the pressure half-time and area. Instead the results supported our theory, and we found a close linear relationship between area and V/(T0.5 square root of delta po) (correlation coefficient [r] = 0.998), as predicted in the theory (T0.5 = pressure half-time). Clinical examples in which the pressure half-time may be misleading in the assessment of severity of mitral stenosis are presented.

A numerical analysis of calendering

Abstract: An analysis of calendering of inelastic (power-law) and viscoelastic sheets of finite initial thickness has been carried out using (i) a perturbation method based on lubrication theory; (ii) an approximate treatment including normal stress effects; (ii) a full numerical analysis using the boundary element method. The Phan-Thien-Tanner (PTT) fluid model was used in the viscoelastic analyses. Attention is focused on the separation criterion at the roll exit plane. While it is usual to assume in the inelastic case that separation occurs when the pressure and pressure gradient vanish simultaneously, it is not clear that this is appropriate in the viscoelastic model. The main new results are (a) a method of determining the separation point numerically using the criterion of zero tangential traction; (b) a computation of welling (∼ 5%) after the sheet leaves the nip; (c) a demonstration that the roll force first decreases as Weissenberg number (roll speed) rises, and then increases.

Explicit upwind algorithm for the parabolized Navier-Stokes equations

Abstract: A new explicit upwind algorithm based on Roe's flux-difference splitting (FDS) method has been developed for the three-dimensional Parabolized Navier-Stokes (PNS) equations. For three-dimensional flows, FDS's are determined separately for the two nonmarching directions and modified to account for the calculated shock angle in the crossflow plane. Second-order FDS is applied to the pressure and convection terms with the streamwise pressure gradient limited in the subsonic region to maintain a hyperbolic inviscid equation set. Second-order central differencing is obtained in the two-step algorithm for the shear and heat flux terms. The new algorithm is demonstrated for three laminar flow test cases: supersonic flow over a flat plate, hypersonic flow over a 15 deg ramp, and hypersonic flow past a 10 deg cone at a 24 deg angle of attack. The computed results agree well with experimental measurements.

Transition Modeling Effects on Viscous/Inviscid Interaction Analysis of Low Reynolds Number Airfoil Flows Involving Laminar Separation Bubbles

Abstract: Estimating the low Reynolds number and off-design performance of axial turbomachine blades requires an accurate prediction of separation phenomena occurring on the blade surface. This paper discusses a viscous/inviscid interaction analysis of flow over a NACA 65-213 airfoil at a chord Reynolds number of 240,000 using a calculation method of Cebeci et al. The computed characteristics of a mid-chord laminar separation bubble are compared with experimental laser-doppler anemometer measurements of Hoheisel et al. Attention is focused on problems of modeling the laminar-turbulent transition zone within the viscous layer.A parametric study is undertaken to determine the location and extent of the transition zone which best models the observed separation bubble behavior. The required transition length is almost an order of magnitude smaller than that predicted from conventional transition length correlations. A physical model for this greatly reduced transition length in positive pressure gradient flows is proposed.The computational model correctly predicts most features of the separation bubble flow, but there are some significant discrepancies at reattachment which point to the need for improved turbulence modeling in this area. The inclusion of transverse pressure gradients associated with flow curvature in the viscous regions also appears very desirable for airfoils operating at Reynolds numbers around 105.Copyright © 1988 by ASME

Model structure of a cosmic-ray mediated stellar or solar wind

Abstract: An idealized hydrodynamic model is presented for the mediation of a free-streaming stellar wind by galactic cosmic rays or energetic particles accelerated at the stellar wind termination shock. The spherically-symmetric stellar wind is taken to be cold; the only body force is the cosmic ray pressure gradient. The cosmic rays are treated as a massless fluid with an effective mean diffusion coefficient k proportional to radial distance r. The structure of the governing equations is investigated both analytically and numerically. Solutions for a range of values of k are presented which describe the deceleration of the stellar wind and a transition to nearly incompressible flow and constant cosmic ray pressure at large r. In the limit of small k the transition steepens to a strong stellar wind termination shock. For large k the stellar wind is decelerated gradually with no shock transition. It is argued that the solutions provide a simple model for the mediation of the solar wind by interstellar ions as both pickup ions and the cosmic ray anomalous component which together dominate the pressure of the solar wind at large r.

Depth Dependence of Bottom Stress and Quadratic Drag Coefficient for Barotropic Pressure-Driven Currents

Abstract: A level 2½ turbulence closure model is used to investigate the dependence on water depth H of bottom stress τb and quadratic drag coefficient Cd for a steady barotropic pressure-driven current in unstratified water when the current is the primary source of turbulence. For spatially uniform pressure gradient and bottom roughness z0 the magnitude |τb| increases from small values in shallow water to a maximum (at a depth ∼0.004 U0/f where U0 is the geostrophic current speed derived from the pressure gradient and f is the Coriolis parameter) at which the dynamics changes from being depth-limited to being controlled by similarity scales. As the depth increases further, |τb| decreases to its deep-water value that is 15% to 19% less than the maximum. The angle θ of the bottom stress relative to the geostrophic direction decreases rapidly from 90° in very shallow water, reaching its deep-water value (∼11°–21°) at a somewhat shallower depth than does |τb|. At the maximum stress θ is 8° larger than the dee...

Time-periodic convection in porous media: transition mechanism

Abstract: We resolve the disturbance structures that destabilize steady convection rolls in favour of a time-periodic pattern in two-dimensional containers of fluid-saturated porous material. Analysis of these structures shows that instability occurs as a travelling wave propagating in a closed loop outside the nearly motionless core. The travelling wave consists of five pairs of thermal cells and four pairs of vorticity disturbances in the case of a square container. The wave speed of the thermal disturbances is determined by an average base-state velocity and their structure by a balance between convection and thermal diffusion. Interpretation of the ‘exact’ solution is aided by a one-dimensional convection-loop model which correlates (i) point of transition, (ii) disturbance wavenumber, and (iii) oscillation frequency given the base-state temperature and velocity profiles. The resulting modified Mathieu-Hill equation clarifies the role of the vertical pressure gradient, induced by the impenetrable walls, and the role of the base-state thermal layer.

Balloon valvuloplasty for pulmonic valve stenosis--two-year follow-up: hemodynamic and Doppler evaluation.

Abstract: The purpose of this investigation was to evaluate the efficacy, technique, and follow-up results of balloon dilation angioplasty for valvular pulmonary stenosis. Percutaneous dilation was performed on 63 patients with pulmonary stenosis (ages 3 months to 76 years, mean = 4.3 years). In 43 patients, a single balloon was used; in 20 patients two balloons were used simultaneously. The pressure gradient across the pulmonary valve was determined with right ventricular and main pulmonary artery catheters. Pressure gradients simultaneously were estimated by continuous wave Doppler (CWD) during catheterization. The peak systolic ejection gradient was obtained by both techniques both pre- and postangioplasty. There was excellent linear correlation between the simultaneous catheter pressure gradient and the pressure gradient estimated by Doppler (r = 0.99). Follow-up pressure gradient estimations by Doppler echocardiogram were obtained in 30 patients between 6 months and 30 months postcatheterization (mean = 13 months). The mean preangioplasty gradient of 64 mm Hg (range 30-160 mm Hg) was reduced to 22 mm Hg (range 2-31 mm Hg). A significant reduction of transvalvular gradient (52-95%, mean 68%) occurred in each patient. A linear correlation was found between the predilation gradient and the pressure gradient drop (r = 0.92). Mean follow-up gradient by Doppler was 20 mm Hg (range 0-31 mm Hg), and there was no significant difference between these gradients and the postdilation gradient. No important complications were noted. These data confirm that balloon dilation angioplasty for valvular pulmonary stenosis is safe and effective, and suggest that stenosis does not recur.