Showing papers on "Pressure gradient published in 1983"
TL;DR: In this paper, the performance of a constant-temperature normal hotwire in a supersonic flow is critically examined and it is shown that this instrument is inherently unsuitable for measuring turbulent temperature correlations because of the highly non-linear response to temperature fluctuations, particularly at low overheat ratios.
Abstract: The performance of a constant-temperature normal hotwire in a supersonic flow is critically examined. It is shown that this instrument is inherently unsuitable for measuring turbulent temperature correlations because of the highly non-linear response to temperature fluctuations, particularly at low overheat ratios. The instrument is therefore limited to measurements of mean and fluctuating mass-flow rates. Suitable calibration procedures. as well as the limits on spatial and temporal resolution are discussed. and corrections for mean stagnation temperature changes are suggested. The instrument was used to measure the mass-flow fluctuations in a zero pressure gradient Mach 2.9 turbulent boundary layer. A comparison with the available data suggests good agreement.
161 citations
TL;DR: In this paper, a wall-pressure fluctuation pattern associated with the burst-sweep cycle of events in the wall region is identified. But the phase relationship with velocity fluctuations close to the wall and the wall shear-stress fluctuations during the burst cycle has not been established.
Abstract: The wall-pressure fluctuations beneath a turbulent boundary layer have been conditionally sampled on a basis of the high-frequency activity of the pressure fluctuations themselves, the high-frequency activity of the streamwise velocity fluctuations in the vicinity of the wall, and the excursions in velocity in the vicinity of the wall. This has led to the identification of a characteristic wall-pressure fluctuation pattern which is associated with the burst–sweep cycle of events in the wall region. The pattern has the form of an overpressure over a streamwise extent of about 1·5−2·0δ * , with a region of underpressure and a pressure minimum to either side of it, the distance between pressure minima being about 3·0−3·5δ * . This pattern is convected at a velocity 0·67 times the freestream velocity. Its phase relationship with velocity fluctuations close to the wall and the wall shear-stress fluctuations during the burst–sweep cycle have been established. It appears to be produced by the inclined shear layer which forms the upstream surface of the large organized structures in the layer, and calculated pressure patterns support this conclusion. The phase relationships indicate that fluid involved in the bursting process is subjected to a favourable streamwise pressure gradient by the characteristic wall-pressure pattern at the time that the lift-up of low-speed streaks in the wall region begins. In addition, order-of-magnitude estimates suggest that the adverse pressure gradients associated with the characteristic pressure pattern, even if their phasing with streak lift-up were appropriate, would be insufficient to initiate the lift-up. It is therefore concluded that the streamwise pressure gradients associated with the pressure patterns do not play an active role in the dynamics of the wall flow and are not the direct cause of the bursting process.
124 citations
TL;DR: In this paper, a novel and efficient numerical method is used to investigate the nonlinear equations of motion for the upper layer of a two-layer ocean in which the lower layer is infinitely deep and at rest.
Abstract: A novel and efficient numerical method is used to investigate the nonlinear equations of motion for the upper layer of a two-layer ocean in which the lower layer is infinitely deep and at rest. The efficiency is achieved by seeking solutions that are in a steady state, translating in equilibrium with the storm. Oscillations are found in the wake of the storm. Two features of the response are attributed to the nonlinear terms in the equation of motion: 1) a rapid transition from a maximum in the downwelling phase, to a maximum in the upwelling phase of each oscillation, followed by a gradual relaxation to the next downwelling maximum; and 2) a displacement of the maximum response, usually to the right of the storm track, by ∼40 km. It is shown that the horizontal pressure gradient terms can be neglected from the momentum equations for “fast”, “large” storms, in which case a Lagrangian integration can be performed, following fluid particles. This enables feature 1) to be attributed to the along-tra...
120 citations
TL;DR: In this article, the variable-interval time-averaging (VITA) technique was applied to data obtained from large-eddy simulation of turbulent channel flow in an investigation of the organized structures associated with the bursting phenomenon in the nearwall region.
Abstract: The variable-interval time-averaging (VITA) technique developed by Blackwelder and Kaplan is applied to data obtained from large-eddy simulation of turbulent channel flow in an investigation of the organized structures associated with the bursting phenomenon in the near-wall region. Conditionally averaged velocities, shear stress, pressure, and vorticity are discussed in conjunction with the bursting phenomenon detected by the VITA technique. The conditionally averaged pressure reveals that the ejection process is associated with a localized adverse pressure gradient. In the plane perpendicular to the flow direction, the conditionally averaged vorticity field indicates that a pair of counterrotating streamwise vorticity is being lifted through the ejection process.
109 citations
01 Oct 1983
TL;DR: A four-month record from a current meter mooring on the continental slope west of Scotland displayed a steady northward current of 0.16 m s−1 which appears to be associated with a core of warm light water normally observed over the shallow side of the slope.
Abstract: A four-month record from a current meter mooring on the continental slope west of Scotland displayed a steady northward current of 0.16 m s−1 which appears to be associated with a core of warm light water normally observed over the shallow side of the slope. Although the tides change considerably between deep water and the continental shelf, the horizontal velocity gradients are not sufficient to explain the observed average current. One possible explanation, which displays the tendency to enhance the current over the shallow side of the slope, is a boundary current driven by a north-south pressure gradient, with pressure increasing towards the north. Such a pressure gradient could be a surface level gradient which contributes to a large term in the vorticity equation in regions with large bottom slopes.
108 citations
98 citations
TL;DR: The boundary layer transition under instationary afflux conditions as present in the stages of turbomachines is investigated in this paper by means of time-space distributions of the turbulent spots during transition and schematic drawings of the instantaneous boundary layer thicknesses.
Abstract: The boundary layer transition under instationary afflux conditions as present in the stages of turbomachines is investigated. A model for the transition process is introduced by means of time-space distributions of the turbulent spots during transition and schematic drawings of the instantaneous boundary layer thicknesses. To confirm this model, measurements of the transition with zero and favorable pressure gradient are performed.
80 citations
TL;DR: In this paper, the authors demonstrate the importance of the seasonal barotropic longshore pressure gradient force to Pacific Northwest coastal dynamics and show that three important features of seasonal circulation depend on the existence of the pressure gradient.
Abstract: In this paper we demonstrate the importance of the seasonal barotropic longshore pressure gradient force to Pacific Northwest coastal dynamics. Values of the seasonal longshore pressure gradient corrected for gauge height relative to a level surface (Hickey and Pola, 1982) and for year-to-year variations (Enfield and Alien, 1980) were included in the two-dimensional, time-dependent, baroclinic finite-difference model of Hamilton and Rattray (1978) as an external force. Observed wind stress, stratification and bottom topography were included in the model, and comparisons were made with current meter data in each of the three seasonal situations observed in the Northwest: pressure gradient force southward opposing local wind stress (winter), pressure gradient force northward opposing local wind stress (summer), and pressure gradient force and local wind stress both southward (spring). Three important features of the seasonal circulation are shown to depend on the existence of the pressure gradient ...
57 citations
TL;DR: A particle fluid suspension model is applied to the problem of pulsatile blood flow through a rigid circular tube with entrance effects, and steady pulsatile velocities for both cases are deduced by taking time t -greater than .
57 citations
TL;DR: In this paper, the bounce-averaged particle drifts in an axisymmetric toroidal plasma were calculated for the case of a large-aspect-ratio tokamak equilibrium with circular surfaces but a local pressure gradient comparable with the ideal MHD ballooning limit.
Abstract: The bounce-averaged particle drifts in an axisymmetric toroidal plasma are calculated for the case of a large-aspect-ratio tokamak equilibrium with circular surfaces but a local pressure gradient comparable with the ideal MHD ballooning limit: dp/dr ~B2/Rq2
55 citations
TL;DR: The transstenotic gradient measured at angioplasty overestimates "true" resting gradient in a predictable manner, which is dependent on the ratio of Dc to Ds.
Abstract: Pressure gradient measurement across a stenosis is used during angioplasty to aid catheter positioning and estimate dilatation efficacy. The angioplasty catheter itself, however, further reduces lumen size, and therefore augments the transstenotic gradient. To more precisely define the catheter influence on gradient, we derived a theoretical expression relating the measured gradient with the angioplasty catheter in situ to the "true" gradient; that is, the gradient in the absence of the angioplasty catheter. We then tested this theoretical construct in a canine femoral artery angioplasty model. Fifty-four measurements were performed using 23 separate, 3-mm-long, 40 to 70% stenoses. As predicted by the theoretic model, "true" gradient is compounded by the angioplasty catheter principally as a function of the angioplasty catheter diameter (Dc) and the stenosis diameter (Ds). The best-fit curve of data points relating "true" and compounded gradients to various Dc and Ds combinations can be expressed as: Measured gradient = K X true gradient, where K = 0.25 (e)4.47 (Dc divided by Ds) and e = 2.718. Thus, the transstenotic gradient measured at angioplasty overestimates "true" resting gradient in a predictable manner, which is dependent on the ratio of Dc to Ds.
TL;DR: In this article, an area-averaged tidal bottom stress is made for four channel segments of the Great Bay Estuary, N.H. The analysis shows that while throughout the estuary the principal force balance is between the frictional stress and the pressure gradient forcing, RMS values of total bottom stress range from 2·67 to 10·38 Nm−2 and friction coefficients vary from 0·015 to 0·054.
Abstract: Estimates of area-averaged tidal bottom stress are made for four channel segments of the Great Bay Estuary, N.H. Current and sealevel measurements are used to estimate acceleration and pressure gradient terms in the equation of motion, while the equation of motion itself is used to infer the remaining stress term. Dynamic terms, bottom stress values, friction coefficients and energy dissipation rates are estimated for each site. The analysis shows that while throughout the estuary the principal force balance is between the frictional stress and the pressure gradient forcing, RMS values of total bottom stress range from 2·67 to 10·38 Nm−2 and friction coefficients vary from 0·015 to 0·054. Both stress and energy dissipation are largest in the seaward portion of the estuary with an order of magnitude decrease in dissipation at the most inland site. These distributions of stress and energy dissipation are consistent with cotidal charts of the principal semi-diurnal tidal constituent (M2) which indicate that the estuary is composed of a highly dissipative more progressive tidal wave regime seaward and a less dissipative standing wave regime landward.
TL;DR: In this article, a thermal theory is developed to explain the experimentally observed effect of pressure changes on the propagation velocity of premixed flames using a one-step reaction and large activation energy asymptotics in the framework of a slowly varying flame analysis.
Abstract: A thermal theory is developed to explain the experimentally observed effect of pressure changes on the propagation velocity of premixed flames. The theory uses a one-step reaction and large activation energy asymptotics in the framework of a slowly varying flame analysis. A one-dimensional unsteady flame with Lewis number ℒ equal or unequal to unity is considered. The Mach number is assumed to be low enough such that spatial pressure gradients can be neglected. The asymptotic analysis requires a small temperature variation of the unburnt gas due to pressure changes-this condition is satisfied in the limit of large heat release. The analysis shows that pressure changes affect the flame velocity in two direrent ways, through the change of the enthalpy balance in the preheat zone and through the change of flame temperature. An ordinary differential equation for the time variation of the normalized mass burning rate is derived. This equation is analysed for ℒ=l and constant relative pressure decrease...
TL;DR: In this article, a one-layer primitive equation model is presented for the atmospheric boundary layer over the marginal ice zone (MIZ), which simulates the slow rate of inversion growth and rate of warming of the boundary layer seaward of an ice edge for off-ice winds observed on two cruises in the Bering Sea by the NOAA R/V Surveyor.
Abstract: A one-layer, primitive equation model is presented for the atmospheric boundary layer over the marginal ice zone (MIZ). The model simulates the slow rate of inversion growth and rate of warming of the boundary layer seaward of an ice edge for off-ice winds observed on two cruises in the Bering Sea by the NOAA R/V Surveyor. The horizontal temperature gradient in the boundary layer, caused by the oceanic heat flux seaward of an ice edge, induced an increase in wind speed with a maximum increase of 8% at 50 km seaward of the edge. At 100 km off ice, a momentum balance is established between accelerative terms (boundary layer baroclinity, momentum entrainment, synoptic-scale scale pressure gradient) and decelerative terms (surface drag and the local pressure force resulting from inversion rise). Wind velocity in the boundary later over the MIZ during off-ice winds is sensitive to changes in surface roughness. When an MIZ is modeled as a smooth interior (CD = 2 × 10−3) and a 30-km-wide rough marginal ice zone (CD = 3.8 × 10−3) with an unstable surface layer over the ocean, the model shows a decrease in wind speed of 9% at the windward side of the MIZ and an 18% increase in wind speed from 5 km interior to the ice edge to 40 km seaward of the edge. These results suggest an atmospheric mechanism for rafting at the windward side of the marginal ice zone, divergence of the ice at the edge, and ice-band formation seaward of the edge.
TL;DR: Water transport in the arterial wall is studied using a mathematical model based on the theory for the consolidation of water saturated soils and the pulsatile flow is found to be confined to the boundary layer while a smaller mean flow exists throughout the wall.
TL;DR: In this paper, the authors have numerically studied the reconnection process at an interface where the total pressure is in balance but where the thermal pressure is higher on one side than on the other.
Abstract: To examine the basic characteristics of reconnection of the dayside magnetopause, we have numerically studied the reconnection process at an interface where the total pressure is in balance but where the thermal pressure is higher on one side than on the other. Reconnection is caused by anomalous resistivity that is assumed to operate only in a localized region at the interface. Boundaries are assumed to be free boundaries, but reflection of the perturbations originating from inside the simulation region is suppressed by placement of an absorbing region. Results can be expressed as follows, denoting the high-pressure and low-pressure regions as magnetosheath and magnetosphere, respectively. (1) a slow shock is formed in magnetosheath and a slow expansion fan is formed in magnetosphere; (2) at the slow shock the Lorentz force and pressure gradient are of comparable importance in accelerating plasma, but at the expansion fan the pressure gradient has the dominant effect, and (3) the acceleration depends on the direction of the interplanetary magnetic field (IMF) and when the reconnection line is assumed to be directed at the half angle (theta/2) between IMF and geomagnetic field, the velocity of the accelerated plasma is roughly proportional to sin/sup 2/(theta/2)+sin/sup 3/(theta/2).
TL;DR: Findings indicate that the permeability of a cultured endothelium to water and solutes is pressure- and Ca2+-dependent.
TL;DR: In this paper, a steady viscous flow through a two-dimensional, infinite channel consisting of an uneven wall and a plane wall is theoretically investigated under a given pressure gradient, where the profile of the uneven wall is assumed to be periodic in the direction of the mean flow.
Abstract: A steady viscous flow through a two-dimensional, infinite channel consisting of an uneven wall and a plane wall is theoretically investigated under a given pressure gradient. The profile of the uneven wall is assumed to be periodic in the direction of the mean flow. A systematic expansion procedure is developed for the case where the ratio of the mean channel width to the period of the uneven wall, k, is small. An approximate solution is obtained up to the order of k2 . As a result, the flow rate is explicitly found as a function of the profile of the uneven wall, k and the Reynolds number. The unevenness of the wall always decreases the flow rate in proportion to k2 . The inertia effect on the flow rate first arises from the second order of k. The flow rate and the stream line are concretely obtained for the case of a sinusoidal wall. The approximate solutions show in good agreement with the solutions obtained by a numerical calculation.
TL;DR: In this article, a hybrid of hydrodynamics and kinetics was used to study the effect of finite plasma pressure on the pressuregradient driven toroidal drift modes and the characteristic frequencies, growth rates, and polarization of the electromagnetic modes were investigated as functions of the parameters of toroidicity, plasma gradients, and plasma pressure.
Abstract: A hybrid of hydrodynamics and kinetics is used to study the effect of finite plasma pressure on the pressure‐gradient driven toroidal drift modes. The linear drift modes of the system are given by a fifth‐order polynomial describing the coupling of the electron‐drift, the ion‐acoustic, and the shear Alfven oscillations. The characteristic frequencies, growth rates, and polarization of the electromagnetic modes are investigated as functions of the parameters of toroidicity, plasma gradients, and plasma pressure.
01 Jan 1983
TL;DR: In this article, a series of low pressure drop, permeable surfaces have been conducted to characterize their surface interaction with a turbulent boundary layer, and direct drag balance measurements of skin friction indicate that the general effect of surface permeability is to increase drag above that of a smooth plate reference level.
Abstract: Experimental boundary-layer studies of a series of low pressure drop, permeable surfaces have been conducted to characterize their surface interaction with a turbulent boundary layer. The models were flat and tested at nominally zero pressure gradient in low speed air. The surfaces were thin metal sheets with discrete perforations. Direct drag balance measurements of skin friction indicate that the general effect of surface permeability is to increase drag above that of a smooth plate reference level. Heuristic arguments are presented to show that this type of behavior is to be expected. Other boundary-layer data are also presented including mean velocity profiles and conditionally sampled streamwise velocity fluctuations (hot wire) for selected models.
TL;DR: In this article, an extended study of the gas jet developed above the pressure limiting aperture is described, and the influence of the jet deflectors to control the effects of this jet on the microscope system is quantitatively using a specifically designed apparatus.
TL;DR: In this article, an axisymmetric magnetic mirror with large diameter and easily variable mirror ratio is described, and the magnetic field configuration is a simple mirror with an average minimum −B field region on the outer surface.
Abstract: An axisymmetric magnetic mirror with large diameter and easily variable mirror ratio is described. The magnetic field configuration is a simple mirror with an average minimum‐B field region on the outer surface. Experimental results are given which demonstrate that the surface field region ensures interchange stability. Stability can be maintained with both positive and negative radial plasma pressure gradients provided that the product of the pressure gradient and specific flux volume gradient is positive.
TL;DR: In this article, the authors developed and tested a three-dimensional model of the correlation between velocity and pressure gradient which is of interest in stress equation modeling of turbulent flows, for which prior models, based on constant density turbulence, have some shortcomings.
Abstract: The paper develops and tests a three-dimensional model of the correlation between velocity and pressure gradient which is of interest in stress equation modeling of turbulent flows. In particular, the correlation is developed with reference to variable density reacting flows, for which prior models, based on constant density turbulence, have some shortcomings. The model is tested against data on H2-air diffusion flames, using a form of the developed correlation appropriate for k—ϵ modeling of the flame. It is found that inclusion of the correlation is imperative for satisfactory predictions of some of the flame features and the current model is satisfactory for such predictions.
Patent•
08 Jun 1983
TL;DR: In this paper, the specific permeability of the bed and the pressure gradient are selected so that in combination they result in an equivalent average wall shear stress of about 1 dyn/cm2 or less.
Abstract: Apparatus for rapidly measuring blood viscosity including a hollow column (12) of narrow bore in fluid communication with a chamber (14) containing a porous bed (16) and means for measuring blood flow rate (54, 56, 60, 62) within the column (12). The specific permeability of the bed (16) and the pressure gradient are selected so that in combination they result in an equivalent average wall shear stress of about 1 dyn/cm2 or less.
TL;DR: In this article, the magnetic field of a spheromak was analyzed and compared with expectations for the ratio of δ( √ √ n/B) from the pressure-gradient-free Taylor model and a model with pressure due to Morikawa.
Abstract: Spatially resolved measurements of the magnetic field of a spheromak have been analyzed and compared with expectations for the ratio of $\frac{{j}_{\ensuremath{\parallel}}}{B}$ from the pressure-gradient-free Taylor model and a model with pressure due to Morikawa. Better agreement is found with the model containing finite pressure.
TL;DR: In this article, a two-dimensional model is combined with an analytical solution of the Ekman equations, which at each grid point provides an expression for the time-dependent flow at any depth in terms of a convolution integral over the sea surface slope and wind stress.
Abstract: Earlier models of the circulation in Bass Strait have been extended to include vertical structure. Time- dependent circulation fields in Bass Strait, induced by wind driving at the surface and tidal oscillations along open-sea boundaries, are computed at a number of selected depths. The original two-dimensional model is combined with an analytical solution of the Ekman equations, which at each grid point provides an expression for the time-dependent flow at any depth in terms of a convolution integral over the sea- surface slope and wind stress. This model should be applicable to winter conditions when the strait is well mixed vertically and hence the dynamical effects of density stratification negligible. The predicted wind-induced circulation fields are highly depth dependent, with equilibrium surface currents in the central Bass Strait basin flowing in a direction approximately 45o to the left of the wind. At lower levels, currents are controlled by pressure gradient forces due to the sea-surface slope and friction. Significant upwelling and downwelling motions along the Victorian and Tasmanian coastlines can be inferred from these circulation fields. In the deep water off the continental shelf, currents in the upper 100 m are dominated by the (Ekman) drift current which rotates in an anticlockwise direction with increasing depth, such that the wind drift at the surface is accompanied by a measure of return flow at depth. Tidal currents are predicted in the absence of wind stress, but include the effects of bottom topography. Considerable variation with depth is found and the distinctive features are explained in terms of the relative importance of Coriolis force, bottom friction, and water depth. Comparison with the few existing observations reveals that the present model is producing realistic results.
01 Jan 1983
TL;DR: In this article, an experimental investigation of two phase flow through porous layers formed of non-heated glass particles (nominal diameter 1 to 6 mm) has been made, where the effect of particle size, particle size distribution and bed porosity on void fraction and pressure drop through a cylindrical test section has been investigated.
Abstract: An experimental investigation of two phase flow through porous layers formed of non-heated glass particles (nominal diameter 1 to 6 mm) has been made. Particulate bed depths of 30 cm and 70 cm were used. The effect of particle size, particle size distribution and bed porosity on void fraction and pressure drop through a particulate bed formed in a cylindrical test section has been investigated. The superficial velocity of liquid (water) is varied from 1.83 to 18.3 mm/s while the superficial velocity of gas (air) is varied from 0 to 68.4 mm/s. These superficial velocities were chosen so that pressure drop and void fraction measurement could be made for the porous layer in fixed and fluidized states. A model based on drift flux approach has been developed for the void fraction. Using the two phase friction pressure drop data, the relative permeabilities of the two phases have been concluded with void fraction. The void fraction and two phase friction pressure gradient in beds composed of mixtures of spherical particles as well as sharps of different nominal sizes have also been examined. It is found that the models for single size particles are also applicable to mixtures of particles if amore » mean particle diameter for the mixture is defined.« less
TL;DR: In this paper, the ability of 10 near wall similarity models to describe the near wall velocity field for the measured flow under a wide range of skewing conditions and a variety of pressure gradient and wall shear vector orientations was used.
Abstract: Mean velocity, measured wall pressure and wall shear stress fields were made in a three dimensional pressure-driven turbulent boundary layer created by a cylinder with trailing edge placed normal to a flat plate floor. The direct force wall shear stress measurements were made with floating element direct force sensing shear meter that responded to both the magnitude and direction of the local wall shear stress. The ability of 10 near wall similarity models to describe the near wall velocity field for the measured flow under a wide range of skewing conditions and a variety of pressure gradient and wall shear vector orientations was used.
TL;DR: In this article, the authors developed and tested a numerical model to predict diurnal cycles of boundary-layer flows of synoptic horizontal scale above sloping terrain, where the truncation errors of the pressure gradient term are of special concern.
Abstract: We document the development and Sensitivity testing of a numerical model designed to predict diurnal cycles of boundary-layer flows of synoptic horizontal scale above sloping terrain. This application requires detailed vertical resolution of low-level jets, mixed layers and sharp inversions above terrain with large pointwise variations. The truncation errors of the pressure gradient term are of special concern. This term is computed using a thermodynamic state carried as a deviation from a standard state. Turbulent mixing is based upon a simplified diagnostic treatment. The final model produces rather stable solutions because of the rather careful pressure gradient calculation and strong physical dissipation. Sensitivity testing indicates that the model prediction of diurnal convergence cycles depends upon soil parameters. Results are also sensitive to absolute rotation and mixing parameterizations, but not equally sensitive to longwave radiative flux divergence. Applications over complex North A...