Showing papers on "Pressure gradient published in 1989"

Gas-particle flow in a vertical pipe with particle-particle interactions

Abstract: A theory is presented for the fully-developed flow of gas and particles in a vertical pipe. The relation between gas pressure gradient and the flow rates of the two phases is predicted, over the whole range of cocurrent and countercurrent flows, together with velocity profiles for both phases and the radial concentration profile for the particles. The gas and the particles interact through a drag force depending on their relative velocity, and there are mutual interactions between pairs of particles through inelastic collisions. This model is shown to account for marked segregation of gas and particles in the radial direction, and the predicted relation between the pressure gradient and the flow rates of the two phases is surprisingly complex.

Near-wall structure of a turbulent boundary layer with riblets

Abstract: A detailed wind tunnel study has been carried out on the near-wall turbulence structure over smooth and riblet wall surfaces under zero pressure gradient. Time-average quantities as ‘well as conditionally sampled profiles were obtained using hotwire/film anemometry, along with a simultaneous flow visualization using the smoke-wire technique and a sheet of laser light. The experimental results indicated a significant change of the structure in the turbulent boundary layer near the riblet surface. The change was confined within a small volume of the flow close to the wall surface. A conceptual model for the sequence of the bursts was then proposed based on an extensive study of the flow visualization, and was supported by the results of conditionally sampled velocity fields. A possible mechanism of turbulent drag reduction by riblets is discussed.

On the structure of pressure fluctuations in simulated turbulent channel flow

Abstract: Pressure fluctuations in a turbulent channel flow are investigated by analyzing a database obtained from a direct numerical simulation. Detailed statistics associated with the pressure fluctuations are presented. Characteristics associated with the rapid (linear) and slow (nonlinear) pressure are discussed. It is found that the slow pressure fluctuations are larger than the rapid pressure fluctuations throughout the channel except very near the wall, where they are about the same magnitude. This is contrary to the common belief that the nonlinear source terms are negligible compared to the linear source terms. Probability density distributions, power spectra, and two-point correlations are examined to reveal the characteristics of the pressure fluctuations. The global dependence of the pressure fluctuations and pressure-strain correlations are also examined by evaluating the integral associated with Green's function representations of them. In the wall region where the pressure-strain terms are large, most contributions to the pressure-strain terms are from the wall region (i.e., local), whereas away from the wall where the pressure-strain terms are small, contributions are global. Structures of instantaneous pressure and pressure gradients at the wall and the corresponding vorticity field are examined.

Slow spreading of a sheet of Bingham fluid on an inclined plane

Abstract: To study the dynamics of fluid mud with a high concentration of cohesive clay particles, we present a theory for a thin sheet of Bingham-plastic fluid flowing slowly on an inclined plane. The physics is discussed on the approximate basis of the lubrication theory. Because of the yield stress, the free surface need not be horizontal when the Bingham fluid is in static equilibrium, nor parallel to the plane bed when in steady flow. We then show that there is a variety of gravity currents that can advance at a constant speed and with the same profile. Experimental confirmation of one type is presented. By solving a nonlinear partial differential equation, transient flows due either to a steady upstream discharge or to the sudden release of a finite fluid mass on another fluid layer are studied. In the first case there is a mud front which ultimately propagates as a constant speed as a steady gravity current. In the second case, when the ambient layer is sufficiently shallow that there is no initial motion, the flow induced by the new fluid can terminate after the disturbance has travelled a finite distance. The extent of the final spread is examined. Disturbances due to an external pressure travelling parallel to the free surface are also examined. It is found in particular that a travelling localized pulse of pressure gradient not only generates a localized mud disturbance which travels along with the forcing pressure, but further leaves behind a permanent footprint.

Effects of heat release in a turbulent, reacting shear layer

Abstract: Experiments were conducted to study the effects of heat release in a planar, gas-phase, reacting mixing layer formed between two free streams, one containing hydrogen in an inert diluent, the other, fluorine in an inert diluent. Sufficiently high concentrations of reactants were utilized to produce adiabatic flame temperature rises of up to 940 K (corresponding to 1240 K absolute). The temperature field was measured at eight fixed points across the layer. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from Pitot-probe dynamic pressure measurements. The results showed that the growth rate of the layer, for conditions of zero streamwise pressure gradient, decreased slightly with increasing heat release. The overall entrainment into the layer was substantially reduced as a consequence of heat release. A posteriori calculations suggest that the decrease in layer growth rate is consistent with a corresponding reduction in turbulent shear stress. Large-scale coherent structures were observed at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease was more than the corresponding decrease in shear-layer growth rate, and suggests that the mechanisms of vortex amalgamation are, in some manner, inhibited by heat release. The mean temperature rise profiles; normalized by the adiabatic flame temperature rise, were not greatly changed in shape over the range of heat release of this investigation. A small decrease in normalized mean temperature rise with heat release was however observed. Imposition of a favourable pressure gradient in a mixing layer with heat release resulted in an additional decrease in layer growth rate, and caused only a very slight increase in the mixing and amount of chemical product formation. The additional decrease in layer growth rate is shown to be accounted for in terms of the change in free-stream velocity ratio induced by the pressure gradient.

Stability and equation of state of CaSiO3-perovskite to 134 GPa

Abstract: The stability and P-V equation of state of CaSiO3 have been investigated using in situ diamond-anvil X ray diffraction techniques to 134 GPa, a pressure equivalent to that at the core-mantle boundary. Samples were heated by YAG laser at each pressure increment at high pressures to accelerate phase transitions. X ray diffraction measurements were carried out at 300 K using both energy-dispersive synchrotron and sealed-tube film techniques. Quenched CaSiO3-perovskite was observed to remain metastable close to 0.1 MPa, and to convert rapidly to an amorphous phase on pressure release. The simple cubic perovskite phase of CaSiO3 was found to be the stable phase for all lower mantle pressure conditions. All 47 P-V data points were used to obtain a third-order Birch-Murnaghan equation of state with zero-pressure parameters: unit cell volume V0 = 45.37±(0.08) A3, density ρ0 = 4.252(±0.008) Mg/m3, and bulk modulus K0 = 281(±4) GPa, with an assumed bulk modulus pressure K0′ = 4. These parameters are close to those of (Mg0.88Fe0.12)SiO3-perovskite and to those inferred by the Preliminary Reference Earth Model for the lower mantle. Hence, CaSiO3 must be considered an invisible component, in terms of density and bulk modulus constraints, in the lower mantle. Mantle composition models with both high and low calcium content can satisfy existing seismological constraints for the lower mantle.

Peristaltic Transport of a Particle-Fluid Suspension

Abstract: Peristaltic pumping by a sinusoidal traveling wave in the walls of a two-dimensional channel filled with a viscous incompressible fluid in which are distributed identical rigid spherical particles, is investigated theoretically. A perturbation solution is obtained which satisfies the momentum equations for the case in which amplitude ratio (wave amplitude/channel half width) is small. The results show that the fluid phase mean axial velocity decreases with increase in the particle concentration. The phenomenon of reflux (the mean flow reversal) is discussed. A reversal of velocity in the neighborhood of the centerline occurs when the pressure gradient is greater than that of the critical reflux condition. It is found that the critical reflux pressure is lower for the particle-fluid suspension than for the particle-free fluid. It is further observed that the mean flow reversal is strongly dependent on the particle concentration and the presence of particles in the fluid favors the reversal flow. A motivation of the present analysis has been the hope that such a theory of two-phase flow process is very useful in understanding the role of peristaltic muscular contraction in transporting bio-fluid behaving like a particle-fluid mixture. Also the theory is important to the engineering applications of pumping solid-fluid mixtures by peristalsis.

Reynolds stress development in pressure-driven three-dimensional turbulent boundary layers

Abstract: The development of the Reynolds stress field was studied for flows in which an initially two-dimensional boundary layer was skewed sideways by a spanwise pressure gradient ahead of an upstream-facing wedge. Two different wedges were used, providing a variation in the boundary-layer skewing. Measurements of all components of the Reynolds stress tensor and all ten triple products were measured using a rotatable cross-wire anemometer. The results show the expected lag of the shear stress vector behind the strain rate. Comparison of the two present experiments with previous data suggests that the lag can be estimated if the radius of curvature of the free-stream streamline is known. The magnitude of the shear stress vector in the plane of the wall is seen to decrease rapidly as the boundary-layer skewing increases. The amount of decrease is apparently related to the skewing angle between the wall and the free stream. The triple products evolve rapidly and profiles in the three-dimensional boundary layer are considerably different than two-dimensional profiles, leaving little hope for gradient transport models for the Reynolds stresses. The simplified model presented by Rotta (1979) performs reasonably well providing that an appropriate value of the T-parameter is chosen.

Horizontal-well pressure analysis

Abstract: This paper presents an analysis of the pressure-transient behavior of a horizontal well or a drainhole. The performances of horizontal wells and fully penetrating vertical fractures are compared. Dimensionless wellbore pressures are computed for two classic boundary conditions: infinite conductivity and uniform flux. Results are presented as pseudoskin factors and as type curves. In addition to conventional pressure-vs.-time type curves, derivative type curves from pressure/time predictions are presented. The derivative approach the authors discuss is applicable to a broader range of problems than considered here.

The Effect of Density Ratio on the Heat Transfer Coefficient From a Film Cooled Flat Plate

Abstract: The effect of density ratio of cooling films on the heat transfer coefficient on a flat plate is investigated using a heat-mass transfer analogy. The experimental technique employed uses a swollen polymer surface and laser holographic interferometry. A density ratio of 1.0 was achieved using air as the injectant. Density ratios of 1.38 and 1.52, representative of turbine operating conditions, were obtained by using foreign gases.The coolant fluids were injected at various blowing rates through a single normal hole or through a row of holes spaced at three-diameter intervals, and inclined at 35° or 90° to the mainstream direction. The experiments were conducted under isothermal conditions in a subsonic, zero mainstream pressure gradient turbulent boundary layer. The results indicated large differences in behaviour between the two injection angles. For normal injection, the heat transfer coefficient at a fixed blowing parameter was insensitive to the variation of density ratio, whereas for 35° injection strong dependence was observed. Scaling parameters for the heat transfer data have been proposed so that use can be made of data obtained at density ratios not representative of gas turbine practice. In addition, a correlation for normal injection data has been formulated.Copyright © 1989 by ASME

Atmospheric Structure and Momentum Balance during a Gap-Wind Event in Shelikof Strait, Alaska

Abstract: Gap winds occur in topographically restricted channels when a component of the pressure gradient is parallel to the channel axis. Aircraft flight-level data are used to examine atmospheric structure and momentum balance during an early spring gap-wind event in Shelikof Strait, Alaska. Alongshore sea level pressure ridging was observed. Vertical cross sections show that across-strait gradients of boundary-layer temperature and depth accounted for the pressure distribution. Geostrophic adjustment of the mass field to the along-strait wind component contributed to development of the observed pressure pattern. Boundary-layer structure and force balance during this event was similar to that often observed along isolated barriers. However, the Rossby radius was lager than the strait width, and atmospheric structure in the strait exit region indicates transition of the flow to open coastline conditions. Two across-strait momentum budgets show that the Coriolis force and across-strait pressure gradient w...

Modelling tidal energetics of the Columbia River Estuary

Abstract: The Columbia River Estuary is shallow and has a mixed diurnal and semidiurnal tide, strong riverflow, broad tidal flats and variable channel cross-section. It is weakly non-linear with respect to tidal forcing, as measured by a ratio of tidal amplitude to depth. These features, common to many shallow estuaries, were incorporated into a one-dimensional harmonic transport model. The harmonic method utilizes the fact that nearly all of the energy in the system is in a few fundamental tidal frequencies, the first overtides thereof and the mean flow. This allows the representation of the flow as a series of harmonic components whose spatial variability is determined by the model. The model provides a qualitative explantation for and accurate quantitative predictions of along-channel variations in tidal properties in terms of the momentum balance. Near the mouth of the estuary, the boundary shear stress, the pressure gradient and the acceleration terms are all important in the force balance, and the tide behaves like a damped oscillator. Far upriver the pressure gradient is primarily balanced by friction at the bed and tidal wave propagation can be described as a diffusion process. Changes in the channel width in mid-estuary cause partial reflection of the tidal wave and a maximum in tidal range. This partial reflection decreases the tidal energy flux in the upstream direction. The distribution of the time averaged energy fluxes was also determined from the model. The analysis shows that there are two regions of high energy dissipation. Energy is supplied near the mouth of the estuary by the divergence of the landward flux of tidal potential energy and far upriver by the divergence of the seaward flux of fluvial potential energy. These two regions of high dissipation are separated by a region of low dissipation, the energy flux divergence minimum. This spatial division of the estuary is mirrored in the system biology and geology. It is likely that the energy flux divergence minimum is a common feature of shallow estuaries with strong riverflow.

Perturbation by and recovery from bend curvature of a fully developed turbulent pipe flow

Abstract: An experimental investigation was carried out to study the perturbation by and recovery from bend curvature of a fully developed turbulent pipe flow. The pipe Reynolds number and the pipe‐to‐bend radius ratio were 50 000 and 0.077, respectively. Hot‐wire techniques were used to measure the three components of mean velocity and the six components of the Reynolds stress along a horizontal as well as along a normal plane in streamwise locations extending from 18 diameters upstream to 18 diameters downstream of the U‐bend. Results show that a Dean‐type secondary motion is established in the bend and is confirmed by a numerical modeling of the flow based on a two‐equation closure. Evidence of a second cell is obtained within the bend and again verified by numerical results and previous total pressure measurements. Therefore it is postulated that the pressure field is also responsible for the second cell. The second cell disappears at about one diameter downstream of the curved bend. Turbulent kinetic energy is greatly enhanced by additional production due to extra strain rates created by bend curvature. Since viscous dissipation is the only way in which this extra energy can be destroyed, recovery from bend curvature takes a long distance.

Zonal Momentum Balance at the Equator

Abstract: The conventional view of equatorial dynamics requires that the zonal equatorial wind stress be balanced, in the mean, by the vertical integral of “large-scale” terms, such as the zonal pressure gradient, mesoscale eddy flux, and mean advection, over the upper few hundred meters. It is usually presumed that the surface wind stress is communicated to the interior by turbulent processes. Turbulent kinetic energy dissipation rates measured at 140°W during the TROPIC HEAT I experiment and a production rate–dissipation rate balance argument have been used to calculate the zonal turbulent stress at 30 to 90 m depth. The calculated turbulent stress at 30 m depth amounts to only 20% of the wind stress and decreases exponentially with depth below 30 m. Typical large-scale estimates of the zonal pressure gradient, mesoscale eddy flux, and advection have a depth scale larger than the turbulent stress, and are inconsistent with the vertical divergence of the stress as estimated from the dissipation rate measu...

The Role of Valley Geometry and Energy Budget in the Formation of Nocturnal Valley Winds

Abstract: Diurnally varying up and down-valley winds are a commonly observed feature of mountain meteorology. These winds are produced through the heating and cooling of the land surface but direct connections from the topography to the winds have been difficult to establish. A concept has been proposed which theoretically relates the energy budget and valley geometry to the rate of atmospheric cooling in the valley. The gradient of the along-valley cooling rate will then lead to an along-valley pressure gradient which provides a topographic control of the wind. The ratio of valley width to cross-section area is shown to be the critical topographic parameter which is proportional to the valley cooling rate. Net radiation and the ground heat flux are also critical to the valley cooling rate. An example is given which illustrates that this new concept can produce pressure gradients about 60% larger than the mountain-plain mechanism. Observations of wind and temperature in three valleys in Colorado which incl...

Prospective Doppler echocardiographic evaluation of pulmonary artery diastolic pressure in the medical intensive care unit.

Abstract: To test the hypothesis that the noninvasive evaluation of pulmonary regurgitation can provide accurate estimates of pulmonary artery (PA) diastolic pressures and PA wedge pressures, Doppler echocardiographic studies were performed immediately before bedside PA catheterization in 29 medical intensive care unit patients. The characteristic color flow Doppler signal of pulmonary regurgitation was detected in 19 (66%) patients. In 17 of the 29 patients (59%), the gradient between the right ventricle and PA at end-diastole could be calculated from the pulsed-wave Doppler signal of pulmonary regurgitation using the simplified Bernoulli equation. Right atrial pressure was then estimated by examination of the jugular venous pulse or by electronic transduction of the pressure signal from a previously placed central venous catheter. A noninvasive estimate of PA diastolic pressure was made by adding the clinical estimate of right atrial pressure to the end-diastolic pressure gradient across the pulmonary valve. Pulmonary artery catheterization was then performed and stripchart recordings were interpreted by a physician who was unaware of the noninvasively-estimated PA diastolic pressure. The PA diastolic pressure estimated by Doppler echocardiography correlated closely with that found at catheterization (r = 0.94, mean absolute difference 3.3 mm Hg). The noninvasive estimate of PA diastolic pressure also correlated with the PA wedge pressure (r = 0.87, mean absolute difference 3.8 mm Hg). Therefore, in 59% of medical intensive care unit patients, Doppler echocardiographic evaluation of pulmonary regurgitation allowed accurate noninvasive estimation of PA diastolic pressure.

Thermal diffusivity induced by resistive pressure‐gradient‐driven turbulence

Abstract: The calculation of the thermal diffusivity induced by resistive pressure‐gradient‐driven turbulence is revisited. The relevant correlation length for the magnetic fluctuations is shown to be different from estimates made with a mixing‐length approach and with dimensional analysis. This leads to different scaling predictions for the electron heat diffusivity. The root of this difference is the fact that the flux and current perturbations are related by two scale lengths, the poloidal wavelength and the mode width.

The relaxation of a turbulent boundary layer in an adverse pressure gradient

Abstract: Reattached turbulent boundary layer relaxation downstream of a wall fence is investigated. An adverse pressure gradient is imposed upon it which is adjusted to bring the boundary layer into equilibrium. The pressure gradient is adjusted so as to bring the Clauser parameter G down to a value of about 11.4 and then maintain it constant. In the region from the reattachment point to 2 or 3 reattachment lengths downstream, the boundary layer recovers from the initial major effects of reattachment. Farther downstream, where G is constant, the pressure-gradient parameter changes very slowly and profiles of non-dimensionalized eddy viscosity appear self-similar. However, pressure gradient and eddy viscosity are both roughly twice as large as expected on the basis of previous equilibrium turbulent boundary layer studies.

A mathematical model for pressure evaluation in an infinite-conductivity horizontal well

Abstract: A mathematical model was developed to study the transient pressure behavior in a well with an infinite-conductivity horizontal drainhole in an infinite slab reservoir. The physical model includes a fluid of small and constant compressibility flowing through an infinitely large anisotropic reservoir with upper and lower impermeable boundaries. The analytical solution is obtained by applying the concepts of instantaneous sources and Green's functions. The authors discuss the uniform-flux model, a special case of the infinite-conductivity model, and present a simple way to use it to calculate the pressure at the wellbore face. They suggest that the pressure for the infinite-conductivity case can be evaluated with the uniform flux model at a fixed point along the wellbore with an error of less than 1% for combinations of the various parameters that may be encountered in real situations. They show to what extent the accuracy of the model will be affected by neglecting gravitational effects.

On the Pressure Gradient Force Error in σ-Coordinate Spectral Models

Abstract: The pressure gradient force error of the spectral technique used in combination with the σ vertical coordinate was examined in an idealized case of an atmosphere at rest and in hydrostatic equilibrium. Small-scale (one-point and three-point) mountains were used in the tests. With such definitions of topography, difficulties could be expected with the spectral method due to slow convergence of the Fourier series. For reference, the finite-difference pressure gradient force errors were also computed. In the examples considered, it. was found that the errors of the spectral method can be large. In the rms sense, the spectral pressure gradient force errors were larger than those of the finite-difference method.

Low-level wind profiles at an Antarctic coastal station

Abstract: Wind and temperature profiles in the lowest 2000 m of the atmosphere at Halley (75°35′S, 26°50′W) have been analysed. Surface winds blow most frequently from the sector 090° ± 45° but the 2000 m wind direction is much more evenly distributed and appears to be determined by synoptic-scale pressure gradients. A simple one-dimensional boundary layer model, which includes the effects of stably-stratified air overlying a sloping surface, is able to reproduce some of the features of the observed profiles.

Topology of a turbulent boundary layer with and without wall suction

Abstract: Measurements of velocity and temperature fluctuations are made in a turbulent boundary layer with nominally zero pressure gradient for two different slightly heated wall conditions: impermeable and porous surfaces. The temperature fluctuations are measured at three points in the flow to permit the identification of two spatially coherent events: coolings and heatings. Conditional velocity vectors in the plane of mean shear are viewed in a reference frame which translates at a constant velocity. Conditioning is on coolings, heatings or a combination of these events. Sectional streamlines, derived from the velocity vector data, show a succession of critical points: saddles and unstable foci. Coolings are aligned with diverging separatrices through the saddles whereas heatings are identified with the foci. Coolings are associated with a large strain rate and also a large spanwise vorticity: this result seems consistent with the presence of hairpin vortices which extend to different distances from the wall. In contrast, the strain rate and spanwise vorticity are small at the foci. The stabilising influence of suction is observed in the topology of the organised motion and in the contribution from this motion to the conventional stresses, temperature variance and heat fluxes.

Nucleation phenomena in flowing high-pressure steam: experimental results

Abstract: The paper describes the results of an experimental investigation of limiting supersaturation in high-pressure steam. It follows an earlier investigation and to avoid the uncertainties associated with leakage past sliding profiles, the test section has been redesigned and the measurements taken with fixed nozzles. Three convergent-divergent nozzles with nominal rates of expansion of 3000, 5000 and 10000 per second have been used and the inlet stagnation pressures cover the range 25–35 bar. The data consist mainly of axial pressure distributions but some droplet measurements have also been recorded.

Weakly nonlinear behavior of periodic disturbances in two‐layer Couette–Poiseuille flow

Abstract: The stability of a plane Couette–Poiseuille flow consisting of two layers of different fluids is analyzed using methods of bifurcation theory. The fluids have different viscosities and densities, and there is surface tension at the interface. The center manifold theorem is used to justify the derivation of the final amplitude evolution equation. The nonlinear calculations are carried out with two alternative approaches. One approach is to keep the combined volume flux fixed, and the other is to keep the pressure gradient in the horizontal direction fixed. Numerical results are presented for some Couette flow profiles and a Poiseuille flow profile at low speeds, showing that traveling waves are supported at the interface. A computation at a high speed is also presented. The derivation and numerical results are compared with those of a formal approach, employing multiple scales, which has been used on related problems.

Pressure gradient-driven modes in finite beta toroidal plasmas

Abstract: When the ion temperature gradient is finite, the ideal kinetic theory plasma destabilizes the MHD mode below the critical beta of MHD theory. For a temperature gradient eta i>2/3 and inverse aspect ratio epsilon (Ti)<0.35 the electrostatic toroidal eta i-mode is unstable. The coupling between the two modes is investigated by solving the fourth-order system describing the shear Alfven-drift wave and the ion acoustic wave in the finite beta Tokamak. The ion kinetic velocity integral including the gyroradius effects and the ion magnetic drift resonances are used to obtain gamma k(k,q, beta , eta i,s, epsilon n,q, tau ) and gamma k/(kx2) for the modes. The study emphasizes the beta - and q-dependence of the transport associated with gamma k/(kx2).

Origin of the Catalina Eddy

Abstract: During late springs through early fall the normal regime of low-level westerly and northwesterly flow within the bight of southern California is occasionally interrupted by periods of southerly flow, elevated marine layers, and increased low-level cloudiness. This transition, often termed the Catalina Eddy, is limited to narrow zone of ∼100 km from the coastal mountains and brings cooler temperatures and improved air quality. This paper describes the initiation and evolution of Catalina Eddy events using both composite and case study approaches. It is found that this phenomenon is produced by the interaction between the synoptic-scale flow and the formidable topography of the region. As a result of synoptic-scale pressure falls along the central California coast and/or mesoscale lee troughing southeast of Point Conception, an alongshore pressure gradient with lower pressure to the north becomes established. The result of such a pressure gradient in a coastal zone with an adjacent topographic barr...