Showing papers on "Pressure gradient published in 1990"

The structure of two-dimensional separation

Abstract: The separation of a two-dimensional laminar boundary layer under the influence of a suddenly imposed external adverse pressure gradient was studied by time-accurate numerical solutions of the Navier–Stokes equations. It was found that a strong adverse pressure gradient created periodic vortex shedding from the separation. The general features of the time-averaged results were similar to experimental results for laminar separation bubbles. Comparisons were made with the ‘steady’ separation experiments of Gaster (1966). It was found that his ‘bursting’ occurs under the same conditions as our periodic shedding, suggesting that bursting is actually periodic shedding which has been time-averaged. The Strouhal number based on the shedding frequency, local free-stream velocity, and boundary-layer momentum thickness at separation was independent of the Reynolds number and the pressure gradient. A criterion for onset of shedding was established. The shedding frequency was the same as that predicted for the most amplified linear inviscid instability of the separated shear layer.

Friction, overpressure and fault normal compression

Abstract: More than twenty-five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thickness of fault gouge. For example, a clay-filled fault zone about 1 km wide at the base of the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near-zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near-lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will not occur.

Analysis of Turbulence Structure in the Surface Layer with a Modified Similarity Formulation for Near Neutral Conditions

Abstract: Data from a recent detailed surface layer experiment are critically examined in terms of the turbulent kinetic energy budget and the other second order moment budgets formed by the three velocity components and temperature. In moderately unstable and slightly stable conditions nondimensional terms of all the moment budgets studied agree reasonably well with results reported from the Kansas study (after application of a flow distortion correction). In the near-neutral range, where the present experiment contains a large amount of data, results deviate significantly from previous studies in general and, in particular, for ideal, zero-pressure gradient turbulent boundary layers. Several moments, such as u2W, v2w and W2 are not, as expected, constant in the surface layer, but vary logarithmically with height, making instead their non dimensional vertical gradients constant. Some moments scale with the roughness length and others with a length scale containing the large-scale pressure gradient or, wit...

Percolation theory of creation and mobilization of foams in porous media

Abstract: A percolation model of foam mobilization in porous media is developed. This model indicates that there is a minimum pressure gradient or, equivalently, a minimum gas velocity required to initiate mobilization of foam. As a result, for most foam enhanced oil recovery processes, where the surface tension is not low, deep foam penetration depends on propagation of foam formed at a high pressure gradient near the well. Low surface tension makes mobilization of CO2 foams feasible, however, at pressure gradients found throughout much of the formation in a typical field application. The theory further predicts, and data confirm, that the minimum velocity for foam mobilization during steady flow of liquid and gas decreases as injected liquid volume fraction increases. The theory suggests a better strategy for foam generation: alternate injection of small slugs of liquid and gas.

An improved k-e model for boundary layer flows

Abstract: An improvement of the k-e model has been made in conjunction with an accurate prediction of the near-wall limiting behaviour of turbulence and the final period of the decay law of free turbulence. The present improved k-e model has been extended to predict the effects of adverse pressure gradients on shear layers, which most previously proposed models failed to do correctly. The proposed model was tested by application to a turbulent pipe flow, a flat plate boundary layer, a relaminarising flow and a diffuser flow with a strong adverse pressure gradient. Agreement with the experiments was generally very satisfactory.

Mechanics of local scour around submarine pipelines

Abstract: The primary objective of this study is to improve understanding of the mechanism causing scour in unidirectional current. Experiments have shown how local scour develops around submarine pipelines in noncohesive sediments. The study shows that piping is the dominant cause of the initiation of scour. Piping and the stagnation eddy combine to undermine the pipeline, and mark the onset of scour. The critical hydraulic gradient associated with the initiation of scour is equal to the flotation gradient of the bed sediment. The pressure drop between the stagnation pressure upstream and wake pressure downstream of the pipe induces this hydraulic gradient. When a pipe is just embedded, the onset of scour does not occur if the ratio of the flow depth to pipe diameter exceeds 3.5. Similarly, the onset of scour does not occur for half‐buried pipes. The reduction in pressure gradient across the pipeline for these flow/pipe combinations accounts for the lack of scour. The onset of scour can be prevented by placing an ...

The influence of Reynolds number upon the apparent permeability of spatially periodic arrays of cylinders

Abstract: Flow fields within spatially periodic arrays of cylinders arranged in square and hexagonal lattices are calculated, with microscale Reynolds number ranging between zero and 200, employing a finite element numerical scheme. The terminology of an ‘‘apparent permeability’’ is introduced to establish a relationship existing between mean velocity and macroscopic pressure gradient characterized by a finite Reynolds number flow. In contrast with the low Reynolds number ‘‘true ’’ permeability, the apparent permeability is shown here to generally depend upon the direction of the applied pressure gradient, owing to nonlinearities existing within the local fluid motion. The orientation‐dependent permeabilities of both square and hexagonal monodisperse arrays are observed to diminish with increasing Reynolds number. Similar behavior is also observed for a bidisperse square array, though the apparent permeability of the latter is shown less sensitive to Darcy velocity orientation at large Reynolds numbers in comparison to the corresponding monodisperse square array, for all cylinder concentrations examined.

Air flow over and through a forest edge: A steady-state numerical simulation

Abstract: A numerical model was developed to simulate neutrally stratified air flow over and through a forest edge. The spatially averaged equations for turbulent flow in vegetation canopies are derived as the governing equations. A first-order closure scheme with the capability of accounting for the bulk momentum transport process in vegetation canopies is employed. The averaged equations are solved numerically by a fractional time-step method and successive relaxation. The asymptotic solution in time is regarded as the steady-state solution. Comparisons of model output to the field measurements of Raynor (1971) indicate that the model provides a realistic mean flow. Momentum balance computations show that the pressure gradient induced by the wind blowing against the forest edge is significant and has the same order of magnitude as the drag force in the edge region. The edge effect involves the generation of drag forces, the appearance of a large pressure gradient, the upward deflection of mean flow and the transport of momentum into the edge of the canopy.

The Influence of Curvature on Film Cooling Performance

Abstract: The effects of injection rate and strength of curvature on film cooling performance of gas injected through a row of holes on a convex surface is studied. Comparisons are made to film cooling of concave and flat surfaces. Three different relative strengths of curvature (ratio of radius of curvature to radius of injection hole), two density ratios (0.95 and 2.0), and a wide range of blowing rates (0.3 to 2.7) are considered. A foreign gas injection technique (mass transfer analogy) is used. The strength of curvature was controlled by varying the injection hole diameter. At low blowing rates, film cooling is more effective on the convex surface than on a flat or a concave surface. The cross stream pressure gradient present in curved flows tends to push the jet into the convex wall. As the injection rate is increased, normal and tangential jet momentum promote lift-off from the convex surface, thereby lowering performance. In contrast, previous studies show that on a concave surface, tangential jet momentum, flow instabilities, and blockage improve performance on a concave surface as blowing rate is increased.Copyright © 1990 by ASME

Darcy's law for slow viscous flow past a stationary array of bubbles

Abstract: We examine slow viscous flow past a concentrated bed of small stationary viscous bubbles of a second fluid, and derive Darcy's law relating the average fluid velocity to the overall pressure gradient and body force.

A supersonic turbulent boundary layer in an adverse pressure gradient

Abstract: This investigation describes the effects of an adverse pressure gradient on a flat plate supersonic turbulent boundary layer (Mf ≈ 2.9, βx ≈ 5.8, Reθ, ref ≈ 75600). Single normal hot wires and crossed wires were used to study the Reynolds stress behaviour, and the features of the large-scale structures in the boundary layer were investigated by measuring space–time correlations in the normal and spanwise directions. Both the mean flow and the turbulence were strongly affected by the pressure gradient. However, the turbulent stress ratios showed much less variation than the stresses, and the essential nature of the large-scale structures was unaffected by the pressure gradient. The wall pressure distribution in the current experiment was designed to match the pressure distribution on a previously studied curved-wall model where streamline curvature acted in combination with bulk compression. The addition of streamline curvature affects the turbulence strongly, although its influence on the mean velocity field is less pronounced and the modifications to the skin-friction distribution seem to follow the empirical correlations developed by Bradshaw (1974) reasonably well.

Pressure‐Balanced structures between 1 AU and 24 AU and their implications for solar wind electrons and interstellar pickup ions

Abstract: 'Pressure-balanced structures' (PBSs) in the heliosphere are microscale phenomena with a characteristic length along the radial dimension of the order of 0.05 AU across which the sum of the thermal pressures and the magnetic pressure is constant. PBSs have been identified in the Voyager data from 1 AU to 24 AU. If the density of pickup ions were proportional to the proton density across a PBS, and if the electron pressure were negligible, then from measurements of the magnetic field and the proton density and temperature across pressure balanced structures a pickup ion density is derived which is significantly smaller than the density predicted by a model based on the same assumption. This suggests that there are processes which 'smear out' the pressure of the pickup ions so that it is nearly constant on the scale of a PBS. If the pressure of the pickup ions is uniform across a PBS, then one can determine the electron temperature from the pressure balance condition.

On the evolution of the turbulent spot in a laminar boundary layer with a favourable pressure gradient

Abstract: The evolution of a turbulent spot in an accelerating laminar boundary-layer flow was investigated. The type of boundary layer chosen for this experiment resembles in every respect the flow in the vicinity of a stagnation point theoretically described by Falkner and Skan. The rate of growth of the spot was significantly inhibited by the favourable pressure gradient in all three directions. It became much shorter and narrower in comparison with a similar spot generated in a Blasius boundary layer at comparable distances from its origin and comparable Reynolds numbers. The celerities of its boundaries did not scale with the local free-stream velocity as they do in the absence of a pressure gradient. Dimensional analysis was used to identify and correlate the independent variables determining the size, the convection speed, and the relative rate of growth of this spotThe familiar arrowhead shape of the spot gave way to a rounded triangular shape with the trailing interface being straight and perpendicular to the direction of streaming. The familiar Tollmien-Schlichting wave packet was not observed in this pressure gradient because the surrounding boundary layer was very stable at the Re considered. Since the arrowhead shape of the spot is associated with the breakdown of the waves within the packet it cannot occur below the critical Re. The relative size of the ‘calmed region’ following the spot also diminished; however, one could only speculate as to the origin of this region.

Film Cooling in the Presence of Mainstream Pressure Gradients

Abstract: Film-cooling in the presence of mainstream pressure gradients typical of gas turbines has been studied experimentally on a flat plate This paper describes, measurements of the spanwise averaged effectiveness and heat transfer coefficient for an inclined slot and a single row of holes in the presence of favourable, zero and adverse pressure gradients. Acceleration parameters of K = 2.62×10−6 and - 0.22 × 10−6 were achieved at the point of injection where the freestream unit Reynolds number was held constant at Re/m = 2.7 × 107. The flow was accelerated to high Mach number and results are analysed using a superposition model of film-cooling which included the effects of viscous energy dissipation. The experimental results show the effects of pressure gradient differ between the geometries and a discussion of these results is included. The unblown turbulent boundary layer with pressure gradient were also studied. Experiments were performed using the Isentropic Light Piston Tunnel, a transient facility which enables conditions representative of those in the engine to be attained.Copyright © 1990 by ASME

Lubricated pipelining. Part 3 Stability of core-annular flow in vertical pipes

Abstract: The stability of core-annular flow in vertical pipes is analysed using the linearized theory of stability. In previous studies instabilities due to interfacial friction, interfacial tension and Reynolds stresses in the bulk fluid were identified and associated with observed instabilities. In this study we include and analyse the effects of gravity. In one case gravity opposes and in the other aids the applied pressure gradient. Some preliminary results from our experiments are also presented. The prediction of stability for perfect core-annular flow in a carefully selected window of parameters is verified for the case of free fall in which the applied pressure gradient vanishes.

Numerical simulations of ion temperature gradient‐driven turbulence

Abstract: High resolution numerical simulations of plasma turbulence driven by ion temperature gradients in the presence of magnetic field inhomogeneities have been performed with special attention to the behavior of the anomalous ion energy flux. The pressure gradient evolution is treated consistently with energy transport, allowing for the study of the saturated state in situations of relevance to tokamak plasmas. It is found that the presence of large‐scale coherent structures significantly affects the turbulent losses, leading to a reduction of the flux with respect to mixing length estimates.

Effects of Adverse Pressure Gradients on the Nature and Length of Boundary Layer Transition

Abstract: Existing transition models are surveyed and deficiencies in previous predictions, which seriously overestimate transition length under an adverse pressure gradient, are discussed. A new model for transition in an adverse pressure gradient situation is proposed and experimental results are provideed that confirm its validity. A correlation for transition lenght is advanced that incorporates both Reynolds number and pressure gradient effects

Minimum pressure gradient for foam flow in porous media : effect of interactions with stationary lamellae

Abstract: As foam bubbles flowing through a porous medium at low pressure gradients pass trapped bubbles in adjacent pores, the moving lamellae interact with the stationary lamellae that separate the flowing from the trapped bubbles. These interactions distort the shapes of the flowing bubbles and thereby alter the capillary resistance to their flow. For incompressible foams at high capillary pressure, these interactions reduce the minimum pressure gradient required to maintain foam flow, (∇p)min; they may, in fact, reduce (∇p)min nearly to zero for these foams. We base this conclusion on a comparison of a two-dimensional (2-D) model for lamella shapes with observations of three-dimensional (3-D) lamellae in a model pore. The simplified 2-D model offers insights into the interactions between moving and stationary lamellae, but gives incorrect estimates of the effect of these interactions on (▿p)min.

Instability waves in twin supersonic jets

Abstract: Calculations are presented for the characteristics of instability waves in the initial mixing region of twin circular supersonic jets. Two models for the basic jet flow are used. In the first, the jets are modeled as two circular vortex sheets. In the second, realistic velocity and density profiles are used. It is shown that the unsteady flow fields of the two jets interact before the time-averaged jets flows have merged. The normal modes or instability waves are classified by their symmetry properties in the twin-jet case and their asymptotic behavior for large jet separations. Calculations of the growth rates and phase velocities are made for these modes as a function of jet separation and mixing-layer thickness. The associated pressure distributions are also presented. In the realistic jet profile calculations the effect of jet separation is found to be relatively weak. For modes that are even about the symmetry plane between the two jets the pressure levels are found to increase near this plane as the jet separation decreases.

Influence of shape and skin of matrix-rock blocks on pressure transients in fractured reservoirs

Abstract: A formulation of pressure transients in terms of the intrinsic, or core, properties of the two media that compose the fractured reservoir, establishes the influence of these properties, and reciprocally, their corroboration from - the pressure-time relationship observed in well tests and interference tests. The following reservoir characteristics are analyzed: the area of fractures transverse to flow; the dimensions, shape and properties of rectangular parallelepiped matrix-rock blocks; and a permeability reduction in the blocks surface. A restatement of the so-called pseudo-steady state inter-media flow gives to parameters alfa and lambda in the theory of a previous study the physical meaning they lacked, and allows a direct determination of the blocks minimum dimension.

On the generation of helical waves in circular pipe flow

Abstract: The question of the existence of helical waves in fluid flow through a circular pipe is discussed. In the case of pipe flow forced only by an axial pressure gradient, direct simulations have failed to find any self‐sustaining helical waves. The effect of residual swirl in the inlet flow is investigated, by performing linear stability calculations and direct simulations of a forced basic flow. The results are qualitatively similar to that of flow in a rotating pipe, in which stable helical waves are known to exist. These results suggest that residual rotation may be an important mechanism in triggering laminar‐turbulent transition in circular pipe flow.

Effects of Pressure Gradient on Reattaching Flow Downstream of a Rearward-Facing Step

Abstract: Reattachment problems of separated flow on a solid surface such as an airfoil, diffuser, cavity wall, etc. are of interest because a rapid rise of pressure and heat transfer takes place at the reattachment zone. Among the solid surface models for the study of the separated flow reattachment, the rearward-facing step is one of the simplest since the separation point is readily known. A number of investigations for the rearward-facing step is one of the simplest since the separation point is readily known. A number of investigations for the rearward-facing step flow showed that the reattachment length is strongly affected by the pressure gradient. This paper presents the experimental results of reattachment length and wall static pressure distribution affected by the constant streamwise pressure gradients.

Significance of the Doppler-derived gradient across a residual aortic coarctation

Abstract: Few data exist which address the significance of the Doppler gradient across a residual narrowing in older children who have had a coarctation repaired. Therefore, we evaluated 11 patients with repaired aortic coarctation with and without residual obstruction by Doppler echocardiography. The Doppler-derived transcoarctation pressure gradient correlated poorly with catheter-measured peak-to-peak and catheter maximal instantaneous gradients when only the maximal velocity across the repair was utilized in the simplified Bernoulli equation, [r=0.73, standard of error of the estimate (SEE)=5.0 mmHg andr=0.56, SEE=7.6 mmHg, respectively]. However, when the precoarctation velocity was included in the simplified Bernoulli equation, the correlation between Doppler-derived and catheter-measured gradients became excellent. The maximal Doppler gradient correlated well with catheter peak-to-peak gradient (r=0.95, SEE=2.2 mmHg) and catheter maximal instantaneous gradient (r=0.94, SEE=3.2 mmHg). However, the maximal Doppler gradient slightly overestimated the catheter peak-to-peak gradient and underestimated the catheter maximal instantaneous gradient. The Doppler mean gradient showed excellent correlation with the catheter mean gradient (r=0.97, SEE=0.85 mmHg). Precoarctation velocities were generally twofold greater than published normals due to a spatial acceleration phenomenon. All subjects had residual hypoplasia of the transverse aorta such that its transverse diameter was 29% less than, and its crosssectional area was 50% less than, the ascending and descending aorta. Thus, it is apparent that proximal velocities must be accounted for in the modified Bernoulli equation in order to achieve an accurate gradient determination, and that this gradient is representative of a value that is between the maximal instantaneous and peak-to-peak catheter gradient. Flow acceleration within the transverse aorta may be related to significant reductions in luminal radius and crosssectional area.

Supersonic gas-particle two-phase flow around a sphere

Abstract: This paper describes supersonic flows of a gas-particle mixture around a sphere. The Euler equations for a gas-phase interacting with a particle one are solved by using a TVD (Total Variation Diminishing) scheme developed by Chakravarthy & Osher, and the particle phase is solved by applying a discrete particle-cloud model. First, steady two-phase flows with a finite loading ratio are simulated. By comparing in detail the dusty results with the dust-free ones, the effects of the presence of particles on the flow field in the shock layer are clarified. Also an attempt to correlate the particle behaviours is made with universal parameters such as the Stokes number and the particle loading ratio. Next, non-steady two-phase flows are treated. Impingement of a large particle-cloud on a shock layer of a dust-free gas in front of a sphere is numerically simulated. The effect of particles rebounded from the sphere is taken into account. It is shown that a temporal reverse flow region of the gas is induced near the body axis in the shock layer, which is responsible for the appearance of the gas flow region where the pressure gradient becomes negative along the body surface. These phenomena are consistent with the previous experimental observations. It will be shown that the present results support a flow model for the particle-induced flow field postulated in connection with ‘heating augmentation ’ found in the heat transfer measurement in hypersonic particle erosion environments. The particle behaviour in such flows is so complicated that it is almost impossible to treat the particle phase as an ordinary continuum medium.

Flow of Non-Newtonian Fluids Through Eccentric Annuli

Abstract: The laminar eccentric annular flow of non-Newtonian fluids is analyzed with a new method where an eccentric ammulus is represented by an infinite number of concentric annuli with variable outer radii. The analytical solutions for the shear stress, shear rate, velocity, and volumetric flow rate/pressure gradient are obtained for both power-law and Binham-plastic fluids. This method is shown to provide more accurate approximations for various profiles and good predictions of the volumetric flow rate/pressure gradient in eccentric annular flow. In addition, turbulent eccentric annular flow is discussed.

Flow in curved ducts. Part 2. Rotating ducts

Abstract: When a coiled tube is rotated about the coil axis, the effects of rotation interact with centrifugal and viscous effects to complicate the flow characteristics beyond those seen in stationary curved ducts. The phenomena encountered are examined for steady, fully developed Newtonian flow in circular tubes of small curvature. The governing equations are solved using orthogonal collocation, and the results presented cover both the nature of the flow and the bifurcation structure. When rotation is in the same direction as the axial flow imposed by a pressure gradient, the flow structure remains similar to that seen in stationary ducts, i.e. with two- or four-vortex secondary flows in addition to the axial flow. There are, however, quantitative changes, which are due to the Coriolis forces resulting from rotation. The bifurcation structure also shows only quantitative changes from that for stationary ducts at all values of Taylor number examined. More complex behaviour is possible when rotation opposes the flow due to the pressure gradient. In particular, the direction of the secondary flow may be reversed at higher rotational strengths, and the mechanism of the flow reversal is explored. The flow reversal occurs smoothly at low Taylor numbers, but at higher rotational strengths a cusp appears in the primary solution branch in the vicinity of the flow reversal.

Travelling wave convection in a rotating layer

Abstract: Small amplitude two-dimensional Boussinesq convection in a plane layer with stress-free boundaries rotating uniformly about the vertical is studied. A horizontally unbounded layer is modelled by periodic boundary conditions. When the centrifugal force is balanced by an appropriate pressure gradient the resulting equations are translation invariant, and overstable convection can take the form of travelling waves. In the Prandtl number regime 0.53 < [sgrave] < 0.68 such solutions are preferred over the more usual standing waves. For [sgrave] < 0.53, travelling waves are stable provided the Taylor number is sufficiently large.