Showing papers on "Pressure gradient published in 1986"

Scrutinizing the k-ε Turbulence Model Under Adverse Pressure Gradient Conditions

Abstract: The k-e model and a one-equation model have been used to predict adverse pressure gradient boundary layers. While the one-equation model gives generally good results, the k-e model reveals systematic discrepancies, e.g. too high skin friction coefficients, for these relatively simple flows. These shortcomings are examined and it is shown by an analytical analysis for the log-law region that the generation term of the e-equation has to be increased to conform with experimental evidence under adverse pressure gradient conditions. A corresponding modification to the e-equation emphasizing the generation rate due to deceleration was employed in the present investigation and resulted in improved predictions for both moderately and strongly decelerated flows.

Methods and apparatus for moving and separating materials exhibiting different physical properties

Abstract: Methods and apparatus for controlling the movement of materials having different physical properties when one of the materials is a fluid. The invention does not rely on flocculation, sedimentation, centrifugation, the buoyancy of the materials, or any other gravity dependent characteristic, in order to achieve its desired results. The methods of the present invention provide that a first acoustic wave is progpagated through a vessel containing the materials. A second acoustic wave, at a frequency different than the first acoustic wave, is also propagated through the vessel so that the two acoustic waves are superimposed upon each other. The superimposition of the two waves creates a beat frequency wave. The beat frequency wave comprises pressure gradients dividing regions of maximum and minimum pressure. The pressure gradients and the regions of maximum and minimum pressure move through space and time at a group velocity. The moving pressure gradients and regions of maximum and minimum pressure act upon the marterials so as to move one of the materials towards a predetermined location in the vessel. The present invention provides that the materials may be controllably moved toward a location, aggreated at a particular location, or physically separated from each other.

Boundary-layer receptivity to unsteady pressure gradients: experiments and overview

Abstract: The experimental evidence on the mechanisms of forcing of unstable vorticity waves (the Tollmien–Schlichting–Schubauer or TS waves) of circular frequency ω and wavelength λTS in wall layers by unsteady pressure gradients of amplitude A and frequency ω is reviewed and found to be confused and contradictory. It is proposed that a likely effective receptivity mechanism rests on the fact that under realistic conditions A varies with distance x along any body of finite thickness, A(x), and introduces thereby additional characteristic lengths which can match λTS. Heuristic arguments suggest that through A(x) the pressure gradient infuses vorticity at the wall and forces spatial growth of the TS mean-square vorticity and is proportional to the imaginary part of κTSΔAF(KTS).The proposition is consistent in all currently verifiable respects with one numerical and a series of laboratory experiments. In the laboratory experiments, various configurations of a pulsating pressure source and shielding plates located in the free stream supplied the variable-amplitude pressure gradients over the nearby flat-wall boundary layer. Three of the cases presented here demonstrate that stationary unsteady pressure fields induce Stokes-like sublayers when the boundary layer is stable and self-excited vorticity waves when it is unstable. The results of a fourth experiment suggest that unsteady pressure sources in wakes near the boundary layer can force the growth of unstable wall waves at the wake frequencies even though their propagation speeds differ. Material is also presented on key Soviet experiments and views on receptivity. Finally, these experiments and ours are examined for consistency and complementarity.

Measurement of transstenotic pressure gradient during percutaneous transluminal coronary angioplasty.

Abstract: Obstruction to blood flow is accompanied by a pressure gradient across the obstructed site. In certain clinical settings, magnitude of pressure gradient has been used to judge severity of obstruction, and gradient reduction to judge success of an interventional procedure. In percutaneous transluminal coronary angioplasty (PTCA) the relationships between transstenotic pressure gradient, diameter stenosis, and lesion length are imprecisely known. We therefore examined 4263 sets of measurements in patients who underwent PTCA on single, discrete coronary arterial lesions. Multivariate regression analysis demonstrated that pressure gradient was artifactually elevated by about 12 mm Hg at low values of diameter stenosis but increased by the 4th power of stenosis as expected from fluid dynamics models. Pressure gradient was dampened and relatively constant at values of diameter stenosis of 60% or higher, probably because of total or near-total occlusion of the artery. Lesion length was not found to influence pressure gradient. Reductions in diameter stenosis (delta D) and pressure gradient (delta G) were related nonlinearly, with delta D proportional to the square root of delta G, suggesting that a reduction in gradient is directly proportional to an increase in cross-sectional area of the stenosis. The predictive value of final post-PTCA pressure gradients was found: a final gradient of 15 mm Hg or less predicted a final post-PTCA diameter stenosis of 30% or less, with 75% sensitivity and 29% specificity (p less than .01). The results of this study suggest that (1) pressure gradient as currently measured during PTCA is related to diameter stenosis but not to lesion length (2) reductions in pressure gradient and diameter stenosis are nonlinearly related.(ABSTRACT TRUNCATED AT 250 WORDS)

Laboratory Studies of Gas Flow Through a Single Natural Fracture

Abstract: The relationship between the aperture and gas conductivity of a single natural fracture was investigated in the laboratory. Fracture conductivity was evaluated as a function of both the applied fluid pressure gradient and average fracture aperture, the latter ranging from 600 to 200 μm. Fracture apertures were determined independently on the basis of both fracture deformation and fracture volume measurements. Flow generally occurred in the linear and transitional flow regime between linear and fully nonlinear flow. The transition was found to be smooth and well described by an equation of the form: −(dp/dx) = av + bv2, where dp/dx is the pressure gradient and v is the fluid velocity. The linear and nonlinear fracture conductivities were found to be functions of the aperture and surface roughness of the fracture in agreement with existing empirical equations. A new physical model for fracture flow is also formulated based on an analogy to pipe flow.

Cocurrent downflow of air and water in a two‐dimensional packed column

Abstract: Multiple hydrodynamic states were observed during cocurrent down-flow of air and water under trickling flow conditions in a packed bed of rectangular cross section. Although the multiplicity was exhibited by both pressure gradient and liquid holdup, the pressure gradient showed the largest variations at identical conditions. The multiplicity is interpreted as being due to the liquid flowing in two different modes, namely, film flow and rivulet flow. The characteristics of pulsing flow in a packed bed of rectangular cross section were found to be appreciably different from those reported in the literature for flow in small-diameter cylindrical columns. The most significant observation in the packed bed of rectangular cross section was that the pulses did not always span the column cross section, unlike the case in small-diameter columns. The lower pressure drop and pulse velocity in the packed bed of rectangular cross section are believed to result from the bypassing of gas around the edge of the pulses. The location of the pulses was found to depend on the quality of gas and liquid distribution at the top of the column, and the shape of the top surface of the packing. A distributor configuration in which the gas was injected directly into the bed was found to be the most desirable, and is recommended in industrial practice.

Vertical upward cocurrent gas-liquid annular flow

Abstract: Upward cocurrent gas-liquid annular flow was investigated in a 50.8 mm ID cylindrical vertical pipe. The film flow was studied by measuring instantaneous local film thickness, wall shear stress, and pressure gradient. Analysis of these data revealed that at low gas flow rates the film motion is controlled by a switching mechanism, as speculated by Moalem-Maron and Dukler (1984). In the region of high gas flow rates the switching process is suppressed and traveling roll waves characterize the film motion.

Continuous-wave Doppler in children with ventricular septal defect: noninvasive estimation of interventricular pressure gradient.

Abstract: Continuous-wave Doppler was used to estimate the pressure gradient between the right and left ventricles in 28 children with ventricular septal defect (VSD). Doppler measurement of maximal velocity was performed during cardiac catheterization and the Doppler-predicted gradient was compared with the peak-to-peak gradient measured simultaneously by catheter. Doppler gradients ranged from 10 to 71 mm Hg and correlated well with measured gradient (r = 0.97, p greater than or equal to 0.001). Fourteen patients had isolated VSD, and in these patients Doppler measurements of gradient allowed accurate estimation of right ventricular pressure (r = 0.93). There was an inverse correlation between the ratio of pulmonary to systemic resistance and maximal velocity (r = -0.77). Thus, continuous-wave Doppler is an accurate means of measuring instantaneous VSD pressure gradient in children with congenital heart disease and can be used to estimate the right ventricular and pulmonary artery pressure in children with isolated VSD. This noninvasive method can be used to distinguish restrictive from nonrestrictive VSD.

Influence of Free Stream Turbulence and Blade Pressure Gradient on Boundary Layer and Loss Behaviour of Turbine Cascades

Abstract: The optimization of the blade surface velocity distribution is promising for a reduction of turbine cascade losses. Theoretical and experimental investigations on three turbine cascades with the same blade loading show the important influence of the blade pressure gradient and the free-stream turbulence on the loss behavior. The results presented demonstrate that it is the boundary layer transition behavior that determines the losses on turbine cascades. An enormous effort in measuring technique is required in order to define the location of transition from cascade experiments very accurately.

On the measurement of wall shear stress

Abstract: The differential pressure reading from a static hole pair is utilized for determination of the local wall shear stress Both the hole diameter and forward-facing angle are varied to test the sensitivity of the device The static hole pair in tested in a two-dimensional zero pressure gradient turbulent boundary layer on a smooth surface The calibrating values for the local wall shear is determined from the universal scaling laws for the mean velocity profile in the inner part of the turbulent boundary layer The static hole pair is found to be sensitive to imperfections in the manufacturing process, and needs an individual calibration in order to make accurate measurements of the local skin friction possible

Application of the Pressure Derivative Function in the Pressure Transient Testing of Fractured Wells

Abstract: A new technique is presented for the analysis of wells with finite conductivity fractures. The technique simultaneously uses the pressure and pressure derivative for the cases of no fracture face skin and no wellbore storage, and with fracture face skin and wellbore storage during bilinear flow. New type curves are presented and applied to three field cases. It concludes that using the pressure derivative with pressure behaviour type curves reduces the uniqueness problem in type curve matching and gives greater confidence in the results.

Transport due to ion pressure gradient turbulence

Abstract: Turbulent transport due to the ion pressure gradient (or temperature drift) instability is thought to be significant when ηi= d(ln Ti)/d(ln n) > 1. The invariance properties of the governing equations under scale transformations are used to discuss the characteristics of this turbulence. This approach not only clarifies the relationships between earlier treatments but also, in certain limits, completely determines the scaling properties of the fluctuations and the consequent thermal transport.

The physics of the cometary contact surface

Abstract: The contact surface, which separates outflowing cometary plasma from solar wind controlled cometary plasma, is explained in terms of a balance between the magnetic pressure gradient force and ion-neutral drag. Giotto data indicate that the plasma pressure inside the contact surface cannot balance the sum of the external plasma and magnetic pressures, and therefore a Venus-like ionopause is not present at the contact surface. An expression for the magnetic field strength as a function of cometocentric distance is derived from the momentum equation. The theoretical magnetic field profile agrees quite well with the profile measured by the Giotto magnetometer in the vicinity of the contact surface.

Droplet entrainment in vertical annular flow and its contribution to momentum transfer

Abstract: Simultaneous measurements were made of the size, axial and radial velocity of drops entrained by the gas in annular flow. A model is developed to use these data to compute the rate of deposition or entrainment and the pressure gradient, del p/sub E/, due to drop interchange. del p/sub E/ is a significant fraction of the measured total shear del p.

Diffusion of bound water in wood

Abstract: Fick's law states that during steady-state diffusion, moisture flux is proportional to the gradient in moisture concentration, making concentration gradient the driving force for diffusion. Although Fick's law permits convenient comparisons between predicted and measured moisture contents, fundamental and experimental difficulties with it have been reported. In this paper, the dependence of spreading pressure on vapor pressure and temperature is developed. This information is used to show that gradients in spreading pressure and chemical potential are not proportional. Spreading pressure gradient is considered the correct driving force because the associated transport law can be derived from first principles.

Pressure Loss Due to the Tip Clearance of Impeller Blades in Centrifugal and Axial Blowers

Abstract: The pressure loss based on the tip clearance of impeller blades consists of the pressure loss induced by the leakage flow through the clearance and the pressure loss for supporting fluid against the pressure gradient in the channels and in the thin annular clearance space between the shroud and the impeller. Equations to evaluate these losses are derived and the predicted efficiency drop is compared with experimental data for two types of centrifugal impellers. Furthermore, the equations are simplified for axial impellers as a special case, and the predicted efficiency drop is compared with the experimental data for seven cases in the literature. Fair agreement demonstrates plausibility of the present model.

Effect of extravascular pressure gradients on capillary fluid exchange

Abstract: A mathematical model is developed to examine the effect of pressure gradients in the extravascular space on fluid exchange from capillaries. Starling's hypothesis describes fluid transport through the capillary wall, and D'Arcy's law governs flow in the tissue. Using physiologically reasonable values for the wall and tissue hydraulic conductivities and other parameters, it is shown that the tissue hydrostatic pressure is not uniform throughout the tissue space. The greatest deviations from the background pressure occur near the capillary, and the magnitude of deviations increases with increasing ratios of the two conductivities. The model also shows that the fluid exchange behavior is modified by the presence of other capillaries. This capillary-capillary interaction is influenced by the ratio of conductivities, capillary proximity, and the number of existing capillaries.

Numerical solution procedure for viscous incompressible flows

Abstract: A calculation procedure for solving the time-dependent incompressible Navier-Stokes equations in arbitrary shapes is presented. The conservative form of the primitive-variable formulation is adopted. The numerical scheme is based on an overlapping grid with forward and backward differencing for mass and pressure gradients, respectively. This structure allows use of the same computational cell for both the continuity and momentum equations and storage of the pressure and velocity components at the same grid location. The result is a stable and accurate algorithm, and no oscillations on the velocity or pressure field are detected. The computed results are compared with numerical and experimental data.

Growth of a gas bubble in a viscous fluid

Abstract: The cavitation pressure of spherical gas bubbles is correlated to the fluid viscosity in a theoretical investigation of the viscous Rayleigh–Plesset equation. The analysis is complicated by the absence of a first integral for the dissipative system. A qualitative normal form bifurcation analysis about a double‐zero codimension 2 singularity is used to guide a quantitative estimate of the cavitation pressure by numerical and Melnikov perturbation techniques.

Turbulent boundary layers on rough surfaces with and without a pliable overlayer: a simulation of marine fouling

Abstract: A wind tunnel simulation of two boundary layer flows (with and without an adverse pressure gradient) on two surfaces with different roughness was carried out. One of the surfaces was entirely covered with an irregular coarse roughness. The other was identical but additionally overlaid with many deformable nylon tufts to simulate the growth of biological fouling on a marine object. Detailed mean flow and turbulence measurements were made to study the effect of the overlayer on the boundary layer development with different pressure gradient conditions.

Measurement of wall shear stress in turbulent boundary layers subject to strong pressure gradients

Abstract: Measurements of the wall shear stress with a floating element and with Preston tubes were conducted in turbulent boundary layers. Sudden application and removal of adverse pressure gradients resulted in boundary layers far from equilibrium. Positive and negative errors of the Preston-tube results were observed for adverse pressure gradients. The negative errors occurred mainly in regions with dτ w /d x > 0. The relation between the error, the pressure gradient and the tube size (1.1) suggested by Frei & Thomann (1980) predicts only positive errors for d p /d x > 0. Therefore, it cannot be used for the present pressure distributions and is not as general as was expected. The present results show that indirect methods to determine the wall shear stress should not extend beyond y + - 3 if accuracies of ± 1% are required for pressure distributions similar to the ones used in the present investigation. Predictions from Ludwieg & Tillmann's relation (4.21) agree to within ± 10% with the present measurements. The Preston-tube readings indicate velocities below the law of the wall in regions with a decreasing adverse pressure gradient. No local parameters could be found that correlated the errors of the Preston-tube results for the large pressure gradients used in the present investigation.

Role of a hydrostatic pressure gradient in the formation of early ischemic brain edema.

Abstract: We studied whether a hydrostatic pressure gradient between arterial blood and brain tissue plays a role in the formation of early ischemic cerebral edema after middle cerebral artery (MCA) occlusion in cats. Tissue pressure, regional CBF, and water content were measured from the cortex in the core and the peripheral zone of brain normally perfused by the MCA. Intraluminal arterial pressure was altered at intervals by inflation of an aortic balloon to vary the blood-tissue pressure gradient in the ischemic zone. Brain water content in the ischemic core, where flow fell to 5.5 ml/100 g/min, increased within 1 h of occlusion. After occlusion tissue pressure rose from 7.95 +/- 0.72 mm Hg at 1 h to 13.16 +/- 1.13 mm Hg at 3 h. When intraluminal pressure was increased, water content increased further, but only at 1 h after occlusion. In the periphery where flow was 18.9 ml/100 g/min during normotension, neither water content nor tissue pressure rose within 3 h of occlusion. Increased intraluminal pressure was accompanied by increased water content only at 3 h. This study indicates that a hydrostatic pressure gradient is an important element in the development of ischemic brain edema, exerting its major effect during the initial phase of the edema process.