Positive Pressure Gradient Research Articles

Eddy covariance measurements of surface energy budget and evaporation in a cool season over southern open water in Mississippi

Eddy covariance measurements of sensible (H) and latent (LE) heat fluxes were made over a large southern open water surface of Ross Barnett Reservoir (the Reservoir hereafter) in Mississippi during the cool season with frequent incursions of cold fronts from 1 September 2007 to 31 January 2008. The eddy covariance tower was located in the middle of the main body of the Reservoir with the tower fetches exceeding 2.0 km in all directions. The Reservoir was ice‐free in winter and the water temperatures always decreased with depth. Over the entire cool season, the averaged water surface temperatures were 1.8°C higher than the overlying air (i.e., positive temperature gradients that led to thermally convective conditions) and the averaged vapor pressure near the water surface was 0.8 kPa greater than the overlying air (i.e., positive vapor pressure gradients), though occasionally negative gradients for temperature and vapor pressure were also observed for short periods. On average, the wind speeds were considerably large (3.9 m s−1) to maintain adequate turbulent mixing mechanically. As a consequence of the combined effect of thermally and mechanically generated turbulent mixing, consistently positive H (with a mean H of 20.0 W m−2) and LE (with a mean LE of 80.0 W m−2) occurred during the entire season. These continuous energy losses via H and LE resulted in release of a large amount of energy stored in the water to the atmosphere. The mean Bowen ratio was low for this open water surface (i.e., 0.3), suggesting that most of the energy released from the water fueled evaporation rather than sensible heating of the atmosphere. Nighttime evaporative water losses were substantial, contributing to 45% of the total evaporative water loss in this cool season. Frequent incursions of cold fronts with windy, cold, and dry air masses significantly promoted turbulent exchanges of sensible and latent heat through enhanced turbulent mixing thermally and mechanically, leading to large H and LE events. Daily H and LE (i.e., evaporation) during the passages of cold fronts were on average 2.7 and 7.3 times those in nonevent days, respectively. Given the fact that large H and LE events occurred 26% of the time for our site, these cold front events caused an increase in the seasonal H by 42% and LE by 157%. Therefore changes in frequency, intensity, and duration of synoptic weather events, particularly the incursions of cold fronts, have significant impacts on the surface energy budget and evaporation over water at this site.

Hypobaric pressure exposure effects on cochlear frequency selectivity in fluctuating, low-frequency hearing loss

To study the effects of hypobaric pressure chamber exposure on the cochlear frequency selectivity of subjects with monaural, fluctuating, low-frequency hearing loss, such as occurs in Ménière's disease. We used a hypobaric pressure chamber to create relative underpressure in the ear canal, in order to impose positive pressure gradients on the inner ear. Psychophysical tuning curves, transiently evoked otoacoustic emissions and speech recognition scores in noise were measured in 10 subjects with fluctuating, low-frequency hearing loss, before and after pressure exposure. After the exposure, subjects' overall pure tone averages showed no improvement, but individual results showed improved speech recognition scores in noise (six subjects), increased transiently evoked otoacoustic emission strength (three subjects) and increased psychophysical tuning curve steepness (two subjects). Deteriorations were also seen, mainly in psychophysical tuning curves. No association could be established between the different tests, and the measured parameters could not predict subjective improvement. The results suggest variable effects of hypobaric pressure exposure on inner-ear physiology.

Urethral instability.

Urethral instability is still evolving as a clinical entity. Using pressure variation of 15 cm water or more at the point of maximum urethral pressure (MUP), urethral pressure profilometry on patients referred for urodynamic assessment for lower urinary tract symptoms revealed urethral instability in 6.4% of 608 patients. The close association between urethral and detrusor instability was noted. Urethral instability appears to be a cause of frequency and urgency of micturition, and its presence increase the risk of urinary incontinence.

Aerodynamic optimization of Laval nozzle flow with shocks: Numerical investigation of active/passive shock control via expansion fans

AbstractInstantaneous ignition in the supersonic part of a 3‐D Laval nozzle realized by a well‐defined sudden temperature rise across a normal shock is the focus of the present study. Unfortunately, the divergence of the supersonic nozzle part is necessarily smooth. Therefore, the turbulent boundary layer ahead of the shock is thick and causes substantial shock/boundary layer interactions. The non‐homogeneous temperature increase across the shock, caused by the boundary layer thickening ahead of the shock and the resulting pre‐compression prevents the quasi 1‐D evolution of the flow downstream. Additionally, due to multiple boundary layer interactions the single shock disintegrates into a so called pseudo‐shock system; i.e., into a sequence of periodic weak compression and expansion regions. To avoid this drawback and to establish homogeneous thermodynamic conditions throughout the entire cross section and flow domain downstream of the shock we apply active and passive control techniques in the area of shock boundary layer interaction. The central idea of the control technique described below is compensation of the thickening of the boundary layer by quantitative appropriate inverse effects, i.e. by superimposing negative and positive pressure gradients in the near wall region close to the shock position. In a first approach the additional expansion fan is created by active suction slots in flow direction and through all sidewalls of the 3‐D nozzle. The resulting shock remains straight with exception of the near wall region. Suction creates an effective concave wall curvature. In supersonic flow the resulting local expansion tends to compensate the pre‐compression. Because suction in high temperature environment is difficult to realize, we alter the wall curvature to create a negative bump with the same effect on the effective wall curvature. Under these conditions a normal shock in the channel core could be established without active or passive suction. The paper compares these active/passive control techniques with the unmodified setup. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract 2230: Lower Extremity Counterpulsation During The Decompression Phase of CPR Improves Hemodynamics And Provides Continuous Forward Carotid Blood Flow.

Background: We hypothesize that lower extremity counterpulsations (LECP) during the decompression phase of CPR will increase diastolic aortic pressure without raising the intracranial pressure (ICP) and significantly increase forward carotid blood flow. Methods: In 6 (24±4Kg) pigs, 4 minutes of untreated VF were followed by 2 minutes epochs of standard (STD) CPR, STD +LECP, active compression decompression (ACD) +ITD and ACD + inspiratory impedance threshold device (ITD)+LECP. Compressions and ventilations were performed according to the 2005 AHA guidelines. A large blood pressure cuff that was circumferentially placed around the pig’s thighs avoiding the abdominal cavity was inflated with air to the point that manometer pressure started to rise. Compressions over the cuff were performed manually and synchronized with the thoracic decompression phase. The cuff was compressed up to 200mmHg. Basic hemodynamic parameters and common carotid blood flow (CCBF) (ml/min) were continuously measured. Cerebral perfusion pressure (CerPP) (mean arterial pressure - mean ICP) and CPP (diastolic AoP - diastolic RAP) were calculated at the end of the 2 minute epochs. Statistical analysis was performed with ANOVA. Results: LECP significantly increased diastolic aortic pressure, CPP and CerPP both when added on STD and on ACD+ITD CPR. The largest benefit was observed with ACD+ITD+LECP were there was a 2-fold increase in CPP and CerPP with a 2.5-fold increase in common carotid blood flow compared to STD CPR. LECP did not alter ICP. (Table 1 ). Decompression phase CCBF during ACD+ITD+LECP CPR was positive and reached pre cardiac arrest levels. Conclusion: LECP during thoracic decompression significantly augmented aortic diastolic pressure, CPP and CerPP without increasing the ICP . LECP led to a continuous positive cerebral perfusion pressure gradient during CPR cycles and in combination with ACD + ITD CPR common carotid blood flow reached pre-arrest levels. Hemodynamic Parameters

Numerical investigations of electro-osmotic flows in triangle microchannels

Small triangle channels are frequently encountered in microelectromechanical systems (MEMS). In this paper, the mixed electro-osmotic/pressure-driven flows in triangle microchannels are numerically investigated. Poisson and Navier–Stokes equations were numerically solved employing the Galerkin algorithm. The accuracy of the numerical algorithm was validated by refining the grids and using Richardson extrapolation. The numerical results show the mass flux of electrolytic solution increases with the increase of imposed positive pressure gradient. At small pressure gradients, the relative mass flux increment induced by the imposed positive pressure gradient is sensitive to the change in the pressure gradient. However, when the pressure gradient increases beyond a critical value, the relative mass flux increment increases smoothly with the increase of pressure gradient. It is also found that the mass flux increases with increasing the length ratio, κD h, at a given pressure gradient.

Tidal barrier and the asymptotic mass of proto gas-giant planets

AbstractAlthough late stage gap formation reduces the surface density in the vicinity of protoplanets, simulations suggest gas may continue to leak through the protoplanets tidal barrier, replenishing the gas supply and allowing protoplanets to acquire masses comparable to or larger than that of Jupiter. Global gas depletion is a possible explanation for gaseous planets with lower masses in weak-line T-Tauri disks and ice giants in our own solar system, but it is unlikely to have stalled the growth of multiple systems around nearby stars that contain relatively low-mass, close-in planets along with more massive and longer period companions. Here, we suggest a potential solution. We show that supersonic infall of surrounding gas onto a protoplanet is only possible interior to both its Bondi and Roche radii. Although the initial Bondi radius is much smaller than its Roche radius, the former overtakes the latter during its growth. Thereafter, a positive pressure gradient is required to induce the gas to enter the Roche lobe of the protoplanet and flow is significantly reduced. We present the results of analysis and numerical simulations to show that the accretion rate increases rapidly with the ratio of the protoplanets Roche to Bondi radii. Based on these results we suggest that in regions with low geometric aspect ratios gas accretion is quenched, resulting in relatively low protoplanetary masses.

Air core formation in the hydrocyclone

Air core formation has been investigated in hydrocyclones operated with clear water and a lucid suspension of glass balls. Hydrocyclones form a central air core which extends over the complete hydrocyclone length. Air is sucked in the core at the underflow discharge. The air core diameter can be determined balancing the positive pressure gradient and the centrifugal force in the rotational flow field. In dense flow separation (high feed solids content) the air core in the conical part of the hydrocyclone is suppressed. The hydrocyclone operates as it is air sealed because the solids are discharged trough the underflow as a rope. Then, air can be introduced to the hydrocyclone only on the feed side. In practice, feed suspension always contains more or less dissolved or dispersed air. Observations in a transparent hydrocyclone show that dissolved gas is released due to the pressure drop inside the hydrocyclone. The generated micro bubbles grow by coalescence and move in the centrifugal field toward the centre, where an air core is formed.

Intrinsic pump-conduit behavior of lymphangions

Lymphangions, segments of lymphatic vessels bounded by valves, have characteristics of both ventricles and arteries. They can act primarily like pumps when actively transporting lymph against a pressure gradient. They also can act as conduit vessels when passively transporting lymph down a pressure gradient. This duality has implications for clinical treatment of several types of edema, since the strategy to optimize lymph flow may depend on whether it is most beneficial for lymphangions to act as pumps or conduits. To address this duality, we employed a simple computational model of a contracting lymphangion, predicted the flows at both positive and negative axial pressure gradients, and validated the results with in vitro experiments on bovine mesenteric vessels. This model illustrates that contraction increases flow for normal axial pressure gradients. With edema, limb elevation, or external compression, however, the pressure gradient might reverse, and lymph may flow passively down a pressure gradient. In such cases, the valves may be forced open during the entire contraction cycle. The vessel thus acts as a conduit, and contraction has the effect of increasing resistance to passive flow, thus inhibiting flow rather than promoting it. This analysis may explain a possible physiological benefit of the observed flow-mediated inhibition of the lymphatic pump at high flow rates.

Inleakage of a vortex-ring bunch onto a plane screen

A method is suggested for simulating axisymmetric laminar or turbulent flows formed during the motion of a vortex-ring bunch of given geometry and circulation toward a plane screen. Earlier, similar problems were simulated with the numerical solution of the Navier-Stokes equations for laminar flows. Turbulent flows have remained unconsidered until now. When a vortex ring approaches the screen, the secondary nonstationary flow is induced near the screen’s surface and this secondary flow causes the formation of the radial boundary layer (provided that air viscosity is taken into account). First, the medium spreads out from the critical point at the screen’s center with the negative pressure gradient along the radial coordinate and then detaches in the region of the positive pressure gradient. This radial wall flow and the corresponding boundary layer are considered in the quasi-stationary approximation. When the boundary layer detaches at successive instances, the flow is replenished with the radially moving secondary vortex rings whose circulations have the sign opposite to that of the circulation of the primary vortex ring. It is the interaction of the primary and secondary vortices that governs process dynamics, which differs substantially from that in the case when the formation of secondary vortices is disregarded. The suggested method is based on the method of discrete vortices (a perfect liquid) and the boundary-layer (laminar or turbulent) theory. During the development of the flow under investigation, the nonstationary ascending flow in the direction perpendicular to the screen’s plane is formed and then this flow decays and dissipates. Simulations for large Reynolds numbers corresponding to the formation of the turbulent boundary layer show that the velocity of ascending vortices in the plane of the initial vortex bunch is less than one-tenth of the initial velocity of the descending vortex ring. The boundary layer is introduced into calculations with the sole goal of determining the parameters of the secondary vortex rings formed during boundary-layer detachments. The interaction of the primary and secondary vortices is then considered within the framework of a perfect medium. Simulations for large Reynolds numbers corresponding to the formation of the turbulent boundary layer on the screen were correlated with the available data obtained in laboratory experiments for small Reynolds numbers. Qualitative agreement between the simulations and experiments is fairly satisfactory. The simulation for one combination of the circulation and vortex-ring geometry takes at most 10–15 min with the use of an average PC.

Transient Lower Esophageal Sphincter Relaxations and Reflux: Mechanistic Analysis Using Concurrent Fluoroscopy and High-Resolution Manometry

The aim of this study was to perform a detailed analysis of the mechanics leading to esophagogastric junction (EGJ) opening during transient lower esophageal sphincter relaxations (tLESRs) using high-resolution manometry coupled with simultaneous fluoroscopy. Six subjects without hiatus hernia had endoclips placed at the squamocolumnar junction and 10 cm proximal. A 36-channel solid-state manometric assembly was placed spanning from stomach to pharynx, and subjects were studied for 2 hours after a high-fat meal. An esophageal pH electrode also was placed and fluoroscopy was initiated at the onset of a tLESR. Axial clip movement was measured during replay of the videotaped fluoroscopy and was correlated with manometric data. Ninety-three tLESRs were recorded, 62 tLESRs of which had good fluoroscopic visualization. Seventy-eight tLESRs had manometric evidence of flow and the majority had evidence of a common cavity (88%), but few were detected by the pH electrode. Esophageal shortening and crural diaphragm inhibition always preceded EGJ opening and common cavity. A positive pressure gradient between the stomach and the EGJ lumen of 7.1 mm Hg (interquartile range, 4.1-9.1 mm Hg) preceded the EGJ opening. Key events leading to the EGJ opening during tLESRs were LES relaxation, crural diaphragm inhibition, esophageal shortening, and a positive pressure gradient between the stomach and the EGJ lumen. The manometric signature of opening was pressure equalization within the EGJ, but this only occasionally was associated with pH evidence of reflux. Future investigations will need to analyze how this delicately balanced anatomic-physiologic system is perturbed in subjects with reflux disease.

Focusing of laser-generated ion beams by a plasma cylinder: Similarity theory and the thick lens formula

It is shown that plasma-based optics can be used to guide and focus highly divergent laser-generated ion beams. A hollow cylinder is considered, which initially contains a hot electron population. Plasma streaming toward the cylinder axis maintains a focusing electrostatic field due to the positive radial pressure gradient. The cylinder works as a thick lens, whose parameters are obtained from similarity theory for freely expanding plasma in cylindrical geometry. Because the lens parameters are energy dependent, the lens focuses a selected energy range of ions and works as a monochromator. Because the focusing is due to the quasineutral part of the expanding plasma, the lens parameters depend on the hot electron temperature Te only, and not their density.

Saphenous--peritoneal shunt in refractory ascites: a neglected technique.

Management of intractable ascites has always been a challenge. Peritoneovenous shunt (PVS) plays a major role in the surgery of intractable ascites in patients with liver cirrhosis. Positive pressure gradient between the ascitic fluid and venous pressure leading to one-way drainage of ascitic fluid into venous circulation is the mainstay. Over decades, various modifications of shunting technique have been done. Here we report our experience with this procedure which is safe, easy and effective. Here the long saphenous vein is used as a drainage system. One-way ascites flow is ensured by a natural valve in the saphenous orifice.

Stimulation of the lymphatic pump inhibits passive flow

Lymphatic vessels act as pumps to promote lymph flow up a pressure gradient and as conduits for flow down a pressure gradient. We hypothesized that stimulation of the pumping lymphatic will decrease average diameter, increasing vessel resistance to passive flow. Bovine post-nodal mesenteric lymphatic vessels (n = 4) were harvested locally from freshly slaughtered cattle. Vessels were instrumented in a tubular organ bath and bathed with a balanced polyionic 1% albumin solution set to 37° C, gassed with room air, and buffered to a pH of 7.4. Inlet pressure to the vessel was set at 5 cm H2O while outlet pressure was varied from about 6.5 to 3.5 cm H2O. Lymphatic vessels were exposed to solution containing 10−7 M U46619, a thromboxane analogue, and solution without U46619 in random order and instantaneous diameter and flow rate were recorded. Mean diameter of the lymphatic vessel was significantly decreased in the presence of U46619 (1.81±0.032 mm vs. 1.55±0.057 mm, p = 0.001, Student's t-test). Flow rates were significantly inhibited during positive pressure gradients (mean difference 2.63±0.62 g/min, p < 0.001, ANCOVA) while no difference in flow was detected during pumping against a pressure gradient (p = 0.26, ANCOVA). We conclude that stimulation of post-nodal mesenteric lymphatic vessels inhibits passive flow by decreasing average diameter and thereby increasing resistance to flow. TAMU-LSTF, AHA-0365127Y, CDC-623086.

Shallow water hydrodynamic models for hyperconcentrated sediment-laden floods over erodible bed

The majority of the huge annual sediment load of the Yellow River in China is transported by a few hyperconcentrated sediment-laden floods. Being hyperconcentrated, these floods are still so “starved” as to entrain enormous volumes of sediment from the bed, triggering quick and extensive bed-tearing scour. In the recession period of the floods, river blockage might occur as characterized by an abrupt halt of the flow. The physics of these fluvial processes has remained unclear for several decades. Previous hydrodynamic models were built upon simplified conservation laws and are applicable only for processes with weak sediment transport. A complete shallow water hydrodynamic model is deployed here to reveal new insights into the phenomena. A self-amplifying mechanism of the interaction between the flow and bed scour is identified, which explains how bed-tearing scour occurs. River blockage is ascribable to the longitudinally positive pressure gradient due to a non-uniform distribution of sediment concentration. The spatial and temporal development of the system of flow, sediment transport and morphology is far more complicated than represented by previous models that have evolved from fixed-bed, single-phase hydrodynamics or involved a capacity description of sediment transport. The present approach may facilitate a better understanding of active sediment transport by flash floods in ephemeral desert rivers and by subaqueous turbidity currents.

The Laminar Boundary Layer over a Permeable Wall

An analysis is given of the laminar boundary layer over a permeable/porous wall. The porous wall is passive in the sense that no suction or blowing velocity is imposed. To describe the flow inside and above the porous wall a continuum approach is employed based on the Volume-Averaging Method (S. Whitaker The method of volume averaging). With help of an order-of-magnitude analysis the boundary-layer equations are derived. The analysis is constrained by: (a) a low wall permeability; (b) a low Reynolds number for the flow inside the porous wall; (c) a sufficiently high Reynolds number for the freestream flow above the porous wall. Two boundary layers lying on top of each other can be distinguished: the Prandtl boundary layer above the porous wall, and the Brinkman boundary layer inside the porous wall. Based on the analytical solution for the Brinkman boundary layer in combination with the momentum transfer model of Ochoa-Tapia and Whitaker (Int. J. Heat Mass Transfer 38 (1995) 2635). for the interface region, a closed set of equations is derived for the Prandtl boundary layer. For the stream function a power series expansion in the perturbation parameter κ is adopted, where κ is proportional to ratio of the Brinkman to the Prandtl boundary-layer thickness. A generalization of the Falkner–Skan equation for boundary-layer flow past a wedge is derived, in which wall permeability is incorporated. Numerical solutions of the Falkner–Skan equation for various wedge angles are presented. Up to the first order in κ wall permeability causes a positive streamwise velocity at the interface and inside the porous wall, but a wall-normal interface velocity is a second-order effect. Furthermore, wall permeability causes a decrease in the wall shear stress when the freestream flow accelerates, but an increase in the wall shear stress when the freestream flow decelerates. From the latter it follows that separation, as indicated by zero wall shear stress, is delayed to a larger positive pressure gradient.

A Critical Assessment of Reynolds Analogy for Turbine Flows

The application of Reynolds analogy 2St/cf≅1 for turbine flows is critically evaluated using experimental data collected in a low-speed wind tunnel. Independent measurements of St and cf over a wide variety of test conditions permit assessments of the variation of the Reynolds analogy factor (i.e., 2St/cf) with Reynolds number, freestream pressure gradient, surface roughness, and freestream turbulence. While the factor is fairly independent of Reynolds number, it increases with positive (adverse) pressure gradient and decreases with negative (favorable) pressure gradient. This variation can be traced directly to the governing equations for momentum and energy which dictate a more direct influence of pressure gradient on wall shear than on energy (heat) transfer. Surface roughness introduces a large pressure drag component to the net skin friction measurement without a corresponding mechanism for a comparable increase in heat transfer. Accordingly, the Reynolds analogy factor decreases dramatically with surface roughness (by as much as 50% as roughness elements become more prominent). Freestream turbulence has the opposite effect of increasing heat transfer more than skin friction, thus the Reynolds analogy factor increases with turbulence level (by up to 35% at a level of 11% freestream turbulence). Physical mechanisms responsible for the observed variations are offered in each case. Finally, synergies resulting from the combinations of pressure gradient and freestream turbulence with surface roughness are evaluated. With this added insight, the Reynolds analogy remains a useful tool for qualitative assessments of complex turbine flows where both heat load management and aerodynamic efficiency are critical design parameters.

Influence of Bulk Fluid Velocity on the Efficiency of Electrohydrodynamic Pumping

Abstract The efficiency of conversion of electrical power into fluidic power in an electrohydrodynamic (EHD) pump depends on the bulk fluid velocity. An analytical formulation is developed for calculation of the efficiency of an EHD pump, with and without the presence of a superimposed flow due to an externally imposed pressure gradient. This formulation is implemented into a numerical model, which is used to investigate the effect of bulk fluid velocity on the efficiency of the EHD action. In particular, the net flow due to the combined action of EHD and a positive or negative external pressure gradient is computed. Both ion-drag pumps and induction EHD pumps are considered. Pumps based on the ion-drag principle that are studied include a one-dimensional pump, a two-dimensional pump driven by a stationary potential gradient, and another driven by a traveling potential wave. Two-dimensional repulsion-type and attraction-type induction pumping caused by a gradual variation in the electrical conductivity of the fluid is also investigated. The efficiency of EHD pumps exhibited a strong dependence on bulk fluid velocity: for the two-dimensional steady ion-drag pump, for example, the efficiency increased from less than 2% to 22% under the influence of an external pressure gradient. The corresponding increase in efficiency for a two-dimensional repulsion-type EHD pump was from 0.26% to 24.5%.

Universal Velocity Defect Law for the Turbulent Boundary Layer

Turbulent plane boundary layer flows of an incompressible fluid are considered. A refinement of the known Coles wake law is proposed. This refinement makes it possible to ensure the smooth matching of the turbulent boundary layer velocity profile with the outer flow and to extend the range of validity of the law to the case of large positive pressure gradients. The accuracy of the analytical approximation obtained is verified by comparison with the known experimental equilibrium velocity profiles. Using the approximation proposed, a relation for calculating the cross-sectional distribution of the Reynolds stress in the equilibrium boundary layer is derived. The pressure distributions for which the equilibrium turbulent boundary layer flows are single- and two-valued are distinguished.

Effect of Nozzle Contour on Flow Separation in Overexpanded Rocket Nozzles

In the present paper, parametric numerical analyses of the separated transonic flows in overexpanded rocket nozzles are conducted. Several types of nozzles are examined numerically to clarify the detailed mechanisms of the occurrence of restricted shock separation (RSS) in compressed truncated perfect (CTP) nozzles. The effects of nozzle length and geometric compression factor are examined. In CTP nozzles with large length and small compression factor, a positive pressure gradient appears downstream of the Mach disk before the occurrence of RSS. Under the positive pressure gradient, the flow, which passed through the Mach disk, rolls up and forms a large vortex ring around the nozzle exit. The vortex ring pushes the separated jet towards the nozzle wall and results in RSS.