Reversal Of Pressure Gradient Research Articles

Numerical modeling studies of lee mesolows, mesovortices and mesocyclones with application to the formation of Taiwan mesolows

Numerical experiments are performed for inviscid flow past an idealized topography to investigate the formation and development of lee mesolows, mesovortices and mesocyclones. For a nonrotating, low-Froude number flow over a bell-shaped moutain, a pair of mesovortices form on the lee slope move downstream and weaken at later times. The advection speed of the lee vortices is found to be about two-thirds of the basic wind velocity, which is due to the existence of a reversed pressure gradient just upstream of the vortices. The lee vortices do not concur with the upstream stagnation point in time, but rather form at a later time. It is found that a pair of lee vortices form for a flow withFr=0.66, but take a longer time to form than in lower-Froude number flows. Since the lee vortices are formed rather progressively, their formation may be explained by the baroclinically-induced vorticity tilting as the mountain waves become more and more nonlinear.

A case study of pollutant transport from Los Angeles to the desert South-West

The transport of pollutants from the Los Angeles basin to remote sites in southern Nevada and northwestern Arizona has been fingerprinted by halocarbons. One instance of episodic transport driven by a synoptic scale pressure gradient reversal is discussed wherein there was minimal mixing and broadening of the pollution front during transit. The particulate loading within the polluted air parcel was found to be vertically homogeneous suggesting that it did not originate from subsidence of an elevated pollution layer. The relationship between this event and the ‘climatological’ data presented by White et al. (1991, Conf. Proc. Tropospheric Ozone and the Environment) is discussed with particular regard to the atypically short transport time.

Diastolic mitral regurgitation with atrioventricular conduction abnormalities: Relation of mitral flow velocity to transmitral pressure gradients in conscious dogs

Diastolic mitral regurgitation is a common finding that can be detected with use of Doppler echocardiographic techniques in patients with atrioventricular (AV) conduction abnormalities. With use of simultaneous hemodynamic and Doppler techniques, mitral flow velocity, mitral valve motion and transmitral pressure gradient were studied during 50 cardiac cycles each of spontaneous or atrial paced first- and second-degree AV block in five lightly sedated dogs.Diastolic mitral regurgitation was detected during atrial relaxation on all beats in which ventricular contraction was delayed >190 ms. In all dogs the diastolic regurgitation was associated with a reverse transmitral pressure gradient (3.7 ± 1.1 mm Hg in first-degree AV block and 3.2 ± 1.5 mm Hg in second-degree AV block) that occurred primarily as the result of a decrease in atrial pressure with atrial relaxation. These reverse pressure gradients were as large as the maximal forward transmitral gradients in early diastole (2.9 ± 0.9 mm Hg in first-degree AV block and 3.1 ± 0.7 mm Hg in second-degree AV block) and larger than the maximal forward pressure gradients at atrial contraction (1.7 ± 0.5 and 1.4 ± 0.6 mm Hg, respectively, p < 0.05). The maximal reverse pressure gradient during atrial relaxation was also as large as the reverse pressure gradient in mid-diastole (2.7 ± 0.9 and 2.8 ± 1.0 mm Hg, respectively), associated with deceleration of early diastolic mitral flow.Peak diastolic mitral regurgitation velocity coincided with the maximal reverse transmitral gradient and was usually larger than anterograde mitral flow velocity. In second-degree AV block the velocity and duration of diastolic regurgitation were variable, with smaller reverse transmitral gradients and peak flow velocities on beats with a shorter QP interval. The mitral valve opened widely with a forward transmitral pressure gradient, and demonstrated rapid and near closure with a reverse transmitral pressure gradient. The smallest valve opening coincided with the maximal reverse pressure gradient and the peak velocity of diastolic regurgitation. Complete valve closure occurred only with ventricular systole.These results demonstrate that a reverse transmitral pressure gradient is present during diastolic mitral regurgitation in AV block, is related to the peak velocity and duration of regurgitation and is equal to or larger than the diastolic forward transmitral gradient. The results also suggest a role for a reverse diastolic transmitral gradient in causing near closure of the mitral valve leaflets and preventing significant regurgitant volume during diastole. However, complete closure of the mitral valve appears to occur only during ventricular systole.

Uncontrolled ignition mechanisms in liquid propellant guns

AbstractTwo potential mechanisms are described which may result in uncontrolled ignition in liquid propellant regenerative guns. The two mechanisms are related to reverse burn in the injection orifice. One mechanism is associated with a poorly designed injection orifice, which may result in a low discharge coefficient, boundary layer separation, and flame holding in the orifice. The second mechanism is related to the response of the liquid propellant reservoir to the ignition pressure, which may excite pressure oscillations in the reservoir resulting in a reverse orifice pressure gradient. Both mechanisms may result in a reverse orifice burn or flash back to the reservoir.

Evaluation of transmitral pressure gradients at different heart rates: divergent action of isoprenaline and atropine

The aim was to record diastolic transmitral pressure gradients at high sensitivity to quantitate the effect on transmitral pressure gradients of changing the heart rate. Diastolic left atrial and left ventricular pressures were recorded at high sensitivity (40 mm Hg = 10 mm recorded deflection) in control conditions (heart rate 70 beats.min-1) and after intravenous administration of atropine or isoprenaline (heart rate 110 beats.min-1). A special ventricular extrasystole protocol enabled the zero level of the transmitral pressure gradients to be unequivocally determined at high heart rates. The effect of atropine and isoprenaline on the pressure gradients, absolute diastolic pressures, and diastolic time intervals was investigated. 16 mongrel dogs, 16-25 kg, were used. Below a heart rate of 110 beats.min-1, four distinct periods were identified, during which a pressure gradient existed. During early and late diastole, a positive pressure gradient was consistently followed by a negative pressure gradient. Mean negative pressure gradient during early diastole correlated with the pressure difference of rapid filling wave (r = 0.72, p less than 0.01) and with mean positive pressure gradient during early diastole (r = 0.66, p less than 0.01). At a heart rate of 110 beats.min-1, isoprenaline augmented, while atropine reduced, the mean positive pressure gradient during early diastole without affecting the time interval over which the gradient occurred. This divergent action of the two drugs was related to their different effects on the decay of left ventricular pressure, which fell faster and deeper with isoprenaline but not with atropine. Both drugs shortened the time interval of the negative pressure gradient in early diastole without significantly affecting the mean negative pressure gradient during this period. In late diastole, atropine augmented the mean positive pressure gradient more than isoprenaline, reflecting the higher afterload after administration of atropine. Neither drug affected the time interval of the positive pressure gradient. As a result of a shortening of the P-R interval, isoprenaline shortened the time interval of the negative pressure gradient and reduced its mean value. Such an effect was not observed with the dose of atropine used. We conclude that a pressure gradient reversal in early diastole is always observed below a heart rate of 110 beats.min-1 and that isoprenaline and atropine affect the pattern of transmitral pressure gradients in a different way.

Doppler Atrial Shunt Flow Patterns in Patients with Secundum Atrial Septal Defect: Determinants, Limitations, and Pitfalls

Fifteen patients with uncomplicated secundum atrial septal defect underwent studies with real-time color-coded two dimensional flow imaging, pulsed Doppler echocardiographic examination, and simultaneous pressure recordings from the left and right atrium to determine the flow-pressure dynamics of the atrial shunt flow. In all 15 patients both the color flow mapping and pulsed Doppler studies revealed that the shunt flow was mainly from left to right, occurring both during ventricular systole and diastole. It started in early systole, reached a peak in late systole to early diastole, and lasted throughout diastole with an accentuation in late diastole during atrial contraction. The amplitude of the flow velocity, the direction, and the magnitude of the shunt flow, however, changed from phase to phase during the cardiac cycle. It correlated well with the phasic variation of the interatrial pressure difference, which usually revealed a peak pressure gradient that occurred in early systole between the x descent and v wave and during the period of v wave and a wave of the left atrial pressure tracing. Right to left shunt was not detected in any of the 15 patients by color flow mapping studies. A minor reversal of the shunt flow, however, was frequently detected at the beginning of ventricular systole and sometimes also in the middle of diastole by pulsed Doppler echocardiography. The reversal of shunt flow correlated with the minor reversal of pressure gradient that occurred during the z point, x descent, and y descent of the left atrial pressure tracing.In conclusion, left to right shunt flow occurs both during ventricular systole and diastole in uncomplicated secundum atrial septal defect. The amplitude of flow velocity, the magnitude, and the direction of the shunt flow changes from phase to phase and reflects the phasic variation of the interatrial pressure difference during the cardiac cycle.

Effects of Near-Bottom Return Flows on Wind-Induced Currents

Abstract Wind, currents, and surface waves have been measured in a circulating wind-wave tank. Conditions corresponding to both coast and ocean, with and without bottom return flows respectively, have been simulated. Spectra of currents were found to exhibit a −5/3 slope outside the frequency range of motions induced by dominant waves. For the coastal condition, the Reynolds stress decreases linearly with depth, and the turbulence energy decays with a z−1 trend, where z is the depth. Much of this results from the piling up of water at the downwind coast, with the vertical distribution of momentum significantly affected by return flows and the reverse pressure gradient. For the oceanic condition, the wind-induced drift currents are extended much further downward; a region of constant momentum flux prevails, and a uniform vertical distribution of turbulence energy was found. Below this surface region, the turbulence energy decays with a z−2 trend, possibly implying a dissipative mechanism.

Late diastolic mitral regurgitation studied by pulsed Doppler echocardiography

To evaluate the usefulness of pulsed Doppler echocardiography in assessing late diastolic mitral regurgitation (MR) and to clarify the pathophysiology of MR, 226 consecutive patients who had undergone left ventriculography were studied. By investigating blood flow patterns at the left atrial outflow tract, late diastolic disturbed flow suggesting MR was detected in 15 patients (7%), including 10 (4%) with positive left ventriculographic findings. Among these 15 patients, 14 (93%) had atrial fibrillation and had late diastolic MR in the cardiac cycle with prolonged RR interval. The limitation in number of cardiac cycles that could be analyzed and the rapid heart rate eliminating appearance of the beat with prolonged RR interval may be the reasons for the paucity of late diastolic MR by left ventriculography. Ten patients (66%) with late diastolic MR, including 1 with sinus rhythm, had aortic regurgitation, 3 had high-grade systolic MR and 2 had atrial septal defect. Simultaneous recording of pulmonary artery wedge pressure and left ventricular pressure in 3 patients showed a reversal of pressure gradient in late diastole when the RR interval was prolonged. In conclusion, pulsed Doppler echocardiography was useful for detecting late diastolic MR and in reducing overestimation of systolic MR in left ventriculography induced by erroneous addition of late diastolic MR. The difference of the incidence of this flow between left ventriculography and Doppler examination indicated that this flow depends primarily on heart rate and may come and go in a patient.

Experimental model of chronic limb lymphedema and determination of lymphatic and venous pressures in normal and lymphedematous limbs.

Experimental studies on chronic lymphedema of the extremities were performed using dogs, and an improved technique after the method of Danese in the preparation of chronic limb lymphedema is described. The incidence of chronic lymphedema in this model was 81.8%, whereas with Danese's method, it was only 66.7%. In addition, the measurement of lymphatic and venous pressure was studied in experimental animals. In the normal limb, the lymphatic pressure was always lower than the venous pressure, while in the chronic lymphedematous limb, the lymphatic pressure was higher than the venous pressure. Thus, after a lymphaticovenous anastomosis, a pressure gradient reversal exists between the two sides of the anastomosis. Although the high lymphatic pressure gradually dropped as the anastomosis became patent, ultimately the venous pressure was often higher than the lymphatic pressure. A venous back flow can be ensured by the anastomosis and the resulting thrombosis, which obstructs the lymphaticovenous anastomosis, is inevitable.

Bacterial Transport Within and Among Various Teatcup and Cluster Assemblies During Milking

For evaluation of bacterial transport, two experiments were conducted. In Experiment 1, two nonconventional cluster designs were compared with a conventional assembly. These two units were: 1) conventional unit without a claw and 2) linerless unit without a claw. In Experiment 2, two nonconventional teatcup designs were compared with a conventional teatcup, all attached to a conventional claw. These two were: 1) partial liner teatcup assembly and 2) conventional teatcup with restricted liner wall movement. The three teatcups in Experiment 2 differed only in the lower part of the liner barrel; mouthpiece and short milk tube were identical.A culture of Serratia marcescens bacteria was infused throughout milking into the short milk tube at the right front teatcup assembly. Swabs of liners and teats were used to culture for Serratia marcescens. For Experiment 1, the number of contaminated teats (and liners) was 32 of 144 and 88 of 144 for the conventional cluster, 20 of 144 and 37 of 144 for the clawless cluster, and 10 of 144 and 16 of 144 for the linerless cluster. For Experiment 2, the total number of bacterial transfers was 14 of 25 for the conventional teatcup, 7 of 25 for the restricted liner, and 3 of 25 for the partial liner. These occurrences of transfer correlate well with previously measured reverse pressure gradients across the short milk tube.

Relationships Between Radar-Derived Thermodynamic Variables and Tornadogenesis

A methodology for obtaining pressure perturbations and buoyancy in thunderstorms observed with Doppler radar is described and applied to two tornadic thunderstorms. Extracted thermodynamic information is combined with kinematic analyses to study observed severe storm processes. The intensification of tornado parental circulations (mesocyclones) during tornadogenesis is found to be associated with deepening pressure deficits in lower storm levels. Upward directed perturbation pressure forces in the vicinity of the mesocyclone are reduced and can be reversed as the low-level vorticity amplifies. The sudden formation of concentrated rear downdrafts, commonly observed in tornadic thunderstorms, apparently stems from the vertical pressure gradient reversal. Reduced upward pressure forces decrease the storm's ability to lift low-level negatively buoyant air at the base of the updraft. Further, the restructured pressure forces create a flux of air parcels into the mesocyclone from higher levels on the storms' rear. In the final stages, downdrafts fill the mesocyclone, and updrafts in neighboring regions weaken. Qualitative examination of retrieved buoyancy distributions suggests that horizontal vorticity created by buoyancy gradients in inflow regions is not essential for mesocyclone intensification or for tornadogenesis.

Milking Characteristics of Teatcups with Reduced Liner Wall Movement

To evaluate effects of reduced liner wall movement on milking performance and pressure gradient across the short milk tube, we developed and compared four experimental teatcup assemblies to a conventional unit. These assemblies were 1) a conventional liner with restricted liner wall movement, 2, 3) two partial liners of different lenghts, and 4) a partial liner equipped with a mouthpiece adapter. In three of the four experiments, milking performance was evaluated. In the fourth experiment vacuum conditions and reverse pressure gradients across the short milk tube were measured.The long partial liner resulted in longer milking time and lower peak milk flow rate than the control (7.6 versus 5.9min and 2.5 versus 3.2 kg/min). The difference in milking time was eliminated when the mouthpiece adapter was included. Characteristics of milking performance of the restricted liner were similar to the control with the exception of peak milk flow rate, which was decreased for the restricted liner (3.3 versus 3.6 kg/min).Reverse pressure gradients were measured across the short milk tubes of the conventional liner (−4 kPa) and the restricted liner (−2 kPa). Reverse gradients were practically nonexistent for the partial liner assemblies.

Investigation of combustion phenomena associated with the flow of hot propellant gases. III: Experimental survey of the formation and decay of muzzle flow fields and of pressure measurements

The development of the flow fields generated by firing a 7.62-mm caliber rifle is measured using a time-resolved spark shadograph technique. Two precursor flow fields are forced out ahead of the projectile. The propellant gas flow field forms upon separation of the projectile from the muzzle. While the first precursor develops in a nearly spherical manner, the development of the second precursor and the propellant gas blast field occur anisotropically. In the axial downstream direction the propagation velocity is higher than along the lateral direction. Motion of observable discontinuities is shown to follow time-displacement laws. In addition, the local and temporal pressure distribution in the muzzle blast field of the rifle was determined by immersing quartz probes in the flow. Within the main expansion zone of the flow, subatmospheric pressures of p > 0.2 × 10 5N/m 2 are observed. After passing through the shock fronts which terminate this expansion zone, the pressure increases to p > 1.5 × 10 5N/m 2. Expansion starts anew at some distance from the shock front, and the pressure decreases. This decrease is rapidly disturbed by the onset of combustion, and the buildup of reverse pressure gradients between the shock front and the reaction zone is observed. Further downstream, expansion commences anew until the gas-air interface is reached. At this interface the pressure increases indicating the deceleration of flow within the “contact surface.” The overpressure of the blast was shown to decrease both in-axis and lateral from p > 3.5 × 10 5N/m 2 near the muzzle to about p = 1.1 × 10 5N/m 2 at distance 2 m. The noted features of the flow and the interaction with the combustion processes are discussed in detail.

Investigation of combustion phenomena associated with the flow of hot propellant gases-II: Gas velocity measurements by laser-induced gas breakdown

The local and temporal distribution of gas velocity along the axis of the propellant gas flow ahead of the muzzle of a caliber 7.62 mm rifle is determined by using luminous tracers produced by a laser pulse. Within the main expansion zone of the flow field, maximum gas velocities of 2400 m/s are observed. After passing through the shock front which terminates this expansion zone, the flow is decelerated to ν < 150 m/s. Expansion starts anew at some distance from the shock front leading to gas velocities of several hundred m/s. This expansion is rapidly disturbed by the onset of secondary combustion causing a ‘breakdown’ of the flow. The effect is explained by the buildup of reverse pressure gradients between the shock front and the reaction zone.

Pressure as a function of time and distance in a vented vessel

Rates of fall in pressure have been measured at different positions in a number of differently-shaped vessels vented by means of bursting diaphragms. It is demonstrated that the qualitative features of the experimental pressure histories are predicted by characteristic theory. Attention is drawn to the importance of the reflection of the expansion wave which may partly explain the existence of twin pressure peaks typical of vented explosions and which can result in the reversal of pressure gradients within the vessel.

Reversal of pressure gradients and wind circulation across India and the southwest monsoon

AbstractThis paper describes some features of the climatology of the free atmosphere over India which are related to the onset and withdrawal of the monsoon.

The valve-closing function of the right atrium: A study of pressures and atrial sounds in patients with heart block

I N second degree or complete heart block, prolonged ventricular diastoles allow the atria1 contraction cycle to be studied in a pure form. Auscuitation may reveal two sounds in association with these isolated atria1 contractions (Fig. l), here referred to as the first and second atria1 sounds; Lewis [7] suggested that the second atria1 sound was due to closure of the atrioventricular valves. To test whether the tricuspid valve can be closed by atria1 contraction, four subjects with heart block have been studied by right heart catheterization. Theory suggests that if the tricuspid valve does close following atria1 contraction it will trap blood in the ventricle, ventricular pressure will remain elevated and a reversed pressure gradient will develop across the valve with the lower pressure in the atrium. The catheter pressure measurements were designed to detect such a reverse gradient, if present.