Peak Pressure Drop Research Articles

Three-dimensional cerebrospinal fluid flow within the human central nervous system

This work describes a three-dimensional (3D) numerical simulation of the flow field of the complete enclosed Central Nervous System (CNS) including the ventricular system, the spinal cord and spinal sub-arachnoid space (SAS). Previous works on the topic consider only parts of the complete system imposing artificial boundary conditions at internal cross sections. The computational domain was constructed from MRI data using Materialise Software Mimics. In this work pulsatile velocity inlets in the lateral ventricles, due to the cardiac cycle, were used to simulate the dynamic nature of the CSF, whilst pressure outlets were used to model the areas of CSF re-absorption. A porous medium formulation (Darcy flow) was considered in the SAS to account for the effect of the arachnoid trabeculae within these areas. The simulation was run using the commercial CFD code Fluent using the laminar solver and transient simulation. A maximum CSF velocity was found to be in the region 11.8 mm/s, with a peak pressure drop through the aqueduct of the order of 2.8 Pa corresponding to a calculated peak Reynolds number of 12. CSF pressure at the exits of Magendie and Luschke were found to vary over the cardiac cycle, with pressure at the exits of Luschke being higher than Magendie for large periods of the cycle. CSF was seen to enter the SAS as a laminar jet from the exits of Magendie and Luschke. By only considering the cardiac cycle a very slow CSF motion within the spinal SAS was observed with magnitudes significantly reduced after a depth of 50mm down the column from the exit of Magendie. This result suggests that pulsating wall motion in the region of the spinal cord, due to respiratory effects, needs to be considered in order to predict experimentally observed flow recirculation within the spinal sac. 287 words.

Pacemaker Lead-Induced Severe Tricuspid Valve Stenosis

A 73-year-old man had a DDD pacemaker implanted in 1991 for symptomatic high-degree atrioventricular block. Because of atrial lead dysfunction, a new atrial lead was implanted in 1998. Ten years later, the patient experienced unexplained ascites and edema with progressive exertional dyspnea, for which he was referred. Chest radiography showed a loop of the ventricular lead at the level of the tricuspid valve (Figure 1). Transthoracic echocardiography demonstrated an enlarged right atrium. The mean diastolic gradient across the tricuspid valve was 15 mm Hg with a peak pressure drop of 29 mm Hg without tricuspid regurgitation (Figure 2). No other abnormalities were found. Transesophageal echocardiogram revealed looping of one of atrial leads at the …

Hydrodynamics of gas–solid fluidization in tapered beds

AbstractGas–solid fluidization has a wide range of industrial applications like catalytic reactions, combustion, gasification, etc. In a number of these applications, there is particle size reduction during the operation leading to severe entrainment and limitation of operating velocity. The various problems associated with particles of different sizes or changing particles sizes could be overcome by adopting tapered beds in fluidization operation.In the present investigation, the fluidization phenomenon in tapered beds has been critically assessed through experimental investigations using particles of different sizes and materials and wide range of apex angles of the vessels. The effect of particle size and apex angle on the fluidization behaviour is clearly brought out which has not been reported so far in literature. The importance of compressive force existing in tapered beds is highlighted.In addition, correlations for all hydrodynamic characteristics, viz. critical fluidization velocity, minimum velocity for full fluidization, maximum velocity for defluidization, peak pressure drop, fluctuation ratio, compressive force, and hysteresis have been developed some of which are proposed for the first time.

Hydrodynamics of Conical Spouted Beds Using Different Types of Internal Devices

AbstractThe hydrodynamics and performance of conical spouted beds provided with different types of draft‐tubes have been studied for the treatment of fine particles. Correlations for calculation of the minimum spouting velocity, operating pressure drop, and peak pressure drop have been developed as a function of dimensionless moduli that take into account the geometric factors, particle characteristics, and operating conditions. A statistical analysis of the data obtained following a design of experiments shows that the height of the entrainment zone is highly significant in the nonporous draft‐tubes, whereas the stagnant bed height is the counterpart in the open‐sided draft‐tubes.

Right ventricular myocardial velocities and timing estimate pulmonary artery systolic pressure

Background Non-invasive estimation of pulmonary artery systolic pressure (PASP) is important for identifying and following up patients. We aimed at revisiting the accuracy of various right ventricular (RV) Doppler echocardiographic measurements of PASP. Methods Twenty-eight patients were studied with simultaneous right heart catheterization (RHC), conventional and tissue Doppler echocardiography (TDE). We measured RV–right atrial (RA) peak pressure drop, RV spectral filling and myocardial velocities and timings. Results RV–RA peak pressure drop ( r = 0.89, p < 0.001) strongly correlated with PASP. Both RV spectral and myocardial measurements of isovolumic relaxation time (IVRT) modestly correlated with PASP ( r = 0.63, p < 0.01 and < 0.001). Time interval measurements missed 6 and 9 cases with normal PASP by using proposed cut off values. Combining myocardial IVRT and isovolumic contraction velocity (IVCV) in a formula, predicted PASP in all but 3 of our patients. In addition, TDE measurements were obtainable in all cases compared to RV–RA gradient which were measurable in only 64% of patients. Conclusion RV–RA peak pressure drop is the most accurate non-invasive method for assessing PASP. Combining myocardial IVCV and IVRT can be used accurately in estimating PASP being more feasible than RV–RA drop. Such additional measurement might be important in patients follow-up when RV–RA gradient is difficult to obtain.

A First Approach to CFD Simulation of Hydrodynamic Behaviour in a Conical Spouted Bed Contactor

Spouted beds have a widespread application in the processing industry for efficient contacting of large particles with a gas. However, there is no detailed understanding of the complex behaviour of these systems, especially for fine particles, which means significant scale-up problems in industry. This paper approaches fundamental aspects of the computational fluid dynamic simulation of fine particle spouting. Using the commercial CFD simulation package Fluent (version 6.3), the spouting hydrodynamics of fine particles in a conical spouted bed is simulated and compared with experimental data.Fluent code offers a variety of models to describe the physical phenomena occurring in these kinds of reactors. In some cases, the choice is straightforward, whereas in other cases more than one option is valid a priori, and so the best one has to be selected. The main choices are the Lun et al. (1984) approach for granular kinetics and the Gidaspow model for drag force.In order to validate this selection process, model predictions are compared with experimentally observed hydrodynamic patterns, whereby CFD model parameters can be tuned. It has been proven that after this tuning, the model explains the hydrodynamic behaviour of the bed and the influence of geometric parameters and bed nature (sand, glass beads) concerning spout stability and minimum spouting velocity. Nevertheless, peak pressure drop values predicted by the model are considerably smaller than the experimental values. The model also provides reasonable predictions for spout shape, local bed voidage and air and particle velocities.

Influence of Multiple Stenoses on Echo-Doppler Functional Diagnosis of Peripheral Arterial Disease: A Numerical and Experimental Study

The objective of this paper was to evaluate the ability of the peak systolic velocity ratio (PSVR) and pressure drop (DeltaP) to detect and grade multiple stenoses in lower limb mimicking arteries. Numerical simulations and experiments in vascular phantoms allowing ultrasound duplex scanning and pressure measurements were used to investigate simple and double stenotic arterial segments. Inter-stenotic distance, severity of the distal stenosis, flow rate and flow profile (steady or pulsatile) were the tested parameters. The three-dimensional simulations considered the turbulent two-equation Wilcox model. Agreements were observed between the experimental and numerical results for DeltaP and PSVR. The maximum PSVR along the artery was shown to be mainly influenced by the severity of the most important stenosis. However, mutual interactions of both stenoses on hemodynamics were noted. By using the clinical PSVR threshold used to diagnose critical lesions (PSVR > or = 2), its longitudinal evolution along the artery poorly reflected the length of the lesion or the impact of surrounding stenoses. This investigation confirms the interaction between adjacent stenoses on hemodynamics and its impact on the Doppler ultrasound index PSVR.

Statistical Correlation Between Transient Pressure Drop and Cavitation at Closure of a Mechanical Heart Valve

Cavitation on a mechanical heart valve (MHV) is attributable to transient regional pressure drop at the instant of valve closure. As a cavitation bubble collapses, it emits shock waves, which have the characteristics of high frequency oscillations (HFO) on a pressure time trace. The potential for such HFO bursts to cause material damage on an MHV can be measured by the cavitation impulse I, which is defined as the area under the trace of the HFO bursts. In the present study, experiments were conducted on a bileaflet MHV in a durability tester, operated at pulse rates from 300-1,000 bpm. In each case, the transient pressure near an occluder was monitored for 60,000 beats via a transducer. The peak pressure drop Pm and the corresponding cavitation impulse I obtained for the 60,000 beat sequence are found to resemble sample records of two stationary stochastic processes, each of which follows a log normal distribution. Their first order probability density functions are estimated from the records. The correlation is investigated between I and Pm associated with each beat, which is found to be of statistical significance.

Transient Pressure and Flow Predictions for Concentrated Packed Absorbers Using a Dynamic Nonequilibrium Model

A dynamic, nonisothermal, nonequilibrium model is developed to predict detailed transient behavior during startup and transition to standby for concentrated random-packed absorbers. The model accounts for solvent evaporation and transient hydraulic behavior and is used to simulate the aqueous absorption of gaseous HCl. During startup, the simulations show a large decrease in the gas flow rate followed by a recovery due to high rates of solvent evaporation. The model predicts a spike in the solute composition in the gas during the startup transient. At low mass-transfer rates, this spike can cause unacceptable solute levels in the vent. During the transition to standby operation (where feed gas is replaced with carrier gas), there is an appreciable increase in the gas flow rate and pressure in the column. The peak transient gas flow rates are about 3 times greater than the feed rate, and the peak pressure drops are about 20 times greater than those at flooding. These spikes may be too brief to flood the column, but mechanical damage is possible. The transient also shows solute being desorbed from the bottom part of the column and re-absorbed in the upper part of the column, which causes a burp of solute in the vent gas. In less concentrated columns, the burp can result in unacceptable solute levels in the vent. These problems can be addressed by initially reducing the carrier gas flow rate. The model also applies to the use of scrubbers for the control of toxic vent emissions.

Prediction of symptom-onset in aortic stenosis: a comparison of pressure drop/flow slope and haemodynamic measures at rest

To compare the pressure drop/flow slope with peak and mean pressure drop, effective orifice area and aortic valve resistance for the prediction of symptom-onset we performed resting and dobutamine stress echocardiography in 49 asymptomatic patients with aortic stenosis (peak aortic velocity>2.5 m/s). The end-point was progression to symptoms requiring surgery and patients were followed for a mean 21.2 (5.2) months. A total of 23 (47%) patients progressed to symptoms requiring aortic valve replacement and 26 remained asymptomatic. There was no significant difference in age, gender, fractional shortening or the presence or absence of coronary artery disease between these groups. There were differences in peak aortic velocity ( P<0.0001), peak and mean pressure drop ( P<0.0001), effective orifice area ( P=0.03), aortic valve resistance ( P=0.001) and pressure drop/flow slope ( P<0.0001). On Cox regression analysis, the pressure drop/flow slope ( P<0.0001), peak aortic velocity ( P=0.005) and peak pressure gradient ( P=0.02) were independent predictors. Mean event-free survival at 2 years for peak velocity >4.0 m/s was 17% and for pressure drop/flow slope >0.10 mmHg/ms −1 was 20%. Of 13 patients reporting symptoms during dobutamine stress, 10 (77%) developed spontaneous symptoms during follow-up compared with 13 of 36 (36%) with no symptoms ( P=0.11). The pressure drop/flow slope is a better independent predictor of symptom onset than resistance, mean pressure difference and effective orifice area, but is similar to peak velocity.

Hydrodynamic characteristics of a rotating jet annular spouted bed

A novel Rotating Jet Annular Spouted Bed (RJASB) dryer was developed for drying of particulates in the falling rate period. This gas-particle contactor consists of one rotating spouting air jet in the annular region of a cylindrical vessel. Effects of the spouting jet rotational speed, bed height, and nozzle diameter as well as particle size, shape and density on the minimum spouting velocity as well as peak and steady spouting pressure drops were determined experimentally and correlated empirically.

Aerodynamics of a novel rotating jet spouted bed

A novel rotating jet spouted bed (RJSB) is developed and tested. It consists of a rotating air distributor with two radially located spouting air nozzles. The effects of bed height, distributor rotational speed, nozzle diameter and particle properties on the flow characteristics were examined. Various flow regimes were mapped as functions of distributor rotational speed and superficial air velocity for different materials and column dimensions. Empirical correlations were developed for the minimum spouting velocity, peak pressure drop and steady spouting pressure drop.

Influence of the size of aortic valve prostheses on hemodynamics and change in left ventricular mass: Implications for the surgical management of aortic stenosis

Discussion of aortic valve replacement has primarily concerned the choice between tissue and mechanical prostheses. Less emphasis has been placed on prosthesis size. Despite technical advances increasing prosthesis orifice area, small valves implanted in the unenlarged aortic root may not be significantly less obstructive than the stenotic native valves they replace. Methods: In this work we studied 52 patients (31 women, 21 men; mean age 59.2 years) in whom valve prostheses sized 19, 21, 23, or 25 mm (30 bioprostheses and 22 tilting disc valves) had been implanted to replace stenotic aortic valves. Most patients with 19 or 21 mm prostheses were women. Doppler and conventional echocardiographic studies were performed in the 10 days preceding the operation and between 10 and 40 months (mean 18 months) after the operation. The patients receiving larger valve sizes had significantly larger body surface areas than those receiving smaller valve sizes (mainly women). Results: No significant differences were observed between preoperative and postoperative diameters or left ventricular systolic function parameters, but left ventricular mass and mass index decreased in all four groups (albeit nonsignificantly in the 19 mm group, and with less statistical significance in the 21 mm group than in the 23 and 25 mm groups). Postoperative peak and mean transvalvular pressure drops were significantly greater in the 19 mm group than in the other groups, and the 21 mm group had significantly greater transvalvular pressure drops than the 25 mm group. Postoperative effective valve area was significantly smaller in the 19 mm group than in the 21 mm group, and significantly smaller in the 21 mm group than in the 23 and 25 mm groups. Conclusion: We conclude that despite undeniable recent improvements in the design of artificial heart valves, 19 mm aortic prostheses continue to create significant obstruction of the left ventricular outflow tract and, possibly as a consequence of this, fail to bring about significant reduction in left ventricular hypertrophy. (J T horac C ardiovasc S urg 1996;112:273-80)

Early diastolic left ventricular inflow pressures in normal subjects and patients with dilated cardiomyopathy. Reconstruction from pulsed Doppler echocardiography.

To estimate early diastolic left ventricular inflow pressures in normal subjects and patients with dilated cardiomyopathy, and thus to assess the potential effect of restoring forces. Early diastolic left ventricular inflow pressures were reconstructed using the ventricular blood as an accelerometer, by measuring velocity at 1 cm intervals within the left ventricle from mitral ring to apex by pulsed Doppler echocardiography, and differentiating the records to obtain the acceleration. Aortic component of second heart sound (A2) was used to fix relative timings. The local pressure gradient was determined from the acceleration at each level, and the total pressure drop during the acceleration (+ peak PD) and deceleration (- peak PD) phases of the filling interval were determined by summing the local increments. The total stroke volume (SV) at the left ventricular outflow tract and the mitral stroke distances (MSD) were also determined, using the time-velocity integral at mitral ring level. Effective flow orifice area was thus SV/MSD. Inflow jet width across the mitral valve was estimated by cross sectional colour Doppler flow mapping. 32 patients with dilated cardiomyopathy with a dominant mitral E or summation wave, and 24 normal subjects of similar ages. Normal + peak PD was 3.9 (SD 0.7) v 7.4 (2.2) mm Hg in dilated cardiomyopathy (P < 0.01). Normal - peak PD was 2.5 (0.9) v 5.6 (2.8) mm Hg in cardiomyopathy (P < 0.01). Normal effective flow orifice area was 5.9 (1.3) v 1.9 (0.8) [range 0.9 approximately 3.7] cm2 in cardiomyopathy (P < 0.01). This corresponded to 71 (18)% of the end systolic cavity cross section in normals v 11 (6)% in dilated cardiomyopathy (P < 0.01). Normal cross sectional colour inflow jet width was 2.7 (0.3) v 1.5 (0.4) cm in cardiomyopathy (P < 0.01). The jet width correlated with flow width calculated from effective flow orifice area (r = 0.82, P < 0.01). (1) Total early diastolic positive and negative peak pressure drop are normally low, so that significant negative left ventricular pressures are not needed to explain normal resting early diastolic mitral flow velocities. (2) These low pressure drops are only possible with a large effective orifice area approaching end systolic left ventricular cavity area. (3) Atrioventricular pressure drops are much greater in dilated cardiomyopathy, where increased inflow accelerations are due to reduced effective flow orifice area. These disturbances will impair filling independently of any abnormality of relaxation or compliance.

Doppler Echocardiographic Comparison of Small (19 mm) Bileaflet and Pericardial Heart Valve Prostheses in Aortic Position

The resting haemodynamics of five types of small (19 mm) aortic valve prosthesis (2 bileaflet, 3 pericardial) were evaluated with Doppler echocardiography in 43 patients. Two received St Jude Medical and six CarboMedics bileaflet valves and 35 were given bioprostheses--16 Ionescu-Shiley, four Mitroflow and 15 Labcor-Santiago. No significant differences in peak or mean transvalvular pressure drop or in effective valve area were found between the bileaflet and the pericardial valves or among the three types of bioprosthesis. All but one of the bileaflet prostheses showed a characteristic regurgitation pattern, with two lateral and one central jet, and 16 (46%) of the bioprostheses showed central regurgitation, but in no case were these jets haemodynamically significant. Thus the 19 mm bileaflet and the studied pericardial prostheses all have satisfactory resting haemodynamics, and all are suitable for implanting in small aortic roots.

Limitations of Doppler ultrasound in the diagnosis of neonatal coarctation of the aorta

Blood flow velocities were recorded using Doppler ultrasound in 40 babies aged between 1 day and 4 weeks who had coarctation of the aorta or interruption of the aortic arch. High velocity jets of 3 m/sec or more in the descending aorta were detected in only 3 patients, two of whom had interruption of the aortic arch. There was very poor agreement between upper and lower limb blood pressure difference and Doppler derived peak pressure drop. Doppler ultrasound may provide useful information on coexisting intracardiac abnormalities, but in the presence of ductal patency is of little value in the diagnosis of coarctation itself.

Day to day reproducibility of Doppler sonographic measurements in patients with valvular aortic stenosis.

High reproducibility of Doppler gradient measurements is necessary for both the reliable noninvasive assessment of the severity of aortic stenosis and for repeated follow-up examinations in individual patients. We therefore studied day to day reproducibility of Doppler sonographically measured peak pressure drops in 46 patients with valvular aortic stenosis. Clinically stable patients were examined twice within 29 +/- 18.2 days by the same examiner. Peak pressure drop (PPD) and peak flow velocity differed between the two examinations by 8.6 +/- 7.0 (range 0-29) mmHg and by 0.25 +/- 0.18 (range 0-0.7) m/s, respectively. Reproducibility was comparable in patients with excellent, good, and moderate quality examinations, but was lower in the 6 patients with poor quality examination. Variability of PPD, but not of peak flow velocity was higher (p less than 0.05) in patients with severe (PPD greater than 60 mmHg) stenosis. Reproducibility was comparable in patients with or without concomitant aortic incompetence and in patients with normal or reduced left ventricular function. Similar reproducibility was obtained in patients with heart rate changes below or above 10 beats/min between the two examinations. It is concluded that good reproducibility of Doppler measurements in patients with aortic stenosis allows reliable noninvasive assessment of the severity of the stenosis. In follow-up studies of patients with mild to moderate aortic stenosis increases in peak flow velocity in excess of 15% (mean day to day variability +2 SD) are highly indicative of the true progress of the stenosis.

Pressure drop and power dissipation in oscillatory wavy-walled-tube flows

Pressure drops occurring in oscillatory viscous flows in wavy-walled tubes have been studied experimentally, for Reynolds numbers up to 1500 and Strouhal numbers in the range 0.005 to 0.02, and by numerical solution of the Navier-Stokes equations, for Reynolds numbers up to 200 and Strouhal numbers between 0.005 and 0.1. Agreement was good for values of the mean modulus of the pressure drop at lower Strouhal numbers and for values of the mean power dissipation at all Strouhal numbers.Numerical solutions have shown that the pressure drop may vary non-sinusoidally, even though the imposed variation in flow rate is sinusoidal. This cannot be explained by the nonlinearity of the steady pressure drop-flow rate relationship, and arises because the velocity field is not quasi-steady. In particular energy may be stored in strong vortices formed during the acceleration phase of the flow cycle, and partially returned to the main flow later. The peak pressure drops in such flows, which are associated with the formation of these vortices, can be almost twice as large as values predicted by adding the appropriate quasi-steady and unsteady inertial contributions. This finding is important in the wider context of unsteady conduit flow.The dependences of the mean modulus of the pressure drop and the mean power dissipation on the Strouhal number and frequency parameter were investigated in detail numerically for two geometries. It was not possible to reduce either dependence to a function of a single parameter. The ‘equivalent’ straight-walled tube for power dissipation was found to have a smaller bore than that for pressure drop, leading to smaller ‘phase angles’ than might have been expected at large values of the frequency parameter. This is because as the pressure drop becomes increasingly dominated by unsteady inertia, there remain relatively large recirculations in which energy is dissipated.

Noninvasive estimation of systolic pressure in the right ventricle in patients with tricuspid regurgitation.

In 70 patients, tricuspid regurgitation was diagnosed with Doppler by recording reverse flow in systole originating at the tricuspid orifice, directed into the right atrium. The peak velocities were recorded, and the peak pressure drop from the right ventricle to the right atrium in systole was calculated from the simplified Bernoulli equation (pressure drop = 4 X 4 X Vmax2), and was found to correlate well with invasive measurements (r = 0.97, SEE +/- 6.1 mmHg). Central venous pressure was judged by neck vein congestion and added to the pressure drop to obtain a noninvasive estimation of systolic right ventricular pressure. Correlation with catheterization measurements was good (r = 0.96, SEE +/- 7.1 mmHg). In patients in whom the tricuspid regurgitation was judged as mild with Doppler, the correlation between noninvasive and invasive measurements of the transtricuspid pressure drop was still good (r = 0.95, SEE +/- 5.1 mmHg), indicating that the viscosity of the blood does not invalidate the use of the simplified Bernoulli equation when the regurgitant area of the valve is small.

Noninvasive assessment and differentiation of left ventricular outflow obstruction with Doppler ultrasound.

Blood flow velocities in the left ventricle and the ascending aorta were recorded noninvasively with Doppler ultrasound. The ultrasound beam was aligned as much as possible to the direction of velocity, using the frequency shift in the audio signal as a guide to obtain velocities as close as possible to those present. From the maximal velocity recorded by continuous-wave Doppler, a peak pressure drop was calculated in 24 patients with aortic valve stenosis and nine with fixed subaortic stenosis. Fourteen patients with aortic stenosis and three with fixed subaortic stenosis were catheterized. In these patients, the correlation between calculated pressure drops and those obtained by pressure recording was good (r = 0.85). The pressure drop can be underestimated by underestimating velocity, but cannot be overestimated. With pulsed Doppler, the level of obstruction can be determined.