Pressure Dilatation Research Articles

Systemic arterial pressure and brachial arterial flow-mediated dilatation in young cardiac transplant recipients

Hypertension is a frequent complication of orthotopic cardiac transplantation. Recent studies in the pediatric heart transplant population showed that 63% of pediatric heart transplant recipients require long-term antihypertensive therapy.1 Similar studies in an older cardiac transplant population2,3 demonstrated that >90% of patients require antihypertensive therapy. Nonetheless, there is a paucity of published data regarding the mechanism of postcardiac transplant hypertension. A limited number of studies in adult cardiac transplant patients with hypertension suggest that vascular endothelial dysfunction may play a significant role.4,5 Recently, in humans and in animal models with genetic hypertension, endothelial dysfunction has been shown to be associated with a specific decrease in nitric oxide (NO)-dependent vasodilatation.6–8 This study evaluated endothelial function and NO production in a group of young, relatively healthy cardiac transplant recipients without preexisting hypertension.

The effects of heat release on the energy exchange in reacting turbulent shear flow

The energy exchange between the kinetic and internal energies in non-premixed reacting compressible homogeneous turbulent shear flow is studied via data generated by direct numerical simulations (DNS). The chemical reaction is modelled by a one- step exothermic irreversible reaction with Arrhenius-type reaction rate. The results show that the heat release has a damping effect on the turbulent kinetic energy for the cases with variable transport properties. The growth rate of the turbulent kinetic energy is primarily in uenced by the reaction through temperature-induced changes in the solenoidal dissipation and modifications in the explicit dilatational terms (pressure–dilatation and dilatational dissipation). The production term in the scaled kinetic energy equation, which is proportional to the Reynolds shear stress anisotropy, is less affected by the heat release. However, the dilatational part of the production term increases during the time when the reaction is important. Additionally, the pressure–dilatation correlation, unlike the non-reacting case, transfers energy in the reacting cases, on the average, from the internal to the kinetic energy. Consequently, the dilatational part of the kinetic energy is enhanced by the reaction. On the contrary, the solenoidal part of the kinetic energy decreases in the reacting cases mainly due to an enhanced viscous dissipation. Similarly to the non-reacting case, it is found that the direct coupling between the solenoidal and dilatational parts of the kinetic energy is small. The structure of the flow with regard to the normal Reynolds stresses is affected by the heat of reaction. Compared to the non-reacting case, the kinetic energy in the direction of the mean velocity decreases during the time when the reaction is important, while it increases in the direction of the shear. This increase is due to the amplification of the dilatational kinetic energy in the x2-direction by the reaction. Moreover, the dilatational effects occur primarily in the direction of the shear. These effects are amplified if the heat release is increased or the reaction occurs at later times. The non-reacting models tested for the explicit dilatational terms are not supported by the DNS data for the reacting cases, although it appears that some of the assumptions employed in these models hold also in the presence of heat of reaction.

Flame acceleration and overpressure development in a semiopen tube with repeated obstacles

An investigation of the dependence of turbulent flame acceleration and overpressure in a semiopen tube on the configuration of obstacles has been performed in a flame propagation tube with one end closed and the other open, having an 80 mm inner diameter, a 5 m length, and repeated obstacles. Three kinds of obstacle shapes have been used for various blockage ratios and spacing of obstacles. There is evidence that the influence of the obstruction characteristics on the flame speed and overpressure is different in the various flame regimes. In the low-speed combustion regime, the highest terminal flame speed is obtained with the blockage ratio of BR=0.3–0.4. In the choking regime, the maximum flame speed is insensitive to the blockage ratio, and a blockage ratio of about 0.5 generates the highest peak overpressure. In the detonation regime, the maximum flame speed and overpressure decrease with increasing blockage ratio due to the severe momentum losses induced by the blockage effect of the obstacles, and at the same time, the detonative range is observed to become narrower. The steady flame speed is independent of the shape of obstacles when the blockage ratio is the same, and the spacing of obstacles plays a role only in determining the flame acceleration rate rather than the steady flame speed: the highest mean flame speed and peak overpressure are obtained when the spacing of obstacles is about equal to the inner diameter of the flame tube. In addition, the unsteady compressible flow model with correction for the influence of turbulence Mach number on viscosity dissipation and pressure dilatation was formulated, and numerical studies using the eddy-break-up combustion model were made to predict the flame acceleration and the development of overpressure in the tube. The comparison of calculated overpressure and flame speeds with experimental data shows good agreement.

Vulvar angiokeratoma following radical hysterectomy and radiotherapy

Vulvar angiokeratoma following radical hysterectomy and radiotherapy

Modeling of flame-generated turbulence based on direct numerical simulation databases

Turbulent premixed flames propagating in homogeneous isotropic turbulent flows were simulated directly with a single-step irreversible reaction. Two cases were calculated, case H, with a high-density ratio of flame p u / p b =7.53, and case L, low-density ratio of flame p u / p b =2.50, while u′/u L was nearly equal to unity. We obtained databases of fully developed stationary turbulent flames. These databases were investigated by analyzing the transport equation for turbulent kinetic energy to study flame-generated turbulence and its models. We found that turbulent fluctuations of all components, especially the streamwise component, were amplified in the flame brush and that flame-generated turbulence increased for a larger density ratio of the flame. Analysis based on the Favre-averaged transport equation for turbulent kinetic energy showed that pressure-related terms produced kinetic energy in the flame brush, the mean pressure gradient term was most important in case H and the pressure work term was most important in case L. On the other hand, the diffusion and dissipation term and velocity gradient term decreased kinetic energy. Next, modeling of the important terms in the balance equations were discussed. The mean pressure gradient term, pressure dilatation term, and additional dissipation components were modeled and compared with the direct numerical simulation (DNS) results. The mean pressure gradient term was modeled with assumption on the density, and the model was in good agreement with DNS. The other two terms were also modeled by scaling and these models mimicked DNS well.

Direct simulation of turbulent supersonic boundary layers by an extended temporal approach

The present paper addresses the direct numerical simulation of turbulent zero-pressure-gradient boundary layers on a flat plate at Mach numbers 3, 4.5 and 6 with momentum-thickness Reynolds numbers of about 3000. Simulations are performed with an extended temporal direct numerical simulation (ETDNS) method. Assuming that the slow streamwise variation of the mean boundary layer is governed by parabolized Navier–Stokes equations, the equations solved locally in time with a temporal DNS are modified by a distributed forcing term so that the parabolized Navier–Stokes equations are recovered for the spatial average. The correct mean flow is obtained without a priori knowledge, the streamwise mean-flow evolution being approximated from its upstream history. ETDNS reduces the computational effort by up to two orders of magnitude compared to a fully spatial simulation.We present results for a constant wall temperature Tw chosen to be equal to its laminar adiabatic value, which is about 2.5 T∞, 4.4 T∞ and 7 T∞, respectively, where T∞ is the free-stream temperature for the three Mach numbers considered. The simulations are initialized with transition-simulation data or with re-scaled turbulent data at different parameters. We find that the ETDNS results closely match experimental mean-flow data. The van Driest transformed velocity profiles follow the incompressible law of the wall with small logarithmic regions.Of particular interest is the significance of compressibility effects in a Mach number range around the limit of M∞ ≃ 5, up to which Morkovin's hypothesis is believed to be valid. The results show that pressure dilatation and dilatational dissipation correlations are small throughout the considered Mach number range. On the other hand, correlations derived from Morkovin's hypothesis are not necessarily valid, as is shown for the strong Reynolds analogy.

Turbulent transport in spatially developing reacting shear layers

Direct numerical simulation (DNS) of turbulent reacting shear layers with significant heat release and local Reynolds numbers up to 1000 based on the Favre-averaged vorticity thickness was performed. The combustion is simulated by a single-step, second-order reaction with an Arrhenius reaction rate. The transport equations are solved using a low Mach number approximation in which the effects of heat release are accounted for through variable density. Two-dimensional simulations illustrate how growth rate decreases with increasing heat release and how the shear layer is sensitive to reaction rate. A three-dimensional simulation is used to generate data to study in detail how the flame alters the turbulence using budgets of turbulent kinetic energy. The three-dimensional results reveal that turbulent transport and pressure transport have a unique role in the overall energy balance in that they tend to increase kinetic energy near the edges of the layer while decreasing it at the center. Opposite behavior is found for pressure dilatation. Dissiplation and convection everywhere act as sinks for turbulence. The DNS results are compared with the standard k-∈ model for production and turbulent transport. The Boussinnesq approximation with a typical eddy viscosity constant yields the proper profile for production but, in general, overpredicts the peak value. The evaluation also reveals that the dilatation dissipation is small compared with the solenoidal dissipation. The standard gradient-diffusion model for turbulent transport/molecular diffusion is satisfactory when the molecular diffusion is small. The similarity between pressuren transport and turbulent transport suggests that pressure transport could also be modeled using a gradient-diffusion expression.

Combustion effects on turbulence in a partially premixed supersonic diffusion flame

Effects of chemical heat release on turbulence in a partially premixed diffusion flame are investigated using direct numerical simulation (DNS). The full three-dimensional time-dependent compressible Navier-Stokes equations are employed to simulate the coupling between supersonic turbulence and heat release from a one-step chemical reaction governed by the Arrhenius kinetics. Four reacting cases with increasing heat release and one nonreacting case have been studied. Combustion is found to produce strong coupling among fluctuations in velocity, pressure, density, and other thermochemical quantities. In the Reynolds stress budget, the pressure–strain term becomes dominant as the heat release increases. Despite its wave behavior and its alternate roles as a source and a sink, the pressure–strain serves to promote energy transfer from the streamwise to the transverse and spanwise directions, and from the diagonal to the off-diagonal components of the Reynolds stresses, thus reducing anisotropy. Moreover, the pressure–strain, which in the case of the turbulent kinetic energy budget can be split up into a mean pressure work and a pressure–dilatation, helps to convert chemical energy from combustion into turbulence energy, leading to the phenomenon of “combustion-generated turbulence.” Both the solenoidal dissipation and the dilatational dissipation in particular increase as the heat release increases, but their effects are secondary to those of the pressure–strain within the main combustion period. The mixing layer growth rate can be predicted directly through the integrated Reynolds stress generation, with the accuracy of prediction depending on the heat release rate and the Reynolds number. A simple mechanism for the interactions of combustion and turbulence is proposed, while the modeling difficulties involved are also outlined. Finally, the differences and similarities of heat release effects and compressibility effects are clarified.

Effects of pressure fluctuations on turbulence growth in compressible homogeneous shear flow

Direct numerical simulation results are used to study compressibility effects on turbulent energy growth in homogeneous shear flow. Normalized amplitude of the pressure fluctuations is shown to decrease as the turbulent Mach number increases. This decrease causes the reduction in the pressure–strain term, changes the anisotropy of the Reynolds stress, and reduces the growth rate of turbulent kinetic energy as is the case of mixing layers. The transport equation for the pressure variance is examined in detail. It is shown that when the turbulent Mach number is high the pressure-variance dissipation term is not negligible and contributes to the reduction in the pressure fluctuations. The pressure-variance dissipation is closely related to the high-wavenumber part of the pressure-variance spectrum. Profile of the spectrum is observed to depend on the turbulent Mach number. A statistical theory called the two-scale direct-interaction approximation is applied to obtain model expressions for pressure-related terms. The pressure–dilatation correlation is a dominant term in the pressure-variance equation; its model expression as derived contains the material derivative of turbulent kinetic energy. This quantity represents nonequilibrium properties of turbulent field and can explain the very small value of the pressure dilatation in boundary layers and channel flows. The model expression for the pressure–strain correlation is also derived; the normalized pressure variance can be a parameter that describes compressibility effect on the pressure strain.

Numerical simulation of shock‐turbulence interaction

AbstractWe outline results of numerical and theoretical investigation of shock‐turbulence interaction. Following the strategy of increasing complexity we first consider isotropic turbulence interacting with a normal shock. In the presence of mean‐shear and shock‐obliqueness, turbulence‐structure becomes dependent on additional parameters such as shock‐angle and strain‐rate. Presently, the most complex situation achievable by direct numerical simulation is the interaction of a turbulent boundary layer with a shock along a compression ramp. Based on recent results for a low‐Reynolds number direct simulation it is found that the simplified cases predict correct trends for turbulent structure. The DNS results indicate that explicit compressibility effects such as pressure dilatation and dilatation dissipation are non‐negligible in the interaction region.

The relation between CSF pressure and ventricular dilatation in hydrocephalic HTx rats.

The relation between increased cerebrospinal fluid (CSF) pressure and ventricular enlargement in infantile hydrocephalus is uncertain, variable, and probably dependent on the etiology of the condition. The HTx rat has early-onset hydrocephalus due to aqueduct stenosis in late gestation with ventricular dilatation present in fetuses. Increased CSF pressure, however, is first detected at postnatal day 10 or later. How the transition from low pressure to raised pressure hydrocephalus takes place, is not clear. In order to study this, we investigated how CSF pressure is related to the magnitude of ventricular dilatation using hydrocephalic and control HTx rats at postnatal days 15 and 21. At 15 days, the CSF pressure for hydrocephalics was 31.1 +/- 3.6 mm H2O (mean +/- SEM) which was not significantly higher than the controls at 19.9 +/- 2.8 mm H2O. At 21 days, the pressure was significantly higher in hydrocephalics at 50.9 +/- 7.5 mm H2O compared to 24.3 +/- 3.6 mm H2O for controls, p < 0.05. The mean volume of the lateral ventricles was 409.3 mm3 at day 15 and 478.8 mm3 at day 21, whereas age-matched control rats in a previous study had ventricles of 14 and 25 mm3. At each age there was a significant linear relationship between CSF pressure and ventricle volume, p < 0.05 at day 15 and p < 0.01 at day 21. Closer examination of the data, however, showed that 5/10 hydrocephalics at day 15 and 4/10 at day 21 had CSF pressures that were within the 95% confidence limits of the mean pressures for the control groups. The ventricle volumes of these low pressure hydrocephalics were between 100 and 280 mm3 which was several times larger than the ventricles in control rats. CSF pressure in the remaining 15 day rats ranged from 30 to 53 mm H2O and in the 21 day rats from 46 to 90 mm H2O and the ventricles were between 280 and 1050 mm3. The results suggest that up to a critical volume the infant rat brain can accommodate additional CSF without an increase in pressure. At this volume there is a "break point" beyond which additional fluid results in further expansion along with an increase in CSF pressure.

High Versus Normal Balloon Pressure Dilatation for Coronary Stent Placement. 6-Month Clinical and Angiographic Results From a Randomized Multicenter Trial

High Versus Normal Balloon Pressure Dilatation for Coronary Stent Placement. 6-Month Clinical and Angiographic Results From a Randomized Multicenter Trial

Combustion-induced pressure effects in supersonic diffusion flames

A turbulent diffusion flame at a convective Mach number 1.2 was investigated using direct numerical simulation (DNS). The DNS employed the full time-dependent compressible Navier-Stokes equations coupled with a one-step chemical reaction governed by the Arrhenius kinetics. Detailed study of combustion-induced pressure effects on turbulence generation, conserved scalar, and stagnation enthalpy transport was conducted. Local countergradient diffusion (CGD) of a conserved scalar flux was observed for the first time in a diffusion flame where heat release was strong enough while gradient diffusion prevailed when heat release was zero or weak. The CGD occurred in spite of the absence of an externally imposed mean pressure gradient and was attributed to combustion-induced pressure fluctuations. The balance of the turbulent kinetic energy budget was strongly influenced by the pressure dilatation and the (combustion-induced) mean pressure work when heat release was strong. Both terms can be a source or a sink of turbulence, depending on the intricate interactions between turbulence and combustion. However, the temporal change in pressure ϱp/ϱt had an insignificant influence on the stagnation enthalpy transport. A linear relation between the stagnation enthalpy and the mixture fraction was confirmed, which could lead to considerable simplification in modeling high-speed turbulent combustion.

High versus normal balloon pressure dilatation for coronary stent placement. 6-Month clinical and angiographic results from a randomized multicenter trial

High versus normal balloon pressure dilatation for coronary stent placement. 6-Month clinical and angiographic results from a randomized multicenter trial

A pseudo-sound constitutive relationship for the dilatational covariances in compressible turbulence

The mathematical consequences of a few simple scaling assumptions regarding the effects of compressibility are explored using a singular perturbation idea and the methods of statistical fluid mechanics. Representations for the pressure–dilatation and dilatational dissipation appearing in single-point moment closures for compressible turbulence are obtained. The results obtained, in as much as they come from the same underlying procedure, represent a unified development for both dilatational covariances. While the results are expressed in the context of a statistical turbulence closure they provide, with very few phenomenological assumptions, an interesting and clear mathematical model for the ‘scalar’ effects of compressibility. For homogeneous turbulence with quasi-normal large scales the expressions derived are – in the small turbulent Mach number squared isotropic limit – exact. The expressions obtained contain constants that have a precise physical significance and are defined in terms of integrals of the longitudinal velocity correlation. The pressure–dilatation covariance is found to be a non-equilibrium phenomenon related to the time rate of change of the kinetic energy and internal energy of the turbulence; it is seen to scale with α2M2t εs [Pk/ε−1] (Sk/εs)2. Implicit in the scaling is a dependence on the square of a gradient Mach number, S[lscr ]/c. A new feature indicated by the analysis is the appearance of the Kolmogorov scaling coefficient, α, suggesting that large-scale quantities embodied in the well-established ε∼u˜3/[lscr ] relationship provide a link to the structural dependence of the effects of compressibility. The expressions for the dilatational dissipation are found to depend on the turbulent Reynolds number and scale as M4t (Sk/εs)4R−1t. The scalings for the pressure–dilatation are found to produce an excellent collapse of the pressure–dilatation data from direct numerical simulation.

Towards modeling inhomogeneous compressible turbulence using a two-scale statistical theory

Turbulence models for compressible flows are investigated using a statistical theory called the two-scale direct-interaction approximation. Inertial-range spectra for velocity and density variances are assumed to derive models for several correlations systematically; they include the dilatation dissipation, mass flux, Reynolds stress, and pressure–dilatation correlation. Model expressions are shown to contain two important parameters: the turbulent Mach number and the density variance normalized by the mean density. Typical terms are material derivatives of the turbulent kinetic energy and its dissipation rate as well as the mean velocity divergence. The statistical theory is also applied to a two-time velocity correlation to derive a transport equation for the eddy viscosity. The equation is combined with the turbulent kinetic energy equation to derive a model equation for the dissipation rate. Direct numerical simulation data of isotropic and homogeneous shear turbulence are used to examine models for the dilatation dissipation and the pressure dilatation. The normalized density variance is shown to be useful to explain results from two runs of isotropic turbulence with different initial conditions.

Study of compressibility modifications to the k−ε turbulence model

In modeling the effect of compressibility on the turbulence structure, additional physical mechanisms arise in production and dissipation of turbulent kinetic energy. Several proposed treatments dealing with the dilatation dissipation and the pressure dilatation correlation are discussed in the context of the k−ε two-equation model. Modifications accounting for the turbulent mass flux, enthalpic production, and baroclinic torque are also assessed along with the nonequilibrium treatment. These compressibility models are evaluated with the aid of experimental data for supersonic flow over an axisymmetric afterbody. With the free-stream Mach number of 2.46, the compressibility and nonequilibrium modifications give marginally better results for the turbulence structure and offer added insight into the mean and fluctuating flow fields.

Simultaneous monitoring of head-to-cervix forces, intrauterine pressure and cervical dilatation during labour

In many westernized countries, the caesarean section rate has now reached 15% or more, most commonly because of slow progress in labour. In order for labour to result in a vaginal delivery, the uterine cervix must dilate to allow the foetus to travel through the birth canal. This process is driven by uterine contractions, but the mechanisms by which the contractions result in cervical dilatation are still far from clear. The force exerted by the presenting part (foetal head) on the cervical tissue during contractions (head-to-cervix force, HCF) has been shown to be the variable with the best correlation with cervical dilatation. Unfortunately, the mechanism by which these two variables are related is still poorly understood. In order to investigate the relationship between head-to-cervix force, intrauterine pressure (IUP) and cervical dilatation, we have developed a system for their simultaneous and continuous monitoring during labour. The HCF is measured by using a novel intrauterine probe which is slipped alongside the foetal head so as to lie sandwiched between the latter and the cervix. The probe is fitted with six specially designed miniature force sensors, spaced 1.8 cm apart, which respond linearly and approximate the behaviour of load cells. They are interfaced with a PC by circuitry that allows auto-zeroing and drift compensation. The system enables simultaneous acquisition of intrauterine pressure and foetal heart rate (measured using a Sonicaid Meridian foetal monitor) via a serial link, together with continuous cervical dilatation measured by a caliper-like device applied to the cervix. Some preliminary data are presented, which suggest that the system can be used to investigate the role played by head-to-cervix force and intrauterine pressure in the cervix dilatation process.

A coherent flame model of premixed turbulent combustion in a counterflow geometry

Premixed combustion in turbulent counterflowing streams is studied theoretically by adopting a coherent flame model. With this model, a flame surface density Σ -described by a transport equation—is used to calculate the mean chemical reaction rate relative to the distribution of a Reynolds mean progress variable c . The value of a model constant β, which is connected to flame surface stretch and annihilation, is selected to match numerical solutions with experiments at different nozzle exit conditions giving various values of the mean progress variables at the stagnation point c ̄ (0) . The k ̃ − ϵ ̃ equations are used to predict the turbulence strain rate for the Σ balance equation and the turbulent viscosity μ t = C μ p ̄ k ̃ 2 ϵ ̃ for second-moment terms. In this turbulence model, the term representing correlation of pressure fluctuations and dilatation is found to be important to turbulence production in the reaction zone. It is found that two different regions exist in the counterflow field: an outer region, which is far from the stagnation point and without chemical reaction, and an inner sublayer, which is close to the stagnation point, which experiences a large bulk strain rate and is located in the combustion zone. In order to provide an appropriate theoretical description of the experimental conditions, the inner region is allowed to have a finite thickness, thus imposing a finite displacement on the outer solution, due to thermal expansion in the sublayer. A composite similarity solution is obtained by matching the boundary conditions of the inner and outer solutions at the cold flame edge. The effective mean bulk strain rate is increased slightly by the displacement effect. Theoretical predictions are found to be in good agreement with experimental data. The constant β is found to be approximately proportional to the mean bulk strain rate.

Cutting balloon angioplasty and stenting for aorto-ostial lesions.

OBJECTIVE: To investigate the use of a cutting balloon to overcome aorto-ostial lesions before stent implantation and thereby reduce the restenosis rate. DESIGN: Observational follow up study. SETTING: Tertiary referral...