Conduit Resistance Research Articles

Review of: a brief history of aqueducts and conduit resistance laws by Michele Mossa

Review of: a brief history of aqueducts and conduit resistance laws by Michele Mossa

Shunt resistance is associated with clinically important outcomes after the Norwood operation

BackgroundIn single-ventricle physiology, focus on pulmonary vascular resistance neglects the resistance in the conduit supplying the pulmonary inflow. MethodsConduit length and diameter, which can approximate conduit resistance, are available in the public dataset of Single Ventricle Reconstruction (SVR) trial. Conduit resistance was then calculated for SVR trial participants and the relationship with clinically important variables (death or transplant at 1 year, pulmonary artery size at second-stage palliation, pulmonary-to-systemic blood flow ratio, and supplemental oxygen requirement) was explored. To validate this calculated resistance, calculated resistance was compared with catheterization measurements at a single institution (not included in the SVR trial). ResultsIn the institutional dataset, calculated and measured resistances had an intraclass correlation of 0.78 for modified Blalock–Taussig shunts (MBTS). Within the SVR trial, transplant-free survivors had a lower MBTS resistance (median, 8.3 Woods Units [WU]. interquartile range [IQR], 6.5-11.1 WU) than patients who died or required transplantation (median, 13.0 WU; IQR, 9.4-16.6 WU, P = .0001). When we controlled for left pulmonary artery diameter after the Norwood procedure in the SVR trial, for each unit increase in MBTS resistance, the left pulmonary artery diameter at stage II decreased (–0.006 ± 0.002 cm, P = .005). When we controlled for pulmonary vascular resistance, greater MBTS resistance was associated with a decrease in log pulmonary-to-systemic blood flow ratio (–0.04 ± 0.015, P = .0048) in the SVR trial. Patients in the SVR trial requiring supplemental oxygen on admission for stage II palliation had greater MBTS resistance (median. 11.1 WU; IQR, 6.6-16.6 WU) than patients not requiring oxygen (median 8.3, WU; IQR, 6.5-11.1 WU, P = .015). ConclusionsConduit resistance is associated with important clinical outcomes after Norwood; however, further studies are required to guide conduit resistance optimization.

Tip-to-base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities.

In the stems of terrestrial vascular plants studied to date, the diameter of xylem water-conducting conduits D widens predictably with distance from the stem tip L approximating D∝Lb , with b≈0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip-to-base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip-to-base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip-to-base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes.

Hydraulic tuning of vein cell microstructure in the evolution of angiosperm venation networks

High vein density (D(V)) evolution in angiosperms represented a key functional transition. Yet, a mechanistic account on how this hydraulic transformation evolved remains lacking. We demonstrate that a consequence of producing high D(V is that veins must become very small to fit inside the leaf, and that angiosperms are the only clade that evolved the specific type of vessel required to yield sufficiently conductive miniature leaf veins. From 111 species spanning key divergences in vascular plant evolution, we show, using analyses of vein conduit evolution in relation to vein packing, that a key xylem innovation associated with high D(V) evolution is a strong reduction in vein thickness and simplification of the perforation plates of primary xylem vessels. Simple perforation plates in the leaf xylem occurred only in derived angiosperm clades exhibiting high D(V) (> 12 mm mm(-2)). Perforation plates in the vessels of other species, including extant basal angiosperms, consisted of resistive scalariform types that were associated with thicker veins and much lower D(V). We conclude that a reduction in within-vein conduit resistance allowed vein size to decrease. We suggest that this adaptation may have been a critical evolutionary step that enabled dramatic D(V) elaboration in angiosperms.

Method to Evaluate CIC Conductor Performance by Voltage Taps Using CSMC Facility

A method for evaluating the critical current of conductors is discussed in the context of the relationship between the electric field of the conduit surface and the superconducting cable in CIC conductors used for nuclear fusion reactors. The potential detected by voltage taps located on the surface of the conduit is the average potential of strands that have contact with the virtual surface area of the cable. This area is defined in terms of contact resistance between the conduit and strands as well as conduit resistance. Considering that a strand is uniformly distributed in a conductor due to the cabling effect, it is possible to replace in calculations the distribution of the electric field of a strand along the length of the conductor with the distribution of a cross section of the conductor, a replacement which renders possible statistical approach. As a result, the voltage measured by a pair of voltage taps located along the length of the conductor can be expressed in terms of a general formula which establishes a relationship between the electric field of the conduit surface and the superconducting cable including statistic errors. Using this formula, the electric field of the cable and the consequent superconducting properties of the conductor may be estimated with an acceptable degree of accuracy. Assessment of the experimental results and performance of the ITER conductors are introduced with discussion of temperature measurement issue, and the validity of the formula is discussed.

Computational study on the hemodynamics of the bypass shunt directly connecting the left ventricle to a coronary artery

A shunt from the left ventricle to the left anterior descending artery is being developed for coronary artery occlusive disease, in which the shunt or conduit connects the the left ventricle (LV) with the diseased artery directly at a point distal to the obstruction. To aid in assessing and optimizing its benefit, a computational model of the cardiovascular system was developed and used to explore various design conditions. Computational fluid dynamic analysis for the shunt hemodynamics was also done using a commercial finite element package. Simulation results indicate that in complete left anterior descending artery (LAD) occlusion, flow can be returned to approximately 65% of normal, if the conduit resistance is equal for forward and reverse flow. The net coronary flow can increase to 80% when the backflow resistance is infinite. The increases in flow rate produced by asymmetric flow resistance are enhanced considerably for a partial LAD obstruction, since the primary effect of resistance asymmetry is to prevent leakage back into the ventricle during diastole. Increased arterial compliance has little effect on net flow with a symmetric shunt, but considerably augments it when the resistance is asymmetric. The computational results suggest that an LV-LAD conduit will be beneficial when the resistance due to artery stenosis exceeds 27 PRU. if the resistance is symmetric. Fluid dynamic simulations for the shunt flow show that a recirculating region generated near the junction of the coronary artery with the bypass shunt. The secondary flow is induced at the cutting plane perpendicular to the axis direction and it is in the attenuated of coronary artery.

Dynamics of magma flow inside volcanic conduits with bubble overpressure buildup and gas loss through permeable magma

Many volcanic eruptions show transitions between extrusive and explosive behaviour. We develop a new generic model that considers concurrence between pressure buildup in the bubbles due to the viscous resistance to their growth and gas escape through the bubble network as they become interconnected. When the pressure difference between bubbles and magma reaches the strength of the material fragmentation occurs. The effect of grain size distribution on the flow in gas-particle dispersion is modelled by two populations of particles which strongly influence the velocity of sound in the mixture. Solutions to the steady-state boundary value problem show non-uniqueness. There are at least two regimes for the fixed parameters in the magma chamber. In the low discharge rate regime, fragmentation does not occur and magma rises with partial gas escape. This regime corresponds to extrusive activity. The upper regime corresponds to explosive activity. The simulations using the parameters defined at the workshop produced the following results for a rhyolitic magma composition: discharge rate 5.5×10 7 kg/s; fragmentation at depth of 2585 m with magma vesicularity of 0.74; exit gas velocity varies from 200 to 450 m/s depending on the mass fraction of small particles in the fragmented mixture; exit pressure is in the range 1.5 to 3 MPa. Variation of conduit diameter d in the range 40 to 70 m gives a mass flow rate Q which depends on the diameter as d 2.8, less strongly than for the case of viscous flow of Newtonian liquid in a cylindrical pipe where Q∼ d 4. With the increase in conduit diameter, fragmentation happens later in the flow and conduit resistance remains high. Changes in magma temperature from 700 to 950 °C lead to increase in discharge rate only by a factor of 4 whereas viscosity decreases by more then 8000 times.

Effect of viscous dissipation on nonisothermal high-viscosity magma flow in a volcanic conduit

A two-dimensional nonisothermal model of magma flow in a volcanic conduit is proposed. The model makes it possible to investigate the effect of the processes of viscous dissipation and heat conduction on the magma flow. It is established that the effect of these processes is significant, particularly in the case of high flow rates. It is shown that in this case the conduit resistance calculated from the Poiseuille formula widely used in one-dimensional models is highly overestimated. This is related to the formation of a strongly heated fluid layer with reduced viscosity in the near-wall conduit zone. Within the framework of the proposed model it is possible to describe eruptions with flow rates which are several times higher than the flow rates obtained within the framework of one-dimensional models.

Longitudinal Impedance Is Independent of Outflow Resistance

Background. Many investigators have measured outflow resistance ( R) following peripheral bypass procedures, but correlations with graft patency have been weak. This is because the primary determinants of graft patency are the size and quality of the conduit, not its outflow bed. Efforts at separating conduit resistance from outflow resistance have been unsuccessful. Recently, the concept of longitudinal impedance (∫ Z L) has been suggested as a measure of conduit resistance independent of outflow resistance. The purpose of this in vitro experiment was to test the hypothesis that ∫ Z L is independent of R within physiologically relevant ranges. Methods. Rigid polyethylene tubing of known internal diameter and length (4.3 mm, 375 cm) was perfused with a glycerin/saline mixture mimicking the viscosity of blood (4.1 cp), utilizing a variable pulsatile pump and Windkessel, with outflow into multiply branched tubes of decreasing diameter simulating the hemodynamic conditions of arterial bypass. Flow and pressure were measured using ultrasonic transit time and catheter transduction, respectively, and waveforms digitized at 200 Hz. Flow was varied while maintaining “systemic” pressure and resistance. After Fourier transformation, ∫ Z L was calculated as Δ P/ Q at each harmonic and integrated over 4 Hz. Results. ∫ Z L calculations were remarkably reproducible within the same day with a coefficient of variation (CV) = 4.0% (at 100 dyne · s/cm 5; n = 4) or over 4 successive days (CV = 4.3%). Furthermore, ∫ Z L was largely independent of R over the physiologic range tested, with ∫ Z L remaining relatively constant as R was increased sixfold. Conclusion. ∫ Z L is a consistent and reproducible measure of conduit resistance independent of R over a wide physiologic range. It may be useful for measuring the adequacy of bypass graft conduits.

Periodic behavior in lava dome eruptions

Lava dome eruptions commonly display fairly regular alternations between periods of high activity and periods of low or no activity. The time scale for these alternations is typically months to several years. Here we develop a generic model of magma discharge through a conduit from an open-system magma chamber with continuous replenishment. The model takes account of the principal controls on flow, namely the replenishment rate, magma chamber size, elastic deformation of the chamber walls, conduit resistance, and variations of magma viscosity, which are controlled by degassing during ascent and kinetics of crystallization. The analysis indicates a rich diversity of behavior with periodic patterns similar to those observed. Magma chamber size can be estimated from the period with longer periods implying larger chambers. Many features observed in volcanic eruptions such as alternations between periodic behaviors and continuous discharge, sharp changes in discharge rate, and transitions from effusive to catastrophic explosive eruption can be understood in terms of the non-linear dynamics of conduit flows from open-system magma chambers. The dynamics of lava dome growth at Mount St. Helens (1980–1987) and Santiaguito (1922–2000) was analyzed with the help of the model. The best-fit models give magma chamber volumes of ∼0.6 km3 for Mount St. Helens and ∼65 km3 for Santiaguito. The larger magma chamber volume is the major factor in explaining why Santiaguito is a long-lived eruption with a longer periodicity of pulsations in comparison with Mount St. Helens.