Variable Flow Resistance Research Articles

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

AbstractThe bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large‐scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large‐scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood‐ and ebb‐oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune‐induced directional bed roughness in numerical models of estuarine and tidal environments.

A lumped-parameter model for kidney pressure during stone removal

AbstractIn this paper, we consider a lumped-parameter model to predict renal pressures and flow rate during a minimally invasive surgery for kidney stone removal, ureterorenoscopy. A ureteroscope is an endoscope designed to work within the ureter and the kidney and consists of a long shaft containing a narrow, cylindrical pipe, called the working channel. Fluid flows through the working channel into the kidney. A second pipe, the ‘access sheath’, surrounds the shaft of the scope, allowing fluid to flow back out of the urinary system. We modify and extend a previously developed model ( Oratis et al., 2018) through the use of an exponential, instead of linear, constitutive law for kidney compliance and by exploring the effects of variable flow resistance, dependent on the presence of auxiliary ‘working tools’ in the working channel and the cross-sectional shapes of the tools, working channel, scope shaft and access sheath. We motivate the chosen function for kidney compliance and validate the model predictions, with ex vivo experimental data. Although the predicted and measured flow rates agree, we find some disagreement between theory and experiment for kidney pressure. We hypothesize that this is caused by spatial pressure variation in the renal pelvis, i.e. unaccounted for in the lumped-parameter model. We support this hypothesis through numerical simulations of the steady Navier–Stokes equations in a simplified geometry. We also determine the optimal cross-sectional shapes for the scope and access sheath (for fixed areas) to minimize kidney pressure and maximize flow rate.

A multi-scale model of gas transport in the lung to study heterogeneous lung ventilation during the multiple-breath washout test.

The multiple-breath washout (MBW) is a lung function test that measures the degree of ventilation inhomogeneity (VI). The test is used to identify small airway impairment in patients with lung diseases like cystic fibrosis. However, the physical and physiological factors that influence the test outcomes and differentiate health from disease are not well understood. Computational models have been used to better understand the interaction between anatomical structure and physiological properties of the lung, but none of them has dealt in depth with the tracer gas washout test in a whole. Thus, our aim was to create a lung model that simulates the entire MBW and investigate the role of lung morphology and tissue mechanics on the tracer gas washout procedure. To this end, we developed a multi-scale lung model to simulate the inert gas transport in airways of all size. We then applied systematically different modifications to geometrical and mechanical properties of the lung model (compliance, residual airway volume and flow resistance) which have been associated with VI. The modifications were applied to distinct parts of the model, and their effects on the gas distribution within the lung and on the gas concentration profile were assessed. We found that variability in compliance and residual volume of the airways, as well as the spatial distribution of this variability in the lung had a direct influence on gas distribution among airways and on the MBW pattern (washout duration, characteristic concentration profile during each expiration), while the effects of variable flow resistance were negligible. Based on these findings, it is possible to classify different types of inhomogeneities in the lung and relate them to specific features of the MBW pattern, which builds the basis for a more detailed association of lung function and structure.

A Hybrid Computational and Analytical Model of Inline Drip Emitters

This paper details a hybrid computational and analytical model to predict the performance of inline pressure compensating drip irrigation emitters. Pressure compensating emitters deliver a constant flow rate over a range of applied pressures to accurately meter water to crops. Flow rate is controlled within the emitter via a fixed resistance tortuous path, and a variable flow resistance composed of a flexible membrane that deflects under changes in pressure, restricting the flow path. A pressure resistance parameter was derived using an experimentally validated computational fluid dynamics (CFD) model to describe the flow behavior in tortuous paths. The bending mechanics of the membrane were modeled analytically and refined by deriving a correction factor using finite element analysis (FEA). A matrix formulation that calculates the force applied by a line or a patch load of any shape on a rectangular membrane, along which there is a prescribed deflection, was derived and was found to be accurate to be 1%. The combined hybrid computational–analytical model reduces the computational time of modeling emitters from hours to less than 30 min, dramatically lowering the time required to iterate and select optimal designs. The model was validated experimentally using three commercially available drip emitters and was accurate to within 12% of the experimental results.

Sawtooth wave-like pressure changes in a syrup eruption experiment: implications for periodic and nonperiodic volcanic oscillations

This study is based on the observation of sawtooth wave-like pressure changes (STW) observed during repetitive gas emissions in a syrup eruption experiment. Similar waveforms are observed at many active volcanoes as geodetic signals. By studying the physics of such experiments, we often find new ideas and insights that are applicable to natural volcanic phenomena. We consequently try identifying the features common to both our experimental system and natural volcanic systems. We infer that the oscillatory mechanism in our experiment is similar to flow-induced oscillation controlled by a coupling between elastic capacitance and variable flow resistance. We developed an elementary pipe–chamber system to quantitatively test this hypothesis. We observed three distinct oscillatory patterns: periodic STW, non-STW, and nonperiodic STW. A mathematical model is constructed to support the hypothesis and to enable comparison with existing models of volcanic systems. Models of flow-induced volcanic oscillations are mathematically similar to the model derived from our experimental system. Our results indicate that flow pattern transitions are essential for oscillatory behavior during gas emission. An important finding is that cycle periodicity and fluctuation are controlled by interactions between upward and downward flow within the pipe, especially during the termination of gas emission and the transition to the next cycle. Similar interactions occur in natural volcanoes: during the termination of an explosive eruption, fall-back or drain-back of ejected materials may modulate the explosivity and periodicity of the next eruption cycle. Our experimental system may provide a useful tool for understanding the oscillatory behavior of natural volcanic systems. This study may also give a proof that the original eruption experiment provides a useful tool for explaining the dynamics of oscillatory and eruptive behaviors of natural volcanic systems in educational (e.g., Open Day) demonstrations.

Implications of variable fluid resistance caused by start-up flow in microfluidic networks

Electrical circuit analogies are often used to design microfluidic systems because they simplify device design, providing simple relationships between fluid flow rate, driving forces, and channel dimensions. However, such approximations often significantly overestimate flow rates in situations where start-up energy losses from establishing kinetic head are similar in magnitude to the energy required to overcome viscous shear stresses, as is often the case within complex microfluidic networks. These reduced flows can be more accurately predicted within an electrical analogy framework that accounts for the nonlinear flow resistance generated on the transient regime of start-up flow. In this work, standard flow resistance expressions are modified to account for such effects, and the onset of nonlinear resistance is predicted by a dimensionless parameter, \(\xi = Re\frac{D}{L},\) which is dependent on the Reynolds number and the channel length. As a demonstration, variable fluid resistance is shown to dramatically affect the flow performance of common microfluidic units such as T-junctions and serpentine channels, and the change in performance is accurately predicted. Experimental and theoretical analysis of T-junctions further shows that variable flow resistance causes the ratio of flows through the junction to converge toward unity with respect to an increasing total flow rate. In addition, serpentine channels are shown to exaggerate these start-up effects, owing to compounded energetic demand associated with changing a flow’s direction. As a result, serpentine channels cause the ratio of flow rates exiting a T-junction to diverge from unity with respect to an increasing flow rate.

The lymphatic vessels’ winning philosophy: different challenges, tailored solutions

The nature and function of the lymphatic vessels were correctly described for the first time by the Scottish anatomist and physician William Hunter (1718–1783) who proposed that the lymphatics were not ‘continuations from arteries, but a particular system of vessels by themselves’ and that lymph was formed by the absorption of fluid from the tissues into peripheral lymphatics (Hunter, 1762). Nowadays we know that the system originates with initial lymphatics, which drain fluid, particulates and cells from the extracellular space. The newly formed lymph is subsequently discharged into fewer and larger collecting lymphatics that transport the lymph back to the venous blood stream by means of well-synchronized contractions of the smooth muscle cells in their vessel wall. Much emphasis has recently been placed on the actual function of the lymphatic system, that not only controls tissue fluid homeostasis, but also: (a) maintains plasma volume constant, contributing to the regulation of arterial pressure and cardiovascular function; (b) returns leukocytes and cells, including tumoural ones, to the blood stream, thus contributing to host immune defence in lymph nodes; (c) may serve, particularly in some species, as a reservoir for extracellular fluid. What clearly emerged since their first systematic description is the extreme heterogeneity of the lymphatic structures, a specific characteristic of this vascular network that still makes it very difficult to deepen our knowledge of the physiology and patho-physiology of many lymphatic districts. Indeed, vessel shape, size and complexity is extremely variable from tissue to tissue. It has been clearly shown that this variability reflects various factors such as the specific microanatomy, the mechanical behaviour and, additionally and most importantly from the functional standpoint, the specific drainage requirements of the tissue (Aukland & Reed, 1993). The elegant study by Nepiyushchikh et al. in a recent issue of The Journal of Physiology (Nepiyushchikh et al. 2011), by looking deeper into the core mechanism of lymphatic contractility, introduces an additional degree of complexity into the already quite intricate frame and extends our knowledge about the contractile features of the lymphatic smooth muscle cells. The results reveal that the same contractile apparatus, i.e. the phosphorylation of myosin light chain 20 (MLC20), is exploited in a quite different manner in proximal collecting lymphatics such as the thoracic duct or the cervical lymphatics, compared to distal mesenteric lymphatics (Gashev et al. 2004). Indeed, inhibition of this contractile machinery more profoundly impairs muscle tone and contractile frequency in the thoracic duct than in cervical lymphatics, while such a MLC20 phosphorylation dependence was not observed in distal mesenteric vessels (Wang et al. 2009). Muscle tone and phasic contractile activity are also differently regulated: when the MLC20 phosphorylation pathway is highly expressed, as in thoracic duct and, to a lesser extent, in cervical lymphatics, the tonic activity prevails over the phasic one. Conversely, if the percentage MLC20 phosphorylation is low as in mesenteric lymphatics (Wang et al. 2009), the phasic activity prevails. Therefore, when coupled to the results of a previous study from the same group showing that the contractile mechanism of lymphangions in the wall of collecting lymphatics differs from that encountered in blood vessels (Muthuchamy et al. 2003), the study by Nepiyushchikh et al. reveals that function-dependent specifically tailored contractile machineries had developed in vascular smooth muscle cells. In blood vessels, blood flow is guaranteed by the cardiac pump and smooth muscle cell contraction modulates downstream tissue perfusion and upstream and/or downstream perfusion pressures. In the lymphatics, whose muscle cells posses both vascular and cardiac muscle contractile elements, contraction of smooth muscle cells is meant to sustain centripetal lymph progression against an adverse pressure gradient and extremely variable flow resistance. Each vessel is then equipped with a contractile apparatus to best exploit its function. The existence of differently regulated contractile machinery between distal mesenteric vessels and more proximal ones and even between differing proximal vessels is a significant example of how the lymphatic structures form an extremely sophisticated system able to separately adjust contraction force and frequency to cope with anatomical variability, functional flow requirements, and biodynamic flow transport properties.

Flow resistance due to lateral momentum transfer in partially vegetated rivers

In the study of natural rivers, accurate conveyance estimation is challenging because of complex cross‐sectional geometry and variable flow resistance. The aim of this study was to investigate the additional friction factor due to lateral momentum transfer at the interface of vegetated and nonvegetated channel parts in two rivers: (1) the Rhine River, a large dredged river with partially vegetated floodplains, and (2) the River Päntäneenjoki, a small boreal lowland river with dense bank vegetation and an undulating longitudinal profile. The friction factors of the interfaces were computed backward from the topographical field data, while the measured depth‐flow regime data were computed with the help of an unsteady one‐dimensional (1‐D) flow model. Laboratory experiments were carried out to verify the effects of longitudinal spacing of plants on the friction factors. In the researched rivers the values of the friction factors at the interface were higher than the ones presented in the literature.

Studies on a new type of silencer for air ducts

The research work deals with a study of a specific type of silencer previously designed and tested by Shenoda. It consists of a rectangular duct, which is treated at its side walls with a gradually variable flow resistance. It was realized by increasing the perforated area along the duct wall combined with an homogeneous sound absorbing material. Based on the assumptions that only plane waves propagate in the duct, ignoring the effect of the mean flow velocity (very low) and applying linear acoustic equations, the sound propagation in such a silencer was studied. Therefore the wave impedance, the sound reflection coefficient at the input of the silencer, and the transmission loss introduced by it were calculated, measured, and compared with a silencer of comparable dimensions which is treated with a constant flow resistance along its length. To optimize the design different configurations were theoretically studied, computed, and compared. An important result: The silencer performance depends on a unique design parameter, namely, the silencer characteristic area. The methods of realizing a variable wall flow resistance were introduced. The effect of using different kinds of sound absorbing materials and changing the graduality of the variable flow resistance were measured.

Fine-Scale Patterns of Water Velocity within Macrophyte Patches in Streams

Rooted macrophytes in temperate lowland streams are often distributed in monospecific patches which control flow, carbon fluxes, and the abundance of invertebrates and fish. Small high-resolution hot-wire probes provided detailed measures of flow velocities within and around macrophyte patches of four plant species of contrasting morphologies in Danish streams. Flow velocity declined rapidly at the surface of the plant patches and species with large leaf area on bushy shoots (e.g. Callitriche cophocarpa and Elodea canadensis) reduced the flow more than species with streamlined, strap-formed leaves (e.g. Sparganium emersum). Variable flow-resistance resulted in flow velocities at 2 cm above the sediment which were 11-fold lower inside C. cophocarpa patches than upstream of the patches, whereas no significant differences in near-bed velocities were found inside and outside the more open patches of S. emersum. The reduced velocity within flow-resistant patches remains sufficiently fast (i.e. >1 cm s -1 ) to prevent carbon depletion and oxygen accumulation and should be optimal to photosynthesis and plant growth. The deflected flow is accelerated around the patches and contributes to form a mosaic of highly variable plant cover, flow and substrate conditions. These relations have important implications for flow resistance, areal expansion of patches and spatial variability of sediment and invertebrate composition in streams.

Zweistufiges Regelsystem zur druckkonstanten Perfusion von Koronargefäßen - A Double-loop Servo-System for Constant-pressure Perfusion of the Coronary Circulation

This paper describes a double-loop servo-controlled pump system for the constant-pressure perfusion of a coronary artery. Due to the transient nature of changes in coronary vasomotor tone, such a perfusion system must have a fast regulatory response. In the first stage, a servo-controlled pump primes a windkessel having a volume of 35 ml with blood. The pumping rate is electronically controlled to maintain a constant pressure within the windkessel max. 700 mmHg. The maximal flow rate is 300 ml/min. To reduce the high pressure in the windkessel to the desired coronary perfusion pressure, a variable flow resistance, comprising a clamped thin-walled silicone tube, is provided in the output line of the system. A fast servo-motor drives the clamp and is controlled by an electronic regulator, using a second feedback loop from the pressure signal measured at the tip of the perfusion cannula. The system stabilizes the coronary perfusion pressure within 300 ms. An additional modulation of the setpoint signal in synchrony with the cardiac cycle improves the phasic pattern of the blood flow, and thus prevents changes in transmural blood flow distribution. The dead volume of the overall system is about 60 ml. Hemolysis caused by this system during five hours of perfusion in vivo is negligible.

Sound transmission loss in a rectangular attenuator symmetrically lined with continuously variable flow resistance

The sound attenuation in a free‐path rectangular duct of constant cross‐sectional area was theoretically studied for the case of a symmetrically decreasing wall resistance at its both ends. The change of the wall conductance from either side was thereby taken to be parabolic. In addition to the high transmission loss it possesses, this attenuator was found to be wideband and to have small reflection coefficients at both ends, if it is inserted between two similar hard ducts. The solution of the obtained Bessel differential equation consists of a combination of modified Hankel functions of the order 1/3. The attenuator characteristic at a certain frequency was found to depend only on both the duct length and an attenuator characteristic area (A) defined as A = S/g, where S is the actual attenuator cross‐sectional area and g is a normalized wall conductance at a distance of 1 cm from attenuator ends. Compared to the case of the homogeneous lined attenuator, the insertion of the proposed attenuator between two hard ducts was found to give a much smaller reflection coefficient at comparable values of sound transmission loss. This is due to its good matching characteristics at both ends. As an example of calculations, a duct of 1‐m length having a characteristic area of 500 cm2 was found to have an average transmission loss of about 17 dB for the frequency range up to 600 Hz. Meanwhile, the reflection coefficient was found to decrease with frequency and to have a value less than 0.1 for frequencies above 350 Hz. Experimental results were found to be in fair agreement with the theoretical results. It is worth mentioning that parabolic change of the wall conductance can be realized by means of a homogeneous sound absorbing flow resistance together with a slit which changes parabolically along the length of the attenuator. By the experimental implementation of this slit, a variable perforation area having the same dependence on length and perforation ratio of more than 35% was used. Due to the small reflection coefficient it possesses at both ends, this attenuator can be used as an exhaust silencer.

Variable Flow Resistance With Adjustable Multihole Orifice Plates in Series

Research Papers Variable Flow Resistance With Adjustable Multihole Orifice Plates in Series J. Sherman, J. Sherman PWR Core 2 Design, Bettis Atomic Power Division, Westinghouse Electric Corporation, Pittsburgh, Pa. Search for other works by this author on: This Site PubMed Google Scholar F. A. Grochowski, F. A. Grochowski PWR Core 2 Design, Bettis Atomic Power Division, Westinghouse Electric Corporation, Pittsburgh, Pa. Search for other works by this author on: This Site PubMed Google Scholar J. E. Sharbaugh J. E. Sharbaugh PWR Core 2 Design, Bettis Atomic Power Division, Westinghouse Electric Corporation, Pittsburgh, Pa. Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information J. Sherman PWR Core 2 Design, Bettis Atomic Power Division, Westinghouse Electric Corporation, Pittsburgh, Pa. F. A. Grochowski PWR Core 2 Design, Bettis Atomic Power Division, Westinghouse Electric Corporation, Pittsburgh, Pa. J. E. Sharbaugh PWR Core 2 Design, Bettis Atomic Power Division, Westinghouse Electric Corporation, Pittsburgh, Pa. J. Basic Eng. Sep 1960, 82(3): 645-653 (9 pages) https://doi.org/10.1115/1.3662688 Published Online: September 1, 1960 Article history Received: July 26, 1959 Online: November 4, 2011