Total Flow Resistance Research Articles

Motion effect on the dynamic contact angles in a capillary tube

A mathematical model is developed to predict the effect of motion of a liquid plug in a capillary tube on the dynamic contact angles including the effects of driving pressure, liquid plug length, tube diameter, and operating temperature. Results show that the advancing contact angle significantly affects the flow resistance of the liquid plug occurring in a capillary tube. For a given driving pressure difference, the pressure difference due to the increase of the advancing contact angle is the primary contribution to the flow resistance of capillary flow in a capillary tube. For example, when the liquid plug length is equal to 5 mm at a driving pressure difference of 10 Pa and a tube radius of 500 μm, the flow resistance caused by the dynamic contact angle can be 92% of the total flow resistance at an operating temperature of 60°C. It can be concluded that the effect of the dynamic contact angle on the fluid flow of a liquid plug in a capillary tube must be considered.

Spectral‐ALS data fusion for different roughness parameterizations of forested floodplains

AbstractNatural river floodplains and adjacent wetlands grow typically a diverse and heterogeneous combination of herbs, shrubs and trees, which play an essential role in determining the total flow resistance. Hydrodynamic effects of trees in forested floodplains can provide the majority of flow resistance during flood events. Nevertheless, ground‐based techniques to acquire vegetation parameters are expensive and difficult to apply over long reaches. This paper presents a novel method of automated roughness parameterization of riparian woody vegetation by fusion of Quickbird multi‐spectral image with airborne laser scanning (ALS) data. The data fusion approach includes: individual tree detection and estimation of vegetation metrics from light detection and ranging (LiDAR) data, assessment of predictive models for the vegetation parameters and spatial mapping of the vegetation parameters for the forest plants in the riparian corridor. The proposed method focuses on estimation of plant density (d), crown diameters (DC), tree height (h), stem diameter (DS), crown base height (cbh) and leaf area index (LAI). The procedure is tested along a 14‐km reach of the Sieve River (Tuscany, Italy) characterized by high woody plant density. Due to the complex study area, the data fusion approach explains with variable reliability the local vegetation properties (R2(DC) = 0.14, R2(h) = 0.84, R2(DS) = 0.25, R2(cbh) = 0.66). The generated structural parameter maps represent spatially explicit data layers that can be used as inputs to hydrodynamic models used to analyse flow resistance effects in different submergence conditions of vegetation. A simple flow resistance model was applied over a test area comparing the results of the proposed method and a traditional ground‐based approach. The modelling results showed that the new method is able to provide accurate output data to describe the interaction between water levels and bio‐mechanical characteristics of vegetation. The proposed methodology provides a fast, repeatable and accurate way of obtaining floodplain roughness, which enables regular updating of vegetation characteristics. Copyright © 2010 John Wiley & Sons, Ltd.

Comparative analysis of bed resistance partitioning in high‐gradient streams

Total flow resistance can be partitioned into its components of grain (ffgrain), form (ffstep), wood (ffwood), and spill (ffspill) resistance. Methods for partitioning flow resistance developed for low‐gradient streams are commonly applied to high‐gradient systems. We examined the most widely used methods for calculating each component of resistance, along with the limitations of these methods, using data gathered from 15 high‐gradient (0.02 < S0 < 0.195) step‐pool, cascade, and plane‐bed reaches in Fraser Experimental Forest. We calculated grain resistance using three equations that relate relative submergence (R/Dm) to ffgrain as well as using an additive drag approach. The drag approach was also used for calculating ffwood and ffstep. The ffgrain contributed the smallest amount toward all reaches at all flows, although the value varied with the method used. The Parker and Peterson (1980) equation using D90 best represented ffgrain at high flows, whereas the Keulegan (1938) equation using D50 best characterized ffgrain at base flows, giving a lower bound for grain resistance. This suggests that ffgrain may be better represented if two grain sizes are used to calculate this component of resistance. The drag approach, which is used to calculate wood resistance, overestimated the significance of individual logs in the channel. The contribution of ffspill was reduced at higher flows when form drag around the step is accounted for at higher flows. We propose a method for evaluating the contribution of ffstep that accounts for form drag around the steps once they are submerged at higher flows. We evaluated the potential sources of error for the estimation of each component of resistance. Determination of the drag coefficient was one of the major sources of error when calculating drag around wood, steps, or boulders.

Effects of bed-load movement on flow resistance over bed forms

The effect of bed-load transport on flow resistance of alluvial channels with undulated bed was experimentally investigated. The experiments were carried out in a tilting flume 250mm wide and 12.5m long with glass-sides of rectangular cross-section and artificial dune shaped floor that was made from Plexi-glass. Steady flow of clear as against sediment-laden water with different flow depths and velocities were studied in the experiments with a fine sand (d 50 = 0.5mm). The results indicate that the transport of fine particles (d 50 = 0.5mm) can decrease the friction factor by 22% and 24% respectively for smooth and rough beds. Increasing the bed-load size (d 50 = 2.84 mm) can decrease the friction factor by 32% and 39% respectively for smooth and rough beds. The decrease in flow resistance is due to filling up of the troughs of dunes. This separation zone is responsible for increasing the flow resistance. On the upstream side of dunes condition is similar to plane bed. Presence of bed-load causes to increase the shear velocity and hence increasing flow resistance. But decreasing in flow resistance is more and it causes to decrease the total flow resistance. Grains saturated the troughs in the bed topography, effectively helping in smoothening of bed irregularities.

Heat transfer enhancement by filling metal porous medium in central area of tubes

Abstract Given that the fluid within the tubes of some industrial heat exchangers is under a state of fully developed laminar flow with a constant Nu number, increasing the surface area for heat transfer will significantly increase the flow resistance. In this paper, we filled metal porous medium with high thermal conductivity, high porosity and high filling radius in the central area of fully developed laminar flow within the tube, and established corresponding numerical models for fluid flow and heat transfer. Numerical simulation results indicate that after filling the tube with metal porous medium, the temperature profiles within the porous medium area are very uniform, and the temperature difference between the tube wall and the fluid decreases significantly which correspondingly results in a notable increase of Nu number; meanwhile, the characteristic of flow field redistribution occurs within the enhanced tube, but the total flow resistance composed of the Darcy resistance and inertial resis...

Visual Exploration of Nasal Airflow

Rhinologists are often faced with the challenge of assessing nasal breathing from a functional point of view to derive effective therapeutic interventions. While the complex nasal anatomy can be revealed by visual inspection and medical imaging, only vague information is available regarding the nasal airflow itself: Rhinomanometry delivers rather unspecific integral information on the pressure gradient as well as on total flow and nasal flow resistance. In this article we demonstrate how the understanding of physiological nasal breathing can be improved by simulating and visually analyzing nasal airflow, based on an anatomically correct model of the upper human respiratory tract. In particular we demonstrate how various Information Visualization (InfoVis) techniques, such as a highly scalable implementation of parallel coordinates, time series visualizations, as well as unstructured grid multi-volume rendering, all integrated within a multiple linked views framework, can be utilized to gain a deeper understanding of nasal breathing. Evaluation is accomplished by visual exploration of spatio-temporal airflow characteristics that include not only information on flow features but also on accompanying quantities such as temperature and humidity. To our knowledge, this is the first in-depth visual exploration of the physiological function of the nose over several simulated breathing cycles under consideration of a complete model of the nasal airways, realistic boundary conditions, and all physically relevant time-varying quantities.

Drag coefficient of unsubmerged rigid vegetation stems in open channel flows

Results of an experimental study on the drag force measurement involving a single stem kept in a channel flow stem array are presented. The data collected herein and those from literature indicate that the stem drag coefficient logarithmically increases with the areal stem density. The stem Reynolds number is noticed to have only a small effect on the stem drag coefficient which was however found to depend on the stem staggering pattern. The drag coefficient is less influenced by the Froude number in subcritical flows but it decreases with the Froude number in supercritical flows. New relationships are proposed for the stem drag coefficient which appear useful in partitioning the total flow resistance of vegetated bed streams into the stem and the bed particle resistances. The bed particle resistance applies to sediment transport through such vegetated flows for which the average flow velocity is available.

An increase in thermodynamic efficiency and compactness of contact apparatuses via spatially organized structure of the active material bed

The possibility of filling the hollow input (diffuser) and output (confuser) sections of contact apparatuses by an active material is analyzed. The equation determining the distribution function of the sizes of material particles along the axis of a conelike apparatus, which provides the minimum volume of the bed of particles at the specified values of the total surface area of the interfacial contact in the bed and total flow resistance of the bed, is derived. It is shown that the change in the conventional shape of the apparatus to a construction in the form of a junction of two truncated cones filled with the active material allows one to decrease the total volume of the apparatus by 30%. The dependence of the thermodynamic efficiency and compactness of the regenerative facility on the particle size of the active material bed is numerically analyzed. The existence of a critical particle size which determines the minimum necessary volume of the bed providing high thermodynamic efficiency of the process is shown.

Flow resistance of one-line emergent vegetation along the floodplain edge of a compound open channel

Experiments have been conducted in straight compound channels with and without one-line emergent vegetation along the floodplain edge, in which stream-wise velocities and boundary shear stresses have been measured. The experimental results show that the velocity distribution in the vegetation case is considerably different from that in the no vegetation case and the boundary shear stress is also significantly reduced by the additional flow resistance caused by the vegetation at a similar relative water depth. The apparent shear stress distribution which has been calculated with the boundary shear stress and weight component in the vegetation case is totally different from that in the no-vegetation case. New formulae for friction factors for the with and without vegetation cases are developed using vegetation density and flow parameters. The drag force caused by the vegetation is obtained for two different vegetation density cases and the magnitude of its effect on total flow resistance is then investigated. The force balance method is used to predict discharge and this is compared with the discharge predicted by the new formula. A further analysis of the selection of vegetation spacing is carried out, determining its effect on stage-discharge.

Hemodynamic changes in visceral organs following closed head trauma in rats

Gastrointestinal (GI) tract dysfunction is well documented following head injury. Our study sought to determine whether head injury causes an immediate impairment of the splanchnic circulation which may contribute to later GI sequelae. Three groups of eight rats each received either no closed head trauma (CHT) (group 1) or CHT (groups 2 and 3) immediately following baseline measurements at time 0. The primary measures of interest - individual organ blood flows and cardiac output (radioactive microspheres), and individual organ and systemic vascular resistances - were determined in the control group, at 5 min after CHT in group 2, and at 15 min after CHT in group 3. CHT caused no significant change in portal venous inflow (flows were 2.40+/-0.36, 2.38+/-0.54, and 2.33+/-0.62 ml min(-1) 100g(-1)bw, mean+/-S.D., in groups 1, 2, and 3, respectively). Individual organ and total hepatic blood flow, cardiac index, splanchnic, portal, and total peripheral resistance, and mean arterial or portal venous pressure also did not differ significantly among groups. We found no significant changes in splanchnic circulation immediately after CHT in this rat model. Our results do not support the hypothesis that the splanchnic circulation is impaired immediately after head injury and that splanchnic blood flow impairment immediately after head injury may contribute to post-head injury GI dysfunction.

Flow resistance dynamics in step‐pool stream channels: 1. Large woody debris and controls on total resistance

Flow resistance dynamics in step‐pool channels were investigated through physical modeling using a laboratory flume. Variables contributing to flow resistance in step‐pool channels were manipulated in order to measure the effects of various large woody debris (LWD) configurations, steps, grains, discharge, and slope on total flow resistance. This entailed nearly 400 flume runs, organized into a series of factorial experiments. Factorial analyses of variance indicated significant two‐way and three‐way interaction effects between steps, grains, and LWD, illustrating the complexity of flow resistance in these channels. Interactions between steps and LWD resulted in substantially greater flow resistance for steps with LWD than for steps lacking LWD. LWD position contributed to these interactions, whereby LWD pieces located near the lip of steps, analogous to step‐forming debris in natural channels, increased the effective height of steps and created substantially higher flow resistance than pieces located farther upstream on step treads. Step geometry and LWD density and orientation also had highly significant effects on flow resistance. Flow resistance dynamics and the resistance effect of bed roughness configurations were strongly discharge‐dependent; discharge had both highly significant main effects on resistance and highly significant interactions with all other variables.

Resistance to overland flow due to bed‐load transport on plane mobile beds

AbstractDuring bed‐load transport by overland flow, momentum is transferred from the flow to the bed via grain collisions, resulting in a decrease in flow velocity and an increase in flow resistance, herein termed bed‐load transport resistance. In overland flow on mobile plane beds, total flow resistance f consists of grain resistance fg and bed‐load transport resistance fbt. In order to identify and evaluate the relative importance of the factors controlling fbt, 38 flume experiments were performed on slopes of 2·7 and 5·5° using sediment with median diameters of 0·74 and 1·16 mm. All flows were supercritical and turbulent.This study is an extension of a recent study by Gao and Abrahams (Earth Surface Processes and Landforms 2004, vol. 29, pp. 423–435). These authors found that fbt is controlled by three factors: sediment concentration C, dimensionless sediment diameter D*, and relative submergence h/D, where h is flow depth, D is median sediment diameter. However, a new dimensional analysis identifies two additional factors: Froude number F and slope S. Multiple regression analyses reveal (1) that these five factors together explain 97 per cent of the variance of fbt, and (2) that S controls fbt entirely through C. The variable C is therefore redundant, and a new functional equation relating fbt to D*, h/D, S and F is developed. This equation may be used to predict fbt. An advantage of this equation is that it may be used to predict fbt without measuring bed‐load transport rate. Copyright © 2004 John Wiley & Sons, Ltd.

The frictional resistance characteristics of a bedrock‐influenced river channel

AbstractMuch of the published literature relating to reach‐scale total flow resistance concentrates on alluvial rivers and little mention is made of the more extreme resistance found in bedrock‐influenced river systems. This paper presents the results of a detailed investigation of total flow resistance variation for five channel types on the Sabie River, South Africa (cohesive mixed anastomosed, uncohesive mixed anastomosed, mixed pool–rapid, alluvial braided and alluvial single thread) over a range of discharges. Manning's ‘n’ and Darcy‐Weisbach f resistance coefficients are calculated using Barnes' methodology (H. H. Barnes, US Geological Survey Water Supply Paper 1849, 1967). These data are analysed at the scales of channel type and morphologic unit and the resultant resistance values are compared with the literature. Generally the low‐flow friction values estimated for the bedrock‐influenced channel types are higher than any reported. Flow resistance quantification of the alluvial channel type study sites on the Sabie River produced values slightly higher than earlier studies of similar morphologies. In situations where significant areas of irregular bedrock are exposed (cohesive mixed anastomosed and mixed pool–rapid channel types) the high flow resistance coefficients at low discharges probably reflect energy dissipation due to hydraulic jumps and internal distortions; however, these values fall as discharge increases to values comparable to those recorded for step–pool rivers. It is suggested that the slightly elevated resistance coefficients recorded in the alluvial channel types are the result of the vegetation roughness component, operating when the macro‐channel bar features are inundated. Copyright © 2004 John Wiley & Sons, Ltd.

Experimental Study of Bed Load Transport through Emergent Vegetation

Vegetation is an important agent in fluvial geomorphology and sedimentary processes, through its influence on the local hydraulics that determine sediment transport. Within stands of emergent vegetation, bed shear is substantially reduced through the absorption of momentum by drag on the stems. This stimulates deposition of sediment and reduces capacity for bed load transport. The effect of emergent vegetation on hydraulic parameters (including equilibrium bed gradient, flow depth, and velocity) and on bed load transport rate has been investigated experimentally for one sediment size, stem diameter, and stem spacing. Bed load transport rate was found to be closely related to bed-shear stress, which must be estimated by partitioning total flow resistance between stem drag and bed shear.

Influence of step composition on step geometry and flow resistance in step‐pool streams of the Washington Cascades

Step‐pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step‐pool reaches without large woody debris (LWD) to 20 step‐pool reaches with LWD. Non‐LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water‐surface drop created by steps, larger grain sizes, and smaller Darcy‐Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non‐LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD‐loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step‐pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.

Involvement of nitric oxide in the regulation of regional hemodynamics in streptozotocin-diabetic rats

In experimental and human diabetes mellitus, evidence for an impaired function of the vascular endothelium has been found and has been suggested to contribute to the development of vascular complications in this disease. The aim of the study was to evaluate possible regional hemodynamic in vivo differences between healthy and diabetic rats which would involve nitric oxide (NO). Central hemodynamics and regional blood flow (RBF) were studied using radioactive microspheres in early streptozotocin (STZ)-diabetic rats and compared to findings in healthy control animals. This method provides a possibility to study the total blood flow and vascular resistance (VR) in several different organs simultaneously. L-NAME iv induced widespread vasoconstriction to a similar extent in both groups. In the masseter muscle of both groups, acetylcholine 2 g/kg per min, induced a RBF increase, which was abolished by pretreatment with L-NAME, suggesting NO as a mediator of vasodilation. In the heart muscle of both groups, acetylcholine alone was without effect while the combined infusion of acetylcholine and L-arginine induced an L-NAME-sensitive increase in RBF. The vasodilation induced by high-dose acetylcholine (10 g/kg per min) in the kidney was more pronounced in the STZ-diabetic rats. The results indicate no reduction in basal vasodilating NO-tone in the circulation of early diabetic rats. The sensitivity to vasodilating effects of acetylcholine at the level of small resistance arterioles vary between tissues but was not impaired in the diabetic rats. In the heart muscle the availability of L-arginine was found to limit the vasodilatory effect of acetylcholine in both healthy and diabetic rats. In conclusion, the results indicate a normal action of NO in the investigated tissues of the early STZ-diabetic rat.

Large woody debris and flow resistance in step-pool channels, Cascade Range, Washington

Total flow resistance, measured as Darcy–Weisbach f, in 20 step-pool channels with large woody debris (LWD) in Washington, ranged from 5 to 380 during summer low flows. Step risers in the study streams consist of either (1) large and relatively immobile woody debris, bedrock, or roots that form fixed, or “forced,” steps, or (2) smaller and relatively mobile wood or clasts, or a mixture of both, arranged across the channel by the stream. Flow resistance in step-pool channels may be partitioned into grain, form, and spill resistance. Grain resistance is calculated as a function of particle size, and form resistance is calculated as large woody debris drag. Combined, grain and form resistance account for less than 10% of the total flow resistance. We initially assumed that the substantial remaining portion is spill resistance attributable to steps. However, measured step characteristics could not explain between-reach variations in flow resistance. This suggests that other factors may be significant; the coefficient of variation of the hydraulic radius explained 43% of the variation in friction factors between streams, for example. Large woody debris generates form resistance on step treads and spill resistance at step risers. Because the form resistance of step-pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height, we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel. Hence, the distribution and function, not just abundance, of large woody debris is critical in steep, step-pool channels.

Immediate sensory nerve-mediated respiratory responses to irritants in healthy and allergic airway-diseased mice.

The immediate responses of the upper respiratory tract (URT) to the irritants acrolein and acetic acid were examined in healthy and allergic airway-diseased C57Bl/6J mice. Acrolein (1.1 ppm) and acetic acid (330 ppm) vapors induced an immediate increase in flow resistance, as measured in the surgically isolated URT of urethane-anesthetized healthy animals. Acrolein, but not acetic acid, induced a small URT vasodilatory response. In awake spontaneously breathing mice, both vapors induced a prolonged pause at the start of expiration (a response mediated via stimulation of nasal trigeminal nerves) and an increase in total respiratory specific airway flow resistance, the magnitude of which was similar to that observed in the isolated URT. Both responses were significantly reduced in animals pretreated with large doses of capsaicin to defunctionalize sensory nerves, strongly suggesting a role for sensory nerves in development of these responses. The breathing pattern and/or obstructive responses were enhanced in mice with ovalbumin-induced allergic airway disease. These results suggest that the primary responses to acrolein and acetic acid vapors are altered breathing patterns and airway obstruction, that sensory nerves play an important role in these responses, and that these responses are enhanced in animals with allergic airway disease.

AN EX-VIVO EVALUATION OF VASCULAR CATHETERS FOR CONTINUOUS HEMOFILTRATION

Objectives: To measure outflow and inflow hydraulic resistance in double-lumen catheters used for hemofiltration under standardized laboratory conditions. Setting: ICU Laboratory of tertiary unit. Methods: Heparinized spent red cells diluted in polygeline solution to a constant hematocrit of 32% at 37°C were pumped using a standard Prisma M60 circuit through several hemofiltration catheters. Blood pump speed was increased and decreased in steps of 30 mL/min (30, 60, 90, 120, 150, and 180 mL/min) and catheter outflow and inflow pressures recorded and used to define the pressure flow relationship (line of hydraulic resistance) for each. Results: Double-lumen catheters posed different resistances to outflow or inflow. Among the <15 cm long catheters, the 11.5 Fr Quinton-Mahurkar (0.56 mmHg/mL/min) catheter offered the least resistance to outflow, while the Medcomp 11.5 Fr catheter offered the least resistance to inflow (0.78 mmHg/mL/min). Among the >19 cm long catheters, the 13.5 Fr Vascath Niagara catheter showed the lowest blood flow resistance to both outflow (0.63 mmHg/mL/min) and inflow (0.83 mmHg/mL/min). Longer catheters did not pose statistically greater resistance to both outflow and inflow. Resistance to inflow was consistently greater than resistance to outflow (p = 0.003). Overall, the Prisma M60 blood circuit alone accounted for 40% of the total extracorporeal circuit blood flow resistance. Conclusions: Proprietary hemofiltration catheters have variable resistance to blood flow under standard ex-vivo conditions. This ex-vivo information might be useful to clinicians in guiding their choice of catheters for clinical use.

Stress partitioning in streams by large woody debris

Using simple theoretical models and field measurements from a spring‐dominated stream, we quantify how large woody debris affect channel hydraulics and morphology at both the local and reach‐averaged scales. Because spring‐dominated streams have nearly constant discharge, they provide a unique natural opportunity to study flow and transport processes near the channel‐forming flow. We first show that the drag on a floating log is identical to the theoretical value for widely separated cylinders at similar Reynolds numbers. We then use simple theoretical models to estimate the partitioning of flow shear stress between woody debris and streambeds. The inferred stress partitioning is consistent with an estimate based on a comparison of local and reach‐averaged measurements of the water surface slope. Our measurements show that even though large woody debris cover less than 2% of the streambed, they provide roughly half of the total flow resistance. As large woody debris are added to a stream, the total shear stress increases (because the water depth increases), but the shear stress borne by the bed decreases, as a growing fraction of the total shear stress is borne by the debris. Our analysis shows that simple theoretical models of stress partitioning may provide a convenient mathematical framework for assessing how changes in debris loading affect streams.