Variation Of Flow Resistance Research Articles

Development of CFD porous model parameters of Korean PWR spent nuclear fuel assemblies for thermal analysis

This study focused on developing and validating porous model parameters to accurately estimate the thermal behavior of spent nuclear fuel assemblies for computational fluid dynamics simulations. These assemblies are stored in Korean pressurized water reactors (PWRs) under dry storage conditions. Korean PWR fuel assemblies were categorized into four distinct types based on their configuration, facilitating the calculation of effective thermal conductivities and flow resistance coefficients. These parameters are crucial for simulating heat transfer and determining the peak cladding temperature (PCT), ensuring the thermal safety of stored nuclear fuel. The validation of the proposed porous model against explicit two-dimensional and three-dimensional simulations demonstrated its efficacy and reliability in enhancing the thermal analysis of spent fuel assemblies, highlighting the significance of accurately calculating the porous model parameters in the safety assessment of nuclear fuel storage. Furthermore, the study assessed the sensitivity of variables influencing temperature during spent nuclear fuel storage, focusing on flow resistance coefficients, control rod effect, and basket size. The findings revealed that variations in flow resistance and the presence of control rods minimally impacted PCT, whereas basket size significantly influenced temperature, underscoring its importance in the thermal analysis of nuclear fuel storage systems.

A logarithmic law for the velocity- and retention-dependency of the eddy dispersion in chromatographic columns

Applying the recently introduced patchwork model for porous media, we present a new step forward in the modelling of eddy dispersion in chromatographic columns. The logarithmic law describing the velocity dependency emerging from this patchwork model is supplemented with a retention factor dependency via first principles modelling of the variations in flow resistance and retention capacity caused by the packing disorder. Furthermore, it is shown the derived expression is also able to fit the eddy dispersion originating from the wall effect on the packing. When applied to literature data of eddy dispersion, the newly introduced logarithmic law has a goodness of fit that is at least equal to that of Knox’ empirical power law (R2>0.98). The main difference is that, whereas Knox’ power law requires a separate fit for each component due to the retention factor dependency, the present model simultaneously fits all plate height curves measured on one chromatographic column, using only two parameters with a clear physical meaning.

CuO NWs boosted triboelectric microfluidic nanosensor functionalized by collagen-protein interactions for real-time platelet count monitoring

Blood platelet count significantly affects the development of severe conditions like myocardial infarction, peripheral arterial ischemia, respiratory compromise, stroke, diabetes, coronavirus disease 2019 (COVID-19), along with chemotherapy patients and those suffering cardiovascular diseases (CVDs). These conditions necessitate frequent monitoring of platelet counts to guide diagnostic and therapeutic decisions. However, existing techniques are relatively time-consuming, lack of accuracy and require precise operation. The emergence of these severe diseases underscored the need to develop advanced platelet count-monitoring techniques which are rapid, highly precise, and conveniently portable for point-of-care applications. In this study, we emphasized the development of a triboelectric microfluidic nanosensor (TMNS) for platelet quantification through the assessment of flow resistance. The functionality of TMNS device is based on immobilization of platelets on a collagen layer coated inside a microfluidic channel. The triboelectric voltage output is measured as a detection signal of the flow resistance and is enhanced by incorporating high surface area copper oxide nanowires (CuO NWs) on the interior of copper tubes. These copper tubes serve as terminal electrodes and for flow guiding. The flow resistance of plasma solutions is elevated when the platelet concentration increases due to heightened adherence of platelets onto the collagen layer. Variations in flow resistance induce alterations in contact electrification, causing changes in output voltage at load terminals. Fine-tuning of the TMNS device was achieved by optimizing the channel width and length, flowing liquid viscosity, and voltage measurement technique. Platelet quantification sensing data were acquired through the combination of platelet-rich plasma (PRP) and platelet-poor plasma (PPP) solutions. The described device exhibits promising capabilities for platelet-count monitoring in whole-blood samples collected from three distinct patient groups, showcasing its potential impact in precise point-of-care applications.

Energy transfer mechanism during ethanol evaporation with external tangential temperature gradient under low pressure environment

For the purpose of revealing the energy-transfer process during ethanol evaporation under low pressure environment, experimental measurement and numerical simulation are adopted for ethanol evaporation at different liquid depths and external radial temperature differences in annular pools. The results show that two thermocapillary vortexes appear below the liquid–vapor interface. Most of the latent heat of vaporization is from the liquid phase, but little from the gas phase. With the increase of liquid layer depth, the steady axisymmetric flow will transform into three-dimensional time-dependent convection. The mechanism of flow instability should be attributed to the variation lag of velocity behind the variation of flow resistance. The critical vapor pressure of flow destabilization depends on radial temperature difference and liquid layer depth. Furthermore, the magnitude of temperature discontinuity across the liquid–vapor interface is the largest near the hot inner cylinder. With the increase of the liquid layer depth, the temperature discontinuity decreases.

Study on the influence of radius of curvature on flow resistance in spiral pipe

With the continuous development of west of China, a new kind of tunnel, namely spiral tunnel, has appeared in the expressway. Due to the special linear pattern, the resistance of airflow in spiral tunnel has been found to change, but there are few relevant researches at present. Therefore, numerical calculation method is used to study the variation of flow resistance in spiral pipe with different curvature. The results show that when the fluid flows in the spiral pipe, the wind speed is not uniformly distributed. The highest speed is not in the center of the pipe, but on the outside of the pipe, and the offset distance decreases with the increase of the radius of curvature. In addition, the change of flow resistance in spiral pipe is studied, and it is found that the change rate of flow resistance decreases with the increase of curvature radius. It shows that the radius of curvature is negatively correlated with the flow resistance.

Modeling of Spontaneous–Imbibition Experiments With Porous Disk—On the Validity of Exponential Prediction

Summary Imbibition experiments with porous disk can be used to derive accurate capillary pressure curves for porous media. An experimental setup is considered in which brine spontaneously imbibes cocurrently through a water-wet porous disk and into a mixed-wet core. Oil is produced from the core's top surface, which is exposed to oil. The capillary pressure is reduced in steps to determine points on the capillary pressure curve. A mathematical model is presented to interpret and design such experiments. The model was used to history match experimental data from Ahsan et al. (2012). An analytical model was then derived from a simplification of the general model, and validated by comparing the two by use of parameters from history matching. The main assumption of the analytical model is that the imbibition rate is sufficiently low, allowing fluids to redistribute inside the core, leading to a negligible capillary pressure gradient. This results in an exponential imbibition time profile with a time scale τ. Exponential matching has been applied earlier in the literature, but, for the first time, we derive this expression theoretically and provide an explicit formulation for the time scale. The numerical simulations show that, at low saturations, there can be a significant flow resistance in the core. A capillary-pressure gradient then forms, and the analytical solution overestimates the rate of recovery. At higher saturations, the fluids are more mobile, and imbibition rate is restricted by the disk. Under such conditions, the exponential solution is a good approximation. The demonstrated ability to predict the time scale in the late stage of the experiment is of significant benefit, because this part of the test is also the most time-consuming and most important to estimate. A method is presented to derive capillary pressure data point by point from measured imbibition data. It provides reliable data between the equilibrium points, and demonstrates consistent variations in flow resistance during the imbibition tests. Gravity had minor influence on the considered experimental data, but generally implies that equilibrium points have higher capillary pressure than the phase pressure difference defined by the boundary conditions.

MANNING'S 'n' FOR STAGGERED FLEXIBLE EMERGENT VEGETATION

Vegetation along a coastline could offer significant protection of the adjoining land area against natural hazards such as storm surge and tsunami. In this context, the flexibility of the individual that stems within the green belt is understood to play an important role in the attenuation of momentum of the incoming waves. The physics of which, is yet to be understood completely. Difficulty in modeling the rigidity of the plantations, both numerically and experimentally, is the main cause for this lack of understanding. In the present work, a detailed laboratory study is taken up to examine the resistance characteristics of a group of model slender flexible cylinders. The individual cylinders of the group were fixed to the bed in a staggered configuration. The size, vegetation density and the elastic modulus of the individual stems were chosen such that the tests covered the practical ranges of vegetation in coastal forestry. The Manning's n for different flow conditions as well as for vegetative parameters was obtained from the physical tests in uniform steady current. The results clearly bring out the variation of flow resistance in terms of flow velocity, density of plantation, individual stem diameter and its elastic property. A new empirical relationship is proposed for estimation of Manning's n for staggered flexible emerging plantations which is valid for depths of flow greater than 0.8 times the undeflected plant height.

Multiple states of finger propagation in partially occluded tubes

Recent experiments by Pailha et al. [Phys. Fluids 24, 021702 (2012)10.1063/1.3682772] uncovered a rich array of propagation modes when air displaces oil from axially uniform tubes that have local variations in flow resistance within their cross-sections. The behaviour is particularly surprising because only a single, symmetric mode has been observed in tubes of regular cross-section, e.g., circular, elliptical, rectangular, and polygonal. In this paper, we present experimental results describing a new mode, an asymmetric localised air finger, that persists in the limit of zero propagation speed. We show that the experimental observations are consistent with a model based on capillary static calculations within the tube's cross-section, and the observed bistability is a consequence of the existence of multiple solutions to the Young–Laplace equations. The model also provides an upper bound for the previously reported symmetry-breaking bifurcation [A. de Lózar, A. Heap, F. Box, A. L. Hazel, and A. Juel, Phys. Fluids 21, 101702 (2009)10.1063/1.3247879].

Macroroughness and variations in reach‐averaged flow resistance in steep mountain streams

Steep mountain streams typically feature macroroughness elements like large immobile boulders or channel‐spanning bedforms such as step‐pool sequences. The effects of macroroughness on resistance and flow velocity are not well understood and appropriate field parameters for representing macroroughness in flow velocity equations have not been identified. The prediction of flow velocity in rough and steep streams therefore remains challenging. We measured flow velocity and several macroroughness parameters, i.e., boulder concentration, boulder diameter and protrusion, and roughness of longitudinal channel profiles in six reaches of steep mountain streams with plane bed/riffle, step‐pool, and cascade channel morphologies. The between‐site variations in flow resistance can be explained to a large degree by nondimensionalization of discharge and flow velocity using channel slope and a characteristic roughness length. Using any of our roughness parameters as the characteristic roughness length, this nondimensionalization leads to a similarity collapse of the entire data set. The remaining differences in flow resistance among the streams are related to dimensionless measures of macroroughness that describe the concentration of boulders or step density in a reach. Boulder concentration represents the measure best describing the data and is used in a simple regression equation for flow velocity. The predictions were better than predictions by the variable power law equation proposed by Ferguson. Although the regression might not be statistically significant, the observed trends suggest that boulder concentration partly explains the residual variance of between‐site variation of flow resistance.

Do alternate bars affect sediment transport and flow resistance in gravel‐bed rivers?

ABSTRACTIn natural gravel‐bed streams, the complex topography of the bed can cause variation of flow resistance and sediment transport. Previous studies have shown that in addition to grain resistance (skin friction), flow resistance is also caused by bank roughness, channel bars, bed undulations and channel curvature. Sediment transport is similarly influenced by the complex topography, and the transport rate can vary spatially. A three‐dimensional (3D) numerical model was used to generate a detailed description of the flow and bedload transport fields in gravel‐bed rivers. Here we quantify the reach‐averaged hydraulic resistance and sediment transport regime that prevails when self‐formed alternate river bars are present, and compare with the regime that would prevail were no bars are present. We do this by comparing the results of (i) a 3D morphodynamic model in which bars form as a consequence of flow‐bed interaction and (ii) an ‘equivalent’ one‐dimensional (1D) case, which refers to flat bed conditions, but otherwise corresponding to identical average velocity and bed slope. The 3D numerical model is applied to generate different bed topographies of alternate bars at regime morphological equilibrium, then extended to non‐equilibrium conditions for decreasing shear stress within a sensitivity analysis context. The contribution of grain resistance is estimated with the local values of the bed shear stress, while bar resistance results from the overall deviation of the flow field from that occurring in the flat bed configuration. The local sediment transport in both the longitudinal and transverse directions is computed with the local Shields stress and local bed inclination. The calculations result in a method for correcting 1D models to account for the total sediment transport and resistance in a cross‐section due to 3D effects of alternate bars. We term the resulting relations ‘morphologically averaged’ sediment transport and resistance equations. Copyright © 2012 John Wiley & Sons, Ltd.

Controls on spatial variations in flow resistance along steep mountain streams

Detailed channel and water surface surveys were conducted on 15 mountain stream reaches (9 step‐pool, 5 cascade, and 1 plane‐bed) using a tripod‐mounted Light Detection and Ranging scanner and laser theodolite. Reach‐average velocities were measured at varying discharges with dye tracers and fluorometers. Multiple regressions and analysis of variance tests were used to test hypothesized correlations between Darcy‐Weisbach friction coefficient, f, and potential control variables. Gradient (S0) and relative grain submergence (Rh/D84) individually explained a low proportion of the variability in f (R2 = 0.18), where Rh is hydraulic radius, D84 is the 84th percentile of the cumulative grain size distribution, and R2 is equal to the coefficient of determination. Because channel type, grain size, and S0 are interrelated, we tested the hypothesis that f is highly correlated with all three of these variables or a combination of the above variables with flow period (a categorical variable) or dimensionless unit discharge (q*). Total resistance correlated strongly (adj‐R2 = 0.74, 0.69, and 0.64) with S0, flow period, wood load (volume of wood/m2 of channel), q*, and channel type (step‐pool, cascade, plane‐bed). Total resistance differed between step‐pool and plane‐bed and between cascade and plane‐bed reaches. Significant differences in f in step‐pool and cascade reaches were found at the same values of flow and S0. The regression analyses indicate that discharge explains the most variability in f, followed by S0 when discharge is similar among channel reaches, but that Rh/D84 is not an appropriate variable in these steep mountain streams to represent variations in both resistance and discharge. Results also indicate that the forms of resistance among channel types are sufficiently different to change the relationship of the control variables with f in each channel type. These results can be used to further the development of predictive equations for high‐gradient mountain streams.

Determination of apparent and composite friction factors for flooded equatorial natural rivers

This paper presents results, calculated from field measurements taken in several frequently flooded natural rivers, which include D and R relationships, variation of flow resistance with depth of flow, the apparent friction factor, and the composite friction factor for flooded natural rivers. The results obtained have shown the complexity of flow resistance in natural rivers due to the interaction between the main channel and floodplain flow. The interaction has given rise to a pair of apparent shear stresses at the interface region, which can significantly reduce the discharge capacity of the rivers. The apparent shear was quantified in terms of an apparent friction factor, fa , and it was found that the apparent shear stress is many times greater than the averaged boundary shear stress of the rivers. Based on the averaged boundary shear stress and apparent shear stress, the composite (actual) friction factor for the rivers can be estimated accurately (R2 = 0.99) using a statistical method that had been derived.

The Unusual Channel Resistance of the Texas Hill Country and its Effect on Flood Flow Predictions

The importance of hydraulic roughness estimates in modeling river flow cannot be overstated. Spatial and temporal variation of flow resistance can have a dramatic effect on the prediction of floodplain width, depth, and other hydraulic properties. The Texas Hill Country, a physiographic region of uplifted and differentially eroded limestone in south-central Texas, is the site of considerably heterogeneous fluvial forms and processes. Predictions of the spatial extent of floods in this area are difficult given this topographic and fluvial heterogeneity. In this report, we present a new method to extract spatial maps of hydraulic roughness using channel geometry and newly accessible USGS National Water Information System (NWIS) data. We use these data to construct maps of hydraulic roughness at different discharges and to test roughness prediction methods suggested by previous authors. Our method for extracting roughness values is general and should be applicable in most areas of the United States. The patterns of hydraulic resistance around the Texas Hill Country and through its eastern boundary, the Balcones Escarpment, are spatially diverse, ranging from n ≅ 0.016 to 0.213, and no simple spatial pattern of roughness exists. The most unexpected finding is the high values of Manning's roughness value (n) in most areas of the Hill Country, with n values often two or three times greater (n ≅ 0.04-0.06) than what would be expected using common n-estimation techniques (n ≅ 0.01-0.02). This simple finding is of practical importance because the too-low values of Manning's roughness are almost certainly translated into floodplain maps as too narrowly delineated floodplains.

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.

A 2-D, diffusion-based, wetland flow model

Surface water flow in wetlands occurs typically in a low gradient environment where topographic and flow resistance variations control the discharge distribution. It is within this fluid mechanical regime that the biochemical processes constituting wetland ecology take place. Presented herein are the basics of a mathematical model that takes advantage of the hydraulics typical of wetlands. The resulting model, in the form of a 2-D, non-linear diffusion equation, allows incorporation of spatial variations in flow resistance and topography. The applicability of the model to one- and two-dimensional wetland type flow is demonstrated using two cases: a laboratory experiment and a wetland pond. Results illustrate the versatility of the formulation and the important influence of topography variations on flow depth and velocity variations. The use of a fixed rectangular calculation domain to simulate the flow pattern in wetlands with irregular wet boundaries is the most significant practical aspect of the WETFLOW model. Future research will concentrate on dealing with extreme heterogeneity in the elevation and flow resistance distributions, through a combination of measurements and use of stochastic fractals to represent property distributions ( Molz and Boman, 1993, Water Resour. Res., 29: 3769–3774). Experimental studies are needed to quantify the relationship of flow resistance to different combinations of slope, water depth, and a suitable measure of vegetation density for a wetland ecosystem.

Discussion of “ Variation of Flow Resistance Through Curved Channels ” by Howard H. Chang (December, 1984, Vol. 110, No. 12)

Discussion of “ <i>Variation of Flow Resistance Through Curved Channels</i> ” by Howard H. Chang (December, 1984, Vol. 110, No. 12)

Pumping Action of Aligned Smooth Face Seals Due to Axial Vibrations—Theory

This is an analytical study of the mechanism of pumping action produced in two axially symmetrical disks. The disks have a continuous parallel film thickness between them, and one of them has a recess at the center and is subjected to a normal sinusoidal oscillation. It is found out that whether the pumping action is inward or outward depends upon a shift of phase between a variation of flow resistance due to a periodic fluctuation of the film thickness between seal faces and a variation of pressure in the recess.

Shape of porous region to control cooling along curved exit boundary

A cooled porous insert in a curved wall has a specified spatially varying heat flux applied to one side. It is desired to control the distribution of coolant flow out through this curved surface so that the surface will be kept at a desired uniform temperature. The flow regulation is accomplished by shaping the surface through which the coolant enters the region to obtain the required variation of flow resistance within the region. The proper surface shape is found by solving a Cauchy boundary value problem. Analytical solutions are given in two dimensions for various shapes of the heated boundary subjected to different heating distributions.

Electric Network Analog Study of Viscous Flow Normal to Parallel, Evenly Spaced Cylinders

In connection with the study of viscous flow of liquids and gases between textile fibers, an electric resistance network analog was employed to solve the Navier-Stokes equations for creeping flow normal to an infinite array of circular cylinders arranged in a hexagonal pattern. The results of the analog study predict flow resistances approximately four times as great as those measured with real packages of fibers. The factor of four diner ence between predicted and experimentally measured resistances is very likely due to the uneven spacing of fibers in the real case, giving some very large passages which offer relatively little resistance to a significant fraction of the flow. However, the variation of flow resistance with porosity appears the same, on a per centage basis, for the analog prediction based on equally spaced fibers as for the experi mentally measured resistance of real fiber masses which are unequally spaced in a random fashion. Thus neither the predicted nor measured flow resistance varies with fiber mass porosity in accordance with the Carmen-Kozeny formula which has been successfully applied to flows through granular beds; there is now little reason to believe that the large variation of the Kozeny "constant" with porosity of the fiber mass is due simply to shift of passage size distribution with degree of packing of the textile fibers.

The Measurement of Flow Resistance of Porous Acoustic Materials

The flow resistance is one of the three physical parameters of a uniform, rigid, porous material which completely determine its acoustic behavior; for more complicated materials and sound absorbing structures the flow resistance gives at least some acoustic information. The specific flow resistivity or flow resistance per unit cube is given by the equation r = pAt/Vt, in which p is the pressure drop in dynes per cm2 across the thickness l cm of a sample of area A cm2 when a volume V cc of air flows steadily through the sample in a time t sec. An accurate method has been developed for obtaining r by direct measurements of the quantities p, A, t, V, and l. Using this method flow resistance measurements have been made on twenty-four materials over a wide range of velocities. The measurements show that for nearly all materials there is a variation of flow resistance with velocity. For most materials the resistivity is constant below 0.1 cm per sec. but increases considerably at higher velocities. It is shown that previously described comparison methods, which have the advantages of simplicity and speed, might yield results in serious error unless certain suggested precautions are observed. As indicated previously, a direct correlation is found between the steady state flow resistance values and the resistive component of acoustic impedance for several materials. Analysis of the data also yields additional information bearing on the basic physical processes involved in the absorption of sound in these materials. In addition, the measurement of flow resistance has practical applications, such as the rapid routine checking of uniformity of samples of porous materials.