Flow Resistance Coefficient Research Articles

Debris Flow Characteristics in Flume Experiments Considering Berm Installation

This study was conducted to identify the characteristics and mobility of debris flows and analyze the performance of a berm as a debris flow mitigation measure. The debris flow velocity, flow depth, Froude number, flow resistance coefficients, and mobility ratio were accordingly determined using the results of flume tests. To analyze the influence of the berm, the results for a straight channel test without a berm were compared with those for a single-berm channel test. The debris flow velocity was observed to increase with increasing channel slope and decreasing volumetric concentration of sediment, whereas the mobility ratio was observed to increase with increasing channel slope and volumetric concentration of sediment. In addition, it was confirmed that the installation of a berm significantly decreased the debris flow velocity and mobility ratio. This indicates that a berm is an effective method for reducing damage to areas downstream of a debris flow by decreasing its potential mobility. By identifying the effects of berms on debris flow characteristics according to the channel slope and volumetric concentration of sediment, this study supports the development of berms to serve as debris flow damage mitigation measures.

Morphology, flow and heat transfer in triply periodic minimal surface based porous structures

Porous structures are ubiquitous in many thermal management and energy conversion systems. The morphology of a porous structure has significant impact on the fluid flow, heat/mass transport, and strength performance. However, the available fabrication techniques are not capable of directly tailoring porous structures with well-controlled pore features and functional graded pore morphology, thereby limiting the performance enhancements for these systems. In this study, a triply periodic minimal surface (TPMS) based method was developed to customize the morphology of porous media with well-controlled pore features and to fabricate these parts with additive manufacturing. Porous structures with designed pore parameters were built based on the mathematically defined iWP surface, primitive surface, diamond surface and gyroid surface. Before performing flow and heat transfer simulations, morphological analysis was conducted to establish the connections between the geometrical parameters and the performance of the porous structure (flow resistance, heat transfer, and strength). The porous structures were compared in terms of their structural strength, specific pressure drop, interstitial/volumetric heat transfer coefficients and the ratio of the Colburn factor relative to the friction factor. The TPMS porous structures indicated that much higher strength than the simple cubic packing structure (approximation of sintered metal particles) due to their reasonable struct connectivity. Computations demonstrated that the type P structure had the lowest flow resistance and highest comprehensive heat transfer coefficient (j/f). The high specific surface area, continuous changes in the flow direction, periodic mixing/redistribution and flow acceleration contributed to the higher volumetric heat transfer coefficients in the type W/G structures.

Thermo-hydraulic performance of nanofluids under adjustable magnetic field

The effects of an adjustable magnetic field on convective heat transfer performance and flow resistance coefficient of water-based Fe3O4 nanofluids under varying Reynolds numbers (800–10,000) are experimentally investigated. Additionally, the effects of Reynolds number, mass fractions, magnetic field strengths, electromagnets arrangement modes and pitch curvature ratios are discussed. The obtained results indicate that the convective heat transfer performance is directly related to the mass fraction, magnetic field strength and pitch curvature ratio. The Nusselt number and friction resistance coefficient can be increased by 25.0% and 8.1% at best by the use of ferrofluid. The maximum enhancement ratio of heat transfer is 13.8% by increasing magnetic field strength. The spiral tube with P/D = 1.5 can improve the Nusselt number by 57.8% at best when compared with the spiral tube with P/D = 0.6. To effectively reveal the relation between convective heat transfer and flow resistance, a thermal efficiency index is applied to analyze the thermo-hydraulic performance of spiral tubes, thermal efficiency index decreases with Reynolds number in laminar flow, and then increases until reaching peak value at Re = 8000, finally decreases slightly. Our findings are useful for optimizing the enhanced heat transfer mechanism of ferrofluids and improving heat exchanger heat transfer efficiency.

Investigation of Flow Behavior and Porous Medium Resistance Coefficients for Metallic-Cloth Fibers

The flow through porous metallic-cloth fibers influences the cloth seal leakage performance. Measuring the actual seal leakage proves difficult with challenging turbine operating conditions. A non-Darcian porous medium Computational Fluid Dynamics (CFD) model was employed for the flow within porous metallic-cloth fibers. CFD analyses need leakage data depending on the pressure load to calibrate flow resistance coefficients. A test rig was built to measure leakage with respect to the pressure load and weave orientation in four directions. The Sutherland-ideal gas approach was utilized to determine the flow resistance coefficients for Dutch twill metallic-cloth fibers as a function of pressure load. The results show that metallic-cloth fiber leakage is a linear function of pressure load. The best–worst order for leakage performance was the warp, diagonal, shute, and cross directions. For the best sealing performance, the flow direction in metallic-cloth fibers would be the warp direction. The flow resistance coefficients depend on the evaluation of the pressure level, which changes over the weave flow thickness. This is represented with the pressure constant (Cdown). The best match between the test and CFD leakages was obtained for the weave directions of warp (0.9), shute (0.9), diagonal (0.7), and cross (0.0). Calibrating the resistance coefficients with respect to the pressure and temperature enables performing CFD analyses in turbine conditions.

An Excel VBA‐based educational module for bed roughness predictors

AbstractEstimation of flow resistance coefficients plays an inevitable role in open‐channel flows in hydraulic modeling and water resources engineering. Despite many equations and methods recommended for estimating roughness coefficients, only two standard resistance equations (original Manning's and Keulegan's equations) have been widely taught and utilized in practice. There can be various reasons for not using new bed roughness predictors (BRPs), one of which is the absence of teaching BRPs. A rigorous analysis of available BRPs for estimating grain and form roughness coefficients delineates that most of them are comprised of a scheme rather than a single equation. Also, each BRP is tailored to solve a distinct type of problem; however, a proper definition of a problem statement helps shedding light on this application. This study presents an Excel VBA‐based educational estimator to address the gap. For this purpose, the problem statement is first revisited to accomplish a better understanding in light of proper problem classification. On the basis of different groups of input and output data, each BRP is implemented into an Excel spreadsheet and VBA codes are presented for BRPs incorporating both grain and form roughness. This computer application can help students in open‐channel engineering courses not only to become familiar with modified BRPs but also to acknowledge the merit of the Excel spreadsheet for estimating flow resistance coefficients. Finally, as the VBA codes are presented in detail, they can be modified by users in accordance with their priorities for both educational and practical purposes.

Numerical approach for prediction of turbulent flow resistance coefficient of 90° pipe bends

In the present study turbulent flow in a circular cross-sectioned 90° smooth-walled pipe bend has been investigated numerically. The geometry of the model consisted of three sections: a straight inlet pipe of length 52 diameters, a 90° bend with radius of curvature 1 diameter, and a straight downstream pipe of length 52 diameters. Reynolds numbers from 5,000 to 120,000 were considered. The numerical model was developed using COMSOL Multiphysics and all turbulent flow simulations were performed using the standard k- ε turbulence model. The main objective of this work was to estimate the resistance coefficient of pipe bends using not only pressure drop but also information of the flow behavior far downstream in the outlet pipe. Wall static pressure and velocity fields of the secondary flow were illustrated to provide information of the flow behavior in the bend and downstream of the pipe bend. Furthermore, swirl intensity of secondary flow was presented in order to describe its behavior as the flow develops in the outlet pipe. Estimations of resistance coefficient were calculated and compared to other studies. It was found that the procedure used in this study is a viable method for a more accurate estimation of resistance coefficient values for turbulent pipe flows containing 90° bends. For best accuracy in either experimental or computational analyses, it is recommended that pressure in the downstream tangent be measured at no less than fifty diameters downstream of the bend to avoid influence of the secondary flow.

Effect of varying wheatgrass density on resistance to overland flow

Vegetation cover plays an essential role in control of soil erosion in the Loess Plateau of China. The hydraulic resistance to overland flow under disturbed vegetation is critical in the study of water and soil conservation. A series of experiments on runoff hydraulics was conducted on exposed slopes and surfaces with grass cover. Wheatgrass patches with vegetation density ranging from 0.58% to 5.76% and inflows ranging from 5 to 40 L/min, incorporating slopes ranging from 4° to 12° were used in this study. A total of 264 groups of hydraulic runoff tests were conducted. The test results showed that, with increasing vegetation density, the flow pattern evolved from transient to turbulent flow. Both vegetation density and slope affected the Froude number (Fr). Vegetation density is an essential factor affecting flow resistance. On the premise of variation in vegetation density, the Kendall coefficient of Re and flow resistance exceeded 0.727, and the Spearman correlation coefficient exceeded 0.872. The total resistance of overland flow under vegetation cover can be divided into form resistance and grain resistance. With increased vegetation density, the proportion of form resistance of the total resistance increased from 95% to 99%. Vegetation was the main factor that constitutes flow resistance and controls soil erosion. These results are helpful for understanding the influence of vegetation community distribution on mechanisms of slope runoff and soil erosion.

A Comparison between Numerical Simulation Models for the Prediction of Acoustic Behavior of Giant Reeds Shredded

Giant reeds represent a natural fiber widely available in some areas of the world. Its use can be particularly useful as the uncontrolled growth of giant reeds can be a problem because large areas are invaded by them and the crops are damaged. In this study, two models of numerical simulation of the acoustic behavior of giant reeds were put in comparison: the Hamet model and a model based on artificial neural networks. First, the characteristics of the reeds were examined and the procedures for the preparation of the samples to be analyzed were described. Then air flow resistance, porosity and sound absorption coefficient were measured and analyzed in detail. Finally, the results of the numerical modeling of the acoustic coefficient were compared. The neural network-based model showed high Pearson correlation coefficient value, indicating a large number of correct predictions.

Analysis of air flow resistance through a porous stone bed

The results of analysis of airflow resistance through a porous stone accumulator bed are presented with studies conducted in laboratory and in a full-scale facility. The Darcy–Weisbach equation and Ergun's equation were used to determine the flow resistance coefficients. It was found that a simplification of the Ergun's equation by omitting the viscosity term did not result in a significant error in estimating the flow resistance. Dimensional analysis was applied to analyse the flow resistance in the full-scale facility and to determine the form and parameters of equation allowing a calculation of airflow resistance as a function of independent variables. The analysis showed a significant convergence between the friction factors and flow resistance calculated with the use of proposed equations and the values obtained from measurements. It is concluded that the following factors have the largest impact on the airflow resistance in order: bed particle diameter, followed by airflow velocity, effective section diameter and, to the same degree, layer height and bed mass.

Calculated determination of the flow coefficient through the intake system of the internal combustion engine

Taking into account the complexity and duration of creating the optimal shape of the flow part of the intake system, it is advisable to use mathematical models in its design and modernization. This not only reduces the cost and speeds up the creation of an optimal air path, but also provides conditions for automating the design process. The calculated determination of the flow coefficient using modern methods allows you to significantly reduce the amount of experimental research and solve the main problems by choosing the optimal combination of structural elements of the intake channels at the design and finishing stage of the engine. A method is proposed for calculating the flow rate through the intake system of an internal combustion engine (ICE) depending on the design features of the intake system elements: the flow resistance coefficient, the average speed of the piston, the cross-section area of the cylinder, the average effective flow of the valve slot, the number of valves, the value of the valve lift, the diameter of the neck. The influence of total losses from gas dynamic resistance is estimated.

Resistance of velocity slip flow in pipe/channel with a sudden contraction

A novel approach based on the local entropy generation rate, also known as the second law analysis (SLA), is proposed to compute and visualize the flow resistance in mass transfer through a pipe/channel with a sudden contraction component (SCC) at low Reynolds number (Re) featuring velocity slip. The linear Navier velocity slip boundary condition is implemented using the explicit scheme. At small Reynolds number, i.e., Re ≤ 10.0, the flow resistance coefficient of the SCC, KSCC, is found to be a function of the dimensionless velocity slip length Lslip* and Re−1, and gradually increase to a constant value at contraction ratio Rarea ≥ 8, reaching a formula KSCC=(0.4454Lslip* 3−1.894Lslip* 2+2.917Lslip*+8.909)/Re. Over this range of Re, the equivalent length of the flow resistance is almost independent of Re, while out of this range, the equivalent length increases monotonically with Re. Moreover, the dimensionless drag force work around the SCC is negative and reaches a minimum at a critical Lslip*. The SLA reveals that the regions affected by the SCC mainly concentrate around the end section of the upstream pipe/channel rather than the initial partition of the downstream section reported in large Re turbulent flow, and this non-dimensional affected upstream length increases with Lslip*. The fluid physics are further examined using SLA to evaluate the energy loss over the entire domain, decomposed as the viscous dissipation inside the domain and the drag work on the wall boundary.

Experimental analysis of flow and convective heat transfer in the water-cooled packed pebble bed nuclear reactor core

A randomly packed pebble bed test facility is set up to study the flow and convective heat transfer in the water-cooled pebble-bed type nuclear reactor core. Water is used as the test fluid. The electrical induction heating system provides internal heat source for the packed pebble bed. The effects of input power, porosity, fluid velocity and inlet temperature are studied on the flow resistance and convective heat transfer coefficients. A new correlation is proposed to predict the pressure drop in the random packed pebble bed. The correlations given by Wakao et al. and KTA agree well with the experimental convective heat transfer coefficients in the high-velocity and low-velocity conditions respectively. A new correlation is presented based on the trend similarity between the Colburn j factor and friction factor, to predict the pebble bed convective heat transfer characteristics, with a discrepancy no more than 10%.

Convective heat transfer characteristics of twin-web turbine disk with pin fins in the inner cavity

In order to enhance the convective heat transfer effect of high pressure turbine disk, a novel twin-web turbine disk with pin fins in the inner cavity is developed. The convective heat transfer characteristics of the traditional TWD and the TWD with pin fins are analyzed by numerical method verified by a hollow disk model. The results show that there is a circular flow in the inner cavity which impacts the back and front web to enhance the local convective heat transfer coefficient. The Nusselt number of the back web is remarkably higher than that of the front web, and the temperature of the back web is visibly lower than that of the front web. The pin fins in the inner cavity compress the circular flow and make it move downward, so that the Nusselt number in the region below the pin fins is improved; meanwhile, local convective heat transfer coefficient and the area of heat exchange have both increased to improve the convective heat transfer effect. The convective heat transfer characteristics of the developed TWD with six Reynolds numbers are analyzed, the average Nusselt number of the inner cavity surface with pin fins are as much as about 10% larger than that of the traditional TWD, the flow resistance coefficients of the inner cavity of TWD with pin fins is about 11% higher than that of the traditional TWD.

Modelling sound absorption properties of broom fibers using artificial neural networks

The use of broom to produce fibers has ancient roots. The Greeks appreciated its resistance to water and for this reason they used it to manufacture sailing ropes. But broom fiber was also appreciated for its sound absorption qualities. In this study, a new methodology was developed for the numerical modeling of the acoustic behavior of broom fibers. First, the characteristics of the different varieties of broom were examined and the procedures for processing the samples to be analyzed were described. Subsequently, the results of the measurements of the following acoustic properties of the material were analyzed: air flow resistance, porosity, and sound absorption coefficient. Finally, the results of the numerical modeling of the acoustic coefficient were reported using an algorithm based on artificial neural networks. The results obtained are compared with a model based on linear regression. The model based on neural networks showed high values of the Pearson correlation coefficient (0.989), indicating a high number of correct predictions.

Evaluation of Flow Resistance Models Based on Field Experiments in a Partly Vegetated Reclamation Channel

This study presents a methodology for improving the efficiency of Baptist and Stone and Shen models in predicting the global water flow resistance of a reclamation channel partly vegetated by rigid and emergent riparian plants. The results of the two resistance models are compared with the measurements collected during an experimental campaign conducted in a reclamation channel colonized by Common reed (Phragmites australis (Cav.) Trin. ex Steud.). Experimental vegetative Chézy’s flow resistance coefficients have been retrieved from the analysis of instantaneous flow velocity measurements, acquired by means of a downlooking 3-component acoustic Doppler velocimeter (ADV) located at the channel upstream cross section, and by water level measurements obtained through four piezometers distributed along the reclamation channel. The main morphometrical vegetation features (i.e., stem diameters and heights, and bed surface density) have been measured at six cross sections of the vegetated reclamation channel. Following the theoretical assumptions of the divided channel method (DCM), three sub-sections have been delineated in the reference cross section to represent the impact of the partial vegetation cover on the cross sectional variability of the flow field, as observed with the ADV measurements. The global vegetative Chézy’s flow resistance coefficients have been then computed by combining each resistance model with four different composite cross section methods, respectively suggested by Colebatch, Horton, Pavlovskii, and Yen. The comparative analysis between the modeled and the experimental vegetative Chézy’s coefficients has been performed by computing the relative prediction error (εr, expressed in %) under two flow rate regimes. Stone and Shen model combined with the Horton composite cross section method provides vegetative Chézy’s coefficients with the lowest εr.

Analysis of an internal structure for refrigerated container: Improving distribution of cooling capacity

An improved internal structure is proposed to improve the distribution of cooling capacity in refrigerated container. Firstly, a computational fluid dynamics model was established and the fruit stacks was simplified to be porous medium. The flow resistance coefficient was obtained by combining theory and numerical simulation and then verified. Secondly, three baffles were added to reduce the loss of cold airflow by blocking the airflow through air-gaps. The velocity and temperature of the previous refrigerated container and the improved refrigerated container were obtained and compared. The cold airflow distribution in the improved refrigerated container is more uniform. The improvement of vertical velocity in all stacks is in the range of 5.24% – 425.04%. The cooling performance of the two refrigerated containers were also obtained. The variation of temperature distribution is within ±1 °C and the cooling time is also reduced by at least 22.9% in the improved refrigerated container. Finally, the proposed structure showed good applicability for different fruits and vegetables. For all produce tested, the decrease ratio of temperature rise in the improved refrigerated container relative to the previous refrigerated container ranges from 34.0% to 59.5%.

A Deep Neural Network Based Glottal Flow Model for Predicting Fluid-Structure Interactions during Voice Production.

This paper proposes a machine-learning based reduced-order model that can provide fast and accurate prediction of the glottal flow during voice production. The model is based on the Bernoulli equation with a viscous loss term predicted by a deep neural network (DNN) model. The training data of the DNN model is a Navier-Stokes (N-S) equation-based three-dimensional simulation of glottal flows in various glottal shapes generated by a synthetic shape function, which can be obtained by superimposing the instantaneous modal displacements during vibration on the prephonatory geometry of the glottal shape. The input parameters of the DNN model are the geometric and flow parameters extracted from discretized cross sections of the glottal shapes and the output target is the corresponding flow resistance coefficient. With this trained DNN-Bernoulli model, the flow resistance coefficient as well as the flow rate and pressure distribution in any given glottal shape generated by the synthetic shape function can be predicted. The model is further coupled with a finite-element method based solid dynamics solver for simulating fluid-structure interactions (FSI). The prediction performance of the model for both static shape and FSI simulations is evaluated by comparing the solutions to those obtained by the Bernoulli and N-S model. The model shows a good prediction performance in accuracy and efficiency, suggesting a promise for future clinical use.

Analysis ofWater Transport inside a Plant Xylem Vessel with Pitted Thickening

In this article, computational fluid dynamics (CFD) are used to explore the dynamics of water transport inside the pitted thickening of a plant xylem vessel. A pitted thickening model combined with the Bernoulli equation is used to analyze the influence of various factors (namely, the inner diameter, thickening width, thickening height, thickening spacing, number of laps and adjacent pit axial rotation). The pressure drop and the flow resistance coefficient are the variable parameters for our analysis. The results show that these two parameters are proportional to the thickening height and thickening width, and inversely proportional to the inner diameter, thickening spacing and number of laps. Three different wall thickening structures of the vessel are compared and the pitted thickening vessel is shown to provide the largest structural flow resistance, the annular thickening vessel has the second largest resistance and the helical thickening vessel corresponds to the smallest resistance of the three structures.

Influence of rotation angle of a triangular tube with a built‐in twisted tape on the thermal‐exergy efficiency and entropy generation of nanofluids in the heat exchange system

AbstractIn order to investigate the thermal‐exergy efficiency and entropy generation of nanofluids flowing through a triangular tube with a built‐in twisted tape, a heat exchange system is designed and established. Deionized water and three mass concentration nanofluids (ω = 0.1 wt%, 0.3 wt%, and 0.5 wt%) are applied as the heat‐transfer mediums flowing through three different rotation angle (α = 0°, 30°, and 60°) triangular tubes, respectively. Thermal efficiency index (η) and entropy generation (S) are used to characterize the performance of the heat exchange system. It is proved that the increase of nanoparticles can not only effectively promote the heat transfer effect but also augment the pressure loss. Nusselt number of nanofluids is increased by 20.2% on average, whereas the flow resistance coefficient is increased by 10.1% on average. Based on the thermal efficiency index, the data demonstrate that ω = 0.5 wt% nanofluids in the triangular tube at α = 0° can make the most effective use of energy, whose value could reach 1.32 at most. Exergy efficiency of nanofluids in the heat exchange system can be evidently heightened under the same pumping power and flow rate. Entropy production of 0.1 wt% nanofluids is the smallest.

A non-linear rheological model of plant tissues

A sophisticated relationship between strain rate and stress in plant tissues subjected to mechanical loads reflects the structural complexity and properties of biological tissues. The aim of this study was to develop a mathematical model explaining the above relationship. A simplified, non-linear rheological model of a cellular solid characterised by elastic cell walls and the presence of incompressible fluid in cells was developed. In this model, changes in tissue deformation are determined by the elasticity of the cell wall, the ratio of cell wall thickness to cell diameter, the flow resistance coefficient and tissue porosity. Plant materials (discs cut out from fresh sugar beet roots, celery roots and potato tubers) were analysed to verify the model's adequacy. The developed model indicates that in samples with a constant volume during compression, stress in compressed tissue is determined not only by strain and strain rate, but also by the cellular structure and the shape of the sample. Further research is needed to test the model in other types of materials. The model should also be analysed for its sensitivity to cell size and shape, and the morphological structure of biological tissues.