Mean Flow Path Research Articles

Smoothing of digital elevation models and the alteration of overland flow path length distributions

AbstractDEM smoothing is a common pre‐processing technique used to remove undesirable roughness from a DEM. However, it is hypothesized that smoothing straightens and reduces the length of overland flow paths, which is an important factor controlling modelled time‐to‐peak flow. Currently, there is a lack of research comparing how different smoothing techniques alter the distribution of overland flow path length. Four low‐pass filtering techniques were applied to three fine‐resolution LiDAR DEMs of varying relief: the mean filter, the median filter, the Gaussian filter, and the feature‐preserving DEM smoothing (FPDEMS) filter, each with different degrees of smoothing. Downslope‐distance‐to‐stream distributions were then derived using D8 and D∞ flow directions and statistically compared to distributions derived from the unsmoothed DEM for each study site. The results indicate that the alteration of flow path length distributions as a result of smoothing is complex. Mean flow path lengths may decrease or increase in response to smoothing, depending on landscape relief and the derivation of flow directions, and generalized flow paths may become longer. The largest increase in mean flow path lengths was 19.2 m using the 21 × 21 median filter and D8 flow directions in the high‐relief study site, relative to an unsmoothed mean length of 138.6 m in this site. The largest decrease in mean flow path length was 48.9 m using the 21 × 21 mean filter and D∞ flow directions in the low‐relief study site, relative to an unsmoothed mean length of 290.9 m in this site. Furthermore, minimal flow path length alterations were achieved with the Gaussian filter when gentle smoothing is required, and with the FPDEMS filter when moderate to aggressive smoothing is required. These results suggest that an appropriate smoothing method should be chosen based on the relief of the landscape and the degree of smoothing required.

Discordance of Tracer Transport and Particulate Matter Fate in a Baffled Clarification System

Abstract Large eddy simulation (LES) and coupled physical laboratory-scale modeling are performed to elucidate tracer transport and particulate matter (PM) fate in a baffled clarification system. Such baffled systems are common for urban water unit operations and processes. Flow hydrodynamic indices of these systems such as short-circuiting are often examined with measurement of inert tracer transport as a surrogate for chemical or PM transport and fate. Results of this study illustrate complex interactions between turbulent flow, tracer, and various PM diameters at the system scale. PM preferential accumulation and the discordance of PM transport with respect to flow hydrodynamics are observed based on the modeling results; otherwise not practical with physical model testing. Results demonstrate that baffling can promote system tracer mixing and improve volumetric utilization by extending the mean flow path through flow separation and bifurcation. The baffle tested produced high turbulence kinetic energy near the sedimentation floor and reduced PM separation (clarification) as compared to the unbaffled system used as a control. The unbaffled system in this study yields the highest PM separation, even though significant short-circuiting occurs during the residence time distribution (RTD) of the tracer. Further analysis demonstrates the mechanistic difference between the tracer transport and the finer suspended PM as compared to larger settleable and sediment PM diameters. Results illustrate that the tracer RTD, residence time (RT) and hydraulic efficiency indices are not reliable surrogates for PM or PM-bound chemical/pathogen separation. In addition, simulations suggest a site, system or condition-specific design approach given the coupled dependence on flow and design geometry.

Discrete unified gas kinetic scheme for all Knudsen number flows. IV. Strongly inhomogeneous fluids.

This work is an extension of the discrete unified gas kinetic scheme (DUGKS) from rarefied gas dynamics to strongly inhomogeneous dense fluid systems. The fluid molecular size can be ignored for dilute gases, while the nonlocal intermolecular collisions and the competition of solid-fluid and fluid-fluid interactions play an important role for surface-confined fluid flows at the nanometer scale. The nonequilibrium state induces strong fluid structural-confined inhomogeneity and anomalous fluid flow dynamics. According to the previous kinetic model [Guo etal., Phys. Rev. E 71, 035301(R) (2005)10.1103/PhysRevE.71.035301], the long-range intermolecular attraction is modeled by the mean-field approximation, and the volume exclusion effect is considered by the hard-sphere potential in the collision operator. The kinetic model is solved by the DUGKS, which has the characteristics of asymptotic preserving, low dissipation, second-order accuracy, and multidimensional nature. Both static fluid structure and dynamic flow behaviors are calculated and validated with Monte Carlo or molecular dynamics results. It is shown that the flow of dense fluid systems tends to that of rarefied gases as the dense degree decreases or the mean flow path increases. The DUGKS is proved to be applicable to simulate such nonequilibrium dense fluid systems.

Simulation of a Radial Pump Fast Startup and Analysis of the Loop Response Using a Transient 1D Mean Stream Line Based Model

A predictive transient two-phase flow rotodynamic pump model has been developed in the Code for Analysis of THermalhydraulics during an Accident of Reactor and safety Evaluation (CATHARE-3). Flow inside parts of the pump (suction, impeller, diffuser and volute) is computed according to a one-dimensional discretisation following a mean flow path. Transient governing equations of the model are solved using an implicit resolution method and integrated along the curvilinear abscissa of the element. This model has been previously qualified at the component scale by comparison to an existing experimental database. The present study aims at extending the validation at the system scale: a whole experimental test loop is modelled. The ability of the transient pump model to predict flow rate, head and torque as a function of time during a 1-s pump fast start-up is evaluated. The transient evolution of the pressure upstream and downstream from the centrifugal pump is well predicted by the simulation compared to the measurements. Local quantities such as pressure and velocity inside elements of the circuit are analysed. In the considered case, inertial effects of the global circuit are dominant when compared to pump inertial effects due to the high characteristic lengths of the pipes. The main perspective of this work consists in the simulation of similar pump transients, in cavitating conditions.

Convective instability in layered sloping warm-water aquifers

In a recent paper (McKibbin et al., 2011) some questions were posed about fluid and heat flows in a stratified groundwater aquifer with a small slope, subject to a perpendicular temperature gradient. The strength of shear flows in the direction of the maximum slope and the associated convected heat flux were quantified. This paper provides a method to quantify the stability of such flows and the shape and amplitude of convective rolls that may form when the critical Rayleigh number is exceeded. The associated issue of how the mean flow-path of a soluble species introduced into the aquifer is affected by the convective rolls, is also considered. The models formulated are for buoyancy-driven fluid flow in long, sloping warm-water aquifers with both smoothly- and discretely-layered structures.

Changes in ecosystem structure, function and hydrological connectivity control water, soil and carbon losses in semi‐arid grass to woody vegetation transitions

ABSTRACTConnectivity has recently emerged as a key concept for understanding hydrological response to vegetation change in semi‐arid environments, providing an explanatory link between abiotic and biotic, structure and function. Reduced vegetation cover following woody encroachment, generally promotes longer, more connected overland flow pathways, which has the potential to result in an accentuated rainfall‐runoff response and fluxes of both soil erosion and carbon.This paper investigates changing hydrological connectivity as an emergent property of changing ecosystem structure over two contrasting semi‐arid grass to woody vegetation transitions in New Mexico, USA. Vegetation structure is quantified to evaluate if it can be used to explain observed variations in water, sediment and carbon fluxes. Hydrological connectivity is quantified using a flow length metric, combining topographic and vegetation cover data.Results demonstrate that the two woody‐dominated sites have significantly longer mean flowpath lengths (4 · 3 m), than the grass‐dominated sites (2 · 4 m). Mean flowpath lengths illustrate a significant positive relationship with the functional response. The woody‐dominated sites lost more water, soil and carbon than their grassland counterparts. Woody sites erode more, with mean event‐based sediment yields of 1203 g, compared to 295 g from grasslands. In addition, the woody sites lost more organic carbon, with mean event yields of 39 g compared to 5 g from grassland sites.Finally, hydrological connectivity (expressed as mean flowpath length) is discussed as a meaningful measure of the interaction between structure and function and how this manifests under the extreme rainfall that occurs in semi‐arid deserts. In combination with rainfall characteristics, connectivity emerges as a useful tool to explain the impact of vegetation change on water, soil and carbon losses across semi‐arid environments. Copyright © 2013 John Wiley & Sons, Ltd.

Uptake of picophytoplankton, bacterioplankton and virioplankton by a fringing coral reef community (Ningaloo Reef, Australia)

We examined the importance of picoplankton and virioplankton to reef trophodynamics at Ningaloo Reef, (north-western Australia), in May and November 2008. Picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes), bacterioplankton (inclusive of bacteria and Archaea), virioplankton and chlorophyll a (Chl a) were measured at five stations following the consistent wave-driven unidirectional mean flow path of seawater across the reef and into the lagoon. Prochlorococcus, Synechococcus, picoeukaryotes and bacterioplankton were depleted to similar levels (~40% on average) over the fore reef, reef crest and reef flat (=‘active reef’), with negligible uptake occurring over the sandy bottom lagoon. Depletion of virioplankton also occurred but to more variable levels. Highest uptake rates, m, of picoplankton occurred over the reef crest, while uptake coefficients, S (independent of cell concentration), were similarly scaled over the reef zones, indicating no preferential uptake of any one group. Collectively, picophytoplankton, bacterioplankton and virioplankton accounted for the uptake of 29 mmol C m−2 day−1, with Synechococcus contributing the highest proportion of the removed C. Picoplankton and virioplankton accounted for 1–5 mmol N m−2 day−1 of the removed N, with bacterioplankton estimated to be a highly rich source of N. Results indicate the importance of ocean–reef interactions and the dependence of certain reef organisms on picoplanktonic supply for reef-level biogeochemistry processes.

Tracer distributions of line advected thermals

A Light Attenuation system is employed to study the behaviour of strongly advected buoyant jets, focusing in particular on line advected thermals. A double-Gaussian approximation is employed to characterize the cross-sectional form of the tracer distributions. The new data highlights distinct differences in the form of line advected thermals, when compared to advected line momentum puffs. Thermals have a wider peak separation and a relatively large cross-sectional distortion that decays as the flow becomes fully established. It is therefore possible to improve predictions of mean minimum dilution, spread and flow path through separate parameterization of these strongly-advected flows. However, there are notable inconsistencies in recently obtained trajectory data, which require further investigation before such improvements can be made.

Evaluation of analytical solute discharge moments using numerical modeling in absolute and relative dispersion frameworks

Two‐dimensional numerical simulations are used to validate the analytical solutions for the solute discharge moments. In addition to the analysis of classical absolute dispersion we also consider relative dispersion whereby plume meandering (deviation from mean flow path caused by velocity variations at scales larger than plume size) is removed. The numerical simulations are used within a Monte Carlo framework to assess the accuracy and robustness of the analytical predictions of the solute discharge moments (mean and variance). Results show that the analytical predictions deviate from the numerical simulations as the log conductivity variance increases. Deviation occurs for the mean as well as the variance of the solute discharge. The absolute dispersion formulation, however, shows better agreement with the numerical simulations than does the relative dispersion for strong heterogeneity and vice versa for small variability. The relative dispersion results, however, depend on the prediction of the ensemble mean of the plume arrival time, which differs between simulations and analytical solution. Using the first‐order analytical estimate for this parameter leads to a much better agreement between the numerical and the analytical results for solute discharge moments in the relative dispersion case.

Computational issues in the determination of solute discharge moments and implications for comparison to analytical solutions

Solute discharge moments (mean and variance) are computed using numerical modeling of flow and advective transport in two-dimensional heterogeneous aquifers and are compared to theoretical results. The solute discharge quantifies the temporal evolution of the total contaminant mass crossing a certain compliance boundary. In addition to analyzing the solute discharge moments within a classical absolute dispersion framework, we also analyze relative dispersion formulation, whereby plume meandering (deviation from mean flow path caused by velocity variations at scales larger than plume size) is removed. This study addresses some important issues related to the computation of solute discharge moments from random walk particle tracking experiments, and highlights some of the important differences between absolute and relative dispersion frameworks. Relative dispersion formulation produces maximum uncertainty that coincides with the peak mean discharge. Absolute dispersion, however, results in earlier arrival of the uncertainty peak as compared to the first moment peak. Simulations show that the standard deviation of solute discharge in a relative dispersion framework requires increasingly large temporal sampling windows to smooth out some of the large fluctuations in breakthrough curves associated with advective transport. Using smoothing techniques in particle tracking to distribute the particle mass over a volume rather than at a point significantly reduces the noise in the numerical simulations and removes the need to use large temporal windows. Same effect can be obtained by adding a local dispersion process to the particle tracking experiments used to model advective transport. The effect of the temporal sampling window bears some relevance and important consequences for evaluating risk-related parameters. The expected value of peak solute discharge and its standard deviation are very sensitive to this sampling window and so will be the risk distribution relying on such numerical models.

A study of the shifting transients of a torque converter equipped vehicle

A torque converter, the major component of an automatic transmission, is a hydrodynamic device that has a great influence on the transient characteristics of a vehicle during gear shifting. To predict accurately the vehicle characteristics, detailed analysis of the torque converter is required. In this study, the classical one-dimensional modelling of the torque converter, based on the concept of the mean flow path, is used to analyse the shifting transients. It is proved that the equivalent parameters included in the modelling can be determined from the experimental results by using nonlinear optimisation methods. Dynamic modelling of power transmission components, such as clutches and planetary gear systems, are also considered. The clutch engagement condition, which determines the degree of freedom of the system during shifting, is proposed. In the modelling of the power transmission system, the stiffness of the shaft and various control logics are included. Using the developed simulation method, the effects of the design variables and the control conditions are evaluated. Finally, the results of the driving simulation are compared with those of the dynamometer-based experiments.

A Study on the Correction Factor of Flow Angel by using the One Dimentional Performance Model of Torque Converter

One dimensional performance model has been used for the design of torque converter. The model is based on the concept of constant mean flow path and constant flow angle. These constant-assumed para meters make the design procedure to be simple. In practice, some parameters are usually replaced with geometric raw data and, the constant experiential correction factors have been used to minimize the design error. These factors have no definite physical meaning and so they cannot be applied confidently to the other design condition. In this study, the detail dynamic model of torque converter is presented to establish the theoretical background of correction factors. To verify the validity of theoretical model, steady state performance test was carried out on the several input speed. The oil temperature effect on the performance is analysed and adjusted. The constant equivalent flow angles are determined at a part of performance region by comparing the theoretical model and the test data. The sensitivity of correction factors to the input speeds are studied and the change of torus flow is presented.

A Study on the Basic Design of a Torque Converter Using Equivalent Performance Model

The torque converter, a major part of automatic transmissions, has many difficulties in analysis due to the factors such as power transmission through fluid flow, complex internal geometry, and various operating conditions. Because of such difficulties, the dynamic analysis and design of a torque converter are generally carried out by using equivalent performance model which is based on the concept of mean flow path. Since the design procedures of a torque converter are essential technology of automotive industry, the details of the procedures are rarely published. In this study, the basic design procedures of a torque converter are systemized and coded based on the equivalent performance model. The mathematical methods to deal with mean flow path determination and the core-shape are developed. And by using this model, the method of determination of performance parameters satisfying the requested performance is proposed. Finally, to embody the three-dimensional shape, the intermediate blade angles which maximize the tractive performance are determined and laid out.

New Tools for Modeling Fracture Networks and Simulating Gas Flow in Low-Permeability Sand and Shale Reservoirs: ABSTRACT

The U.S. Department of Energy, Morgantown Energy Technology Center, has an on-going project to model and simulate gas flow in low-permeability sands and shales that contain irregular, sometimes discontinuous, fracture networks (i.e., the types of networks not adequately represented by existing models/simulators). A FORTRAN code and methodology for modeling and simulating flow in these fracture networks has been developed. The goal was to convert the locations and orientations of fractures, as observed along a horizontal well bore, into two-dimensional, geometrically and hydraulically equivalent networks, which can be used to study variability in yield and drainage pattern. The fracture network generator implements four models of increasing complexity through a Monte Carlo process of selecting fracture network attributes from fitted statistical distributions. A process of shifting fracture end-point locations along the axes of fractures provides a partial control of fracture intersection/termination frequencies. Output consists of fracture end-points and apertures. The flow simulator divides each fracture-bounded matrix block into subregions that drain to the midpoint of the adjacent fracture segment in accordance with a one-dimensional, unsteady idealization. The idealization approximates both the volume and the mean flow path length of each subregion. Volumetric flow rate in the fractures is modeled as a linearmore » function of the pressure difference between the recharge points and the fracture intersections. The requirement of material balance between all intersections couples the individual recharge models together, and the resulting equations are solved by a Newton-Raphson technique.« less

Numerical simulations of Twin Lake Natural‐Gradient Tracer Tests: A comparison of methods

The primary objectives of the Twin Lake experiments have been to study geologic heterogeneities and field‐scale dispersion and to provide data for developing and evaluating groundwater flow and transport models. The experiment reported here was undertaken in 1987–1988 to expand the studies of heterogeneity and dispersion from a 40‐m scale investigated in 1983 to a full 270‐m scale from the point of tracer injection to the groundwater discharge zone. The hydrogeologic setting, the experimental and analytical methods, and the interpretation of experimental data are the topics of discussion in this paper. The 1983 and 1987–1988 tracer tests were used to evaluate the predictive abilities of various numerical groundwater flow and transport models. The modeling is done along the mean flow direction in the vertical plane. The two‐dimensional finite‐element modeling was successful in simulating the hydraulic head distribution but not the groundwater motion or the tracer transport. In addition, owing to the advection‐dominated nature of the groundwater flow, the transport simulation suffered from numerical dispersion. Significant improvements in simulations were obtained using method‐of‐characteristics‐and random‐walk‐based computer models. The influence of dimensionality on the transport simulation was evaluated by comparing the two‐ and three‐dimensional random‐walk simulations. It was found that over the first 40 m along the mean flow path, the three‐dimensional simulation does not reproduce the plume migration any better than the two‐dimensional simulation does. Random‐walk simulations of the transport observed in the 1983 and 1987–1988 tests are also done on the basis of the advection‐dispersion equation and tracer test‐derived velocities without invoking the concept of hydraulic conductivity and Darcy's law. These simulations were found to be superior to simulations based on the coupled flow and transport models. It was also found that the simulated hydraulic head distribution is insensitive to spatial variations in hydraulic conductivity and the simulated groundwater velocity distribution is inappropriate for transport modeling. It was concluded that the advection‐dispersion models of the local‐scale transport perform better in predicting the tracer migration at the Twin Lake aquifer using measured velocities rather than velocities simulated by a flow model.

Assessment of the blade skew and the through flow velocity gradients in hydrodynamic torque converters

The well established one-dimensional mean flow approach in the analysis of hydrodynamic torque converters has been supplemented by equations regarding the balance in the transverse direction of the flow path. By these equations the validity of the design mean flow path can be considered as well as the impact of the skew of the blades. Also additional information about the flow is obtained in the form of through flow velocity gradients, which allows a refined calculation of the performance to be undertaken.

An Improved Accelerated Test Chamber for Electrolytic Metal Migration and Corrosion

Many accelerated electrolytic metal migration and corrosion investigations have been performed in accelerated test chambers with unknown gas concentration and flow characteristics. This paper outlines the development of a well‐characterized accelerated test chamber. The length (mean flow path) to hydraulic diameter ratio of the closed‐circuit rectangular flow channel is 20. Velocity and concentration measurements showed minimal horizontal or vertical gradients in the test section. The range of standard deviations (m/s) for the velocity profiles in the test section was 0.0037–0.16 at mean velocities of 0.11–2.9 m/s, respectively. Similarly, the range of standard deviations [ppm(vol.) for the concentration profiles was 1.0–1.2 for a standard gas mixture of in the air. The effect of gas velocity upon the rate of metal migration was demonstrated in the described test chamber using a new accelerated test in an environment. This test, the PEG‐400 drop test, is a moderate accelerated test with metal migration rates that are less than the water‐drop test but greater than a temperature humidity bias test with or without pollutants. The gas velocity was found to have a significant effect upon the rate of copper migration. The effects of gas velocity, humidity, and pollutant concentrations upon the metal migration and corrosion rates can be investigated in this accelerated test chamber.

Brownian diffusion of aerosols to the face of a nuclepore filter

An approximate solution of the transport equation is found for aerosols in the flow approaching a Nuclepore filter. Small aerosols are shown to be collected on the front face of the filter owing to Brownian diffusion relative to to the mean flow path lines. A formula is derived for the diffusive collection efficiency of the filter, and it is valid for filter porosities in the range 0.05–0.64.

A theoretical model for the performance prediction of fully filled fluid couplings

A detailed performance prediction procedure, applicable fo fully filled fluid couplings is presented. In this procedure the fluid flow is considered to have two components; one circumferentially about the coupling axis, and the other passing fluid from the pump impeller to the turbine runner in the plane of the coupling axis. Whilst the procedure is essentially a one-dimensional analysis, a mean flow path approach is not used; instead the velocity component giving the circulation through each element is considered to increase linearly with distance from the mean radius R m . The inclusion of empirical specific energy losses into this flow model is described in detail and the full performance prediction compared with experimental results.

A performance prediction procedure for three element torque converters

A detailed performance prediction procedure for three element torque converters is presented. Whilst the procedure is essentially a one-dimensional analysis a mean flow path approach is not used and instead the fluid is assumed to circulate through each element with a velocity which varies linearly with distance from a mean radius R m . The results predicted with this detailed flow model are compared with the more conventional single mean streamline approach and with available experimental data.