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We describe the flow paths of negatively buoyant river inflows in a small lake with multiple basins separated by sills (Toolik Lake, Alaska) using field data and three-dimensional simulations. Comparisons of field observations, analytical computations, and simulations show that in small basins in which the timescale for filling ([[tau].sub.f]) is less than the timescale of an event ([[tau].sub.0]), overflow is the dominant mechanism of interbasin exchange. Exchange is mediated by internal wave displacements if upwelling reaches the height of the sills, [z.sub.s]. This criterion is met if the Lake number, the inverse of which is proportional to the degree of metalimnetic tilting, is less than [z.sub.i]/([z.sub.i] - [z.sub.s]), where [z.sub.i] is the vertical displacement of the intrusion. Entrainment followed by horizontal dispersion is the dominant mechanism of interbasin transport and can account for ~65% of the exchange between large subbasins. Entrainment is enhanced when submerged sills force river water to flow close to the surface layer. Depths of intrusions depend upon discharge. They occurred in the lower metalimnion for the coldest events analyzed but were near the top of the metalimnion during low-discharge events, such as during brief cold fronts. Persistence of intrusions depends upon the time interval between cold fronts; they ranged from a few days to three weeks. Timescales of horizontal mixing vary with meteorological forcing; they ranged from on the order of 1 d for wind events with speeds of up to 6-8 m [s.sup.-1] to on the order of 10 d for light winds.

The effects of the slot hot blowing of the hollow stator blades on the size of secondary water droplets and the thickness of the water film were experimentally investigated in this paper. The experiment was carried out on the turbine blade cascades in a wet air tunnel with an inlet air wetness fraction of 7.9%, an outlet air velocity of 170 m/s, a slot width of 1.0 mm, and a slot angle of 45 deg to blade surface. The Malvern droplet antiparticle size analyzer was utilized to measure the secondary water droplet size and distribution downstream of the hollow stator blades in the experimental tests. The experimental results show that the maximum diameter and Sauter mean diameter of the secondary water droplets were reduced greatly, and the water droplet size distribution became narrower. The larger blowing pressure difference resulted in the smaller secondary water droplets and the narrower water droplet size distributions. In addition, the efficiency of water separation from the hollow stator blade surfaces was higher for slot on the suction side than that of the pressure side case. Another simplified experimental test was also carried out on the flat plate to investigate the effect of slot hot blowing on the thickness of the water film downstream of the slot. The conductivity probes were used to measure the thickness of the water film downstream and upstream of the blowing slot. The results show that the slot hot blowing reduced the thickness of the water film downstream of the slot, which was affected by the blowing pressure difference and temperature difference between the hot blowing air and the main airflow. In conclusion, the slot hot blowing of the hollow stator blades has reduced the size of the secondary water droplets and secondarily has evaporated a little water film on the blade surfaces. Both effects are beneficial to reduce the erosion damage to the rotor blades. [DOI: 10.1115/1.3030877] Keywords: slot hot blowing, secondary water droplet, hollow stator blade

A numerical study of in-cylinder soot formation and oxidation processes in n-heptane lifted flames using various soot inception species has been conducted. In a recent study by the authors, it was found that the soot formation and growth regions in lifted flames were not adequately represented by using acetylene alone as the soot inception species. Comparisons with a conceptual model and available experimental data suggested that the location of soot formation regions could be better represented if polycyclic aromatic hydrocarbon (PAH) species were considered as alternatives to acetylene for soot formation processes. Since the local temperatures are much lower under low temperature combustion conditions, it is believed that significant soot mass contribution can be attributed to PAH rather than to acetylene. To quantify and validate the above observations, a reduced n-heptane chemistry mechanism has been extended to include PAH species up to four fused aromatic rings (pyrene). The resulting chemistry mechanism was integrated into the multidimensional computational fluid dynamics code KIVA-CHEMKIN for modeling soot formation in lifted flames in a constant volume chamber. The investigation revealed that a simpler model that only considers up to phenanthrene (three fused rings) as the soot inception species has good possibilities for better soot location predictions. The present work highlights and illustrates the various research challenges toward accurate qualitative and quantitative predictions of the soot for new low emission combustion strategies for internal combustion engines. [DOI: 10.1115/1.3043806] Keywords: soot modeling, low temperature combustion, polycyclic aromatic hydrocarbons, lift-off length, reduced chemistry

Migmatitic lower-crustal rocks in the Ediacaran intracontinental Petermann orogen, central Australia, are bounded by regional-scale, low-angle shear fabrics that record opposing shear senses. New sensitive high-resolution ion microprobe (SHRIMP) titanite geochronology suggests that the contrasting shear systems developed synchronously. We argue that the macroscopic structural and metamorphic architecture satisfies the diagnostic criteria outlined by recently proposed channel flow models, and thus the remarkably well-preserved Petermann orogen is potentially an example of ancient intracontinental channel flow.

Problems related to sedimentation and deposition can be minimized by using a system where weirs and gates are combined. Given its applications, the hydraulics of simultaneous flow over weir and under gate, in particular, the determination of the stage--discharge relationship, is of interest. Although previous approaches have been based on regression or dimensional analysis, the current work describes a physically based approach. Models of sharp-edged weirs and gates with no lateral contraction are combined. To calibrate and validate the proposed model, experiments have been carried out in a laboratory flume applying different submergence conditions. It was found that the model is able to predict the stage---discharge relationship with reasonable accuracy. CE Database subject headings: Flow; Weirs; Gates; Submerging; Sediment; Hydraulic models.

This paper reports optimal bifurcation shapes (T and Y) in turbulent regime of tree-shaped flows. Unlike earlier studies of T and Y constructs, here the effect of pressure losses at the junction is taken into account, and the wall roughness and duct cross-sectional shapes are free to vary. The optimal ratio of duct cross-sectional areas (as a generalization of Murray's law), the optimal ratio of duct lengths, and the optimal angle between the branches of the Y are presented. These optimal geometrical features are reported as functions of the flow direction (splitting flow versus merging flow), wall roughness, duct cross-sectional shape, and svelteness. The svelteness, Sv, is a global property defined as the external length scale of the flow construct divided by the internal length scale. It is shown that the effect of junction pressure losses on the optimized architecture can be neglected when [Sv.sup.3/2] is greater than approximately [10.sup.4]. Two dimensionless terms are introduced and shown to be useful for the optimization of flow networks. DOI: 10.1061/(ASCE)0733-9429(2009)135:2(132) CE Database subject headings: Cross sections; Bifurcations; Walls; Roughness; Hydraulic networks.

Choosing a suitable set of cross sections for the representation of the natural geometry of a river is important for the efficiency of one-dimensional (1D) hydraulic models, but only few guidelines are available for the selection of the most suitable distance between cross sections, depending on the hydraulic problem at hand. This issue is investigated by examining models of a ~55 km reach of the River Po, Italy, and a ~ 16 km reach of the River Severn, United Kingdom, for both of which high quality laser scanning altimetry are available. The high-resolution digital terrain models of the two river reaches enabled the construction of a series of hypothetical topographical ground surveys with different spacing between cross sections, which could be used as input to a standard 1D model (UNET). Both historical and synthetic flood events for the two river reaches were simulated, and the results were then analyzed to quantify the accuracy associated with each resolution and to assess how survey resolution impacts the performance of standard 1D models. The study results agree with the available suggestions in the literature and provide useful guidelines for 1D hydrodynamic modeling. DOI: 10.1061/(ASCE)0733-9429(2009)135:2(96) CE Database subject headings: One-dimensional flow; Unsteady flow; Numerical models; Cross sections; River systems; Floods.

The direct numerical simulation of turbulent flows in a compound open channel is described. Mean flows and turbulence structures are provided, and are compared with numerical and measured data available in the literature. The simulated results show that twin vortices are generated near the interface of the main channel and the floodplain and that their maximum magnitude is about 5% of the bulk streamwise velocity. Near the interface, the simulated wall shear stress reaches a maximum, contrary to experimental data. A quadrant analysis shows that both sweeps and ejections become the main contributor to the production of Reynolds shear stresses near the interface. Through the conditional quadrant analysis, it is demonstrated how the directional tendency of dominant coherent structures determines the production of Reynolds shear stress and the pattern of twin vortices near the interface. In addition, the time-dependent characteristics of three-dimensional vortical structures in a compound open-channel flow were investigated using direct numerical simulation (DNS) data. DOI: 10.1061/(ASCE)0733-9429(2008)134:12(1744) CE Database subject headings: Simulation; Turbulence; Open channel flows; Secondary flow; Vortices; Data analysis.

An iterative step method for solving the nonlinear ordinary differential equation, governing spatially varied flows with decreasing discharge, like the flow over side weirs, is developed. In the procedure, starting at a known flow depth and discharge in the control section, the analytical integration of the dynamic equation with bed and friction slope is carried out. The specific energy, the weir coefficient and the velocity distribution coefficient are considered as local variables, then for the explicit integration, the respective average values along the short side weir elements are assumed. The water surface profiles and the discharges for flow over side weirs, obtained with the proposed relation and valid for rectangular channels, are compared with experimental data for subcritical and supercritical flow conditions. The validation of the method is accomplished by the comparison with the solution obtained by De Marchi's classical hypothesis, about the specific energy, which is constant along a side weir. In addition, the influence of the coefficient velocity distribution is considered. DOI: 10.1061/(ASCE)0733-9437(2008)134:6(840) CE Database subject headings: Coefficients: Velocity; Weirs: Slopes; Subcritical flow; Supercritical flow.

The critical flow principle is a useful approach for the hydraulic analysis of round-crested weirs due to their single head-discharge relationships. The hydraulics of circular-crested weirs is examined using simplified models incorporating streamline curvature effects, comparing their predictions with experimental data. A generalized one-dimensional model based on the critical flow in curvilinear motion has been developed. The discharge coefficient increases with the specific energy normalized with the radius of curvature, E/R, when streamline curvature effects are included. The relative flow depth at the crest decreases as E/R increases. The flow at the weir crest is only critical for a normalized specific energy value of E/R [approximately equal to] 0.5-0.6. For larger heads, the flow at the weir crest has been found to be supercritical. CE Database subject headings: Open channels; Critical flow; Weirs.

Results of a numerical simulation investigating the complicated flushing process of an isolated trapped volume of salt water from a bar-blocked estuary are presented. A multiphase model, a part of the commercial code FLUENT 6.2, is applied. The governing equations together with initial and boundary conditions and the numerical scheme are described. The time-dependent salt-wedge position, vertical-density distribution, and proportion of total input kinetic energy converted into potential energy are examined for various incoming flow densimetric Froude number and estuary bed slope. The vertical position and thickness of the interfacial mixed layer between freshwater and salt water as well as the local gradient Richardson number are determined from simulated density profiles and velocity fields. The good agreement between the simulated and measured results indicates that the numerical model can be successfully applied to investigate the complex flushing process involving stratified flow. CE Database subject headings: Flushing; Numerical models; Simulation; Salt water; Estuaries; Stratified flow; Boundaries.

Highway drainage channels often approach slopes of 0.5 (50%). Single layer riprap-lined channels at this slope have been constructed for highway drainage and appear to perform satisfactorily, but no quantitative information is available to guide their hydraulic design. A 1/6 scale laboratory hydraulic model, using a single layer of sized crushed limestone, was constructed to determine the relationship between depth of flow and discharge. The flow range investigated took place at depths near or below the top of the riprap, since riprap instability occurred at greater depths. Standing waves and localized hydraulic jumps dominated the surface of the flow, which held streamwise-averaged Froude numbers in a narrow range near the critical condition for those flow depths for which the channel is designed. This resulted in a simple relationship between unit discharge and depth of flow that applies to the model and prototype channels. CE Database subject headings: Hydraulics; Riprap; Drainage; Channels; Slopes; Highways and roads.

The temporal development of plunge pool scour was investigated using a novel experimental approach. Longitudinal profiles along the scour hole were recorded with an optical method to allow its definition at any time, from the initiation of scour to nearly the end-scour condition. The characteristics of the scour hole geometry were investigated, namely the maximum scour hole depth, the maximum ridge height, and their locations relative to the scour hole origin. It is demonstrated that the evolution is logarithmic, similar to that found for bridge pier and abutment scour. A distinction is further made between the developing and the developed scour hole phases. The main issue of the present research was to define the developed scour hole characteristics because the developing scour phase is influenced by turbulence features that may be difficult to assess. This work therefore allows for an appreciation of the temporal evolution of a scour process of engineering interest. CE Database subject headings: Hydraulic structures; Scour; Sediment; Spillways; Water flow; Pools.

On August 29, 2005 Hurricane Katrina resulted in several breaches in the levees and flood walls protecting New Orleans. Of the 20 breaches, the 17th Street Canal breach caused much of the city flooding. In this paper, the results of studies on a 1:50 scale hydraulic model of this breach based on the Froude similitude relationships are presented. It is assumed in the model that the bed is fixed and the levee below the flood wall remains intact during breach closure. This was the case in the 17th Street Canal breach. Because of the many uncertainties in the values of various variables, a range of conditions were run on the model in an attempt to bracket the results for the flooding depths and the initial failed attempts to close the breach. Then, various possible methods for breach closure were investigated utilizing the procedures developed for cofferdam closure for river diversion, e.g., toe dumping, transverse dumping, single- and multi-barrier embankments, etc. Closures of the breach and the closure of the canal at the Old Hammond Highway Bridge were investigated. Results from the case study show that some of these methods could have been utilized for closing the Katrina breaches. However, special care should be exercised when extending them for breach closure at other sites. CE Database subject headings: Scale models; Cofferdams; Open channel flow; Levees.

Turbulent boundary layer modification in a channel flow using injection of microbubbles as a means to achieve drag reduction was studied. The physical mechanism of this phenomenon is not yet fully understood. To obtain some information related to this phenomenon, single-phase (pure water) flow and two-phase (water and microbubbles) channel flow measurements are taken. The void fraction conditions were varied while maintaining a Reynolds number of 5128 based on the half channel height. The study indicates that the presence of microbubbles within the boundary layer modifies the turbulence structure such that variations in time and space turbulent scales are observed, as well as ejection and sweep phenomena. [DOI: 10.1115/1.2969444] Keywords: drag reduction, microbubbles, PIV, channel flow

Due to adverse pressure gradient along the diverging walls of wide-angled diffusers, the attached flow separates from one wall and remains attached permanently to the other wall in a process called stalling. Separated diffuser flows provide a classical case of pressure driven flow separation. Such flows present a very serious challenge to fluid dynamics modelers. This paper provides a data bank contribution for the streamwise mean velocity field and pressure recovery data in wide-angled diffusers. Turbulent mean flow measurements were carried out at Reynolds numbers between 1.07 x [10.sup.5] and 2.14 x [10.sup.5] based on inlet hydraulic diameter and centerline velocity for diffusers whose divergence angles were between 30 deg and 50 deg. The results presented provide a reliable validation data bank for computational fluid dynamics studies for pressure driven flow separation studies.

A procedure is proposed to perform ship hydrodynamics computations for a wide range of velocities in a single run, herein called the computational towing tank. The method is based on solving the fluid flow equations using an inertial earth-fixed reference frame, and ramping up the ship speed slowly such that the time derivatives become negligible and the local solution corresponds to a quasi steady-state. The procedure is used for the computation of resistance and propulsion curves, in both cases allowing for dynamic calculation of the sinkage and trim. Computational tests are performed for the Athena R/V model DTMB 5365, in both bare hull with skeg and fully appended configurations, including two speed ramps and extensive comparison with experimental data. Comparison is also performed against steady-state points, demonstrating that the quasisteady solutions obtained match well the single-velocity computations. A verification study using seven systematically refined grids was performed for one Froude number, and grid convergence for resistance coefficient, sinkage, and trim were analyzed. The verification study concluded that finer grids are needed to reach the asymptotic range, though validation was achieved for resistance coefficient and sinkage but not for trim. Overall results prove that for medium and high Froude numbers the computational towing tank is an efficient and accurate tool to predict curves of resistance and propulsion for ship flows using a single run. The procedure is not possible or highly difficult using a physical towing tank suggesting a potential of using the computational towing tank to aid the design process. Keywords: computational towing tank, resistance, propulsion, ship flow

The energy loss of the vertical drop with subcritical flow in the upstream channel has been studied experimentally with a physical model of 0.20 m drop height. Experimental data were collected in a range where few data exist. Based on a similarity between a turbulent surface jet and the flow over the drop, a model is developed using the theories of the shear layer and fully developed surface jet to estimate the energy loss. The results were compared with previous and current experimental data and were found to agree well with the proposed model. CE Database subject headings: Hydraulic structures; Subcritical flow; Energy; Hydraulic jets; Turbulence; Experimental data.

This note concerns variations of the friction factor in the two transitional regimes, one between laminar and turbulent flows and the other between fully smooth and fully rough turbulent flows. An interpolation approach is developed to derive a single explicit formula for computing the friction factor in all flow regimes. The results obtained for pipe flows give a better representation of Nikuradse's experimental data, in comparison with other implicit formulas available in the literature. Certain modifications are also made for applying the obtained friction formula to open-channel flows. DOI: 10.1061/(ASCE)0733-9429(2008) 134:9(1357) CE Database subject headings: Friction; Pipe flow; Open channel flow; Laminar flow: Turbulent flow; Reynolds number.

The main flow features of' three-dimensional plunge pool scour are explored in this experimental research for steady flow conditions. These include the maximum depth of the scour hole, its streamwise geometry, and the maximum width, the maximum height of the ridge, its shape in plan view, and its profile. Expressions for all these parameters are presented in terms of the basic scour variables, including the approach flow densimetric Froude number, the jet impact angle, the jet diameter, and the tailwater elevation above the originally horizontal sediment bed. This research is based on a previous work relating to two-dimensional plunge pool scour. Differences between the two phenomena are outlined, and the results are discussed in terms of engineering applications. The results of the two works allow for the prediction of the most salient features of plunge pool scour for both the dynamic and the static scour holes. DOI: 10.1061/(ASCE)0733-9429(2008) 134:9(1275) CE Database subject headings: Hydraulics; Hydraulic structures; Scour; Sediment; Spillways; Water flow.

Results are presented from a laboratory investigation of a continuous discharge gravity current moving down an inclined plane into a linearly stratified fluid; the density of the inflow decreasing linearly with time, initially larger and finally smaller than the bottom ambient density. The inflowing water was observed to follow both underflowing and intrusive flow regimes. Hence, during the time in which the inflow was denser than the water in the stratified reservoir, an underflow was observed to descend down the sloping bottom with a speed that was consistent with that given by the theory for a buoyancy-conserving gravity current on gentle slopes. However, the continuous decrease of the density at the source shortly lead to an unstable density distribution within the initial underflow, which then collapsed into an intrusion that traveled as a horizontal gravity intrusion. Scaling arguments were used to identify both the position and time to the breakup of the underflow. To the end of the experiment, multiple intrusions were established successively at different depths in between the initial underflow and the surface buoyant plume. DOI: 10.1061/(ASCE)0733-9429(2008)134:9(1191) CE Database subject headings: Unsteady flow; Slopes; Velocity; Shear waves; Water discharge; Plumes; Buoyancy; Laboratory tests.

This paper is devoted to nonlinear observer and controller design for water level control of open-channel flow in irrigation canals or dam-river systems. A finite-dimensional model, previously developed by orthogonal collocation methods, based on Saint Venant equations and used for control design, is now further used for online flow rate and water infiltration estimation. This is done by a so-called state observer. In particular, the estimates obtained in this way can successfully be used in a controller previously proposed, resulting in a water level control law using only two level measurements along the canal (instead of the four measurements previously needed). The study is restricted to the case of a rectangular wetted section and subcritical flow. The results have been validated by simulations, on an implicit finite difference simulator based on a Preissmann scheme for various scenarios. DOI: 10.1061/(ASCE)0733-9429(2008) 134:9(1267) CE Database subject headings: Open channel flow; Shallow water; Water levels; Control systems; Approximation methods.

A well-tested numerical model based on the Reynolds averaged Navier-Stokes equations with k-[epsilon] turbulence closure is further validated against three groups of experimental data on lock-exchange gravity flows. The model is then applied to study larger-scale turbidity currents with sudden-release and/or sustained inflow mechanisms. At the field-scale, sudden-release turbidity currents are found to be essentially unsteady with depth-averaged flow variables varying with time and distance from the upstream boundary. Turbidity currents with sustained inflows reach a quasi-equilibrium state in the body of the current. The simulation of a turbidity current initiated by sudden-release and then fed by sustained inflow reveals that, initially, two bore heads appear in the current with the second bore head resulting from sustained inflow and eventually catches up with the first one. Model results show that, depending on onset mechanism and slope, if the erosion rate of sediment exceeds the deposition rate, turbidity currents at the field scale can experience self-acceleration, a process predicted earlier by theory but not demonstrated in laboratory experiments. DOI: 10.1061/(ASCE)0733-9429(2008) 134:9(1199) CE Database subject headings: Gravity waves; Turbidity; Slopes; Hydraulic models; Numerical model; Turbulence; Inflow.

Combining single-vessel perfusion technique with confocal microscopy, this study presents a new approach that allows three-dimensional visualization and quantification of endothelial gaps under experimental conditions identical to those used to measure permeability coefficients, endothelial calcium concentration, and nitric oxide production in individually perfused intact microvessels. This approach provides an efficient means for defining the transport pathways and cellular mechanisms of increased microvascular permeability during inflammation. Platelet-activating factor (PAF) was used to increase the permeability of individually perfused rat mesenteric venules. Fluorescent microspheres (FMs, 100 nm) were used as leakage markers, and confocal images were acquired at successive focal planes through the perfused microvessel. Perfusion of FMs under control conditions produced a thin, uniform layer of FMs in the vessel lumen, but in PAF-stimulated microvessels significant amounts of FMs accumulated at endothelial junctions. Reconstructed confocal images three-dimensionally delineated the temporal and spatial development of endothelial gaps in PAF-stimulated microvessels. The FM accumulation, quantified as the total fluorescence intensity per square micrometer of vessel wall, was 8.4 [+ or -] 1.8 times the control value within 10 min of PAF perfusion and declined to 5.0 [+ or -] 0.6 and 1.4 [+ or -] 0.2 times the control value when FMs were applied 30 and 60 min after PAF perfusion. The changes in the magnitude of FM accumulation closely correlated with the time course of PAF-induced increases in hydraulic conductivity ([L.sub.p]), indicating that the opening and closing of endothelial gaps contributed to the transient increase in [L.sub.p], in PAF-stimulated microvessels. Electron microscopic evaluations confirmed PAF-induced gap formation and FM accumulation at endothelial clefts. hydraulic conductivity; microvessel permeability; fluorescent microspheres; confocal microscopy; endothelial gap formation

Experimental studies on the hydraulics of flow through and over gabion-stepped weirs are presented. Two flow components were observed, i.e., base flow through the void between filled stones and overflow on the gabions. The energy loss ratios in the gabion-stepped weirs are greater than those in the corresponding horizontal stepped weirs by approximately 7, 10, and 14% for weir slopes of 30, 45, and 60[degrees], respectively. As a result, the velocity at the outlet is lower. Stone size and shape have little influence on the energy loss and flow velocity as compared to the increasing effect of the weir slope. The pressure acting on the step face for the gabion-stepped weirs is less than that of the horizontal steps owing to the cushioning effect generated by filled stones. To reduce pressure on a step face at a fixed discharge for different weir slopes, the void ratio of the filled stones should be low to allow a small amount of base flow. The pressure distribution pattern on the horizontal face of a step is provided. DOI: 10.1061/(ASCE)0733-9429(2008) 134:8(1147) CE Database subject headings: Weirs; Hydraulic structures; Overflow; Energy; Pressure measurement.

A method is proposed for predicting the backwater effect due to a single, submerged spur dike located within an open channel flow. A theoretical analysis based on the momentum principle relates the backwater effect to the drag force exerted by the spur dike on the flow. Experimental data obtained in laboratory flumes having subcritical flow conditions throughout the flow field have been used in developing predictive relationships for the spur dike drag coefficient, which is found to be strongly correlated to the blockage created by the spur dike within the flow cross section. The predictive relationships provide a means of obtaining a first-level estimate of the backwater effect due to a single, submerged spur dike in an open channel flow. DOI: 10.1061/(ASCE)0733-9429(2008)134:8(1153) CE Database subject headings: Dikes; Groins, structures; Drag; Submerged flow; Open channels; Backwater.

A three-dimensional twisted hydrofoil with an attached cavitaty closely related to propellers was observed with a high-speed camera at the University of Delft Cavitation Tunnel. Reentrant flow coming from the sides of the cavity aimed at the center plane--termed side-entrant flow--collided in the closure region of the cavity, pinching off a part of the sheet resulting in a periodic shedding. The collapse of the remainder of the sheet appears to be a mixing layer at the location of the colliding reentrant flows. Collision of side-entrant jets in the closure region of a cavity is identified as a second shedding mechanism, in addition to reentrant flow impinging the sheet interface at the leading edge. [DOI: 10.1115/1.2928345]

A series of detailed laboratory measurements were made under steady-state flow conditions through a 0.914-m (3-ft) Cutthroat flume in an attempt to more accurately define transition submergence for four standard throat widths. It was found that the change from free to submerged flow, and vice versa, is gradual and that there is no easily observable transition point. The gradual transition between the flow regimes suggested a new calibration approach in which a single equation could more elegantly and more accurately fit the laboratory measurements, eschewing the need for separate tree- and submerged-flow equations, and obviating the need to define transition submergence. Such an equation was found, providing greater calibration accuracy up to 95% submergence in 0.914-In Cutthroat flumes. DOI: 10.1061/(ASCE)0733-9437(2008)134:4(521) CE Database subject headings: Flumes; Calibration; Measurement; Submerged flow; Steady flow.

The specific energy equation has many applications in rectangular open-channel flow problems. The existing methods of solving this equation are as follows: trial-and-error solution, graphical solution, and design tables prepared from the specific energy equation expressed in dimensionless form. No direct solutions are available in the technical literature for this equation to date because it was presumed that finding roots of this equation requires a series of substitutions. In this technical note, the writer develops an analytical solution for transitions located in rectangular open channels: (1) to avoid the inconvenience in available solutions; (2) to derive a less time consuming method; and (3) to achieve more accurate results than those as a result of using existing methods. DOI: 10.1061/(ASCE)0733-9437(2008)134:4(533) CE Database subject headings: Channel design; Contraction; Control structures; Critical flow; Hydraulic engineering; Open channel.

This paper deals with the underlying mechanism of flow resistance in an alluvial channel: The effects of sidewall and bed form on flow resistance, Einstein's divided hydraulic radius approach and Engelund's energy slope division approach are reexamined. These two approaches assume that the shear stress on a mobile bed is the summation of shear stresses caused by skin friction and bed form. Using a different approach, this paper presents a theoretical relationship between the total bed shear stress with grain and bed-form shear stresses. The contribution of sidewall on the total bed shear stress is also discussed. The writers found that the size of bed form plays a significant role for the flow resistance, and developed relevant expressions for the length of the separation zone behind the bed forms. In addition, a systematical approach has been developed to compute the flow velocity in an alluvial channel. This approach is tested and verified against 5,989 flume and field measurements. The computed and measured discharge/velocity are in good agreement and 83.0% of all data sets fall within the [+ or -] 20% error band. CE Database subject headings: Boundary shear; Velocity; Open channel flow; Turbulent diffusion; Flow resistance.

Unsatisfactory features of the equation that is recommended in a standard reference on bridge scour (commonly known as HEC-18) for predicting pressure-flow scour are pointed out. These are found to stem from the basic model of the original regression analysis, which is argued to have suffered from a spurious correlation. An alternative best-fit and a separate design equation are proposed, together with a definition of the region where such equations may be expected to be applicable. CE Database subject headings: Scour; Bridges; Regression analysis.

The free surface flow of two conceptual fish passage designs are investigated numerically by a level-set finite-element method. One design has a right-angled entrance from the reservoir to the fish passage chute, and the other has a curved-shaped entrance. The numerical results are validated with hydraulic experiments through comparison of the free surface location and the pressure distribution in the spillway. It is found that the right-angled design yields a curved free surface and pressure distribution in the vicinity of the entrance due to large strains, whereas the curved-shaped design yields a smooth flow transition with small strains. The negative pressure distributions near the ogee crest for both designs exhibit similar characteristics. Further downstream the free surface is elevated near the wall and is associated with counterrotating vortices. It is concluded that the curved-shaped design is favorable for fish passage because of the feature of a smooth flow transition with small strains. CE Database subject headings: Finite element method; Free surfaces; Spillways; Fish management.

Maintenance or repair of water mains may require the interruption of flow. Reinitiation of flow may present opportunities for the interaction of air and the accelerating water, resulting in undesirable pressure transients. This study focuses on air intrusion during flow startup in an initially filled horizontal pipeline supplied by a constant-head reservoir. Flow was initiated by opening the downstream end, allowing a free discharge at that point. During the initial phases of flow startup, an air cavity intruded in the upper portion of the pipe cross section while water exited the pipeline through the lower portion. The initial reservoir head controlled the rate of water velocity increase and, thus, the air intrusion advance. Air intrusion occurred until the water velocity was sufficient to arrest the intrusion. Higher reservoir heads resulted in the subsequent expulsion of the intrusion from the pipeline. However, for lower reservoir heads, the intrusion was not expelled, but readvanced to a final position that was controlled by shear forces. Criteria are developed for the flow conditions necessary to arrest the air intrusion in a horizontal pipeline as well as the intrusion lengths to be expected. CE Database subject headings: Transient flow; Water Pipelines; Open channel flow; Pressurized flow; Intrusion.

To improve knowledge of the flow and scour processes associated with spur dikes, three-dimensional flow velocities were measured at 2,592 points using an acoustic Doppler velocimeter over a fixed flat bed with a trapezoidal shaped submerged spur dike in a laboratory flume. General velocity distribution and detailed near field flow structures were revealed by the measurements and numerical simulations performed using a free surface turbulent flow model with a k-[epsilon] closure scheme. The three- dimensional flow separation characterized in this study was found to yield forces on the bed that were significantly different from nonsubmerged vertical obstructions that have been measured in other studies. Values of bed shear stress derived from both measured and simulated values were similar but indicated that local scour would be initiated in one rather than in the two locations of initial local scour measured in previous experiments with a similar flow. CE Database subject headings: Three-dimensional flow; Turbulent flow; Flow patterns; Three-dimensional models; Dikes; Flow measurement; Simulation.

Predicting the runback threshold for liquid drops in aerodynamic boundary layers is a challenging problem with numerous applications including aircraft icing simulations. The critical parameters that govern drop runback are investigated in this experiment by using a wind tunnel that provides a turbulent accelerated flow similar to flows near an unswept wing's leading edge. The experiments feature water drops on aluminum with a contact angle of 70 [+ or -] 5 deg. Results show that significant water/air interface unsteadiness precedes drop runback. This is likely due to air-flow separation in the drop wakes. For displacement-thickness-scaled Reynolds numbers ranging from 348 to 429, a constant-Weber-number runback threshold We =3.45 [+ or -] 0.09 is found to adequately correlate the runback results. [DOI: 10.1115l/1.2917429] Keywords: drop, runback, boundary layer

The relation between the equivalent roughness and different grain size percentiles of the sediment in gravel-bed rivers was determined under the hypothesis that the vertical distribution of the flow velocity follows a logarithmic law. A set of 954 data points was selected from rivers with gravel size sediment or larger, with a nonsinuous alignment and free of vegetation or obstacles. According to the results, the [k.sub.s] roughness is equivalent to approximately 2.4[D.sub.90], 2.8[D.sub.84], and 6.1[D.sub.50]. No correlation was detected between the sediment sorting and the sediment mobility index on one hand and, on the other, the coefficient of proportionality of each grain size percentile. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(847) CE Database subject headings: Bed roughness; Flow resistance; Gravel; Mountain streams; Fluvial hydraulics.

Ambient flow field and circulation patterns induced by circular bubble plumes and water jets in tanks of different sizes were studied in rectangular and square water tanks. A nonstationary nature of the flow was observed in all experiments and its dominant oscillation frequency was found to directly relate to the tank size. The flow circulation patterns were similar for bubble plumes and water jets, but changed significantly with tank size and geometry. Strong three-dimensional effects were observed in a rectangular tank, resulting in flow entraining in the longer plane and flow detraining in the shorter plane, especially for the bubble plume tests. A relationship was developed to relate the tank size to the patterns of circulation cells. Nearly isotropic turbulent flow conditions were obtained in all experiments, but the effect of tank size and geometry on the magnitude of the turbulent stresses was more pronounced in the bubble plume tests. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(833) CE Database subject headings: Bubbles; Water circulation; Water tanks; Water jets; Mixing; Plumes; Geometry; Turbulent flow.

The hydraulic performance of pipelines can be significantly affected by the presence of biological growth on internal surfaces. The change in wall roughness brought about by the biofilms has been studied by the use of headloss tests, precleaning and postcleaning of the pipelines in three Tasmanian hydroelectric schemes. Results of the headloss testing show that improvements to hydraulic efficiency can be achieved from the cleaning of biofouling material. The data, when plotted as a Moody diagram, shows that the friction law for conduits roughened by biological growths may not always follow a Colebrook-White type relationship, although the results are too narrow in Reynolds number to be conclusive. It was found that bacteria made up the majority of the biofilm biomass in the pipelines studied. Based on molecular analysis, members of the class Alphaproteobacteria were the most frequently detected followed by members of the phylum Chloroflexi. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(852) CE Database subject headings: Hydraulic Roughness; Biofilm; Pipe flow; Water pipelines.

An understanding of bridge scour mechanisms during floods in a fluvial river is very important for cost-effective bridge foundation design. Reliable bridge scour data for flood events are limited. In this study, field experiments were performed at the Si-Lo Bridge in the lower Cho-Shui River, the longest river in Taiwan, to collect scour-depth data using a sliding magnetic collar, a steel rod, and a numbered-brick column. By separating each scour component, a methodology for simulating the temporal variations of the total scour depth under unsteady flow conditions is proposed. The proposed total-scour model integrates three scour components, namely general scour, contraction scour, and local scour. The collected field data, comprising both general scour and total scour depths, are used to validate the applicability of the proposed model. Based on the peak flow discharges during floods, a comparison of the local scour depths calculated using several commonly used equilibrium local scour formulas indicates that most equations may overestimate the local scour depth. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(810) CE Database subject headings: Bridges, piers; Scour; Floods; Measurements; Erosion; Deposition; Unsteady flow.

A robust computational model for simulating the long-term evolution of incised and restored or rehabilitated stream corridors is presented. The physically based model simulates the three main processes that shape incised streams: hydraulics, sediment transport, and streambed and bank adjustments. A generalized implicit Preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals. The solution method introduces several enhancements that increase its robustness, specifically to simulate flashy flows. Transport of cohesive or cohesionless graded bed material is based on a total-load concept, and suspended and bed load transport modes are accounted for through nonequilibrium effects. The model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material. The present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds, and reports simulations of published experiments on degrading and aggrading channels with graded bed material. Description and validation of the model's streambank erosion component and the application of the model to incised stream systems are presented elsewhere. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(749) CE Database subject headings: Computer models; Channel morphology; Open channel flow; Bank erosion; Sediment transport; Streams.

A numerical model is proposed to compute one-dimensional open channel flows in natural streams involving steep, nonrectangular, and nonprismatic channels and including subcritical, supercritical, and transcritical flows. The Saint-Venant equations, written in a conservative form, are solved by employing a predictor-corrector finite volume method. A recently proposed reformulation of the source terms related to the channel topography allows the mass and momentum fluxes to be precisely balanced. Conceptually and algorithmically simple, the present model requires neither the solution of the Riemann problem at each cell interface nor any special additional correction to capture discontinuities in the solution such as artificial viscosity or shock-capturing techniques. The resulting scheme has been extensively tested under steady and unsteady flow conditions by reproducing various open channel geometries, both ideal and real, with nonuniform grids and without any interpolation of topographic survey data. The proposed model provides a versatile, stable, and robust tool for simulating transcritical sections and conserving mass. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(736) CE Database subject headings: Open channels; Unsteady flow; Numerical models; Geometry; Channel flow; Wave propagation; Two-dimensional models.

Godunov-based shallow-water models utilize a discontinuous reconstruction of data at cell faces even for smooth flow, which can cause energy dissipation and degrade accuracy. Analysis of discrete equations shows that jumps (and therefore error) can be minimized by adaptively selecting either primitive or conservative variables for slope limiting and reconstruction according to the local Froude number. Therefore, a Godunov-based model with an adaptive scheme of slope limiting and variable reconstruction is presented. Two practical flood modeling applications are used to compare the performance of the adaptive scheme against two nonadaptive schemes. In addition, performance of second-order accurate schemes is compared to first-order schemes that utilize a second-order accurate description of terrain. Results show that the first-order adaptive scheme possesses the best combination of robustness, efficiency, and accuracy of the models tested. DOI: 10.1061/(ASCE)0733-9429(2008) 134:6(714) CE Database subject headings: Floods; Shallow water; Algorithms; Hydraulic models.

This study presents a novel, simple, but rather accurate approximation of the eigenvalues of the system formed by the Saint-Venant--Exner equations, based on the comparison between eigenvalues for the complete system and eigenvalues for the water phase only. Moreover, a strategy is proposed to compute efficiently the intercell fluxes by properly adapting a Harten. Lax, and van Leer scheme for each equation. Two examples of transient transcritical flows are developed: the erosive migration of a knickpoint induced by an increase in the bed slope, and the evolution of a hydraulic jump over a mobile bed. DOI: 10.1061/(ASCE)0733-9429(2008)134:6(726) CE Database subject headings: Shallow water; Sediment transport; Transcritical flow; Bed load; Eigenvalues; Hydraulic jump.

This paper describes the flow structures that occur in a 90[degrees] junction of four open channels with supercritical flow in two orthogonal inlet channels. An experimental facility was constructed to permit the measurement of flow rates, water depths, and the positions of hydraulic jumps in the channels. The various flow patterns which appear depend on the characteristics of the incoming flows and can be classified into three main types, depending on the location and shape of the hydraulic jumps that develop. These jumps can either be normal to the flow and located in the upstream channels, or can be oblique and confined within the junction. The explanation for the existence of various flow patterns is derived from previous studies of the rapid deflection of supercritical flow. A detailed description of each flow regime is provided, with information on the surface elevations, the behavior of the hydraulic jumps, the deflections of the incoming flows, and the formation and characteristics of recirculation zones. DOI: 10.1061/(ASCE)0733-9429(2008) 134:6(701) CE Database subject headings: Flow patterns; Supercritical flow; Channels; Hydraulic jump; Flow rates; Deflection.

Thermohydraulic phenomena of a steam-water natural-circulation (SWNC) system are very complicated, particularly, during its start-up and shutdown. Its performance strongly depends on the circulation inside it. Accurate quantification of the flow, void fraction, two-phase level, boiling boundary, etc., is difficult at both steady state and transient states like load variation, start-up, and shutdown. Attempts have been made to develop a high-fidelity thermohydraulic model (five-equation scheme) that caters to non-homogeneous and thermal nonequilibrium flow to derive the dynamic effect of heating rate on the performance of the SWNC loop of steam generator of an Indian nuclear reactor during steaming-up period. The proposed work also attempts to predict boiling height, flow reversal, and density-wave oscillation (DWO). The boiling channel of the SWNC loop is modeled based on the moving boundary analysis using finite volume method. In this moving boundary problem, both control volumes of single-phase zone and two-phase zone change with time. Numerical results have been presented in this paper. The results indicate that both circulation flow variation and two-phase level variation in steam drum have strong dependency on void fraction in the boiling channel. Flow-reversal phenomenon is identified during the initial stage of boiling. Two-phase swelling and collapse that occur during the start-up are predicted. Above a critical heating rate, DWO has been observed. All these phenomena have been explained. Keywords: two-phase natural circulation, moving boundary analysis, finite volume method, start-up simulation

Abstract: Well injection replenishes depleting water levels in a well field. Observation well water levels some distance away from the injection well are the indicators of the success of a well injection program. Simulation of the observation well response, located a few tens of meters from the injection well, is likely to be affected by the effects of nonhomogeneous medium, inclined initial water table, and aquifer clogging. Existing algorithms, such as the U.S. Geological Survey groundwater flow software MODFLOW, are capable of handling the first two conditions, whereas time-dependent clogging effects are yet to be introduced in the groundwater flow models. Elsewhere, aquifer clogging is extensively researched in theory of filtration; scope for its application in a well field is a potential research problem. In the present paper, coupling of one such filtration theory to MODFLOW is introduced. Simulation of clogging effects during 'Hansol' well recharge in the parts of western India is found to be encouraging. DOI: 10.1061/(ASCE)0733-9437(2008)134:3(400) CE Database subject headings: Ground-water flow: Heterogeneity; Clogging; Numerical models; Simulation.

Abstract: A flooding probability based cost effective design of open channel section has been proposed using freeboard as an additional design variable. The freeboard of the channel is calculated based on the flooding probability value. The proposed model is solved using classical optimization techniques as well as a nondominated sorting genetic algorithm. The results of the model are compared with an earlier reported model to demonstrate its superiority and field applicability. DOI: 10.1061/(ASCE)0733-9437(2008)134:3(405) CE Database subject headings: Channel design; Open-channel flow; Optimization; Uniform flow; Cross sections.

Abstract: A power-law channel section is very versatile. It can model a wide range of familiar man-made or natural channel shapes. However estimating the wetted perimeter of a power-law channel section is difficult. The problem gets complicated further when considering the freeboard in the design process. In this paper, the wetted perimeter is estimated using the isoperimetric theorem which results in a simple and accurate expression for the wetted perimeter that does not lead to discontinuity in the optimal solution. For unconstrained optimum power-law channels, it is shown that the exponent and side slope at the water surface have values of 0.314 and 0.352, respectively, independent of either the maximum side slope or the relative freeboard. The analyses have also shown that the most hydraulically efficient power-law channel sections come closest to the semicircle. They tend to be U shaped and narrow with small relative freeboard ([less than or equal to] 0.30). A design procedure and two design charts are presented together with two illustrative examples to demonstrate the simplicity of the method. DOI: 10.1061/(ASCE)0733-9437(2008)134:3(380) CE Database subject headings: Channel design; Open-channel flow; Optimization; Uniform flow; Cross sections.

Abstract: The article proposes a mathematical model of an automatic upstream water level control gate designed to operate under free and submerged flow conditions, called the Vlugter gate. This automatic gate controls the upstream level close to a design level, using a counterweight to compensate for the hydraulic pressure on the gate. The proposed gate model is an extension of an existing model for free flow conditions (Begemann gate) to submerged conditions. Submergence effects are introduced both in the discharge law and the equilibrium law. The mathematical model is validated on experimental data from two small-scale gates and on data from literature, in order to show the ability of the model to simulate the behavior of gates of various dimensions. DOI: 10.1061/(ASCE)0733-9437(2008)134:3(315) CE Database subject headings: Submerged flow; Gates; Hydraulic models; Mathematical models; Analytical techniques; Laboratory test.