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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.

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.

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.

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.

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.

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

This paper analyzes the structure of turbulence and secondary, flows at the exit of an axial flow fan with variable pitch blades. The influence of changing the blades' pitch angle over the turbulent structures is assessed by means of turbulence intensity values and integral length scales, obtained by using hot-wire anemometry for several test conditions. Since total unsteadiness is composed of both periodic and random unsteadiness, it is necessary to filter deterministic unsteadiness from the raw velocity traces in order to obtain turbulence data. Consequently, coherent flow structures were decoupled and thus, levels of turbulence--rms values of random fluctuations--were determined using a filtering procedure that removes all the contributions stemming from the rotational frequency, the blade passing frequency, and its harmonics. The results, shown in terms of phase-averaged distributions in the relative frame of reference, revealed valuable information about the transport of the turbulent structures in the unsteady, deterministic flow patterns. The anisotropic turbulence generated at the shear layers of the blade wakes was identified as a major mechanism of turbulence generation, and significant links between the blade pitch angle and the wake turbulent intensity were established. In addition, the autocorrelation analysis of random fluctuations was also used to estimate integral length scales--larger eddy sizes--of turbulence, providing useful data for computational fluid dynamics applications based on large eddy simulation algorithms. Finally, contours of radial vorticity and helicity gave a detailed picture of the vortical characteristics of the flow patterns, and the definition of secondary flow as the deviation of the streamwise component from the inviscid kinematics was introduced to determine the efficiency of the blade design in the energy exchange of the rotor. [DOI: 10.1115/1.2903523]

The results of an experimental investigation on the flow field in submerged jumps on horizontal rough beds, detected by an acoustic Doppler velocimeter, are presented. Experiments were conducted for the conditions of submerged jumps, having submergence factors from 0.96 to 1.85 and jet Froude numbers from 2.58 to 4.87, over rough beds of Nikuradse's equivalent sand roughness equaling 0.49, 0.8, 1.86, and 3 mm. The vertical distributions of time-averaged velocity components, turbulence intensity components, and Reynolds stress at different streamwise distances from the sluice opening and the horizontal distribution of bed-shear stress are plotted. Vector plots of the flow field show that the rate of decay of jet velocity in a submerged jump increases with increase in bed roughness. The flow characteristics on rough beds, being different from those on smooth bed, are discussed from the point of view of similarity, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The most important observation is that the flow in the fully developed zone is found to be self-preserving. CE Database subject headings: Boundary layers; Hydraulic jumps; Open channels; Steady flow; Submerged jets; Turbulent flow; Channel beds.

In transitional rough pipes, the present work deals with alternate four new scales, the inner wall transitional roughness variable [xi]=[Z.sub.+]/ [phi], associated with a particular roughness level, defined by roughness scale [phi] connected with roughness function [DELTA][U.sub.+], the roughness friction Reynolds number [R.sub.[phi]] (based on roughness friction velocity), and roughness Reynolds number [Re.sub.[phi]] (based on roughness average velocity) where the mean turbulent flow, little above the roughness sublayer, does not depend on pipes transitional roughness. In these alternate variables, a two layer mean momentum theory is analyzed by the method of matched asymptotic expansions for large Reynolds numbers. The matching of the velocity profile and friction factor by Izakson-Millikan-Kolmogorov hypothesis gives universal log laws that are explicitly independent of pipe roughness. The data of the velocity profile and friction factor on transitional rough pipes provide strong support to universal log laws, having the same constants as for smooth walls. There is no universality of scalings in traditional variables and different expressions are needed for various types of roughness, as suggested, for example, with inflectional-type roughness, monotonic Colebrook-Moody roughness, etc. In traditional variables, the roughness scale, velocity profile, and friction factor prediction for inflectional pipes roughness are supported very well by experimental data. [DOI: 10.1115/1.2375129]

An experiment was conducted to examine a round hypersaline fountain. When the discharge was turbulent, and for large densimetric Froude numbers, the results have indicated that the maximum height of the fountain [Z.sub.m]/[L.sub.m], in a quiescent and homogeneous surrounding, was a function of relative density difference. The height [Z.sub.m]/[L.sub.m] decreased from 3.06 at [DELTA][rho]/[[rho].sub.a]=0.001 to 2.59 at [DELTA][rho]/[[rho[.sub.a] =0.1. The observed density effects on the height of the fountain have indicated that the Boussinesq's approximation was valid when the relative density difference was typically less than 0.003. DOI: 10.1061/(ASCE)0733-9429(2009) 135:1 (57) CE Database subject headings: Density currents; Boussinesq equation; Desalination; Stratified flow.