Journal of Hydraulic Engineering

The validity of assumed frozen-viscosity conditions underpinning an important class of theoretical models of unsteady wall shear stress in transient flows in pipes and channels is assessed using detailed computational fluid dynamics (CFD) simulations. The need for approximate one-dimensional $(1\mathrm{D})\{x,t\}$ models of the wall stress is unavoidable in analyses of transient flows in extensive pipe networks because it would be economically impracticable to use higher order methods of analysis. However, the bases of the various models have never been established rigorously. It is shown herein that a commonly used approach developed by the first authors is flawed in the case of smooth-wall flows although it is more plausible for rough-wall flows. The assessment process is undertaken for a particular, but important, unsteady flow case, namely, a uniform acceleration from an initially steady turbulent flow. First, detailed predictions from a validated CFD method are used to derive baseline solutions with which predictions based on approximate models can be compared. Then, alternative solutions are obtained using various prescribed frozen-viscosity distributions. Differences between these solutions and the baseline solutions are used to determine which frozen-viscosity distributions are the most promising starting points for developing $1\mathrm{D}\{x,t\}$ models of unsteady components of wall shear stress. It is shown that no frozen-viscosity distribution performs well for large times after the commencement of an acceleration. However, even the simplest approximation (laminar) performs well for short durations—which is when the greatest amplitudes of the unsteady components occur.

The use of fish baffles in HDPE slipliners is growing in popularity to improve hydraulic conditions for fish passage, yet little is known on how baffles affect the outlet-controlled discharge capacity of these types of culverts. To fill this gap in knowledge, roughness coefficients (Manning’s $n$ and friction factor $f$ values) were experimentally determined for weir baffle, slotted weir baffle, and spoiler baffle configurations at four relative spacings (${\lambda }^{+}=0.6$, 1.2, 1.8, 2.4) and three values of relative roughness height ($h^{+}=0.15$, 0.10, 0.05). Relative roughness height ($h^{+}=h/D$ where $D$ is the pipe diameter) was found to be the determinant geometric parameter affecting energy losses. Relative spacing (${\lambda }^{+}=\lambda /D$) was found to play an important secondary role. An analytical model was developed and analyzed to determine the effects of the following: roughness reduction ($\alpha =n/n_o$) between the Manning’s coefficients of the host and baffle equipped slipline culvert; diameter reduction ($\beta =D/D_o$) between the diameters of the host and slipline culverts; relative length ($L^{+}=L/D$); and inlet treatments ($k_e$) on the hydraulic capacity of corrugated steel culverts after being sliplined with baffled high-density polyethylene (HDPE) culverts. Results demonstrate that many HDPE slipliner culverts can house baffles with $\alpha$ values in the range of 0.5 to 0.9. Design recommendations for the use of baffles in slipline culverts are discussed.

A three-dimensional (3D) numerical model has been developed to simulate dam-break flows with sediment transport over movable beds. The hydrodynamic model solves the 3D Reynolds-averaged Navier-Stokes equations using a finite-volume method on collocated hexahedral meshes. The volume-of-fluid (VOF) technique with the compressive interface capturing scheme for arbitrary meshes is used to track the water surface boundary. The sediment transport model solves the nonequilibrium transport equations of suspended load and bed load separately, and in turn calculates the resulting bed change. A moving mesh technique is adopted to track the time evolution of bed topography. The grid moving velocity and computational cell volume change due to the moving mesh are taken into consideration when the hydrodynamic and sediment transport equations are discretized. The developed model has been tested using several experimental dam-break flows over movable beds. The calculated temporal and spatial variations of water and bed surfaces are in generally good agreement with the measured data. The effects of 3D features of the flow on morphological changes are discussed by comparing the results of the present model with results calculated by one-dimensional (1D) and two-dimensional (2D) models documented in the literature. The comparisons reveal that the present 3D model improves the accuracy of calculated morphological changes at the initial stages of dam-break flow, near the wave front, and around in-stream structures.

Sharp crested notches, particularly rectangular and V-shaped ones, have been used to measure the flow rate of water in open channels for a long time. However, they have not been used to measure the flow rate of non-Newtonian fluids. The aim of this work is to explore the possibility of extending their range of application to non-Newtonian fluids, especially for rectangular and V-shaped notches. An experimental facility in the flow process research laboratory was used to perform the extensive tests reported herein. The notches and an in-line tube viscometer were calibrated using water. The in-line tube viscometer was used to determine the fluid rheology. Three different types of non-Newtonian fluids [namely, aqueous solution of carboxymethyl cellulose (CMC) solutions and water-based suspensions of kaolin and bentonite] were used as model test fluids. The experimental values of the coefficient of discharge ($C_d$) were plotted against three different definitions of the Reynolds number denoting different underlying physical ideas. In the laminar conditions, the discharge coefficient exhibited the classical dependence on the Reynolds number with slopes of approximately 0.43–0.44. On the other hand, the discharge coefficient was nearly constant in the turbulent flow regime. Single composite power-law function was used to correlate the present $C_d-\mathsf{R}$ for each of the two notch shapes used here. Using these correlations, the $C_d$ values could be predicted to within $\pm 5\%$ for the rectangular and the V notches.

Two fundamental one-dimensional (1D) models are proposed and applied to simulate the transient flows with the propagation of an interface in a water-filled duct. The proposed models are developed to simulate the unsteady open channel flows based on finite-volume method (FVM). The models presented herein are based on the continuity and momentum equations of free surface and pressurized flows and the momentum equation of an interface between both flows. However, the highly simplified marker and cell (HSMAC) method with pressure iteration procedures is applied to the pressurized flow region. The numerical simulations are performed under the hydraulic conditions of previous experiments, and then simulated results were compared with the experimental data. It is pointed out that the solitary wave solution is able to reproduce the air cavity profile. In contrast to the hydrostatic model, results of the Boussinesq model compare reasonably well to the experimental observations.

Laboratory experiments are a convenient way to study morphodynamic processes associated with subaqueous fans. However, the flows driving such systems can be difficult to hydraulically characterize due to the shallowness of the flows, the spatial and temporal dynamics of the autogenic channels, and the mixing at the interface of the ambient fluid and density current. The objective of this study is to develop a set of methods to quantify hydraulic variables in laterally unconstrained and morphologically active density currents. Herein, a methodology is presented capable of characterizing a density current flow field with direct application to submarine fan experiments. This is done with a combination of: (1) particle tracking velocimetry (PTV) analysis of tracers within the density current, (2) flow width measurements from overhead images, and (3) solution of the mass and volume conservation equations in one-dimension to obtain layer-averaged velocity, depth, and fractional excess density.

Seepage flow under hydraulic structures provided with intermediate filters has been investigated. The flow through the banks of the canal has been included in the model. Different combinations of intermediate filter and canal width were studied. Different lengths of the floor, differential heads, and depths of the sheet pile driven beneath the floor were also investigated. The introduction of an intermediate filter to the floor of hydraulic structures reduced the uplift force acting on the downstream floor by up to 72%. The maximum uplift reduction occurred when the ratio of the distance of filter location downstream from the cutoff to the differential head was 1. Introducing a second filter in the downstream side resulted in a further reduction in the exit hydraulic gradient and in the uplift force, which reached 90%. The optimum locations of the two filters occurred when the first filter was placed just downstream of the cutoff wall and the second filter was placed nearly at the middistance between the cutoff and the end toe of the floor. The results showed significant differences between the three-dimensional (3D) and the two-dimensional (2D) analyses.

Plunging flow dropshafts are commonly used in municipal drainage systems. In this Note, an exploratory study is reported on the physics of the flow and air demand in two dropshafts of 7.7 and 6.3 m drop height. The flow inside these dropshafts was controlled by the upstream conditions with no outlet control. The water flow distribution and flow velocities in the vertically falling section are presented, and the energy dissipation in plunge flow drop structures is analyzed. The air demand was measured, and an empirical relation is proposed to predict the air demand in these tall dropshafts.

In this study the author derived the thermal theory for non-Boussinesq gravity currents produced from an instantaneous buoyancy source propagating on an inclined boundary. For Boussinesq gravity currents on a slope, it is known that the gravity current front location history follows an asymptotic relationship, $x_f^{3/2}\sim t$, where $x_f$ is the front location, and $t$ is the time, when the gravity current is sufficiently far into the deceleration phase. For non-Boussinesq gravity currents, the distance for the acceleration phase is extended attributable to the non-Boussinesq effects. When the gravity current is sufficiently far into the deceleration phase, this paper shows that the non-Boussinesq gravity currents tend to approach similar asymptote in the deceleration phase as Boussinesq gravity currents, but the approach is less rapid in non-Boussinesq gravity currents.