Search

Flow visualization techniques and velocity measurements of jets in crossflow prove that the jet follows a distinct trajectory as if immiscible with the cross stream. Such observations reveal similarities between open channels receiving lateral inflow and vegetated flows. In the latter, the vegetation effect is accounted for as an increase in resistance, represented by a drag force. Using a similar approach, a new equation accounting for drag is developed for spatially varied flow (SVF) with increasing discharge. Experimental water surface profiles (WSP) from a number of studies with different arrangements of SVF were used to validate the performance of the proposed equation. A sensitivity analysis revealed that for the data considered the influence of nonuniform velocity distributions was minimal thus momentum correction factor, β was assumed equal to unity. The Blasius equation was used to estimate friction slope. WSPs estimated from the proposed equation were a closer match to measurements than the general equation for different channel bottom slopes and lateral inflow rates. The analogous results imply that the proposed equation is generic for any conditions of SVF with increasing discharge. [ABSTRACT FROM AUTHOR]

A one-dimensional (1D) system and three-dimensional (3D) component cosimulation method based on the method of characteristic (MOC) and computational fluid dynamics (CFD) was proposed to study the interaction between valve-induced water hammer and pump during the rapid closing of the valve. In a long pipeline system, the pump was treated as a 3D CFD model using Fluent code, whereas the remaining system models (e.g., pipe, valve) were represented by 1D components using MOC. The numerical coupling and integration of both models were realized by a coupled code written using Visual Basic. The coupling theory and procedure were described in detail. MOC-CFD steady coupling in the case of the fully open state of the valve was first conducted. The corresponding results showed that the steady cosimulation can accurately determine the operating conditions. A transient cosimulation for the rapid closing procedure of the valve was conducted to study the interaction. The relative dynamic behavior obtained by MOC-CFD cosimulation was compared with that using MOC calculation alone. Transient simulation demonstrated that MOC-CFD coupling analysis was closer to real conditions because of considering the effect of fluid inertia. The effect was further explained through a comparison of internal and external characteristics between the transient and quasi-steady assumptions. [ABSTRACT FROM AUTHOR]

As a gravel bed river armors, a bed surface often develops clusters as part of its structure. The influence of sand on armoring and the impact of clusters on the surrounding flow processes were investigated through flume experiments. Armored beds were created from four different sediments, which progressively increased from 1-38% sand in the bed sediment, extending from gravel framework to sand matrix beds. Turbulent kinetic energy (TKE) and -direction Reynolds stress calculated from velocity profiles showed the clusters increased turbulence and induced formation of recirculation cells. Similar recirculation cells formed downstream of every cluster, but the strength and orientation of these cells varied with bed condition. The increase in sediment sand content and change in bed condition from gravel framework to sand matrix correlated to an increase in the energy and momentum in the recirculation cell but a reduction in cluster influence across the flow profile. Results show that the sand content of the bed sediment during armoring was a primary factor affecting the influence of a cluster on the local flow field and the overall bed surface stability. [ABSTRACT FROM AUTHOR]

The authors previously described a new method [based on the new discrete vapor cavity model (new DVCM)] for numerical prediction of pressure changes during the water hammer with liquid column separation together with results of preliminary experimental verification of this method. This paper is a continuation of the research and includes results of additional laboratory tests and visualization of the cavitation zones generated during transient flow with liquid column separation. The results of these studies provide a better understanding of the phenomenon. It is shown that the phenomenon can have a distributed nature, which means that gas-vapor zones may be observed not only locally, in the vicinity of the shutoff valve, but may be spread along the pipeline length, and the intensity of this phenomenon decreases with distance from the valve. Laboratory test results were also used for further verification of the new DVCM. This verification shows that agreement between calculated and experimental results strongly depends on the friction model incorporated into the calculation. This agreement also depends on the intensity of liquid column separation: for cases of severe separation, the differences between numerical and measured pressure changes are small and accepted from the practical point of view. [ABSTRACT FROM AUTHOR]

A three-dimensional computational fluid dynamics model is applied to predict local scour around an abutment in a rectangular laboratory flume. When modeling local scour, steep bed slopes up to the angle of repose occur. To predict the depth and the shape of the local scour correctly, the reduction of the critical shear stress due to the sloping bed must be taken into account. The focus of this study is to investigate different formulas for the threshold of noncohesive sediment motion on sloping beds. Some formulas only take the transversal angle (perpendicular to the flow direction) into account, but others also consider the longitudinal angle (streamwise direction). The numerical model solves the transient Reynolds-averaged Navier-Stokes equations in all three dimensions to compute the water flow. Sediment continuity in combination with an empirical formula is used to capture the bed load transport and the resulting bed changes. When the sloping bed exceeds the angle of repose, the bed slope is corrected with a sand-slide algorithm. The results from the numerical simulations are compared with data from physical experiments. The reduction of the bed shear stress on the sloping bed improves the results of the numerical simulation distinctly. The best results are obtained with the formulas that use both the transversal and the longitudinal angle for the reduction of the critical bed shear stress. [ABSTRACT FROM AUTHOR]

A surge running down a dry wadi bed as a consequence of a controlled water release from a reservoir—e.g., for artificial groundwater recharge—represents a free boundary problem. After some time, when aiming for groundwater recharge, the infiltration equals inflow and thus forms a kind of “standing” wave. The numerical solution of such phenomena generally involves considerable problems. For avoiding the numerical inconvenience resulting from the complex interacting surface/subsurface flow, we present an analytical solution of the slightly modified zero-inertia (ZI) equations. The development introduces a momentum-representative cross section for portraying the transient development of momentum and refers to a channel with constant slope, irregular geometry, and a permeable channel bed with significant infiltration. Due to the structure of the solution, any arbitrary infiltration model can be used for quantifying the infiltration losses. For both synthetic prismatic and nonprismatic test channels, the robust and easy-to-use analytical ZI model shows an excellent match with the results of a comparative numerical simulation. Finally, the ZI model is employed for simulating a surge flow downstream of the Wadi Ahin groundwater recharge dam (Oman), in order to perform a scenario for artificial groundwater recharge in a natural wadi channel reach. This realistic application illustrates the potential of the new approach by even computing an almost standing wave and shows its applicability for an accurate and robust evaluation of release strategies. [ABSTRACT FROM AUTHOR]

The article presents a response to a commentary about a paper on the use of a zero-height side weir and their implications for one-dimensional (1D) modeling which was tested in a laboratory setting. Topics discussed include a new formulation of De Marchi's model for zero-height side weir and the dependence of the discharge ratio, the use of an asymptotic value of the discharge ratio based on the downstream boundary condition, and the downstream outflow system.

The article discusses the use of a zero-height side weir and their implications for one-dimensional (1D) modeling in response to a paper on article about three-dimenisional (3D) flow structures based on steady flow conditions conducted in a laboratory setting. Topics addressed include the use of a Manning coefficient in describing the main channel roughness, validity of a formula for subcritical flows developed by Borghei et al., and the downstream boundary condition.