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In this study, an array of rigid cylindrical rods was used to simulate emergent vegetation stems that were subject to unidirectional open channel flows. The instantaneous drag force experienced by the rods was measured with a load cell. In addition, the particle image velocimetry (PIV) technique was applied to sample the flow information in a horizontal plane, and wave gauges were used to record the fluctuation in the water-surface elevation. The results show that the drag fluctuation normalized by the mean value may reach as high as 133% when the Reynolds number (defined based on the stem diameter) varied in the range from 400 to 1,100. High fluctuations were also observed in the flow velocity and flow depth under similar flow conditions. [ABSTRACT FROM AUTHOR]

This paper reports a study of the turbulent wake of two-dimensional rectangular cylinders in an open channel. Three cylinders with different aspect ratios were mounted vertically at the midspan of an open channel and oriented through four angles of incidence. A particle image velocimetry technique was used to conduct detailed velocity measurements at various streamwise-spanwise planes around the cylinders. The results show that the head loss coefficients increase with aspect ratio and angle of incidence. Self-similarity in the mean velocities was achieved with the proper choice of velocity and length scales, irrespective of the angle of incidence and cylinder aspect ratio. [ABSTRACT FROM AUTHOR]

The emergent vegetation in open-channel flows is usually simulated using arrays of circular cylinders in laboratory experiments. Analysis of recent experimental data reveals that for a given Reynolds number, the drag coefficient of a cylinder in a dense array is larger than that of an isolated cylinder. A new approach is applied to parameterize the drag coefficient and Reynolds number for flows through arrays of emergent cylinders. The approach is developed based on the concept of pseudofluid, for which an analogy is made between the cylinder-induced drag in an open-channel flow and that induced by the cylinder settling in a stationary fluid. With the proposed parameterization, the experimental database is successfully reorganized in such a way that a generalized drag coefficient is related to a generalized Reynolds number by one single curve, which is valid for a wide range of solid fractions and Reynolds numbers. However, it should be mentioned that only rigid circular stems are considered in this study and their induced drag is assumed to be dominant in comparison with the channel bed resistance. [ABSTRACT FROM AUTHOR]

The flow characteristics around two partially buried objects, a short cylinder (SC) and a truncated cone (TC) were examined experimentally and numerically. Experiments were conducted in a large tilting flume with vertical walls made of acrylic plastic and steel floor covered by a 24-m long sand pit. For each experiment, the object was placed on a flat sandy bed, the flume was filled to a water depth of 41 cm, and a steady unidirectional current was established at high Reynolds numbers while sinking of the object was tracked over time. Velocity records were taken with an acoustic Doppler velocimeter (ADV) probe at the final stage of burial and scour hole evolution. The flow structure around each object was further investigated using a model that solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with a turbulence closure model, and results were compared to the experimental measurements. Measurements and numerical simulations allowed identification of regions of high and low velocity and turbulent kinetic energy accounting for zones of potential sediment erosion and deposition that dictate the burial of the object. The turbulence closure model was shown to reproduce the experimental mean flow velocity and turbulent kinetic energy with acceptable accuracy. [ABSTRACT FROM AUTHOR]

This paper investigates the drag exerted by randomly distributed, rigid, emergent circular cylinders of uniform diameter d. Laboratory measurements are presented for solid volume fraction [lowercase_phi_synonym]=0.091, 0.15, 0.20, 0.27, and 0.35 and cylinder Reynolds number Rep≡Upd/ν=25 to 685, where Up=temporally and cross-sectionally averaged pore velocity and ν=kinematic viscosity. These ranges coincide with conditions in aquatic plant canopies. The temporally and cross-sectionally averaged drag coefficient, CD, decreased with increasing Rep and increased with increasing [lowercase_phi_synonym] under the flow conditions investigated. The dimensionless ratio of the mean drag per unit cylinder length <fD>H to the product of the viscosity, μ, and Up exhibits a linear Rep dependence of the form <fD>H(μUp)=α0+α1Rep, consistent with Ergun’s formulation for packed columns. In the range of experimental conditions, α1 increases monotonically with [lowercase_phi_synonym]. In contrast, α0 is constant within uncertainty for 0.15<=[lowercase_phi_synonym]<=0.35, which suggests that viscous drag per unit cylinder length is independent of [lowercase_phi_synonym] in this range. [ABSTRACT FROM AUTHOR]

A physical model study was performed in which wood dowels were used to model rigid vegetation. The dowel configurations used in the flume were intended to simulate the effects of willow post systems (i.e., collections of rigid cylinders placed along a streambank to reduce streambank erosion). In addition, an analytical model is presented for predicting depth-averaged velocity distributions in straight trapezoidal or rectangular channels with newly constructed willow post systems. The analytical model is founded on wake theory and is applicable to channels with submerged and unsubmerged rigid cylinders. Data from three independent physical model studies were used to validate the analytical model. Depth-averaged velocities predicted using the analytical model, Upred, were compared to velocities observed in the physical models, Uobsd, and yielded discrepancy ratios, Upred/Uobsd, that were typically between 0.80 and 1.20. Results from this study are that significant variables for reducing local velocities are cylinder height, diameter, and density (number of cylinders per unit area); and the arrangement of the cylinders (i.e., rectangular versus staggered grid) is inconsequential. [ABSTRACT FROM AUTHOR]

The flow characteristics around short, bottom-mounted cylinders were examined with a three-dimensional nonhydrostatic model for Reynolds numbers of 300–3,900 and different obstacle geometries. Simulations using a large-eddy simulation closure scheme reproduce the vortex shedding in the wake, with reasonable Strouhal number comparisons to available laboratory data. The length of the recirculation region is also in good agreement with the data. The sensitivity of the flow to obstacle cross section and length is investigated for a Reynolds number of 500 with square and circular cylinder geometries for nondimensional lengths of 1–15. Model predicted upstream separation was found to be most sensitive to cross section. The predicted downstream recirculation lengths reproduced the nondimensional length dependency of available observations. To highlight the processes relevant to scour, the time-dependent bed shear was evaluated with the mean and peak Shields parameters. Significant wake variability was found which would not be described by the mean alone, indicating that sediment transport models assuming a mean shear will underpredict the transport in the wake. [ABSTRACT FROM AUTHOR]

The writers’ experiments on local scour at vertical cylinders placed in a sand bed show that similitude of large-scale turbulence is an important consideration influencing equilibrium depth of local scour. For the range of cylinder diameters used in their experiments, the writers identify a direct trend between equilibrium scour depth (normalized with cylinder diameter) and the intensity and frequency of large-scale turbulence shed from each cylinder; values of normalized scour depth increased when cylinder diameter decreased. The writers offer a scour-depth adjustment factor to account for this trend, which essentially is a scale effect incurred with experiments involving three independent length scales: cylinder diameter, bed-particle diameter, and flow depth. The consequent similitude consideration, or scale effect, has general significance for laboratory studies of local scour associated with hydraulic structures in sediment beds. [ABSTRACT FROM AUTHOR]

The effects of unsteadiness in the turbulent flow through a staggered array of circular cylinders, modeling an ultraviolet disinfection system, are studied by means of solutions of the two-dimensional Reynolds-averaged Navier–Stokes equations incorporating the standard k–ε turbulence model. Time averaging is applied to the unsteady solution, and the time-averaged characteristics are compared with a solution where a steady flow is a priori assumed, as well as with time-averaged measurements. Differences between the predictions of time-averaged and the steady-flow models are found to be largest in the entrance region of the array, and to decline in importance in the downstream direction. Comparison with measurements indicate that, while the time-averaged unsteady model predictions exhibited better agreement in some respects, the turbulent kinetic energy remained substantially underpredicted. Predictions of head losses through the array are also discussed. [ABSTRACT FROM AUTHOR]