Bed Shear Stress Measurements Research Articles

Velocity distribution in non-prismatic compound channels

River-flow modelling relies on accurate prediction of the stage–discharge relationship. Additionally, the lateral distribution of the longitudinal velocity component should be estimated for sediment transport investigations. In compound channels, the shear stress at the interface between the floodplains and the main channel strongly affects both conveyance and velocity distribution. This effect is reinforced when mass transfer occurs between subsections due to channel non-prismaticity. The stage–discharge relationship and the velocity distribution in prismatic compound channels can be computed using the lateral distribution method (LDM). The present paper investigates an extension of the LDM to non-prismatic channels. The physical meaning of the so-called secondary-current term is discussed. Numerical results are compared with previous velocity measurements in a symmetrically narrowing compound channel and to new bed shear-stress measurements in the same geometry.

Distribution of Bed Shear Stress in Rotating Circular Flume

Distributions of bed shear stress across the width of a rotating circular flume with smooth and rough bed surfaces were obtained by measurement and model prediction. Results with flows over smooth beds showed that the flow in the central part may be considered to be two-dimensional and that effects of flow depth over the operating range of the flume are minor for flow depths not exceeding 0.14 m. For rough beds, the bed shear stress distributions were found to be skewed toward the inner wall. This can be corrected if a compensating roughness is added to the bottom of the ring. Such measures are also effective for flumes with smooth beds. Measured bed shear stress distributions agreed well with the predicted distributions for smooth beds and reasonably well for rough beds. The modified Preston tube, for measurement of bed shear stress in flows over rough beds, was found to give promising results. Further tests are required to completely define the uncertainty in bed shear stress measurements made with this instrument.

Numerical Modeling of Three-Dimensional Flow Field Around Circular Piers

A three-dimensional numerical model \IFLUENT\N is used to simulate the separated turbulent flow around vertical circular piers in clear water. Computations are performed using different turbulence models and results are compared with several sets of experimental data available in the literature. Despite commonly perceived weakness of the \Ik-e\N model in resolving three-dimensional (3D) open channel and geophysical flows, several variants of this turbulence model are found to have performed satisfactorily in reproducing the measured velocity profiles. However, model results obtained using the k- models show some discrepancy with the measured bed shear stress. The Reynolds stress model performed quite well in simulating velocity distribution on flat bed and scour hole as well as shear stress distribution on flat bed around circular piers. The study demonstrates that a robust 3D hydrodynamic model can effectively supplement experimental studies in understanding the complex flow field and the scour initiation process around piers of various size, shape, and dimension.

BED SHEAR STRESS DISTRIBUTIONS IN A COMPOUND MEANDERING CHANNEL FLOW

As the bed shape depends on the distribution of the bed shear stress, it is important to evaluate the accurate that. However, the bed shear stress in a compound meandering channel has not been cleared yet. In order to clarify the characteristic of flow turbulence structure and bed shear stress, a measurement device is developed by meansof LDV. In this paper, the measured three-dimensional flow turbulence and shear stress are explained in detail and flow structure are cleared. A three-dimensional numerical analysis is performed to reproduced flow and shear stress, and numerical results are compared with experimental results. It was shown that measured bed shear stress distribution is different from that in numerical results though the time averaged flow in numerical results can explain that in experiments.

Effect of externally generated turbulence on wave boundary layer

This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer (WBL) under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel with a smooth bottom. Turbulence was generated ‘externally’ as the flow in the oscillator was passed through a series of grids that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity measurements were carried out. For the velocity measurements, mean and turbulence properties were measured in both the streamwise direction and in the direction perpendicular to the bed. A supplementary measurement for the undisturbed (without grids) case was also carried out, for comparison with the grid results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore the friction coefficient. Other features related to the bed shear stress, such as transition, the friction factor and phase lead, are discussed. The range of the Reynolds number studied is Re=1×10 4–2×10 6.

A Floating-Element Drag Plate for Direct Measurement of Bed Shear Stress During Eolian Transport

ABSTRACT This report outlines the design of an inexpensive, floating-element drag plate developed to obtain direct measurements of bed shear stress during eolian saltation. Shear stress measurements obtained from the drag plate during a field test are evaluated in comparison with estimates of shear velocity derived from an adjacent mast of rotating-cup anemometers, in terms of their correspondence with measurements of the transport rate obtained from a load cell-based electronic sediment trap. The two approaches provide a comparable level of explanation for transport rates averaged over intervals of several minutes. However, the drag plate provides a substantially improved level of explanation for 15-second intervals, and also for the instantaneous (1 Hz) transport measurements. It is suggested that the drag-plate approach has considerable potential for investigating high-frequency fluctuations in eolian transport rates.

Measurement of local bed shear stress in streams using a Preston‐static tube

Local bed shear stress (Τw) is a fluid dynamic parameter of importance in determining the physical and biological characteristics of stream‐bed environments. Unfortunately, it is often difficult to measure Τw under field conditions. Herein we describe the use of a Preston‐static tube, which is essentially a surface‐mounted Pitot‐static tube, to measure Τw in mountain streams. Our results indicate that it is possible to measure local shear stress quickly, consistently, and inexpensively in the field. This technique also provides high spatial resolution, which should allow for detailed in situ studies of local shear stress at scales relevant to lotic organisms. Such information will be invaluable in studies of benthic organisms and hydraulically relevant phenomena in the near‐bottom zone of lotic systems.

A simple method for measuring shear stress on rough boundaries

This technical note presents a simple method for the real time measurement of bed shear stress with a LabView Program for turbulent flow over uniformly rough boundaries, based on the classical logarithmic velocity distribution equation. The method is based on a step-wise linearization of the additive coefficient in the classical logarithmic velocity distribution equation.

Wave Scour around Group of Vertical Piles

The paper presents the results of an experimental investigation on scour around pile groups with different configurations exposed to waves. Two kinds of tests were carried out: the actual scour experiments, and bed shear stress measurements. Seven kinds of pile-group arrangements were tested, including the side-by-side arrangements of piles, tandem arrangements of piles, and triangular and square group arrangements with two-, three-, and four-pile groups. The variations with the pile spacing as well as the Keulegan-Carpenter number, \IKC\N, were investigated. The scour depth can increase with respect to its single-pile value by as much as a factor of 3 for moderate \IKC\N numbers [such as \iO(10)] for most of the pile-group configurations for small pile spacings. For small \IKC\N numbers, this increase can be even higher, by as much as a factor of 10 or more. For a given pile spacing, the scour depth is mainly governed by the \IKC\N number. In agreement with the single-pile case, the larger the \IKC\N number, the larger the scour depth. The bed was live in all the tests except a few cases.

Control of estuarine sediment dynamics by interactions between currents and waves at several scales

The effect of interactions between continuous (tidal currents) and intermittent (waves) processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves, boundary-layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in Manukau Harbour, New Zealand. The aim is to determine in what ways it is necessary to couple waves and currents in numerical models, and thereby put limits on the fundamental structure of process-based estuarine sediment transport models. Waves were important on the intertidal flat: turbidity switched on and off with the appearance and disappearance of waves; wave groups dominated entrainment of bed sediment; a wave-current boundary-layer model explained measured bed shear stress and hydraulic roughness; and the measured near-bed time-averaged suspended-sediment concentration was mostly well predicted by a pure-wave model. Both the waves themselves and wave-related processes varied markedly over the tidal cycle. The variation in the former (principally changes in wave height) was related to changes in fetch caused by the harbour-wide emergence and submergence of intertidal regions. The variation in the latter was related to changes in water depth relative to the wavelength of the waves, which controlled the penetration to the bed of wave-orbital currents. In addition to the variation over the tidal cycle in the ‘intensity’ of wave processes, there was also a change in ‘kind’, which occurred with the arrival at the measurement site of the ‘turbid fringe’, which is the narrow, highly turbid edge of the estuarine water body. The relationship between suspended-sediment concentration and wave-orbital velocity in the turbid fringe was radically different to the relationship in the estuarine water body proper, which suggests a change in dynamics, perhaps related to breaking waves. A ‘hybrid’ modelling approach is required, i.e. one that treats discrete events but resolves tidal-cycle-scale variation within the event. There is a need to resolve the variation in the wave train over the tidal cycle and the penetration to the bed of wave-orbital motions, both of which could only be done adequately within an estuary tidal model. In contrast to the situation on the intertidal flat where waves intermittently entrained sediment, sediment transport in the channel was continuous, driven by tidal currents. To predict sediment flux in the channel we need to know the upstream sediment-transporting capacity of the flow (including that contributed by waves), the character of the bed sediment, and the sediment-settling time scale. These factors confounded even the simplest notion of flood and ebb dominance, which frequently has been applied to understand estuarine morphodynamics.

The horseshoe vortex and vortex shedding around a vertical wall-mounted cylinder exposed to waves

This study concerns the flow around the base of a vertical, wall-mounted cylinder - a pile - exposed to waves. The study comprises (i) flow visualization of horseshoe-vortex flow in front of and the lee-wake-vortex flow behind the pile and (ii) bed shear stress measurements around the pile conducted in a wave flume, plus supplementary bed shear stress measurements carried out in an oscillatory-flow water tunnel. The Reynolds number range of the flume experiments is ReD = (2-9) x 103 and that of the tunnel experiments is ReD= 103—5 x 104, in which ReD is based on the pile size. Steadycurrent tests were also carried out for reference. The horseshoe-vortex flow (like leewake-vortex flow) is governed primarily by the Keulegan-Carpenter number, KC. The range of KC was from 0 to about 25 in the flume experiments, and from 4 to 120 in the tunnel experiments. The experiments were conducted mainly with circular piles. The results indicate that no horseshoe vortex exists for KC < 6. The size and lifespan of the horseshoe vortex increase with KC. The influence of the cross-sectional shape of the pile on the horseshoe vortex was investigated. The results show that a square pile with 90° orientation produces the largest horseshoe vortex while that with 45° orientation produces the smallest one, the circular-pile result being between the two. The influence of a superimposed current on the horseshoe vortex was also investigated. The range of the current-to-wave-induced-velocity ratio, Uc/Um, was from 0 to about 0.8. The overall effect of the superimposed current is to increase the size and lifespan of the horseshoe vortex. This effect increases with increasing Uc/Um. Regarding the near-bed lee-wake flow, the flow regimes observed for the two-dimensional free-cylinder case exist for the present case, too, but with one exception: in the present case, no transverse vortex street was observed in the so-called single-pair regime. The results show that the bed shear stress beneath the horseshoe vortex and in the lee-wake area is heavily influenced by KC. The amplification of the bed shear stress with respect to its undisturbed value is maximum (O(4)) at the side edges of the pile, in contrast to what occurs in steady currents where the maximum occurs at an angle of about 45° from the upstream edge of the pile with an amplification of O(10).

Bed shear stress measurements for vims Sea Carousel

The results of bed shear stress measurements for vims Sea Carousel under clear-water and flat-bed conditions are presented. The measurements were made using a flush mounted hot film shear stress sensor mounted on the bottom of a laboratory version of the vims Sea Carousel. The experimental results confirm the relationship between the spatial-average bed shear stress, τ (Pascal), and the ring rotation speed, Ω (rpm), from a previous numerical study: τ = 0.0114 Ω 1.69 . The measurements show a maximum of 15% difference with that given by the equation. We also found the side wall effects extend outward less than 2.5 cm for all ring speeds and the bed shear stress distributions are reasonably uniform.

Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements

High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.

Bed shear stress measurements over a smooth bed in three-dimensional wave-current motion

Wave and current experiments have been carried out with the purpose of measuring the instantaneous bed shear stress over a hydraulically smooth bed, in order to evaluate the extent of the interaction. Measurements have been carried out with a dual-component hot-film probe which, with some constraints, enables the magnitude and direction of the instantaneous bed shear stress to be measured. Results are presented as maximum obtained bed shear stress values over one cycle and the corresponding mean value in the direction of the current for various combinations of waves and current.

Shape and Dimensions of Stationary Dunes in Rivers

The shape and dimensions (length, height) of sand dunes in rivers are calculated by use of the measured bed shear stress distribution downstream a rearward-facing step. The transport of sediment is split up into bed load and suspended load, which makes it possible to explain the transition to plane bed. The model can predict flow-resistance curves in alluvial streams.