An Experimental Study of the Flow Structures in Vegetated Open Channels

2014 ◽  
Vol 522-524 ◽  
pp. 941-949
Author(s):  
Xu Yue Hu ◽  
Kun Jiang ◽  
Hua Qiang Ren ◽  
Xiao Xiong Shen
Keyword(s):  
Shear Stress ◽  
Reynolds Stress ◽  
Turbulence Intensity ◽  
Flow Structures ◽  
Vegetation Distribution ◽  
Measuring Point ◽  
Vegetation Height ◽  
Water Flows ◽  
Viscous Shear Stress ◽  
Viscous Shear

To investigate the effects of vegetation on flow Reynolds stress and turbulence intensity,an experiment was performed with plastic rods and artificial waterweeds in a slope-variable laboratory flume; an acoustic Doppler velocimeter was used to measure the instantaneous velocity at different points on the vertical line under different conditions; Turbulence parameters at each measuring point were calculated, such as Reynolds stress and turbulence intensity; The effects of vegetation on flow structures were analyzed through comparison with the turbulence characteristics of uniform open channel flows without vegetation distribution. The experimental results show that the turbulent constant Reynolds stress layer exists in water flows with vegetation distribution compared with the water flows without vegetation distribution. Without vegetation distribution, the viscous shear stress at the flume bed mainly affects the area between the bed and the level at a depth about 30% of the water depth. With vegetation distribution, the effect of the viscous shear stress at the bed weakens.The highest flow turbulence intensity with vegetation distribution occurs within the range of vegetation height.

Theory of free surface flow over rough seeping beds

2006 ◽  
Vol 463 (2078) ◽  
pp. 369-383 ◽  
Author(s):  
Sujit K Bose ◽  
Subhasish Dey
Keyword(s):  
Experimental Data ◽  
Shear Stress ◽  
Free Surface ◽  
Reynolds Stress ◽  
Outer Layer ◽  
Free Surface Flow ◽  
Surface Flow ◽  
Viscous Shear Stress ◽  
Viscous Shear ◽  
Logarithmic Law

A new theory is developed for the steady free surface flow over a horizontal rough bed with uniform upward seepage normal to the bed. The theory is based on the Reynolds averaged Navier–Stokes (RANS) equations applied to the flow domain that is divided into a fully turbulent outer layer and an inner layer (viscous sublayer plus buffer layer), which is a transition zone from viscous to turbulent regime. In the outer layer, the Reynolds stress far exceeds viscous shear stress, varying gradually with vertical distance. Near the free surface, the velocity gradient in vertical direction becomes lesser giving rise to wake flow. On the other hand, in the composite inner layer close to the bed, the viscous shear stress exists together with the turbulent stress. Thus, for the outer layer, a logarithmic law having modified coefficients from the traditional logarithmic law is obtained for the streamwise velocity, whereas for the inner layer, a fifth-degree polynomial including effective height of protrusions holds. The exact velocity expressions for inner and outer layer, which contain principal terms in addition to infinitesimally small terms, are in agreement with the experimental data obtained from laboratory measurements through an acoustic Doppler velocimeter. The experiments were run on two conditions of no seepage and a low upward seepage. Expressions for the Reynolds stress are also derived and computed for validation by the experimental data.

scholarly journals An Experimental Study of Turbulent Structures in a Flat-Crested Weir-Type Fishway

2019 ◽  
Vol 9 (19) ◽  
pp. 4040
Author(s):  
Zhiping Guo ◽  
Xihuan Sun ◽  
Zhiyong Dong
Keyword(s):  
Surface Layer ◽  
Reynolds Stress ◽  
Fish Species ◽  
Turbulence Intensity ◽  
Intensity Distribution ◽  
Three Dimensional ◽  
Turbulent Intensity ◽  
Flow Structures ◽  
Measuring Point ◽  
Flow Rates

Fishways can assist fish species to overcome obstacles for performing spawning, feeding, and overwintering migrations. Flow structures in a flat-crested weir-type fishway were experimentally studied. Variations of time-averaged velocity, turbulence intensity, and Reynolds stress with longitudinal and vertical directions and flow rates were analyzed. Also, flow patterns in the longitudinal profile were given. The experiments were carried out in a large scale fishway model in the Hydraulics Laboratory at Zhejiang University of Technology. Two typical flow rates corresponding to detection and preference velocities of fish species were considered. Five different horizontal planes for each flow rate were taken. Eleven transverse lines were arranged for each horizontal plane. Ten measuring points were laid along each transverse line. Three-dimensional velocity at each measuring point was measured by acoustic Doppler velocimeter (ADV). Longitudinal and vertical time-averaged velocity distributions, longitudinal turbulence intensity distribution on the different horizontal planes, vertical turbulent intensity distribution along flow depth, and Reynolds stress distributions on the different horizontal planes and on the different cross-sections in the pool of fishway were analyzed. The experimental results showed that distribution of longitudinal velocity was characterized by topology, which constituted an apparent vertical vortex. Weir flow exhibited skimming flow in the fishway pool. Peak-value range of longitudinal turbulent intensity existed. The amplitude of variation in Reynolds stress near the surface layer reached the maximum, which provided a certain hydraulic condition for fish that favor jumping near the surface layer. This study uncovers three-dimensional flow structures, especially for turbulence characteristics, which can contribute to improving the design of crested weir fishway and to assisting fish species to pass smoothly through fishway, being of potential application value.

scholarly journals Shear models and parametric analysis of the PVC geomembrane-cushion interface in a high rock-fill dam

PLoS ONE ◽  
2021 ◽  
Vol 16 (1) ◽  
pp. e0245245
Author(s):  
Yun-Feng Liu ◽  
Ke Gu ◽  
Yi-Ming Shu ◽  
Xian-Lei Zhang ◽  
Xin-Xin Liu ◽  
...  
Keyword(s):  
Shear Stress ◽  
Friction Coefficient ◽  
Three Dimensional ◽  
Viscous Friction ◽  
Hydraulic Pressure ◽  
Viscous Shear Stress ◽  
Box Counting ◽  
Viscous Shear ◽  
The Stability ◽  
Hysteresis Friction

As a type of flexible impermeable material, a PVC geomembrane must be cooperatively used with cushion materials. The contact interface between a PVC geomembrane and cushion easily loses stability. In this present paper, we analyzed the shear models and parameters of the interface to study the stability. Two different cushion materials were used: the common extrusion sidewall and non-fines concrete. To simulate real working conditions, flexible silicone cushions were added under the loading plates to simulate hydraulic pressure loading, and the loading effect of flexible silicone cushions was demonstrated by measuring the actual contact areas under different normal pressures between the geomembrane and cushion using the thin-film pressure sensor. According to elastomer shear stress, there are two main types of shear stress between the PVC geomembrane and the cushion: viscous shear stress and hysteresis shear stress. The viscous shear stress between the geomembrane and the cement grout was measured using a dry, smooth concrete sample, then the precise formula parameters of the viscous shear stress and viscous friction coefficient were obtained. The hysteresis shear stress between the geomembrane and the cushion was calculated by subtracting the viscous shear stress from the total shear stress. The formula parameters of the hysteresis shear stress and hysteresis friction coefficient were calculated. The three-dimensional box-counting dimensions of the cushion surface were calculated, and the formula parameters of the hysteresis friction were positively correlated with the three-dimensional box dimensions.

scholarly journals Measurement and Characteristic Analysis of Turbulent Flow near Permeable Spur Dike

2020 ◽  
Vol 19 (5) ◽  
pp. 337-345
Author(s):  
Jielong Hu ◽  
Pingyi Wang ◽  
Jie Zhang ◽  
Meili Wang ◽  
Yafei Chen ◽  
...  
Keyword(s):  
Reynolds Stress ◽  
Turbulence Intensity ◽  
Three Dimensional ◽  
Characteristic Analysis ◽  
Measuring Point ◽  
Spur Dike ◽  
Turbulence Structures ◽  
Body Regions ◽  
Distribution Features ◽  
River Bottom

To disclose the turbulence features near a permeable spur dike, this paper measures the three-dimensional (3D) instantaneous velocity at different vertical lines and depths around the permeable spur dike through acoustic Doppler velocimetry (ADV). The relative turbulence intensity and Reynolds stress were calculated for each measuring point, and the distribution features of different turbulence structures were analyzed along the water depth. Experimental results show that: within the length of the dike, the turbulence intensity and Reynolds stress in the downstream of the dike are much greater than those in the upstream. In the downstream of the dike, the turbulence intensity in the root and body regions peaks in the dike crest layer, but that at other positions minimizes in that layer (but peaks on the water surface or river bottom). In the upstream, ejection and sweeping are the dominant turbulence structures; the two structures also dominate near the free layer at the bottom in the downstream; outward and inward interactions dominate near the dike crest layer in the downstream.

On the radial structure of planetary rings

1984 ◽  
Vol 75 ◽  
pp. 431-437 ◽  
Author(s):  
A.W. Harris ◽  
W.R. Ward
Keyword(s):  
Shear Stress ◽  
Optical Depth ◽  
Orbital Velocity ◽  
Planetary Rings ◽  
Radial Gradient ◽  
Viscous Shear Stress ◽  
Viscous Shear ◽  
Radial Structure ◽  
Uniform Ring ◽  
Increasing Function

ABSTRACTA ring of particles in orbit about a planet experiences a viscous shear stress due to the radial gradient of orbital velocity. This stress tends to spread the ring with time. At low optical depth (τ ≲ 0.5), and again at high optical depth (τ ≳ 2), the shear stress is an increasing function of optical depth. In the intermediate range (0.5 ≲ x ≲ 2), stress may decrease with increasing τ, leading to a diffusive instability which will tend to break an Initially uniform ring into ringlets of high and low optical depths.

scholarly journals Three-dimensional extent of flow stagnation in transcatheter heart valves

2019 ◽  
Vol 16 (154) ◽  
pp. 20190063 ◽  
Author(s):  
Vrishank Raghav ◽  
Chris Clifford ◽  
Prem Midha ◽  
Ikechukwu Okafor ◽  
Brian Thurow ◽  
...  
Keyword(s):  
Shear Stress ◽  
Heart Valves ◽  
Imaging Techniques ◽  
Three Dimensional ◽  
Flow Loop ◽  
Low Velocity ◽  
Viscous Shear Stress ◽  
Velocity Magnitude ◽  
Flow Stagnation ◽  
Viscous Shear

The recent unexpected discovery of thrombosis in transcatheter heart valves (THVs) has led to increased concerns of long-term valve durability. Based on the clinical evidence combined with Virchow's triad, the primary hypothesis is that low-velocity blood flow around the valve could be a primary cause for thrombosis. However, due to limited optical access in such unsteady three-dimensional biomedical flows, measurements are challenging. In this study, for the first time, we employ a novel single camera volumetric velocimetry technique to investigate unsteady three-dimensional cardiovascular flows. Validation of the novel volumetric velocimetry technique with standard planar particle image velocimetry (PIV) technique demonstrated the feasibility of adopting this new technique to investigate biomedical flows. This technique was used to quantify the three-dimensional velocity field in the vicinity of a validated, custom developed, transparent THV in a bench-top pulsatile flow loop. Large volumetric regions of flow stagnation were observed in the neo-sinus throughout the cardiac cycle, with stagnation defined as a velocity magnitude lower than 0.05 m s −1 . The volumetric scalar viscous shear stress quantified via the three-dimensional shear stress tensor was within the range of low shear-inducing thrombosis observed in the literature. Such high-fidelity volumetric quantitative data and novel imaging techniques used to obtain it will enable fundamental investigation of heart valve thrombosis in addition to providing a reliable and robust database for validation of computational tools.

High-Resolution Measurements of Velocity and Shear Stress in Leakage Jets From Bileaflet Mechanical Heart Valve Hinge Models

2015 ◽  
Vol 137 (11) ◽  
Author(s):  
Ewa Klusak ◽  
Alessandro Bellofiore ◽  
Sarah Loughnane ◽  
Nathan J. Quinlan
Keyword(s):  
Shear Stress ◽  
High Resolution ◽  
Flow Field ◽  
Heart Valve ◽  
Temporal Resolution ◽  
Mechanical Heart Valve ◽  
Small Scale ◽  
Bileaflet Mechanical Heart Valve ◽  
Viscous Shear Stress ◽  
Viscous Shear

In flow through cardiovascular implants, hemolysis, and thrombosis may be initiated by nonphysiological shear stress on blood elements. To enhance understanding of the small-scale flow structures that stimulate cellular responses, and ultimately to design devices for reduced blood damage, it is necessary to study the flow-field at high spatial and temporal resolution. In this work, we investigate flow in the reverse leakage jet from the hinge of a bileaflet mechanical heart valve (BMHV). Scaled-up model hinges are employed, enabling measurement of the flow-field at effective spatial resolution of 167 μm and temporal resolution of 594 μs using two-component particle image velocimetry (PIV). High-velocity jets were observed at the hinge outflow, with time-average velocity up to 5.7 m/s, higher than reported in previous literature. Mean viscous shear stress is up to 60 Pa. For the first time, strongly unsteady flow has been observed in the leakage jet. Peak instantaneous shear stress is up to 120 Pa, twice as high as the average value. These high-resolution measurements identify the hinge leakage jet as a region of very high fluctuating shear stress which is likely to be thrombogenic and should be an important target for future design improvement.

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