Turbulence Characteristics of Vegetated Channel With Downward Seepage

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Thokchom Bebina Devi ◽  
Anurag Sharma ◽  
Bimlesh Kumar
Keyword(s):  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Experimental Studies ◽  
Flow Characteristics ◽  
Distribution Functions ◽  
Quadrant Analysis ◽  
Reynolds Stresses ◽  
Vegetation Zone ◽  
Increase With Increase ◽  
Downward Seepage

Experimental studies were carried out for investigating changes in flow characteristics with the presence of flexible vegetation in a channel. The study focuses on the effect of introducing downward seepage on velocity profiles, Reynolds shear stress (RSS), and different turbulent length scales in a vegetative channel. The presence of vegetation provides drag and reduces the flow velocity. The turbulence generation mainly comes from the oscillations occurring near the top of the vegetation as is evident from the achievement of maximum Reynolds stress near the top of the vegetation. Application of downward seepage results in a higher velocity zone in the lower vegetation zone and a higher Reynolds stress. Quadrant analysis shows that sweep and ejection contribute most to Reynolds stress. The dominance of sweep event over ejection event is more with the application of downward seepage which means more bed transport. Different turbulent length and time scales increase with increase in downward seepage percentage due to reduction in energy dissipation. The increase in the length scale and time scale with downward seepage infers that higher level of turbulence prevail near the bed with an increased eddy size resulting in higher Reynolds stresses with downward seepage. The universal probability distribution functions (PDFs) of velocity fluctuations, RSS, and conditional RSS of vegetative channel follow Gram Charlier (GC) series based on exponential distribution except that a slight departure of inward and outward interactions of conditional RSS is observed which may be due to weaker events.

Experimental Study on the Near Wake Behind Two Staggered Cylinders of Unequal Diameters

2012 ◽  
Author(s):  
Yangyang Gao ◽  
Xikun Wang ◽  
Soon Keat Tan
Keyword(s):  
Reynolds Stress ◽  
Incident Angle ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Circular Cylinders ◽  
Mean Flow ◽  
Near Wake ◽  
Particle Image Velocimetry Technique ◽  
Image Velocimetry ◽  
Spacing Ratio

The wake structure behind two staggered circular cylinders with unequal diameters was investigated experimentally using the particle image velocimetry technique (PIV). This investigation was focused on the variations of flow patterns in terms of incident angle at Reynolds number Re = 1200. Comparisons of the time-averaged flow field of two staggered cylinders with unequal diameters at different angles were made to elucidate the mean flow characteristics. The characteristics of Reynolds shear stress contours at different incident angles and spacing ratios were also investigated. The results showed that with increasing of incident angle, the scale of Reynolds stress contours behind the upstream cylinder becomes larger, as well as the effect of spacing ratio on Reynolds stress contours.

Reynolds stress distribution and turbulence generated secondary flow in the turbulent three-dimensional wall jet

2016 ◽  
Vol 800 ◽  
pp. 613-644 ◽  
Author(s):  
L. Namgyal ◽  
J. W. Hall
Keyword(s):  
Secondary Flow ◽  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Secondary Flows ◽  
Half Width ◽  
Stereoscopic Particle Image Velocimetry ◽  
Wall Jet ◽  
Normal Stresses ◽  
Reynolds Stresses

The lateral half-width of the turbulent three-dimensional wall jet is typically five to eight times larger than the vertical half-width normal to the wall. Although the reason for this behaviour is not fully understood, it is caused by mean secondary flows that develop in the jet due to the presence of the wall. The origin of the secondary flow has been associated previously with both vorticity reorientation and also gradients in the Reynolds stresses, although this has not been directly quantified as yet. The present investigation focuses on a wall jet formed using a circular contoured nozzle with exit Reynolds number of 250 000. Stereoscopic particle image velocimetry measurements are used herein to measure the three-component velocity, thereby allowing access to the full Reynolds stress tensor that contributes to the secondary flow in a turbulent three-dimensional wall jet. Throughout the jet, the Reynolds normal stress ($\overline{u^{2}}$) makes the largest contribution to the Reynolds stress field whereas Reynolds shear stress ($\overline{vw}$) is found to be negligible when compared with other stresses. In particular, the differences in the Reynolds normal stresses ($\overline{v^{2}}-\overline{w^{2}}$) are found to be significantly larger than $\overline{vw}$; these terms are important for the generation of turbulence secondary flow in the wall jet. Above all, the differences in the Reynolds normal stresses are oriented to reinforce the near-wall streamwise vorticity, and thus contribute to the large lateral growth of this flow. The contours of the turbulent kinetic budget indicate that the turbulent energy budget obtained on the jet centreline is different from that obtained off of the jet centreline.

scholarly journals Hydrodynamics of submerged vegetated alluvial channel with downward seepage

2017 ◽  
Vol 44 (3) ◽  
pp. 174-181 ◽  
Author(s):  
Thokchom Bebina Devi ◽  
Bimlesh Kumar
Keyword(s):  
Oryza Sativa ◽  
Flow Velocity ◽  
Reynolds Stress ◽  
Time Scales ◽  
Flow Conditions ◽  
Vegetation Zone ◽  
Flow Measurements ◽  
Downward Seepage ◽  
Alluvial Channel ◽  
Zone Centre

The flow conditions in a channel covered with fully submerged Oryza sativa has been investigated considering downward seepage. The flow measurements taken at the upstream free vegetation zone, centre of the vegetation zone, and downstream free vegetation zone will provide a better understanding regarding the flow hydrodynamics in a vegetated channel. The flow velocity, Reynolds stress, and turbulent intensities are reduced at the downstream vegetated section. Downward seepage increases the occurrence of streamwise flux along the length of the channel and vertical flux in the vertically downward direction. Integral length and time scales increase as the percentage of downward seepage increases.

Effects of Gap Ratio on Flow Past a Square Cylinder

2014 ◽  
Author(s):  
Ebenezer E. Essel ◽  
Liam Sharkey ◽  
Mark F. Tachie
Keyword(s):  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Flow Characteristics ◽  
Reynolds Stresses ◽  
Square Cylinder ◽  
Velocity Measurements ◽  
Freestream Velocity ◽  
Gap Ratio ◽  
Image Velocimetry ◽  
Bottom Face

The present study investigates the effect of gap ratio on the turbulent flow characteristics downstream of a square cylinder positioned in an open channel. Detailed velocity measurements were performed using a particle image velocimetry (PIV) system for gap ratios, G/h = 0.5 and 1, where G is the distance from the bottom face of the cylinder to the nearby wall and h is the cylinder height. Each set of experiments was conducted using a water depth of 65 mm and a Reynolds number of 2000 based on the height of the cylinder and the freestream velocity. Mean velocities, Reynolds stresses and Reynolds shear stress producing events of the quadrant decompositions were compared for the different gap ratios investigated. The results showed that as gap ratio decreased from 1 to 0.5 cylinder heights, the length of the separated region increased by 50%. Furthermore, the Reynolds stresses were found to decrease with decreasing gap ratio. Further downstream of the cylinder, the turbulent kinetic energy decreased, while the Reynolds shear stress increased for G/h = 0.5 compared to G/h = 1.

Mean and Fluctuating Velocity Characteristics of a Separated Shear Layer Past a Surface Mounted Block

2006 ◽  
Vol 129 (2) ◽  
pp. 200-208 ◽  
Author(s):  
Ü. Özkol ◽  
C. Wark ◽  
D. Fabris
Keyword(s):  
Shear Layer ◽  
Reynolds Stress ◽  
Reynolds Shear Stress ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Separated Shear Layer ◽  
Block Height ◽  
High Reynolds

The mean velocity, Reynolds stress, and mean vorticity regions of a separated shear layer over a surface mounted block are investigated by 2D Digital Particle Image Velocimetry (DPIV) for three Reynolds numbers (Rea=500, 1000, and 2500) and two channel-to-block height ratios (H∕a=1.825 and 4.6). The recirculation region’s height and length are determined for the separated shear layer by means of U¯=0 contours. It is observed that the high Reynolds stress regions lay just outside of the U¯=0 contours. The flow visualization and DPIV measurement of vorticity indicate that the differing normalized Reynolds stresses between Rea=500 and 1000 are most probably due to the initiation of the vortex shedding between these two Reynolds numbers while, differences are minimal between Rea=1000 and 2500. A sign change in the Reynolds shear stress distribution of the separated shear layer near the leading edge of the block was recognized for every Reynolds number and channel width.

Drag and Turbulent Characteristics of Mobile Bed Channel With Mixed Vegetation Densities Under Downward Seepage

2016 ◽  
Vol 138 (7) ◽  
Author(s):  
Thokchom Bebina Devi ◽  
Rishabh Daga ◽  
Sumit Kumar Mahto ◽  
Bimlesh Kumar
Keyword(s):  
Drag Coefficient ◽  
Reynolds Stress ◽  
Flow Resistance ◽  
Flow Characteristics ◽  
Vegetation Density ◽  
Streamwise Direction ◽  
Mobile Bed ◽  
Downward Seepage ◽  
Turbulent Characteristics ◽  
Sediment Particles

The present study addresses the drag owing to the presence of vegetation and turbulent characteristics in a mobile bed channel, characterized by fully submerged vegetation formed by nonuniform vegetation densities. The influence of seepage on the velocity profiles, Reynolds stress, and turbulence intensities is discussed. Experimental results show that vegetation density is one of the important parameters that affect the flow resistance. It is found that higher vegetation density when placed at the downstream side leads to a reduction in velocity, Reynolds stress, and turbulent intensities. Downward seepage increases the near bed velocity, Reynolds stress, and turbulent intensities. Moment analysis shows that there is an increase in the inrush of flow, and sediment particles are transported more toward the streamwise direction with the application of seepage. The dominance of sweep events over ejection events increases more sediment transport. However, high vegetation density when placed at the downstream portion slightly decreases the dominance of sweep event. Drag coefficient decreases near the vegetation top and increases near the bed. Downward seepage decreases the effect of drag offered by the vegetation stems. The reduction in flow characteristics, viz., velocity, Reynolds stress, turbulent intensities, in the downstream portion of lesser spacing vegetation stems is attributed an increased drag coefficient.

The Influence of the Aortic Root Geometry on Flow Characteristics of a Prosthetic Heart Valve

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Oleksandr Barannyk ◽  
Peter Oshkai
Keyword(s):  
Aortic Valve ◽  
Heart Valve ◽  
Aortic Root ◽  
Reynolds Shear Stress ◽  
Prosthetic Heart Valve ◽  
Flow Characteristics ◽  
Aortic Pressure ◽  
Valve Prosthesis ◽  
Aortic Heart Valve ◽  
Valve Diseases

In this paper, performance of aortic heart valve prosthesis in different geometries of the aortic root is investigated experimentally. The objective of this investigation is to establish a set of parameters, which are associated with abnormal flow patterns due to the flow through a prosthetic heart valve implanted in the patients that had certain types of valve diseases prior to the valve replacement. Specific valve diseases were classified into two clinical categories and were correlated with the corresponding changes in aortic root geometry while keeping the aortic base diameter fixed. These categories correspond to aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. Experiments were performed at test conditions corresponding to 70 beats/min, 5.5 L/min target cardiac output, and a mean aortic pressure of 100 mmHg. By varying the aortic root geometry, while keeping the diameter of the orifice constant, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots.

Quantitative Experimental Study of SiO2-Water Nanofluids on Backward-Facing Step Flow under Low Reynolds Number

2018 ◽  
Vol 916 ◽  
pp. 221-225
Author(s):  
Ji Zu Lv ◽  
Liang Yu Li ◽  
Cheng Zhi Hu ◽  
Min Li Bai ◽  
Sheng Nan Chang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Reynolds Number ◽  
Volume Fraction ◽  
Pure Water ◽  
Experimental Studies ◽  
Flow Characteristics ◽  
Thermal Engineering ◽  
Heat Transfer Medium ◽  
Step Flow

Nanofluids is an innovative study of nanotechnology applied to the traditional field of thermal engineering. It refers to the metal or non-metallic nanopowder was dispersed into water, alcohol, oil and other traditional heat transfer medium, to prepared as a new heat transfer medium with high thermal conductivity. The role of nanofluids in strengthening heat transfer has been confirmed by a large number of experimental studies. Its heat transfer mechanism is mainly divided into two aspects. On the one hand, the addition of nanoparticles enhances the thermal conductivity. On the other hand, due to the interaction between the nanoparticles and base fluid causing the changes in the flow characteristics, which is also the main factor affecting the heat transfer of nanofluids. Therefore, a intensive study on the flow characteristics of nanofluids will make the study of heat transfer more meaningful. In this experiment, the flow characteristics of SiO2-water nanofluids in two-dimensional backward step flow are quantitatively studied by PIV. The results show that under the same Reynolds number, the turbulence of nanofluids is larger than that of pure water. With the increase of nanofluids volume fraction, the flow characteristics are constantly changing. The quantitative analysis proved that the nanofluids disturbance was enhanced compared with the base liquid, which resulting in the heat transfer enhancement.

Education Committee Best Paper of 1995 Award: Methods of Classical Mechanics Applied to Turbulence Stresses in a Tip Leakage Vortex

1996 ◽  
Vol 118 (4) ◽  
pp. 622-629 ◽  
Author(s):  
J. G. Moore ◽  
S. A. Schorn ◽  
J. Moore
Keyword(s):  
Stress Tensor ◽  
Reynolds Stress ◽  
Classical Mechanics ◽  
Three Dimensional ◽  
Surface Model ◽  
Reynolds Stresses ◽  
Tip Leakage Vortex ◽  
Tip Leakage ◽  
Stress Tensors ◽  
Principal Directions

Moore et al. measured the six Reynolds stresses in a tip leakage vortex in a linear turbine cascade. Stress tensor analysis, as used in classical mechanics, has been applied to the measured turbulence stress tensors. Principal directions and principal normal stresses are found. A solid surface model, or three-dimensional glyph, for the Reynolds stress tensor is proposed and used to view the stresses throughout the tip leakage vortex. Modeled Reynolds stresses using the Boussinesq approximation are obtained from the measured mean velocity strain rate tensor. The comparison of the principal directions and the three-dimensional graphic representations of the strain and Reynolds stress tensors aids in the understanding of the turbulence and what is required to model it.

scholarly journals Characterization of the Nonaerated Flow Region in a Stepped Spillway by PIV

2006 ◽  
Vol 128 (6) ◽  
pp. 1266-1273 ◽  
Author(s):  
António Amador ◽  
Martí Sánchez-Juny ◽  
Josep Dolz
Keyword(s):  
Flow Region ◽  
Design Guidelines ◽  
Quadrant Analysis ◽  
Stepped Spillway ◽  
Main Stream ◽  
Reynolds Stresses ◽  
Particle Image Velocimetry Technique ◽  
Mean Velocity ◽  
Local Flow ◽  
Stepped Spillways

The development of the roller-compacted concrete (RCC) as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewed interest in stepped spillways. Previous research has focused on studying the air-water flow down the stepped chute with the objective of obtaining better design guidelines. The nonaerated flow region enlarges as the flow rate increases, and there is a lack of knowledge on the hydraulic performance of stepped spillways at high velocities that undermines its use in fear of cavitation damage. In the present, study the developing flow region in a stepped channel with a slope 1v:0.8h is characterized using a particle image velocimetry technique. An expression for the growth of the boundary layer thickness is proposed based on the streamwise distance from the channel crest and the roughness height. The local flow resistance coefficient is calculated by application of the von Kármán integral momentum equation. The shear strain, vorticity, and swirling strength maps obtained from the mean velocity gradient tensor are presented. Also, the fluctuating velocity field is assessed. The turbulent kinetic energy map indicates the region near the pseudobottom (imaginary line joining two adjacent step edges) as the most active in terms of Reynolds stresses. The turbulence was found to be very intense with maximum levels of turbulence intensity from 0.40 to 0.65 measured near the pseudobottom. Finally, the quadrant analysis of the velocity fluctuations suggests the presence of strong outflows of fluid from the cavities as well as inflows into the cavities. It is conjectured that the mass transfer/exchange between cavities and main stream, play an important role in the high levels of turbulent energy observed.

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