scholarly journals Characteristics of Turbulence in the Downstream Region of a Vegetation Patch

Water ◽  
2021 ◽  
Vol 13 (23) ◽  
pp. 3468
Author(s):  
Masoud Kazem ◽  
Hossein Afzalimehr ◽  
Jueyi Sui
Keyword(s):  
Kinetic Energy ◽  
Velocity Profile ◽  
Turbulent Kinetic Energy ◽  
Mixing Layer ◽  
Quadrant Analysis ◽  
Width Ratio ◽  
Shear Stresses ◽  
Vegetation Patch ◽  
Downstream Region ◽  
Vegetation Patches

In presence of vegetation patches in a channel bed, different flow–morphology interactions in the river will result. The investigation of the nature and intensity of these structures is a crucial part of the research works of river engineering. In this experimental study, the characteristics of turbulence in the non-developed region downstream of a vegetation patch suffering from a gradual fade have been investigated. The changes in turbulent structure were tracked in sequential patterns by reducing the patch size. The model vegetation was selected carefully to simulate the aquatic vegetation patches in natural rivers. Velocity profile, TKE (Turbulent Kinetic Energy), turbulent power spectra and quadrant analysis have been used to investigate the behavior and intensity of the turbulent structures. The results of the velocity profile and TKE indicate that there are three different flow layers in the region downstream of the vegetation patch, including the wake layer, mixing layer and shear layer. When the vegetation patch is wide enough (Dv/Dc > 0.5, termed as the patch width ratio, where Dv is the width of a vegetation patch and Dc is the width of the channel), highly intermittent anisotropic turbulent events appear in the mixing layer at the depth of z/Hv = 0.7~1.1 and distance of x/Hv = 8~12 (where x is streamwise distance from the patch edge, z is vertical distance from channel bed and Hv is the height of a vegetation patch). The results of quadrant analysis show that these structures are associated with the dominance of the outward interactions (Q1). Moreover, these structures accompany large coherent Reynolds shear stresses, anomalies in streamwise velocity, increases in the standard deviation of TKE and increases in intermittent Turbulent Kinetic Energy (TKEi). The intensity and extents of these structures fade with the decrease in the size of a vegetation patch. On the other hand, as the size of the vegetation patch decreases, von Karman vortexes appear in the wake layer and form the dominant flow structures in the downstream region of a vegetation patch.

Interaction between a spatially growing turbulent boundary layer and embedded streamwise vortices

1996 ◽  
Vol 326 ◽  
pp. 151-179 ◽  
Author(s):  
Junhui Liu ◽  
Ugo Piomelli ◽  
Philippe R. Spalart
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Turbulent Boundary Layer ◽  
Turbulent Kinetic Energy ◽  
Vortex Core ◽  
Eddy Viscosity ◽  
Shear Stresses ◽  
Streamwise Vortices ◽  
Mean Velocity ◽  
Production Term

The interaction between a zero-pressure-gradient turbulent boundary layer and a pair of strong, common-flow-down, streamwise vortices with a sizeable velocity deficit is studied by large-eddy simulation. The subgrid-scale stresses are modelled by a localized dynamic eddy-viscosity model. The results agree well with experimental data. The vortices drastically distort the boundary layer, and produce large spanwise variations of the skin friction. The Reynolds stresses are highly three-dimensional. High levels of kinetic energy are found both in the upwash region and in the vortex core. The two secondary shear stresses are significant in the vortex region, with magnitudes comparable to the primary one. Turbulent transport from the immediate upwash region is partly responsible for the high levels of turbulent kinetic energy in the vortex core; its effect on the primary stress 〈u′v′〉 is less significant. The mean velocity gradients play an important role in the generation of 〈u′v′〉 in all regions, while they are negligible in the generation of turbulent kinetic energy in the vortex core. The pressure-strain correlations are generally of opposite sign to the production terms except in the vortex core, where they have the same sign as the production term in the budget of 〈u′v′〉. The results highlight the limitations of the eddy-viscosity assumption (in a Reynolds-averaged context) for flows of this type, as well as the excessive diffusion predicted by typical turbulence models.

Experimental Investigation of Unsteady Flow Field Within a Two Stage Axial Turbomachine Using Particle Image Velocimetry

2002 ◽  
Author(s):  
Oguz Uzol ◽  
Yi-Chih Chow ◽  
Joseph Katz ◽  
Charles Meneveau
Keyword(s):  
Particle Image Velocimetry ◽  
Kinetic Energy ◽  
Flow Field ◽  
Turbulent Kinetic Energy ◽  
Particle Image ◽  
Energy Levels ◽  
Shear Stresses ◽  
Image Velocimetry ◽  
Entire Flow ◽  
The Difference

Detailed measurements of the flow field within the entire 2nd stage of a two stage axial turbomachine are performed using Particle Image Velocimetry. The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. The entire flow field is composed of a “lattice of wakes”, and the resulting wake-wake and wake-blade interactions cause major flow and turbulence non-uniformities. The paper presents data on the phase averaged velocity and turbulent kinetic energy distributions, as well as the average-passage velocity and deterministic stresses. The phase-dependent turbulence parameters are determined from the difference between instantaneous and the phase-averaged data. The distributions of average-passage flow field over the entire stage in both the stator and rotor frames of reference are calculated by averaging the phase-averaged data. The deterministic stresses are calculated from the difference between the phase-averaged and average-passage velocity distributions. Clearly, wake-wake and wake-blade interactions are the dominant contributors to generation of high deterministic stresses and tangential non-uniformities, in the rotor-stator gap, near the blades and in the wakes behind them. The turbulent kinetic energy levels are generally higher than the deterministic kinetic energy levels, whereas the shear stress levels are comparable, both in the rotor and stator frames of references. At certain locations the deterministic shear stresses are substantially higher than the turbulent shear stresses, such as close to the stator blade in the rotor frame of reference. The non-uniformities in the lateral velocity component due to the interaction of the rotor blade with the 1st stage rotor-stator wakes, result in 13% variations in the specific work input of the rotor. Thus, in spite of the relatively large blade row spacings in the present turbomachine, the non-uniformities in flow structure have significant effects on the overall performance of the system.

Simulation of a propelled wake with moderate excess momentum in a stratified fluid

2011 ◽  
Vol 692 ◽  
pp. 28-52 ◽  
Author(s):  
Matthew B. de Stadler ◽  
Sutanu Sarkar
Keyword(s):  
Kinetic Energy ◽  
Velocity Profile ◽  
Turbulent Kinetic Energy ◽  
Vertical Direction ◽  
Qualitative Change ◽  
Stratified Fluid ◽  
Mean Velocity ◽  
Moderate Amount ◽  
Excess Momentum ◽  
The Mean

AbstractDirect numerical simulation is used to simulate the turbulent wake behind an accelerating axisymmetric self-propelled body in a stratified fluid. Acceleration is modelled by adding a velocity profile corresponding to net thrust to a self-propelled velocity profile resulting in a wake with excess momentum. The effect of a small to moderate amount of excess momentum on the initially momentumless self-propelled wake is investigated to evaluate if the addition of excess momentum leads to a large qualitative change in wake dynamics. Both the amount and shape of excess momentum are varied. Increasing the amount of excess momentum and/or decreasing the radial extent of excess momentum was found to increase the defect velocity, mean kinetic energy, shear in the velocity gradient and the wake width. The increased shear in the mean profile resulted in increased production of turbulent kinetic energy leading to an increase in turbulent kinetic energy and its dissipation. Slightly larger vorticity structures were observed in the late wake with excess momentum although the differences between vorticity structures in the self-propelled and 40 % excess momentum cases was significantly smaller than suggested by previous experiments. Buoyancy was found to preserve the doubly inflected velocity profile in the vertical direction, and similarity for the mean velocity and turbulent kinetic energy was found to occur in both horizontal and vertical directions. While quantitative differences were observed between cases with and without excess momentum, qualitatively similar evolution was found to occur.

Measurements of Secondary Flows and Turbulence in a Turbine Cascade Passage

1987 ◽  
Author(s):  
Pietro Zunino ◽  
Marina Ubaldi ◽  
Antonio Satta
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Turbine Rotor ◽  
Secondary Flows ◽  
Rotor Blades ◽  
Turbulent Shear ◽  
Pressure Probe ◽  
Shear Stresses ◽  
Passage Vortex ◽  
High Level

The results of an experimental investigation on secondary flows in a linear cascade of turbine rotor blades are presented. To gain information on the turbulence related to the secondary flow development in turbine cascades, measurements have been carried out at eight normal sections in the passage and at a plane downstream of the trailing edge, using a constant temperature hot wire anemometer and a total pressure probe. A high level of turbulent kinetic energy and turbulent shear stresses has been found to be associated with the passage vortex. This result confirms that secondary loss generation in cascades is related to the production of turbulent kinetic energy in the vortex flow and to its viscous dissipation.

Double averaged turbulence characteristics of alluvial channel with downward seepage

2018 ◽  
Vol 45 (2) ◽  
pp. 135-151 ◽  
Author(s):  
Anurag Sharma ◽  
Bimlesh Kumar
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Seepage Flow ◽  
Quadrant Analysis ◽  
Turbulence Characteristics ◽  
Downward Seepage ◽  
Turbulent Statistics ◽  
Alluvial Channel ◽  
Bursting Events ◽  
Flow Turbulence

Present work evaluates the double averaged turbulence characteristics of the sand bed channel subjected to the downward seepage through permeable bed. Measures of turbulent statistics are observed to increase with the application of downward seepage. The form induced stress in near bed has a reducing effect with no seepage and an increasing effect with seepage. The seepage increases the turbulent kinetic energy and turbulent intensities causing the bed particles to move rapidly. The quadrant analysis suggests that at near bed, the sweep events in flows with seepage are the main bursting events towards the Reynolds stress production, while ejection and sweep events in no seepage flow have almost equal contribution. The increase in sediment transport with seepage is caused by an increase in flow turbulence production and an associated decrease in turbulent kinetic energy dissipation and turbulent diffusion.

A K—ε—γ equation turbulence model

1992 ◽  
Vol 237 ◽  
pp. 301-322 ◽  
Author(s):  
Ji Ryong Cho ◽  
Myung Kyoon Chung
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Turbulence Model ◽  
Mixing Layer ◽  
Spreading Rate ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Model Equations ◽  
Entrainment Effect ◽  
Intermittency Factor

By considering the entrainment effect on the intermittency in the free boundary of shear layers, a set of turbulence model equations for the turbulent kinetic energy k, the dissipation rate ε, and the intermittency factor γ is proposed. This enables us to incorporate explicitly the intermittency effect in the conventional K–ε turbulence model equations. The eddy viscosity νt is estimated by a function of K, ε and γ. In contrast to the closure schemes of previous intermittency modelling which employ conditional zone averaged moments, the present model equations are based on the conventional Reynolds averaged moments. This method is more economical in the sense that it halves the number of partial differential equations to be solved. The proposed K–ε–γ model has been applied to compute a plane jet, a round jet, a plane far wake and a plane mixing layer. The computational results of the model show considerable improvement over previous models for all these shear flows. In particular, the spreading rate, the centreline mean velocity and the profiles of Reynolds stresses and turbulent kinetic energy are calculated with significantly improved accuracy.

Experimental Study of Reynolds Number Effects on Three-Dimensional Offset Jets

2014 ◽  
Author(s):  
Zacharie M. J. Durand ◽  
Shawn P. Clark ◽  
Mark F. Tachie ◽  
Jarrod Malenchak ◽  
Getnet Muluye
Keyword(s):  
Experimental Study ◽  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Shear Stresses ◽  
Mean Velocity ◽  
Flow Measurements ◽  
Reynolds Number Effects

The effect of Reynolds number on three-dimensional offset jets was investigated in this study. An acoustic Doppler velocimeter simultaneously measured all three components of velocity, U, V and W, and turbulence intensity, urms, vrms, and wrms, and all three Reynolds shear stresses, uv, uw, and vw. Turbulent kinetic energy, k, was calculated with all three values of turbulence intensities. Flow measurements were performed at Reynolds numbers of 34,000, 53,000 and 86,000. Results of this experimental study indicate the wall-normal location of maximum mean velocity and jet spread to be independent of Reynolds number. The effects on maximum mean velocity decay are reduced with increasing Reynolds number. Profiles of mean velocities, U, V and W, turbulence intensities, urms, vrms, and wrms, and turbulent kinetic energy, k, show independence of Reynolds number. Reynolds shear stress uv was independent of Reynolds number while the magnitude of uw was reduced at higher Reynolds number.

scholarly journals Large-eddy simulation of a mildly curved open-channel flow

2009 ◽  
Vol 630 ◽  
pp. 413-442 ◽  
Author(s):  
W. VAN BALEN ◽  
W. S. J. UIJTTEWAAL ◽  
K. BLANCKAERT
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Open Channel ◽  
Scale Model ◽  
Shear Stresses ◽  
Subgrid Scale ◽  
Eddy Simulation ◽  
Outer Bank ◽  
Large Eddy ◽  
Subgrid Scale Model

After validation with experimental data, large-eddy simulation (LES) is used to study in detail the open-channel flow through a curved flume. Based on the LES results, the present paper addresses four issues. Firstly, features of the complex bicellular pattern of the secondary flow, occurring in curved open-channel flows, and its origin are investigated. Secondly, the turbulence characteristics of the flow are studied in detail, incorporating the anisotropy of the turbulence stresses, as well as the distribution of the kinetic energy and the turbulent kinetic energy. Moreover, the implications of the pattern of the production of turbulent kinetic energy is discussed within this context. Thirdly, the distribution of the wall shear stresses at the bottom and sidewalls is computed. Fourthly, the effects of changes in the subgrid-scale model and the boundary conditions are investigated. It turns out that the counter-rotating secondary flow cell near the outer bank is a result of the complex interaction between the spatial distribution of turbulence stresses and centrifugal effects. Moreover, it is found that this outer bank cell forms a region of a local increase of turbulent kinetic energy and of its production. Furthermore, it is shown that the bed shear stresses are amplified in the bend. The distribution of the wall shear stresses is deformed throughout the bend due to curvature. Finally, it is shown that changes in the subgrid-scale model, as well as changes in the boundary conditions, have no strong effect on the results.

PIV Investigation of Separated and Reattached Turbulent Flows Over Ribs of Various Aspect Ratio

2014 ◽  
Author(s):  
Kathryn M. Atamanchuk ◽  
Mark F. Tachie
Keyword(s):  
Kinetic Energy ◽  
Aspect Ratio ◽  
Turbulent Kinetic Energy ◽  
Turbulent Flows ◽  
Reynolds Shear Stress ◽  
Separated Flow ◽  
Particle Image Velocimetry System ◽  
Shear Stresses ◽  
Mean Velocity ◽  
Freestream Velocity

An experimental study is undertaken to investigate the features of separated and reattached flow over surface mounted traverse ribs of varying aspect ratio (1:1, 1:2, and 1:4) in a recirculating open channel turbulent flow. A particle image velocimetry system was used to conduct the velocity measurements. Upstream conditions were kept consistent among all three test cases. The reattachment length of the separated flow was found to decrease as rib aspect ratio increased, primarily as a result of a secondary separation reattachment formation on the ribs of increased aspect ratio. Contour plots of mean velocities, turbulence intensities, turbulent kinetic energy and Reynolds shear stresses, as well as one-dimensional profiles of streamwise mean velocity, turbulent kinetic energy and Reynolds shear stress in the recirculation and reattachment region are presented and discussed. The results show that maximum wall-normal mean velocities are approximately 40% of the approach freestream velocity. The results also indicate that the turbulence levels downstream of the block tend to decrease as the rib aspect ratio increases.

Average Passage Flow Field and Deterministic Stresses in the Tip and Hub Regions of a Multistage Turbomachine

2003 ◽  
Vol 125 (4) ◽  
pp. 714-725 ◽  
Author(s):  
Oguz Uzol ◽  
Yi-Chih Chow ◽  
Joseph Katz ◽  
Charles Meneveau
Keyword(s):  
Kinetic Energy ◽  
Flow Field ◽  
Turbulent Kinetic Energy ◽  
Energy Levels ◽  
Decay Rates ◽  
Tip Vortex ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
Order Of Magnitude ◽  
Source Sink

This paper continues our effort to study the dynamics of deterministic stresses in a multistage turbomachine using experimental data. Here we focus on the tip and hub regions and compare them to midspan data obtained in previous studies. The analysis is based on data obtained in particle image velocimetry (PIV) measurements performed in the second stage of a two-stage turbomachine. A complete data set is obtained using blades and fluid with matched optical index of refraction. Previous measurements at midspan have shown that at midspan and close to design conditions, the deterministic kinetic energy is smaller than the turbulent kinetic energy. The primary contributor to the deterministic stresses at midspan is the interaction of a blade with the upstream wakes. Conversely, we find that the tip vortex is the dominant source of phase-dependent unsteadiness and deterministic stresses in the tip region. Along the trajectory of the tip vortex, the deterministic kinetic energy levels are more than one order of magnitude higher than the levels measured in the hub and midspan, and are of the same order of magnitude as the turbulent kinetic energy levels. Reasons for this trend are explained using a sample distribution of phase-averaged flow variables. Outside of the region affected by tip-vortex transport, within the rotor-stator gap and within the stator passages, the turbulent kinetic energy is still 3–4 times higher than the deterministic kinetic energy. The deterministic and turbulent shear stress levels are comparable in all spanwise locations, except for the wakes of the stator blades, where the turbulent stresses are higher. However, along the direction of tip-vortex transport, the deterministic shear stresses are about an order of magnitude higher than the turbulent shear stresses. The decay rates of deterministic kinetic energy in the hub and midspan regions are comparable to each other, whereas at the tip the decay rate is higher. The decay rates of turbulent kinetic energy are much smaller than those of the deterministic kinetic energy. The paper also examines terms in the deterministic kinetic energy transport equation. The data indicate that “deterministic production” and a new term, here called “dissipation due to turbulence,” are the dominant source/sink terms. Regions with alternating signs of deterministic production indicate that the energy transfer between the phase-averaged and average-passage flow fields can occur in both directions. The divergence of the pressure-velocity correlation, obtained from a balance of all the other terms, is dominant and appears to be much larger than the deterministic production (source/sink) term. This trend indicates that there are substantial deterministic pressure fluctuations in the flow field, especially within the rotor-stator gap and within the stator passage.

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