scholarly journals Shoaling Waves Interacting with an Orthogonal Current

2020 ◽  
Vol 8 (4) ◽  
pp. 281
Author(s):  
Massimiliano Marino ◽  
Carla Faraci ◽  
Rosaria Ester Musumeci
Keyword(s):  
Turbulent Mixing ◽  
Current Velocity ◽  
Quadrant Analysis ◽  
Phase Lag ◽  
Mean Velocity ◽  
Experimental Campaign ◽  
Turbulence Mixing ◽  
Bursting Events ◽  
Shoaling Waves ◽  
Ejections And Sweeps

In the present work, an experimental investigation on the hydrodynamics of shoaling waves superposed on a steady orthogonal current is carried out. An experimental campaign in a wave tank has been performed, with waves and current interacting at a right angle over a sloping planar beach. Velocity data have been gathered during the experiments in order to investigate mean, phase and turbulent flow. A detailed preliminary analysis of the time- and space-variability of the experiments is presented. Results show that a complex interaction between waves and current occurs as the wave shoals, in terms of sheer production, momentum transfer and turbulent mixing. Superposition of waves determines a shear increase at the bottom due to an enhanced turbulence mixing, nonetheless as depth decreases and the current velocity consequently increases, shoaling waves may be less efficient in enhancing shear at the bottom. Moreover, the superposition of waves determines the current to oscillate around its mean velocity value. Nevertheless, as wave shoals and simultaneously current velocity increases with decreasing depth, waves and current oscillatory motion experience a phase lag, as a response of the larger momentum of the current to the changing of the shoaling waves acceleration distribution along the wave phase. Moreover, the turbulent bursting events of the combined flow in proximity of the bed have been investigated by means of quadrant analysis, showing an increase of the turbulent ejections and sweeps due to the superposition of the shoaling waves.

Investigation of Turbulent Mixing in a Macro-Scale Multi-Inlet Vortex Nanoprecipitation Reactor by Stereoscopic-PIV

2014 ◽  
Author(s):  
Zhenping Liu ◽  
James C. Hill ◽  
Rodney O. Fox ◽  
Michael G. Olsen
Keyword(s):  
Reynolds Number ◽  
Turbulent Mixing ◽  
Three Dimensional ◽  
Stereoscopic Particle Image Velocimetry ◽  
Vortex Center ◽  
Mean Velocity ◽  
Vortex Model ◽  
Functional Nanoparticles ◽  
Turbulent Characteristics ◽  
Macro Scale

Flash Nanoprecipitation (FNP) is a technique to produce monodisperse functional nanoparticles through rapidly mixing a saturated solution and a non-solvent. Multi-inlet vortex reactors (MIVR) have been effectively applied to FNP due to their ability to provide both rapid mixing and the flexibility of inlet flow conditions. Until recently, only micro-scale MIVRs have been demonstrated to be effective in FNP. A scaled-up MIVR could potentially generate large quantities of functional nanoparticles, giving FNP wider applicability in the industry. In the present research, turbulent mixing inside a scaled-up, macro-scale MIVR was measured by stereoscopic particle image velocimetry (SPIV). Reynolds number of this reactor is defined based on the bulk inlet velocity, ranging from 3290 to 8225. It is the first time that the three-dimensional velocity field of a MIVR was experimentally measured. The influence of Reynolds number on mean velocity becomes more linear as Reynolds number increases. An analytical vortex model was proposed to well describe the mean velocity profile. The turbulent characteristics such as turbulent kinematic energy and Reynolds stress are also presented. The wandering motion of vortex center was found to have a significant contribution to the turbulent kinetic energy of flow near the center area.

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.

scholarly journals An Improved Logistic Model Illustrating Microcystis aeruginosa Growth Under Different Turbulent Mixing Conditions

Water ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 669
Author(s):  
Haiping Zhang ◽  
Fan Huang ◽  
Feipeng Li ◽  
Zhujun Gu ◽  
Ruihong Chen ◽  
...  
Keyword(s):  
Microcystis Aeruginosa ◽  
Growth Model ◽  
Turbulent Mixing ◽  
Logistic Model ◽  
Logistic Equation ◽  
Logistic Growth ◽  
Turbulence Mixing ◽  
Population Dynamics Models ◽  
Dynamics Models ◽  
Relationship Of

To overcome the limitations of the normal logistic equation, we aimed to improve the logistic model under hydrodynamic conditions for the examination of the responses of cyanobacterium, coupled turbulence mixing, and growth of cyanobacterium in population dynamics models. Selecting Microcystis aeruginosa and experimenting with the ideal conditions in a laboratory beaker, the chlorophyll-a concentration reached the corresponding maximum under each turbulent condition compared with the control. According to the experiment results, the theory of mass transfer, turbulence mixing, and the logistic equation are organically combined. The improved logistic growth model of Microcystis aeruginosa and competition growth model in the symbiont Scenedesmus quadricauda under turbulent conditions were established. Using the MATLAB multi-parameter surface fitting device, both models produced good fitting effects, with R > 0.95, proving that the results fit the models, and demonstrating the relationship of the unity of nutrient transfer and algae growth affected by turbulence mixing. With continuous increases in turbulent mixing, the fitted curve became smoother and steadier. Algae stimulated by turbulence accelerate reproduction and fission to achieve population dominance. The improved logistic model quantitatively explains the Microcystis aeruginosa response to turbulence and provides a basis to represent ecological and biogeochemical processes in enclosed eutrophic water bodies.

scholarly journals The Effect of Habitat Structure Boulder Spacing on Near-Bed Shear Stress and Turbulent Events in a Gravel Bed Channel

Water ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 1423
Author(s):  
Amir Golpira ◽  
Fengbin Huang ◽  
Abul B.M. Baki
Keyword(s):  
Shear Stress ◽  
Habitat Structure ◽  
Open Channel ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Quadrant Analysis ◽  
Bed Shear Stress ◽  
Gravel Bed ◽  
Restoration Practices ◽  
Ejections And Sweeps

This study experimentally investigated the effect of boulder spacing and boulder submergence ratio on the near-bed shear stress in a single array of boulders in a gravel bed open channel flume. An acoustic Doppler velocimeter (ADV) was used to measure the instantaneous three-dimensional velocity components. Four methods of estimating near-bed shear stress were compared. The results suggested a significant effect of boulder spacing and boulder submergence ratio on the near-bed shear stress estimations and their spatial distributions. It was found that at unsubmerged condition, the turbulent kinetic energy (TKE) and modified TKE methods can be used interchangeably to estimate the near-bed shear stress. At both submerged and unsubmerged conditions, the Reynolds method performed differently from the other point-methods. Moreover, a quadrant analysis was performed to examine the turbulent events and their contribution to the near-bed Reynolds shear stress with the effect of boulder spacing. Generally, the burst events (ejections and sweeps) were reduced in the presence of boulders. This study may improve the understanding of the effect of the boulder spacing and boulder submergence ratio on the near-bed shear stress estimations of stream restoration practices.

Turbulent Mixing Process of a Round Jet With Slot Lobes

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Paul J. Kristo ◽  
Coleman D. Hoff ◽  
Ian G. R. Craig ◽  
Mark L. Kimber
Keyword(s):  
Turbulent Mixing ◽  
Near Field ◽  
Streamwise Velocity ◽  
Velocity Profiles ◽  
Separate Flow ◽  
Geometric Feature ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Round Jet ◽  
Turbulent Statistics

Abstract Turbulent mixing in the near region of a round jet with three slot lobes is examined via mean velocity and turbulent statistics and structures at a Reynolds number of 15,000. The design utilizes separate flow motivations upstream of each geometric feature, deviating from conventional nozzles or orifice plates. Immediate outlet velocity profiles are heavily influenced by opposing pressure gradients between the neighboring round and slot streams. Spanwise mean velocity profiles reveal the majority of the convective exchange between a given slot and the round center occurs in the immediate near field, but has lasting effects on the axial centerline profiles downstream. This is also reflected by the velocity half-widths, exhibiting asymmetry across the entirety of available measurements. Centerline turbulence intensities exhibit strong and short-lived isotropy. The increasingly anisotropic intensities found downstream are lower than similar geometries from the literature, implying that mixing development is inhibited. Reynolds stresses at the round-slot interface are significantly smaller than the round-stagnant exchange, but achieve a symmetric condition at x/D ≅ 4. Two-point spatial correlations of the fluctuating streamwise velocity exhibit stronger dependence toward the axial centerline at the round-slot interface in comparison to the nominal round radius. In contrast, spanwise velocity fluctuations exhibit nearly identical, localized behaviors on each side of the jet. Corresponding differences in streamwise integral length scale peak in the range 1.0 ≤ x/D ≤ 1.5, and so too do the turbulent structures in this area, as a result of the collated jet geometry.

scholarly journals Structure of the Airflow above Surface Waves

2016 ◽  
Vol 46 (5) ◽  
pp. 1377-1397 ◽  
Author(s):  
Marc P. Buckley ◽  
Fabrice Veron
Keyword(s):  
Surface Waves ◽  
Wind Waves ◽  
Quadrant Analysis ◽  
Momentum Exchange ◽  
Turbulent Structures ◽  
Mean Velocity ◽  
Law Of The Wall ◽  
Generate Surface ◽  
Turbulent Momentum ◽  
Sheltering Effect

AbstractIn recent years, much progress has been made to quantify the momentum exchange between the atmosphere and the oceans. The role of surface waves on the airflow dynamics is known to be significant, but our physical understanding remains incomplete. The authors present detailed airflow measurements taken in the laboratory for 17 different wind wave conditions with wave ages [determined by the ratio of the speed of the peak waves Cp to the air friction velocity u* (Cp/u*)] ranging from 1.4 to 66.7. For these experiments, a combined particle image velocimetry (PIV) and laser-induced fluorescence (LIF) technique was developed. Two-dimensional airflow velocity fields were obtained as low as 100 μm above the air–water interface. Temporal and spatial wave field characteristics were also obtained. When the wind stress is too weak to generate surface waves, the mean velocity profile follows the law of the wall. With waves present, turbulent structures are directly observed in the airflow, whereby low-horizontal-velocity air is ejected away from the surface and high-velocity fluid is swept downward. Quadrant analysis shows that such downward turbulent momentum flux events dominate the turbulent boundary layer. Airflow separation is observed above young wind waves (Cp/u*< 3.7), and the resulting spanwise vorticity layers detached from the surface produce intense wave-coherent turbulence. On average, the airflow over young waves (with Cp/u* = 3.7 and 6.5) is sheltered downwind of wave crests, above the height of the critical layer zc [defined by 〈u(zc)〉 = Cp]. Near the surface, the coupling of the airflow with the waves causes a reversed, upwind sheltering effect.

scholarly journals Turbulent mixing and beyond: non-equilibrium processes from atomistic to astrophysical scales I

2013 ◽  
Vol 371 (1982) ◽  
pp. 20120436 ◽  
Author(s):  
S. I. Abarzhi ◽  
S. Gauthier ◽  
K. R. Sreenivasan
Keyword(s):  
Present State ◽  
Turbulent Mixing ◽  
Specific Problem ◽  
Equilibrium Dynamics ◽  
Equilibrium Processes ◽  
Turbulence Mixing ◽  
Broad Variety ◽  
The Subject ◽  
Non Equilibrium

In this Introduction, we summarize and provide a perspective on 11 articles on ‘Turbulent mixing and beyond’. The papers represent the broad variety of themes of the subject, and are concerned with fundamental aspects of turbulence, mixing and non-equilibrium dynamics. While each paper deals with a specific problem, the collection gives a panoramic overview of the subject at its present state of understanding.

Numerical Investigation of Turbulent Flow in a T-Junction

2011 ◽  
Author(s):  
Jungwoo Kim ◽  
Jae Jun Jeong
Keyword(s):  
Thermal Fatigue ◽  
Turbulent Mixing ◽  
Azimuthal Angle ◽  
Dominant Frequency ◽  
Frequency Characteristics ◽  
Turbulent Pipe Flow ◽  
Energy Spectra ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Large Eddy

In the present study, a large eddy simulation, at conditions of experiments by Vattenfall, was performed in order to investigate the phenomenon of turbulent mixing affecting the thermal fatigue in a T-junction. Mean velocity and turbulence intensity from the simulation are compared against those measured in the Vattenfall experiment, and the observed agreement is good. Based on the numerical results from the LES, the energy spectra were examined because they have been regarded as being of primary importance in the study on the thermal fatigue according to previous studies. The energy spectra showed the dominant frequency of around St = 0.5 agrees with the value reported in experiments. Also, near the T-junction the frequency characteristics are shown to depend on the azimuthal angle. However, this dependency of the frequency characteristics on the azimuthal angle is disappeared in downstream because the turbulence in the T-junction eventually approaches the fully-developed turbulent pipe flow. On the other hand, although not shown here, the dominant frequency near the wall is different from that reported at the centerline (St∼1), meaning that the thermal fatigue may not result from the wake structure, and is affected by the vertical structures existing near the wall. This result is consistent with one stated by [1] that the frequency characteristics are closely associated with arched vortices.

scholarly journals Structure of turbulent flow over regular arrays of cubical roughness

2007 ◽  
Vol 589 ◽  
pp. 375-409 ◽  
Author(s):  
O. COCEAL ◽  
A. DOBRE ◽  
T. G. THOMAS ◽  
S. E. BELCHER
Keyword(s):  
Turbulent Flow ◽  
Order Statistics ◽  
Spatial Organization ◽  
Second Order ◽  
Quadrant Analysis ◽  
Simple Estimate ◽  
Second Order Statistics ◽  
Ejections And Sweeps ◽  
Conditional Averaging ◽  
Regular Arrays

The structure of turbulent flow over large roughness consisting of regular arrays of cubical obstacles is investigated numerically under constant pressure gradient conditions. Results are analysed in terms of first- and second-order statistics, by visualization of instantaneous flow fields and by conditional averaging. The accuracy of the simulations is established by detailed comparisons of first- and second-order statistics with wind-tunnel measurements. Coherent structures in the log region are investigated. Structure angles are computed from two-point correlations, and quadrant analysis is performed to determine the relative importance of Q2 and Q4 events (ejections and sweeps) as a function of height above the roughness. Flow visualization shows the existence of low-momentum regions (LMRs) as well as vortical structures throughout the log layer. Filtering techniques are used to reveal instantaneous examples of the association of the vortices with the LMRs, and linear stochastic estimation and conditional averaging are employed to deduce their statistical properties. The conditional averaging results reveal the presence of LMRs and regions of Q2 and Q4 events that appear to be associated with hairpin-like vortices, but a quantitative correspondence between the sizes of the vortices and those of the LMRs is difficult to establish; a simple estimate of the ratio of the vortex width to the LMR width gives a value that is several times larger than the corresponding ratio over smooth walls. The shape and inclination of the vortices and their spatial organization are compared to recent findings over smooth walls. Characteristic length scales are shown to scale linearly with height in the log region. Whilst there are striking qualitative similarities with smooth walls, there are also important differences in detail regarding: (i) structure angles and sizes and their dependence on distance from the rough surface; (ii) the flow structure close to the roughness; (iii) the roles of inflows into and outflows from cavities within the roughness; (iv) larger vortices on the rough wall compared to the smooth wall; (v) the effect of the different generation mechanism at the wall in setting the scales of structures.

scholarly journals Direct numerical simulation of developed shear driven turbulence

2000 ◽  
Vol 18 (2) ◽  
pp. 189-195
Author(s):  
D.V. NEUVAZHAYEV ◽  
N.S. ESKOV ◽  
A.S. KOZLOVSKIKH
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Turbulent Mixing ◽  
Temporal Evolution ◽  
Program Package ◽  
Density Difference ◽  
Gas Flows ◽  
Plane Parallel ◽  
Turbulence Mixing ◽  
Zone Growth

The work is devoted to direct numerical simulation of turbulent mixing by shear driven instability at an interface of two plane-parallel gas flows. The work presents the results obtained in 2D simulations of turbulence being developed at the interface of two almost incompressible gases using the MAX program package. Spatial and temporal evolution of the turbulence zone resulted from shear driven instability is studied. We calculated the constant of shear driven turbulence mixing and investigated how the rate of turbulence zone growth depended on density difference of mixed fluids. Heterogeneity coefficient of the mixture was calculated for all considered density differences.

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