scholarly journals On the role of turbulent large-scale streaks in generating sediment ridges

2021 ◽  
Vol 930 ◽  
Author(s):  
Markus Scherer ◽  
Markus Uhlmann ◽  
Aman G. Kidanemariam ◽  
Michael Krayer
Keyword(s):  
Large Scale ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Top Down ◽  
Outer Flow ◽  
Secondary Currents ◽  
Sediment Bed ◽  
Low Speed Streaks ◽  
O 10

The role of turbulent large-scale streaks or large-scale motions in forming subaqueous sediment ridges on an initially flat sediment bed is investigated with the aid of particle resolved direct numerical simulations of open channel flow at bulk Reynolds numbers up to 9500. The regular arrangement of quasi-streamwise ridges and troughs at a characteristic spanwise spacing between 1 and 1.5 times the mean fluid height is found to be a consequence of the spanwise organisation of turbulence in large-scale streamwise velocity streaks. Ridges predominantly appear in regions of weaker erosion below large-scale low-speed streaks and vice versa for troughs. The interaction between the dynamics of the large-scale streaks in the bulk flow and the evolution of sediment ridges on the sediment bed is best described as ‘top-down’ process, as the arrangement of the sediment bedforms is seen to adapt to changes in the outer flow with a time delay of several bulk time units. The observed ‘top-down’ interaction between the outer flow and the bed agrees fairly well with the conceptual model on causality in canonical channel flows proposed by Jiménez (J. Fluid Mech., vol. 842, 2018, P1, § 5.6). Mean secondary currents of Prandtl's second kind of comparable intensity and lateral spacing are found over developed sediment ridges and in single-phase smooth-wall channels alike in averages over ${O}(10)$ bulk time units. This indicates that the secondary flow commonly observed together with sediment ridges is the statistical footprint of the regularly organised large-scale streaks.

scholarly journals Water-Worked Gravel Bed: State-of-the-Art Review

Water ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 694 ◽  
Author(s):  
Ellora Padhi ◽  
Subhasish Dey ◽  
Venkappayya R. Desai ◽  
Nadia Penna ◽  
Roberto Gaudio
Keyword(s):  
State Of The Art ◽  
Flow Characteristics ◽  
Streamwise Velocity ◽  
Secondary Currents ◽  
Gravel Bed ◽  
Natural Stream ◽  
Art Research ◽  
The Comparative Study ◽  
Water Work

In a natural gravel-bed stream, the bed that has an organized roughness structure created by the streamflow is called the water-worked gravel bed (WGB). Such a bed is entirely different from that created in a laboratory by depositing and spreading gravels in the experimental flume, called the screeded gravel bed (SGB). In this paper, a review on the state-of-the-art research on WGBs is presented, highlighting the role of water-work in determining the bed topographical structures and the turbulence characteristics in the flow. In doing so, various methods used to analyze the bed topographical structures are described. Besides, the effects of the water-work on the turbulent flow characteristics, such as streamwise velocity, Reynolds and form-induced stresses, conditional turbulent events and secondary currents in WGBs are discussed. Further, the results form WGBs and SGBs are compared critically. The comparative study infers that a WGB exhibits a higher roughness than an SGB. Consequently, the former has a higher magnitude of turbulence parameters than the latter. Finally, as a future scope of research, laboratory experiments should be conducted in WGBs rather than in SGBs to have an appropriate representation of the flow field close to a natural stream.

scholarly journals The Impact of Mobile Phone Uses in the Developing World

2017 ◽  
Vol 9 (2) ◽  
pp. 20-48 ◽  
Author(s):  
Sylvain K. Cibangu ◽  
Mark Hepworth ◽  
Donna Champion
Keyword(s):  
Information And Communication Technologies ◽  
Mobile Phones ◽  
Large Scale ◽  
Communication Technologies ◽  
Structured Interviews ◽  
Top Down ◽  
Information And Communication ◽  
The Subject ◽  
The Impact

In recent years, the rise of information and communication technologies (ICTs) contrasted with the dire living conditions of the world's poorest has been the subject of debate among industry and academia. However, despite the amount of writings produced on mobile phones, Western bias is surprisingly unbridledly prevailing alongside the fêted dissemination of mobile phones. Expansive literature tends to present the rapid adoption of mobile phones among rural individuals, with little to no indication of how local values and voices are respected or promoted. We undertook semi-structured interviews with 16 rural chiefs to inquire into ways in which mobile phones enabled socio-economic development in the rural Congo. Rather than using quantitative, large-scale, or top-down data, we sought to give voice to chiefs themselves about the role of mobile phones. We found that Western bias dominates the literature and deployment of mobile phones more than usually acknowledged. We suggested some paths forward, while bringing the African communal Utu or Ubuntu culture to the center stage.

On the role of the outer region in the turbulent-boundary-layer bursting process

1994 ◽  
Vol 259 ◽  
pp. 345-373 ◽  
Author(s):  
ROY Y. Myose ◽  
Ron F. Blackwelder
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Outer Region ◽  
Low Speed ◽  
Moderate Reynolds Number ◽  
Eddy Structures ◽  
Low Speed Streaks ◽  
Bursting Process

The dynamics and interaction of turbulent-boundary-layer eddy structures was experimentally emulated. Counter-rotating streamwise vortices and low-speed streaks emulating turbulent-boundary-layer wall eddies were generated by a Görtler instability mechanism. Large-scale motions associated with the outer region of turbulent boundary layer were emulated with — ωzspanwise vortical eddies shed by a periodic non-sinusoidal oscillation of an airfoil. The scales of the resulting eddy structures were comparable to a moderate-Reynolds-number turbulent boundary layer. Results show that the emulated wall-eddy breakdown was triggered by streamwise acceleration associated with the outer region of turbulent boundary layer. This breakdown involved violent mixing between low-speed fluid from the wall eddy and accelerated fluid associated with the outer structure. Although wall eddies can break down autonomously, the presence of and interaction with outer-region — ωzeddies hastened their breakdown. Increasing the — ωzeddy strength resulted in further hastening of the breakdown. Conversely, + ωzeddies were found to delay wall-eddy breakdown locally, with further delays resulting from stronger + ωzeddies. This suggests that the outer region of turbulent boundary layers plays a role in the bursting process.

scholarly journals A predictive inner–outer model for streamwise turbulence statistics in wall-bounded flows

2011 ◽  
Vol 681 ◽  
pp. 537-566 ◽  
Author(s):  
ROMAIN MATHIS ◽  
NICHOLAS HUTCHINS ◽  
IVAN MARUSIC
Keyword(s):  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Gradient Flows ◽  
Streamwise Pressure Gradient ◽  
Logarithmic Region ◽  
Inner Region

A model is proposed with which the statistics of the fluctuating streamwise velocity in the inner region of wall-bounded turbulent flows are predicted from a measured large-scale velocity signature from an outer position in the logarithmic region of the flow. Results, including spectra and all moments up to sixth order, are shown and compared to experimental data for zero-pressure-gradient flows over a large range of Reynolds numbers. The model uses universal time-series and constants that were empirically determined from zero-pressure-gradient boundary layer data. In order to test the applicability of these for other flows, the model is also applied to channel, pipe and adverse-pressure-gradient flows. The results support the concept of a universal inner region that is modified through a modulation and superposition of the large-scale outer motions, which are specific to the geometry or imposed streamwise pressure gradient acting on the flow.

Large-scale structures in a developed flow over a wavy wall

2003 ◽  
Vol 478 ◽  
pp. 257-285 ◽  
Author(s):  
AXEL GÜNTHER ◽  
PHILIPP RUDOLF VON ROHR
Keyword(s):  
Velocity Field ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Water Channel ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Wavy Surface ◽  
Large Scale Structures ◽  
Stress Maximum ◽  
Scale Structures

We address – motivated in part by the findings of Gong et al. (1996) and Miller (1995) – the role of streamwise-oriented large-scale structures in a developed flow between a sinusoidal bottom wall and a flat top wall. Particle image velocimetry (PIV) is used to examine the spatial variation of the velocity in different planes of the flow through a water channel with an aspect ratio of 12:1. The wave amplitude is equal to one tenth of the wall wavelength, Λ, and Reynolds numbers between 500 and 7300, defined with the bulk velocity and the half-height of the channel, are considered. To examine streamwise-oriented structures, the spanwise variation of the velocity field is studied in a plane parallel to the top wall, and in one that intersects the wavy surface at an uphill location. From a proper orthogonal decomposition (POD) of the streamwise velocity fluctuations, we obtain the dominant eigenfunctions with a characteristic spanwise scale of O(1.5Λ), which agrees with the scale of perturbations for the streamwise velocity at laminar conditions. A decomposition of the turbulent velocity field close to the uphill section of the wavy surface reveals smaller structures at a location that coincides with the Reynolds shear stress maximum.

Wall shear stress and velocity in a turbulent axisymmetric boundary layer

1994 ◽  
Vol 259 ◽  
pp. 191-218 ◽  
Author(s):  
Anthony Wietrzak ◽  
Richard M. Lueptow
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Large Scale ◽  
Low Frequency ◽  
Streamwise Velocity ◽  
Outer Flow ◽  
Planar Wall ◽  
Wall Bounded Flows ◽  
Wall Shear

Instantaneous streamwise fluctuations of the wall shear stress have been measured using a hot-element probe in a thick axisymmetric turbulent boundary layer on a cylinder aligned parallel to the flow. The measurements were made at a momentum-thickness Reynolds number Rθ = 3050 and a ratio of boundary-layer thickness to cylinder radius of δ/a = 5.7. The ratio of the r.m.s. of the fluctuation to the mean value of the wall shear stress, $\tau_{rms}/\bar{\tau}$, is about 0.32, a value slightly lower than that for recent measurements for flow over a flat plate. The probability density function of the wall shear stress is similar to that for planar wall-bounded flows within experimental error. The power spectral density of the wall shear stress shows that a cylindrical boundary layer contains less energy at lower frequencies and more energy at higher frequencies than other wall-bounded flows. Analysis of simultaneous measurement of the streamwise wall shear stress and the streamwise velocity using VITA and peak detection suggests that transverse curvature has little effect on the near-wall burst–sweep cycle compared to planar wall-bounded flows. The angle of inclination of the structures is similar to that measured for large-scale structures in planar wall-bounded flows. However, measurements of the cross-correlation between the shear stress and the velocity suggest the existence of smaller structures yawed to the axis of the cylinder. The coherence between shear stress and velocity shows a low frequency associated with the inclined structures and a higher frequency associated with the yawed structures. The yawed structures could have an arrowhead or half-arrowhead shape and may be associated with fluid from the outer flow washing over the cylinder.

scholarly journals Turbulent fluctuations above the buffer layer of wall-bounded flows

2008 ◽  
Vol 611 ◽  
pp. 215-236 ◽  
Author(s):  
JAVIER JIMÉNEZ ◽  
SERGIO HOYAS
Keyword(s):  
Reynolds Number ◽  
Buffer Layer ◽  
Turbulent Flows ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Strong Support ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Normal Velocity ◽  
Channel Statistics

The behaviour of the velocity and pressure fluctuations in the logarithmic and outer layers of turbulent flows is analysed using spectral information and probability density functions from channel simulations at Reτ≤2000. Comparisons are made with experimental data at higher Reynolds numbers. It is found, in agreement with previous investigations, that the intensity profiles of the streamwise and spanwise velocity components have logarithmic ranges that are traced to the widening spectral range of scales as the wall is approached. The same is true for the pressure, both theoretically and observationally, but not for the normal velocity or for the tangential stress cospectrum, although even those two quantities have structures with lengths of the order of several hundred times the wall distance. Because the logarithmic range grows longer as the Reynolds number increases, variables which are ‘attached’ in this sense scale in the buffer layer in mixed units. These results give strong support to the attached-eddy scenario proposed by Townsend (1976), but they are not linked to any particular eddy model. The scaling of the outer modes is also examined. The intensity of the streamwise velocity at fixed y/h increases with the Reynolds number. This is traced to the large-scale modes, and to an increased intensity of the ejections but not of the sweeps. Several differences are found between the outer structures of different flows. The outer modes of the spanwise and wall-normal velocities in boundary layers are stronger than in internal flows, and their streamwise velocities penetrate closer to the wall. As a consequence, their logarithmic layers are thinner, and some of their logarithmic slopes are different. The channel statistics are available electronically at http://torroja.dmt.upm.es/ftp/channels/.

Spatial organization of large- and very-large-scale motions in a turbulent channel flow

2014 ◽  
Vol 749 ◽  
pp. 818-840 ◽  
Author(s):  
Jin Lee ◽  
Jae Hwa Lee ◽  
Jung-Il Choi ◽  
Hyung Jin Sung
Keyword(s):  
Channel Flow ◽  
Large Scale ◽  
Spatial Organization ◽  
Outer Region ◽  
Turbulent Channel Flow ◽  
Streamwise Velocity ◽  
Velocity Fluctuations ◽  
Spatial Features ◽  
Temporal Features ◽  
Low Speed Streaks

AbstractDirect numerical simulations were carried out to investigate the spatial features of large- and very-large-scale motions (LSMs and VLSMs) in a turbulent channel flow ($\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\mathit{Re}_{\tau }=930$). A streak detection method based on the streamwise velocity fluctuations was used to individually trace the cores of LSMs and VLSMs. We found that both the LSM and VLSM populations were large. Several of the wall-attached LSMs stretched toward the outer regions of the channel. The VLSMs consisted of inclined outer LSMs and near-wall streaks. The number of outer LSMs increased linearly with the streamwise length of the VLSMs. The temporal features of the low-speed streaks in the outer region revealed that growing and merging events dominated the large-scale (1–$3\delta $) structures. The VLSMs $({>}3\delta )$ were primarily created by merging events, and the statistical analysis of these events supported that the merging of large-scale upstream structures contributed to the formation of VLSMs. Because the local convection velocity is proportional to the streamwise velocity fluctuations, the streamwise-aligned structures of the positive- and negative-$u$ patches suggested a primary mechanism underlying the merging events. The alignment of the positive- and negative-$u$ structures may be an essential prerequisite for the formation of VLSMs.

Streamwise inclination angle of large wall-attached structures in turbulent boundary layers

2019 ◽  
Vol 877 ◽  
Author(s):  
Rahul Deshpande ◽  
Jason P. Monty ◽  
Ivan Marusic
Keyword(s):  
Boundary Layer ◽  
Inclination Angle ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Turbulent Boundary Layers ◽  
Coherence Analysis ◽  
Data Sets ◽  
Hot Wire ◽  
Spectral Coherence ◽  
O 10

The streamwise inclination angle of large wall-attached structures, in the log region of a canonical turbulent boundary layer, is estimated via spectral coherence analysis, and is found to be approximately $45^{\circ }$. This is consistent with assumptions used in prior attached eddy model-based simulations. Given that the inclination angle obtained via standard two-point correlations is influenced by the range of scales in the turbulent flow (Marusic, Phys. Fluids, vol. 13 (3), 2001, pp. 735–743), the present result is obtained by isolating the large wall-attached structures from the rest of the turbulence. This is achieved by introducing a spanwise offset between two hot-wire probes, synchronously measuring the streamwise velocity at a near-wall and log-region reference location, to assess the wall coherence. The methodology is shown to be effective by applying it to data sets across Reynolds numbers, $Re_{\unicode[STIX]{x1D70F}}\sim O(10^{3})$–$O(10^{6})$.

The Impact of Mobile Phone Uses in the Developing World

2021 ◽  
pp. 315-338
Author(s):  
Sylvain K. Cibangu ◽  
Donna Champion ◽  
Mark Hepworth
Keyword(s):  
Economic Development ◽  
Mobile Phone ◽  
Mobile Phones ◽  
Large Scale ◽  
Structured Interviews ◽  
Top Down ◽  
The World ◽  
Socio Economic Development ◽  
The Impact

Around the turn of the year 2020, unprecedented challenges rocked the world, casting a spotlight on large swathes of the world's populations still unconnected and those connected being gravely plagued by inequalities and racism. The promises of rapid/digital mobile phone dissemination around the world have evaporated. To add to these woes, despite the amount of writings produced on mobile phones, Western bias is surprisingly unbridledly prevailing alongside the fêted wireless connections/apps. Expansive literature tends to present the rapid adoption of mobile phones among rural individuals, with little to no indication of how local values and voices are respected or promoted. This chapter is a study that undertook semi-structured interviews with 16 rural chiefs to inquire into ways in which mobile phones enabled socio-economic development in the rural Congo. Rather than using quantitative, large-scale, or top-down data, the study sought to give voice to chiefs themselves about the role of mobile phones.

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