Two mechanisms of modulation of very-large-scale motions by inertial particles in open channel flow

2019 ◽  
Vol 868 ◽  
pp. 538-559 ◽  
Author(s):  
G. Wang ◽  
D. H. Richter
Keyword(s):  
Channel Flow ◽  
Large Scale ◽  
Open Channel ◽  
Reynolds Shear Stress ◽  
Outer Region ◽  
Open Channel Flow ◽  
Inertial Particles ◽  
Turbulent Structures ◽  
Outer Flow ◽  
Particle Distributions

Very-large-scale motions (VLSMs) and large-scale motions (LSMs) coexist at moderate Reynolds numbers in a very long open channel flow. Direct numerical simulations two-way coupled with inertial particles are analysed using spectral information to investigate the modulation of VLSMs. In the wall-normal direction, particle distributions (mean/preferential concentration) exhibit two distinct behaviours in the inner flow and outer flow, corresponding to two highly anisotropic turbulent structures, LSMs and VLSMs. This results in particle inertia’s non-monotonic effects on the VLSMs: low inertia (based on the inner scale) and high inertia (based on the outer scale) both strengthen the VLSMs, whereas moderate and very high inertia have little influence. Through conditional tests, low- and high-inertia particles enhance VLSMs following two distinct routes. Low-inertia particles promote VLSMs indirectly through the enhancement of the regeneration cycle (the self-sustaining mechanism of LSMs) in the inner region, whereas high-inertia particles enhance the VLSM directly through contribution to the Reynolds shear stress at similar temporal scales in the outer region. This understanding also provides more general insight into inner–outer interaction in high-Reynolds-number, wall-bounded flows.

scholarly journals Experimental study on the role of spanwise vorticity and vortex filaments in the outer region of open-channel flow

2014 ◽  
Vol 52 (4) ◽  
pp. 476-489 ◽  
Author(s):  
Qigang Chen ◽  
Ronald J. Adrian ◽  
Qiang Zhong ◽  
Danxun Li ◽  
Xingkui Wang
Keyword(s):  
Experimental Study ◽  
Channel Flow ◽  
Open Channel ◽  
Outer Region ◽  
Open Channel Flow ◽  
Vortex Filaments ◽  
Spanwise Vorticity

scholarly journals Flow Partitioning in Rectangular Open Channel Flow

2018 ◽  
Vol 2018 ◽  
pp. 1-7
Author(s):  
Yu Han ◽  
Shu-Qing Yang ◽  
Muttucumaru Sivakumar ◽  
Liu-Chao Qiu ◽  
Jian Chen
Keyword(s):  
Shear Stress ◽  
Channel Flow ◽  
Open Channel ◽  
Laser Doppler Anemometry ◽  
Reynolds Shear Stress ◽  
Three Dimensional ◽  
Open Channel Flow ◽  
Flow Region ◽  
Mathematical Tool ◽  
Measured Velocity

Hydraulic engineers often divide a flow region into subregions to simplify calculations. However, the implementation of flow divisibility remains an open issue and has not yet been implemented as a fully developed mathematical tool for modeling complex channel flows independently of experimental verification. This paper addresses whether a three-dimensional flow is physically divisible, meaning that division lines with zero Reynolds shear stress exist. An intensive laboratory investigation was conducted to carefully measure the time-averaged velocity in a rectangular open channel flow using a laser Doppler anemometry system. Two innovative methods are employed to determine the locations of division lines based on the measured velocity profile. The results clearly reveal that lines with zero total shear stress are discernible, indicating that the flow is physically divisible. Moreover, the experimental data were employed to test previously proposed methods of calculating division lines, and the results show that Yang and Lim’s method is the most reasonable predictor.

scholarly journals Effect of aspect ratio on higher order moments of velocity fluctuations in hydraulically rough open channel flow

2018 ◽  
Vol 40 ◽  
pp. 05045
Author(s):  
Minakshee Mahananda ◽  
Prashanth Reddy Hanmaiahgari
Keyword(s):  
Aspect Ratio ◽  
Order Statistics ◽  
Channel Flow ◽  
Open Channel ◽  
Outer Region ◽  
Open Channel Flow ◽  
Higher Order ◽  
Higher Order Statistics ◽  
Velocity Fluctuations ◽  
Bed Roughness

The effect of aspect ratio on the higher order statistics of velocity fluctuations in a hydraulically rough turbulent open channel flow is investigated. In this regard, an experiment was conducted in a rough bed narrow open channel flow of aspect ratio equal to three and the instantaneous flow velocities were measured using a Nortek Vectrino+ Acoustic Doppler Velocimeter. To understand the effect of aspect ratio, the results obtained from the present study are compared with the literature data of approximately same Reynolds number and bed roughness in a wide open channel flow for turbulence intensities and higher order statistics of velocity fluctuations. Comparison of turbulence intensities between Narrow OCF and Wide OCF shows occurrence of higher streamwise and vertical turbulence intensities in the outer region of Narrow OCF. The results of third order moments of velocity fluctuations are sensitive to aspect ratio in the outer region.

scholarly journals Drag forces on a bed particle in open-channel flow: effects of pressure spatial fluctuations and very-large-scale motions

2019 ◽  
Vol 863 ◽  
pp. 494-512 ◽  
Author(s):  
S. M. Cameron ◽  
V. I. Nikora ◽  
I. Marusic
Keyword(s):  
Channel Flow ◽  
Drag Force ◽  
High Frequency ◽  
Large Scale ◽  
Open Channel ◽  
Low Frequency ◽  
Open Channel Flow ◽  
Stereoscopic Particle Image Velocimetry ◽  
Force Fluctuations ◽  
Drag Forces

The fluctuating drag forces acting on spherical roughness elements comprising the bed of an open-channel flow have been recorded along with synchronous measurements of the surrounding velocity field using stereoscopic particle image velocimetry. The protrusion of the target particle, equipped with a force sensor, was systematically varied between zero and one-half diameter relative to the hexagonally packed adjacent spheres. Premultiplied spectra of drag force fluctuations were found to have bimodal shapes with a low-frequency (${\approx}0.5~\text{Hz}$) peak corresponding to the presence of very-large-scale motions (VLSMs) in the turbulent flow. The high-frequency ($\gtrapprox 4~\text{Hz}$) region of the drag force spectra cannot be explained by velocity time series extracted from points around the particle, but instead appears to be dominated by the action of pressure gradients in the overlying flow field. For small particle protrusions, this high-frequency region contributes a majority of the drag force variance, while the relative importance of the low-frequency drag force fluctuations increases with increasing protrusion. The amplitude of high-frequency drag force fluctuations is modulated by the VLSMs irrespective of particle protrusion. These results provide some insight into the mechanics of bed particle stability and indicate that the optimum conditions for particle entrainment may occur when a low-pressure region embedded in the high-velocity portion of a VLSM overlays a particle.

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