Critical velocity shear flow for triggering L-H transition and its parametric dependence in the HL-2A tokamak

In natural streams, vegetation considerably has an influence on the flow characteristics in a variety of ways. For example, vegetation distorts flow structure in both lateral and vertical directions and changes the magnitude of turbulence and shear flow. Due to these effects, diluted contaminants in river transport and disperse differently. Accordingly, many previous researchers have investigated the impact of vegetation on the mixing process. Most of them have estimated the dispersion coefficient since this is the crucial parameter to quantify the degree of dispersion of contaminants numerically. They mainly studied in diverse characteristics of vegetation, such as density or submergence, etc., and identified the change in hydraulic parameters involving the dispersion coefficient.In this work, considering the vegetation distributed in various forms in the natural river, we studied the effect of vegetation patterns on the longitudinal mixing coefficient. Six types of spatial patterns considered in this study are represented numerically by introducing the standardized Morisita index. Laboratory experiments with artificial emergent vegetation were performed in multiple vegetation patterns, and the longitudinal dispersion coefficient was estimated from the measured concentration curves by applying the routing technique. And we analyzed the cause of change in dispersion coefficient by calculating not only the dispersion coefficient but also the magnitude of mean velocity, shear flow, turbulence, etc.According to the experimental results, the mean velocity in the vegetated channel is almost the same regardless of the type of pattern but is always lower than that in the non-vegetated channel. The longitudinal dispersion coefficient gets larger as the arrangement changes from uniform to 2D clumped pattern. The cause of change in coefficient is closely related to the spatial velocity gradients in both lateral and vertical directions since the spatial heterogeneity of velocity increases the magnitude of shear flow.

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Physics of Magnetopause Reconnection: A Study of the Combined Effects of Density Asymmetry, Velocity Shear, and Guide Field

International Journal of Geophysics ◽

10.1155/2010/202583 ◽

2010 ◽

Vol 2010 ◽

pp. 1-17 ◽

Cited By ~ 13

Author(s):

Kentaro G. Tanaka ◽

Masaki Fujimoto ◽

Iku Shinohara

Keyword(s):

Shear Flow ◽

Flow Direction ◽

Electron Flow ◽

Velocity Shear ◽

Field Effects ◽

Guide Field ◽

Sliding Motion ◽

Particle Simulations ◽

Flow Effects ◽

Magnetopause Reconnection

Magnetopause reconnection would be characterized by the density jump across the current sheet, the flow shear across the boundary, and nonzero guide field. While effects of each of these elements have been studied, the effects arising from the combination of these are still unexplored. Two-dimensional full-particle simulations show that the combination of shear flow and/or guide field with density asymmetry induces the sliding motion of theX-line along the magnetopause. The direction of theX-line motion is controlled either by the ion flow at theX-line when the shear flow effects dominate or by the electron flow at theX-line when the guide field effects dominate. The shear flow effects and the guide field effects may counteract each other in determining the direction of theX-line motion and, in the close proximity of the subsolar region where the flow is slow, theX-line motion can be opposite to the flow direction.

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Vortex Shedding From Bluff Bodies in a Shear Flow: A Review

Journal of Fluids Engineering ◽

10.1115/1.3242481 ◽

1985 ◽

Vol 107 (3) ◽

pp. 298-306 ◽

Cited By ~ 59

Author(s):

Owen M. Griffin

Keyword(s):

Shear Flow ◽

Cross Sections ◽

Vortex Shedding ◽

Practical Importance ◽

Flexible Structures ◽

Velocity Shear ◽

Bluff Bodies ◽

End Conditions ◽

Cable Systems ◽

Rectangular Cylinders

This paper examines the effects of velocity shear on vortex shedding from stationary and vibrating bluff bodies. Experiments with circular cylindrical bodies and other cross sections such as D-section cylinders and rectangular cylinders, which were limited to conditions with length/diameter ratios less than L/D = 15 to 20, have shown that the spanwise cellular structure of the vortex shedding is dependent upon end conditions. The vortex shedding also is influenced strongly by the shear flow steepness parameter β¯ which is based upon the incident flow velocity gradient. Experimental evidence is available to show that moderate shear levels of practical importance (β¯∼0.01 to 0.015) do not appreciably decrease the probability of occurrence of vortex-excited oscillations for flexible structures and cables. The effects of incident shear on vortex shedding from stationary and vibrating bluff structures in both fluid media should be investigated further for long cylinders which have minimal end boundary effects. More definitive bounds for and details of this fluid-structure interaction are needed for applications in the wind engineering design of buildings and structures, and in the design of marine structures and cable systems.

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Shear flow instabilities in the earth's plasma sheet region

Annales Geophysicae ◽

10.1007/s00585-994-0025-4 ◽

1994 ◽

Vol 12 (1) ◽

pp. 25-32 ◽

Cited By ~ 1

Author(s):

Sunayna Kalra ◽

G. S. Lakhina

Keyword(s):

Shear Flow ◽

Plasma Sheet ◽

Flow Instability ◽

Velocity Shear ◽

Mhd Equations ◽

Data Sets ◽

Neutral Sheet ◽

Data Set ◽

Central Plasma ◽

Central Plasma Sheet

Abstract. Shear flow instability arising from the velocity shear between the inner and the outer central plasma sheet regions is studied by treating the plasma as compressible. Based on the linearized MHD equations, dispersion relations for the surface wave modes occurring at the boundary of the inner central plasma sheet (ICPS) and the outer central plasma sheet (OCPS) are derived. The growth rates and the eigenmode frequencies are obtained numerically. Three data sets consisting of parameters relevant to the earth's magnetotail are considered. The plasma sheet region is found to be stable for constant plasma flows unless MA>9.6, where MA is the Alfvén Mach number in the ICPS. However, for a continuously varying flow velocity profile in the ICPS, the instability is excited for MA\\geq1.4. The excited modes have oscillation periods of 2-10 min and 1.5-6 s, and typical transverse wavelengths of 30-100 RE and 0.5-6 RE for data sets 1 and 2 (i.e., case of no neutral sheet) respectively. For the data set 3, which corresponds to a neutral sheet at the center of the plasma sheet, the excited oscillations have periods of 2 s-1 min with transverse wavelengths of 0.02-1 RE.

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Effects of Submerged Vegetation Density on Turbulent Flow Characteristics in an Open Channel

Water ◽

10.3390/w11102154 ◽

2019 ◽

Vol 11 (10) ◽

pp. 2154 ◽

Cited By ~ 4

Author(s):

Hanqing Zhao ◽

Jing Yan ◽

Saiyu Yuan ◽

Jiefu Liu ◽

Jinyu Zheng

Keyword(s):

Shear Flow ◽

Turbulent Flow ◽

Low Frequency ◽

Velocity Shear ◽

Generation Rate ◽

Vegetation Density ◽

Frequency Range ◽

Spectral Curves ◽

Flow Type ◽

Turbulent Statistics

The vegetation density λ affects turbulent flow type in the submerged vegetated river. This laboratory study investigates different types of vegetated turbulent flow, especially the flow at 0.04 < λ < 0.1 and λ = 1.44 by setting the experimental λ within a large range. Vertical distributions of turbulent statistics (velocity, shear stress and skewness coefficients), turbulence kinetic generation rate and turbulence spectra in different λ conditions have been presented and compared. Results indicate that for flow at 0.04 < λ < 0.1, the profiles of turbulent statistics manifest characteristics that are similar to those of both the bed-shear flow and the free-shear flow, and the turbulence spectral curves are characterized with some slight humps within the low-frequency range. For λ = 1.44, the turbulent statistics above the vegetation top demonstrate the characteristics of boundary-shear flow. The spectral curves fluctuate intensely within the low-frequency range, and the spectra of low-frequency eddies above vegetation top are significantly larger than the values below. The change of turbulent flow type induced by an increase of λ would increase the maximum value of turbulence kinetic generation rate GS and change the point where GS is vertically maximum upwards to the vegetation top.

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Velocity shear flow over rectangular cylinders with different side ratios

Computers & Fluids ◽

10.1016/j.compfluid.2014.03.002 ◽

2014 ◽

Vol 96 ◽

pp. 35-46 ◽

Cited By ~ 13

Author(s):

Shuyang Cao ◽

Qiang Zhou ◽

Zhiyong Zhou

Keyword(s):

Shear Flow ◽

Velocity Shear ◽

Rectangular Cylinders

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Kinetic Alfvén wave generation by velocity shear in collisionless plasmas

Journal of Plasma Physics ◽

10.1017/s002237782000032x ◽

2020 ◽

Vol 86 (2) ◽

Cited By ~ 1

Author(s):

Teresa Maiorano ◽

Adriana Settino ◽

Francesco Malara ◽

Oreste Pezzi ◽

Francesco Pucci ◽

...

Keyword(s):

Shear Flow ◽

Initial Perturbation ◽

Magnetized Plasma ◽

Velocity Shear ◽

Alfven Wave ◽

Larmor Radius ◽

Small Scale ◽

Linear Wave ◽

Initial State ◽

Kinetic Effects

The evolution of a linearly polarized, long-wavelength Alfvén wave propagating in a collisionless magnetized plasma with a sheared parallel-directed velocity flow is here studied by means of two-dimensional hybrid Vlasov–Maxwell (HVM) simulations. The unperturbed sheared flow has been represented by an exact solution of the HVM set of equations of (Malara et al., Phys. Rev. E, vol. 97, 2018, 053212), thus avoiding spurious oscillations that would arise from the non-stationary initial state and inevitably affect the dynamics of the system. We have considered the evolution of both a small and a moderate amplitude Alfvén wave, in order to separate linear wave–shear flow couplings from kinetic effects, the latter being more relevant for larger wave amplitudes. The phase mixing generated by the shear flow modifies the initial perturbation, leading to the formation of small-scale transverse fluctuations at scales comparable with the proton inertial length/Larmor radius. By analysing both the polarization and group velocity of perturbations in the shear regions, we identify them as kinetic Alfvén waves (KAWs). In the moderate amplitude run, kinetic effects distort the proton distribution function in the shear region. This leads to the formation of a proton beam, at the Alfvén speed and parallel to the magnetic field. Such a feature, due to the parallel electric field associated with KAWs, positively compares with solar wind observations of suprathermal ion populations, suggesting that it may be related to the presence of ion-scale KAW-like fluctuations.

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