scholarly journals Formation Mechanism of Dust Devil–Like Vortices in Idealized Convective Mixed Layers

2013 ◽  
Vol 70 (4) ◽  
pp. 1173-1186 ◽  
Author(s):  
Junshi Ito ◽  
Hiroshi Niino ◽  
Mikio Nakanishi
Keyword(s):  
Formation Mechanism ◽  
Mixed Layer ◽  
Weather Conditions ◽  
Small Scale ◽  
Material Surface ◽  
Dust Devil ◽  
Dust Devils ◽  
Eddy Simulation ◽  
Convective Mixed Layer ◽  
Mixed Layers

Abstract Dust devils are small-scale vertical vortices often observed over deserts or bare land during the daytime under fair weather conditions. Previous numerical studies have demonstrated that dust devil–like vertical vortices can be simulated in idealized convective mixed layers in the absence of background winds or environmental shear. Their formation mechanism, however, has not been completely clarified. In this paper, the authors attempt to clarify the vorticity source of a dust devil–like vortex by means of a large-eddy simulation, in which a material surface initially placed in the vortex is tracked backward and the circulation on the material surface is examined. The material surface is found to originate from downdrafts, which already have sufficient circulation. As the material surface converges toward the vortex, the vorticity is increased because of conservation of circulation. It is shown that a convective mixed layer is inherently accompanied by circulation, which is scaled by a product of the convective velocity scale and the depth of the convective mixed layer. This circulation is considered to be originally generated by tilting of baroclinically generated horizontal vorticity principally at middepths of the convective mixed layer.

Large Eddy Simulation of Dust Devils in a Diurnally-Evolving Convective Mixed Layer

1500 ◽  
Vol 999991 (9991) ◽  
pp. 9963-9977 ◽  
Author(s):  
Junshi dummyITO ◽  
Ryo dummyTANAKA ◽  
Hiroshi dummyNIINO ◽  
Mikio dummyNAKANISHI
Keyword(s):  
Large Eddy Simulation ◽  
Mixed Layer ◽  
Dust Devils ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Convective Mixed Layer

scholarly journals Investigating microscale patchiness of motile microbes driven by the interaction of turbulence and gyrotaxis in a 3D simulated convective mixed layer.

2021 ◽  
Author(s):  
Alexander Christensen ◽  
Matthew Piggott ◽  
Erik van Sebille ◽  
Maarten van Reeuwijk ◽  
Samraat Pawar
Keyword(s):  
Mixed Layer ◽  
Fluid Model ◽  
Spatial Dynamics ◽  
Critical Factor ◽  
Trophic Levels ◽  
Small Scale ◽  
Swimming Velocity ◽  
Primary Role ◽  
Fluid Surface ◽  
Convective Mixed Layer

Abstract Microbes play a primary role in aquatic ecosystems and biogeochemical cycles. Spatial patchiness is a critical factor underlying these activities, influencing biological productivity, nutrient cycling and dynamics across trophic levels. Incorporating spatial dynamics into microbial models is a long-standing challenge, particularly where small-scale turbulence is involved. Here, we combine a fully 3D direct numerical simulation of convective mixed layer turbulence, with an individual-based microbial model to test the key hypothesis that the coupling of gyrotactic motility and turbulence drives intense microscale patchiness. The fluid model simulates turbulent convection caused by heat loss through the fluid surface, for example during the night, during autumnal or winter cooling or during a cold-air outbreak. We find that under such conditions, turbulence-driven patchiness is depth-structured and requires high motility: Near the fluid surface, intense convective turbulence overpowers motility, homogenising motile and non-motile microbes approximately equally. At greater depth, in conditions analogous to a thermocline, highly motile microbes can be over twice as patch-concentrated as non-motile microbes, and can substantially amplify their swimming velocity by efficiently exploiting fast-moving packets of fluid. Our results substantiate the predictions of earlier studies, and demonstrate that turbulence-driven patchiness is not a ubiquitous consequence of motility but rather a delicate balance of motility and turbulent intensity.

Small-scale diffusion experiments in the Baltic surface-mixed layer under different weather conditions

1978 ◽  
Vol 31 (6) ◽  
pp. 195-215 ◽  
Author(s):  
Friedrich Schott ◽  
Mafred Ehlers ◽  
Lutz Hubrich ◽  
Detlef Quadfasel
Keyword(s):  
Mixed Layer ◽  
Weather Conditions ◽  
Small Scale ◽  
Surface Mixed Layer ◽  
The Baltic

scholarly journals Baroclinic Instability in the Presence of Convection

2018 ◽  
Vol 48 (1) ◽  
pp. 45-60 ◽  
Author(s):  
Jörn Callies ◽  
Raffaele Ferrari
Keyword(s):  
Numerical Simulations ◽  
Mixed Layer ◽  
Baroclinic Instability ◽  
Relative Strength ◽  
Small Scale ◽  
Upper Ocean ◽  
Bulk Properties ◽  
Scale Turbulence ◽  
Balanced Dynamics ◽  
Mixed Layers

AbstractBaroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.

scholarly journals Large Eddy Simulation on Dust Suspension in a Convective Mixed Layer

SOLA ◽  
2010 ◽  
Vol 6 ◽  
pp. 133-136 ◽  
Author(s):  
Junshi Ito ◽  
Hiroshi Niino ◽  
Mikio Nakanishi
Keyword(s):  
Large Eddy Simulation ◽  
Mixed Layer ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Convective Mixed Layer

scholarly journals Large-Eddy Simulation Study of Flow and Heat Transfer in Swirling and Non-Swirling Impinging Jets on a Semi-Cylinder Concave Target

2021 ◽  
Vol 11 (15) ◽  
pp. 7167
Author(s):  
Liang Xu ◽  
Xu Zhao ◽  
Lei Xi ◽  
Yonghao Ma ◽  
Jianmin Gao ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Wall Temperature ◽  
Target Surface ◽  
Impinging Jets ◽  
Small Scale ◽  
Complex Flow ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy

Swirling impinging jet (SIJ) is considered as an effective means to achieve uniform cooling at high heat transfer rates, and the complex flow structure and its mechanism of enhancing heat transfer have attracted much attention in recent years. The large eddy simulation (LES) technique is employed to analyze the flow fields of swirling and non-swirling impinging jet emanating from a hole with four spiral and straight grooves, respectively, at a relatively high Reynolds number (Re) of 16,000 and a small jet spacing of H/D = 2 on a concave surface with uniform heat flux. Firstly, this work analyzes two different sub-grid stress models, and LES with the wall-adapting local eddy-viscosity model (WALEM) is established for accurately predicting flow and heat transfer performance of SIJ on a flat surface. The complex flow field structures, spectral characteristics, time-averaged flow characteristics and heat transfer on the target surface for the swirling and non-swirling impinging jets are compared in detail using the established method. The results show that small-scale recirculation vortices near the wall change the nearby flow into an unstable microwave state, resulting in small-scale fluctuation of the local Nusselt number (Nu) of the wall. There is a stable recirculation vortex at the stagnation point of the target surface, and the axial and radial fluctuating speeds are consistent with the fluctuating wall temperature. With the increase in the radial radius away from the stagnation point, the main frequency of the fluctuation of wall temperature coincides with the main frequency of the fluctuation of radial fluctuating velocity at x/D = 0.5. Compared with 0° straight hole, 45° spiral hole has a larger fluctuating speed because of speed deflection, resulting in a larger turbulence intensity and a stronger air transport capacity. The heat transfer intensity of the 45° spiral hole on the target surface is slightly improved within 5–10%.

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