scholarly journals Caracterização da transição laminar-turbulenta em um escoamento de Couette plano devido à Estratificação estável utilizando CFD

2020 ◽  
Vol 42 ◽  
pp. e10
Author(s):  
Jean Jonathan Schuster ◽  
Áttila Leães Rodrigues ◽  
Luis Fernando Camponogara ◽  
Luis Eduardo Medeiros ◽  
Felipe Denardin Costa
Keyword(s):  
Boundary Layer ◽  
Flow Regime ◽  
Thermal Stratification ◽  
Flow Regimes ◽  
Temperature Fields ◽  
Intermittent Flow ◽  
Turbulent Transition ◽  
Numerical Studies ◽  
Stable Conditions ◽  
Current Proposal

Despite recent advances in understanding the physical capacity of the thermal and mechanical parameters that control or isolate the nocturnal boundary layer, these are not yet fully understood. The emergence of natural intermittence in runoff is also not a consensus in the boundary layer scientific community. Many of the studies that present numerical studies on intermittence make use of external flow, forcing that is responsible for the resurgence of turbulence. Thus, the current proposal aims to develop a numerical experiment to study the laminar-turbulent transition using computational fluid dynamics. In this case, the thermal stratification will be applied to a turbulent flow entirely generated to obtain conditions of robust stability and to reproduce an intermittent flow. The results show that when the flow regime is thoroughly turbulent, all levels are coupled by turbulence, making speed and temperature fields more homogeneous in the center of the domain. The results show that when the flow regime is completely turbulent, all levels are coupled by turbulence, making speed and temperature fields more homogeneous in the center of the domain. As a temperature gradient is introduced into the flow, the vertical levels become uncoupled, and under very stable conditions, the turbulence is wholly suppressed. While the distinction between flow regimes is evident, the transition between flow regimes occurs intermittently.

Experimental Investigation of Vertical Downward Two-Phase Flow in Annulus

2020 ◽  
Vol 142 (7) ◽  
Author(s):  
Renato P. Coutinho ◽  
Ligia Tornisiello ◽  
Paulo J. Waltrich
Keyword(s):  
Experimental Data ◽  
Pressure Gradient ◽  
Flow Regime ◽  
Two Phase Flow ◽  
Flow Regimes ◽  
Intermittent Flow ◽  
Phase Flow ◽  
Liquid Holdup ◽  
Two Phase ◽  
Downward Flow

Abstract A limited amount of work exists on gas–liquid flow in vertical pipe annulus, and, to the knowledge of the authors, there is no work on the literature to characterize vertical downward two-phase flow in pipe annulus. In the petroleum industry, downward two-phase in annulus is encountered on liquid-assisted gas-lift (LAGL) unloading and production operations. This study presents experimental data for pressure gradient, liquid holdup, and flow regimes for vertical downward two-phase (air and water) flow in pipe annulus. Also, the applicability of two-phase flow models are evaluated. The experimental results show that the liquid holdup is consistently higher for downward flow in annulus than in pipes for the annular flow regime, and these differences are as high as 45%. When the flow regime map for downward flow in annulus is compared with the ones in the literature for flow in pipes, it is observed that the intermittent flow in pipes occurs at lower liquid velocities than flow in annulus. The comparison between experimental data and model results also shows some discrepancy for liquid holdup and pressure gradient. These differences are high for annular and intermittent flow regimes, with errors of 100% for the liquid holdup and 200% for pressure gradient. However, the errors for bubble flow regime are much smaller, generally lower than 20%.

Gas-Liquid Two-Phase Flow Regimes in Microchannels

2002 ◽  
Author(s):  
M. K. Akbar ◽  
D. A. Plummer ◽  
S. M. Ghiaasiaan
Keyword(s):  
Flow Regime ◽  
Two Phase Flow ◽  
Plug Flow ◽  
Flow Regimes ◽  
Flow Patterns ◽  
Intermittent Flow ◽  
Recent Experimental Data ◽  
Phase Flow ◽  
Two Phase ◽  
Cross Sectional

Recent experimental data dealing with gas-liquid two-phase flow regimes and their transitions in microchannels with circular and near-circular cross-sections are reviewed and compared. It is shown that, for microchannels with hydraulic diameters close to 1 mm, the available data are in good agreement. These data are used as the basis for the development of a simple Weber number-based flow regime map that divides the entire flow map into four zones: a surface tension dominated zone including bubbly and plug flow patterns; an inertia dominated zone representing the annular flow regime; a dispersed/churn flow zone; and a transition zone that consists of other intermittent flow patterns. Comparison is als o made with the limited available data representing channels with slightly larger hydraulic diameters or different cross-sectional geometries, and the effects of channel cross-sectional geometry and size are examined and discussed. The areas in need of further systematic experimental investigation are identified.

Effect of Velocity Ratio on Vortex Shedding From a Short Stack

2006 ◽  
Author(s):  
M. S. Adaramola ◽  
A. O. Oladeinde ◽  
D. Sumner ◽  
D. J. Bergstrom
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Flow Regime ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Velocity Ratio ◽  
Plate Boundary ◽  
Cross Flow ◽  
Flow Regimes ◽  
Flat Plate Boundary Layer

The influence of the jet-to-cross-flow velocity ratio, R, on vortex shedding from a cylindrical stack of aspect ratio AR = 9 was investigated using hot-wire anemometry. The cross-flow Reynolds number was ReD = 2.3×104 and R was varied from 0 to 3. The stack was partially immersed in a flat-plate boundary layer, where the boundary layer thickness-to-height ratio at the location of the stack was δ/H = 0.5. In the downwash flow regime, when R < 0.7, a single Strouhal number (e.g., St = 0.167 at R = 0) was measured along the entire stack height. In the crosswind-dominated flow regime, when 0.7 ≤ R < 1.5, a higher Strouhal number was obtained (e.g., St = 0.185 at R = 1.0). In the transitional and jet-dominated flow regimes, a jump in Strouhal number (e.g., from St = 0.176 to 0.193 at R = 2.0), occurred within the flat-plate boundary layer. The power spectra showed that the shape and strength of the vortex shedding peak changed along the stack height. In general, the peak was more broad-banded near the base of the stack, became sharper and more distinct within the middle of the combined stack and jet wakes, and then reduced in strength in the jet wake and jet regions. In the transitional and jet-dominated flow regimes, the jump in Strouhal number was seen as a gradual change in dominance between two closely spaced peaks, with both peaks co-existing near the edge of the boundary layer.

Numerical Studies on Behavior of Lorentz-Force-Applied Boundary Layer in Supersonic Flow

2009 ◽  
Vol 129 (6) ◽  
pp. 831-839
Author(s):  
Keisuke Udagawa ◽  
Sadatake Tomioka ◽  
Hiroyuki Yamasaki
Keyword(s):  
Boundary Layer ◽  
Supersonic Flow ◽  
Lorentz Force ◽  
Numerical Studies

INFLUENCE OF THE PLATE LEADING-EDGE SHAPE AND THICKNESS ON THE BOUNDARY LAYER LAMINAR-TURBULENT TRANSITION IN HYPERSONIC FLOW

2019 ◽  
Vol 50 (5) ◽  
pp. 461-481
Author(s):  
Sergei Vasilyevich Aleksandrov ◽  
Evgeniya Andreevna Aleksandrova ◽  
Volf Ya. Borovoy ◽  
Andrey Vyacheslavovich Gubernatenko ◽  
Vladimir Evguenyevich Mosharov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Hypersonic Flow ◽  
Leading Edge ◽  
Turbulent Transition ◽  
Laminar Turbulent Transition

Removal of trichloroethylene and trichloroethane using a novel hollow-fibre gas-permeable membrane

2003 ◽  
Vol 3 (5-6) ◽  
pp. 67-72
Author(s):  
S. Takizawa ◽  
T. Win
Keyword(s):  
Boundary Layer ◽  
Flow Regime ◽  
Removal Efficiency ◽  
Residence Time ◽  
Flow Rate ◽  
Air Flow ◽  
Hollow Fibre ◽  
Permeation Rate ◽  
Permeable Membrane ◽  
Diffusional Boundary Layer

In order to evaluate effects of operational parameters on the removal efficiency of trichloroethylene and 1,1,1-trichloroethene from water, lab-scale experiments were conducted using a novel hollow-fibre gaspermeable membrane system, which has a very thin gas-permeable membrane held between microporous support membranes. The permeation rate of chlorinated hydrocarbons increased at higher temperature and water flow rate. On the other hand, the effects of the operational conditions in the permeate side were complex. When the permeate side was kept at low pressure without sweeping air (pervaporation), the removal efficiency of chlorinated hydrocarbon, as well as water permeation rate, was low probably due to lower level of membrane swelling on the permeate side. But when a very small amount of air was swept on the membrane (air perstripping) under a low pressure, it showed a higher efficiency than in any other conditions. Three factors affecting the permeation rate are: 1) reduction of diffusional boundary layer within the microporous support membrane, 2) air/vapour flow regime and short cutting, and 3) the extent of membrane swelling on the permeate side. A higher air flow, in general, reduces the diffusional boundary layer, but at the same time disrupts the flow regime, causes short cutting, and makes the membrane dryer. Due to these multiple effects on gas permeation, there is an optimum operational condition concerning the vacuum pressure and the air flow rate. Under the optimum operational condition, the residence time within the hollow-fibre membrane to achieve 99% removal of TCE was 5.25 minutes. The log (removal rate) was linearly correlated with the average hydraulic residence time within the membrane, and 1 mg/L of TCE can be reduced to 1 μg/L (99.9% removal).

Convective diffusion to a slowly rotating spherical electrode; Basic model for Re → O, Pe → ∞

1983 ◽  
Vol 48 (6) ◽  
pp. 1571-1578 ◽  
Author(s):  
Ondřej Wein
Keyword(s):  
Boundary Layer ◽  
Flow Regime ◽  
Peclet Number ◽  
Convective Diffusion ◽  
Creeping Flow ◽  
Péclet Number ◽  
Spherical Electrode ◽  
Basic Model ◽  
Boundary Layer Solution ◽  
Layer Solution

Theory has been formulated of a convective rotating spherical electrode in the creeping flow regime (Re → 0). The currently available boundary layer solution for Pe → ∞ has been confronted with an improved similarity description applicable in the whole range of the Peclet number.

scholarly journals Ventilation of a Mid-Size City under Stable Boundary Layer Conditions: A Simulation Using the LES Model PALM

Atmosphere ◽  
2021 ◽  
Vol 12 (3) ◽  
pp. 401
Author(s):  
Jonathan Biehl ◽  
Bastian Paas ◽  
Otto Klemm
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Passive Scalar ◽  
City Center ◽  
Street Canyons ◽  
Stable Boundary ◽  
Northwest Germany ◽  
Stable Conditions ◽  
Les Model ◽  
The City

City centers have to cope with an increasing amount of air pollution. The supply of fresh air is crucial yet difficult to ensure, especially under stable conditions of the atmospheric boundary layer. This case study used the PArallelized Large eddy simulation (LES) Model PALM to investigate the wind field over an urban lake that had once been built as a designated fresh air corridor for the city center of Münster, northwest, Germany. The model initialization was performed using the main wind direction and stable boundary layer conditions as input. The initial wind and temperature profiles included a weak nocturnal low-level jet. By emitting a passive scalar at one point on top of a bridge, the dispersion of fresh air could be traced over the lake’s surface, within street canyons leading to the city center and within the urban boundary layer above. The concept of city ventilation was confirmed in principle, but the air took a direct route from the shore of the lake to the city center above a former river bed and its adjoining streets rather than through the street canyons. According to the dispersion of the passive scalar, half of the city center was supplied with fresh air originating from the lake. PALM proved to be a useful tool to study fresh air corridors under stable boundary layer conditions.

Observations of Small-Scale Turbulence and Energy Dissipation Rates in the Cloudy Boundary Layer

2006 ◽  
Vol 63 (5) ◽  
pp. 1451-1466 ◽  
Author(s):  
Holger Siebert ◽  
Katrin Lehmann ◽  
Manfred Wendisch
Keyword(s):  
Boundary Layer ◽  
Energy Dissipation ◽  
Wind Velocity ◽  
Turbulence Theory ◽  
Horizontal Wind ◽  
Inertial Subrange ◽  
Intermittent Flow ◽  
Small Scale ◽  
Dissipation Rates ◽  
Wide Range

Abstract Tethered balloon–borne measurements with a resolution in the order of 10 cm in a cloudy boundary layer are presented. Two examples sampled under different conditions concerning the clouds' stage of life are discussed. The hypothesis tested here is that basic ideas of classical turbulence theory in boundary layer clouds are valid even to the decimeter scale. Power spectral densities S( f ) of air temperature, liquid water content, and wind velocity components show an inertial subrange behavior down to ≈20 cm. The mean energy dissipation rates are ∼10−3 m2 s−3 for both datasets. Estimated Taylor Reynolds numbers (Reλ) are ∼104, which indicates the turbulence is fully developed. The ratios between longitudinal and transversal S( f ) converge to a value close to 4/3, which is predicted by classical turbulence theory for local isotropic conditions. Probability density functions (PDFs) of wind velocity increments Δu are derived. The PDFs show significant deviations from a Gaussian distribution with longer tails typical for an intermittent flow. Local energy dissipation rates ɛτ are derived from subsequences with a duration of τ = 1 s. With a mean horizontal wind velocity of 8 m s−1, τ corresponds to a spatial scale of 8 m. The PDFs of ɛτ can be well approximated with a lognormal distribution that agrees with classical theory. Maximum values of ɛτ ≈ 10−1 m2 s−3 are found in the analyzed clouds. The consequences of this wide range of ɛτ values for particle–turbulence interaction are discussed.

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