scholarly journals Estudo do colapso da turbulência em um canal aberto estratificado

2020 ◽  
Vol 42 ◽  
pp. e6
Author(s):  
Cristhian Hilberto Kirinus ◽  
Felipe Pereira Lamaizon ◽  
Luis Fernando Camponogara ◽  
Danilo Nogueira Lemes Junior ◽  
Felipe Denardin Costa
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Thermal Stratification ◽  
Open Channel ◽  
Longwave Radiation ◽  
Temperature Gradients ◽  
Stable Boundary ◽  
Air Layers ◽  
Temperature Homogenization

Shortly after sunset the surface starts its cooling by the emission of longwave radiation. As a result of this process, the air layers along with the surface are cooled, thus giving rise to a Stable Boundary Layer (SBL). This paper aims to perform a numerical simulation of a turbulent flow and observe the effects of stratification on it. To perform this study, a reproduction of a flow in an open channel was made due the fact that the upper wall did not influence the flow, in which a temperature gradient between the lower and upper plates was applied. The CFD software used in this paper was OpenFOAM 2.4.0. The most appropriate solver for the present study is the buoyantPimpleFoam, which is suitable for compressible, turbulent, transient and heat transfer flows. Analyzing the results, it can be observed that before the thermal stratification process was performed the levels are coupled as a function of the turbulence. When this occurs, it causes greater velocity and temperature homogenization in the central region of the domain. For larger temperature gradients, it can be observed a laminarization of the flow due the thermal forcing being bigger than the mechanical ones.

scholarly journals Thermal Submesoscale Motions in the Nocturnal Stable Boundary Layer. Part 1: Detection and Mean Statistics

2021 ◽  
Author(s):  
Lena Pfister ◽  
Karl Lapo ◽  
Larry Mahrt ◽  
Christoph K. Thomas
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Static Stability ◽  
Detection Algorithm ◽  
Cold Pool ◽  
Stable Boundary ◽  
Novel Approach ◽  
Order Of Magnitude ◽  
Air Layers ◽  
Objective Detection

AbstractSubmesoscale motions within the stable boundary layer were detected during the Shallow Cold Pool Experiment conducted in the Colorado plains, Colorado, U.S.A. in 2012. The submesoscale motion consisted of two air layers creating a well-defined front with a sharp temperature gradient, and further-on referred to as a thermal submesofront (TSF). The semi-stationary TSFs and their advective velocities are detected and determined by the fibre-optic distributed-sensing (FODS) technique. An objective detection algorithm utilizing FODS measurements is able to detect the TSF boundary, which enables a detailed investigation of its spatio–temporal statistics. The novel approach in data processing is to conditionally average any parameter depending on the distance between a TSF boundary and the measurement location. By doing this, a spatially-distributed feature like TSFs can be characterized by point observations and processes at the TSF boundary can be investigated. At the TSF boundary, the air layers converge, creating an updraft, strong static stability, and vigorous mixing. Further, the TSF advective velocity of TSFs is an order of magnitude lower than the mean wind speed. Despite being gentle, the topography plays an important role in TSF formation. Details on generating mechanisms and implications of TSFs on the stable boundary layer are discussed in Part 2.

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.

scholarly journals Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

2021 ◽  
Author(s):  
Lena Pfister ◽  
Karl Lapo ◽  
Larry Mahrt ◽  
Christoph K. Thomas
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Side Wall ◽  
Cold Pool ◽  
Continuous Data ◽  
Valley Bottom ◽  
Near Surface ◽  
Cold Air ◽  
Stable Boundary ◽  
Surface Temperatures

AbstractIn the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

Tethered Balloon Observations Of The Nocturnal Stable Boundary Layer In A Valley

2000 ◽  
Vol 97 (1) ◽  
pp. 1-24 ◽  
Author(s):  
J. J. Holden ◽  
S. H. Derbyshire ◽  
S. E. Belcher
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Tethered Balloon ◽  
Stable Boundary

On stable boundary-layer height estimation using backscatter lidar data and variance processing

2021 ◽  
Author(s):  
Marcos Paulo Araujo da Silva ◽  
Constantino Muñoz-Porcar ◽  
Umar Saeed ◽  
Francesc Rey ◽  
Maria Teresa Pay ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Adaptive Estimation ◽  
Microwave Radiometer ◽  
Minimum Variance ◽  
Initial Guess ◽  
Layer Height ◽  
Stable Boundary ◽  
Boundary Layer Height ◽  
Temporal Variance

<p>This study describes a method to estimate the nocturnal stable boundary layer height (SBLH) by means of lidar observations. The method permits two approaches which yield independent retrievals through either spatial or temporal variance vertical profiles of the attenuated backscatter. Then, the minimum variance region (MVR) on this profile is identified. Eventually, when multiple MVRs are detected, a temperature-based SBLH estimation derived from radiosonde, launched within the searching time, is used to disambiguate the initial guess. In order to test the method, two study cases employing lidar-ceilometer (Jenoptik CHM 15k Nimbus) measurements are investigated. Temperature-based estimates from a collocated microwave radiometer permitted validation, using either temporal or spatial backscatter variances. The dataset was collected during the HD(CP)2 Observational Prototype Experiment (HOPE) [1].   </p><p>[1] U. Saeed, F. Rocadenbosch, and S. Crewell, “Adaptive Estimation of the Stable Boundary Layer Height Using Combined Lidar and Microwave Radiometer Observations,” IEEE Trans. Geosci. Remote Sens., 54(12), 6895–6906 (2016), DOI: 10.1109/TGRS.2016.2586298.</p><p>[2] U. Löhnert, J. H. Schween, C. Acquistapace, K. Ebell, M. Maahn, M. Barrera-Verdejo, A. Hirsikko, B. Bohn, A. Knaps, E. O’Connor, C. Simmer, A. Wahner, and S. Crewell, “JOYCE: Jülich Observatory for Cloud Evolution,” Bulletin of the American Meteorological Society, 96(7), 1157-1174 (2015). DOI: 10.1175/BAMS-D-14-00105.1</p>

scholarly journals Local Characteristics of the Nocturnal Boundary Layer in Response to External Pressure Forcing

2017 ◽  
Vol 56 (11) ◽  
pp. 3035-3047 ◽  
Author(s):  
Steven J. A. van der Linden ◽  
Peter Baas ◽  
J. Antoon van Hooft ◽  
Ivo G. S. van Hooijdonk ◽  
Fred C. Bosveld ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Stable Boundary Layer ◽  
Dynamical Behavior ◽  
Geostrophic Wind ◽  
External Pressure ◽  
Near Surface ◽  
Stable Boundary ◽  
Wind Speeds ◽  
Turbulent Characteristics

AbstractGeostrophic wind speed data, derived from pressure observations, are used in combination with tower measurements to investigate the nocturnal stable boundary layer at Cabauw, the Netherlands. Since the geostrophic wind speed is not directly influenced by local nocturnal stability, it may be regarded as an external forcing parameter of the nocturnal stable boundary layer. This is in contrast to local parameters such as in situ wind speed, the Monin–Obukhov stability parameter (z/L), or the local Richardson number. To characterize the stable boundary layer, ensemble averages of clear-sky nights with similar geostrophic wind speeds are formed. In this manner, the mean dynamical behavior of near-surface turbulent characteristics and composite profiles of wind and temperature are systematically investigated. The classification is found to result in a gradual ordering of the diagnosed variables in terms of the geostrophic wind speed. In an ensemble sense the transition from the weakly stable to very stable boundary layer is more gradual than expected. Interestingly, for very weak geostrophic winds, turbulent activity is found to be negligibly small while the resulting boundary cooling stays finite. Realistic numerical simulations for those cases should therefore have a comprehensive description of other thermodynamic processes such as soil heat conduction and radiative transfer.

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