Atmospheric Boundary Layer Profiles Research Articles

Wind Loads on Rooftop Solar Panels for a Flat-roof Cubic Building

Objective: Rooftop solar installations may be susceptible to significant damage during strong winds. With the increase in solar photovoltaic generation, most building wind codes need to be updated to provide relevant wind resistance design information. The present study aims to estimate wind loads on rooftop solar panels for a cubic building under the design wind speed specified by the Swiss wind code. Methods: Wind tunnel tests and computational fluid dynamics modeling were carried out to determine lift force coefficients for rooftop solar panels on a cubic building. The tests involved a single panel connected to a force balance, and in a subsequent phase, a panel array with pressure taps located on the upper and lower surfaces of the panels. The computational model was initially validated using measured data, and then wind loads were computed under an atmospheric boundary layer profile for the design wind speed. Results: Lift force coefficients for rooftop solar panels on a cubic building were measured and computed for various building orientations. When the flow directly impinged on the lower face of the panels, maximum lift coefficients as high as 0.7 were computed. Conclusion: Adding shielding backplates, ensuring system porosity, and mounting panels together are effective strategies for reducing loads.

Surrogate Modeling of the Aeroacoustics of an NM80 Wind Turbine

Wind turbines play a major role in the European Green Deal for clean energy transition. Noise is a critical aspect among open technological issues, as it determines the possibility of onshore installations near inhabited places and the possible detrimental effects on wildlife when offshore. This paper assesses the accuracy of different approaches to predicting the sound pressure level (SPL) of a wind turbine. The 2.75 MW Neg Micon NM80 horizontal axis wind turbine (HWAT) was simulated in OpenFOAM, modeling the turbine with the actuator line method (ALM) implemented in the turbinesFoam library. Two different inflow conditions were considered: a stationary inflow with a typical atmospheric boundary layer profile and a time-dependent inflow derived from a precursor channel with fully turbulent conditions. The surrogate model for noise prediction used for this work is based on the synthetic/surrogate acoustics models (SAMs) of Amiet and Brooks-Pope-Marcolini (BPM). This approach allows for blade motion modeling and the prediction of the SPL of the URANS postprocessing results. The SPL spectrum obtained was then compared to the results from the other aeroacoustic solvers of IEA Task 39 participants, showing the best performance in the fully turbulent case. The results demonstrate that coupling between the ALM and surrogate acoustics provides more accurate results than the blade element momentum (BEM) approach.

Effect of upstream flow characteristics on the wake topology of a square-back truck

The influence of upstream flow characteristics on the bi-stable flow structure in the wake region of a simplified square-back heavy vehicle model at a Reynolds number of 2.7 × 104 was investigated by using the improved delayed detached eddy simulation method. The asymmetric wake structure of this model and its corresponding aerodynamic response were examined, aiming to identify the effect mechanism of three inlet profiles on the asymmetric wake structure of the named ground transportation system (GTS) model in simulations. The accuracy of the numerical method used in this study was validated by comparison with wake structure data, including the flow states, vortex core's location, and aerodynamic drag obtained from previous large eddy simulations and water channel experiments. The numerical results show that different turbulent inlet velocity profiles lead to different wake topologies. When the turbulent velocity profile with a turbulence intensity of 15% generated by TurbSim, a stochastic inflow turbulence tool for generating turbulent velocity inlet on an atmospheric boundary layer profile, is used, the expected bi-stable flow topology is still observed, but it is not shown in the case by means of the turbulence generator incorporated into ANSYS Fluent. Those turbulent inlet velocity profiles contribute to the increase in GTS model's aerodynamic drag forces. Compared to the uniform velocity profile, the TurbSim velocity profile can achieve a drag increase in 7.23%. In addition, this turbulent profile intensifies the flow fluctuations in the wake region and enhances the transient response frequency of the wake region. Thus, when assessing the vehicle aerodynamic performance in open air, especially under crosswinds, the real turbulence velocity profile, e.g., the profile generated by TurbSim in the current study, is recommended to be used for a more accurate prediction in numerical simulations.

Large-eddy simulation of wind-driven flame in the atmospheric boundary layer

The interaction between the natural ambient winds found in the atmospheric boundary layer (ABL) and buoyant flames is crucial in broad applications in the scientific and engineering fields. Unlike the buoyancy-driven pool fires in still air that have been studied extensively, the complexity of physics changes significantly in wind presence. This study aims at analysing the characteristics of boundary-layer turbulence in the presence of large fires. The eddy dissipation concept, finite volume discrete ordinate method, and one k-equation model are used for combustion, thermal radiation, and sub-grid scale closure using the Large-Eddy Simulation (LES), respectively. A numerical model on simple cases is validated first to assess its capability to reproduce available experimental observations for a purely buoyant fire in still air. A forced-flow boundary layer combustion in a small chamber with a smooth inflow is further considered. In general, good agreement between the simulation results and available experimental data was achieved for temperature and velocity profiles. The unsteady inflow condition used to consider incoming atmospheric turbulence is generated through a precursor simulation. The wind interaction with the line fire changes the atmospheric boundary layer profile affecting the heat transfer ahead of the flame, thereby creating counter-rotating structures downstream. It is shown that the buoyancy-dominated flow due to the flame reaction induced local pressure variation and perturbed shear flow near the ground, thereby altering the wind speed through which the plume rises. Richardson number was also used as a dominant non-dimensional group to analyse the variation of enhanced flow vertical velocity with distance from the fire source. Thus, it is understood that the pronounced longitudinal shear spreading at the surface affects the behaviour of short term or puff releases, suggesting the shedding of small eddies during the combustion process.

2D-CFD Analyses of Flow Controlling Plates on Gable Roof Geometry Cross-Sections for Light Air to Strong Breeze Wind Speed Classifications-an Unsteady RANS Approach

The flow field analysis has been numerically performed on the effectiveness of a flow control mechanism called the Flow Controlling Plate (FCP) on buildings. For this purpose, the gable roof geometry has been considered as a common urban element in the western residential architecture. As the justification step towards the functionality of the concept of FCPs, the 2D numerical investigation of the flow field under the realistic assumptions of atmospheric boundary layer profiles for the spectrum ranging from the so-called light air to strong breeze wind speed classifications have been performed. The CFD (Computational Fluid Dynamics) field calculations have been conveyed as an unsteady case for the flow around a bluff body, using RANS (Reynolds Average Navier-Stokes) averaging methods targeting a solution of Navier-Stokes equations of the fluid flow. The results have proven the hypotheses of the contribution of the FCPs on preventing the flow separation on a partial region of the surface and improving the boundary layer development on the rest of the gable roof facades, which have led to a drastic reduction in the convective heat transfer coefficient as well as the drag force exerted on the roof

One-way fluid-structure interaction of a medium-sized heliostat using scale-resolving CFD simulation

Vortex shedding and the resultant transient loadings on a medium-sized heliostat are investigated in this paper. Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) of a simplified version of an operational heliostat, the LH-2, is used as a validation case for mean loads and to define the orientations used for fluid-structure interaction (FSI): maximum drag in the upright position, and maximum torque-tube moment at an elevation of 60° relative to the vertical. Optimized Atmospheric Boundary Layer profiles for mean flow and turbulence intensity were implemented as the inlet flow boundary condition for both the RANS and Stress-Blended Eddy Simulation (SBES) simulations, with the latter using the synthetic turbulence technique, the Vortex Method, at the inlet. The SBES results show a strong likeness to the experimental results of Peterka et al. (1986) with a comparable mean and peak loading distribution. The transient SBES CFD pressure was implemented in a one-way FSI simulation to obtain the structural response of the heliostat to the transient wind loading. The results show that the response of the heliostat conformed to and depended on the mode shapes and frequencies of the heliostat structure more so than the vortex shedding frequencies. For the upright case, the Strouhal number obtained was within 8% of that obtained from an experimental study in literature, with the main vortex-shedding frequency shown to be 0.5 Hz. When excited with this wind, the structure responded with the third modal frequency of 2.7 Hz. The 60° elevation case also responded mainly with the third mode of 2.6 Hz being excited, but the response was significantly influenced by a combination of higher modes located around 6 Hz. The results from the transient structural analysis using the temporal SBES heliostat surface pressure fields as input indicate that the method holds promise in predicting the transient response of heliostats. Importantly it can be concluded that due to the difference in frequencies between the vortex shedding and modal frequencies, the structure is safe from self-excitation.

Design and Evaluation of Sensor Housing for Boundary Layer Profiling Using Multirotors.

Traditional configurations for mounting Temperature–Humidity (TH) sensors on multirotor Unmanned Aerial Systems (UASs) often suffer from insufficient radiation shielding, exposure to mixed and turbulent air from propellers, and inconsistent aspiration while situated in the wake of the UAS. Descent profiles using traditional methods are unreliable (when compared to an ascent profile) due to the turbulent mixing of air by the UAS while descending into that flow field. Consequently, atmospheric boundary layer profiles that rely on such configurations are bias-prone and unreliable in certain flight patterns (such as descent). This article describes and evaluates a novel sensor housing designed to shield airborne sensors from artificial heat sources and artificial wet-bulbing while pulling air from outside the rotor wash influence. The housing is mounted above the propellers to exploit the rotor-induced pressure deficits that passively induce a high-speed laminar airflow to aspirate the sensor consistently. Our design is modular, accommodates a variety of other sensors, and would be compatible with a wide range of commercially available multirotors. Extensive flight tests conducted at altitudes up to 500 m Above Ground Level (AGL) show that the housing facilitates reliable measurements of the boundary layer phenomena and is invariant in orientation to the ambient wind, even at high vertical/horizontal speeds (up to 5 m/s) for the UAS. A low standard deviation of errors shows a good agreement between the ascent and descent profiles and proves our unique design is reliable for various UAS missions.

On the Horizontal Homogeneity of the Atmospheric Boundary Layer Profile in CFD Simulations

On the Horizontal Homogeneity of the Atmospheric Boundary Layer Profile in CFD Simulations

CFD analysis for siting of wind turbines on high-rise buildings

Extracting energy from wind by placing wind turbines over tall buildings is an emerging idea for ”greener” cities. This paper discusses the best location for placing wind turbines based on numerical simulations. Different building geometries are considered by varying the dimension and number of buildings. Here real atmospheric boundary layer profiles are created with the help of the Weather Research and Forecasting model and also compared with the theoretical profiles from power law equation. Outputs from the simulations show the location where wind velocity is maximum and the possibility of placing wind turbines on the top as well as in the gap between the buildings. The paper also describes the significance of using realistic profiles instead of theoretical profiles.

Realistic boundary conditions for the simulation of atmospheric boundary layer flows using an improved k–ε model

Atmospheric flow simulations in engineering practice – such as simulations of pollutant dispersion, wind load on buildings, wind climate of urban areas – are fairly sensitive to the inlet boundary conditions. The objective of the present paper is to introduce suitable profiles for velocity and turbulent quantities, which do not only fit the measured vertical distributions, but also guarantee the overall consistency of the turbulence model. If the latter aspect is not verified, the inlet profiles deteriorate rapidly within the computational domain, leading to unrealistic wind profiles in the area of interest.Considering that, in general, the atmospheric boundary layer profiles might not satisfy the equations describing an unperturbed atmospheric boundary layer, turbulence models need to be corrected to guarantee the overall consistency of the approach. Former studies have addressed such a problem via the introduction of source terms in the transport equations of kinetic energy and turbulent dissipation rate, to guarantee consistency between the turbulence model and inlet profiles. In the present study, the formulation is further extended to improve the numerical stability of the approach as well as the realistic character of the produced profiles.

Geophysical controls on C band polarimetric backscatter from melt pond covered Arctic first‐year sea ice: Assessment using high‐resolution scatterometry

Geophysical controls on C band polarimetric backscatter from the discrete surface cover types which comprise advanced melt first‐year sea ice (FYI): snow covered ice, bare ice, and melt pond; are assessed using polarimetric radar scatterometry from test sites representing high Arctic and marginal ice zones in the Canadian Arctic. Surface characterization data is used to evaluate the interaction of polarized radiation with each feature, and dominant scattering mechanisms are assessed in a regional context. High‐resolution time series (diurnal) scatterometry and coincident atmospheric boundary layer profile data are used to explain linkages between ice‐atmosphere interactions and polarimetric backscatter in a marginal ice zone. The co‐polarization ratio for FYI melt ponds is shown to be distinct from snow covered ice or bare ice during early and peak phases of advanced melt, making it a candidate parameter for the unambiguous detection of pond formation and the inversion of melt pond fraction. The ratio displays an increasing trend with radar incidence angle in a manner consistent with Bragg surface scattering theory, though it is not predictable by a Bragg model. Cross‐polarization backscatter intensity shows potential for discriminating the onset and duration of freeze events in a marginal ice zone, due to dominant backscatter from the snow cover adjacent to melt ponds. Preliminary results here outline the potential of covariance matrix derived polarimetric measurements for the inversion of advanced melt sea ice geophysical parameters, and provide a basis for the investigation of distributed targets in late season spaceborne polarimetric SAR scenes.

An Integrated System for the Study of Wind-Wave Source Terms in Finite-Depth Water

AbstractA field experiment to study the spectral balance of the source terms for wind-generated waves in finite water depth was carried out in Lake George, Australia. The measurements were made from a shore-connected platform at varying water depths from 1.2 m down to 20 cm. Wind conditions and the geometry of the lake were such that fetch-limited conditions with fetches ranging from approximately 10 km down to 1 km prevailed. The resulting waves were intermediate-depth wind waves with inverse wave ages in the range 1 < U10/Cp < 8. The atmospheric input, bottom friction, and whitecap dissipation were measured directly and synchronously by an integrated measurement system, described in the paper. In addition, simultaneous data defining the directional wave spectrum, atmospheric boundary layer profile, and atmospheric turbulence were available. The contribution to the spectral evolution due to nonlinear interactions of various orders is investigated by a combination of bispectral analysis of the data and numerical modeling. The relatively small scale of the lake enabled experimental conditions such as the wind field and bathymetry to be well defined. The observations were conducted over a 3-yr period, from September 1997 to August 2000, with a designated intensive measurement period [the Australian Shallow Water Experiment (AUSWEX)] carried out in August–September 1999. High data return was achieved.

First- and second-generation agrometeorological models for the prairies and simulated water-demand for potatoes

On the Canadian prairies, moisture is usually the limiting factor in crop production. Occasionally, the thermal regime limits yields. At the Winnipeg Climate Centre, Environment Canada, a root-zone water-balance model simulates crop development, water-demand and water-use with daily climatological observations from across the agricultural portion of the Canadian Prairie Provinces. Simulations are used for regional-scale monitoring of the prairie’s major crops (spring wheat, barley and canola) and they have been used to quantify climatological risks. This agrometeorological model is termed a first-generation model as potential evapotranspiration is estimated empirically from Baier and Robertson’s simplest regression equation.A coupled atmosphere-crop-soil agrometeorological model has also been developed for crop monitoring on the prairies. This model generates atmospheric boundary layer profiles at climatological-sites using upper-air analyses and surface characteristics. The crop-soil boundary layer consists of the growing crop, and the top, root and sub-zones of the soil. Evapotranspiration is calculated deterministically from the air’s water vapour density deficit. The calculation is modulated by aerodynamic, canopy and soil resistances. The coupled approach is termed a second-generation model.The re-formulation of the coupled model to simulate the phenology and water-demand of a newly significant crop, potatoes, is described. Comparison with 1994 and 1995 test-plot observations of fractional leaf area and rooting depth suggests that the potato phenology simulation requires further development. Estimates of daily crop water-demand are similar but show greater day-today variation than values generated by a first-generation Baier and Robertson procedure. Key words: Regional modelling, soil moisture, phenology, crop water-demand, potatoes

Turbulent boundary layer profiles over uniform rough surfaces

Historically, atmospheric boundary layer profiles have been most commonly expressed by logarithmic and power law forms. This paper examines several ways in which the wind profile can be “derived” and shows that all the methods lead either to logarithmic or power law expressions. The logarithmic form is seen to be best near the surface, with the power law a better fit at greater heights or over a larger range of elevations. Results show that the power law exponent is a function of the boundary layer thickness and the surface roughness. The various expressions obtained are compared with the log-polynomial profile proposed by Deaves and Harris (CIRIA Rep. 76, 1978) since it has been verified with many atmospheric profiles measured to as much as half the boundary layer thickness. When compared over the entire boundary layer, the power law agrees with the Deaves and Harris model to within a maximum of 5% and generally much less; a weighted log-power law expression agrees with the Deaves and Harris expression to within 1%. The velocity defect forms of the several expressions are examined, and all show similar values and variation.