Simulated Atmospheric Boundary Layer Research Articles

Experimental study on the aerodynamic forces on railway vehicles in presence of turbulence

The present research work is based on an experimental campaign of wind tunnel tests aimed at investigating the crosswind aerodynamic of a lightweight railway vehicle. The performed tests allow to measure the aerodynamic forces on the vehicles for different wind exposures and over a wide range of Reynolds number. In addition, different Atmospheric boundary layer simulations are performed by varying the turbulence intensity indexes and the integral length scales of the wind generated in the wind tunnel. The work focuses on the role of the atmospheric wind turbulence in the definition of the crosswind loads on this typology of railway vehicles.

On atmospheric stability in the dynamic wake meandering model

ABSTRACTThe present study investigates a new approach for capturing the effects of atmospheric stability on wind turbine wake evolution and wake meandering by using the dynamic wake meandering model. The most notable impact of atmospheric stability on the wind is the changes in length and velocity scales of the atmospheric turbulence. The length and velocity scales in the turbulence are largely responsible for the way in which wind turbine wakes meander as they convect downstream. The hypothesis of the present work is that appropriate turbulence scales can be extracted from the oncoming atmospheric turbulence spectra and applied to the dynamic wake meandering model to capture the correct wake meandering behaviour. The ambient turbulence in all stability classes is generated using the Mann turbulence model, where the effects of non‐neutral atmospheric stability are approximated by the selection of input parameters.In order to isolate the effect of atmospheric stability, simulations of neutral and unstable atmospheric boundary layers using large‐eddy simulation are performed at the same streamwise turbulence intensity level. The turbulence intensity is kept constant by calibrating the surface roughness in the computational domain. The changes in the turbulent length scales due to the various atmospheric stability states impact the wake meandering characteristics and thus the power generation by the individual turbines.The proposed method is compared with results from both large‐eddy simulation coupled with an actuator line model and field measurements, where generally good agreement is found with respect to the velocity, turbulence intensity and power predictions. Copyright © 2013 John Wiley & Sons, Ltd.

Experimental Study on the Four Pipes Self-Supporting Steel Chimney

In the first test segment (12m×16m) of 8m×6m wind tunnel with simulation of atmospheric boundary layer, we did first measuring pressure test research on rigid model of four tube self-supporting steel chimney, and got average wind pressure coefficient, maximum and minimum wind pressure coefficient distribution, as well as wind loads body coefficient of this structure, thus gave the body coefficient and extreme wind load envelopes of two species height chimney. The results show that in two wind speeds, the test results have good coincidence, reasonable value, and good regularity.

Computational Fluid Dynamic (CFD) simulation of flow around tall buildings

Significant improvements of computer resources in recent past years allow to use computational fluid dynamics (CFD) methods as an alternative test method for wind tunnel tests in various wind engineering aspects. However the accuracy of CFD simulation mainly depends on careful setup of three main components of a CFD simulation, which are domain size, adopted mesh, and boundary conditions. This paper presents basic theoretical background of use of CFD for atmospheric boundary layer simulations and proper methods recommended for creating domains and meshes for CFD models. It also demonstrates several empirical methods that can be used as boundary conditions in the absence of more accurate data for simulation. The CFD simulation results of pressure distribution of 112m tall buildings is compared with the wind tunnel test results and found that performance of those empirical methods is satisfactory. The use of CFD simulation for flow visualization around a tall building and evaluating pedestrian level wind velocities are also demonstrated in this paper. DOI: http://dx.doi.org/10.4038/engineer.v46i3.6784 ENGINEER, Vol. 46, No.03, pp. 43-54 2013

Soil‐layer configuration requirement for large‐eddy atmosphere and land surface coupled modeling

AbstractTo investigate the soil‐layer configuration requirement for numerical studying large‐eddy scale atmosphere and land surface interactions, we conduct two experiments with an atmosphere and land surface coupled large‐eddy model, one with the ‘usual’ soil‐layer configuration and the other with a finer one. It is found that the former configuration prevents the soil to respond to atmospheric large eddies and the soil acts solely as a fixed external condition to the atmospheric turbulence, and is thus inadequate for large‐eddy atmosphere and land surface coupled modeling. It is shown that soil‐layer configuration has a profound impact on the simulated atmospheric boundary layer dynamics. Copyright © 2013 Royal Meteorological Society

A Numerical Study of Atmospheric and Wake Turbulence Impacts on Wind Turbine Fatigue Loadings

Large-eddy simulations of atmospheric boundary layers under various stability and surface roughness conditions are performed to investigate the turbulence impact on wind turbines. In particular, the aeroelastic responses of the turbines are studied to characterize the fatigue loading of the turbulence present in the boundary layer and in the wake of the turbines. Two utility-scale 5 MW turbines that are separated by seven rotor diameters are placed in a 3 km by 3 km by 1 km domain. They are subjected to atmospheric turbulent boundary layer flow and data are collected on the structural response of the turbine components. The surface roughness was found to increase the fatigue loads while the atmospheric instability had a small influence. Furthermore, the downstream turbines yielded higher fatigue loads indicating that the turbulent wakes generated from the upstream turbines have significant impact.

The impact of coastal phytoplankton blooms on ocean‐atmosphere thermal energy exchange: Evidence from a two‐way coupled numerical modeling system

A set of sensitivity experiments are performed with a two‐way coupled and nested ocean‐atmosphere forecasting system in order to deconvolve how dense phytoplankton stocks in a coastal embayment may impact thermal energy exchange processes. Monterey Bay simulations parameterizing solar shortwave transparency in the surface ocean as an invariant oligotrophic oceanic water type estimate consistently colder sea surface temperature (SST) than simulations utilizing more realistic, spatially varying shortwave attenuation terms based on satellite estimates of surface algal pigment concentration. These SST differences lead to an ∼88% increase in the cumulative turbulent thermal energy transfer from the ocean to the atmosphere over the three month simulation period. The result is a warmer simulated atmospheric boundary layer with respective local air temperature differences approaching ∼2°C. This study suggests that the retention of shortwave solar flux by ocean flora may directly impact even short‐term forecasts of coastal meteorological variables.

Numerical study and comparison with experiment of dispersion of a heavier-than-air gas in a simulated neutral atmospheric boundary layer

This paper presents a Reynolds-averaged Navier–Stokes simulation of the dispersion of a heavier-than-air gas from a ground level line source in a simulated atmospheric boundary layer. A previously published experimental study has been used to define the computational domain and boundary conditions, as well as to compare with the predicted results. The dispersed material is a mixture of 97% carbon-dioxide and 3% propane by concentration, where the latter gas was used as a tracer in the experiments. The floor of the computational domain was populated with vertical fences in order to simulate a rough surface for boundary layer development, as in the experiments. This also helped in obtaining streamwise homogeneity for mean velocity and turbulence kinetic energy. The results and comparisons with the experimental data are presented for concentration profiles as well as a number of derived parameters, such as entrainment velocity. The cases presented are for three Richardson numbers of 0.1, 7 and 16. Sensitivity tests are carried out to show the effects of boundary conditions at the inlet to the flow domain, turbulence model, namely, the standard k–ε model and RNG k–ε model, and the turbulent Schmidt number. The results showed significant sensitivity to the value of turbulent Schmidt number. By optimizing the value of this parameter, it was possible to obtain close comparisons between the predicted and measured parameters.

Formation of tip-vortices on triangular prismatic-shaped cliffs Part 1: A wind tunnel study

Speed-up over topography has long been recognized as significant for structural design and wind energy applications. Here wind flow over cliffs that have a sawtooth plan are studied for speed-up effects under different wind directions. The study undertook PIV measurements to document the mean and turbulence changes in three dimensions for simulated atmospheric boundary layer flow in the University of Sydney BLWT. As the flow approaches the cliffs, it accelerates and moves up to pass the obstruction and generates two strong vortices on the top surface of the cliff. Small changes in wind direction not only lead to significantly different speed-up ratios, but also influence the location of the vortices and the level of turbulence. The complexity of the flow field is revealed in this unique three-dimensional study.

RANS simulation of ABL flow over complex terrains applying an Enhanced k-ε model and wall function formulation: Implementation and comparison for fluent and OpenFOAM

The simulation of Atmospheric Boundary Layer (ABL) flows is commonly performed using commercial CFD codes with RANS turbulence modeling, applying the standard k-ε model. However, when applied to the simulation of the homogenous ABL, this approach may result in an undesired decay of the velocity and turbulent fully–developed profiles specified at the inlet of the computational domain. This behavior is due to an inconsistency between turbulence model, inflow conditions and wall function formulation. An approach has been introduced recently to overcome this problem, which consists in the modification of the turbulence model and wall function formulation to retrieve an overall consistent treatment of the neutral ABL. Such methodology, previously applied to simulation of the atmospheric boundary layer over flat terrain and ground-mounted bluff bodies, is here applied to the simulation of the flow over complex terrains and hills, at wind tunnel and atmosphere scale. In a time of limited scientific funding, the availability of open source CFD software such as OpenFOAM is a very attractive option to investigate; therefore, a comparison between OpenFOAM and the commercial code FLUENT 13.0 has been carried out in the present paper. The potential of the proposed methodology and the satisfactory performances of OpenFOAM are demonstrated.

Improved Experimental Simulation of Wind Characteristics around Tall Buildings

A wind-tunnel study has been carried out to investigate the effects of the barrier wall and surface roughness on the simulated atmospheric boundary-layer (ABL) flow generated using the Counihan method, (i.e., the castellated barrier wall, vortex generators, and surface roughness elements). Experimental results indicate that changing the basic barrier height does not influence the mean-velocity profiles. However, turbulence intensity and absolute values of the Reynolds stress increase with an increase in the barrier height, and simultaneously the integral length scales of turbulence decrease. Variations in the height of the barrier castellation influence the simulated ABL flow similarly to the changes in the basic barrier height. Empirical relations were derived to quantify effects of the surface roughness height on the simulated ABL flows. Increasing the height of surface roughness elements results in a larger velocity defect and stronger turbulence in the near-ground region.

A neural network shelter model for small wind turbine siting near single obstacles

Many potential small wind turbine locations are near obstacles such as buildings and shelterbelts, which can have a significant, detrimental effect on the local wind climate. A neural network-based model has been developed which predicts mean wind speed and turbulence intensity at points in an obstacle's region of influence, relative to unsheltered conditions. The neural network was trained using measurements collected in the wakes of 18 scale building models exposed to a simulated rural atmospheric boundary layer in a wind tunnel. The model obstacles covered a range of heights, widths, depths, and roof pitches typical of rural buildings. A field experiment was conducted using three unique full scale obstacles to validate model predictions and wind tunnel measurements. The accuracy of the neural network model varies with the quantity predicted and position in the obstacle wake. In general, predictions of mean velocity deficit in the far wake region are most accurate. The overall estimated mean uncertainties associated with model predictions of normalized mean wind speed and turbulence intensity are 4.9% and 12.8%, respectively.

Numerical Study of the Neutral Atmospheric Boundary Layer Over Complex Terrain

We evaluate the Reynolds-averaged Navier–Stokes equations available as commercial computational fluid dynamics code for the simulation of a neutral atmospheric boundary layer and attempt to define a proper numerical simulation procedure. Four turbulence models, including two-equation and Reynolds stress models, were evaluated together with two near-wall models. Mesh and map digitization sensitivity tests were also performed. The simulations were compared to experimental field data from the Askervein Hill in Scotland. The results show that the simulations performed with ANSYS CFX 12.1 on a proper mesh and topological map with a Reynolds stress turbulence model provided the best wind-speed predictions when compared to the experimental results.

Employing Taylor and Heisenberg subfilter viscosities to simulate turbulent statistics in LES models

A turbulent subfilter viscosity for Large Eddy Simulation (LES) based on the Taylor statistical diffusion theory is proposed. This viscosity is described in terms of a velocity variance and a time scale, both associated to the inertial subrange. This new subfilter viscosity contains a cutoff wavenumber kc, presenting an identical form (differing by a constant) to the Heisenberg subfilter viscosity. Therefore, both subfilter viscosities are described in terms of a sharp division between large and small wavenumbers of a turbulent flow and, henceforth, Taylor and Heisenberg subfilter viscosities are in agreement with the sharp Fourier filtering operation, frequently employed in LES models. Turbulent statistics of different orders, generated from atmospheric boundary layer simulations employing both Taylor and Heisenberg subfilter viscosities have been compared with observations and results provided by other simulations. The comparison shows that the LES model utilizing the approaches of Taylor and Heisenberg reproduces these turbulent statistics correctly in different vertical regions of a planetary convective boundary layer (CBL).

A Study on the Equivalent Static Wind Loadings on the Arched Roof Frames of Low-Rise Buildings in Atmospheric Boundary Layers

A series of wind tunnel aerodynamic experiments were conducted to investigate the wind loads on the low-rise buildings with arched roofs. The wind loadings were calculated from the simultaneous measured wind pressures data over all of the models surfaces in a simulated open terrain exposure atmospheric boundary layer. With these data the equivalent static wind loads (ESWL) of fluctuating wind pressures acting on buildings frames were evaluated and the results of equivalent static wind loads were compared with the 2006 version design codes of wind loadings on buildings in Taiwan. The design codes suggested the direction of wind acting on the low-rise buildings roofs were normal to the ridges of roofs. In this study results shown, the equivalent static wind loads caused by the approaching turbulent flow may exceed the suggested values by codes. So the peak response of structural frames should be considered carefully.

Characteristics of natural wind simulations in the TUM boundary layer wind tunnel

The boundary layer wind tunnel at the Technische Universitat Munchen was tested for atmospheric boundary layer (ABL) simulations. The ABLs developing above rural, suburban, and urban terrains were reproduced using the Counihan method, i.e., castellated barrier wall, vortex generators, and a fetch of surface roughness elements. A series of flow-characteristic evaluations was performed to investigate the flow development and uniformity. Experimental results presented as mean velocity, turbulence intensity, integral length scale of turbulence, Reynolds stress, and power spectral density of velocity fluctuations were compared with the ESDU data and/or theoretical models. Generated ABL wind-tunnel simulations compare well with the rural, suburban, and urban ABLs. In the test section area used for experiments on structural models, the ABL simulation is developed and uniform. Results of this study indicate the boundary layer wind tunnel at the Technische Universitat Munchen can be successfully employed in a broad spectrum of engineering, environmental, and micrometeorological studies, where it is required to accurately reproduce ABL characteristics.

Improved k– ε model and wall function formulation for the RANS simulation of ABL flows

The simulation of Atmospheric Boundary Layer (ABL) flows is usually performed using the commercial CFD codes with RANS turbulence modelling and standard sand–grain rough wall functions. Such approach generally results in the undesired decay of the velocity and turbulent profiles specified at the domain inlet, before they reach the section of interest within the computational domain. This behaviour is a direct consequence of the inconsistency between the fully developed ABL inlet profiles and the wall function formulation. The present paper addresses the aforementioned issue and proposes a solution to it. A modified formulation of the Richards and Hoxey wall function for turbulence production is presented to avoid the well-documented over-prediction of the turbulent kinetic energy at the wall. Moreover, a modification of the standard k– ε turbulence model is proposed to allow specific arbitrary sets of fully developed profiles at the inlet section of the computational domain. The methodology is implemented and tested in the commercial code FLUENT v6.3 by means of the User Defined Functions (UDF). Results are presented for two neutral boundary layers over flat terrain, at wind tunnel and full scale, and for the flow around a bluff-body immersed into a wind-tunnel ABL. The potential of the proposed methodology in ensuring the homogeneity of velocity and turbulence quantities throughout the computational domain is demonstrated.

Design Wind Loads on Reinforced Concrete Chimney – An Experimental Case Study

The present paper is aimed at providing a better understanding of the effect of interference and influence of strakes for wind load on Thermal Power Station (TPS) chimneys. Measurements of across and along vibration have been made on scale model of Panipat power station chimney, India. The aeroelastic model of the chimney has been tested in a simulated atmospheric boundary layer of height 1.0 m in the Boundary Layer Wind Tunnel at IIT Roorkee, India.The chimney model has been fabricated with FRP (fiber-reinforced plastic) at a geometric scale of 1:150 representing a chimney with height of 100 m in prototype. The wind tunnel tests for the present study have been carried out with the objective of obtaining the maximum along-wind and across-wind response of the chimney and this has been expressed in terms of bending moments. In particular, the interference effect between other nearby structures of the TPS has been investigated. Based on dimensional analysis measured values of chimney model have been converted into prototype values.In the present study, particular attention has been given to bending moment due to across-wind vibration, because it has been found that across-wind vibration is more predominant for the case of interference at a 45o angle of wind incidence. Bending moment due to across-wind vibration for interference is found to be approximately double compared to that of stand-alone condition.To control across wind vibration, strakes have been mounted over the top one-third height of the chimney. It is found that there is a significant decrease in the magnitude of across as well as resultant bending moment in presence of strakes.

Wind effects on atria fires

A series of unsteady atria fire calculations are performed using a finite-volume CFD program on two and three dimensional generic buildings immersed in simulated atmospheric boundary layers. The model results reveal that external winds can modify the infiltration and exfiltration of air through external doors and windows, distort thermal and smoke columns rising above test fires in the atria, cause the plumes to impact directly against atria walls, and modify the resultant filling of elevated atria spaces. In some cases aggressive fire “whirls” form, which can enhance fire strength, enclosure mixing, and exposure. Results are compared qualitatively with similar physical model experiments.

Assessment of Pollutant Dispersion in the Re-entrance Space of a High-rise Residential Building, Using Wind Tunnel Simulations

This paper investigates the likely route of dispersion of airborne pollutants and pathogenic agents in the re-entrance space of a high-rise residential (HRR) building in Hong Kong. A 1 : 150 scaled HRR building, a 30 storeys tall model was tested in a wind tunnel with simulated atmospheric boundary layer flow. Tracer gas, propane released from the lower, middle and upper parts of the building, was measured by flame ionisation detection and this was used to simulate the air pollutant dispersion within the re-entrance space. There were a total of 495 measurements, tested under five different wind directions at 99 measurement points on windows locations of every floor of the modelled building. The tracer gas concentration profiles obtained would illustrate the pollutant dispersion and transmission routes under the different wind effect. The results showed that the airborne pollutants or pathogenic agents could be dispersed mainly along the vertical routes, posing a potential risk to the adjacent neighbours immediately above the source location. Wind direction could also affect the dispersion. The findings of this study should inform the development of ventilation strategies for HRR buildings and would guide building design to minimise transmission of airborne diseases or harmful pollutants within buildings.