Reference Wind Speed Research Articles

Meteorological events causing extreme winds in Brazil

The meteorological events that cause most strong winds in Brazil are extra-tropical cyclones, downbursts and tornadoes. However, one hurricane formed off the coastline of southern Brazil in 2005, a tropical storm formed in 2010 and there are predictions that others may form again. Events such as those described in the paper and which have occurred before 1987, generate data for the wind map presented in the Brazilian wind loading code NBR-6123. This wind map presents the reference wind speeds based on 3-second gust wind speed at 10 m height in open terrain, with 50-year return period, varying from 30 m/s (north half of country) to 50 m/s (extreme south). There is not a separation of the type of climatological event which generated each registered velocity. Therefore, a thunderstorm (TS), an extra-tropical pressure system (EPS) or even a tropical cyclone (TC) are treated the same and its resulting velocities absorbed without differentiation. Since the flow fields generated by each type of meteorological event may be distinct, the indiscriminate combination of the highest wind velocities with aerodynamic coefficients from boundary layer wind tunnels may lead to erroneous loading in buildings.

A New Single-Layer Urban Canopy Model for Use in Mesoscale Atmospheric Models

AbstractA new single-layer urban canopy model for use in mesoscale atmospheric models is developed and validated. The urban canopy model represents a built-up area as a street canyon, two facing buildings, and a road. In this model, the two facing walls are divided into sunlit and shaded walls on the basis of solar azimuth angle and canyon orientation, and individual surface temperature and energy budget are calculated for each wall. In addition, for better estimation of turbulent energy exchange within the canyon, a computational fluid dynamics model is employed to incorporate the effects of canyon aspect ratio (height-to-width ratio) and reference wind direction on canyon wind speed. The model contains the essential physical processes occurring in an urban canopy: absorption and reflection of shortwave and longwave radiation, exchanges of turbulent energy and water between surfaces (roof, two facing walls, and road) and adjacent air, and heat transfer by conduction through substrates. The developed urban canopy model is validated using datasets obtained at two urban sites: Marseille, France, and Basel, Switzerland. The model satisfactorily reproduces canyon air temperatures, surface temperatures, net radiation, sensible heat fluxes, latent heat fluxes, and storage heat fluxes for both sites. Extensive experiments are conducted to examine the sensitivities of the urban surface energy balance to meteorological factors and urban surface parameters. The reference wind speed is found to be a more crucial meteorological factor than the reference air temperature in altering urban surface energy balance, especially for weak winds. The urban surface energy balance is most sensitive to the roof albedo among urban surface parameters. The roof fraction, canyon aspect ratio, and ratio of roughness length for momentum to that for heat for the roof play important roles in altering urban surface energy balance.

Modeling wake effects in large wind farms in complex terrain: the problem, the methods and the issues

ABSTRACTComputational fluid dynamic (CFD) methods are used in this paper to predict the power production from entire wind farms in complex terrain and to shed some light into the wake flow patterns. Two full three‐dimensional Navier–Stokes solvers for incompressible fluid flow, employing k − ϵ and k − ω turbulence closures, are used. The wind turbines are modeled as momentum absorbers by means of their thrust coefficient through the actuator disk approach. Alternative methods for estimating the reference wind speed in the calculation of the thrust are tested. The work presented in this paper is part of the work being undertaken within the UpWind Integrated Project that aims to develop the design tools for next generation of large wind turbines. In this part of UpWind, the performance of wind farm and wake models is being examined in complex terrain environment where there are few pre‐existing relevant measurements. The focus of the work being carried out is to evaluate the performance of CFD models in large wind farm applications in complex terrain and to examine the development of the wakes in a complex terrain environment. Copyright © 2011 John Wiley & Sons, Ltd.

기상탑 차폐 영향에 따른 측정 풍속의 오차 분석

In the performance test for wind turbines of medium and large, The reference met-mast should be installed for measurement reference wind speed as IEC 61400-12-1 standards and design of booms for mounted an anemometer must be considered exactly. Boom-mounted cup anemometer are influenced by flow distortion of the mast and the boom. Therefore design of booms must be important so that flow distortion due to booms should be kept below 0.5%. But, in some cases at size of met-mast structure, the distance of boom from mast is longer then measurement of wind speed is impossible because of oscillation of boom-mounted anemometer. In this paper, We measured a wind speed at several point from mast and boom and we analyzed the error of wind speed at each point of measurement. Also, we will suggest a correction method using the data curve fitting about errors of wind speed between each point of mounted anemometer.

Evaluation of the effects of turbulence model enhancements on wind turbine wake predictions

AbstractThe modelling of wind turbine wakes is investigated in this paper using a Navier–Stokes solver employing the k–ω turbulence model appropriately modified for atmospheric flows. It is common knowledge that even single‐wind turbine wake predictions with computational fluid dynamic methods underestimate the near wake deficit, directly contributing to the overestimation of the power of the downstream turbines. For a single‐wind turbine, alternative modelling enhancements under neutral and stable atmospheric conditions are tested in this paper to account for and eventually correct the turbulence overestimation that is responsible for the faster flow recovery that appears in the numerical predictions. Their effect on the power predictions is evaluated with comparison with existing wake measurements. A second issue addressed in this paper concerns multi‐wake predictions in wind farms, where the estimation of the reference wind speed that is required for the thrust calculation of a turbine located in the wake(s) of other turbines is not obvious. This is overcome by utilizing an induction factor‐based concept: According to it, the definition of the induction factor and its relationship with the thrust coefficient are employed to provide an average wind speed value across the rotor disk for the estimation of the axial force. Application is made on the case of five wind turbines in a row. Copyright © 2010 John Wiley & Sons, Ltd.

Convective heat transfer coefficients for exterior building surfaces: Existing correlations and CFD modelling

Convective heat transfer at exterior building surfaces has an impact on the design and performance of building components such as double-skin facades, solar collectors, solar chimneys and ventilated photovoltaic arrays, and also affects the thermal climate and cooling load in urban areas. In this study, an overview is given of existing correlations of the exterior convective heat transfer coefficient (CHTC) with the wind speed, indicating significant differences between these correlations. As an alternative to using existing correlations, the applicability of CFD to obtain forced CHTC correlations is evaluated, by considering a cubic building in an atmospheric boundary layer. Steady Reynolds-averaged Navier–Stokes simulations are performed and, instead of the commonly used wall functions, low-Reynolds number modelling (LRNM) is used to model the boundary-layer region for reasons of improved accuracy. The flow field is found to become quasi independent of the Reynolds number at Reynolds numbers of about 10 5. This allows limiting the wind speed at which the CHTC is evaluated and thus the grid resolution in the near-wall region, which significantly reduces the computational expense. The distribution of the power-law CHTC- U 10 correlation over the windward and leeward surfaces is presented ( U 10 = reference wind speed at 10 m height). It is shown that these correlations can be accurately determined by simulations with relatively low wind speed values, which avoids the use of excessively fine grids for LRNM, and by using only two or three discrete wind speed values, which limits the required number of CFD simulations.

New Reference Wind Speed for Wind Turbines in Typhoon-Prone Areas in the Philippines

The Philippines has good potential for wind energy utilization. However, it is also prone to strong typhoons. Thus, wind turbines that will be erected in these areas maybe categorized into a special safety class and the reference wind speed is left to the discretion of the designer. Due to this scenario, the writers proposed a new reference wind speed applicable to Philippine climate which take typhoons into account. Annual extreme wind speeds gathered from fifty weather stations around the country were fitted to a generalized extreme value distribution. For each station, buckling failure of a wind turbine was analyzed using reliability theory. Based on these results a new 50-year mean wind speed and a new reference wind speed for a wind turbine in typhoon-prone areas in the Philippines are proposed.

Overview of three state-of-the-art wind-driven rain assessment models and comparison based on model theory

In the past, different calculation models for wind-driven rain (WDR) have been developed and progressively improved. Today, the models that are most advanced and most frequently used are the semi-empirical model in the ISO Standard for WDR (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi, extended by Blocken and Carmeliet. Each of these models is quite different, and to the knowledge of the authors, no comparison of these models has yet been performed. This paper first presents a detailed overview of the three models, including new insights in similarities between these models and relations with recent research results. Based on this overview, it provides a comparison focused on the extent to which the different influencing parameters of WDR are implemented in the models. It shows that the implementation of the influencing parameters is most pronounced for the CFD model, less pronounced for the ISO model and least pronounced for the SB model. It is also shown that in the two semi-empirical models, the values of the wall factor W (for ISO) and the rain admittance function RAF (for SB), which have the same definition in both models, can differ more than a factor 2 from each other. The two models can therefore provide very different results. They also require differently defined reference wind speed values as input. The overview and the comparison in this paper provide the basis for future comparison studies and future improvements of the semi-empirical models.

Moisture response of building facades to wind-driven rain: Field measurements compared with numerical simulations

Wind-driven rain (WDR) is one of the most important boundary conditions governing the hygrothermal behaviour of building facades, which is usually numerically analysed with the so-called Heat-Air-Moisture (HAM) transfer models. In the traditional approach of HAM transfer models, WDR is implemented in a simplified manner: the total mass of all raindrops impinging on a certain surface area of a building facade during the time interval of the meteorological input data (typically 1 h) is spatially and temporally averaged and is supplied to the facade as an averaged moisture flux. However, real WDR is the sum of individual raindrops that impinge on the facade in a spatially and temporally discrete modus, and that do not only spread at impact, but may also splash or bounce off the facade. Therefore the reliability of this simplification can be questioned. To investigate its validity, a new experimental set-up was developed at a full-scale test building. It allows simultaneous and continuous measurements of the reference wind speed and direction, WDR intensity, outdoor air temperature and humidity, as well as the response of facade material samples to these environmental conditions. For this purpose, a measuring device was developed that monitors the weight change of the sample with a resolution of 5 mg. Temperatures at the interior and exterior material surfaces are also monitored. The whole measurement data set is used to check the validity of the traditional numerical approach. Large differences are found between the measurement and simulation results, which cannot solely be attributed to the uncertainty in the convective moisture transfer coefficient, but may be due to two additional reasons: the occurrence of splashing and bouncing at raindrop impact on the facade, which is not included in the model, and/or errors in surface moisture evaporation and absorption due to modelling the actual random and discrete raindrop impingement as a simplified averaged moisture flux.

The mutual influence of two buildings on their wind-driven rain exposure and comments on the obstruction factor

Wind-driven rain (WDR) deposition on a two-building configuration is studied with Computational Fluid Dynamics (CFD). The configuration consists of a high-rise building screened by a low-rise building. Validation of the wind-flow simulations is performed with Particle Image Velocimetry (PIV) measurements in a wind tunnel. Raindrop motion is simulated by Lagrangian particle tracking in the mean wind-flow pattern with a reference wind speed U 10=10 m/s. Horizontal rainfall intensities R h=5 and 30 mm/h are considered. Simulations of WDR are performed for the two-building configuration and for each building separately, to analyse the mutual influence of the buildings on their WDR deposition pattern. The simulation results indicate that this influence is very pronounced and that it is to some extent opposite to what might be expected. The low-rise building influences the deposition on the high-rise building (downstream disturbance), not by partly shielding it from wind and WDR, but by increasing the strength of the standing vortex between the two buildings. This locally increases WDR intensities on the high-rise building facade by more than a factor 2 for both R h=5 and 30 mm/h. On the other hand, the high-rise building influences deposition on the low-rise building facade (upstream disturbance) by the wind-blocking effect. This effect yields a reduction in WDR deposition on the low-rise building facade by up to about 25% for both R h=5 and 30 mm/h. In the European standard draft for WDR assessment, the mutual influence can only be taken into account by a simplified reduction factor, called the obstruction factor. It only considers downstream disturbances, and does not consider the possibility of increased WDR deposition due to neighbouring buildings. Care should therefore be exercised when using the current version of the obstruction factor to determine WDR exposure.

Measurements of particles in the 5–1000 nm range close to road level in an urban street canyon

A newly developed instrument, the ‘fast response differential mobility spectrometer (DMS500)’, was deployed to measure the particles in the 5–1000 nm range in a Cambridge (UK) street canyon. Measurements were taken for 7 weekdays (from 09:00 to 19:00 h) between 8 and 21 June 2006 at three heights close to the road level (i.e. 0.20 m, 1.0 m and 2.60 m). The main aims of the measurements were to investigate the dependence of particle number distributions (PNDs) and concentrations (PNCs) and their vertical variations on wind speed, wind direction, traffic volume, and to estimate the particle number flux (PNF) and the particle number emission factors (PNEF) for typical urban streets and driving conditions. Traffic was the main source of particles at the measurement site. Measured PNCs were inversely proportional to the reference wind speed and directly proportional to the traffic volume. During the periods of cross-canyon flow the PNCs were larger on the leeward side than the windward side of the street canyon showing a possible effect of the vortex circulation. The largest PNCs were unsurprisingly near to road level and the pollution sources. The PNCs measured at 0.20 m and 1.0 m were the same to within 0.5–12.5% indicating a well-mixed region and this was presumably due to the enhanced mixing from traffic produced turbulence. The PNCs at 2.60 m were lower by 10–40% than those at 0.20 m and 1.0 m, suggesting a possible concentration gradient in the upper part of the canyon. The PNFs were estimated using an idealised and an operational approach; they were directly proportional to the traffic volume confirming the traffic to be the main source of particles. The PNEF were estimated using an inverse modelling technique; the reported values were within a factor of 3 of those published in similar studies.

Calibration Procedures and Uncertainty in Wind Power Anemometers

Accurate wind measurements are critical in evaluating wind turbine power performance and site assessment. In a turbine power performance evaluation, wind speed readings are matched with corresponding turbine power measurements to produce a power curve for the turbine. For site assessment, the distribution of measured wind speed is used to determine the predicted annual energy production from the wind. Since wind power is proportional to the cube of the wind speed, a small error in the wind measurement could translate to a much greater error in the predicted wind power, which emphasizes the importance of having accurate wind speed readings. To acquire such precision in wind data, it is recommended that individually calibrated anemometers be employed. With these calibrations, it is also recommended that the uncertainty in the calibration be reported so that it may be used not only in the overall uncertainty for turbine power curves and site assessments, but also in improving the performance of an anemometer. A method of presenting calibration uncertainty is defined in the standard IEC 61400-12-1. However, the standard only refers to the measurement uncertainty of the reference wind speed from the particular test facility. It does not include the uncertainty in the anemometer linear transfer function and the errors directly made by the anemometer signal. This paper will discuss: 1) the details of uncertainty reporting as defined by IEC 61400-12-1, 2) a method of extending the uncertainty to include the errors when using the linear transfer function, and 3) a qualitative description of how to determine the uncertainty in a wind speed measurement in the field.

Design for wind comfort in The Netherlands: Procedures, criteria and open research issues

A code for the assessment of wind comfort and wind danger in the built environment in the Netherlands is presented. The code is not another legal building requirement, but a helping hand to include wind comfort in a building programme. It regulates technical procedures and some quality control. A decision scheme indicates which building plans need professional assessment by wind engineers. Local wind speed data are predicted from wind tunnel or CFD simulations. Statistics of a reference wind speed at the building location is needed to calculate the probability that a threshold wind speed is exceeded. The code prescribes a method which is offered as a software package to calculate the statistics of the reference wind speed at a height of 60 m. It includes the most up-to-date data from meteorological stations in the Netherlands. The probability that the pedestrian wind speed exceeds a threshold value of 5 m/s is a measure for wind comfort. Larger probability means less comfort. Therefore, five grades of wind comfort A–E are defined as function of this probability. In addition, for three different activities of the public (traversing, strolling and sitting) these grades of wind comfort are assessed in terms of a poor, moderate or good local wind climate.

Duration‐ and fetch‐limited growth functions of wind‐generated waves parameterized with three different scaling wind velocities

Under steady wind forcing, wave development follows the duration‐ and fetch‐limited growth laws. These growth functions are used extensively to obtain the sea state information when only limited observations of the environmental variables are available. Validation and verification of wave models also employ numerical experiments of duration‐ and fetch‐limited wave growth as benchmark tests. The reference wind speed reported in most of the wave‐growth data is the equivalent neutral wind speed at 10 m elevation, U10. It is generally believed that a more suitable scaling wind speed is either the wind friction velocity, u*, or the wind speed at an elevation proportional to the wavelength of the ocean surface fluctuation, Uλ/2. The connection among the growth functions using different velocity scales is the drag coefficient of the ocean surface. In this paper, the similarity relation of the drag coefficient based on wavelength scaling is applied to the conversion of the wave growth functions from U10 to Uλ/2 and u* scaling. The results are in good agreement with field measurements that include direct u* measurements. Comparisons with numerical model output are also described.

Meteorological basis for wind loads calculation in Croatia

The results of reference wind speed calculation in Croatia as a base for the revision of the Croatian standards for wind loads upon structures are presented. Wind speed averaged over 10 minutes, at 10 m height, in a flat, open terrain, with a 50-year mean return period is given for 27 meteorological stations in Croatia. It is shown that the greatest part of Croatia is covered with expected reference wind speeds up to 25 m/s. Exceptions are stations with specific anemometer location open to the bura wind which is accelerated due to the channelling effects of local orography and the nearby mountain passes where the expected reference wind speed ranges between 38 m/s and 55 m/s. The methodology for unifying all available information from wind measurements regardless of the averaging period is discussed by analysing wind speed variability at the meteorological station in Hvar.

Comparison of Ocean Surface Wind Stress Computed with Different Parameterization Functions of the Drag Coefficient

Depending on the choice of reference wind speed, the quantitative and qualitative properties of the drag coefficient may vary. On the ocean surface, surface waves are the physical roughness at the air-sea interface, and they play an important role in controlling the air-sea exchange processes. The degree of dynamic influence of surface waves scales with wavelength. Drag coefficient computed with the reference wind speed at an elevation proportional to the wavelength (for example, Uλ/2) is fundamentally different from the drag coefficient computed with the wind speed at fixed 10 m elevation (U10). A comparison has been carried out to quantify the difference in wind stress computation using several different parameterization functions of the drag coefficient. The result indicates that the wind stress computed from U10 input using a drag coefficient referenced to Uλ/2 is more accurate than that computed with drag coefficient functions referenced to U10.

Mass Flux and Vertical Distribution of Currents Caused by Strong Winds in a Wave Tank

Abstract The velocity fields of wind-driven currents under strong winds were measured in a wind-wave tank with a double bottom. The tank has the characteristics to satisfy partially the continuity of the mass flux and to reduce return-flow effects on the currents. The lower part of the double-bottom tank functions as a duct to circulate the currents, allowing the measurement of the return-flow velocity. The velocity measurements were made on the currents just below the mean water level by using a high-resolution particle image velocimetry (PIV) system and tracking floats and on the currents inside of the duct by using a normal PIV system. Thus, accurate data of the vertical distribution and mass flux of the currents driven by strong winds were obtained from the PIV data. As a result, it is found that the flux in the surface layer with the thickness of 2 times the significant wave height amounts to about 30% of the total mass flux of the currents driven by strong winds with a reference wind speed of 12.0 m s−1.

Effects of the wind profile at night on wind turbine sound

Abstract Since the start of the operation of a 30 MW, 17 turbine wind park, residents living 500 m and more from the park have reacted strongly to the noise; residents up to 1900 m distance expressed annoyance. To assess actual sound immission, long term measurements (a total of over 400 night hours in 4 months) have been performed at 400 and 1500 m from the park. In the original sound assessment a fixed relation between wind speed at reference height (10 m) and hub height (98 m) had been used. However, measurements show that the wind speed at hub height at night is up to 2.6 times higher than expected, causing a higher rotational speed of the wind turbines and consequentially up to 15 dB higher sound levels, relative to the same reference wind speed in daytime. Moreover, especially at high rotational speeds the turbines produce a ‘thumping’, impulsive sound, increasing annoyance further. It is concluded that prediction of noise immission at night from (tall) wind turbines is underestimated when measurement data are used (implicitly) assuming a wind profile valid in daytime.

Wind climate micro-zoning: a pilot application to Liguria Region (North Western Italy)

A selection of the results obtained in the framework of the research activities for the realisation of the Liguria (North Western Italy) wind map are presented. In that context, a method was developed and implemented combining wind speed data probabilistic analysis techniques and numerical codes for simulating wind fields over complex orography. By applying that method to several long time series of wind speed data recorded on the Ligurian territory, high resolution wind maps at 10 m height were elaborated for both the data population and the extreme values, as well as a probabilistic model of the reference wind speed, i.e. the mean speed at 10 m height on an ideal flat open terrain. The implemented method was tested comparing the wind map with both literature and the available experimental data. Good agreement was found, especially for extreme values, showing a satisfactory reliability of the method.

Scaling Effects of Standing Crop Residues on the Wind Profile

Standing senescent stems increase the aerodynamic roughness of the surface, reducing wind energy available for momentum transfer at the soil surface, such as for wind erosion, and also the soil–atmosphere convective exchanges of heat, water vapor, and trace gases. We conducted studies to determine the predictive accuracy of an algorithm derived for plant canopies to scale effects of standing crop residues on the wind profile. We used this algorithm to calculate aerodynamic properties (displacement height and roughness length) of standing crop residues related to the log wind profile equation. We also calculated apparent roughness length from wind profiles measured under neutral stability conditions over stems of wheat (Triticum aestivum L.), corn (Zea mays L.), millet (Panicum miliaceum L.), and sunflower (Helianthus annuus L.) using calibrated single‐needle and cup anemometers. A least‐squares fit of roughness length calculated by an algorithm derived for crop canopies indicated a systematic, positive bias when it was applied to standing stems. After adjusting for bias, calculated windspeeds generally were contained in 80% confidence intervals for observations above and within the crop stubble. Predictive root mean square errors (RMSE) within profiles ranged from 0.6 to 4.6% of reference wind speed. The nonlinear forms of the scaling algorithms are consistent with theory and wind tunnel observations, representing an advance over parameterization schemes assuming a linear relation with residue height. This advance warrants evaluation of the adjusted algorithm for simulation of microclimate in the soil–residue–crop canopy regime. Application to momentum transfer problems requires further investigation of drag partitioning.