Issues of contemporary wind engineering and aerodynamics of building structures

The paper reviews nowadays problems and issues of wind engineering and aerodynamics of building structures. The article mainly focuses on aerodynamics of building structures, shortly characterizing theoretical bases, which one must take into account when assuming wind loads. The three different approaches of collecting information in the field of wind loads are described: in-situ measurements, wind tunnel tests and numerical simulations. Also, a review of the most important contemporary issues of wind engineering is presented.


Introduction
Wind loads became an object of special attention in the 60's and 70's of the XX century. This growing interest was mainly induced by a collapse of Tacoma Narrows Bridge and Ferrybridge Cooling Towers. The first event held in 1940 in US (Fig. 1a). The Tacoma Narrows Bridgea suspension bridge of the large span and slender cross-section of the decksuffered vibrations caused by the wind action. The wind speed was not extremely high, but its action and uncommon structure of the bridge led to uncontrolled vibrations. It was one of the first documented collapses caused by so-called flexural-torsional flutter. The second disaster took place in 1965 in UK (Fig. 1b). The cluster compound of eight cooling towers of large dimensions stood one by one in very close distances. The acceleration of the flow between objects and vortices shedding from the windward cooling towers were not considered during the design process. Additional wind loads were produced, and it was the direct cause of the total collapse of the three cooling towers and of serious damages of the other five. The speed increases with the height above the ground, until it reaches the gradient wind speed Ugrad, at the height zgrad. The variation of the mean wind speed with height can be expressed using mathematical formulas known as vertical wind profiles. There are two groups of formulas describing the wind speed profile: theoretically developed logarithmic equation (log-law profile) and power-law equation. The best known and firstly defined (by Alan Davenport), is the power-law formula, which illustrate wind speed changes over different terrain types (Davenport, 1960(Davenport, , 1965. The original values and formula given by Davenport are presented in Fig. 2, whereas profiles recommended by Eurocode (2008) are shown in Fig. 3a. Turbulent character of wind must be defined if a wind action is considered. The following characteristics of the fluctuating part (turbulence) should be given: intensity and scales of turbulence, peak factors, the correlation functions (in the time domain) and power spectral density functions (in the frequency domain). Intensity of turbulence, in general, defines time changes of the wind speed around the mean value. Turbulence length scale is a measure of the size of gusts of the wind speed in space and represents the distance in which the process is correlated. Examples of that functions are presented in Fig. 3b, c. Correlation functions give the detailed description of the wind structure, as a space-time stochastic process in the time domain. On the basis of correlation functions one can get additional information of the wind structure in the frequency domain. Applying Fourier transforms to respective correlations we can get power spectral density functions (PSD). There are several practical PSD functions in wind engineering elaborated on the basis of local measurements. An example of PSD function is presented in Fig. 4.
Characteristics of the wind speed can be described differently depending on civil engineering standards and codes which consider wind actions on structures. Among the most important standards in this field are: 1) Eurocode 1.

Flow around bodies of different shapes
Additionally to the wind characteristics, the character of flow in the close proximity of the objects of different shapes must be considered. In order to explain the flow phenomena that occurs in the vicinity of the body, let's consider the flow around a flat wall (Fig. 5). If the air flows around a flat, smooth surface, then in the effect of the viscous forces the movement slows down in its immediate vicinity. The area in which this phenomenon occurs is called the boundary layer. Within it, the speed varies from 0 at the surface, to the value in the undisturbed flow. The thickness of the boundary layer, i.e. the distance from the body surface to the height of undisturbed flow, ranges from a few millimeters to tens of centimeters (or to several hundred meters in ABL). The thickness of the boundary layer increases with the distance from the first contact area of the air and the obstacle in the flow direction. The thickness of the boundary layer  depends on the size of the body, its surface roughness, air viscosity, flow velocity and the nature of the flow itself, which can be laminar or turbulent. Forces working in the boundary layer are: the inertial force resulting from the mass of the flow and the viscosity force that slows down the flow what causes the formation of a force tangential to the surface, opposite to the flow direction. We may also distinguish forces derived from the pressure, slowing or accelerating the flow depending on the pressure increase or decrease in the flow direction. The resultant effect of these forces may cause a slowdown in the flow, and consequently the possibility of the return movement (reversed flow), known as the boundary layer separation (detachment). The point on the surface to which the flow is returned is the point of separation (detachment) of the boundary layer. Sometimes also the term 'separation bubble' is used since the detachment occurs at a some length of the body. Beyond the point of separation, the reversed flow in a form of a vortex appears. The location of the area at the body surface, where the separation occurs, depends mainly on the shape and surface roughness of the body, velocity and the nature of the flow, which can be laminar or turbulent. Vortices formed in the flow can cause large suction on the surface of the object, of the largest value close to the point of detachment. The shape of the body is crucial for the flow features in its proximity.
If the object has a streamlined cross-section (e.g. airfoils), the air flow adapts to its shape. This kind of sections are used in aviation, or in wind energy engineering as a rotor blades.
The flow around bluff-bodies with sharp edges of a compact cross-sections and crosssections elongated in a direction perpendicular to flow depends on Reynolds number -Re. This dimensionless value represents the ratio of the inertia forces to the viscosity forces. In the field of civil engineering, high values of Re will be taken into account, like Re > 10 5 . In the case of bluff-bodies with sharp edges, at Re > 1000, the turbulent wake region (limited by smaller vortices) is formed. Shear layers separate the streamline flow area (no vortices) from the highly turbulent vortex area called aerodynamic wake (Fig. 6a).
In the case of bluff-bodies with sharp edges elongated in flow direction, boundary layer separates on the windward edges, then at side walls reattaches to the surface (so-called reattachment of the boundary layer), and next at leeward edges separates again to form a narrower wake region. The flow depends not only on the windward surface, but also on the dimensions of the object along the flow direction (Fig. 6b).
The flow around bluff-bodies of oval sections strongly depends on Re. For different ranges of Re, vortex shedding has different character from symmetrical shedding of two vortices, periodical shedding to quasi-periodical vortex shedding. For example, when Re rises above 3.5·10 6 (super-critical range of Re) flow is completely turbulent, while vortex shedding exhibits a quasi-regularity (Fig. 6c). The Navier-Stokes equations describe movement of the fluid (in our casewind) with a use of the principle of mass and momentum conservation. The change of momentum of the fluid element depends on the external pressure and internal viscous forces in the fluid, and it can be represented by three components u, v, w in the following form: Using notations of pressure gradient (grad), divergence (div), Lagrange operator ( 2  ) and a material derivative / D Dt , Navier-Stokes equations take the following form: Taking into account several assumptions, that: 1 The fluid is incompressible (the density is constant or its changes are negligible); 2 Fluid is inviscid, so its viscosity is omitted; 3 Flow is stationary, i.e. the derivatives of the velocity components with respect to time are equal to 0; 4 The external forces are ignored; the Bernoulli equation for the stationary, inviscid and incompressible flow will be obtained: After integration: where: ufluid (wind) speed, 0.5u 2dynamic pressure, pstatic pressure.
On the basis of the Bernoulli equation, the pressure (positive pressure) or suction (negative pressure)the aerodynamic loads acting on the body placed in the air flow which is perpendicular to its surface, can be defined. The pressure on the outer walls of the objects is mainly determined experimentally. It is most convenient to use dimensionless values of pressure coefficients, which are independent on the wind velocity. In practice, the dimensionless coefficient is determined according to the relationship: where: pi -surface pressure at point 'i', p and qstatic pressure and wind speed pressure in undisturbed flow in the front of the object,

Aeroelastic phenomena
Any engineering structure can vibrate due to the action of:  Inertia forces;  Elasticity forces;  Aerodynamic forces. Dynamic phenomena connected with wind action are: 1. Forced vibrationsoccur when the time varying external force (independent from the vibration of the structure) is applied. The example of such behaviour is the dynamic response of the structure. 2. Self-excited vibrationsthe force disappears when the vibrations disappear. The vibrations are controlled by the vibration system itselfthe feedback appears. Examples of such vibrations are flutter, vortex excitation, galloping. The forced, damped vibrations of the given mass are described by the general motion equation: Dynamic features of the structure must be taken into account because dynamic wind action can cause resonance or at least vibrations of unaccepted level. Taking into account dynamic action of the wind and dynamic response of any structure which can vibrate in the wind field the following various wind actions can appear: 1. Dynamic action of gusts. The wind gusts can cause along-wind vibrations of the structure. Fig. 7 illustrates structure's response to wind gusts. The approach was originally developed by Alan Davenport (e.g. 1960Davenport (e.g. , 1965 and currently is used in several wind codes.

Galloping.
A precondition for the galloping is an initial movement of the structure of small stiffness, caused e.g. by detachment of vortices, or by gusts. Most frequently, this phenomenon occurs on overhead power lines, guys of masts, cables of cable-stayed bridges. It is more likely that the galloping will occur if there is ice on a structure, or because of rain, which changes the aerodynamic properties of the cross-section. A scheme of the forces acting on the system during galloping is shown in Fig. 9a. 4. Flutter.
The phenomenon of flutter is generated due to the feedback between vibrations in the direction of different degrees of freedom. In the classic flutter problem which appears for aircraft wings or bridges, the feedback occurs between vertical and torsional vibrations. Such system with two degrees of freedom is shown in Fig. 9b.
Summing up, to assume realistic wind action on engineering structures, all above mentioned aspects (and many more) should be considered.

Experimental methods
When the wind action is analysed, the level of uncertainty is high. To maximize the accuracy, several parameters of wind load must be determined with care. There are three ways which allow to determine features describing wind actions. These are: 1. In-situ (full-scale, real scale) measurements. 2. Model scale measurements. 3. Numerical simulations.

Full-scale tests
Full-scale tests are obviously the most powerful tool in estimation of wind actions. Test could be performed on already erected objects, and thus could provide a database for designers of future structures. This leads to the basic limitation of full-scale tests which results are sometimes impossible to implement in new design or already erected structures. The second limitation is connected to the difficulty of proper instrumentation of the structure. The third, and probably the most important limitation, is a huge cost of measurement installation. Currently, there are limited data gathered from the full-scale experiments.
The majority of results were derived from long-lasting monitoring of bridges, high-rise buildings and roofs of large spans, and also from meteorological measurements of wind field over different terrains. These tests supply data about wind flow around objects of different shapes, wind pressure on outer surfaces of objects or response of the structure to wind action which could be described by vibration accelerations.
To give the better view on various wind engineering full-scale tests, performed during recent years, some examples are enclosed below. The wind features were measured by Roth (2000) who analysed data about wind turbulence in urban areas. Li

Model-scale tests
From the point of view of costs and accuracy, the reasonable alternative to full-scale tests is model-scale testing. The large advantage of such tests is that they are usually carried out when the engineering structure is still in the design stage. Such experiments are performed in water channels or mainly in wind tunnels. First, the scale model of the real structure is created and next it is placed in the special tunnel where artificial wind flow is created with the use of fans.
If the tests in the tunnel are intended to refer to a particular real object, it is necessary to perform appropriate model scaling and flow scaling. Unfortunately, a scaling process raises a number of problems. If one has to determine the external wind pressure on the object, then it is sufficient to scale the object's dimensions and mount the model rigidly in the tunnel. When data about structural response of the object are needed (e.g. structure vibrations induced by the wind load), the simple scaling of geometry is insufficient and appropriate scaling of stiffness and weight distribution along the height/span is necessary. The second scaling problem, often even more difficult to overcome, is the wind structure scaling, so that the boundary layer generated in the tunnel corresponds to the atmospheric boundary layer occurring in the reality. Appropriate (in relation to reality) model and flow scaling, is based on similarity analysis, which gives similarity criteria of objects and flows between model and real scale. Reliable experiments can be conducted, only when similarity criteria are fulfilled.
Through the years, the most common techniques used in wind tunnels were:

Numerical simulation
Computational Wind Engineering (CWE) and Computational Fluid Dynamics (CFD) consist of various types of flow numerical simulations, objects-flow interactions, etc. When talking about CWE one can consider, for example, the simulation of the wind field as the stochastic process. CFD focuses on simulations of turbulent flows around different structures. Recently, the wide summary of CFD past and current achievements, as well as future challenges in wind engineering applications, was described by Blocken (2014).
There are numerous methods which allow to perform simulations of turbulent flows (corresponding to atmospheric flows). CFD methods, adopting different flow models, are: DNS (Direct Numerical Simulation), RANS (Reynolds-Averaged Navier-Stokes), LES (Large Eddy Simulation), DVM (Discrete Vortex Method). Results obtained in wind engineering and building aerodynamics simulations are still not clear and must be based on extensive theoretical knowledge. Therefore, CFD simulations are usually accompanied by model or in situ tests, and the results apply only to the analysed case. Numerical simulations still need to be validated experimentally. Computer simulations are relatively not expensive, but due to the validation necessity, costs can increase dramatically. The large advantage of CFD is that after validation of one case, other cases, e.g. associated with the changes of the angle of wind attack, could be also computed. Graphical summary of methods with respect to the accuracy, the speed of operation and the associated costs, is represented by the diagram shown in Fig. 10.
While conducting CFD analyses, a special attention should be paid to several recommendations, use of which will increase the correctness of obtained results. The most important of them are: 1. For all methods based on grid discretization, the final calculations should be preceded by a grid sensitivity analysis. The correct solution should be grid-independent.  The topics most frequently considered in CFD deal with: atmospheric boundary layer simulations, bluff body aerodynamics, pedestrian-level wind conditions, air pollutant dispersion, flow over complex terrain, ventilation of buildings, wind-driven rain, snow distribution, wind loads on buildings and structures, assessment of wind farms localization, aerodynamics of wind turbines, road vehicle aerodynamics, trains aerodynamics, windborne flying debris, sport aerodynamics, etc.

Current and future issues in wind engineering
The review of recent major topics, undertaken by wind engineering and structure aerodynamics, was made on the basis of the papers published recently, and on the basis of presentations showed during the most important "wind" conferences. The main journal which deals with the subject is Journal of Wind Engineering and Industrial Aerodynamics. The division of main topics of all papers published in JWEIA since 2013 till April, 2016 is compiled in Table 1. Of course, this division is subjective, because in some papers several issues were considered. For example, the development of CFD techniques is presented with the case study related to the cable-stayed bridge or wind turbine, wind loads on roofs are calculated for atmospheric boundary layer as well as for tornado or hurricane, wind structure is described in details on the occasion of wind loads estimation for different structures, etc. Each paper was classified to one topic only, what gave the overall view on major topics. The great majority of papers included: wind tunnel tests, full-scale tests or CFD studies performed for the given structure or the group of structures. In every case, numerical simulations were validated with respect to wind tunnel or full-scale tests.  Table 2. Since many more issues were addressed, the additional topics were added.   The largest number of published papers was devoted to wind energy. The field of wind energy is wide and it is connected with optimization of airfoils and wind turbines. Many of articles referred to CFD simulations and wind tunnel measurements of different types of wind turbines: with horizontal (HAWT) and vertical (VAWT) axis. Large wind farms consisting of clusters of wind turbines were also investigated. Some papers also considered the possible localization of wind farms with respect to energy harvest. More frequently FSI (fluid-structure interaction) and transient numerical simulations were used as the main tool of analyses.
The methods of wind field description are still being developed. Modern measurement techniques allow to perform more exact measurements of wind characteristics in atmospheric boundary layer (ABL). Both, the results of measurements in microscale (less than 2 km), as well as in macroscale (synoptic scale, several hundreds of km) are used in wind engineering. Description of the wind field as the non-Gaussian process is applied to downbursts and is currently being developed. Numerical methods of simulation of Gaussian and non-Gaussian type processes are being expanded. Another problem is connected to the correct implementation of ABL to wind tunnel measurements and to CFD simulations. Whereas, the first matter is rather well known (but still needs investigation), the second is currently under strong interest of researchers. The proper representation of ABL, both in wind tunnels and in numerical domain, results in simulations with the conditions closer to the reality.
Bridges, as one of the most spectacular structures, and moreoverstructures which every year are designed with larger spans, and for which wind action could be a dimensioning load, are under continuous attention of wind engineers. Different models of flatter and buffeting load are developed. Full-scale and wind tunnel results, as well as CFD calculations (sometimes all three experiments together) were presented for various cable-stayed bridges, suspension bridges, footbridges, etc. Optimization of a bridge deck was also investigated.
Many papers dealt with bluff body aerodynamics. Wind tunnel tests concerned flow around circular, square, rectangular 2D cross-sections as well as 3D prisms of different cross-sections. Several papers considered bridge cables of circular cross-section, galloping phenomena of slender elements, etc. The low-rise and medium-rise buildings, which shapes are predominantly rectangular or square, were also included in that topic. Various investigations such as measurements of wind field around buildings, pressures on the outer surfaces or wind impact on claddings were performed.
Large emphasis was also put on extreme winds, like: cyclones, typhoons, tropical storms and tornadoes. Modelling of such extreme wind events concerned mainly downbursts, which precede thunderstorms, and also tornadoes in relation to their impact to the engineering structures. Nowadays, it is possible to model tornadoes in model scale (see WindEEE) as well as in CFD. There were also presented some data from full-scale monitoring of engineering structures, mainly of high-rise buildings during extreme wind events. Another very important issues are: wind hazard assessment and wind vulnerability of structures. These topics are, in many regions of the world, crucial to local people and to strength of structures placed there.
Many papers and conference presentations concerned roofs of various shapes. For the basic rectangular shapes, pressure distribution was investigated, sometimes with respect to so-called conical vorticesvortices which cause large suction on the roof, close to its edge. Surface pressures and the influence of parapets or attics on the flow over the roof were also considered. Different shapes of rectangular roofs, for example stepped roofs, were checked in wind tunnels, full scale or in CFD. Practical problems like different kinds of linings, green roofs with gardens on the top of high-rise buildings, etc. were analysed. Large span roofs over halls or stadiums were measured and also calculated in CFD.
Wind action is dominant in case of high-rise buildings and cannot be neglected. In their case, different combinations of load should be investigated (sometimes crosswind load connected with torsional load could be larger than along-wind load). Modern high-rise buildings are almost always of complicated shapes, based on basic rectangles or ovals. They may have corner modifications, be tilted, tapered, helical, have setbacks, openings or combine various features. It makes every of them a unique structure, vulnerable to the wind load, and that is why they must be modelled and examined in wind tunnel tests.
From the point of view of urban planning and people living in existing settlements, a very important issue is wind comfort at pedestrian level. It is connected to the wind speed which can accelerate significantly in flow contractions (such as those between buildings). The topic of pedestrian comfort is also connected to dispersion of snow during winters, or dispersion of smoke, gases and pollutants all over the year, or to the natural ventilation of the given area. The pollutant dispersion could be analysed in the scale of the building and its nearest surroundings (for example smoke from the chimney on the building roof) or in the scale of the district or even the whole city. Nowadays, CFD simulations play the major role in investigations of pedestrians wind comfort and pollutant dispersion.
Recently, mainly in journal papers, issues connected to road and rail aerodynamics arose. This topic concerns optimization of the aerodynamic shape of road and rail vehicles. Another topic relates to high-speed trains (introduced in many countries in last decade), their shape, but also their impact on the passengers and the vicinity. More frequently, some papers about coupling between vibrations of the structure (for example of the bridge), traffic and wind action were published. These problems can be investigated theoretically or with use of numerical methods, or experimentally. The possibility of the freight railway wagons to roll-over in strong winds was also the matter of interest in recent papers.
Another subject connected with cities and urban planning is an interference phenomenon. It could be interference between high-rise buildings in different configurations, but also between circular hangers of the bridge placed one by one. The influence of the highrise building on the roof of the medium-rise building placed nearby, was also the point of interest of some researchers. Sometimes, the effect of the windward structure on the leeward structure is called shielding effect.
In the civil engineering structures, the group of slender vertical objects containing: towers, chimneys, masts, lattice towers and cranes was distinguished. Different aspects of wind loads, and different aerodynamic phenomena which appear for these objects, were examined in full-scale, model scale or numerically.
Other engineering structures were grouped as uncommon structures. Papers in this group dealt with large structures like domes of various curves, tanks and siloes, cooling towers, pyramids, monument brick structures, membrane structures like umbrellas, inflatable structures, etc. Wind action was also analysed for smaller objects like road signs, traffic lights structures, lighting poles, scaffoldings, cyclone shelters, road windshields, aircooler condensers or for uncommon vehicles like helicopters or air-cushion vehicles. The analyses were sometimes performed in full-scale, but more frequently in model scale in wind tunnels or with use of CFD.
Four groups of other structures described a few times in papers and presentations were distinguished. They are tunnels, solar panels, porous structures and transmission lines. Different experiments with road or rail tunnels were carried out, they included fire propagation, car exhaust propagation, ventilation, etc. Solar panels are another green source of energy beside wind farms. Optimization of pitch angle of solar collectors, their localization on large span-roofs and spacing between collectors, or wind conditions in terrains under solar farms, etc., were the main subjects of interest in this topic. Porous structures are mostly windbreakers placed along roads. The flow through such objects is different than in case of solid ones, and so the wind load is also different. Recently, CFD simulations also addressed this problem. Yet another group of structures are transmission lines and their supporting structures. Dynamic wind action and galloping of lines were main issues of investigations in this topic.
Another part of wind engineering experiments describes flow over complex terrains. Mainly local changes in terrain topography such hills, edges, slopes and valleys were considered (usually investigated with use of CFD simulations). The main results obtained from such tests were wind speed multipliers.
Codification of wind loads is still under elaboration. Calculations carried out according to several standards often give significantly different results. Of course, it is mainly caused by local wind environment and different statistical tools used to describe wind characteristics. There are continuous works on improvements of the wind load description.
The impact of wind on forests, vegetations, single trees and orchards was also taken under consideration. This issue is important mainly for windy places around the world with an intensive agriculture. Respective representation of trees was usually examined in model tests or CFD simulation.
Damping (connected to structures and wind action) was another subject investigated by the researchers. Model tests give reasonable answer how dynamic properties of bridges, highrise buildings or other slender structures would change when dampers are assembled to the structure. In many places around the world, damping of the structure's response to wind action connected with seismic action is one of the most important problems faced by civil engineers.
Damping of structures is accompanied by another interesting questionhow people in high-rise building will react on building vibrations? Full-scale tests of such comfort were performed in high-rise buildings or in special vibrating chambers (where real conditions are simulated). The response of the human body to vibrations was also examined. The internal wind comfort connected with thermal comfort and ventilation are another issues for building occupants.
Environmental actions combined with wind action were also often investigated. This could be wind driven-rain and its impact on buildings. Rivulet or rivulets can form on the surfaces of inclined cables of masts or cable-stayed bridges. They change significantly the aerodynamic properties of the cross-section of the cable and make them susceptible to galloping. Recently, also CFD studies considered this problem. Another environmental action is caused by wind and snow. Accumulation of snow in windy conditions and transport of accumulated snow caused by wind action in terrain or on roofs (so-called snowdrifts) were widely examined by full-scale tests, model tests and CFD simulations. Yet another issue was ice load and ice accretion. Ice storms are relatively rare, but ice accretion on lattice structures, transmission lines or cables can cause dangerous behaviour of the structures and even lead to collapse.
Windborne flying debris are another matter which was considered more frequently. Hurricanes, cyclones, tornadoes, and even strong (not extreme winds), can raise parts of elements or whole small elements from the ground and carry them. They can strike buildings and destroy their façades. This problem was studied in full-scale, and recently also in wind tunnels and with use of CFD.
The relatively new issue for wind engineers is sport aerodynamics. Various experiments on soccer balls, cyclists, ski jumpers or downhill runners were performed. More money is invested in sports, so the need of new investigations also appears. Sail aerodynamic was also considered.
There are always many papers and conference articles about theoretical development of the given problem. Recently, mainly CFD development is theoretically described, and concerns governing equations, methods of solutions or grid improvements in simulations. Systematically, the reviews of achievements of wind engineering or presentations of the new facilities for wind study, or new measurement techniques appeared.

Conclusions
This paper gives a short description of the bases of wind engineering and aerodynamics of structures. Wind field characteristics, the flow around different bodies, governing equations and aeroelastic phenomena are also shortly explained. Three methods of investigations are described: full-scale tests, wind tunnel tests and CFD simulations. Finally, the review of the contemporary topics considered in the wind engineering is given. The number of published papers and given presentations during conferences is an indicator of the significance of the wind engineering topic.
It seems that CFD techniques are in constant development. The computer power rises, new modelling methods appear and there are more and more case studies well validated with model or full-scale tests. More advanced CFD techniques like LES, DES or unsteady RANS give better results (better validation) of many wind engineering problems. Also fullscale test are on rise because of new possibilities and more common monitoring of large structures. It seems that wind tunnel tests will remain on the same level as previously or their number will also rise. The major argument for this is that the measurement techniques in wind tunnel test are more diverse nowadays (e.g. PIV, LDA, etc.) and give more exact results. The necessity of CFD validation makes tunnel tests still a basic experimental tool in wind engineering. New wind facilities, like WOW or WindEEE, open the new perspectives for scientists. Moreover, the tendencies to build longer and higher, and to design structures of unexpected and futuristic shapes will definitely ensure the work to wind engineers.
The very up-to-date problem is the risk evaluation of extreme wind events and the assessment of hazards connected to it. For engineers, the main purpose is to design structures resistant to even extreme winds. Such issues, like windborne flying debris or the description of non-Gaussian processes, associated with extreme winds, must be considered.
The issue of green energy would develop more intensively in the next years, due to climate changes. In developed countriesthere is a necessity to reduce various pollutions, in more poor countriesthere is a need for cheaper energy. That is why different matters associated with wind and solar energy will be investigated even more frequently in the near future.