Reliability-based fatigue design of wind-turbine rotor blades

A probabilistic model for analysis of the safety of a wind-turbine rotor blade against fatigue failure in flapwise bending is presented. The model is based on a Miner's rule approach to cumulative damage and capitalizes on a conventional S–N curve formulation for fatigue resistance in conjunction with a new ‘distorted Weibull' distribution for representation of wind-induced bending moment ranges. The model accounts for inherent variability and statistical uncertainty in load and resistance, and model uncertainties are also included. The model is applied to an analysis of the reliability of a site-specific wind turbine of a prescribed make. A 20-year design lifetime is considered. The probability of fatigue failure in flapwise bending of the rotor blade is calculated by means of a first-order reliability method. It is demonstrated how the reliability analysis results can be used to calibrate partial safety factors for load and resistance for use in conventional deterministic fatigue design.     

Simulation of a spatially correlated turbulent velocity field using biorthogonal decomposition

This paper presents a method for generating a turbulent velocity field that can be used as an input for the temporal simulation in wind-excited structure problems. Temporal simulations become necessary when nonlinear behaviour, in the structure or in aeroelastic forces, must be accounted for. The main difficulty is then to reproduce correctly the statistical properties of the atmospheric turbulence, especially the spatial correlation. These properties constitute here the targets that the generated signal has to satisfy. We propose to use the biorthogonal decomposition technique which possesses interesting features to reach this objective, notably the space-time symmetry. Moreover, the convergence in energy is obtained rapidly with few terms of the decomposition, particularly in the low-frequency range. Thus the method is found suitable for application to large civil engineering structures, such as bridges. Examples are provided for two different kinds of wind.     

大跨度桥梁的等效脉动风速模拟

鉴于脉动风速时程模拟是计算大跨桥梁结构在脉动风作用下响应的前提,采用Deodatis’谐波合成法,通过模拟桥面处水平和竖向的等效风速时程曲线,为进一步计算桥梁的抖振响应提供了方便,并验证了该方法的有效性。     

An experimental study of low concentration sludge settling velocity under turbulent condition

Particle Image Velocimetry (PIV) was used to study the settling of activated sludge flocs under turbulent flow conditions. Experimental results showed that a larger particle diameter led to a higher settling velocity while the higher turbulence intensity led to lower settling velocity. Based on the measurements a mathematical relation has been derived which correlates the settling velocity for individual sludge flocs under turbulent conditions through a modified Vesilind equation. Settling velocity shows a power-type relation to sludge particle diameter and an exponential-type relation with turbulence intensity and sludge concentration.     

Evaluation of the indoor temperature field using a given air velocity distribution

A new method for compensating the space discretization error introduced when the fixed flow field is considered for the dynamic models of temperature distribution is presented. It is proved that the method generally used in literature is a particular solution of the proposed one. Moreover, it results in a continuous-time model, for which the integrating method becomes a free choice and a state-space representation is possible. The numerical model was experimentally validated, the comparison, both in the time and in the frequency domains, between simulation and measured results showing good agreement. The presented dynamic model increases the calculation speed and it can be analysed with the tools developed in control theory.     

Mean velocity field around prismatic bodies in tandem arrangement

This experimental investigation reports the wind tunnel measurements of the mean velocity fields around (a) an isolated square prismatic body with height to width ratio (h/b) of 3 and (b) the same body in tandem with an identical interfering body. In the case of the studies involving interference, the ratios of the centre to centre distance to the body width (G/b) are 4 and 8. The mean velocity components are measured using a Conrad six-hole probe. The approach flow is atmospheric boundary layer with the velocity profile satisfying th power law. It is observed that the effect of interference is considerably large when G/b is small. Due to interference (i) the re-attachment distance shortens and (ii) the maximum values of transverse and vertical velocities are small. The data should be useful for validating computational codes.     

旋风除尘器内流场测试中三维速度的计算方法

在利用五孔探针测试旋风除尘器内流场时，探针标定情况及实验模型不同，所应采用的一一维速度计算公式也就不同，这一点在测试研究中常常被饭视，从而得出错误结论而该错误又不易被发现。给出了各种情况下三维速度的计算公式及探针测也布和β角正负规定的自我判别方法。     

Characteristics of some organised structures in the turbulent wind above and within a spruce forest from field measurements

The present paper describes an evaluation of turbulent wind data acquired at the measuring tower installed in the Solling spruce forest by the Institute of Bioclimatology at the Georg Augustus University at Goettingen. Simultaneous wind velocity data were analysed from two measuring points: at the mixing layer above the forest canopy, and at a height of 2 m, well within the forest, by means of quadrant analysis, correlation analysis and wavelet techniques.The results suggest two types of phenomena coexisting in the upper mixing layer region of the flow: large-scale oscillations independent of the roughness type, and smaller-scale structures, of frequent penetration, related to vertical transport phenomena.The aim of the present work was to describe aspects of the mechanism involved in the generation of the known canopy size vortices, which transport momentum from the mixing layer region deep within the canopy. This work discusses aspects of the interaction between the mixing layer region above the forest and a particular low mean velocity region near the ground, submitted to intermittent shear by the nearby vortex structures and influenced by features of the canopy top mixing layer.     

Modelling and simulation of the near-wall velocity of a turbulent ceiling attached plane jet after its impingement with the corner

At present, ceiling-mounted diffusers are very popular for indoor air distribution, particularly in offices, owing to greater efficiency in the distribution of the air supply and a more comfortable indoor environment. The objective of this study is to construct an effective model to design the indoor airflow of an attached plane jet after its impingement with the corner in a room. In this study, a full-scale test facility was set up to obtain detailed experimental data. One commercial CFD tool, CFX 11.0, was used to simulate the air velocity distribution of an attached plane air jet bounded by the ceiling and an insulated wall. One semi-empirical model was also constructed to predict the impingement jet velocity. The results show that bout the semi-empirical model and CFX 11.0 were able to predict the maximum velocity of an impinging jet at low Reynolds numbers, 1000 and 2000, with an inaccuracy of ±11%. However, the semi-empirical model could be more conveniently used to predict the maximum jet velocity decay after its impingement the corner in a room than CFD simulation in terms of accuracy and the time required to design the indoor airflow pattern.     

地下车库诱导通风系统流场特性实验研究

针对地下车库诱导通风系统的工作过程,建立了实体实验模型,测试了4种工况下流场中2个断面的速度场分布情况,分析了不同情况下诱导通风系统速度场的变化规律,得出地下车库诱导通风系统作用下浓度场的可能变化规律。     

置换通风的实验研究

通过对单一热源情况的实验,得到了置换通风时垂直方向的温度梯度和三维温度场,并对不同墙壁的传热系数和不同的外环境温度作了４个对比实验,结果表明：①热源不影响房间温度水平方向上的均匀度,热源或污染源无横向扩散;②除热源上方有较大的上升气流外,整个速度场均匀平稳;③围护结构的热损失对温跃层高度无明显影响,外部环境温度则使室内温度垂直分布有所改变。     

Laboratory measurements of velocity and turbulence field behind porous fences

Flow characteristics of turbulent wake behind porous fences have been investigated experimentally. The velocity fields were measured using the two-frame PTV method in a circulating water channel. The fence models used in this study have geometric porosity () of 0%, 20%, 40% and 65%, respectively. Each fence model was located in uniform flow whose boundary layer thickness (δ) at the fence location was about 0.1 of the fence height (H). Among the porous fences used in this study, the porous fence with porosity =20% shows the maximum reduction of mean streamwise velocity, but it has the highest vertical mean velocity at about x/H=1 location and large turbulence intensity in the near wake region. However, the porous fence with =40% has good flow characteristics for abating wind erosion with small turbulent fluctuations and a relatively large reduction in mean velocity. Except for the solid fence (=0%), two shear layers develop from the porous fences. As the fence porosity () increases, the height of the shear layer and the streamline curvature decrease. When the porosity () is greater than 40%, there is no re-circulation flow behind the fence due to the strong bleed flow, the Reynolds shear stress is nearly negligible in the entire near-wake region and relatively small turbulent kinetic energies are concentrated in the region just behind the fence (x/H<0.5). When the fence porosity is less than 20%, the Reynolds shear stress and turbulent kinetic energy are strong over the fence and in the shear layer near the reattachment region.     

On the significance of transient heat release rate excursions above a set limit

When a heat release rate limit for a consumer product is set by a regulatory agency, it is of interest to know whether small excursions above that limit, such as may occur due to production line variability, represent a disproportionate increase in fire hazard. This paper presents a methodology to examine this issue. The heat release rate curve of the object is described by a Gaussian time variation; a perturbation peak, also Gaussian, is added to this main peak. The impacts of the perturbation peak on the build up of hazardous conditions in a room fire (where the object is the only item burning) and on the threat of ignition of secondary items are examined. For the peak heat release rate domain studied here, only the ignition threat is significantly affected by the perturbation peak. The results quantify the trade-off between the height of the perturbation peak and its duration for a fixed percentage of increase in the room area threatened by secondary object ignition. The results show that the increased threat is of the same order as the relative perturbation in heat release rate.     

Ventilation measurements at model scale in a turbulent flow

Ventilation rates have been measured in a model building in a wind tunnel. Two types of opening, circular holes and model windows, have been tested under two wind conditions. One wind condition was selected to give the maximum flow through the model, while for the other condition the ventilation was due mainly to turbulent pressure fluctuations. The different characteristics of the two types of opening are illustrated. Comparisons are made between the measurements and theoretical predictions. The use of wind tunnels for ventilation studies is discussed.     

Flow around a cubic body immersed in a turbulent boundary layer

This paper reports the results of an experiment concerning flow in the vicinity of a cubic body immersed in a turbulent boundary layer which is fully developed along a tunnel floor. By changing the flow direction and body height, measurements were made of the pressure and drag acting on the body, and of the flow pattern and pressure distribution on the floor. The results indicate the changes taking place in the pressure distribution over the surface of a cubic body, the pressure drag acting on it, and the flow pattern and pressure distribution over the floor surface as the angle of incidence α is varied.     

Orthogonal expansion of Gaussian wind velocity field and PDEM-based vibration analysis of wind-excited structures

An effective procedure for simulation of random wind velocity field by the orthogonal expansion method is proposed in this paper. The procedure starts with decomposing the fluctuating wind velocity field into a product of a stochastic process and a random field, which represent the time property and the spatial correlation property of wind velocity fluctuations, respectively. By an innovative orthogonal expansion technology, the stochastic process for wind velocity fluctuations may be represented as a finite sum of deterministic time functions with corresponding uncorrelated random coefficients. Similarly, the random field can be expressed as a combination form with only a few random variables by the Karhunen-Loeve decomposition. This approach actually simulates the wind velocity field with stochastic functions other than methods such as spectral representation and proper orthogonal decomposition. In the second part of the paper, the probability density evolution method (PDEM) is employed to predict the stochastic dynamic response of structures subjected to wind excitations. In the PDEM, a completely uncoupled one-dimensional partial differential equation, the generalized density evolution equation, plays a central role in governing the stochastic responses of structures. The solution of this equation will give rise to instantaneous probability density function of the responses. Finally, the accuracy and effectiveness of the approach in representing the random wind velocity field and PDEM-based dynamic response of wind-excited building are investigated.     

An analysis of a jet into a turbulent ambient fluid

A comparison of the results of an analysis for the mixing of a passive tracer in a uniform current with the results of two jet analyses, one based on the concept of entrainment, the other based on the concept of turbulent diffusion, suggests that the effect of mixing due to turbulence in the ambient fluid can be included in the jet analysis based on entrainment by adding a term to the integrated continuity equation. For a two-dimensional jet that is discharged parallel to the mean velocity of a turbulent ambient fluid, the local ratio of the effects of jet mixing to ambient mixing is given by 0·40 Eb•u•/•, where E is the entrainment coefficient, b• is a local measure of the jet width, u• is the local maximum excess jet velocity, and • is the local ambient turbulent diffusion coefficient. A numerical solution shows that an increase in the ambient turbulent diffusion coefficient causes a decrease in the maximum concentrations and an increase in the lateral spreading of the discharged fluid.     

Experimental investigation of the velocity field and airflow pattern generated by cooling ceiling beams

In the modern office environment there are numerous heat generating equipment, heat loads from solar radiation and heat produced by people. The loads will often exceed the load the ventilation system can cope with. To meet this demand on extra cooling capacity the commercial market provides cooling ceiling panels and chilled beams. A chilled beam is a source of natural convection, creating a flow, vulnerable to disturbances, of cold air into the occupied zone. Experiments have been conducted in a mock up of an office room; qualitative information has been obtained by visualisation. Instantaneous velocity profiles of the airflow generated by the chilled beam has been registered. In addition, the temperature field below the chilled beam has been measured with a whole field measuring technique. The results show that the airflow from the chilled beam has behaviour similar to a two-dimensional plume but exhibits strong oscillation both sideways and along the chilled beam. These oscillations (intermittence) might cause a sensation of draught but in order to clarify this further investigations are required. Furthermore, airflow generated by heat sources in the room may reverse the flow generated by the chilled beam.