ASSESSMENT OF THE INFLUENCE OF BUILDING FACADE FACETING ON THE ACCURACY OF WIND LOAD SIMULATION

Introduction: Despite the fact that wind tunnel testing is quite expensive and time-consuming, physical modeling in wind tunnels remains the primary method for determining wind effects on unique buildings and structures. Computational fluid dynamics (CFD) provides more variability, calculations are performed faster and at a lower cost. However, the issue of accuracy of integral characteristics obtained as a result of numerical modeling and, accordingly, verification procedure remains open. Currently, when using numerical modeling results in structural aerodynamics, it is mandatory to verify them with experimental data. In recent years, studies have explored the CFD potential for accurate wind load predictions, but there have not been studies presenting a comprehensive description and implementation of a verification and validation system to analyze wind effects on unique buildings and structures. The purpose of the study was to compare the CFD results with the wind tunnel test data for three different objects, analyze the results, and propose a method for verification and validation of CFD analysis of wind effects on unique buildings and structures. The following methods were used: physical testing of models of unique buildings and structures in a wind tunnel, including a detailed method of experimental studies to determine integral aerodynamic characteristics, as well as numerical modeling of wind effects using ANSYS. Numerical modeling was performed in two setups: with and without virtual wind tunnel modeling. As a result, it is shown that virtual wind tunnel modeling makes it possible to achieve better data consistency when verifying numerical modeling results with physical modeling data, and the proper use of numerical modeling technology can significantly reduce the time and cost of experimental studies in a wind tunnel and/or reduce the design time by decreasing the number of considered loading scenarios.

Wind loading and its effects on single-layer reticulated cylindrical shells

Wind loads represent an important contribution to the design loads of large floating offshore structures during installation, normal operation and in extreme conditions. It impacts the intact stability of the floating structure as well as its mooring loads. In many cases the wind loads are even the dominant load factor. Wind loads can cause large overturning moments, which may have important safety implications during installation and operation. Planning of required tug force capacity for offshore installation, maneuvering, berthing and offloading activities may often strongly depend on reliable wind load predictions. Traditionally, the wind loads acting on floating offshore structures are studied experimentally in wind tunnels, but recent developments in Computational Fluid Dynamics (CFD) make it possible to also compute the wind flow around structures and to also derive the wind loads numerically. In general, the numerical and experimental approaches to establishing wind loads may be considered to be complimentary methods. The Wind Load JIP, in 2014, was initiated with the aim of investigating the accuracy level and consistency of wind tunnel tests and CFD predictions for offshore applications. Wind tunnel tests are commonly required towards the end of the design process and provide the final wind loads for qualification. However, there is today no common industry guidance for setting-up a wind tunnel test experiment for large blunt bodies in an atmospheric boundary layer flow. There is particular uncertainty on how to set up the inflow conditions for the undisturbed atmospheric boundary layer velocity profile and associated turbulence intensity. End users of wind tunnel tests have reported occasions where unexpected relatively large variation in forces was observed for fairly similar geometries. For example, large variations in wind forces have also been reported by Croonenborghs et al. (2013) and Andrillon et al. (2015). The Wind Load JIP facilitated a carefully planned benchmark study for wind tunnel tests. Contributions were obtained from three established large boundary-layer wind tunnel test facilities in Europe. A scale 1:230 model of a modern FPSO design with complex topside geometry was tested on the force balance in the wind tunnel. The tests were conducted with carefully adjusted ISO/Frøya velocity profiles, for a target wind speed of respectively 25 m/s and 45 m/s at 10 m reference height. The ‘blind’ wind tunnel tests were planned in such a way that the wind tunnel test facilities were not informed of each other’s results until after handing in their contribution. The scale 1:230 model was shipped around to be sure of testing the exact same geometry. The JIP also facilitated a CFD benchmark study for the same geometry. The CFD benchmark study was performed ‘blind’ as well and the CFD providers were not informed of each other’s results and also not of the wind tunnel test results. The different CFD participants used different choices for the numerical settings in their calculations. Commonly they opted for steady RANS, mesh size around 30M cells, unstructured meshes, meshes generated with mesh wrapper functionality, low y+, avoidance of wall functions on the test geometry, Menter SST k-co turbulence model and about ten thousand iterations for convergence. The CFD participants had the choice between modeling the exact FPSO geometry as tested in the wind tunnel, or a simplified geometry with equivalent wind area. Both geometry variations were used. The CFD benchmark study showed that CFD can predict the wind tunnel test results fairly well. The shape of the wind load coefficient curves was adequately predicted by all CFD contributions for all six degrees of freedom (CX, CY, CZ, CNX, CNY and CNZ) and for all wind headings between 0 to 360 degrees. The scatter between the CFD contributions was about 10%, which is not too much different from the scatter in the wind tunnel test results. Apart from the random scatter, a 5 to 10% systematic error was observed between the CFD results and the wind tunnel test results. The CFD results slightly under predicted the wind tunnel test results.

Membrane structures are used in the built environment as roof or canopy and must therefore be designed to resist the external conditions. Nonetheless, the topologies of membrane structures are not covered by existing wind load standards and relevant wind load distributions for the basic shapes of these structures are almost not available. To have a realistic analysis of the wind loading, wind tunnel tests can be performed for each design. However, due to the lack of resources or time, for many projects the wind analysis will be based on rough approximations by relying on conventional shapes in the Eurocodes, with applying very high safety factors or designing unsafe structures as risk. Therefore, this paper presents a study of the orientation and curvature dependency of the wind load distributions over hypar roof and canopy structures. This study is performed with a numerical wind tunnel, using CFD with Reynolds averaged Navier Stokes equations. The outcomes are summarised in pressure coefficient distribution plots for most important wind orientations for hypar roofs and canopies with different curvature. The presented pressure coefficient distributions can be used in line with the Eurocode to derive more relevant wind load estimations for hypar membrane structures. These wind load estimations will give the engineer information about the average response of these structures under wind loading and will facilitate more reliable wind design of membrane structures.

Wind loading is very important, even dominant in some cases, to large-span single-layer reticulated shells. At present, usually equivalent static methods based on quasi-steady assumption, as the same as the wind-resistant design of low-rise buildings, are used in the structural design. However, it is not easy to estimate a suitable equivalent static wind load so that the effects of fluctuating component of wind on the structural behaviors, especially on structural stability, can be well considered. In this paper, the effects of fluctuating component of wind load on the stability of a single-layer reticulated spherical shell model are investigated based on wind pressure distribution measured simultaneously in the wind tunnel. Several methods used to estimate the equivalent static wind load distribution for equivalent static wind-resistant design are reviewed. A new simple method from the stability point of view is presented to estimate the most unfavorable wind load distribution considering the effects of fluctuating component on the stability of shells. Finally, with comparisive analyses using different methods, the efficiency of the presented method for wind-resistant analysis of single-layer reticulated shells is established.

Wind action on circular cross section was described in many publications. It finds an application for flue gas ducts, pipelines, silo or chimneys. This study concentrate on elements with diameter greater than 1m. There are well recognized analytical solutions of static calculation with uses Fourier-series for wind distribution. Although during last 10 years numerical methods of solving problems get more popular, especially among young engineers. For surface structures ability of analytical finding internal forces disappears, and Finish Element Method substitutes analytical calculation. Modelling of wind load in FEM programs cause several problems. Using wind load distribution proposed in Eurocode 1-4, or from laboratory test, it is usually necessary to divide circular cross-section into 32 up to 72 rectangular elements. Applying load in that way is the most accurate method to imitate wind load in FE model. From the other hand that take much time, and requires preparing data about distribution before modelling. Applying wind on complicated model, with many independent parts of piping, for at least 2 load cases cause faults and slows down work. This paper shows and compares a few proposal wind load models for numerical calculation. Those models were built to obtain accurate internal forces in compare to Eurocode procedure. Proposed models offers simplification of Geometry in numerical model, and saves of time. It also helps to make FE mesh become independent from structural nodes, lines or divisions. This paper concern on one case of one Reynolds number, with refers to 2m wide cylinder, wind velocity of 22m/s and surface roughness of steel plate – 0,05 mm. This paper compares different wind load distributions, in terms of required number of division of model, time consuming, precision of results. Author selected one proposal load distribution, with give equivalent internal forces as wind load distribution obtained from Wind Flow simulation (for example CFD method). Proposed model is useful for structural engineers and statics in offer stage of project. With some safety factor it can be also used as wind load as case for detailing cylindrical structures.

Structural monitoring of an instrumented experimental single-storey wood building provided important information regarding the behaviour and response of low-rise buildings subjected to wind loads. Foundation wind-induced loads were captured by twenty-seven three-dimensional load cells simultaneously with the envelope pressures and weather characteristics. The distribution of the total wind load to each wall is examined and normalized to the wind speed for each direction. The correlation between loads acting on different wall segments is also quantified in the form of participation factors. In parallel with full-scale findings, a scaled model of the experimental building and its surroundings is tested in a boundary layer wind tunnel. The detailed pressure distribution information for thirty-six wind angles of attack obtained through three different upstream terrain simulations is used to generate expected total uplift wind force. The findings are compared to data acquired directly from the foundation load cells located in the test house. The comparison revealed that in most cases wind tunnel values are within the range of the field results. Discrepancies are somewhat higher for wind tunnel tests conducted using the open/suburban terrain compared to the urban terrain simulation. The results are significant for the improvement of the wind tunnel testing and simulation procedures, for the development of code and standard provisions, as well as for the verification of the finite element numerical model of the test house.

CFD wind tunnel investigation for wind loads of steel television tower with grid structure

This paper investigates the transmission of wind loads through a roofing system of a contemporary house type built in Australia. The distribution of wind loads and associated structural response of batten-to-truss connections, including the effect of failures is investigated. The study found that calculations based on normal design practices can significantly underestimate connection loads, when highly correlated wind pressures act on the roof. A process for assessing the fragility of roof components to wind loads is described. The fragility of components can be combined with the response to progressive modes of failure to develop vulnerability functions for these contemporary houses.

This paper investigates wind load distribution in float PV plants. Wave and wind load are dominant environmental load factors in determining design load in float PV plants. In particular, wind load is determined based on the numerical analysis results. The literature indicates that several input parameters exist, such as inlet angle and space between PV modules. An exemplary structure with ten arrays of PV modules was generated in this study. To investigate the wind load distribution in a float PV plant, the computational fluid dynamic (CFD) analysis was conducted with variables including wind direction (inlet angles) and three wind speeds (36.2, 51.7, and 70 m/s) in PV modules in the floating structure. Based on the numerical analysis, the wind load distribution of PV modules can be characterized with respect to the inlet angle and wind speed. The numerical results show that the wind loads in the central arrays are dominant.

Recently, energy-efficient technologies are employed to buildings in terms of reduction of environmental burdens. Vertical fins on walls of tall building are environmentally-friendly components as sunshade louver. As these components are exposed to outer air, wind loads should be evaluated for wind resistant design of cladding. However, the fluctuating wind pressures or forces acting on vertical fins are generally difficult to predict with accuracy because of the reproducibility in wind tunnel experiments which require geometrically-accurate models and installations of measurement system. On the other hand, recent development of techniques on computational fluid dynamics (CFD) has enabled us to simulate the complicated flow such as the wind around buildings. Large Eddy Simulation (LES) is expected to be adopted as an effective tool to evaluate wind load for wind-resistant design of buildings. As numerical computations can overcome the difficulties of experimental prediction for complicated models of cladding and components, wind loads of vertical fins are thought to be evaluated accurately by LES. For numerical prediction of pressure fields around vertical fins by LES, unstructured grid systems are effective in terms of flexibility in generating the computational model. Present authors previously discussed the applicability of unstructured LES for prediction of the fluctuating wind pressures on building models (isolated square cylinder, two square cylinders and actual building model in a real urban district) by comparison with experiments. In this paper, the fluctuating wind forces acting on the vertical fins on walls of tall building are examined using unstructured LES. Especially, focusing on the size effects of fins, the pressure fluctuations and the flow fields are studied to evaluate wind loads for practical wind resistant design. First, the applicability of unstructured LES for wind force estimation of fins is validated by comparison with wind tunnel experiments. Here, the computation is carried out using the same geometry as the experimental model which has relatively larger fins than actual situation. Strong wind forces act on the fins placed at the corners of building and the computed force distributions coincide with experiments with accuracy. Next, the wind forces acting on the downscaled fins are computed to simulate the real situation. The flow fields and the wind force fluctuations are quite different from those of experimental (large-scaled fin) model. For the case of downscaled fins, the pressure fields around fins are dominantly influenced by the turbulence structures formed by the building scale. The scale effects on the wind load evaluation of cladding components, which can be examined by only numerical simulations, is clarified. Moreover, some cases for practical investigation are also computed (corner geometry and wind direction). As the overall trend, the maximum forces of fins at lower height become larger than other heights. In order to elucidate these phenomena, flow fields at the upwind region are visualized. The vertical flow promoted by the configuration of fins intensifies the horseshoe vortex in upwind region and the secondary vortex appears near the upwind wall on the ground level. As a conclusion of this study, for evaluation of design wind load of cladding components such as vertical fins, the scale effects should be considered for accurate prediction of pressure fluctuation. Unstructured LES is available to evaluate the wind loads of vertical fins on walls of buildings and effective to elucidate the flow mechanism on the complicated surfaces of buildings.

Prototyping of thin shell wind tunnel models to facilitate experimental wind load analysis on curved canopy structures

Large-span flexible photovoltaic (PV) arrays exhibit irregular chaotic behavior due to temporal variability and spatial non-uniformity, making the prediction of extreme wind load highly challenging. Current standards and engineering practices also lack effective strategies for wind load mitigation and chaos suppression. To address this, synchronous wind tunnel pressure measurements conducted on large-span flexible PV arrays, with and without four representative flow-altering devices (shaped as >, <, Γ, and L). The distribution of wind load and chaotic characteristics under 0° and 180° inflow angles were compared and analyzed. Computational fluid dynamics simulations were further employed to reveal the mechanisms of load reduction and chaos suppression. Based on the peak factor, chaos–non-Gaussian joint extreme wind load models are developed for five PV array configurations. Results show that the devices significantly reduce the maximum Lyapunov exponent, sample entropy, and extreme positive and negative pressure coefficients, with average reductions of 47.6%, 21.7%, 16.7%, and 11.1%, respectively. Load reduction is primarily due to vortex separation and reattachment-induced low-pressure zones, while stable large-scale vortices and organized flow structures help suppress chaotic fluctuations. The proposed model improves prediction accuracy by 46.6% and 19.6% compared to conventional peak factor and Hermite polynomial methods. Among all devices, the >-type performs best in reducing loads and chaotic intensity, followed by the <-type, with the Γ- and L-types being less effective.

Collating Wind Data for Doubly-curved Shapes of Tensioned Surface Structures (Round Robin Exercise 3)

Numerical prediction on environmental loads for the first 40 MW floating photovoltaic power station built in China is carried out. Based on the verification from the comparison with the measured data in a wind tunnel, a CFD (Computational Fluid Dynamics) method is applied to study the wind load and the current load on a large-scale square array. CFD numerical investigation verifies it feasible to adopt a simplified floating body model instead of the original one. The computational results of the model monomer are in a good agreement with the wind tunnel measured data. The 3D full scale square array's simplified calculations based on a coarse grid are carried out, and the distributions of wind loading on the array under various wind direction angles are obtained. Through the analysis of the distribution of maximum wind loading at north wind, the strategy of 2.5D calculation is put forward and numerically verified. According to the computational results, an 18(row)×11(column) scale model array test plan is designed and performed in a wind tunnel. The measured results are in a good agreement with CFD results, thus a global numerical calculation algorithm is set up. The 2.5D calculation based on a fine grid provides correction coefficients to rectify the 3D computational result to obtain more accurate wind loads. The calculation method for current loads is similar to that of wind loads. The numerical computation and analysis on the wave load are performed by the potential flow method. The wave load variations, at regular waves with unit wave amplitude, along the row or the column are obtained. According to the variation, the whole wave load is estimated. The wave load on the array is predicted at the extreme condition of a fifty-year return period. This study provides a method for the numerical prediction of wind loading, current loading and wave loading on a floating photovoltaic power station. The researches provide a technical support for anchoring the calculation of floating square array and the system design of floating photovoltaic power stations.