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The present study is devoted for the development of two equation Reynolds-Averaged Navier–Stokes (RANS) closures for computational fluid dynamics (CFD) modelling of the atmospheric boundary layer. By using the inflow conditions based on the Monin–Obukhov similarity theory, the closure coefficients of the proposed models are derived from the analytical solutions of simplified turbulent transport equations. Modifications are conducted for three different turbulence models, which are standard k − ϵ , k − ω and Re-Normalisation Group (RNG) k − ϵ . Numerical experiments are performed for the homogeneous atmospheric boundary layer and the results are compared with the theoretical values in comparison to the standard versions of modified turbulence models. Developed models are implemented to open source CFD toolbox OpenFOAM. [ABSTRACT FROM AUTHOR]

Abstract: Full-scale and reduced-scale measurements along with numerical simulations have been carried out in order to increase fire safety in car parks, with particular focus on the smoke back-layering (SBL). SBL is the distance covered by the smoke upstream of the ventilation flow with respect to the fire source. It has been measured in both full- and reduced-scales and the results are compared to the literature. The investigated car park (full- and reduced-scale) has been simplified: ceiling and walls are plain and the final layout is similar to a very wide road tunnel. Horizontal and vertical beams supporting the ceiling are not considered. Therefore the results obtained are only valid for large closed parks with a flat ceiling and uni-directional smoke and ventilation patterns within the investigated heat release rate range. The study serves double purposes: Empirical formulae can be obtained and the best model for SBL can be chosen from the existing correlations for road tunnel fires to be applied to car parks with the above described simplified geometries. If the results of the reduced-scale car park model are in accordance with the full-scale ones, the reduced-scale model can be used for more complex car park layouts, which is a very favorable solution in terms of both time and costs. In addition to the SBL investigation, unexpected flow phenomena at the entrance and extractors, respectively, are also explained by reduced-scale trials. [Copyright &y& Elsevier]

Abstract: The Belgian Antarctic Base Princess Elisabeth is based on an elevated building on top of sloping terrain and connected to an under-snow garage. The integrated design of the base was supported by wind engineering testing that looked into building aerodynamics (pressure taps) and snowdrift management. Wind tunnel modeling using sand erosion technique allowed efficient evaluation of the snow erosion and deposition around different building-block shapes during the conceptual design phase. Parametric testing shows that the positioning of the main building on the ridge has a significant impact on wind loading and snow erosion and deposition. Important reductions in wind loading and snow deposition can be obtained by elevating the building and reducing the windward cantilever. The positioning of the garage roof can further decrease the wind loading by acting as a diffuser in the back of the building. This study shows that, not only for safety and cost reduction but also for the integration of renewable energies, important benefits in the design of a building can be achieved if wind engineering is considered since the conceptual phase of the integrated building design process. [Copyright &y& Elsevier]

Abstract: One of the major challenges of the wind energy sector is to accurately predict the wind potential. This task is especially difficult in mountainous terrains where the topography can imply complex relief-induced flows. Wind tunnel testing is one of the possibilities to simulate and predict the wind for wind turbine micro-siting. Most advanced quantitative measurement techniques can be used in the wind tunnel, however, measuring the whole terrain to find the highest wind potential zones is very time-consuming. This paper proposes to use a very simple, quick and cheap technique to detect and evaluate the high wind speed areas over an entire model. Commonly used for pedestrian wind comfort assessment, the sand erosion technique is here applied to wind resource assessment. The technique can provide valuable qualitative information but can also give an order of magnitude of the local speed-up. It is first applied to a backward facing step flow and then on a mountainous terrain. An amplification factor and the fractional speed-up ratio (FSR) can be calculated over the entire mountain. For high speed positions results extracted from sand erosion appears to be comparable the one calculated by particle image velocimetry. The technique is repeatable, able to perform a detection of the high speed area, and capable of giving an estimation of the amplitude of the wind. The technique allows to restrict the use of quantitative measurements to the most interesting areas. [Copyright &y& Elsevier]

The pollutant dispersion in a two-dimensional street canyon is studied in this project. The principal parameter investigated is the height of the downstream building. The pollutant source is situated in the middle of the street. The investigation is performed in two ways. Experiments have been carried out in the L-2B wind tunnel at von Karman Institute and numerical simulations have been done with the CFD software Fluent 5.2. The concentration measurements have been performed by means of light scattering technique and the velocity field has been measured with particle image velocimetry. In the numerical simulations, a preliminary study about the backward-facing step has been performed in order to select the best turbulence model in Fluent for these complex flows characterized by separation, stagnation, recirculation, reattachment, etc. The best model appeared to be the realizable k–&epsiv; model with the two-layer zonal approach to the wall, which predicts the reattachment length after the step with <1% error in comparison with the value obtained from direct numerical simulation by Le, Moin and Kim (Direct numerical simulation of turbulent flow over a backward-facing step, Report No. TF-58, 1996). This model has been applied in the street canyon simulations. Comparison with the experimental results has been made.Besides the height of the downstream building, the influence of a third building situated upstream of the street canyon in the flow and dispersion inside the street has been investigated. [Copyright &y& Elsevier]

Windblown sand flow interacts with a number of surface-mounted human-built obstacles. The wind-sand flow perturbation and resulting morphodynamic response of the sand bed cannot be assessed in analytical terms. Therefore, wind-sand tunnel studies around scale physical models are often carried out. They should be driven by physical similarity theory based on dimensionless numbers referred to the whole multiphase and multiscale flow. However, similarity requirements cannot be fully satisfied under typical testing conditions and attention should be paid on the extent of the similarity relaxation. In this study, the background of wind-sand tunnel testing of surface-mounted obstacles is recalled by reviewing wind tunnel setups and similarity requirements. Then, a wind-sand tunnel campaign on a Sand Mitigation Measure is described and critically discussed. The setup dimensionless numbers are compared with statistics on those of past studies. The inescapable relaxation of similarity requirements is motivated by the test goals. The time evolution towards in-equilibrium conditions of both sand bed morphodynamics and sand transport is discussed. Finally, the results of engineering interest are described: the Sand Mitigation Measure sand trapping performance is assessed in dimensionless terms through the measurements of the incoming and outgoing sand concentration in air. • Wind-Sand Tunnel Tests are categorized based on obstacle types and setup conditions. • Similarity requirements are reviewed and their relaxation is critically discussed. • In-equilibrium conditions around the tested Sand Mitigation Measure are analysed • The aerodynamic working principle is confirmed through sand bed morphodynamics. • Sand trapping performance is assessed by a dimensionless sedimentation coefficient. [ABSTRACT FROM AUTHOR]

A validation study is carried out for a generalized actuator disk model (GAD) implemented into the Weather Research and Forecasting model, an open-source numerical weather prediction code, in order to simulate the aerodynamic behavior of a real-scale wind turbine under varying atmospheric conditions. Multiple large-eddy simulations (LESs) are performed to resolve energy-containing eddies of turbulent motions utilizing the GAD model, which calculates the wind turbine-induced forces distributed over the rotor disk. The benchmarks defined at the Scaled Wind Technology Facility (SWiFT) campaign, (for details see Doubrawa et al., 2020), were chosen to validate the performance of the GAD model in terms of its capability to reproduce the wake and aerodynamic loading on the rotor. Meteorological data are available from a 60 m meteorological tower located 65 m upstream of the wind turbine, and the aerodynamic data, including scans of downstream velocity profiles, are available for the Vestas V27 wind turbine thanks to DTU's nacelle-mounted, rear-facing SpinnerLidar (Mikkelsen et al., 2013). Rotor performance and wake recovery results obtained from the GAD model are compared with field experiments and other LES data. • Large-eddy simulations of the Vestas V27 wind turbine under varying atmospheric conditions are presented. • A generalized actuator disk model is validated based on field measurements and available LES data. • The wake behavior and aerodynamic response of the wind turbine are investigated for different ABL conditions. • Accurate modeling of wind turbine wakes under stably-stratified ABL is highly dependent on grid resolution. • The generalized actuator disk model performs as good as a generalized actuator line model in certain situations. [ABSTRACT FROM AUTHOR]

A generalized actuator line model (GAL) is implemented in the Weather Research and Forecasting (WRF) (Skamarock and Klemp, 2008) model to carry out high-fidelity large-eddy simulations (LESs) of turbulent wind fields in stratified atmospheric boundary layer (ABL) flows and study the effects of atmospheric stability on the wake characteristics and aerodynamic response of a wind turbine (WT). The performance of Vestas V27 WT has been evaluated paying attention to the wind turbine wake behavior and the aerodynamic performance using the already available generalized actuator disk (GAD) and the recently implemented GAL model. Spatial distributions of mean velocity components, their variances and instantaneous vorticity in the wake of the wind turbine are analyzed by comparing the results from GAD and GAL approaches. Results of the wake velocity deficit profiles and wind turbine aerodynamic response have been compared with the experimental ones obtained in the Scaled Wind Farm Technology campaign and available LES data reported in the works of Doubrawa et al. (2020) and Jézéquel et al. (2021). The results from the GAD and GAL models, including the aerodynamic effects of nacelle and tower, agree reasonably well with the results from other LES codes and experimental data, with minor differences between these two models regarding velocity deficits in the near-wake region and wind turbine response under varying atmospheric stability conditions. Overall, the GAL model is able to reproduce the observed wake behavior and the aerodynamic response of the Vestas V27 for the atmospheric conditions tested. • GAD and GAL model performances were compared under realistic atmospheric conditions. • GAL model was validated with field data, available LES and recent GAD model results. • Vestas V27 WT performance was thoroughly analyzed under varying ABL conditions. • GAD and GAL models had similar wake behavior at relatively coarse grid resolutions. • GAD model slightly outperformed GAL in certain cases at the tested grid resolutions. [ABSTRACT FROM AUTHOR]

The present study was devoted to the development and application of a remote microphone technique for aeroacoustic measurements in large aerodynamic wind tunnels. In this technique, the microphone and its connecting line to the sensing port are fitted within an aerodynamically streamlined fairing. A model-based calibration method was applied to account for the phase lag and dissipation within the microphone connecting line. The proposed system permits to carry out acoustic measurements when the wind tunnel surface cannot be altered, since the fairing can be simply glued on the wall. The fairing is streamlined to minimize flow disturbances as well as the risk of separation which would otherwise contaminate the wind tunnel flow quality and acoustic measurements. It was furthermore hoped that the acceleration of the flow over the fairing surface might reduce the turbulence fluctuations and thereby increase the signal-to-noise ratio of the acoustic measurements. Those aerodynamic effects were investigated with a set of hot-wire measurements performed in the subsonic L2B small wind tunnel of von Karman Institute (VKI). The mean velocity and the turbulence intensity profile were analyzed in the presence of different inflow turbulence characteristics at 5, 10 and 15 m/s freestream velocity. The effect of incident turbulence on the acoustic measurement was investigated applying the calibration procedure and comparing the measurements with a wall-mounted reference microphone signal in similar flow conditions. The results indicate that the wished beneficial effects of the flow acceleration are not significant. As an application, this technique is used to measure the noise emitted from the contra-rotating fans of the large L1 subsonic wind tunnel of the von Karman Institute, where the microphone fairing is placed inside the diffuser and thus subjected to a thick turbulent boundary layer. The results indicate that in spite of the mitigated turbulence-reduction performance of the fairing, it provides a measurement solution that is suitable for large wind tunnels provided the signals are compensated using the measured dynamic calibration. While it should be stressed that the calibration procedure itself is not new, the originality of the present paper stands in the proposed fairing design, and in the aerodynamic/acoustic investigation of its effect on the flow. [ABSTRACT FROM AUTHOR]

Floating Modular Energy Islands (FMEIs) are modularized, interconnected floating structures that function together to produce, store, possibly convert and transport renewable energy. Recent technological advancements in the offshore energy sector indicate that the concept of floating offshore energy islands has the potential to become more cost-effective and more widespread than previously anticipated. This review is specifically meant as a basis for the development of new approaches to the sustainable exploitation of multi-energy sources in the offshore environment leveraging the know-how of existing technologies and, at the same time, exploring new solutions for the specific challenges of FMEIs. The paper critically analyzes the current state of data-driven approaches and structural health monitoring techniques in the offshore energy sector. It also covers topics such as met-ocean data, loads estimation, platform dynamics, coupling actions, nonlinear dynamics of mooring lines, modelling considerations, and control of electrical subsystems. It is believed that this systematic and multidisciplinary review will facilitate synergies and further enhance research and development of offshore renewable energies. • Floating Modular Energy Islands. • Data-driven approaches and Structural Health Monitoring techniques. • Loads estimation, platform dynamics and modelling considerations. • Dynamics of mooring lines and modelling aspects. • Control of electrical subsystem. [ABSTRACT FROM AUTHOR]

Simultaneous measurement of velocity and concentration provides a tool for studying the turbulent diffusion. This quantity until now was mainly modeled and not measured and it plays an essential role in turbulent diffusion of pollutant.We present a new technique to measure the turbulent mass flux vector, u′c′, based on the same PIV/PTV images, employed for both instantaneous velocity and concentration measurements. Then we applied the new technique to check the correctness of the model widely used for the turbulent mass flux in RANS simulations, in a street canyon type flow. We found places where the turbulent diffusion is opposite to the gradient in mean concentration. [Copyright &y& Elsevier]

Offshore wind energy has seen a steady increase in the latest years as it serves as an ideal alternative energy source to meet our renewable energy goals. Due to this increase in interest and in installations of offshore wind farms a better understanding of the impact of waves on the power production of wind farms in necessary. This wave-wind farm power production interaction has scarcely been examined, however, some exceptions of small scale large-eddy-simulations exist. Unfortunately, these studies are not able to take multiple wind farms under real weather conditions into account. Here, we show the power production and wake lengths of 1250 offshore wind turbines located in the German Bight simulated by a stand-alone atmospheric (WRF) model and a coupled atmosphere-wave (WRF-SWAN) model. The coupled atmosphere-wave model estimates larger (smaller) power production in case of waves and wind traveling in the same (opposed) direction compared to the stand-alone atmospheric model. The relative difference between the two models can be as much as 20% for the two-week averaged grid power production, while the difference in total power over this two-week period is equal to 9%. Moreover, the wind farm wake lengths of waves and wind traveling in the same (opposed) direction are longer (shorter) for the coupled atmosphere-wave model compared to the stand-alone atmospheric model. Here, the relative difference of the mean wake length between the two models can be up to 25%. This shows the importance of waves as a part of the offshore wind environment and can be an important factor in future wind assessment or power production estimation studies. [Display omitted] • Coupled model has higher power for aligned wind-wave directions than atmosphere model. • Coupled model has lower power for opposed wind-wave directions than atmosphere model. • Coupled model has longer wakes for aligned wind-wave directions than atmosphere model. • Coupled model has shorter wakes for opposed wind-wave directions than atmosphere model. • Relative differences between models are up to 20% in power and 25% in wake length. [ABSTRACT FROM AUTHOR]

The assessment of the aerodynamic forces acting on transmission towers is of crucial importance for their design. These predictions are mainly based on an extrapolation of wind tunnel measurements done on simple structures which are the base of the present design codes. This extrapolation results in an uncertainty which often leads to insufficiently accurate drag force predictions, because of a lack of agreement between the basis of the design codes and their use for tall and complex tower geometries. Therefore, in the present study drag forces are measured in a wind tunnel on a scaled transmission tower and three representative sections of it. Results show similar drag coefficients for the entire tower and the different sections it is composed of. Additionally, the flow fields in the wake of these structures are measured with PIV (Particle Image Velocimetry). With low lattice densities the effect of each lattice element can be seen in the wake, while the lattice structures act as a porous media for higher densities. Based on these results a CFD (Computational Fluid Dynamics) approach, in which the transmission tower is modelled as a porous media, is proposed in this paper. The CFD simulations are performed substituting the tower geometry with a representative porous model, decreasing considerably the computational time and cost of the simulations. A validation against the experiments and classical CFD simulations, where the detailed geometries are resolved, is performed and shows the applicability of the developed approach. The drag forces as well as the velocity deficit in the wake of the transmission tower are well predicted with the porous CFD simulations. For regions with low lattice densities the porous CFD simulations cannot predict the effect of individual lattice elements on the flow field, because the individual lattice elements are not explicitly resolved. [ABSTRACT FROM AUTHOR]

This study focuses on bridging the gap between the turbulence modelling methodologies of meterological and engineering codes by proposing a novel methodology to define the closure coefficients of Reynolds-Averaged Navier-Stokes turbulence models consistently with the physics of the atmospheric boundary layer. In this framework, different turbulence closures have been developed and tested on different full-scale test cases corresponding to different atmospheric stability conditions by performing microscale simulations with the inflow conditions provided by a numerical weather prediction (NWP) code. Developed turbulence models have been implemented into the open source computational fluid dynamics (CFD) toolbox, OpenFOAM and the inflow conditions have been acquired with another open source code, the Weather Research and Forecasting (WRF) model. [ABSTRACT FROM AUTHOR]

A better understanding of the wind–wave direction effect on the wind field and on the momentum transfer at the air–sea interaction will aid in the development of numerical atmospheric and wave models, important for wind energy studies among other applications. So far, the effect of opposed wind–wave directions on the wind field is not fully understood, even though it is shown to be an occurring phenomenon at the air–sea interface. To investigate the effects of different wind–wave alignments on the wind field and on the momentum transfer, the Wind Gallery of the von Karman Institute (VKI) was modified in order to perform aligned and opposed wind–wave direction experiments. The measurements performed to define the wind field and momentum transfer consisted of a Particle Image Velocimetry (PIV) setup with one laser and one camera. Based on this setup, differences in horizontal and vertical wave perturbation velocity as well as in phase-averaged wave-perturbation stress or momentum were observed between aligned and opposed wind–wave directions in terms of location, structure and vertical depth. This study thus provides support that the wind field and phase-averaged wave-perturbation momentum are dependent on the wind–wave alignment, which in the future should be accounted for in numerical models. • Wave perturbation velocity is dependent on the wind–wave directions. • Wave perturbation velocity differs in terms of height, structure, and location for different wind–wave directions. • No tilting downwind of the wave perturbation velocity for opposed wind–wave directions. • The wave perturbation velocity reaches higher altitudes for opposed wind–wave directions. [ABSTRACT FROM AUTHOR]

Abstract: The main purpose of this research is to manage simultaneous measurement of velocity and concentration in large cross-sections by recording and processing images of cloud structures to provide more detailed information for e.g. validation of CFD simulations. Dispersion from an isolated stack in an Atmospheric Boundary Layer (ABL) was chosen as the test case and investigated both experimentally and numerically in a wind tunnel. Large Scale-Particle Image Velocimetry (LS-PIV), which records cloud structures instead of individual particles, was used to obtain the velocity field in a vertical plane. The concentration field was determined by two methods: Aspiration Probe (AP) measurements and Light Scattering Technique (LST). In the latter approach, the same set of images used in the LS-PIV was employed. The test case was also simulated using the CFD solver FLUENT 6.3. Comparison between AP measurements and CFD revealed that there is good agreement when using a turbulent Schmidt number of 0.4. For the LST measurements, a non-linear relation between concentration and light intensity was observed and a hyperbolic-based function is proposed as correction function. After applying this correction function, a close agreement between CFD and LST measurements is obtained. [Copyright &y& Elsevier]

The correct prediction of air pollutants dispersed in urban areas is of paramount importance to safety, public health and a sustainable environment. Vehicular traffic is one of the main sources of nitrogen oxides (NO x) and particulate matter (PM), strongly related to human morbidity and mortality. In this study, the pollutant level and distribution in a section of one of the main road arteries of Antwerp (Belgium, Europe) are analyzed. The assessment is performed through computational fluid dynamics (CFD), acknowledged as a powerful tool to predict and study dispersion phenomena in complex atmospheric environments. The two main traffic lanes are modeled as emitting sources and the surrounding area is explicitly depicted. A Reynolds-averaged Navier–Stokes (RANS) approach specific for Atmospheric Boundary Layer (ABL) simulations is employed. After a validation on a wind tunnel urban canyon test case, the dispersion within the canopy of two relevant urban pollutants, nitrogen dioxide (NO 2) and particulate matter with an aerodynamic diameter smaller than 10 μ m (PM 10), is studied. An experimental field campaign led to the availability of wind velocity and direction data, as well as PM 10 concentrations in some key locations within the urban canyon. To accurately predict the concentration field, a relevant dispersion parameter, the turbulent Schmidt number, S c t , is prescribed as a locally variable quantity. The pollutant distributions in the area of interest – exhibiting strong heterogeneity – are finally demonstrated, considering one of the most frequent and concerning wind directions. Possible local remedial measures are conceptualized, investigated and implemented and their outcomes are directly compared. A major goal is, by realistically reproducing the district of interest, to identify the locations inside this intricate urban canyon where the pollutants are stagnating and to analyze which solution acts as best mitigation measure. It is demonstrated that removal by electrostatic precipitation (ESP), an active measure, and by enhancing the dilution process through wind catchers, a passive measure, are effective for local pollutant removal in a realistic urban canyon. It is also demonstrated that the applied ABL methodology resolves some well known problems in ABL dispersion modeling. • Realistic urban area modeled with novel Reynolds-Averaged Navier–Stokes method. • Further indications of strong heterogeneity of urban air pollutant distributions. • Innovative modeling of localized urban air pollutant mitigation measures. • Electrostatic precipitation and enhanced natural ventilation effective as measures. • Value of CFD models in atmospheric dispersion studies further demonstrated. [ABSTRACT FROM AUTHOR]

In forming processes, friction is a local phenomenon influenced by the contact conditions at the tool-sheet metal interface. A multi-scale friction model applicable for coated sheets is developed for the boundary lubrication regime which accounts for the physical behavior of coating and measured surface topographies of sheet and tools. The contact patches and therefore the real area of contact is determined at the tool-sheet metal interface for different contact loading conditions. A single asperity micro-scale ploughing model is adapted at each contact patch to determine the friction force from which the overall coefficient of friction is determined. The friction model is validated using different sets of lab-scale friction tests and cup drawing experiments on zinc coated (GI) steel sheets. • 1.A multi-scale friction model is developed for the boundary lubrication regime. • 2.Applicable for coated sheets and accounts for physical behavior of the coatings. • 3.Surface topographies of sheet and tools are used as the input in the model. • 4.Single asperity ploughing model is adopted at macro-scale deformed sheet surface. • 5.The model is validated using lab-scale friction tests and cup drawing experiments. [ABSTRACT FROM AUTHOR]