Simulated Atmospheric Boundary Layer Research Articles

Effect of facet gap on heliostat wind loading

Multi-facet heliostats are more versatile and advantageous in design compared to single facet heliostats, while generating new design challenges. Multi-facet designs can reduce manufacturing costs, allow for decentralisation of power supply, and bring the option of face down stow. Addition of a gap between two facets, changes the flow structure around the heliostat. The effect of gap between two heliostat facets is therefore investigated on wind loads through experimental investigation in a large wind tunnel. A heliostat model was placed in a simulated atmospheric boundary layer at The University of Adelaide wind tunnel. The investigation covered gap ratios between two panels (gap spacing/facet width) from 0 to 2, and panel aspect ratios (facet width/chord length) of 0.35 to 0.65, forming a complete heliostat aspect ratio of 0.7 to 1.3 with no gap. Wind load coefficients were determined using load cell measurements at the base, and application of differential pressure sensors on the surface, of the heliostat model. Findings indicate the presence of a gap, in general, increases the wind load on the heliostat, with facet aspect ratio exacerbating the results. The maximum load cases shift further into the operating range of heliostats where a change in centre of pressure, due to a change in gap ratio, shifts maximum coefficients from an elevation angle of 30° to 45° for lift force and hinge moment, and 90° to 75° for azimuthal moment. Increasing loads on elevation and azimuth drives potentially increases tracking deviations and beam misalignment, reducing concentrated solar power plant performance.

Energy Exascale Computational Fluid Dynamics Simulations With the Spectral Element Method

Abstract Development and application of the open-source GPU-based fluid-thermal simulation code, NekRS, are described. Time advancement is based on an efficient kth-order accurate timesplit formulation coupled with scalable iterative solvers. Spatial discretization is based on the high-order spectral element method (SEM), which affords the use of fast, low-memory, matrix-free operator evaluation. Recent developments include support for nonconforming meshes using overset grids and for GPU-based Lagrangian particle tracking. Results of large-eddy simulations of atmospheric boundary layers for wind-energy applications as well as extensive nuclear energy applications are presented.

Extreme Value Analysis for Peak Heliostat Wind Load Predictions

This study investigated high-frequency load fluctuations on a rectangular heliostat model in a boundary layer wind tunnel experiment. Statistical methods for peak wind load predictions were compared with load distributions through load cell and surface pressure measurements on the heliostat model placed in two simulated atmospheric boundary layers (ABLs) representing a flat desert terrain and open country terrain. It was found that statistical peak predictions based on horizontal wind gust velocity amplitude underestimates the load coefficients in stow and at oblique azimuth angles for maximum azimuth moments. In addition, it was shown that the distributions of the centre of pressure position and the hinge moment in stow position are non-Gaussian. It is therefore recommended that the frequency and amplitude of the vertical wind velocity component must be considered for peak load predictions of hinge moments and azimuth moments.

Coherent subsiding structures in large‐eddy simulations of atmospheric boundary layers

AbstractCoherent structures are characterized in high‐resolution simulations of three atmospheric boundary layers: dry convection, marine cumulus, and stratocumulus. Based on radioactive‐decaying tracers emitted at different altitudes (surface, top of well‐mixed layer, and cloud top), an object‐oriented methodology allows individual characterization of coherent tridimensional plumes within the flow. Each boundary layer shows updraft structures surrounded by subsiding shells that have similar thermodynamical characteristics. Well‐mixed downdrafts are located relatively close to updrafts and entrain dry, warm air from the free troposphere. While updrafts primarily carry the majority of heat and moisture within well‐mixed layers, accounting for 62‐70% of the total resolved flux, it is noteworthy that well‐mixed downdrafts also contibute a significant portion, ranging from 14% to 35%. Identified in all boundary layers, these subsiding structures are triggered by air mass convergence linked to updrafts' divergence and are thus part of an overturning circulation in well‐mixed layers. Close to the surface, downdrafts' divergence constrain updrafts' locations and thus shape a mesoscale cellular organization with cell sizes scaling with the boundary‐layer height (aspect ratio of around 2). Active cumulus formation does not strongly perturb the spatial organization of the sub‐cloud well‐mixed layer. The stratocumulus‐topped boundary layer also shares similarities with the overturning circulation despite having condensation and cloud‐radiation diabatic effects within the mixed layer. However, the visible mesoscale organization of stratocumulus shows larger cells than the boundary‐layer depth (aspect ratio 10) that suggest deviations from the clear‐sky conceptual view. The boundary‐layer decoupling influences mass fluxes of coherent structures and thus potentially plays a role in shaping the spatial organization. Since well‐mixed downdrafts contribute to a significant part of resolved flux of heat and moisture, our results suggest that downdraft properties in well‐mixed layers should be represented at the subgrid scale in climate models through non‐local mass‐flux parametrizations.

대형 비행갑판을 갖는 함정과 풍동시험 결과를 활용한 고신뢰도 함정 Airwake 예측

Developing high-fidelity Computational Fluid Dynamics (CFD) simulation methods used to evaluate the airwake characteristics along a flight deck of a large ship, the various kind of data such as actual ship measurement and wind tunnel results are required to verify the accuracy of CFD simulation. Inflow velocity profile at the bow, local unsteady flow field data around the flight deck, and highly reliable wind tunnel data which were measured after reviewing Atmospheric Boundary Layer (ABL) simulation and Reynolds Number effects were also used to determine the key parameters such as turbulence model, time resolution and accuracy, grid resolution and type, inflow condition, domain size, simulation length, and so on in STAR CCM+. Velocity ratio and turbulent intensity difference between Full-scale CFD and actual ship measurement at the measurement points show less than 2% and 1.7% respectively. And differences in velocity ratio and turbulence intensity between wind tunnel test and small-scale CFD are both less than 2.2%. Based upon this fact, the selected parameters in CFD simulation are highly reliable for a specific wind condition.

POD analysis of the structure of vertical axis wind turbine wakes

Proper orthogonal decomposition (POD) is used to investigate the structure and evolution of the wake behind a model vertical axis wind turbine. The analysis reveals the high level of energy contained in the coherent structures that mark the vortex shedding due to the dynamic stall on the blades and the vortex shedding downstream of the tower, and the dynamics of the wake interactions. Vortex pairing is shown to be an important feature of the wake evolution. Changing the inflow conditions from a uniform flow to a simulated atmospheric boundary layer flow reduced the relative energy contained in the coherent motions, because of the reduced level of dynamic stall. Similarly, changing the blade sweep angle reduces the modal energy associated with dynamic stall, which is desirable in terms of mitigating fatigue due to torque and force variations on the blades.

Improved Simulation of the Polar Atmospheric Boundary Layer by Accounting for Aerodynamic Roughness in the Parameterization of Surface Scalar Exchange Over Sea Ice

AbstractA new, simple parameterization scheme for scalar (heat and moisture) exchange over sea ice and the marginal ice zone is tested in a numerical weather and climate prediction model. This new “Blended A87” scheme accounts for the influence of aerodynamic roughness on the relationship between momentum and scalar exchange over consolidated sea ice, in line with long‐standing theory and recent field observations, and in contrast to the crude schemes currently operational in most models. Using aircraft observations and Met Office Unified Model simulations of cold‐air outbreak (CAO) conditions over aerodynamically rough sea ice, we demonstrate striking improvements in model performance when the Blended A87 scheme replaces the model's operational treatment for surface scalar exchange, provided that the aerodynamic roughness over consolidated ice is appropriately prescribed. The mean biases in surface sensible heat flux, surface latent heat flux, near‐surface air temperature, and surface temperature reduce from 25 to 11 W m−2, 22 to 12 W m−2, 0.8 to 0.0 K, and 1.4 to 0.8 K, respectively. We demonstrate that such impacts on surface exchange over sea ice can have a marked impact on the evolution of the atmospheric boundary layer across hundreds of kilometres downwind of the sea ice during CAO conditions in the model. Our results highlight the importance of spatiotemporal variability in the topography of consolidated sea ice for both momentum and scalar exchange over sea ice; accounting for which remains a key challenge for modeling polar weather and climate.

The effect of wind characteristics on tall buildings with porous double-skin façades

Wind loads on tall buildings with porous double-skin façade (PDSF) systems were studied for various wind characteristics. Experiments were performed on a small-scale building model in a boundary layer wind tunnel. A single-skin building model was used as a reference case alongside three PDSF systems of 25%, 50% and 65% porosity. Two atmospheric boundary layer (ABL) simulations were created corresponding to category I (rural) and category III (suburban) ABLs of the EN-1991-1-4:2005 standard. Wind loads on the building model were studied based on high-frequency force balance measurements of integral moments, while pressure distribution on the building model surfaces was analyzed based on pressure measurements. Experiments were performed for ten flow incidence angles 0° < α < 45° with a 5° increment. The obtained results are considered highly relevant both for research purposes and engineering applications, especially considering the lack of similar studies currently available. In particular, the rural ABL simulation yields larger mean pressure coefficients. The effect of the ABL characteristics on the pressure coefficient standard deviation depends substantially on α, while the most notable difference is observed on the windward building model surface at α = 0°, where the suburban ABL causes a substantially higher standard deviation. The mean across-wind moment coefficient of the suburban ABL simulation is lower at all α points, while its standard deviation is lower for α > 0°. The along-wind moment coefficient in the suburban ABL simulation is ∼20% lower than in the rural ABL simulation, a trend which is observed for all α points. In contrast, the along-wind moment standard deviation is ∼25% larger in the suburban ABL simulation. The power spectral density of the along-wind moment is considerably lower in the suburban ABL simulation.

The simplified $$k-\epsilon $$ turbulence model applied to numerical simulation of the stable atmospheric boundary layer

The simplified $$k-\epsilon $$ turbulence model applied to numerical simulation of the stable atmospheric boundary layer

Space-Time Characterization of Ship Airwakes

The turbulent airwakes produced by a generic navy frigate (Simple Frigate Ship 2) was investigated in a low-speed wind tunnel. The frigate model was subjected to a headwind and immersed in a simulated atmospheric boundary layer. High-speed stereoscopic particle image velocimetry (PIV) was performed at six different spanwise planes over the flight deck. A spatio-temporal analysis of the PIV airwake measurements was conducted by applying the proper orthogonal decomposition (POD) to individual frequency bands of the airwake. Physical interpretations of the leading POD modes were substantiated by other complementary analyses. In particular, it was discovered that turbulent structures were shed from the funnel and superstructure edges. The flow recirculation region behind the superstructure exhibited a strongly three-dimensional bistable behavior. A pair of vortices near the stern also demonstrated low-frequency bistable dynamics. Notably, the bistable flow recirculation region appeared to influence or interact with the low-frequency behavior of the other major flow features.

Effects of Inflow Parameters and Disk Thickness on an Actuator Disk inside the Neutral Atmospheric Boundary Layer

An accurate choice of the inflow parameters has been shown to affect the CFD results significantly. In this study, the actuator disk method (AD) is used to investigate the effects of the widely used inflow formulations, the logarithmic and power-law formulations, in the neutral atmospheric boundary layer simulations. Based on the one-dimensional momentum theory, the AD model is a rapid method that replaces the turbine with a permeable disk and is among the most used methods in the literature. The results of the k-ω AD simulation indicated that in spite of the logarithmic method’s widespread use, the power law formulation gives a better description of the velocity field. Furthermore, an actuator disk thickness study also showed that given the effect of actuator disk thickness on the rate of convergence, more attention should be dedicated towards finding a suitable disk thickness value. The combination of an optimized mesh and a suitable choice of AD thickness can help with the rate of convergence which in turn shortens the simulation’s run time.

Assessment of flow characteristics over complex terrain covered by the heterogeneous forest at slightly varying mean flow directions: (A case study of a Swedish wind farm)

The impact of heterogeneous forest canopies and complex terrain on the horizontal distortion of the inflow is studied. Large-Eddy Simulation (LES) of the neutral Atmospheric Boundary Layer (ABL) flow is performed for a wind farm in Sweden for three cases associated with three different wind directions at the range of the static yaw misalignment (≃±6∘) where the yaw control system is not activated. The ground topography and forest properties for the numerical modeling are extracted from the Airborne Laser Scanning (ALS) 3D data. The wind turbines within the wind farm are introduced using the actuator disk model. To focus on the airflow deflection only by the complex terrain and vegetation, the study is limited to upstream wind turbines without any wake interaction. The predicted mean wind speed and turbulence intensity for the upstream wind turbines are compared against the nacelle-mounted anemometers taken from the wind farm’s turbine SCADA data. To quantify the additional load and moments induced at the rotor blades by the horizontal misalignment of the incoming flow, aero-structural simulation of the upstream wind turbines in the wind farm for all three cases is performed. The results show that the horizontal distortion of the inflow over the rotor swept area is usually kept below the range of static yaw misalignment (≃6∘) for the majority of the upstream wind turbines for all three cases. However, the impact of a large vertical shear exponent leading to misinterpretation of the results must be taken into consideration. Furthermore, the load imbalance of the rotor due to the vertical wind shear has the least direct contribution to the yaw moment. However, for a mean vertical shear exponent larger than α=0.25, contrary to expectation, a positive mean yaw moment under the positive-yawed inflow may be observed.

Near-wall modeling of forests for atmosphere boundary layers using lattice Boltzmann method on GPU

In this paper, the simulation and modeling of the turbulent atmospheric boundary layers (ABLs) in the presence of forests are studied using a lattice Boltzmann method with large eddy simulation, which was implemented in the open-source program GASCANS with the use of Graphic Processing Units (GPU). A method of modeling forests in the form of body forces injected near the wall is revisited, while the effects of leaf area density (LAD) on the model accuracy is further addressed. Since a uniform cell size is applied throughout the computational domain, the wall-normal height of the near-wall cells is very large, theoretically requiring a wall function to model the boundary layer. However, the wall function is disregarded here when the forest is modeled. This approximation is validated based on the comparison with previous experimental and numerical data. It concludes that for the ABL conditions specified in this study as well as a large body of literature, the forest forces overwhelm the wall friction so that the modeling of the latter effect is trivial. Constant and varying LAD profiles across the forest zone are defined with the same total leaf area despite the varying one being studied previously. It is found that the two LAD profiles provide consistent predictions. The present forest modeling can therefore be simplified with the use of the constant LAD without degrading the model accuracy remarkably.

Reconstruction of flow around a high-rise building from wake measurements using Machine Learning techniques

This paper investigates the unsteady flow around a high-rise building using OpenFoam. A Vortex Method is developed to generate upstream unsteady fluctuations that are validated considering the numerical simulation of a neutral atmospheric boundary layer around a high-rise building. The Vortex Method parameters are tuned to produce an upstream velocity profile whose turbulence characteristics are comparable to experimental data. Comparison between mean and fluctuating velocity profiles of the numerical data measured in the building’s wake with experimental data shows satisfactory results.The resulting database is used to reconstruct the flow from limited velocity measurements inside the wake and static wall pressure measurements on the building surface. Machine learning techniques such as linear regressions (Ridge, Lasso, ElastikNet, MultiTaskLasso regression) and Artificial Neural Network are tested and compared. The flow reconstruction using velocity measurements inside the wake leads to a better result compared to the flow reconstruction from the wall pressure measurements. At the same time, it was noticed that the Artificial Neural Network regression does not lead to more satisfactory results compared to linear regression techniques.

Consistency improved random flow generation method for large eddy simulation of atmospheric boundary layer

Realistic turbulence is the key component to obtain accurate simulations for large eddy simulation of atmospheric boundary layer (ABL). At present, synthetic turbulence methods have been widely applied, but there are still some defects such that spatial correlations and cross-correlations between different velocity components cannot be strictly reproduced. Thus, we propose a new inflow turbulence generation (ITG) method, the consistency improved random flow generation (CIRFG) method. The proposed method is derived to model the spatial correlations and cross-correlations between different velocity components by explicitly embedding turbulence characteristics, thus it can be used widely in arbitrary situations without any empirical parameters. Meanwhile, the CIRFG method is maintained to satisfy arbitrary mean velocity, turbulence intensity, frequency spectra, temporal correlation, etc. Further, according to the simulation of ABL flows, it is verified that the new method has good self-sustainability of wind profiles. Finally, compared with the simulations of flow around high-rise building model with wind tunnel tests, it is demonstrated that the proposed method has high accuracy and feasibility.

An improved consistent inflow turbulence generator for LES evaluation of wind effects on buildings

This paper proposes an improved consistent inflow turbulence generator for reproducing spatially correlated turbulence fields with specified wind profiles and spectra for large eddy simulation (LES). The inflow turbulence is developed based on the widely used Random Flow Generation (RFG) technique, and improvements include obtaining more consistent wind spectra, direct control of spatial integral length scales, mitigation of fictitious pressure fluctuations caused by nonsatisfaction of divergence-free conditions, and iteration-based adaption of velocity profiles within the future building area. These improvements are obtained from strictly theoretical derivation, and the correctness of the proposed algorithm is verified by comparing the velocity profiles and the wind spectra of the simulated turbulence atmospheric boundary layer (ABL) with target values. The proposed turbulence generator is adopted to generate inflow turbulence for analyzing wind-induced pressure distributions on isolated low-rise buildings immersed in the turbulent ABL, and the comparison between simulated results and wind tunnel experimental results obtained by Tokyo Polytechnic University (TPU) is presented. Good agreement between the numerical and experimental results verifies the applicability of the proposed generator technique to LES of wind effects on low-rise buildings.

On the Application of an Observations‐Based Machine Learning Parameterization of Surface Layer Fluxes Within an Atmospheric Large‐Eddy Simulation Model

AbstractRecently, machine learning techniques have been employed to develop improved models for predicting surface‐layer fluxes of momentum, heat and moisture based on field observations. Herein we explore refinement to these models, in particular artificial neural networks (NN), and investigate their applicability within an atmospheric large‐eddy simulation model as an alternative to the widely adopted standard of Monin − Obukhov (MO) similarity theory. Atmospheric boundary layer (ABL) simulations under different stability conditions are carried out for a variety of scenarios of increasing complexity, from dry steady neutral boundary layers to moist diurnal cycle. Simulations using the NN models result in predicted flux differences with respect to the corresponding MO simulations that are consistent with NN model skill in predicting the tower observations. These differences lead to notable modifications of mean and turbulence quantities throughout the ABL. While these NN models provide an alternative to improve upon MO, it is demonstrated that thorough scrutiny in design and evaluation is necessary to prevent unphysical predictions and establish a robust and generalizable parameterization. Design aspects considered include input feature engineering, applicability under inputs obtained from different heights, biased predictions due to climatological fingerprints, and sensitivity to the choice of activation function. In this context, it is shown how an atmospheric model can contribute toward efficiently investigating these relevant aspects, including expanding the training data set to generalize the NN model to a range of surface roughness values. Finally, we outline remaining challenges to be addressed toward developing a universal parameterization for surface‐layer fluxes using machine learning techniques.

The effect of coastal terrain on nearshore offshore wind farms: A large-eddy simulation study

Performance of offshore wind farms built in the nearshore region will be affected by onshore terrain with higher turbulence in the flow when wind is blowing from land toward sea. Current study employs large-eddy simulation to investigate the effect of coastal terrain on the performance of large nearshore offshore wind farms. At first, two atmospheric boundary layer (ABL) simulation cases are defined to investigate the evolution of an internal boundary layer (IBL) during the sea-to-land and land-to-sea transition of the flow. The growth rate of the IBL was similar for both ABL simulation cases. However, the mean velocity-based definition of IBL heights, which essentially are the equilibrium layer, were half the height of shear stress-based IBLs. The first wind farm simulation case only considers an offshore surface, while the second case includes the region with land-to-sea transition upstream of the wind farm. Better wake recovery is observed in the case that considers the effect of onshore terrain. This is attributed to the higher inflow turbulence level, which resulted in higher entrainment of kinetic energy from the flow above. The farm-induced IBL for a land-to-sea transition case shows rapid growth for the first few turbine rows, while the offshore only case shows gradual growth. However, the difference between the two IBLs decreases with downstream distance, implying that for sufficiently long wind farms, both IBLs will converge. Total power output of the land-to-sea transition case is 17% higher than the offshore only case for the farm layout and roughness heights considered in this study.

A comparison of major steady RANS approaches to engineering ABL simulations

Engineering atmospheric boundary layer (EABL) simulations performed using Computational Fluid Dynamics (CFD) can give key insights in successfully addressing a variety of topics in environmental and structural aerodynamics. To improve understanding of this complex topic, a computational evaluation of the shear stress-driven (SSD), pressure-driven (PD) and body-force-driven (BFD) EABL flows was performed using the open-source CFD code OpenFOAM®. The EABL simulations were computationally evaluated by using the successor domain technique (SDT) and precursor domain technique (PDT). The boundary conditions applied in the CFD algorithm using OpenFOAM® to computationally model the SSD, PD, and BFD EABL flows were reported and discussed. A developed CFD approach may also be satisfactorily used in other relevant CFD codes, including but not limited to Ansys CFX®, Ansys Fluent®, STAR−CCM+®. The impact of the EABL models on surface pressure distribution on a low-rise cubic building, the total building drag force and the wind velocities in pedestrian-level areas was computationally evaluated by using major Reynolds-averaged Navier-Stokes (RANS) turbulence models, i.e., the standard k−ε, re-normalization group (RNG) k−ε, realizable k−ε, Wilcox's k−ω, and Menter's k−ω shear stress transport (SST) turbulence models. The obtained computational results indicate that the SSD and PD EABL flows may be successfully modeled in cases where additional modifications of the officially released CFD code are not required in regard to the PDT. The PD EABL flow may be successfully modeled by using all studied RANS turbulence models in combination with the PDT. The EABL flow throughout an empty computational domain is homogeneous even without using a body force because the employed CFD code is capable of creating the required pressure gradient that drives the flow along the computational domain. While the surface pressure estimation on the windward body surface and in the recirculation zones was generally proven to be an issue when using steady RANS two-equation turbulence models, surface pressures in the stagnation zone on the windward cubic building surface and pressures on the top and the side building surfaces in the present study agree well with the experiments when using the RNG k−ε and Menter's k−ω SST turbulence models. The pressure distribution on the building surfaces, the total building drag force and wind characteristics in the vicinity of a lift-up building are not considerably affected by the choice of the EABL model, while they are substantially affected by the type of RANS turbulence model. The best agreement with the experiments was achieved when using the RNG k−ε and Menter's k−ω SST turbulence models, so these models may be recommended for future applications.

The Spatial Structure of Passively Simulated Atmospheric Boundary Layer Turbulence

Three types of turbulence fields were investigated using a research method combining wind tunnel tests and theoretical analysis to further explore the spatial structure of atmospheric boundary layer turbulence, which was passively simulated by a wind tunnel. The fundamental theory of turbulence is introduced, and some traditional theoretical coherence models based on isotropic turbulence theory are derived. The difference between the theoretical results and the passive simulation of atmospheric boundary layer turbulence was compared and discussed. The analysis results show that the passively simulated atmospheric turbulence basically conformed to the homogeneous isotropic turbulence assumption on the horizontal plane, but the interference of the nonisotropic turbulence components cannot be ignored either. Finally, some improvements were made to the traditional coherence function model based on the experimental results to apply the passively simulated atmospheric boundary layer turbulence.