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ABSTRACT Fast and accurate large‐eddy simulation (LES) of the atmospheric boundary layer plays a crucial role in advancing wind energy research. Long‐duration wind farm studies at turbine‐resolving scales have become increasingly important to understand the intricate interactions between large wind farms and the atmospheric boundary layer. However, the prohibitive computational cost of these turbulence‐ and turbine‐resolving simulations has precluded such modeling to be exercised on a regular basis. To that end, we implement and validate the generalized actuator disk (GAD) model in the computationally efficient, graphics processing unit (GPU)–resident, LES model FastEddy. We perform single‐turbine simulations under three atmospheric stabilities (neutral, unstable, and stable) and compare them against observations from the Scaled Wind Farm Technology (SWiFT) facility and other LES codes from the recent Wakebench turbine wake model benchmark. Our idealized LES results agree well with observed wake velocity deficit and downstream recovery across stability regimes. Turbine response in terms of rotational speed, generated power, torque, and thrust coefficient are well predicted across stability regimes and are consistent with the LES results from the benchmark. The FastEddy simulations are found to be at least two orders of magnitude more efficient than the traditional CPU‐based LES models, opening the door for realistic LES simulations of full wind plants as a viable standard practice.

This study utilizes the wealth of observational data collected during the recent Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) drift experiment to constrain and evaluate close to two‐hundred daily Large‐Eddy Simulations (LES) of Arctic boundary layers and clouds at high resolutions. A standardized approach is adopted to tightly integrate field measurements into the experimental configuration. Covering the full drift represents a step forward from single‐case LES studies, and allows for a robust assessment of model performance against independent data under a range of atmospheric conditions. A homogeneously forced domain is simulated in a Lagrangian frame of reference, initialized with radiosonde and value‐added cloud profiles. Prescribed boundary conditions include various measured surface characteristics. Time‐constant composite forcing is applied, primarily consisting of subsidence rates sampled from reanalysis data. The simulations run for 3 hours, allowing turbulence and clouds to spin up while still facilitating direct comparison to MOSAiC data. Key aspects such as the vertical thermodynamic structure, cloud properties, and surface energy fluxes are well reproduced and maintained. The model captures the bimodal distribution of atmospheric states that is typical of Arctic climate. Selected days are investigated more closely to assess the model's skill in maintaining the observed boundary layer structure. The sensitivity to various aspects of the experimental configuration and model physics is tested. The model input and output are available to the scientific community, supplementing the MOSAiC data archive. The close agreement with observed meteorology justifies the use of LES for gaining further insight into Arctic boundary layer processes and their role in Arctic climate change. Plain Language Summary: The Arctic is one of the regions most affected by global climate change, warming up to four times as fast as the rest of the globe. It is also a particularly inaccessible region to conduct measurements. Fortunately, between 2019 and 2020 the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign collected an unprecedented amount of data in the Arctic. In this study, numerous of these measurements are incorporated into high‐resolution computer simulations of the lowest part of the Arctic atmosphere. This simulation data complements and contextualizes the observations and enables insight into complex physical processes, for example, cloud formation, cloud ice production, or turbulent mixing. The Arctic is an extreme place, and models often struggle to represent the atmosphere accurately. Therefore, the main achievement of this study is to successfully simulate 190 atmospheric situations as measured during the campaign. The generated data set performs well when compared to independent observations. Single cases deliver information about individual atmospheric conditions, and the collection gives insight into how key climate variables behaved throughout the MOSAiC year. Key Points: A standardized LES setup based on campaign data is developed with an aim to supplement the local measurements during the Multidisciplinary drifting Observatory for the Study of Arctic Climate driftIndependent drift‐long statistics on key aspects of the surface energy budget, thermodynamic structure, and clouds are reproducedSensitivity tests indicate microphysics, ice‐radiation interaction and surface representation are critical for successful daily simulations [ABSTRACT FROM AUTHOR]

Fast and accurate large‐eddy simulation (LES) of the atmospheric boundary layer plays a crucial role in advancing wind energy research. Long‐duration wind farm studies at turbine‐resolving scales have become increasingly important to understand the intricate interactions between large wind farms and the atmospheric boundary layer. However, the prohibitive computational cost of these turbulence‐ and turbine‐resolving simulations has precluded such modeling to be exercised on a regular basis. To that end, we implement and validate the generalized actuator disk (GAD) model in the computationally efficient, graphics processing unit (GPU)–resident, LES model FastEddy. We perform single‐turbine simulations under three atmospheric stabilities (neutral, unstable, and stable) and compare them against observations from the Scaled Wind Farm Technology (SWiFT) facility and other LES codes from the recent Wakebench turbine wake model benchmark. Our idealized LES results agree well with observed wake velocity deficit and downstream recovery across stability regimes. Turbine response in terms of rotational speed, generated power, torque, and thrust coefficient are well predicted across stability regimes and are consistent with the LES results from the benchmark. The FastEddy simulations are found to be at least two orders of magnitude more efficient than the traditional CPU‐based LES models, opening the door for realistic LES simulations of full wind plants as a viable standard practice. [ABSTRACT FROM AUTHOR]

Abstract Recent observations of the trade‐wind regions highlight the covariability between cold‐pool properties and mesoscale cloud organization. Given the covariability of organization with cloud cover and albedo, this suggests a potential impact of cold pools on the cloud radiative effect (CRE). To explore this, we use an ensemble of 103 large‐domain, high‐resolution, large‐eddy simulations and investigate how the variability in cold pools is determined by large‐scale external cloud‐controlling factors and shaped by processes within the mesoscale. It is demonstrated that the size and frequency of occurrence of cold pools are strongly influenced by the near‐surface horizontal wind speed and large‐scale subsidence. The temporal evolution of cold pools is strongly correlated with the diurnality in radiation. Even without external variability, we find a strong intermittent behavior in the evolution of cold pools, governed by a complex interplay between cold pools and clouds which expresses itself in the form of shallow squall lines. These squall lines result from precipitating downdrafts, cold pool outflows and the resulting gust fronts, reinforcing parent clouds. Cold pools influence the CRE of trade cumuli, but only when they exist during the day. This emphasizes the importance of the synchronization between cold‐pool events and the diurnal cycle of insolation for the dependence of the CRE on cold pools.

The paper reports on the computation of the hydrodynamic loads on a full-scale caisson at high Reynolds number in the presence of vortex shedding. The objective was to obtain reliable predictions of the resulting mean and fluctuating forces to guide the design of an actual caisson in the absence of relevant experimental data. A further objective was to investigate the effectiveness of alternative methods for the control of vortex shedding that can be implemented in practice. Two such methods were evaluated: (1) by rounding the corners of the rectangular-sectioned caisson, and (2) by the placement of a splitter plate in the separated wake region. The computations, which were performed using the OpenFOAM open-source software, were for a fixed caisson and hence did not account for motions due to vortex-induced vibrations. The effects of turbulence were accounted for by performing large-eddy simulations, and by using two-equation eddy-viscosity closures, one of which was specifically adapted to account for the interactions between the periodic vortex shedding and the random turbulence. The numerical accuracy was checked using the grid convergence index method, and the computations were extensively validated against data from relevant benchmark flows. The recommendations of this research were implemented in the design of a full-scale caisson that has since been deployed in a bridge construction project.

Supersonic impinging jets are a versatile configuration that can model the compressible flows of cold-spray manufacturing and vertical take-off-and landing strategy. In this work, rhoCentralFoam, solver of the OpenFOAM framework, and a large-eddy simulation formulation were used to simulate an underexpanded supersonic jet of Mach 1.45 and nozzle pressure ratio of 4, impinging on a flat wall situated at 1.5 nozzle diameters away from the jet outlet. Care was taken in the mesh construction to properly capture the characteristic standoff shock and vortical structures. The grid convergence index was evaluated with three meshes of increasing spatial resolution. All meshes can generally be considered as sufficient in terms of results focused on time-averaged values and mean physical properties such as centerline Mach number profile. However, the highest resolution mesh was found to capture fine shear vortical structures and behaviors that are absent in the coarser cases. Therefore, the notion of adequate grid convergence may differ between analyses of time-averaged and transient information, and so should be determined by the user's intention for conducting the simulations. To guide the selection of mesh resolution, scaling analyses were performed, for which the current rhoCentralFoam solver displays a good weak scaling performance and maintains a linear strong scaling up to 4096 cores (32 nodes) for an approximately 40 million-cell mesh. Due to the internode communication bottlenecks of OpenFOAM and improvements in central processing units, this work recommends, for future scaling analyses, adopting a "cells-per-node" basis over the conventional "cells-per-core" basis, with particular attention to the interconnect speed and architecture used. [ABSTRACT FROM AUTHOR]

Numerical simulations and in-situ measurements represent two important and synergistic pillars for the study of flow and transport in plant canopies. Due to model limitations and parameter uncertainty, the alignment of model predictions with actual observations is challenging in practice. The present work proposes a Bayesian uncertainty quantification (UQ) framework that estimates the approaching wind angle parameter for large-eddy simulation (LES) of flow in plant canopies by assimilating data from in-situ measurements. The framework is applied to LES of flow within and above realistic plant canopy, with plant area density derived from light detection and ranging measurements. Uncertainty on approaching wind direction is characterized via a Markov chain Monte Carlo procedure, and propagated through Monte Carlo sampling to wind speed and resolved Reynolds stresses. Given the substantial computational cost of LES, a surrogate model based on an exiguous number of LESs is used for flow simulations within the UQ framework. As a result of the analysis, the UQ solution is given by probability density functions of selected flow statistics at different heights. Profiles of mean ± standard deviation for the considered flow statistics exhibit excellent agreement with corresponding observations, proving that the proposed approach is able to calibrate the approaching wind angle parameter, and that the quantified uncertainty captures discrepancies between observations and model results. Overall, the present work highlights the potential of UQ to enhance predictions of exchange processes between vegetation canopy and atmosphere. [ABSTRACT FROM AUTHOR]

This paper investigates a wind speed oscillation event that occurred near the coastline of central Taiwan in the afternoon of 17 February 2018, using data from observations and numerical simulations. The observed wind speeds at 100-m altitude displayed a fast-oscillating pattern of about 6 cycles between strong winds of approximately 21 m s−1 and weak winds of around 2 m s−1, with periods of about 10 min. The pressure anomalies fluctuated in antiphase with the wind speed anomalies. The synoptic analysis revealed the influence of a continental high pressure system, resulting in a cold-air outbreak over Taiwan. The cold north-northeasterly winds split into two branches upon encountering Taiwan's topography, with ridging off the east coast and a lee trough off the west coast of Taiwan. Wind oscillations were detected in the low-level cold air offshore the west coast of Taiwan, depicted by wavelike structures in wind speeds, sea level pressure, and potential temperature. The perturbations were identified as Kelvin-Helmholtz billows characterized by regions of strong wind speeds, warm and dry air, sinking motions, and low pressure collocated with each other, while regions of weaker wind speeds, cooler and moister air, ascending motions, and high pressure were associated with each other. With terrain contributing to favorable conditions, the large vertical and horizontal wind shears resulted from the southward acceleration of low-level cold air and the northward movement of the lee trough played an important role in initiating the wind oscillations. [ABSTRACT FROM AUTHOR]

We investigate whether and how spatial organization affects the pathway to precipitation in large‐domain hectometer simulations of the North Atlantic trades. We decompose the development of surface precipitation (P) in warm shallow trade cumulus into a formation phase, where cloud condensate is converted into rain, and a sedimentation phase, where rain falls toward the ground while some of it evaporates. With strengthened organization, rain forms in weaker updrafts from smaller cloud droplets so that cloud condensate is less efficiently converted into rain. At the same time, organization creates a locally moister environment and modulates the microphysical conversion processes that determine the raindrops' size. This reduces evaporation and more of the formed rain reaches the ground. Organization thus affects how the two phases contribute to P, but only weakly affects the total precipitation efficiency. We conclude that the pathway to precipitation differs with organization and suggest that organization buffers rain development. Plain Language Summary: Clouds in the trade‐wind region organize into a variety of spatial patterns. We investigate how this spatial organization influences rain development in simulations of trade‐wind convection. We divide the formation of surface precipitation into two phases. In the first phase, rain forms from the collision of cloud droplets or the collection of cloud droplets by raindrops. In the second phase, rain falls toward the ground while some of the rain evaporates. Our study shows that as organization strengthens, rain forms less efficiently, but a larger fraction of that rain reaches the ground as evaporation is reduced. Thus, organization in the simulations affects the way surface rain is generated. Our analyses suggest that it does so by modulating the cloud vertical motion in which rain forms, the local moisture environment through which rain falls and the microphysical conversion processes. Key Points: The development of surface precipitation in simulated trade‐wind convection is decomposed into a formation and sedimentation phaseAs organization strengthens, less cloud condensate is converted into rain, but more rain reaches the ground as evaporation is suppressedOrganization affects rain formation by modulating the local moisture environment, cloud vertical motion and microphysical properties [ABSTRACT FROM AUTHOR]

The noise radiated by the flow around a cylinder in the subcritical regime at R e D = 1 × 10 4 and at a subsonic Mach number of M = 0.5 is here studied. The aerodynamic sound radiated by a cylinder has been studied with a wide range of Reynolds numbers, but there are no studies about how the Mach number affects the acoustic field in the subsonic regime. The flow field is resolved by means of large-eddy simulations of the compressible Navier–Stokes equations. For the study of the noise propagation, formulation 1C of the Ffowcs Williams–Hawkings analogy is used. The fluid flow results show good agreement when comparing the surface pressure coefficient, the recirculation length, the vortex shedding frequency and the force coefficients against other studies performed under similar conditions. The dynamic mode decomposition of the pressure fluctuations is used to relate them with the far-field noise. It is shown that, in contrast to what happens for low Mach numbers, quadrupoles have a significant impact mainly in the observers located in the streamwise direction. This effect leads to a global monopole directivity pattern as the shear fluctuations compensate for the lower value of the aeolian tone away from the cross-stream direction. [ABSTRACT FROM AUTHOR]

Supersonic impinging jets are a versatile configuration that can model the compressible flows of cold-spray manufacturing and vertical take-off-and landing strategy. In this work, rhoCentralFoam, solver of the OpenFOAM framework, and a large-eddy simulation formulation were used to simulate an underexpanded supersonic jet of Mach 1.45 and nozzle pressure ratio of 4, impinging on a flat wall situated at 1.5 nozzle diameters away from the jet outlet. Care was taken in the mesh construction to properly capture the characteristic standoff shock and vortical structures. The grid convergence index was evaluated with three meshes of increasing spatial resolution. All meshes can generally be considered as sufficient in terms of results focused on time-averaged values and mean physical properties such as centerline Mach number profile. However, the highest resolution mesh was found to capture fine shear vortical structures and behaviors that are absent in the coarser cases. Therefore, the notion of adequate grid convergence may differ between analyses of time-averaged and transient information, and so should be determined by the user’s intention for conducting the simulations. To guide the selection of mesh resolution, scaling analyses were performed, for which the current rhoCentralFoam solver displays a good weak scaling performance and maintains a linear strong scaling up to 4096 cores (32 nodes) for an approximately 40 million-cell mesh. Due to the internode communication bottlenecks of OpenFOAM and improvements in central processing units, this work recommends, for future scaling analyses, adopting a “cells-per-node” basis over the conventional “cells-per-core” basis, with particular attention to the interconnect speed and architecture used.

An initially well‐mixed stratocumulus deck can remain overcast for several tens of hours under warm‐advection conditions, although moisture supply is cut off from the ocean due to surface‐atmosphere decoupling (stabilization of the surface‐atmosphere interface). In this study, a set of idealized large‐eddy simulations were performed to investigate the physical mechanism of how warm‐air advection impacts the evolution of a pre‐existing stratocumulus deck. To mimic warm‐air advection, we decrease the sea surface temperature linearly over time in a doubly periodic domain. Given the same initial conditions, the stratocumulus deck is more persistent when experiencing warm‐air advection than cold‐air advection. This persistence is caused by reduced cloud‐top entrainment drying due to decoupling, a process more influential than the decoupling‐induced cutoff of moisture supply. This mechanism is more notable when the free troposphere becomes more humid. The relevance of the mechanism to previous observations of less low‐level cloudiness under warm‐advection conditions is discussed. Plain Language Summary: Marine stratocumulus clouds exert strong radiative cooling on Earth's climate because they reflect much solar radiation back to space. It is important to understand what factors control the cloud properties, or cloud‐controlling factors. Among all cloud‐controlling factors, the least understood is warm‐air advection, meaning winds mobilizing clouds from over warm water to over cold water. A high‐resolution numerical model was used to investigate the response of the stratocumulus evolution to warm‐air advection. A stratocumulus deck was found to persist longer under warm‐advection conditions than its cold counterpart, inconsistent with the decoupling‐induced dissipation mechanism. This persistence is primarily due to the weaker mixing of clouds with the overlying dry air when the atmosphere become decoupled from the sea surface, a consequence of warm‐air advection. This work revises our conventional understanding of how clouds respond to changes in temperature advection that might change as the planet warms, contributing to a more confident projection of future climates. Key Points: A preexisting stratocumulus deck is more persistent when experiencing warm‐air advection than cold‐air advectionThis persistence is due to reduced entrainment drying as a result of decoupling, which outweighs decreased cloud‐base moisture transportThe mechanism is more notable when free‐tropospheric humidity is higher [ABSTRACT FROM AUTHOR]

To characterize the effects of subgrid surface heterogeneity, the blending-height concept has been developed as a coupling strategy for surface parameterization schemes used in numerical weather prediction models. Previous modeling studies have tested this concept using stationary conditions with one-dimensional strips of surface roughness. Here, large-eddy simulations are used to examine the response of the blending height and effective surface roughness to tiled land-cover heterogeneity, or a two-dimensional chessboard pattern of alternating high and low vegetation given a diurnal cycle of solar irradiance in subarctic conditions. In each experiment, the length scale of the roughness elements is increased while the total domain fraction of each vegetation type is kept constant. The effective surface roughness was found to decrease with increasing length scale of surface cover heterogeneity, which is shown to have a significant impact on estimated wind turbine power calculated from logarithmic wind profiles. In stable conditions, the blending height in cases with large heterogeneity length scales was found to exist well above the surface layer. Because the behavior of the blending height has implications for coupled models, a simple model for the blending height as a function of heterogeneity length scale is introduced. [ABSTRACT FROM AUTHOR]

The precipitation hotspot of the orographically locked convection highly depends on the interactions among physical processes governing local energetics and cloud dynamics. Accurately estimating the future change of these hotspots will require a model with sufficient spatial resolution as well as an appropriate representation of the critical physical processes. In this study, ensembles of TaiwanVVM large-eddy simulations (Δ x = 500 m) were designed to capture the summertime diurnal convection in Taiwan when local circulation dominates. The precipitation hotspots identified by long-term observations are well represented by the present-day ensemble simulations with appropriate environment variabilities. A pseudo global warming experiment is carried out to identify changes in convective structures, which results in local rainfall changes. Under the scenario of 3 K uniform warming with conserved relative humidity, the changes in the thermodynamic environment feature an overall higher convective available potential energy and a small decrease in convective inhibition (CIN), owing to the marked increase in low-level water vapor in the marine boundary layer. The results show that mean precipitation and the occurrence of extreme convective systems (ECSs) increase, with hotspots over mountains expanding toward the foothills and plains. The response in cloud dynamics leads to more short-duration, intense rainfall events. The tracking of ECSs with maximum rainfall exceeding 100 mm h ^−1 reveals more numerous short-lived ECSs (lifetime

Abstract We study wind turbine wakes of rotors operating at high thrust coefficients (CT > 24/25) using large‐eddy simulations with a rotating actuator disk model. Wind turbine wakes at high thrust coefficients are different from wakes at low thrust coefficients. Wakes behave differently at high thrust, with increased turbulence and faster recovery. Lower induction in the wake is achieved because wakes in high‐thrust conditions recover much faster than in normal operating conditions. This enhanced recovery is possible thanks to the turbulence generated in the near wake. We explore the mechanism behind this behavior and propose a simple model to reproduce it. We also propose a Gaussian fit for the wakes under high‐thrust conditions and use it use it to initialize an Ainslie type model within the FAST.Farm framework.

We present simulations of cumulus convection over land with shortwave radiation computed by a Monte Carlo ray tracer coupled to the flow solver. Ray tracing delivers very realistic in‐cloud heating rates and global horizontal irradiance fields. The necessary performance of the ray tracer has been enabled by the raw power of graphics processing unit computing and from techniques for accelerating data lookup and ray tracer convergence. We used a case study over a grassland in the Netherlands to compare simulations with a coupled ray tracer to those with a conventional two‐stream solver, and to test ray tracer convergence. We demonstrate that the simulated cloud evolution is insensitive to the convergence of the ray tracing across a wide range of samples per pixel per spectral quadrature point. Furthermore, simulations with a coupled ray tracer produce surface irradiance patterns that resemble observations and that strongly feed back to the evolution of clouds via locally enhanced surface heat fluxes. Plain Language Summary: Clouds absorb and reflect solar radiation and create spatial patterns at the land surface of cloud shadows interspersed with sunny regions. These patterns are currently largely simplified in most weather models because radiation computations cost a lot of computer power. We have developed a fast and realistic numerical radiation model that runs on a modern computer graphics card. By doing so, we can do simulations that produce radiation patterns that closely resemble reality and study how these patterns affect clouds. This enables us to improve our understanding of the complex interactions between clouds, solar radiation, and the Earth's surface. Key Points: We present graphics processing unit‐accelerated cloud‐resolving simulations with a coupled ray tracerSimulated cloud statistics are insensitive to ray tracer convergence across a wide range of sample sizesStrong coupling between surface irradiance patterns and cloud evolution underlines need for coupled 3D radiative transfer [ABSTRACT FROM AUTHOR]

Within the roughness sublayer (RSL) of dense urban canopies composed of uniformly distributed cuboids, the time and planar-averaged mean velocity profile exhibits an approximate exponential shape characterized by a depth-independent attenuation coefficient a. A formulation that links a to the zero-plane displacement d and aerodynamic roughness length z om is proposed using a one-dimensional momentum balance between the background mean horizontal pressure gradient, vertical gradients of total stresses, and the drag force. Dispersive effects on a within the urban RSL are then explored using large-eddy simulations (LESs) that vary independently the planar ( λ p ) and frontal ( λ f ) densities of the cuboids. The LES results are used to compute d and z om by fitting a log-profile to the mean velocity above the canopy. Within the canopy, the LES results are also used to estimate (i) a by fitting an exponential profile to the computed time and planar-averaged velocity, (ii) profiles of drag coefficients, and (iii) turbulent as well as dispersive stresses. The LES results demonstrate that dispersive stresses can be commensurate with turbulent stresses in magnitude and act in the same direction. Moreover, dispersive transport, determined from vertical gradients of dispersive stresses, is some 25–75% of turbulent stress gradients. These dispersive effects impact a (and thus d and z om ) via two mechanisms: (i) reducing the effective adjustment length scale that leads to an increase in a and (ii) increasing the effective mixing length that leads to a reduction in a across a wide range of λ f and λ p . These two effects are shown to be partly compensatory giving rise to an apparent constant a with respect to height inside the canopy. The effects of mean recirculation and the usage of the drag force centroid method to estimate d are discussed. The analysis also evaluates the consequences of a finite roughness sublayer thickness extending above the canopy on the derived expressions. [ABSTRACT FROM AUTHOR]

Intentional release of hygroscopic particles, or seeding, in convective clouds is one of the postulated methods to artificially enhance rainfall. Motivated by the general uncertainty in the underlying physics, this work employs a large-eddy simulation code together with a detailed aerosol–cloud microphysics model to investigate the conditions and processes conducive to seeding in the United Arab Emirates. Mixed-phase processes are identified as the main source for rainfall in convective clouds in this area owing to the continental aerosol characteristics and a high cloud-base altitude relatively close to the freezing level. Subsequently, our model experiments highlight the importance of mixed-phase processes in mediating the effects of hygroscopic seeding on rainfall as well. The seeding particles acted to accelerate riming by increasing the number of large droplets taken above the freezing level by the convective updrafts. The rime fraction was increased by up to 15%, which promotes the growth of the frozen hydrometeors, eventually leading to enhanced rainfall via melting. The peak enhancement in surface rainfall was up to 20%–30%, although this is almost certainly an overestimation relative to real-world operations because of the simplified description of the seeding in the model. The strongest rain enhancement was obtained with a high background aerosol concentration of approximately 4500 cm−3, whereas reduced aerosol resulted in weaker enhancement. The latter case showed an overall higher rime fraction indicating an already efficient precipitation formation process, which suppressed the seeding-induced enhancement. The conclusions of our work encourage more careful consideration of the mixed-phase processes in quantifying the hygroscopic seeding effects in continental convective clouds. [ABSTRACT FROM AUTHOR]

The noise radiated by the flow around a cylinder in the subcritical regime at ReD=1×104 and at a subsonic Mach number of M=0.5 is here studied. The aerodynamic sound radiated by a cylinder has been studied with a wide range of Reynolds numbers, but there are no studies about how the Mach number affects the acoustic field in the subsonic regime. The flow field is resolved by means of large-eddy simulations of the compressible Navier–Stokes equations. For the study of the noise propagation, formulation 1C of the Ffowcs Williams–Hawkings analogy is used. The fluid flow results show good agreement when comparing the surface pressure coefficient, the recirculation length, the vortex shedding frequency and the force coefficients against other studies performed under similar conditions. The dynamic mode decomposition of the pressure fluctuations is used to relate them with the far-field noise. It is shown that, in contrast to what happens for low Mach numbers, quadrupoles have a significant impact mainly in the observers located in the streamwise direction. This effect leads to a global monopole directivity pattern as the shear fluctuations compensate for the lower value of the aeolian tone away from the cross-stream direction.

Scale-resolving hybrid RANS-LES models are increasingly being used to numerically simulate wall-bounded turbulent flows. Such models typically display modeled stress depletion within attached turbulent boundary layers, which in turn leads to underprediction of wall shear stress. The interpolated hybrid RANS-LES (IRL) model, recently proposed by Jaiswal et al. (Computers & Fluids, 2023, 106,086), avoids modeled stress depletion by simultaneously evolving the Large Eddy Simulation (LES) equation for filtered velocity and Reynolds Averaged Navier Stokes (RANS) equation for turbulent eddy viscosity on the same computational grid. A hybrid turbulent eddy viscosity, interpolated from RANS eddy viscosity and effective LES eddy viscosity, is used to correct the mean momentum equation near the wall. In this work, IRL has been extended to non-isothermal flows, and a more general formulation, termed as the Interpolated Reynolds-Stress RANS-LES (IRRL) technique, is also presented, in which the full turbulent Reynolds stress/flux is interpolated over the hybrid region. Results from simulations of canonical turbulent flow geometries have been used to evaluate the performance of non-isothermal IRL and IRRL solvers. [ABSTRACT FROM AUTHOR]

The mesoscale atmospheric model Meso-NH is used to investigate the influence of mesoscale atmospheric turbulence on the mean flow, turbulence, and pollutant dispersion in an idealized urban-like environment, the array of containers investigated during the Mock Urban Setting Test field experiment. First, large-eddy simulations are performed as in typical computational fluid dynamics-like configurations, i.e., without accounting for the atmospheric- boundary-layer (ABL) turbulence on scales larger than the building scale. Second, in a multiscale configuration, turbulence of all scales prevailing in the ABL is accounted for by using the grid-nesting approach to downscale from the mesoscale to the microscale. The building-like obstacles are represented using the immersed boundary method and a new turbulence recycling method is used to enhance the turbulence transition between two nested domains. Upstream of the container array, flow characteristics such as wind speed, direction and turbulence kinetic energy are well reproduced with the multiscale configuration, showing the efficiency of the grid-nesting approach in combination with turbulence recycling for downscaling from the mesoscale to the microscale. Only the multiscale configuration is able to reproduce the mesoscale turbulent structures crossing the container array. The accuracy of the numerical results is evaluated for wind speed, wind direction, and pollutant concentration. The microscale numerical simulation of wind speed and pollutant dispersion in an urban-like environment benefits from taking into account the ABL turbulence. However, this benefit is significantly less important than that described in the literature for the Oklahoma City Joint Urban 2003 real case. The present study highlights that pollutant dispersion simulation improvement when accounting for ABL turbulence is dependent on the specific configuration of the city. [ABSTRACT FROM AUTHOR]

In this work, we investigate the effect of areawide building retrofitting on summertime, street-level outdoor temperatures in an urban district in Berlin, Germany. We perform two building-resolving, weeklong large-eddy simulations: one with nonretrofitted buildings and the other with retrofitted buildings in the entire domain to meet today’s energy efficiency standards. The comparison of the two simulations reveals that the mean outdoor temperatures are higher with retrofitted buildings during daytime conditions. This behavior is caused by the much smaller inertia of the outermost roof/wall layer in the retrofitting case, which is thermally decoupled from the inner roof/wall layers by an insulation layer. As a result, the outermost layer heats up more rigorously during the daytime, leading to increased sensible heat fluxes into the atmosphere. During the nighttime, the outermost layer’s temperature drops down faster, resulting in cooling of the atmosphere. However, as the simulation progresses, the cooling effect becomes smaller and the warming effect becomes larger. After 1 week, we find the mean temperatures to be 4 K higher during the daytime while the cooling effects become negligible. SIGNIFICANCE STATEMENT: Building retrofitting is taking place in Europe and other continents as a measure to reduce energy consumption. The change in the building envelope directly influences the urban atmosphere. Our study reveals that areawide retrofitting in a German city district can have negative effects on the outdoor microclimate in summer by causing higher air temperatures. [ABSTRACT FROM AUTHOR]

This paper provides a numerical study on n-dodecane flames using Large-Eddy Simulations (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popular chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Analogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, ignition delay time, and flame lift-off. Additionally, it captures the "cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance. [ABSTRACT FROM AUTHOR]

In this study, a clear-air turbulence event was reproduced using a high-resolution (250 m) large-eddy simulation in the Weather Research and Forecasting (WRF) Model, and the resulting wind field was used in a flight simulation to estimate the vertical acceleration changes experienced by an aircraft. Conditions were simulated for 16 December 2014 when many intense turbulence encounters (and one accident) associated with an extratropical cyclone were reported over the Tokyo area. Based on observations and the WRF simulation, the turbulence was attributed to shear-layer instability near the jet stream axis. Simulation results confirmed the existence of the instability, which led to horizontal vortices with an amplitude of vertical velocity from +20 to −12 m s−1. The maximum eddy dissipation rate was estimated to be over 0.7, which suggested that the model reproduced turbulence conditions likely to cause strong shaking in large-size aircraft. A flight simulator based on aircraft equations of motion estimated vertical acceleration changes of +1.57 to +0.08 G on a Boeing 777-class aircraft. Although the simulated amplitudes of the vertical acceleration changes were smaller than those reported in the accident (+1.8 to −0.88 G), the model successfully reproduced aircraft motion using a combination of atmospheric and flight simulations. [ABSTRACT FROM AUTHOR]

We study wind turbine wakes of rotors operating at high thrust coefficients (CT > 24/25) using large‐eddy simulations with a rotating actuator disk model. Wind turbine wakes at high thrust coefficients are different from wakes at low thrust coefficients. Wakes behave differently at high thrust, with increased turbulence and faster recovery. Lower induction in the wake is achieved because wakes in high‐thrust conditions recover much faster than in normal operating conditions. This enhanced recovery is possible thanks to the turbulence generated in the near wake. We explore the mechanism behind this behavior and propose a simple model to reproduce it. We also propose a Gaussian fit for the wakes under high‐thrust conditions and use it use it to initialize an Ainslie type model within the FAST.Farm framework. [ABSTRACT FROM AUTHOR]

Abstract The Advanced Microphysics Prediction System (AMPS), which adopts a two‐moment hybrid‐bin habit‐predicting scheme, was previously developed to study cloud microphysical processes that depend on ice habit; however, only one particular atmospheric model, the University of Wisconsin‐Nonhydrostatic Modeling System, has been used to test the AMPS. In this study, AMPS is implemented into the Scalable Computing for Advanced Library and Environment (SCALE) large‐eddy simulation model. The AMPS Eulerian advection scheme for non‐mass characteristic variables of ice particles, such as axis lengths, is refined to minimize numerical artifacts. The resulting SCALE‐AMPS model successfully reproduces features of mixed‐phase clouds observed during the Surface Heat Budget of the Arctic campaign, including liquid water path (LWP), ice particle size distributions, and ice habits, when ice particle number concentrations (Nice) are reproduced. Sensitivity studies show that increases in Nice result in reductions of LWP that are generally consistent with previous results. Interestingly, LWP reductions lead to changes in ice habits through increases in cloud temperature due to weaker cloud top radiative cooling. Furthermore, aspect ratios of precipitating particles also change following LWP reductions, because in Bigg's immersion freezing scheme, adopted in this study, the aspect ratios depend on the initial size of the ice particles and freezing rates depend on both temperature and droplet size. Because habits of ice particles affect their growth rates, fall speeds, and collision rates, the results obtained in this study reveal possible feedback processes of Arctic mixed‐phase clouds operating through ice habits.

It is shown here that a wide, buoyant parcel of air at the surface accelerates far less rapidly than it does aloft. In particular, analytical formulae are derived for the effective buoyancy (i.e. the net vertical acceleration due to parcel buoyancy and environmental response) of idealized cylinders of diameter D and height H, located in free space and at the surface. These formulae quantify the decrease of effective buoyancy with increasing aspect ratio D/H, and show that this effect is more pronounced for surface cylinders, especially when D/H > 1. We gain intuition for these results by considering the pressure fields generated by these buoyant parcels, and we test our results with large-eddy simulations. Our formulae can inform parametrizations of the vertical velocity equation for clouds, and also provide a quantitative map of the 'grey zone' in numerical modelling between hydrostatic and non-hydrostatic regimes.

Abstract Advances in high‐performance computing have enabled large‐eddy simulations (LES) of turbulence, convection, and clouds. However, their potential to improve parameterizations in global climate models (GCMs) is only beginning to be harnessed, with relatively few canonical LES available so far. The purpose of this paper is to begin creating a public LES library that expands the training data available for calibrating and evaluating GCM parameterizations. To do so, we use an experimental setup in which LES are driven by large‐scale forcings from GCMs, which in principle can be used at any location, any time of year, and in any climate state. We use this setup to create a library of LES of clouds across the tropics and subtropics, in the present and in a warmer climate, with a focus on the transition from stratocumulus to shallow cumulus over the East Pacific. The LES results are relatively insensitive to the choice of host GCM driving the LES. Driven with large‐scale forcing under global warming, the LES simulate a positive but weak shortwave cloud feedback, adding to the accumulating evidence that low clouds amplify global warming.

A giga-large-eddy simulation of a cumulus congestus has been performed with a 5-m resolution to examine the fine-scale dynamics and mixing on its edges. At 5-m resolution, the dynamical production of subgrid turbulence clearly dominates over the thermal production, whereas the situation is reversed for resolved turbulence, the tipping point occurring near the 250-m scale. For cloud dynamics, the toroidal circulation already obtained in previous observational and numerical studies remains, with a strong signature on the resolved turbulent fluxes, the most important feature for the exchanges between the cloud and its environment even though numerous smaller eddies are also well resolved. The environment compensates for the upward mass flux through a large-scale compensating subsidence and the so-called subsiding shell composed of cloud-edge downdrafts, both having a significant contribution. A partition is used to characterize the dynamics, buoyancy, and turbulence of the inner and outer edges of the cloud, the cloud interior, and the far environment. On the edges of the cloud, downdrafts caused by the eddies and by evaporative cooling effects coexist with a buoyancy reversal while the cloud interior is mostly rising and positively buoyant. An alternative simulation in which evaporative cooling is suppressed indicates that this process reinforces the downdrafts near the edges of the cloud and causes a general decrease of the convective circulation. Evaporative cooling also has an impact on the buoyancy reversal and on the fate of the engulfed air inside the cloud. [ABSTRACT FROM AUTHOR]

Advances in high‐performance computing have enabled large‐eddy simulations (LES) of turbulence, convection, and clouds. However, their potential to improve parameterizations in global climate models (GCMs) is only beginning to be harnessed, with relatively few canonical LES available so far. The purpose of this paper is to begin creating a public LES library that expands the training data available for calibrating and evaluating GCM parameterizations. To do so, we use an experimental setup in which LES are driven by large‐scale forcings from GCMs, which in principle can be used at any location, any time of year, and in any climate state. We use this setup to create a library of LES of clouds across the tropics and subtropics, in the present and in a warmer climate, with a focus on the transition from stratocumulus to shallow cumulus over the East Pacific. The LES results are relatively insensitive to the choice of host GCM driving the LES. Driven with large‐scale forcing under global warming, the LES simulate a positive but weak shortwave cloud feedback, adding to the accumulating evidence that low clouds amplify global warming. Plain Language Summary: Clouds remain one of the largest uncertainties in our understanding and in predictions of climate change because it is challenging to represent their small‐scale dynamics in climate models. However, high‐resolution simulations can provide faithful simulations of clouds and turbulence in limited areas, and these can be used to calibrate climate models. So far, only a limited set of simulations has been used for calibration of climate models, with focus on a few specific locations. This study presents an experimental setup that allows the high‐resolution simulations to be run anywhere on the globe, in any climate state, driven by output from different climate models. The setup is used to create a library of high‐resolution simulations of clouds across the tropics and subtropics in both the current and a warmer climate. The library substantially expands the data set available for the calibration and evaluation of climate models. Key Points: A library of high‐resolution simulations of clouds is created with large‐eddy simulations (LES) driven by a few global climate models (GCMs)Clouds simulated by LES driven by different GCMs are more similar to one another than the corresponding GCM‐simulated cloudsUnder global warming, LES‐simulated low clouds exhibit a weak but positive feedback on warming [ABSTRACT FROM AUTHOR]

The influence of drifting and blowing snow on surface mass and energy exchange is difficult to quantify due to limitations in both measurements and models, but is still potentially very important over large areas with seasonal or perennial snow cover. We present a unique set of measurements that make possible the calculation of turbulent moisture, heat, and momentum fluxes during conditions of drifting and blowing snow. From the data, Monin–Obukhov estimation of bulk fluxes is compared to eddy-covariance-derived fluxes. In addition, large-eddy simulations with sublimating particles are used to more completely understand the vertical profiles of the fluxes. For a storm period at the Syowa S17 station in East Antarctica, the bulk parametrization severely underestimates near-surface heat and moisture fluxes. The large-eddy simulations agree with the eddy-covariance fluxes when the measurements are minimally disturbed by the snow particles. We conclude that overall exchange over snow surfaces is much more intense than current models suggest, which has implications for the total mass balance of the Antarctic ice sheet and the cryosphere. [ABSTRACT FROM AUTHOR]

Experimental evidence shows that temperature‐humidity (θ−q) similarity in the atmospheric surface layer (ASL) is reduced as Bowen ratio (β) increases over land. However, underlying physical mechanisms remain not well understood. With large‐eddy simulations, θ−q dissimilarity is investigated in the steady‐state, convective boundary layer (CBL) over homogeneous landscape with varying β. As β increases from 0.4 to 2.0, the entrainment ratio for θ slightly decreases but that for q largely increases. As a result, local production of humidity variance is substantially enhanced in the upper CBL and transported to the lower CBL by vigorous large eddies, contributing significantly to nonlocal fraction. However, the increased temperature variance in the ASL associated with strong heat flux is larger than that transported from the upper CBL. Such asymmetry in vertical diffusion induced by varying partitioning of surface fluxes strongly regulates θ−q dissimilarity even under perfect conditions valid for Monin‐Obukhov similarity theory. Plain Language Summary: The behavior of potential temperature (θ) and specific humidity (q) in the atmospheric surface layer (ASL) is assumed to be similar over homogeneous landscape. However, abundant experimental evidence shows that such assumption of θ−q similarity is not satisfied as evaporation decreases (i.e., increased Bowen ratio, β). In order to understand the intrinsic physical mechanism, we investigate θ−q similarity in the steady‐state convective boundary layer (CBL) using the high‐resolution model and analyze the results in various β cases. We confirm that θ−q similarity is reduced across the CBL with increasing or decreasing β from 0.4, with the lowest similarity appearing in the middle or upper CBL. The disproportional variations of σθ2 and σq2 associated with asymmetric contributions by top‐down and bottom‐up transport of θ and q under varying β conditions explain the θ−q dissimilarity in the CBL. The results suggest that varying degrees of validity of similarity assumption with changes in β should be noted in applying Monin‐Obukhov similarity theory and interpreting eddy covariance data even over homogenous landscapes and highlight the influence of the CBL processes on the ASL turbulence structures. Key Points: Temperature‐humidity (θ−q) similarity varies with Bowen ratio (β) in the convective boundary layer (CBL) over homogeneous surfaceθ−q dissimilarity with varying β is mostly linked to the large‐scale eddiesDisproportional variations of σθ2 and σq2 by asymmetric top‐down and bottom‐up transport of θ and q explain the dissimilarity with varying β [ABSTRACT FROM AUTHOR]

The presence of spherical solid particles immersed in an incompressible turbulent flow was numerically investigated from the perspective of the particle mass fraction (PMF or Φm), a measure of the particle-to-fluid mass ratio. Although a number of different changes have been reported to be obtained by the presence of solid particles in incompressible turbulent flows, the present study reports the findings of varying Φm in the the turbulent behaviour of the flow, including aspects such as: turbulent statistics, skin-friction coefficient, and the general dynamics of a particle-laden flow. For this purpose, a particle-laden turbulent channel flow transporting solid particles at three different friction Reynolds numbers, namely Reτ = 180, 365, and 950, with a fixed particle volume fraction of Φv = 10-3, was employed as conceptual flow model and simulated using large eddy simulations. The value adopted Φv for allowed the use of a two-way coupling approach between the particles and the flow or carrier phase. Three different values of Φm were explored in this work Φm ≈ 1, 2.96, and 12.4. Assessment of the effect of Φm was performed by examining changes of mean velocity profiles, velocity fluctuation profiles, and a number of other relevant turbulence statistics. Our results show that attenuation of turbulence activity of the carrier phase is attained, and that such attenuation increases with Φm at fixed Reynolds numbers and Φv. For the smallest Reynolds number case considered, flows carrying particles with higher Φm exhibited lower energy requirements to sustain constant fluid mass flow rate conditions. By examining the flow velocity field, as well as instantaneous velocity components contours, it is shown that the attenuation acts even on the largest scales of the flow dynamics, and not only at the smaller levels. These findings reinforce the concept of a selective stabilising effect induced by the solid particles, particularly enhanced by high values of Φm, which could eventually be exploited for improvement of energetic efficiency of piping or equivalent particles transport systems. [ABSTRACT FROM AUTHOR]

We present a review of existing wind–wave coupling models and parameterizations used for large-eddy simulation of the marine atmospheric boundary layer. The models are classified into two main categories: (i) the wave-phase-averaged, sea surface–roughness models and (ii) the wave-phase-resolved models. Both categories are discussed from their implementation, validity, and computational efficiency viewpoints, with emphasis given on their applicability in offshore wind energy problems. In addition to the various models discussed, a review of laboratory-scale and field-measurement databases is presented thereafter. The majority of the presented data have been gathered over many decades of studying air–sea interaction phenomena, with the most recent ones compiled to reflect an offshore wind energy perspective. Both provide valuable data for model validation. We also discuss the modeling knowledge gaps and computational challenges ahead. [ABSTRACT FROM AUTHOR]

The identification and visualization of airfoil noise sources are critical for comprehending the interplay between flow and acoustics, and for understanding the generation and propagation of sound. In this paper, we introduce an innovative method, specifically designed to identify and visualize flow-induced noise sources associated with airfoil noise. This approach employs cross power spectral density analysis to distinctly identify flow-induced noise sources. The data for our study comes from large-eddy simulations of a NACA 0012 airfoil, characterized by a Reynolds number of 4 × 10 5 , a Mach number of 0.058, and an angle of attack of 6.25 ∘. Different cross-spectrum formulations are examined and evaluated. The analysis includes a comparison of the cross-spectrum method's strengths and limitations relative to established data-driven approaches like dynamic mode decomposition (DMD) and spectral proper orthogonal decomposition (SPOD). We find that the cross-spectrum method provides both spectral magnitudes and phase topologies, allowing it to effectively compare sound intensities among various sources at specific frequencies, while retaining high-resolution spatiotemporal coherent flow and acoustic dynamics. One of the notable advantages of the cross-spectrum method over DMD or SPOD modes is its lesser reliance on extensive data manipulation in the form of large-size data matrix, making it a more efficient and user-friendly approach for practitioners, particularly when dealing with complex systems or high-dimensional datasets. This characteristic enhances its practicality and accessibility in the field of flow and acoustic visualization. Finally, the newly developed method is applied to three distinct flow transition scenarios to evaluate its proficiency in distinguishing acoustic generation and propagation mechanisms, depending on the specific transition case. [ABSTRACT FROM AUTHOR]

Abstract Interactions between clouds and radiation are at the root of many difficulties in numerically predicting future weather and climate and in retrieving the state of the atmosphere from remote sensing observations. The broad range of issues related to these interactions, and to three‐dimensional interactions in particular, has motivated the development of accurate radiative tools able to compute all types of radiative metrics, from monochromatic, local, and directional observables to integrated energetic quantities. Building on this community effort, we present here an open‐source library for general use in Monte Carlo algorithms. This library is devoted to the acceleration of ray tracing in complex data, typically high‐resolution large‐domain grounds and clouds. The main algorithmic advances embedded in the library are related to the construction and traversal of hierarchical grids accelerating the tracing of paths through heterogeneous fields in null‐collision (maximum cross‐section) algorithms. We show that with these hierarchical grids, the computing time is only weakly sensitive to the refinement of the volumetric data. The library is tested with a rendering algorithm that produces synthetic images of cloud radiances. Other examples of implementation are provided to demonstrate potential uses of the library in the context of 3‐D radiation studies and parameterization development, evaluation, and tuning.

The Eulerian stochastic fields (ESF) combustion model can be used in LES in order to evaluate the filtered density function to describe the process of turbulence–chemistry interaction. The method is typically computationally expensive, especially if detailed chemistry mechanisms involving hydrocarbons are used. In this work, expensive computations are avoided by coupling the ESF solver with a reduced chemistry model. The reaction–diffusion manifold (REDIM) is chosen for this purpose, consisting of a passive scalar and a suitable reaction progress variable. The latter allows the use of a constant parametrization matrix when projecting the ESF equations onto the manifold. The piloted flames Sandia D–E were selected for validation using a 2D-REDIM. The results show that the combined solver is able to correctly capture the flame behavior in the investigated sections, although local extinction is underestimated by the ESF close to the injection plate. Hydrogen concentrations are strongly influenced by the transport model selected within the REDIM tabulation. A total solver performance increase by a factor of 81% is observed, compared to a full chemistry ESF simulation with 19 species. An accurate prediction of flame F instead required the extension of the REDIM table to a third variable, the scalar dissipation rate. [ABSTRACT FROM AUTHOR]