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- The Role of Diffusion in Boundary-Layer Turbulence Simulation in the Grey
Zone-
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Abstract: The Large Eddy Simulation (LES) regime of boundary-layer turbulence modelling is expected to be reasonably independent of sub-grid model choices. In contrast, numerical weather predictions at 1 km grid length are in the grey zone of the convective boundary layer where the sub-grid model and other sources of diffusion play a much larger role. Also, the expectation at the limit of the grid length ($$\varDelta x$$) being the same order as the boundary-layer depth (h) is that turbulence should collapse due to being no longer resolved. The rate of transition from resolved to unresolved flow with increasing grid length is a key question in grey-zone research. In this paper we investigate the role of sub-grid diffusion magnitude on the simulation of turbulence at grid lengths ranging from LES to the grey zone, focusing on three key features. For the LES, and at high wavenumbers, we look at the fall-off of the spectrum with respect to the inertial sub-range. In the grey zone, we examine the collapse of turbulence at $$\varDelta x \sim h$$ and the ability to spin up turbulence. Our methodology is to compare a numerical weather prediction model (the Met Office Unified Model) with an established LES model with a completely different dynamical core (MONC) but both using a Smagorinsky sub-grid model. We use an established convective boundary layer case. Both models require an enhancement of diffusion to give a collapse of turbulence at $$\varDelta x \sim h$$. In contrast, the spin up of turbulence within the grey zone ideally requires a reduced diffusion. This indicates that grey-zone diffusion levels are regime dependent. We propose a classification of the useable grey zone in terms of the spectrum. When there is a discernable peak and inertial subrange, we argue there is useful information in the simulation and the turbulence should not be suppressed.
PubDate: 2025-07-11
- Sub-canyon Scale Motions in Urban Turbulence: Influence of Flow
Configurations-
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Abstract: Urban turbulence is a typical form of rough-wall turbulence, in which turbulent motions within the roughness sublayer are strongly influenced by obstacle geometry. Based on large-eddy simulations (LES) and space-scale filtering in wavelet space, this study investigates the energy contributions and interscale interactions associated with obstacle-induced motions under various flow configurations, including different obstacle layouts, height variations, and wind directions. The results reveal that sub-filtered scale (SFS) contributions to turbulent kinetic energy (TKE) generally peak within the canopy layer, with the peak height varying depending on the flow configuration, and decrease sharply above the roof level ($$z/H_{max} = 1$$). In contrast, the SFS Reynolds stress peaks in the near-wall region and exhibits a secondary maximum near the roof level. At a cut-off scale equal to one-quarter of the canyon scale ($$n=3$$), the sub-filtered scale (SFS) contributions, based on the Haar wavelet basis, account for approximately 20% or more of the total TKE and Reynolds stress across the entire vertical domain of interest ($$z/H \le 2.5$$). Following the framework of Meneveau (Meneveau in J Fluid Mech 232:469–520, 1991), interscale interactions between sub-canyon and larger-scale motions are quantified using the effective eddy viscosity defined in the wavelet space. The vertical profiles of SFS energy and effective eddy viscosity demonstrate significant sensitivity to obstacle layout within the canopy layer and to obstacle height near the roof level. These findings advance the understanding of small-scale motions and their role in wall turbulence, emphasizing the necessity of capturing vertical variability and sub-filter scale dynamics in urban boundary layer simulations to improve turbulence modeling.
PubDate: 2025-07-10
- Investigation of Boundary Layer Scale Interactions Over Realistic
Morphology Influenced by An Upstream Tall Building Using the SPOD Method-
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Abstract: The turbulent dynamics of the boundary layer flow over an urban morphological model, under the influence of an upstream tall building, is investigated using Stereoscopic PIV (SPIV) in a boundary layer wind tunnel. A recently proposed method - Spectral Proper Orthogonal Decomposition (SPOD) is performed on the boundary layer velocity fluctuations, to separate the large scales and small scales. The separation method demonstrates reliable performance by revealing distinct patterns of periodic scale interaction within the boundary layer, which align with the most energetic frequencies observed in Hot-Wire Anemometry (HWA) measurements. The influence of the wake of a very tall building on the scale interaction extends across a broad range in the transverse direction, with the exception of the central area of the wake.
PubDate: 2025-07-09
- Penetrative Convection Driven by Radiative Cooling in the Nocturnal
Atmospheric Boundary Layer-
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Abstract: In the nocturnal boundary layer, under calm and clear-sky conditions, the aerosol-rich surface layer cools radiatively to the upper atmosphere. This cooling significantly impacts the vertical temperature profile, affecting areas several hundred meters above the surface. The cooling process results in a stable nocturnal inversion layer. However, the ground surface, with its higher thermal inertia, cools more slowly. This difference in cooling leads to the development of an air layer about one meter thick, which can be $${2-6}^\circ $$C cooler than the surface. Consequently, an unstable convective layer forms at the surface, capped by a stable inversion layer that extends upwards for hundreds of meters. This situation, where the presence of a convective layer below a stably stratified inversion layer, is a classic example of penetrative convection. Micro-meteorological phenomena near the surface, such as heat and moisture transport from the ground, fog formation and deepening are affected by this convection. This study presents numerical simulations of penetrative convection driven by radiative cooling in the nocturnal boundary layer. The results show that entrainment velocity-governing the growth of the surface convective layer-follows the Deardorff model and is influenced by aerosol number density, bulk Richardson number based on the penetrative length scale, and convective layer height.
PubDate: 2025-07-04
- Ensembles in Urban Large Eddy Simulations with Changing Wind Direction
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Abstract: Differences between time-averaged and ensemble-averaged wind are studied for the case of changing wind direction. We consider a flow driven by a temporally turning pressure gradient in both an idealized case of a staggered cube array and a realistic urban environment. The repeating structure of the idealized case allows us to construct a large ensemble of 3240 members with a reasonable compute time. The results indicate that the use of plain time averaging instead of an ensemble average can severely reduce the accuracy of both the mean and variance. These errors are the largest when the averaging time is of the same order as the time scale associated with the turning. Utilizing Taylor diagrams, we show that a reasonable compromise between ensemble size and accuracy can be achieved by calculating the ensemble statistics from temporally averaged results with an averaging time that is clearly smaller than the characteristic time scale. This allows the use of reasonably-sized ensembles with 10–50 members. By applying this approach to the realistic urban geometry, we identify building wakes as the regions most severely affected by the incorrectly use of time averaging.
PubDate: 2025-07-03
- Direct Numerical Simulation of Fog Formation with Turbulence and Longwave
Radiation-
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Abstract: This study investigates turbulent channel flow with condensation at a friction Reynolds number of $$Re_*=590$$, focusing on the influence of longwave radiation on fog formation. Two setups are analyzed: one incorporating ground cooling without direct radiative effects, and the other including cooling due to longwave radiation emitted by the fog. The cooling rate and radiative coefficient are varied independently to assess their impacts. In scenarios with excessive ground cooling, turbulence is suppressed, leading to laminarization and delayed condensation higher up. Additionally, longwave radiation is found to either invigorate or dampen turbulence dynamics, depending on the radiative coefficients. At high radiative coefficients, longwave radiation counterbalances ground cooling effects, producing uniform temperature profiles that drive fog formation. These findings underscore the critical role of longwave radiation in atmospheric dynamics.
PubDate: 2025-06-24
- Clear Low-wind Nocturnal Boundary Layers Over Topography
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Abstract: We examine the diurnal variation of the boundary layer under fair-weather winter conditions using measurements taken from a 30-m tower in the North Park Basin of Colorado, USA. We also examine various instrument networks near the tower and found them to be casually useful. The tower site is near the south end of the basin where an air flow is often controlled by downslope southerly flow which then enters the basin. Northerly cold air from the basin interior also commonly reaches the tower. With weak winds, the wind direction shifts back and forth. The two longest fair weather cases of four-day and five-day periods are examined. Time series of the stratification is computed in terms of potential temperature at two levels. The stratification increases and decreases during the night multiple times corresponding to a semi-periodic structure.
PubDate: 2025-06-10
- Developing New Mixing Length Scheme for Improved Offshore Wind Simulation
in the MYNN PBL Parameterization-
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Abstract: Accurate simulation of wind flow in offshore environments is essential for wind energy resource assessment and wind farm planning. This study introduces a novel mixing length scheme to enhance the MYNN (Mellor-Yamada-Nakanishi-Niino) planetary boundary layer (PBL) parameterization in the Weather Research and Forecasting (WRF) model. The new scheme recalculates the surface mixing length using a combination of two logistic functions with S-shaped curves and revises the buoyancy mixing length for open water areas. The results show that the proposed mixing length scheme effectively mitigates the overestimation of near-surface wind speeds and provides a more realistic wind shear profile compared to the default scheme. Additionally, it improves the estimation of wind power density in far offshore areas by reducing the underestimation by 4%, thereby enhancing the practical relevance of WRF model outputs. The scheme significantly influences turbulence kinetic energy (TKE) transport under unstable and neutral atmospheric conditions, reducing both TKE and its vertical transport by nearly 50%, while its effect remains minimal under stable conditions. Overall, the proposed mixing length scheme offers a more comprehensive representation of wind behavior in offshore environments, with the potential to substantially improve offshore wind flow simulations. These advancements provide valuable insights for wind engineering applications and enable more accurate wind resource assessments.
PubDate: 2025-06-01
- The Turbulent Flow within a Wind-Aligned Street Canyon with Trees: an
Experimental Study-
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Abstract: Most studies on the aerodynamic impact of trees on street canyon ventilation focus on external winds perpendicular to the street axis, as these are crucial for dispersing ground-level pollutants like vehicle emissions. Other wind directions, despite their importance for urban pollutant dispersion, receive less attention. Non-perpendicular winds introduce an advective component along the canyon, which is strongly influenced by tree presence and density, significantly shaping pollutant spread across downwind streets. To address this gap, we conducted velocity measurements in a reduced-scale street canyon aligned with the external flow, varying the canyon’s height-to-width ratio and tree density. The aerodynamic effects of trees were simulated by placing plastic tree miniatures along the canyon’s lateral walls. Our results show that trees within a square canyon reduce the mean velocity by 80% and decrease velocity fluctuations by 15% compared to an empty canyon. At the rooftop, the interaction between the external flow and tree crowns leads to a 30% increase in velocity fluctuations. As the aspect ratio decreases (i.e., for wider canyons), a vegetation-free corridor between the rows of trees results in a 25% decrease in mean longitudinal velocity and a 30% increase in velocity fluctuations compared to the empty large canyon. This induces a street ventilation more efficient with respect to the vegetated square canyon, especially when pollutants are emitted at pedestrian level. This study provides valuable experimental data for evaluating the spatially-averaged mean longitudinal velocity in urban canyons, which can be incorporated into analytical and numerical models for better urban air quality management.
PubDate: 2025-05-26
- Passive Tracer Evolution Under Stable Conditions: Impact of Background
Wind and Valley Geometry in an Idealized Setup-
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Abstract: The impact of a large-scale background wind and valley depth on passive tracer accumulation in a valley during nighttime stable boundary layer (SBL) regimes is investigated using idealized three-dimensional large-eddy simulations (LES). The simulations, representative of the Beromünster area in the Swiss Midlands, are initialized approximately two hours before sunset, with a constant passive tracer flux emitted at the surface. The reference simulation assumes a background wind speed of 5 ms$$^{-1}$$ oblique to the valley axis. The results show that stronger background winds (10 ms$$^{-1}$$) enhance vertical mixing and reduce nighttime tracer accumulation, while weaker winds (2.5 ms$$^{-1}$$) lead to higher peak mixing ratios near the surface due to weaker dilution and trapping within the stable layer. Additionally, the along-valley wind component accelerates tracer export by enhancing anabatic transport. Sensitivity to valley depth reveals that, surprisingly, shallower valleys form deeper cold-air pools (CAPs) relative to their size and with more uniform tracer distributions, whereas deeper valleys develop stronger local circulations that enhance vertical mixing. The dimensionless valley depth parameter effectively captures these trends, with higher values indicating increased potential for CAP formation and tracer entrapment. The results emphasize the critical role of CAP breakup time in determining tracer depletion, as earlier inversion breakup leads to more efficient tracer export. The passive tracer in this study is intended to represent CO$$_2$$, providing insights into how CO$$_2$$ accumulates and disperses under stable boundary conditions in complex terrain.
PubDate: 2025-05-26
- Wake Characteristics of Multiscale Buildings in a Turbulent Boundary Layer
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Abstract: Urban forms characterised by multi-scale roughness can drastically modify the wind structure within cities affecting both pedestrian comfort and air quality at street level. For simplicity, most urban flow studies focus on cuboid buildings with a single length scale. We consider six forms to assess how additional length scales impact urban flow: two reference cuboids that differ in aspect ratio (mean building height to width) cases (Standard, 1; Tall, 3) plus two additional fractal iterations of each. The six models have the same mean building width, height, and frontal area but their length scale characteristics differ. These are used in wind tunnel experiments within a deep turbulent boundary layer. The length scale differences are found to affect the drag force exerted by the buildings in a non-negligible way (up to 5 and 13% for Standard and Tall buildings, respectively). The added length scales also modify the wake lateral spread and intensity of the turbulence fluctuations, with smaller the length scales having the lower (higher) intensity of fluctuations in the near (far) wake. Additionally, the strength of the vortex shedding emanating from the buildings is reduced by introducing systematically smaller length scales. This work suggests that omission of additional length scales can lead to inaccuracies in drag and wake recovery estimations. The reduction in intensity of vortex shedding found with each fractal iteration could have engineering applications (e.g. reducing vibration).
PubDate: 2025-05-05
- Evaluation of Six Subgrid-Scale Models for LES of Wind Farms in Stable and
Conventionally-Neutral Atmospheric Stratification-
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Abstract: The performance of six subgrid-scale (SGS) models is analyzed for large-eddy simulations (LES) of wind-farm flows under stable (SBL) and conventionally-neutral (CNBL) atmospheric conditions. A precursor–concurrent technique is employed to provide fully developed turbulent inflow for simulations of a 40-turbine wind farm. Turbines are represented using the actuator-disc method, employing a baseline grid of 12 cells across the turbine diameter. The SBL precursor flow poses a challenge for LES, as it may not be able to resolve the small turbulent scales featured in this flow if the grid is coarse. For these precursor flows, the baseline grid results of all six SGS models are assessed relative to coarser and finer grids, with 6 and 45 cells across the diameter, respectively. The wall-adapting local eddy-viscosity (WALE) and Lagrangian-averaged scale-dependent dynamic (LASDD) models exhibit high grid sensitivity, while the standard Smagorinsky (Smag.), anisotropic minimum-dissipation (AMD), one-equation turbulent kinetic energy (TKE), and stability-dependent Smagorinsky (SDS) models show low sensitivity. For the wind-farm simulations conducted with the baseline grid, the AMD and SDS models predict similar wind-farm performance. In contrast, the WALE and LASDD models predict nearly 30% less power output, primarily due to their prediction of lower inflow wind speeds. CNBL simulations on the baseline grid show reduced sensitivity to the SGS model due to larger atmospheric turbulence and length scales compared to the SBL flow. Among the six models, the AMD model demonstrates ease of implementation, the least sensitivity to grid size for the SBL precursor flow, and predictions that are consistent with other models and higher-order pseudo-spectral LES solvers, making it a suitable choice for LES of wind-farm flows under both stable and conventionally-neutral conditions.
PubDate: 2025-03-25
- Wind-Tunnel Experiment of Heavy Gas and Passive Scalar Emission in a
Turbulent Boundary Layer-
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Abstract: The aim of this study is to experimentally investigate the dynamics of a heavy gas release and highlight its main differences compared to that of a passive gas. To achieve this, we examine the vertical emission of both heavy and passive gases from an elevated source, situated within a turbulent boundary layer. The wind tunnel experiment is designed to simulate a realistic release of oxygen at T = $$-\,40\,^{\circ }$$C, following a similar setup to the study performed by Schatzmann et al. (Atmos Environ 27:1105–1116, 1993). The release is characterised by means of simultaneous velocity and concentration measurements along vertical and lateral profiles at various downwind distances from the source. The data were used to estimate the turbulent mass flows, the turbulent Schmidt number, and the high-order statistics of the scalar field. Furthermore, we conduct an in-depth analysis of the production and dissipation terms of the concentration variance. These are subsequently used to estimate the typical time scale for turbulent mixing using two different micro-mixing models that parametrize the effects of molecular diffusion. These findings are crucial for improving the accuracy of operational dispersion models that simulate localised releases of dense gases in the atmosphere.
PubDate: 2025-03-22
- Large Eddy Simulation Study of Boundary-Layer Flow Behind a
Smooth-to-Rough Surface Roughness Transition-
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Abstract: The atmospheric boundary layer flow downstream of a smooth-to-rough (S$$\rightarrow $$R) roughness transition is studied using large eddy simulations (LES). Simulating the flow over an S$$\rightarrow $$R transition requires overcoming two numerical challenges. First, the flow behind the transition needs space in the vertical direction for the outer boundary layer to develop without adversely affecting the flow near the surface. Vertical padding of the computational domain is employed for this purpose. Numerical experiments show that it is sufficient to set the padding height equal to the incoming boundary layer height. Second, an abrupt change in aerodynamic roughness length ($$z_0$$) leads to large oscillations in the flow statistics. To reduce these oscillations below an acceptable threshold, the S$$\rightarrow $$R transition needs to be smeared over a few grid points. Numerical experiments for a range of roughness ratios show that smearing the roughness transition such that $$\text {d}z_0/\text {d}x$$ is 0.01 or less keeps the oscillations in the wall shear-stress under 5% of the overall jump across the S$$\rightarrow $$R transition. With these features incorporated, the LES are validated against two experimental datasets with roughness ratios 4.34 and 150. Wind engineering design studies require the use of computationally inexpensive analytical models. Seven internal boundary layer height analytical models are evaluated using the LES data and three of them are found to be reasonable over a range of roughness ratios. An analytical model for the turbulence intensity, previously developed for a rough-to-smooth transition, is found to predict the turbulence intensity reasonably for S$$\rightarrow $$R transitions as well.
PubDate: 2025-03-14
- $${\mathscr {L}}$$-moments Reveal the Scales of Momentum Transport in
Dense Canopy Flows-
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Abstract: The interaction between a dense forest canopy and atmosphere is a complex fluid-dynamical problem with a wide range of practical applications, spanning from the aspects of carbon sequestration to the spread of wildfires through a forest. To delineate the eddy processes specific to canopy flows, we develop an $${\mathscr {L}}$$-moment based event framework and apply it on a suite of observational datasets encompassing both canopy and atmospheric surface layer flows. In this framework, the turbulent fluctuations are considered as a chronicle of positive and negative events having finite lengths or time scales, whose statistical distributions are quantified through the $${\mathscr {L}}$$ moments. $${\mathscr {L}}$$ moments are statistically more robust than the conventional moments and have earlier been used in hydrology applications, but here we show how this concept is useful to identify the contrasting features between canopy and atmospheric surface layer flows. The $${\mathscr {L}}$$-moment framework is complemented with wavelet analysis, revealing how differently the canopy-scale coherent structures modulate the horizontal and vertical velocity components in sub-canopy environments. We hypothesize that a consequence of this phenomenon is the existence of two different eddy processes with distinct scaling properties that transport momentum in the gradient and counter-gradient directions, respectively. These findings shed light on a long-standing issue in canopy flows: why the integral timescale of vertical velocity increases as the heights approach the forest floor'
PubDate: 2025-03-14
- Vertical Distribution Characteristics and Formation/Dissipation Mechanisms
of Air Pollutants in Xingtai, China Based on Multi-source Data: A Case
Study-
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Abstract: The Beijing-Tianjin-Hebei urban agglomeration is one of the regions in China with the most severe air pollution. Using aircraft observations collected over Xingtai in May 2016 and multi-source data, such as aerosol chemical composition and lidar data, we analyzed aerosol composition and optical properties, vertical pollution characteristics of gases within the boundary layer, and their interactions with meteorological parameters. This study focuses on investigating the transport and evolution mechanisms of pollutants during transitions from polluted research flight No.7(RF7) to clean research flight No.8 (RF8) periods in summertime Xingtai. Results show that during RF7, the near-surface submicron aerosol (PM1) mass concentration was generally low (37.5 µg m−3), with the contribution of inorganic salts far exceeding that of organic matter. Aircraft observations indicated weak cold-air activities above 1000 m during RF8, while the southeasterly wind still prevailed below 1000 m, with a slight increase in wind speed. From RF7 to RF8, the overall vertical atmosphere gradually transitioned from polluted to clean conditions. During RF8, an inversion layer appeared in the temperature profile between 1100 and 1300 m, with ~ 21.3% of this inversion coming from black carbon heating. Changes in meteorological and pollution transport conditions led to significant differences in PM1 and gaseous components between RF7 and RF8. Compared to RF7, the contribution of nitrates decreased markedly during RF8, with the organic matter becoming the dominant component of PM1. During RF7, sulfur dioxide (SO2), nitrogen dioxide (NO2), and carbon monoxide (CO) were strongly correlated with the scattering coefficient, with correlation coefficients of 0.92 and 0.96 for SO2 and NO2, respectively. By contrast, during RF8, the correlations between SO2, NO2, and CO and the scattering coefficient decreased, with SO2 having the highest correlation with the scattering coefficient (coefficient of determination = 0.88). From RF7 to RF8, the concentrations of various pollutants within the boundary layer, except for ozone (O3), had decreasing trends. When the relative humidity was below 50%, O3 generally contributed positively to extinction. There was no pronounced correlation between O3 and relative humidity when the relative humidity exceeded 50%.
PubDate: 2025-02-18
- Atmospheric Dispersion Downstream a Two-Dimensional Obstacle: Experimental
Evaluation of Turbulence Closure Models-
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Abstract: This study investigates the turbulent dispersion of pollutants in the wake of a two-dimensional square obstacle. Utilizing Laser Doppler Anemometry and Particle Image Velocimetry, we characterized the flow dynamics, identifying a recirculation zone downstream of the obstacle, marked by high shear and increased turbulent viscosity, and playing a crucial role in turbulent momentum exchange. We evaluated the turbulence kinetic energy budget, estimating its dissipation rate, and found traditional isotropy and Taylor hypothesis methods inadequate within the wake region. Furthermore, we explored pollutant dispersion from a linear source located downstream the obstacle. Analysis of mean concentration and variance revealed that the log-normal distribution is most effective for modelling concentrations within the recirculating region, while the Gamma distribution suits areas outside it. Testing various closure models for turbulent mass fluxes highlighted the limitations of the Simplified Gradient Diffusion Hypothesis model, favouring more complex closure models for longitudinal trends, though these still faced challenges with intensity estimation. The Simplified Gradient Diffusion Hypothesis model proved robust for vertical mass fluxes, with satisfactory results in turbulent diffusivity and turbulent Schmidt number calculations. The experimental results serve as a benchmark for validating numerical simulations and assessing the accuracy of closure models typically employed in pollutant dispersion modelling.
PubDate: 2025-02-18
- Large-Eddy Simulation and Modeling on the Evolution of Large-Scale
Secondary Vortices in Turbulent Boundary Layer-
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Abstract: Spanwise heterogeneity in surface properties, such as surface roughness and temperature, at the scale of boundary-layer height, leads to the formation of large-scale secondary vortices in a turbulent boundary layer. Most prior studies on the topic have focused on the topology of secondary vortices in flows that involve little streamwise variation. By introducing streamwise change at regional scales, this study investigates the evolution of large-scale secondary vortices after the removal of the spanwise heterogeneity that sustains them. A suite of roughness-resolved large-eddy simulations is conducted and the results show both the development and decay processes of the secondary vortices. Through analysis of the Reynolds-averaged vorticity equation, a phenomenological model is proposed. The model predicts a slow, exponential decay of the secondary vortices, which receives support from our data.
PubDate: 2025-02-18
- Scaling the Vertical-Velocity Variance During the Very Late Afternoon
Transition of the Convective Boundary Layer-
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Abstract: In the bulk of the convective boundary layer driven by surface heating, the vertical-velocity variance is known to scale with the convective velocity scale. This scaling relies on the quasi-equilibrium assumption that the surface heat flux (H, the water-vapor contribution to buoyancy is neglected) varies slowly compared to the adjustment time scale of the large scale convective eddies (or the eddy turnover time), a condition which over land is typically satisfied from the late morning to the early afternoon transition. Later on, when the surface heat flux starts decaying moderately compared to the eddy turnover time, a departure from the quasi-equilibrium regime is expected. A recent idealized large-eddy simulations (LES) study proposed a parameter for describing such departure during the late afternoon transition: $${\widetilde{r}} \equiv H^{-1}dH/dt ^{-1}/t_*$$, where $$t_*$$ is the eddy turnover time. The quasi-equilibrium assumption applies when $${\widetilde{r}}$$ $$>>$$ 1, and breaks down when $${\widetilde{r}}$$ $$\sim $$ 1. Building on these results, we further investigate the scaling for the vertical-velocity variance during the very late afternoon transition, i.e. in the regime $${\widetilde{r}}$$ $$
PubDate: 2025-02-07
- Wind Extremes over Built Terrain: Characterization and Geometric
Determinants-
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Abstract: Cities feature complex and heterogeneous topographies that create highly-variable airflow patterns and dynamics. The resulting extreme high and low winds challenge urban design’s quest for maintaining a safe and comfortable environment. This study investigates these urban wind extrema by conducting large-eddy simulations (LESs) in four American neighborhoods with distinct topographic features and links the resulting winds to geometric indicators. By analyzing the profile and spatial variation of the wind extrema, the simulations illustrate that gust zones tend to occur along wind-oriented streets and on both sides of high-rise buildings, while stagnation zones are mostly located in the lee of tall or wide buildings. Potential geometric parameters explaining these winds extrema are investigated, including the plan and frontal area fraction and the sky view factor. These findings advance our ability to predict wind conditions solely based on urban geometry, and ultimately to design more resilient, sustainable, and livable cities.
PubDate: 2025-02-07
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