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Natural ventilation is gaining popularity in response to an increasing demand for a sustainable and healthy built environment, but the design of a naturally ventilated building can be challenging due to the inherent variability in the operating conditions that determine the natural ventilation flow. Large-eddy simulations (LES) have significant pot...
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Turbulent flow over urban-like roughness has been numerically studied for various purposes, such as the clarification of turbulent characteristics over rough walls, visualization of turbulent structures around block arrays, and evaluation of urban ventilation and pedestrian winds. In such simulations, a portion of the developing boundary layer is e...
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... where C V is the Vreman coefficient, α ij is the filtered velocity gradient, and B β is the second invariant of β ij = Δ 2 α ij α ij . The static Vreman model has been successfully deployed in prior ABL studies (Ciarlatani et al., 2023;Y. Hwang & Gorlé, 2022Hochschild & Gorlé, 2024;Oh et al., 2024); however, we include a sensitivity study to a dynamic local Smagorinsky SGS model (Goc et al., 2024) in the Supporting Information S1, where we find insignificant differences in the results. The low-Mach equation of state is given bỹ ...
... In assuming an isentropic approximation of the flow, the formulation provides the benefits of a variable density, compressible solver, while also removing the time-step restriction associated with low-Mach flows (Brès et al., 2023), which are common to ABL flows (Y. Hwang & Gorlé, 2022). For describing statistical quantities, the instantaneous velocity (e.g., U) is decomposed into its time-averaged (〈U〉) and fluctuating (u ʹ ; about the mean) component: U(x, y, z, t) = 〈U〉(x, y, z) + u ʹ (x, y, z, t). ...
... CharLES uses an isotropic Voronoi meshing scheme, which provides means to generate a highly scalable, highquality body-fitted mesh, suitable for complex, irregular geometries (Brès et al., 2023;Cooke et al., 2023;Y. Hwang & Gorlé, 2022. The mesh generation is fully parallelized and automated, allowing for production of a grid with O(10M) elements in O(1) minutes using tens of processors. For mesh generation, a far-field grid spacing is first specified, Δ FF , where this relative length-scale sets the refinement. Subsequent mesh spacing is then determined by Δ FF / 2 n ...
Landforms such as sand dunes act as roughness elements to Atmospheric Boundary Layer (ABL) flows, triggering the development of new scales of turbulent motions. These turbulent motions, in turn, energize and kick‐up sand particles, influencing sediment transport and ultimately the formation and migration of dunes—with knock‐on consequences for dust emission. While feedback between flow and form have been studied at the scale of dunes, research has not examined how the development of an Internal Boundary Layer (IBL) over the entire dune field influences sediment‐transporting turbulence. Here, we deploy a large‐eddy simulation of an ABL encountering a natural roughness transition: the sand dunes at White Sands National Park, New Mexico. We analyze turbulence producing motions and how they change as the IBL grows over the dune field. Frequency spectrum and Reynolds shear stress profiles show that IBL thickness determines the largest scales of turbulence. Moreover, the developing IBL enhances the frequency, magnitude and duration of sweep and ejection events—turbulence producing motions whose peaks systematically migrate away from the wall as the IBL thickens. Because sweep and ejection events are known to drive sediment transport, our findings provide a mechanism for coupling the co‐evolution of the landscape and the ABL flow over it. More broadly, our results have implications for how roughness transitions influence the transport of pollutants, particulates, heat, and moisture.
... The static Vreman model has been successfully deployed in prior ABL studies (Y. Hwang & Gorlé, 2022Ciarlatani et al., 2023;Hochschild & Gorlé, 2024;Oh et al., 2024); however, we include a sensitivity study to a dynamic local Smagorinsky SGS model (Goc et al., 2024) in the Supplemental Material, where we find insignificant differences in the results. The low-Mach equation of state is given byρ ...
... In assuming an isentropic approximation of the flow, the formulation provides the benefits of a variable density, compressible solver, while also removing the timestep restriction associated with low-Mach flows (Brès et al., 2023), which are common to ABL flows (Y. Hwang & Gorlé, 2022). For describing statistical quantities, the instantaneous velocity (e.g., U ) is decomposed into its time-averaged (⟨U ⟩) and fluctuating (u ′ ; about the mean) component: ...
... CharLES uses an isotropic Voronoi meshing scheme, which provides means to generate a highly scalable, high-quality body-fitted mesh, suitable for complex, irregular geometries (Y. Hwang & Gorlé, 2022;Cooke et al., 2023;Brès et al., 2023;Y. Hwang & Gorlé, 2023). ...
Landforms such as sand dunes act as roughness elements to Atmospheric Boundary Layer (ABL) flows, triggering the development of new scales of turbulent motions. These turbulent motions, in turn, energize and kick-up sand particles, influencing sediment transport and ultimately the formation and migration of dunes -- with knock on consequences for dust emission. While feedbacks between flow and form have been studied at the scale of dunes, research has not examined how the development of an Internal Boundary Layer (IBL) over the entire dune field influences sediment-transporting turbulence. Here, we deploy large-eddy simulation of an ABL encountering a natural roughness transition: the sand dunes at White Sands National Park, New Mexico. We analyze turbulence producing motions and how they change as the IBL grows over the dune field. Frequency spectrum and Reynolds shear stress profiles show that IBL thickness determines the largest scales of turbulence. More, the developing IBL enhances the frequency, magnitude and duration of sweep and ejection events -- turbulence producing motions whose peaks systematically migrate away from the wall as the IBL thickens. Because sweep and ejection events are known to drive sediment transport, our findings provide a mechanism for coupling the co-evolution of the landscape and the ABL flow over it. More broadly, our results have implications for how roughness transitions influence the transport of pollutants, particulates, heat, and moisture.
... The first challenge is the validation of CFD predictions of the complex flow phenomena that occur during combined buoyancy-and wind-driven natural ventilation in an urban environment. To date, validation of CFD results for natural ventilation has primarily focused on wind-driven ventilation processes, considering both small-scale (Adachi, Ikegaya, Satonaka, & Hagishima, 2020;Hirose, Ikegaya, Hagishima, & Tanimoto, 2021;Hu, Ohba, & Yoshie, 2008;Hwang & Gorlé, 2022a, 2022bMurakami, Ikegaya, Hagishima, & Tanimoto, 2018;Ramponi & Blocken, 2012;Shirzadi, Tominaga, & Mirzaei, 2020;Tominaga & Blocken, 2016; van Hooff, Blocken, & Tominaga, 2017) and full-scale (Jiang & Chen, 2002;King et al., 2017;Larsen, Nikolopoulos, Nikolopoulos, Strotos, & Nikas, 2011) experiments. Buoyancy-driven ventilation has received comparatively less attention, possibly because the flow is more challenging to model. ...
Natural ventilation can play an important role towards preventing the spread of airborne infections in indoor environments. However, quantifying natural ventilation flow rates is a challenging task due to significant variability in the boundary conditions that drive the flow. In the current study, we propose and validate an efficient strategy for using computational fluid dynamics to assess natural ventilation flow rates under variable conditions, considering the test case of a single-room home in a dense urban slum. The method characterizes the dimensionless ventilation rate as a function of the dimensionless ventilation Richardson number and the wind direction. First, the high-fidelity large-eddy simulation (LES) predictions are validated against full-scale ventilation rate measurements. Next, simulations with identical Richardson numbers, but varying dimensional wind speeds and temperatures, are compared to verify the proposed similarity relationship. Last, the functional form of the similarity relationship is determined based on 32 LES. Validation of the surrogate model against full-scale measurements demonstrates that the proposed strategy can efficiently inform accurate building-specific similarity relationships for natural ventilation flow rates in complex urban environments.
... This section first provides a summary of the simulation set-up presented in Part 1 of this study (Hwang and Gorlé, 2022), including the governing equations and discretization methods, the computational domain and mesh, the boundary conditions, and the quantities of interest. Subsequently, we introduce the additional ventilation configurations that are considered in this paper. ...
... This section briefly summarizes the inflow and boundary conditions, which are identical to those used for the simulations in Part I of this study (Hwang and Gorlé, 2022). For the inflow condition, we combine a divergence-free of a digital filter method (Xie and Castro, 2008;Kim et al., 2013) with a gradient-based optimization technique to obtain the desired turbulence characteristics at the building location (Lamberti et al., 2018). ...
Natural ventilation can contribute to a sustainable and healthy built environment, but the flow can be highly dependent on the ventilation configuration and the outdoor turbulent wind conditions. As a result, quantifying natural ventilation flow rates can be a challenging task. Wind tunnel experiments offer one approach for studying natural ventilation, but measurements are often restricted to a few points or planes in the building, and the data can have limitations due to the intrusive nature of measurement techniques or due to challenges with optical access. Large-eddy simulations (LES) can offer an effective alternative for analyzing natural ventilation flow, since they can provide a precise prediction of turbulent flow at any point in the computational domain and enable accurate estimates of different ventilation measures. The objective of this study is to use a validated LES set-up to investigate the effect of the opening size, opening location and wind direction on the ventilation flow through an isolated building. The effects are quantified in terms of time-averaged and instantaneous ventilation flow rates, age of air, and ventilation efficiency. The LES results indicate that, for this isolated building case, the effect of the wind direction is more pronounced than the effect of the size and position of the ventilation openings. Importantly, when ventilation is primarily driven by turbulent fluctuations, e.g. for the 90° wind direction, an accurate estimation of the ventilation rate requires knowledge of the instantaneous velocity field.
A key component of building design involves understanding the flow field that develops near a building due to synoptic wind flow across it. The wind environment around buildings is important for their structural strength, but it also has an impact on ventilation effectiveness, pedestrian comfort, and the spread of pollutants. Modern structures now have a variety of apertures, including windows, doors, and ventilators, leading to the rise of the green building idea and environmentally friendly living. The flow field becomes more complex as a result of these apertures, which alter the flow properties inside and outside the building. The goal of the current study is to better understand the flow fields surrounding the structures with openings, both internally and externally. This study aims to examine the wind flow over structures with Pyramidal roof rectangular base building that have openings exposed to synoptic winds. Using the Computational Fluid Dynamics (CFD) approach, internal flow within the buildings is also examined for optimal ventilation. Numerical simulations are conducted using ANSYS-FLUENT, a commercially available CFD program. The steady Reynolds Averaged Navier Stokes (RANS) equations are solved by numerical simulations. This study examines the effects of various wall porosities on the building façade as well as the relative vertical placements of the openings. The flow field both within and outside of buildings is significantly impacted by wall porosities and the relative vertical positions of openings on building facades. The internal pressure distribution and air flow rate are greatly influenced by the wall porosity. The study provides information about the flow environment within and outside of a low-rise building with Pyramidal roof rectangular base building with openings, which is useful for estimating wind loads for the building's structural design and for creating a natural ventilation system.
