Inclusion of Building‐Resolving Capabilities Into the FastEddy® GPU‐LES Model Using an Immersed Body Force Method

Abstract

As a first step toward achieving full physics urban weather simulation capabilities within the resident-GPU large-eddy simulation (LES) FastEddy ® model, we have implemented and verified/validated a method for explicit representation of building effects. Herein, we extend the immersed body force method (IBFM) from Chan and Leach (2007, https://doi.org/10.1175/2006JAMC1321.1) to (i) be scale independent and (ii) control building surface temperatures. Through a specific drag-like term in the momentum equations, the IBFM is able to enforce essentially zero velocities within the buildings, in turn resulting in a no-slip boundary condition at the building walls. In addition, we propose similar forcing terms in the energy and mass conservation equations that allow an accurate prescription of the building temperature. The extended IBFM is computationally efficient and has the potential to be coupled to building energy models. The IBFM exhibits excellent agreement with laboratory experiments of an array of staggered cubes at a grid spacing of Δ = 1 mm, demonstrating the applicability of the method beyond the atmospheric scale. In addition, the IBFM is validated at atmospheric scale through simulations of downtown Oklahoma City (Δ = 2 m) using data collected during the Joint Urban 2003 (JU03) field campaign. Our LES IBFM results for mean wind speed, turbulence kinetic energy, and SF 6 transport and dispersion compare well to observations and produce turbulence spectra that are in good agreement with sonic anemometer data. Statistical performance metrics for the JU03 simulations are within the range of other LES models in the literature. Plain Language Summary A significant majority of social and economic activities are logically concentrated around densely populated urban areas. Consequently, accurate modeling and prediction of urban-scale weather entails a tremendous benefit to society in many ways. Herein, we extend the immersed body force method (IBFM), which allows explicit representation of building effects in microscale numerical models, to be applicable to disparate scales and to effectively control building surface temperatures. This computationally efficient method is implemented into the GPU-accelerated large-eddy simulation (LES) FastEddy ® model, with the purpose of facilitating a path toward realistic street-scale operational weather forecasting in the near future. We validate the extended IBFM with observations at laboratory scale and urban-scale field measurements over downtown Oklahoma City during the Joint Urban 2003 field campaign. Our LES IBFM results for mean wind speed, turbulence kinetic energy, and SF 6 transport and dispersion compare well to observations, and the corresponding statistical performance metrics are within the range of other LES models in the literature employing body fitted and immersed boundary approaches.

Large-Eddy Simulations of Stability-Varying Atmospheric Boundary Layer Flow over Isolated Buildings

“…No acoustic filtering, e.g., time-splitting technique, was applied. To parameterize effects of SGS turbulence on the resolved (or filtered) turbulent flow, a Smagorinsky-Lilly SGS model was adopted with a diagnostic SGS TKE equation (e.g., Moeng et al 2007;Muñoz-Esparza et al 2014b). The surface momentum and heat fluxes at ground surfaces were parameterized based on the Monin-Obukhov similarity theory (Monin and Obukhov 1954), with stability correction terms (Dyer and Hicks 1970;Dyer 1974).…”

“…For the explicit representation of building effects, an extended version of the IBFM from Chan and Leach (2007) was implemented in the FastEddy model (Muñoz-Esparza et al 2020). The IBFM from Chan and Leach (2007) was designed to reduce the momentum at inside-building grid points to zero by adding sufficiently large drag forces to the momentum equations through a discrete forcing (Mohd-Yusof 1997).…”

“…The IBFM from Chan and Leach (2007) was designed to reduce the momentum at inside-building grid points to zero by adding sufficiently large drag forces to the momentum equations through a discrete forcing (Mohd-Yusof 1997). Muñoz-Esparza et al (2020) extended the IBFM of Chan and Leach (2007) in the following two aspects. First, the drag force term of the momentum equations, which was originally developed for a specific grid spacing (Chan and Leach 2007), was generalized for application of the IBFM to a wide range of spatial scales (from laboratory to atmospheric scales); refer to Eqs.…”

“…A number of LES studies have contributed to advancing the knowledge on this subject mainly for the flow over horizontally homogeneous and structure-free surfaces (e.g., Nieuwstadt et al 1993;Andren et al 1994;Moeng and Sullivan 1994;Kosović and Curry 2000;Kumar et al 2010;Sullivan et al 2016;Salesky et al 2017). Recently, the LES scope has been expanded to include the flow over urban landscapes and/or complex terrains with improvements to the explicit representation of effects of these heterogeneous surfaces, in response to an increasing demand for microscale information of the urban flows (e.g., Bou-Zeid et al 2009;Lundquist et al 2012;Giometto et al 2016Giometto et al , 2017Gronemeier et al 2017;García-Sánchez et al 2018;Auguste et al 2019;Auvinen et al 2020;Baumman-Stanzer et al 2020;Sauer and Muñoz-Esparza 2020;Muñoz-Esparza et al 2020). However, the majority of these urban-resolving LES studies have focused on neutrally stratified ABL (NBL) conditions.…”

“…As a first step toward bridging this gap in the fundamental understanding of building-induced local flow structures in realistic ABL stability regimes, we explore the response of flow around isolated cuboid buildings under various atmospheric stability conditions, modulated by changing the surface heating and vertical structure of the incoming flow, which is at a quasi-equilibrium state for the surface heating. The NCAR GPU-accelerated LES model, FastEddy Ò (hereafter FastEddy) (Sauer and Muñoz-Esparza 2020), is used with explicit representation of building effects using an immersed body force method (IBFM; Muñoz-Esparza et al 2020) and a Smagorinsky-Lilly subgrid-scale (SGS) model (Lilly 1966;Deardorff 1980) based on a diagnostic SGS turbulence kinetic energy (TKE) equation. The computational efficiency gain of the GPU-based FastEddy LES model compared to CPU-based models, 1 GPU matching 256 CPUs (Sauer and Muñoz-Esparza 2020), enables a comprehensive assessment based on a large number of buildingresolving simulations: a total of 33 simulations with 21 of them at 1-or 2-m grid spacing.…”