Application of the Cell Perturbation Method to Large-Eddy Simulations of a Real Urban Area

Abstract

With the continuous increase in computing capabilities, large-eddy simulation (LES) has recently gained popularity in applications related to flow, turbulence, and dispersion in the urban atmospheric boundary layer (ABL). Herein, we perform high-resolution building-scale LES over the Seoul, South Korea, city area to investigate the impact of inflow turbulence on the resulting turbulent flow field in the urban ABL. To that end, LES using the cell perturbation method for inflow turbulence generation is compared to a case where no turbulence fluctuations in the incoming ABL are present (unperturbed case). Validation of the model results using wind speed and wind direction observations at 3 m above ground level reveals minimal differences irrespective of the presence of incoming ABL turbulence. This is due to the high density of building structures present at the surface level that create shear instabilities in the flow field and therefore induce local turbulence production. In the unperturbed case, turbulent fluctuations are found to slowly propagate in the vertical direction with increasing fetch from the inflow boundaries, creating an internal boundary layer that separates the turbulent region near the building structures and the nonturbulent flow aloft that occupies the rest of the ABL. Analysis of turbulence quantities including energy spectra, velocity correlations, and passive scalar fluxes reveals significant underpredictions that rapidly grow with increasing height within the ABL. These results demonstrate the need for realistic inflow turbulence in building-resolving LES modeling to ensure proper interactions within the ABL.

The FastEddy® Resident‐GPU Accelerated Large‐Eddy Simulation Framework: Model Formulation, Dynamical‐Core Validation and Performance Benchmarks

“…In addition, the CP method is computationally efficient, a desired quality in the context of an accelerated GPU framework targeting predictive capabilities for microscale turbulence phenomena. The CP method has been proven to be an efficient technique to generate inflow turbulence and demonstrated on a variety of problems including full physics coupled mesoscale‐LES atmospheric simulations (Muñoz‐Esparza et al, 2017, 2018) and sea breeze fronts (Chen et al, 2019), other semiidealized scenarios dealing with ocean‐island interactions and cloud formation (Jähn et al, 2016), sea breeze over an urban‐like coast (Jiang et al, 2017), snow formation and precipitation (Chu et al, 2018), and flow and turbulence in urban scenarios (Lee et al, 2019). An example of the use of the CP method to develop forcing‐consistent turbulence in FastEddy is presented in section 4.4, for the validation of flow over heterogeneous terrain.…”

Validation of the PALM model system 6.0 in a real urban environment: a case study in Dejvice, Prague, the Czech Republic

“…Validation of the urban model requires a dataset of measurements of the urban meteorological and air quality conditions, the properties of the urban canopy elements, and the energy exchange among parts of the urban canopy. Several campaigns of comprehensive observations and measurements of the urban atmospheric boundary layer, covering more than one season, have been done in the past: the Basel UrBan Boundary Layer Experiment (BUBBLE) dataset containing observations from Basel is specifically targeted for validation of urban radiation models, urban energy-balance models, and urban canopy parameterizations (Rotach et al, 2005); MUSE (Montreal Urban Snow Experiment) is aimed at the thermoradiative exchanges and the effect of snow cover in the urban atmospheric boundary layer (Lemonsu et al, 2008); and the CAPITOUL (Canopy and Aerosol Particles Interaction in TOulouse Urban Layer) project (Masson et al, 2008) is aimed at the role of aerosol particles in the urban layer.…”

Mesoscale nesting interface of the PALM model system 6.0

“…When anthropogenic heat or chemical compounds are emitted, unrealistic thermodynamic conditions or concentrations would reenter the model domain on the opposite boundary modifying the upstream conditions for the urban environment, which in turn may bias the distribution of heat and mass concentrations. Here, buffer zones help to move the affected flow region outwards (Letzel et al, 2012;Maronga and Raasch, 2013). Schalkwijk et al (2015) used a hybrid nesting approach to minimize scalar and mean flow feedbacks from re-entering wakes.…”