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 inflowturbulence on the resulting turbulent flowfield in the urbanABL. 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 3m above ground level revealsminimal differences irrespective of the presence of incomingABLturbulence. 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 fromthe inflowboundaries, 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.
Bibliographical noteFunding Information:
Acknowledgments. This work was supported by the National Research Foundation of Korea (NRF) (Grant 2015R1A5A1009350) and the Korea Meteorological Administration Research and Development Program under Grant KMI (KMI2018-01410). We would like to acknowledge high-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX)provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation. Two-dimensional figures have been produced using the python libraries Matplotlib (Hunter 2007). Volume-rendered three-dimensional fields were generated using ParaView (Ayachit 2015). We thank S. Raasch, M. Sühring and T. Gronemeier for helpful discussions about PALM through a ticketing system (https:// palm.muk.uni-hannover.de/trac/wiki/tickets). And we also thank H. Yang for discussions about simulation setup and validation, and S. Weon for giving a technical tip for three-dimensional visualization. We appreciate three anonymous reviewers for providing valuable comments that helped improving the clarity of the manuscript.
© 2019 American Meteorological Society.
All Science Journal Classification (ASJC) codes
- Atmospheric Science