Abstract: uDALES is an open-source large-eddy simulation framework designed for outdoor flows in the built environment. It is capable of simulating airflow, sensible and latent heat transfer, and pollutant dispersion within the urban atmospheric boundary layer at microscale resolution. Buildings are resolved using the immersed boundary method (IBM), with wall functions for surface shear stresses and heat fluxes. The latter are two-way coupled with a three-dimensional surface energy balance model that is able to capture both man-made and vegetative materials.

Recently, the framework has been improved in two substantial ways that together constitute a new version: uDALES v2.0. Firstly, by using the library 2DECOMP&FFT it is now possible to decompose the domain in two dimensions for the purpose of parallelisation. This allows the user to fully utilise the resources available on supercomputers such as ARCHER2 and prepares the codebase for exascale computing. Secondly, the IBM has been extended so that the geometry can be specified independently of the Cartesian computational grid, meaning the immersed boundary need not sit on cell edges. This enables greater flexibility and fidelity in object representation, thus increasing the applicability to realistic cases.

This presentation outlines the physical and numerical considerations made in the model development and validates against several idealised studies. It also demonstrates the advantages offered by the new version by comparing the performance and scaling with the previous version, and by varying the numerical (but not physical) set-up of the validation cases such that the immersed boundary does not align with the computational grid.