Dual Navier-Stokes / Lattice-Boltzmann Method for Urban Wind Flow

Scale-resolving turbulent flow simulation for urban environments is computationally expensive and not widely used in industrial practice, despite inherent complexities from geometry and high Reynolds number effects. One way to reduce the computational demand of the simulation is to focus a turbulence-resolving solver on the region of interest, and employ a cheaper less accurate solver elsewhere. We present developments towards a dual Navier-Stokes / lattice-Boltzmann (NS/LB) solver for three-dimensional unsteady urban wind flow. The simulation domain is divided into an NS sub-domain and an LB sub-domain with calculations performed on CPU and GPU respectively. Turbulence is modelled via a large eddy simulation (LES) sub-grid model in the LB sub-domain and an unsteady Reynolds averaged Navier-Stokes model (URANS) in the NS sub-domain. Both sub-domains are bi-directionally coupled on their overlapping boundaries; a synthetic eddy method (SEM) is used to generate instantaneous flow data for the LB sub-domain from the URANS mean flow. Preliminary results of the dual NS/LB model on laminar flow show that the two solvers are able to communicate and simulate continuous flow across the interface. The implementation of the SEM inlet boundary condition successfully demonstrates the convection of turbulence throughout the LB sub-domain. Further work to fully embed the LES region within the URANS domain is ongoing. While motivated by a focus on urban wind flow simulations, this work is expected to have broader relevance where computational domains include flow around a range of geometric scales.