Abstract
A central-moments-based lattice Boltzmann model for large-eddy simulation of neutrally-stratified turbulent flows is described. Through comparative simulations of the airflow within and above a homogeneous plant canopy, the performance of the model is evaluated with respect to a conventional large-eddy-simulation model based on the incompressible Navier–Stokes equations. Simulated turbulence statistics, such as the mean velocity, velocity variances, velocity skewness, and power spectra, are shown to be almost identical between the two models. The spatial structure of coherent eddies and their maintenance processes are also confirmed to be properly represented by the lattice Boltzmann method through analysis of the turbulence kinetic energy budget and spatial two-point correlation functions. Using the simulated results, the energetics of the streamwise-elongated streaky structures commonly observed over vegetation and urban canopies are examined. While the short-wavelength components of the shear-generated streamwise kinetic energy are redirected rapidly by pressure to the lateral and vertical velocity components, long-wavelength energy tends to remain in the streamwise velocity component, which is dissipated in relatively slower processes. Consequently, the streaky structures persist in the streamwise velocity component.
Highlights
The exchange of momentum, heat, water vapour, carbon dioxide and other gasses, and particles between vegetation and the atmosphere, is one of the main processes in land–atmosphere interactions
Velocity skewness in the canopy layer is positive for the streamwise velocity component and negative for the vertical velocity component, and the signs become reversed at a distance well above the canopy
Streaming of the distribution functions in the lattice Boltzmann method is always performed by an exact upstream scheme with a Courant number of unity (Eq 4), which enables the model to simulate three-dimensional advection with almost no numerical error
Summary
The exchange of momentum, heat, water vapour, carbon dioxide and other gasses, and particles between vegetation and the atmosphere, is one of the main processes in land–atmosphere interactions. Collision and forcing processes are expressed in a local manner, in which reference is only to a set of variables defined at a single grid point With these features, together with its simple algorithm and the ease of implementing boundary conditions, the lattice Boltzmann method is feasible for application to high-resolution simulations of atmospheric-boundary-layer flows using advanced parallelprocessing machines. Ahmad et al (2017) and Inagaki et al (2017) used the same code to perform simulations of the spatial development of a neutral boundary layer over an actual urban geometry near Tokyo with 2-m resolution in a domain of 19.2 × 4.8 × 1.0 km3 The latter simulation allowed the authors to conduct simultaneous analysis of the aerodynamic properties both of boundary-layer-scale eddies and of gusty motions in street canyons. This paper describes the development of a new LES code based on the lattice Boltzmann method for the simulation of canopy turbulence in neutrally-stratified conditions, presents the validation of the code with respect to a conventional LES model based on the Navier–Stokes equations, and briefly discusses the maintenance process of the large-scale eddies that dominate canopy turbulence
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