Abstract
A double-distribution-function lattice Boltzmann model for large-eddy simulations of a passive scalar field in a neutrally stratified turbulent flow is described. In simulations of the scalar turbulence within and above a homogeneous plant canopy, the model’s performance is found to be comparable with that of a conventional large-eddy simulation model based on the Navier–Stokes equations and a scalar advection–diffusion equation in terms of the mean turbulence statistics, budgets of the second moments, power spectra, and spatial two-point correlation functions. For a top-down scalar, for which the plant canopy serves as a distributed sink, the variance and flux of the scalar near the canopy top are predominantly determined by sweep motions originating far above the canopy. These sweep motions, which have spatial scales much larger than the canopy height, penetrate deep inside the canopy and cause scalar sweep events near the canopy floor. By contrast, scalar ejection events near the canopy floor are induced by coherent eddies generated near the canopy top. The generation of such eddies is triggered by the downward approach of massive sweep motions to existing wide regions of weak ejective motions from inside to above the canopy. The non-local transport of scalars from above the canopy to the canopy floor, and vice versa, is driven by these eddies of different origins. Such non-local transport has significant implications for the scalar variance and flux budgets within and above the canopy, as well as the transport of scalars emitted from the underlying soils to the atmosphere.
Highlights
A plant canopy produces a peculiar internal environment through physical and biological processes
The counter-gradient fluxes are not simulated in the present settings. These features may not be typical for the scalar statistics of natural canopy turbulence, the figure indicates that the lattice Boltzmann method reproduces the profiles simulated by the Navier–Stokes model, at least in the lower half of the domain
We have described a double-distribution-function lattice Boltzmann model for simulating a passive scalar field in a neutrally stratified turbulent flow within and above a plant canopy using the large-eddy simulation (LES) approach
Summary
A plant canopy produces a peculiar internal environment through physical and biological processes. For the airflow within and above a plant canopy, Watanabe et al (2020) reported that a large-eddy simulation (LES) using a central-moment-based lattice Boltzmann method reproduces the turbulent velocity field as well as a conventional LES model based on the Navier–Stokes equations. This motivated us to extend the lattice Boltzmann method to the advection and diffusion processes of a passive scalar within and above the plant canopy. The lattice Boltzmann method uses a number of prognostic variables (i.e. 7–27 discretized distribution functions depending on the lattice system adopted) to predict a single scalar quantity In this respect, a conventional model based on the advection–diffusion equation for the macroscopic scalar density is more advantageous. The characteristics of the eddy motions that contribute to the scalar variance and fluxes near the canopy top (Sect. 3.2) and the spatio–temporal structure of coherent eddies that promote non-local scalar transport to/from deep inside the canopy from/to the atmosphere above are examined (Sect. 3.3)
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