Large-eddy simulation analysis of turbulence characteristics of atmospheric boundary layers during a diurnal cycle

In the safety assessment for the construction of nuclear facilities in Japan, wind tunnel experiments or computational fluid dynamics (CFD) are required to estimate spatial distribution of air concentrations of a plume emitted from a stack (Nuclear Safety Commission of Japan, 1982). The experimental or CFD results are used to derive effective stack height, which is applied for long-term assessment using a Gaussian plume model. The effective stack height is often found to be lower than the actual height of the stack, considering terrain and building effects in a way that provides a conservative evaluation. Although reliable data on wind velocity and material concentrations are obtained, the effective stack height is estimated under the assumption of neutral stability.

In the atmosphere, heating and cooling within a boundary layer due to solar cycle during a day result in temperature differences, which introduce buoyancy forcing. Plume dispersion within the atmospheric boundary layer is also influenced by roughness elements, terrain, and thermal stability. In terms of thermal stability, atmospheric boundary layers are in general classified into three types; neutral boundary layer (NBL), stable boundary layer (SBL), and convective boundary layer (CBL). In an NBL, turbulence is generated and maintained by wind shear, while in an SBL turbulence is not only maintained by wind shear but also constrained by negative buoyancy. In a CBL, turbulence is mainly produced by shear and/or buoyancy. The most common stability classification scheme is the Pasquill-Gifford (P-G) (Turner, 1970), which defines six stability classes namely A (highly unstable), B (moderately unstable), C (slightly unstable), D (neutral), E (moderately stable), and F (extremely stable). The plume spreads over a flat ground surface in the typical meteorological conditions are determined by the P-G chart. Since atmospheric dispersion behaviors of a plume released from a tall stack are sensitively influenced by atmospheric stability, thermal effects should be incorporated into the effective stack height.

In this study, we perform LESs of a diurnal cycle of atmospheric boundary layer (ABL) flows based on the similar computational conditions to Kumar et al. (2006). As a first step, our objective is to investigate the turbulence characteristics of various thermal-stratified ABL flows and classify them based on the P-G chart.