Large eddy simulation of the neutral atmospheric boundary layer: performance evaluation of three inflow methods for terrains with different roughness

R Vasaturo,I Kalkman,B Blocken,P.J.V Van Wesemael

doi:10.1016/j.jweia.2017.11.025

Abstract

In large eddy simulations (LES) of the atmospheric boundary layer (ABL), the choice of inflow method and the specification of surface roughness are crucial for obtaining accurate results. The consistency between inflow conditions and wall boundary condition has been investigated in depth for the RANS approach, whereas it needs further analysis for LES. This paper investigates the combined effect of inflow method and aerodynamic roughness length for LES of the neutral ABL. Three basic inflow generators in combination with three terrain types are evaluated in terms of streamwise homogeneity of the vertical profiles of mean velocity and turbulence kinetic energy. The results show that the precursor method best preserves the homogeneity of the profiles for all roughness lengths considered, with mean absolute deviations up to 0.7% for mean velocity and 3.5% for turbulence kinetic energy at the outlet of the computational domain. Among the synthetic methods, the Vortex Method performs satisfactorily for mean velocity and, to a lesser extent, turbulence kinetic energy, whereas the random flow generation (RFG) method leads to the largest deviations from the target profiles. Finally, the value of the aerodynamic roughness length is found to only weakly influence the performance of the inflow methods considered.

Highlights

The results show that the precursor method best preserves the homogeneity of the profiles for all roughness lengths considered, with mean absolute deviations up to 0.7% for mean velocity and 3.5% for turbulence kinetic energy at the outlet of the computational domain
Since these incident profiles may have a considerable impact on the results of numerical simulations of flow around buildings or other obstacles, it is nowadays considered best practice to verify the homogeneity of the vertical profiles of the atmospheric boundary layer (ABL) in an empty computational domain (Blocken et al, 2007a, 2007b; Blocken, 2015; Franke et al, 2007) before running
Large eddy simulations of the neutral atmospheric boundary layer are performed at wind tunnel scale using three inflow generation methods: a precursor method and two basic synthetic methods, i.e. the Vortex Method and the Spectral Synthesizer

Summary

Introduction

Large eddy simulation (LES) is increasingly used for a wide range of topics in wind engineering, such as pollutant dispersion (Bazdidi-Tehrani et al, 2016; Gousseau et al, 2011; Moonen et al, 2013; Salim et al, 2011a, 2011b; Tomas et al, 2015; Tominaga and Stathopoulos, 2011; Xie and Castro, 2009), natural ventilation (Blocken, 2014; Jiang et al, 2003), vegetation effects (Kanda and Hino, 1994; Shaw and Schumann, 1992; Tamura et al, 2007; Watanabe, 2004), wind energy (Calaf et al, 2010; Porte-Agel et al, 2011) and wind effects on buildings (Aboshosha et al, 2015a; Dagnew and Bitsuamlak, 2014; Daniels et al, 2013; Huang et al, 2010; Li et al, 2015; Nozawa and Tamura, 2002; Tamura and Ono, 2003; Yan and Li, 2015).In the past much attention has been paid to the accurate CFD modeling of the atmospheric boundary layer (ABL), both using Reynoldsaveraged Navier-Stokes (RANS) and LES approaches, with particular regard to the correct implementation of boundary conditions. As a consequence the incident profiles, defined as the vertical profiles of mean velocity and turbulence quantities detected in an empty computational domain at the position where the obstacle(s) would be placed in a non-empty domain, might differ from those prescribed at the inlet boundary

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Conclusion