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  4. An Intercomparison of LES Subgrid-Scale Models for Simulations of Slope Flows
 
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An Intercomparison of LES Subgrid-Scale Models for Simulations of Slope Flows

Giometto, Marco Giovanni  
•
Parlange, Marc  
•
Fang, Jiannong  
2014
21st Symposium on Boundary Layers and Turbulence

We propose an inter-comparison of Smagorinsky type subgrid-scale models for the simulation of high Reynolds number katabatic flows, with the purpose of testing the validity of some of the model's assumptions for the considered application. Katabatic flows arise along sloping surfaces under stably stratified conditions and are a direct consequence of surface cooling. Understanding their structure is of great interest in meteorology because of the broad band of areas and scales that they cover, influencing from local valleys microclimate (e.g. over Salt Lake and Phoenix valleys) to synoptic scale motions (e.g. over Antarctica). Large-eddy simulation (LES), on the other hand, represents an important tool to study high-Reynolds number flows and has undergone significant developments and validations in the past years, confirming its reliability when adopted to simulate a variety of atmospheric (and industrial) flows. The stable stratification of the environment and the complex dynamics that arise close to the surface in katabatic winds – leading eventually to the formation of the so-called low level jet – represent a tough test case for closure models, questioning their applicability over a broad range of filter (grid) scales and motivating the proposed work. The study is performed in the idealized framework of the Prandtl model, the set of normalized, filtered Boussinesq equations is solved on a regular domain, adopting an in-house mixed pseudo-spectral / finite difference code. The original static Smagorinsky model (SMAG), the standard dynamic model with planar averaging of the coefficient (PASI) and the more recent scale-dependent Smagorinsky model with Lagrangian averaging of the coefficients (LASD) are adopted to independently parametrize the subgrid-scale terms for kinematic momentum and energy, which arise from the filtering operation. Results focus on the statistically steady state solution of the problem and show how mean profiles are consistent across subgrid-scale models and most of the discretizations, whereas second order statistics show strong dependence on the spatial resolution, which is partially alleviated once the scale invariance assumption of the model coefficient is relaxed. For the considered resolutions we show that scale-invariance of the model coefficient does in fact not hold – on average – in proximity of the low level jet and close to the wall. At these location of strong shear production the support of the LES filter is approaching the integral scale and the LASD model is therefore expected to provide a more accurate estimation of the Smagorinsky coefficient. Considerations regarding the dissipation rates of the different subgrid-scale models and on mixing efficiency of resolved and modeled turbulence will be discussed.

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Type
conference presentation
Author(s)
Giometto, Marco Giovanni  
•
Parlange, Marc  
•
Fang, Jiannong  
Date Issued

2014

Subjects

LES

•

slope flows

Written at

OTHER

EPFL units
EFLUM  
WIRE  
Event nameEvent place
21st Symposium on Boundary Layers and Turbulence

Leeds, UK

Available on Infoscience
March 16, 2015
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/112500
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