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A new stochastic backscatter model for large-eddy simulation of neutral atmospheric flows

O’Neill, James Joseph (2016)
Ph.D. thesis, University of Birmingham.

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Abstract

A stochastic backscatter (SB) approach to subgrid-scale (SGS) modelling for large-eddy simulation (LES) of the neutral atmospheric boundary layer (ABL) has previously been shown to reduce excessive velocity shear, as seen with the popular Smagorinsky SGS model, in the under-resolved surface layer. However, previous SB models exhibit unwanted grid-dependency issues, and the range of atmospheric flows tested remains limited. Here, a new SB model is proposed that uses a grid-adaptive filter to control the length-scale, anisotropy and momentum flux of the backscatter fluctuations, independently of the model grid. Model performance is confirmed to be grid-independent in simulations of the neutral ABL, in which an 80% reduction in excessive near-surface velocity shear is achieved.
The model is also applied to street canyon flow, where the shear layer that separates the recirculating vortex within the canyon from the external flow is again typically under-resolved in most LES set-ups. The backscatter acts to increase momentum transfer across the shear layer, bringing the simulated vortex intensity significantly closer towards wind-tunnel observations. A passive tracer is also released to model traffic emissions, and the pollutant exchange velocity between the canyon and the external flow is again found in better agreement with wind-tunnel data. This information can be used to improve operational urban dispersion models.

Type of Work:Ph.D. thesis.
Supervisor(s):Cai, Xiaoming and Kinnersley, Rob
School/Faculty:Colleges (2008 onwards) > College of Life & Environmental Sciences
Department:School of Geography, Earth and Environmental Sciences (GEES)
Additional Information:

Publications arising from this thesis:

O'Neill, J.J., Cai, X.-M., Kinnersley, R. 2015. A generalised stochastic backscatter model: large-eddy simulation of the neutral surface layer. Quarterly Journal of the Royal Meteorological Society. 141: 2617-2629.
http://dx.doi.org/10.1002/qj.2548

O'Neill, J.J., Cai, X.-M., Kinnersley, R. 2016. Improvement of a stochastic backscatter model and application to large-eddy simulation of street canyon flow. Quarterly Journal of the Royal Meteorological Society. 142: 1121-1132.
http://dx.doi.org/10.1002/qj.2715

Subjects:GE Environmental Sciences
Institution:University of Birmingham
ID Code:6849
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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