@article {bnh-5477, title = {A preliminary report on simulation of flows through canopies with varying atmospheric stability}, number = {469}, year = {2019}, month = {03/2019}, institution = {Bushfire and Natural hazards CRC}, address = {Melbourne}, abstract = {

Large eddy simulation is performed for a flow through forest canopy applying various atmospheric stability conditions. The canopy is modelled as a horizontally homogenous region of aerodynamic drag with a leaf-area density (LAD) profile approximating the profile of a Scots pine tree. Varying atmospheric stability is incorporated into the simulation by applying varying heat flux in two different ways; (i) a surface heat flux prescription using Monin-Obukhov similarity functions and, (ii) a canopy heat flux model where heat from the canopy is modelled as distributed volume heat source. When the surface heat flux was prescribed, five stability classes: very unstable, unstable, neutral, stable and very stable are modelled while for canopy heat flux model three classes: stable, unstable and neutral are simulated for this study. We observe, realistically, that the stable and very stable velocity profiles are leaned towards right to neutral velocity profile indicating wind dominated flow. On the other hand, unstable and very unstable velocity profiles become more vertical indicating buoyancy dominated flow. In all velocity profiles, an inflection point is observed in the dense canopy region. Expected variations in the temperature profiles are also observed {\textendash} higher near-ground temperature for unstable and very unstable cases and converse is true for stable and very stable cases. Simulations involving exponential heat source variation vertically for forest canopy is ongoing for validation with experimental data and other similar studies. Once validation is obtained, parametric study with various atmospheric stability can be carried out in order to improve operational models. \ 

}, keywords = {atmospheric surface layer, forest canopy, thermal stratification, turbulence, turbulent kinetic energy}, author = {Nazmul Khan and Duncan Sutherland and Jimmy Philip and Andrew Ooi and Khalid Moinuddin} } @conference {bnh-6144, title = {Direct Numerical Simulation of Confined Wall Plumes}, booktitle = {21st Australasian Fluid Mechanics Conference}, year = {2018}, month = {12/2018}, address = { Adelaide, Australia}, abstract = {

We present results from the direct numerical simulation (DNS) of a wall attached thermal plume in a confined box. The plume originates from a local line heat source of length, L, placed at the bottom left corner of the box. The Reynolds number of the wall plume, based on box height and buoyant velocity scale, is ReH = 14530 and a parametric study is carried out for boxes of two different aspect ratios (ratio of box width to box height) for a particular value of L. In the simulation, the plume develops along the vertical side wall while remaining attached to it before spreading across the top wall to form a buoyant fluid layer and eventually moving downwards and filling the whole box. Further, the original filling box model of Baines and Turner is modified to incorporate the wall shear stress and the results from the DNS are compared against it. A reasonable agreement is observed for the volume and momentum fluxes in the quiescent uniform environment and also for the time-dependent buoyancy profile calculated far away from the plume.

}, keywords = {direct numerical simulation, Simulation modeling, thermal plume}, url = {https://people.eng.unimelb.edu.au/imarusic/proceedings/21/Contribution_790_final.pdf}, author = {Nitheesh George and Jimmy Philip and Andrew Ooi} } @article {bnh-5197, title = {Literature review: modelling and simulation of flow over tree canopies}, year = {2018}, month = {12/2018}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

We review recent literature on the topic of atmospheric boundary-layer flow over forest canopies. Included in this review are brief discussions of flow over rough surfaces and flow over urban canopies (collections of buildings). The purpose of this review is to inform fire behaviour analysts of progress in sub-canopy modelling, with an eye to developing simplified models for wind reduction factors. The wind reduction factor is a parameter that quantified the effect of canopy density on fire spread rate. Simulation of canopy flow is also reviewed and discussed. Simulations provide insight into the flow behavior that is otherwise difficult to obtain from field observations and experiments. The basic principles of Large Eddy Simulation and the validity of the simulation results are discussed. Finally some open problems are posed.

}, issn = {432}, author = {Duncan Sutherland and Jimmy Philip and Andrew Ooi and Khalid Moinuddin} } @conference {bnh-5023, title = {Simulations of the effect of canopy density profile on sub-canopy wind speed profiles}, booktitle = {AFAC18}, year = {2018}, month = {09/2018}, publisher = {Bushfire and Natural Hazards CRC}, organization = {Bushfire and Natural Hazards CRC}, address = {Perth}, author = {Duncan Sutherland and Rahul Wadhwani and Jimmy Philip and Andrew Ooi and Khalid Moinuddin} } @article {bnh-5212, title = {Simulations of the effect of canopy density profile on sub-canopy wind speed profiles}, year = {2018}, month = {12/2018}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

In computational simulations for weather prediction and fire simulation, forest canopies are often modelled as regions of aerodynamic drag. The magnitude of the drag term depends on the Leaf-Area Density (LAD) of the forest. For most forests LAD varies strongly with height; trees typically have more vegetation at the top of the canopy than the bottom. S. Dupont, and Y. Brunet [Influence of foliar density profile on canopy flow: a large-eddy simulation study. Agricultural and forest meteorology, 148(6), pp.976-990. 2008] simulated the flow through three very different profiles of LAD measured from three different Canadian forests.\  K. Moon, T.J. Duff, and K.G. Tolhurst [Sub-canopy forest winds: understanding wind profiles for fire behaviour simulation, Fire Safety Journal, 2016] recently measured the sub-canopy winds and LAD for seven different Australian forest types. Large-Eddy Simulations (LES) of flow through idealised forests are now computationally tractable and an extensive systematic study of different LAD profiles is possible. Here we assume that the LAD can be modelled by a Gaussian with two parameters representing the mean and variance of the distribution of LAD. The total vegetation density is forced to remain constant as the profile changes. We present preliminary simulation results showing how the mean and variance of LAD affects the sub-canopy wind velocity and we discuss a potential modelling approach for sub-canopy wind velocity.

}, issn = {442}, author = {Duncan Sutherland and Rahul Wadhwani and Jimmy Philip and Andrew Ooi and Khalid Moinuddin} } @conference {bnh-3882, title = {Large-eddy simulation of neutral atmospheric surface layer flow over heterogeneous tree canopies}, booktitle = {AFAC17}, year = {2017}, month = {09/2017}, publisher = {Bushfire and Natural Hazards CRC}, organization = {Bushfire and Natural Hazards CRC}, address = {Sydney}, abstract = {

Large-eddy simulation of a neutral atmospheric surface layer (ASL) flow is per- formed over a modelled tree canopy with heterogeneous leaf-area density. The canopy is arranged as a series of equally-sized stripes of different leaf-area density, emulating the study of Bou-Zeid et al. (E. Bou-Zeid, C. Meneveau, and M.B. Parlange. Large-eddy simulation of neutral atmospheric boundary layer flow over heterogeneous surfaces: Blending height and effective surface roughness. Water Resources Research, 40(2), 2004.) over heterogeneous rough surfaces. The simulation results are analysed to understand the qualitative similarities and differences between ASL flows over heterogeneous canopies and heterogeneous roughnesses. This will allow, in the future, the identification of the equivalent roughness length, displacement length, and blending height which parameterises the flow above the heterogeneous canopy. In the present work we restrict attention to the characterisation of the four canopy case and the blending height and β parameter, the ratio of shear stress to velocity at the canopy top. The general characteristics of the four-canopy case are representative of the other cases. Strong vertical velocities (ie up- and down-drafts) exist at the interface between the heterogeneous roughness stripes. However, for a canopy, vertical velocity couplets exist on the vertical interface between two canopies. This implies the presence of sub-canopy recirculation zones at canopy interfaces, which can be confirmed by visualisation of the fluid streamlines. Above the canopy internal boundary layers form over each canopy stripe and exhibit similar features to the characteristic upstream plumes of flow over a rough surface. The shear stress immediately above the canopy varies over the stripes but it varies more smoothly over a canopy than over a heterogeneous roughness. These simulations will allow the development of parameterisations for the near-surface layer and the sub-canopy winds. A better understanding of the effect of heterogeneous canopies on the sub-canopy winds will improve predictions of the wind reduction factor, and in turn, improve operational fire spread predictions.

}, author = {Duncan Sutherland and Khalid Moinuddin and Andrew Ooi} } @article {bnh-3917, title = {Verification of a Lagrangian particle model for short-range firebrand transport}, journal = {Fire Safety Journal}, volume = {91}, year = {2017}, month = {07/2017}, pages = {776-783}, chapter = {776}, abstract = {

Firebrands are a harbinger of damage to infrastructure; their effects cause a particularly important threat to people living within the wildland-urban-interface. Short-range firebrands travel with the wind with little or no lofting, and cause spotfires. In this work, the design of a novel firebrand generator prototype is discussed to achieve a uniform shower of firebrands. The transport of short-range firebrand is studied to verify the existing Lagrangian particle model of Fire Dynamics Simulator. Uniform, non-combusting cubiform and cylindrical firebrands are projected using the firebrand generator. The experimentally observed distribution of particles on the ground is compared with a simulated distribution using the fire dynamic simulator. The results show that the existing Lagrangian model gives a good agreement with the experimental data.

}, doi = {10.1016/j.firesaf.2017.03.019}, url = {http://www.sciencedirect.com/science/article/pii/S0379711217301121}, author = {Rahul Wadhwani and Duncan Sutherland and Andrew Ooi and Khalid Moinuddin and Graham Thorpe} } @article {bnh-3341, title = {Turbulent flow over transitionally rough surfaces with varying roughness densities}, journal = {Journal of Fluid Dynamics}, volume = {October 2016}, year = {2016}, month = {09/2016}, doi = {https://doi.org/10.1017/jfm.2016.459}, url = {https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/div-classtitleturbulent-flow-over-transitionally-rough-surfaces-with-varying-roughness-densitiesdiv/61E757B4C36F15D1541FED8F68220105}, author = {M MacDonald and L Chan and Daniel Chung and N Hutchins and Andrew Ooi} } @article {bnh-2393, title = {The spread of fires in landscapes}, year = {2015}, author = {Daniel Chung and Khalid Moinuddin and Andrew Ooi and Duncan Sutherland and Graham Thorpe and Rahul Wadhwani} }