@article {bnh-6830, title = {Simulation of flows through canopies with varying atmospheric stability}, number = {562}, year = {2020}, month = {04/2020}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

Large eddy simulation is performed of a flow through forest canopy over a range of atmospheric stabilities. A heat source is introduced at the top of the tree canopy to model the heating of canopy top by solar energy, Unlike our previous report [1] where an ideal Monin-Obukhov method was used for the surface heat flux variation, this study has introduced a varying volumetric heat flux in a sub-canopy region. The flow field develops naturally with the applied thermal stratification and pressure-driven flow. The forest canopy modelled using the leaf area density (LAD) of pine trees. The simulation is allowed for a sufficient time, of the order of 20000 s, to adjust with the applied heat flux in the domain. The simulation is attempted for two broad classes: stable and unstable situations with varying negative and positive fluxes, respectively. The simulation results are validated against the numerical study of Nebenfuhr et al. [2] and the field measurement taken at Ryningsnas, Sweden [3]; which shows a good agreement. The effect of canopy top heat flux on different atmospheric stabilities are studied in detail and we present mean velocity and Reynolds stresses. These results suggest that atmospheric stability may affect the rate of spread and pollution dispersion, especially in the case of unstable stratifications. There is a need to understand atmospheric stabilities for accurate analysis of wildland fire spread and fire intensity. Most importantly, flame characteristics must be carefully diagnosed with due account for different atmospheric conditions prevailing in real wildland fire for reducing property damage and loss of lives.

}, keywords = {atmospheric boundary layer, forest canopy, thermal stratification, turbulence, turbulent kinetic energy}, issn = {562}, author = {Nazmul Khan and Duncan Sutherland and Khalid Moinuddin} } @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} } @article {bnh-5771, title = {A Statistical Approach to Understanding Canopy Winds over Complex Terrain}, journal = {Environmental Modeling \& Assessment}, year = {2019}, month = {08/2019}, pages = {1-20}, abstract = {

Winds that flow within a forest canopy in complex terrain exhibit considerable variability that can have significant consequences for environmental processes such as the spread of bushfires, seed dispersal and transport of pollutants. This variability is still poorly understood, due in part to limited observations of canopy winds over complex terrain, and as such is often unaccounted for in deterministic modelling approaches. Probabilistic representation of wind fields can better characterise variability and build understanding of uncertainty in applications such as bushfire prediction. This study introduces two new, publicly available, datasets that support analyses of canopy winds over complex terrain using novel statistical approaches. Wind fields are characterised using bivariate distributions, representing the response of synoptic winds to changing landscapes, including post-bushfire forest regrowth and varying topography across uniform vegetation. For statistical comparison, non-parametric Kolmogorov-Smirnov style tests are considered. A new test is proposed for the comparison of bivariate circular distributions arising from analyses of wind direction. The study broadly reaffirms established theory while highlighting the variability of canopy winds over complex terrain and the importance of developing statistical techniques that result in a better quantitative understanding of these wind fields and the processes driving them.

}, keywords = {Circular statistics, Flow separation, forest canopy, uncertainty, Wind field observations, wind modelling}, doi = {https://doi.org/10.1007/s10666-019-09674-w}, url = {https://link.springer.com/article/10.1007/s10666-019-09674-w}, author = {Rachael Quill and Jason J. Sharples and Leesa Sidhu} }