Wildfire Ember Transport Modeling
Image source: News Tribune; Firebrands from burning tree in 2021 Caldor Fire
Overview
This project is about the numerical modeling of firebrand showers in wildfire simulations.
Problem: Firebrands can spread wildfire through the ignition of spot fires, yet there is a gap in knowledge on where they will land due to turbulent wind
Solution: Model high-resolution turbulent boundary layers at various turbulence intensities, and release firebrands in those domains to study the effect of small-scale turbulence
Results: Novel implementation of firebrand transport model coupled with wildfire simulation WRF-SFIRE for comparison between large-scale and small-scale transport
Problem
Firebrand showers are the fastest and most complex form of wildfire spread, by generating spot fires in random locations.
There is a gap in knowledge on where firebrands land due to turbulent wind.
There is no existing coupled firebrand-wildfire simulation with complex firebrand shapes.
Firebrand shapes
Experimental data has shown that firebrands are made of 3 basic shapes: compact, plate, and rod. Studies have shown that particles of different shapes have different flight trajectories. And in the wildfire research community, these difference have not yet been fully explored.
Image source: NCAR; WRF-Fire: Wildland Fire Modeling
Small & Large-scale Turbulence
Modern wildfire simulations use large grid sizes in their computational meshes (around 250-300 m). This leads to only large-scale turbulence seen in the flow. Understanding the transport of plate and rod shapes in small-scale turbulence is crucial for understanding large-scale transport in wildfire simulations. With this knowledge, researchers will better understand how spot fires are generated.
Turbulent Boundary Layers
Set up
Parameters
U = 2.23 m/s
Re = 284,000
L = 2 m
𝓥 = 1.568E-5
Mesh Resolution
∆x = 0.025 m
∆y = 0.025 m
∆z = 0.025 m
Simulation Cases
4% turbulence intensity
7% turbulence intensity
Validation
Velocity profile validation of turbulent boundary layer simulations with experimental data from Tohidi 2016 and power law fit line.
4% turbulence intensity
7% turbulence intensity
Power spectral density of turbulent boundary layer simulations validated with Kolomorgov -5/3 Spectrum to show a fully developed flow.
Results
Q-Criterion
Iso-surfaces of the vortices in the 4% turbulence intensity simulation case
Firebrand Transport Simulations
Small-scale Transport
A series of 32 tests were conducted in the high-resolution turbulent boundary layer simulations in uniform and turbulent velocity fields. Plates and rods were released at 4 different heights
Impact and Purpose
Climate change is a major issue in our society today and wildfires are a growing problem across the world. Completing my project will help push our understanding of how wildfires spread, and help create better fire models. And this could help firefighters and emergency personnel stop wildfire spread and save lives.