arXiv subjects

Kevin Speer

Publications and source records attributed to Kevin Speer.

7 recordsLinked to original sources

arXiv2026

Models of Wildland Fire and Ember Spread

In this Chapter we explore the nature and theory of ember transport using observations from controlled laboratory settings, prescribed fire, and wildland fire. Specific examples and statistics from fires are used to gain insight and motivate a hierarchy of modeling approaches from simple idealized models to full-physics atmospheric boundary layer models. The emphasis is on the fundamental processes involved in moving embers away from their sources in vegetation and structures on and near the ground or in the atmospheric boundary layer winds and turbulent flows. We describe the problem first in terms of basic theory about the rate of spread of wildland fire, examine the physical principles at work in ember transport for two principle modes of transport near the ground and in the atmosphere well above the surface, and connect these to statistical models for transport.

physics.ao-ph↗

arXiv2026

Statistical Models of Ember Wash and Their Impact on Wildfire Area Growth

Wildfire spread is strongly influenced by the transport and ignition of embers. While long-range spotting driven by plume lofting has received significant attention, embers transported near the surface by turbulent winds can also influence fire propagation. We develop a stochastic model for near-surface ember transport, referred to as ember wash. The model represents ember motion as a sequence of short displacements analogous to saltation-like transport and incorporates a probabilistic ignition process that depends on ember survival during transport. This formulation leads to an exponential distribution of ember flight times. The model is implemented within a simplified fire spread model to examine burn patterns and growth dynamics. Simulations demonstrate that ember wash produces spread behavior that differs fundamentally from classical plume-driven spotting. These results suggest that ember wash provides a plausible mechanism for wildfire spread regimes that differ from those predicted by geometric or plume-driven spread models.

physics.ao-ph↗

arXiv2025

The Ceiling Height of Wildland Fire Plumes in Sheared Boundary Layer Flow

Radar observations from a prescribed fire experiment reveal a large-scale, billow-like vorticity pair associated with the plume head at the onset of plume bending. The bending confines the ceiling height of the plume, delaying its smoke dispersion and increasing fire spotting risks. This study aims to investigate the onset of plume bending in a sheared crossflow and stratified atmospheric conditions, providing insights into smoke dispersion and fire behavior. Large Eddy Simulations (LES) using the Cloud Model 1 (CM1) are conducted to simulate the observed development of plume structure and its associated dynamical fields, with particular focus on the plume head and its evolution from initial plume development under different fire intensities and atmospheric boundary layer (ABL) conditions. A scaling analysis of plume ceiling height is proposed based on a modified Byram's convective number that accounts for sheared crossflow. The proposed scaling agrees well with the LES results, highlighting the critical roles of shear and stratification in controlling plume dynamics.

physics.flu-dyn↗

arXiv2024

Characterizing Turbulence at a Forest Edge: A Vorticity Budget Analysis around a Canopy

Vorticity is a key characteristic of flow patterns that determine wildland fire behavior, frontal evolution, and wind-canopy interaction. Investigating the role of vorticity in the flow fields around vegetation can help us better understand fire-atmosphere feedback and the influences of vegetation on this feedback. In modeling vorticity, ``perhaps the greatest knowledge gap exists in understanding which terms in the vorticity equation dominate [...] (and) when one or the other might dominate" (Potter, 2012). In this study, we investigate the role of vorticity in boundary layer dynamics and canopy/forest edge effects using HIGRAD/FIRETEC, a three-dimensional, two-phase transport model that conserves mass, momentum, energy, and chemical species. A vorticity transport equation was derived and discretized. Simulations were performed over a cuboidal homogeneous canopy surrounded by surface vegetation. This derivation led to the discovery of a drag tilting and stretching term, which shows that gradients in the aerodynamic drag of the vegetation, tied to heterogeneities in surface area-to-volume ratio, play an important role in the generation of vorticity. Results from the vorticity budget analysis show that this term contributes significantly in the areas where these gradients are present, namely the edges of the canopy.

physics.flu-dyn↗

arXiv2024

Characterizing Turbulence at a Forest Edge: Comparing Sub-filter Scale Turbulence Models in Simulations of Flow over a Canopy

In wildfires, atmospheric turbulence plays a major role in the transfer of turbulent kinetic energy. Understanding how turbulence feeds back into a dynamical system is important, down to the varying small scales of fuel structures (i.e. pine needles, grass). Large eddy simulations (LES) are a common way of numerically representing turbulence. The Smagorinsky model (1963) serves as one of the most studied sub-grid scale representations in LES. In this investigation, the Smagorinsky model was implemented in HIGRAD/FIRETEC, LANL's coupled fire-atmosphere model. The Smagorinsky turbulent kinetic energy (TKE) was compared to FIRETEC's 1.5-order TKE eddy-viscosity subgrid-scale model, known as the Linn turbulence model. This was done in simulations of flow over flat terrain with a homogeneous, cuboidal canopy in the center of the domain. Examinations of the modeled vertical TKE profile and turbulent statistics at the leading edge, and throughout the canopy, show that the Smagorinsky model provides comparable results to that of the original closure model posed in FIRETEC.

physics.flu-dyn↗

arXiv2018

Anomalous wave statistics induced by abrupt depth change

Laboratory experiments reveal that variations in bottom topography can qualitatively alter the distribution of randomized surface waves. A normally-distributed, unidirectional wave field becomes highly skewed and non-Gaussian upon encountering an abrupt depth transition. A short distance downstream of the transition, wave statistics conform closely to a gamma distribution, affording simple estimates for skewness, kurtosis, and other statistical properties. Importantly, the exponential decay of the gamma distribution is much slower than Gaussian, signifying that extreme events occur more frequently. Under the conditions considered here, the probability of a rogue wave can increase by a factor of 50 or more. We also report on the surface-slope statistics and the spectral content of the waves produced in the experiments.

physics.flu-dyn↗

arXiv2017

Pixel-Level Statistical Analyses of Prescribed Fire Spread

Wildland fire dynamics is a complex turbulent dimensional process. Cellular automata (CA) is an efficient tool to predict fire dynamics, but the main parameters of the method are challenging to estimate. To overcome this challenge, we compute statistical distributions of the key parameters of a CA model using infrared images from controlled burns. Moreover, we apply this analysis to different spatial scales and compare the experimental results to a simple statistical model. By performing this analysis and making this comparison, several capabilities and limitations of CA are revealed.

nlin.CG↗