Findings about the fluid dynamics of wildfires

That ambient winds influence fire behavior is well known. Less understood is how fire influences the winds and how the feedback affects the fire’s evolution.

wildfire dynamics wind fields
One freeze-frame moment in a simulation illustrating the dynamics of wind fields in a vertical plane as a wildfire approaches — towers and troughs. From the video.
wildfire dynamics wind fields
Towers and troughs, in reality. In this experimental grass fire, the few visible peaks are separated by gaps in which the wind currents sweep downward between the flames and feed the peaks on opposite sides. (Courtesy of Mark Finney, US Forest Service, Missoula Fire Sciences Laboratory.)

The more knowledge firefighters have about the fluid dynamics of wildfires the better equipped they will be to take on the tasks of igniting prescribed fires and suppressing wildfires.

Below is an article written by Rod Linn, who leads development, implementation, testing, and application of computational models of wildfire behavior in the Earth and environmental sciences division at Los Alamos National Laboratory in New Mexico. From Physics Today 72, 11, 70 (2019).  https://doi.org/10.1063/PT.3.4350


Fluid dynamics of wildfires

Wildland fires are an unavoidable and essential feature of the natural environment. They’re also increasingly dangerous as communities continue to spread away from urban areas. Unfortunately, a century of wildfire exclusion—the strategy of putting out fires as fast as they start—has led to a significant buildup of fuel in the form of overgrown forests. Continuing to keep wildfires at bay is simply not sustainable. In 2018, nearly 60,000 fires scorched parts of the continental US. California wildfires exemplify what can happen when they burn through communities: In November alone that year fires killed more than 90 people and destroyed some 14,000 homes and businesses.

Decision makers are striving to find ways to manage the consequences of those fires and yet still allow them to thin out dense, fuel-heavy forests and reset ecosystems. Among other things, the goal requires that land managers be able to predict the behavior of wildland fires and their sensitivity to ever-changing conditions. Many factors, including the interactions between fire, surrounding winds, vegetation, and terrain, complicate those predictions.

That ambient winds influence fire behavior is well known. Less understood is how fire influences the winds and how the feedback affects the fire’s evolution. As the fire rages, it releases energy and heats the air. The rising air draws in air below it to fill the gap in much the same way as air is drawn into a fireplace and rises up a chimney. The interaction between rising air and ambient winds controls the rate at which surrounding vegetation heats up and whether it ignites. The interaction thus determines how quickly a fire spreads.

FUEL MATTERS
The influence of the fire–atmosphere coupling is much greater in wildland fires than in building fires. Wildland fires are fed by fine fuels—typically grasses, needles, leaves, and twigs; often, tree trunks and large branches do not even burn. Buildings burn thicker fuels, such as boards, furniture, and stacks of books. The difference matters because fine fuels exchange energy more efficiently with surrounding hot air and gases. In those hot, fast-moving gases, the fuels’ temperature rises quickly to the point where they ignite.

But the converse is also true. Because wildland fuels are primarily fine, they are also efficiently cooled when the surrounding ambient air is cooler than they are. That means that the indraft of air caused by a fire may actually impede its spread. A rising plume can draw cool air over foliage and litter near a fire line and prevent those fine fuels from heating. The grasses just outside a campfire ring are a case in point: They are continuously exposed to the fire’s radiant heat, but the cool indraft effectively prevents them from reaching the point of ignition.

The spread of a wildfire is sometimes conceptualized as an advancing wall of flame that the wind forces to lean toward unburned fuels that then ignite in front of the fire. Although that wall-of-flame paradigm simplifies models of fire behavior, it is not correct. Convective cooling would prevent the wall of flame from spreading by radiation alone, and for convective heating to spread the fire, the wind would have to be strong enough to lean the flame to the point where it touches the unburned fuel. Were that true, the fires would be unable to spread in low-wind conditions because the buoyancy-driven updrafts would keep the flames too upright.

If you were to look upon an advancing wildfire from the front, you would actually see a series of strong updrafts, visible as towers of flame that are separated by gaps, as shown in figures 1 and 2. The towers are regions where the buoyancy-driven updrafts carry heat upward. They are fed by ambient wind drawn into the gaps between them, as described earlier. When the ambient wind is strong enough, it pushes air through the gaps between the towers, but that air is heated as it blows over burning vegetation. The motion of hot gases through the fire line disrupts the indraft of cool ambient air and ignites grasses and foliage in front of the fire. That’s the primary way a wildfire spreads.

A second factor that influences the spread is the shape of the fire line, because different parts of the blaze compete for wind. The headfire, the portion moving the fastest, often has trailing flanking fires that form a horseshoe shape and open up to the ambient wind. Part of that wind gets redirected toward the flanks of the horseshoe. The strength, length, and proximity of the flanking fires to each other thus help determine how much wind reaches the headfire. The narrower the horseshoe is, the larger the fraction of wind diverted to the flanks, the lower the wind speed reaching the headfire, and the slower it spreads.

Another factor to be considered is the spatial arrangements of fuels. The potential for wildfires spreading from the crown of one tree to another is reduced when the spacing between trees increases. In that case more horizontal wind is required for flames to jump between trees. Indeed, removing trees is a common fire-risk-management practice. But the strategy behind it is more complex than just removing fuel. Gaps in a forest canopy also make it easier for high-speed winds above the canopy to reach fires on the ground. So although reducing the number of trees might reduce the crown-to-crown fire activity, it might increase the spread rate of a surface fire.

PRESCRIBED FIRE
In some regions of the US, land managers counter the threat of wildfires and promote ecosystem sustainability by purposefully lighting fires. Carefully controlled, prescribed burns, which clear duff and deadwood on the forest floor, are often lit at multiple locations; fire-induced indrafts at one location influence fires at other locations. For example, a single line of fire under moderate winds might reach spread rates and intensities that are undesirable or uncontrollable, but the addition of another line of fire upwind can influence how much ambient wind reaches the original fire and thus reduces its intensity.

The spread of the upstream fire line, ignited second, is purposefully limited, as it converges on the area downwind where the first fire has burned off fuel. Practitioners can manipulate the flow of wind between fire lines by adjusting the spacing between ignitions. Fire managers might tie the various ignition lines together—reducing the fresh-air ventilation, increasing the interaction between the lines, and causing fire lines to rapidly pull together—to give themselves more control over the spread.
The interaction between multiple fire lines can even stop a wildfire in its tracks. When firefighters place a new fire line downwind of a fire, they often hope that the indrafts will pull the so-called “counter fire” toward the wildfire and remove fuel in front of it. Unfortunately, the maneuver requires a good understanding of the wildfire’s indraft strength. Too weak an indraft could turn the counter fire into a second wildfire.

After realizing the huge significance of the wind interactions in wildfires over the past two decades, the science community is striving to better account for them. Those efforts should improve predictions of how a wildfire will behave in various conditions. To that end, some researchers, including me, use computer models to explicitly account for the motion of the atmosphere, wildfire processes, and the two-way feedbacks between them. Others perform experiments at scales ranging from meters (such as in wind tunnels) to kilometers (such as in high-intensity fires on rugged topography) for new insight on the nature of those fire–atmosphere interactions or to confirm existing models.

SIMULATION VIDEO

(If you’re having trouble playing the video, you can see it on YouTube)

The [above] simulation illustrates the dynamics of wind fields in a vertical plane, located at the white horizontal line, as a wildfire approaches it. The colors mark the speed u of the wind perpendicular to the plane, with red indicating motion toward the viewer (out of the screen), and blue indicating motion away from the viewer. As the clip shows, the fire starts to influence the winds long before it reaches the plane, and the wind patterns change in scale and character as the fire approaches. As the fire crosses the plane, the towers and trough flow patterns become apparent. Some locations show strong upward motion, whereas others have strong horizontal or even slightly downward motion. The colors on the ground surface illustrate the convective cooling (blue) that occurs as a result of the movement of cool air over the fuel— grasses in this simulation—and locations in front of the fire where the fuels are being convectively heated (red).

DHS studies emerging technology for wildfire response

The project team evaluated over 60 systems

DHS study wildfire technologyIn December of 2017, the Federal Emergency Management Agency Administrator requested the Department of Homeland Security Science and Technology research new and emerging technology that could be applied to wildland fire incident response, given the loss of life that occurred in California during the fall of 2017 in Santa Rosa and Ventura.

The project team identified three overarching conclusions that represent consistent themes captured throughout the course of the table top exercises and expert engagements.

  1. Time Criticality of WUI Fire Incidents: WUI fire incidents require immediate protective and response actions to save lives. The conflagration created when a wildland fire enters populated areas is unpredictable and can rapidly devastate these areas, threatening lives. Interventions and solutions that improve decision making and response in the initial minutes of a WUI fire are vital.
  2. Available Technology Solutions Exist: There exist available technologies (both government and commercial), which—if implemented—could immediately help emergency responders reduce the number of lives lost during WUI fire incidents. In particular, these technologies could immediately support ignition detection, fire tracking, public information and warning, evacuation, and responder safety. Improving capabilities in other elements of the WUI response (i.e. preparedness and critical infrastructure) may require investing in adaptable or developable solutions that are not immediately available.
  3. Public Education and Preparedness Measures are Vital: Public education and preparedness are essential to reducing the number of lives lost to WUI fire incidents. There is no solution more effective than preventing an ignition in the first place and ensuring the at-risk communities are prepared at the grassroots level to face wildland fire dangers.

The principal conclusions of this project are distilled into a set of seven key findings. They describe lines of effort addressing priority capability gaps that, if implemented, could substantially improve immediate life-saving efforts during WUI fire incidents. The key findings listed below are considered equally important to this objective and are not listed in any priority order.

  1. Implement and scale the use of state-of-the-art remote sensing assets to provide state and local stakeholders real-time, accurate, low-cost ignition detection and tracking information— especially fire perimeter using a mix of in situ, aerial, and space-based systems.
  2. Improve the ability of available and adaptable public alert and warning technologies to deliver more targeted and effective message across the whole community, particularly to individuals with disabilities and others with Access and Functional Needs (AFN).
  3. Improve use of key public and private social media and internet resources and capabilities to appropriately share data and adapt existing applications to enable more efficient and effective evacuation—e.g., expand and accelerate public-private partnerships through Integrated Public Alert and Warnings System (IPAWS) to include WUI incident-related evacuations, warning, and alerting.
  4. Support broader use of existing fire modeling and forecasting tools for pre-incident planning; while also advancing efforts to create high-confidence, timely WUI fire-specific models that can be used to inform response tactics during extreme conditions.
  5. Increase infrastructure resilience, especially critical infrastructure lifelines and support functions for wildland fire response—e.g., improve the resilience, interoperability, and reliability of communications, power utilities, digital links, and data center infrastructure.
  6. Integrate private, open, and crowdsourced data, resources, and capabilities to improve public safety situational awareness of WUI fire ignition detection and tracking.
  7. Support wide-scale adoption of interoperable, low-cost blue-force tracking technologies that feed near real-time situational awareness across key stakeholders, missions, and operations.

The project team evaluated over 60 existing systems, products, or solutions. Here is an example of how 10 were ranked for how well they addressed requirements.

technology address wildfire management safety

technology address wildfire management safety
Top ten solutions based on how many requirements that solution addresses.

In addition, the team evaluated the solutions for feasibility, affordability, usability, impact, and technology alignment.

The entire 131-page report can be downloaded. 2.8 MB

Thanks and a tip of the hat go out to LM. Typos or errors, report them HERE.

Research indicates that wildfire smoke may distribute microbial life

That may be what causes allergic reactions in some people who are sensitive to smoke

smoke wildfires Utah and Colorado
Satellite photo at 5:37 p.m. MDT Sept. 13, 2018 showing smoke from wildfires in Utah and Colorado.

We have known for a long time that smoke from wildfires can be harmful to humans, but in recent years that knowledge base has increased significantly. And it may have reached a new level with research conducted by fire ecologist Leda Kobziar. After learning that some snow machines use bacteria as condensation nuclei, she started to wonder if bacteria was a component of smoke. Using petri dishes and drones she collected air and smoke samples at a prescribed fire.

Below is an excerpt from an article at KQED.org:

…Then they compared what was collected to the contents of ambient (non-smoky) air. They sampled for abundance and diversity by culturing colonies and analyzing DNA.

Turns out a surprising amount and diversity of bacterial cells and fungal spores gets lofted into wildfire smoke during a fire. The more severe the burn, the more cells it transports. This is a newly emerging area of research, but Kobziar thinks these microbes have the potential to affect human health.

“There are numerous allergens that we’ve found in the smoke. And so it may be that some people who are sensitive to smoke have that sensitivity, not only because of the particulate matter and the smoke, but also because there are some biological organisms in it.” … Possibly, she says, wildfire smoke has been a driving factor in the global distribution of microbial life.

“We think that the role that wildland fire is playing in transporting organisms through smoke has probably had some influence on the evolution of species as well and development of communities,” Kobziar said.

Let’s be careful out there.

European organization recruiting 15 fire-related PhD candidates

PyroLife will train a new generation of experts in integrated wildfire management

PhD candidates in Europe
Two of the 15 positions available for fire-related PhD candidates in Europe. Click to enlarge.

An organization in Europe is recruiting 15 PhD candidates who have wildfire-related  masters degrees. They will be part of the PyroLife Innovative Training Network (Marie Skłodowska-Curie) involved in integrated fire management.

Ten leading institutions will host and monitor the research done by the 15 individuals who are early-stage researchers. The interdisciplinary and intersectoral consortium spans across Northwest and Southern Europe and beyond, encompassing the key disciplines and actors in fire; from academia and research institutes to small and large businesses, advocacy, governance, and emergency management.

One of the announcements for the 15 positions has already closed, and the others will very soon. Here is a link to the individual announcements.

The project is funded by the European Union’s Horizon 2020 research and innovation programme, Innovative Training Networks.

The applicants will be based in various locations in Europe. Some of them will at times be in one or more of the following countries: Spain, Canada, France, Netherlands, Greece, United States, Poland, UK, Denmark, New Zealand, or Germany.

The positions may have unusual requirements concerning the location of the applicant. Here is an example:

PyroLife as a Marie Curie Action is a researcher mobility programme. You are therefore required to undertake transnational mobility in order to be eligible for recruitment. As such, you must not have resided or carried out your main activity (e.g. work, studies) in the country where you have been recruited for more than 12 months in the 3 years immediately before the recruitment date.

Below is more information from the organization’s website:


Do you have a genuine interest in landscape fires and resilience? Are you up for an interdisciplinary challenge, looking and learning beyond your own field and assumptions? With an international team that is inclusive, collaborative, creative and open minded? Then we are looking for you!

The 2018 wildfire season was a glimpse of what to expect in the future: deadly mega-fires in Mediterranean regions and high fire activity in temperate and boreal areas outside the typical Spring fire season. We cannot solve this challenge with the old mono-disciplinary approach of fire suppression: there is a critical need to change fire management from fire resistance to landscape resilience: Living with Fire. This requires a new type of diverse experts, who not only understand fire, but who are also able to communicate risks, deal with uncertainty, and link scientific disciplines as well as science and practice.

The new Innovative Training Network PyroLife will train the new generation of interdisciplinary experts in integrated fire management, acknowledging that 1) knowledge transfer from southern Europe (and worldwide) to temperate Europe can support the new generation of experts; and 2) fire risk planning, communication and management can learn from cross-risk lessons including temperate European expertise in water management. In doing so, this project combines how the North solves community problems with the fire knowledge of the European South.

We are hiring 15 PhD candidates across Southern and Northwest Europe and across a range of scientific disciplines, from social sciences and policy to environmental sciences and engineering. We are looking for a diverse group of creative and open minded Early Stage Researchers who are able to link innovative science to society, and communicate with media, stakeholders, and policy makers.

These 15 positions are open at 6 universities, 2 research institutes, a foundation and a company across Southern and Northwest Europe. For an overview of all positions, please visit https://pyrolife.lessonsonfire.eu/

This PhD project will help formulate an effective temperate European Fire Danger rating system that is urgently needed to support the management of increased wildfire occurrence expected under changing climatic conditions.  The project will take a hydrological approach, predicting the moisture content of fuels (living and dead vegetation) at a range of spatial scales; from targeted high risk localised plots to temperate European regions. Fuel moisture predictions will be devised from the development of a low-cost wireless fuel moisture sensor network combined with remotely sensed water and vegetation data. Working with secondment partners, the impact of the refined temperate fuel moisture contents on fire behaviour and fire danger will be assessed at exemplar sites. The PhD project will be based at the University of Birmingham, UK, with secondments to both the University of Alberta, Canada, and to industry partners Tecnosylva, Spain.

British Columbia initiates wildland firefighter health research

Above: Firefighters in a smoky environment on the White Tail Fire, March 8, 2019, Black Hills National Forest.

Information from the British Columbia Wildfire Service:

VICTORIA – The BC Wildfire Service has provided $305,000 to help fund two research projects looking into the health and wellness of firefighters and associated personnel. The University of Northern British Columbia and the University of Alberta are conducting these studies to learn more about how firefighting activities affect the health of fire crews.

“Our firefighters have worked hard on the front lines to keep British Columbians safe during difficult and record-setting wildfire seasons,” said Doug Donaldson, Minister of Forests, Lands, Natural Resource Operations and Rural Development. “These studies will help us support their long-term health and well-being.”

Research by the University of Northern British Columbia is led by Chelsea Pelletier, PhD, who is an assistant professor with the School of Health Sciences at the University of Northern British Columbia.

A scoping review will:

  • Look holistically at the existing body of research and knowledge about wildland firefighter health and wellness (including its physical, mental and emotional dimensions) by conducting a global scan of the scientific literature;
  • Identify any modifications (based on the scientific literature and work done by wildfire management agencies elsewhere) that could be implemented in the short term to reduce potential health impacts; and
  • Identify any gaps in the work-related health knowledge of wildland firefighters and associated personnel.

The outcomes of this project and other information will help the BC Wildfire Service establish a long-term research strategy for worker health. This research is expected to be completed in the summer of 2020.

Research by the University of Alberta is led by Nicola Cherry, MD, PhD, who is the tripartite chair of occupational health with the Division of Preventive Medicine at the University of Alberta.

It is also supported by the Alberta government and aims to:

  • Examine the nature and concentration of polycyclic aromatic hydrocarbons in the air that firefighters breathe and accumulate on their skin (polycyclic aromatic hydrocarbons are a suite of organic compounds produced when organic material burns, some of which can be hazardous to human health);
  • Explore the practicality and effectiveness of firefighters using respiratory protective equipment; and
  • Investigate whether wildland firefighters have more chronic lung disease than other people of the same age, gender and geographic location.

So far, about 50 BC Wildfire Service firefighters have taken part in this study. Alberta firefighters are also participating. A progress report on the initial phase of this project should be released in March 2020.

Missoula science lab assists firefighters

Above: Screenshot from the Missoula Fire Sciences Lab presentation. USFS.

The U.S. Forest Service has produced a fancy “story map” about the science being conducted at the Fire Sciences Lab in Missoula, Montana. I have toured the facility but was not aware of the breadth of the work now being conducted in the Wind Tunnel & Combustion Chamber, Soils Lab, Chemistry & Emissions Lab, Ecophysiology Lab, Tree Ring Lab, and Fuels Lab.

The presentation, which only takes a few minutes to scroll through, is mostly photos with brief descriptions of the work going on in the various departments. With science under attack in recent years, it is heart warming to know that some federal employees in Missoula have our backs.

Missoula Fire Sciences Lab
Screenshot from the Missoula Fire Sciences Lab presentation. USFS.