Computational Fluid Dynamics (CFD) Modeling Use in Fire

■ DRI Fire Science and Litigation Seminar, September 9–11, 2021 ■

Computational Fluid Dynamics (CFD) Modeling Use in Fire Investigations

and

Matterport Camera

(Session: Live Burn Technology – Fire Scene Investigations – 3 Quick Hits)

Nicholas A. Nava, P.E., CFEI, CVFI

Exponent, Inc.

17000 Science Dr., Suite 200

Bowie, MD 20715

(301) 291-2525

[email protected]

Drew Paris

Jensen Hughes

3224 Rice St Ste 101

Little Canada, MN 55126

(425) 775-5550

[email protected]

Amy E. Gray, Ph.D.

ESi

8491 NW 17th St Ste 102

Doral, FL 33126

(305) 599-2262

[email protected]

mailto:[email protected]




Nicholas A. Nava, PE, CFEI, is a professional fire protection engineer that applies fire

science and fire protection engineering principles to the analysis of fire protection

systems, consumer product performance, material fire performance, and origin and cause

investigation of residential, commercial, and industrial fires and explosions. Mr. Nava's

project work experience includes fire modeling as it relates to origin and cause fire and

explosion investigations, modeling fire protection system (sprinklers, smoke alarms, heat

detectors) response to fires, and building and fire code compliance.

Drew Paris, PE, CFEI, Director, Fire Forensics - Midwest Region and Principal

Electrical Engineer specializes in investigating and consulting on electrical fires and

failures, electrical design and electrical safety. His investigative casework experience

includes product failure analysis, residential, commercial, and industrial fire and

explosion investigations, heavy equipment accidents and failures, personal injuries,

electrocution and electric shock investigations, appliance fire and failure investigations.

Mr. Paris brings to Jensen Hughes an extensive background in electrical safety

consulting. He has developed safety training procedures for industrial clients as well as

inspected the design and manufacture of specialized industrial machinery. In addition to

his forensic and safety experience, Mr. Paris has worked as a manufacturing and design

engineer where he designed and tested custom and replacement current transformers for a

variety of industrial and utility customers. He provides litigation support and expert

testimony to law firms, insurance companies and manufacturers.

Amy E. Gray, PhD, PE, is a licensed mechanical engineer with expertise in thermal-

fluid sciences. She applies this specialized knowledge to provide clients with consultation

regarding fires, explosions, and mechanical product liability. Failure investigations

conducted by Dr. Gray have included fuel gas explosions, aircraft accidents involving

fires, tank and boiler ruptures, natural gas pipeline and utility incidents, fuel spread and

dispersion, gas-fueled appliances, and chemical processing plant incidents. Additionally,

she has experience in Dust Hazard Analysis (DHA), Process Hazard Analysis (PHA),

Computed Tomography (CT) data analysis, and Computational Fluid Dynamics (CFD).


I. Computational Fluid Dynamics (CFD) Modeling Use in Fire

Investigations

(Nicholas Nava)

Mathematical modeling used during fire investigations can take many forms to include

simple “hand” calculations, zone modeling, and computational fluid dynamic modeling,

each more complex than the next. Regardless of the type of model used by a fire

investigator, there are limitations that will be discussed herein including input

uncertainties, assumptions, and approximations. Modeling has been verified and

validated for numerous scenarios and when used with an appreciation for its limitations,

is an effective tool as it related to hypothesis testing for fire investigators.

What Is CFD?

Computational Fluid Dynamics (CFD) modeling uses the principles of conversation of

mass, energy, and momentum expressed as generalized mathematical equations in the

form of integral or partial differential equations generally referred to as the Navier-Stokes

equations. When modeling with CFD, the volume of the space modeled is divided using

a 3-dimensional grid which creates cubes for solving these equations. These equations

are solved numerically to calculate the physical conditions in each cube for each time

step. CFD uses an interactive approach to calculate a cube’s physical change as it relates

to the surrounding cubes from the previous time step. The grid cube size often corelated

with the computational time for CFD modeling, whereas very small grid cells can be

computationally “expensive” and take days to weeks to complete a model run.

CFD modeling is used in the fire protection engineering community for performance

based design, fire risk analyses, fire investigations, and research. The National Institute

of Standards and Technology’s (NIST) Fire Dynamics Simulator (FDS) is well known in

the fire protection engineering community. Considerable research has been performed by

engineers and scientists from NIST and within the fire protection engineering community

to verify and validate this model for use by comparing experimental data to model data.

In addition to the FDS computational ability, a program known as Smokeview is used as

a tool for visualizing FDS data. Smokeview can be used to visualize particles, smoke,

temperatures, velocities, gas concentrations, detector response, and more as a 2-

dimensional slice or 3-dimensoinal contour. The figure below shows an example of a

Smokeview outputs for a compartment fire.


Figure 1: Smokeview examples. Top) Smoke 3D Files; Bottom) Temperature Slice Files.

(NIST Special Publication 1017-1 Sixth Edition Smokeview, User Guide)


Data Collection - Garbage In, Garbage Out

If modeling is expected or proposed, it is important for a fire investigator or engineer to

collect information from the fire scene that will be used as model inputs. FDS requires

the user to input variables, inputs, and boundary conditions, many of which are

information collected from a fire scene. Important fire scene data that should be

collected includes building dimensions including doors, windows, and vents, building

materials, occupant contents, and fire protection features. Each of these parameters are

important inputs for a fire model. Most of these are intuitive as they are necessary in

building the geometry of the model, specifying model materials, and specifying the

contents inside the room/building. New technologies include 3D scanning such as

Matterport, Leica, and GeoMax scanners that can be used to assist in collecting

dimensional data. The location of doors, windows, and vents are important as it relates to

ventilation in the model and can have a non-negligible effect of fire growth,

development, and combustion product. Typical data collected from FDS modeling

includes smoke production, heat release, plume and surface temperatures, velocities, gas

concentrations, detector response, and sprinkler spray extinction effects.

FDS allows the fire investigator or engineer the ability to set inputs such as when

windows/doors open, HVAC operation, and exhaust vents. The user, for example, can

specify the HVAC duct flows and air movement which can assist in determining how

products of combustion move through a building. Additionally, FDS provides the ability

for investigators to calculate the activation of heat detectors, sprinklers, and smoke

detectors; however, each of these fire protection devices have specific characteristics that

should be understood by the fire investigator or engineer prior to specifying in the model.

A common adage in the computer modeling community is Garbage In, Garbage Out. A

fire investigator or engineer should be mindful of this as it relates to CFD modeling and

its limitations when using a model to test a hypothesis. Inputs into a model that are not

well-founded, rely on incorrect assumptions, or missing inputs are likely to output results

that do not accurately reflect the fire.

Fire Modeling for Fire Investigation

NFPA 921: Guide for Fire and Explosion Investigations addresses the use of fire

modeling for hypothesis testing. The model’s results and predictions of the fire

environment can be used to test a given proposed hypothesis for fire origin, fire

development, and occupant exposure. For example, the results can be compared to

eyewitness accounts, photos and videos of the fire, fire patterns, occupant injuries, fire

protection response, timeline data, and gas concentration. Another benefit of using a fire

model is that it does not require the costs of physical construction and coordination of a

full-scale fire test. Additionally, several models can be run to test several hypotheses, for

example, an origin in several locations in a building, which would require several

buildings in a full-scale fire test and would likely be unreasonable.


The investigator should be mindful of the limitations of the CFD modeling. A

recommended methodology for use of a CFD model is addressed by the Society of Fire

Protection Engineers (SFPE) and the National Fire Protection Association (NFPA) and

includes: defining a problem to be addressed, selecting an appropriate model, ensuring

the model is verified and validated for a specific use, determining uncertainty and user

input effects, and then performing the analysis and confirming the basis for selection.

The verification and validation is a factor that is sometimes overlooked by users and

should be explored prior to use of a model.

For a trial, exhibits can be created using Smokeview to illustrate to the trier of fact the

fire environment that may be better understood visually. Often a visual representation of

fire growth is much easier to understand and key timestamps can be displayed such as

when a smoke alarm activates, when a room becomes untenable for occupants, or the

effect of a window or door being open during evacuation or fire fighter operations.

Conclusion

CFD modeling is a versatile tool that can be used during a fire investigation to test

hypotheses. CFD modeling has extensive capabilities including, but not limited to,

providing data on smoke production, heat release, plume and surface temperatures,

velocities, gas concentrations, detector response, and sprinkler spray extinction effects.

While computational models can be used to test hypotheses, models should not be

utilized as the sole basis of a fire origin and cause determination and their limitations

should be appreciated.

References:

NIST Special Publication 1018-1 Sixth Edition Fire Dynamics Simulator Technical

Reference Guide Volume 1: Mathematical Model, 2021.

NIST Special Publication 1019 Sixth Edition Fire Dynamics Simulator User’s Guide,

2021.

NIST Special Publication 1017-1 Sixth Edition Smokeview, A Tool for Visualizing Fire

Dynamics Simulation Data Volume I: User’s Guide, 2021.

NFPA 921: Guide for Fire and Explosion Investigations, 2021 Edition.

NFPA Fire Protection Handbook, 20th edition.

SFPE Engineering Guide: Guidelines for Substantiating a Fire Model for a Given

Application, 2011.

SFPE Handbook of Fire Protection Engineering, 5th Edition.

II. Matterport Camera

(Drew Paris)

1. Matterport Overview – Matterport was founded in 2011 with the intent to enable

easy 3D capture. In the last few years Matterport and other 3D scanning


technology has become frequently used in the fire forensics arena. These 3D

scans allow clients access to visually examine the scene as well as courtroom

presentations.

2. Equipment – The most common model of Matterport is the Pro2 3D camera. In

addition to the camera the following are recommended to use and transport the

device; Matterport camera and charger, tripod, quick release clamp, iPad and if

needed power bank, external light source and large pelican style case to carry

these items. The Matterport is easily operated by one person.

1

3. Capabilities – The camera system allows you to scan up to 10,000 square feet per

project. The results of the scan are dimensionally accurate, within 1 percent, and

can produce spatially accurate schematic floor plans. The scan allows experts to

use accurate measurements in their fire analysis. Images and floor plans produced

by the scan allow the expert to use these images in expert reports further

explaining their opinion.

4. Operation – Operation of the device is simple and can be taught easily. The first

step is to assemble the camera onto the tripod. Next connect the camera and Ipad

(or other iOS device) to the Matterport Wi-Fi and the expert is ready to scan!

Scanning is a simple process of moving the device as it captures 360o images.

After scanning and uploading the images they are then ready to be shared

amongst parties. Light is very important and an external light in fire scenes is

usually needed. Some limitations exist in fire scenes depending on the severity of

the damage. Missing walls and open light from above can cause some extra steps

the expert would need to accomplish to scan the scene properly.

5. Example – Single Family Dwelling structure fire

1 https://support.matterport.com/hc/en-us/articles/360037428093-Getting-Started-With-Matterport

https://support.matterport.com/hc/en-us/articles/360037428093-Getting-Started-With-Matterport


Dollhouse view of the structure. Viewer can rotate 3600 to view all sides.

Floor selector view. Viewer can select each floor to view them in 360o

orientation. Red circle shows the room of fire origin.


Explore 3D space view. Viewer can walk through the structure. Each white circle

is a scan location the viewer can select to move down the hallway.


View into the room origin from the hallway.


Measuring tool allows users to click on any points to gain an accurate

measurement.


Documents

Computational Fluid Dynamics (CFD) Modeling Use in Fire