Date of Award
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
Committee Chair
Babak Shotorban
Committee Member
Jason Cassibry
Committee Member
Chang-kown Kang
Committee Member
Udaysankar Nair
Committee Member
Sarma Rani
Subject(s)
Flame spread--Mathematical models, Wildfires, Cubic surfaces
Abstract
The spread of wildland and wildland urban interface (WUI) fires can be caused by spotting, where embers from a primary fire ignite spot fires. This study investigates the dispersion and deposition of firebrands over cubic structures in WUI regions under wind condition. Large eddy simulation was used for turbulence while tracing firebrands in Lagrangian framework, employing the open source software Fire Dynamics Simulator (FDS). The software program was revised to include a firebrand model where the rotational motion and thermal degradation of firebrands were taken into account in addition to the translation motion. This was used for simulation of cylindrical. The simulation included a firebrand depositing over single isolated block and multiple neighboring blocks in different configurations with varying wind speeds and separation distances. Both scenarios showed a safe zone on the leeward side of the blocks where no firebrands were deposited. The safe zone had a rectangular shape, with width matching the block and length proportional to the block’s height and wind speed. For tandem blocks, the safe zone extended onto the windward face of the trailing block with smaller separation distances and higher wind speeds. No firebrands were observed to deposit on the leeward face of the blocks in either scenario. The normalized number density (NND) of deposited firebrands was analyzed based on landing positions. At lower wind speeds, fewer firebrands were deposited on the top face of the block compared to the ground. Additionally, for blocks with longer length, the concentration of firebrands increased towards the trailing edge. Another firebrand model was developed and integrated in FDS to investigate the deposition and accumulation of cuboidal firebrands released from a firebrand-generating apparatus (Dragon) onto two adjacent blocks, as observed in previous experiments by Suzuki and Manzello [75]. In this model, firebrands were assumed to be influenced by drag and gravity during flight, and by drag and friction while sliding on the ground. These simulation revealed various flow features such as, the re-circulating region in the dragon’s wake, crossflow region upwind of the blocks and twin re-circulation region on the leeward side of the blocks affecting firebrand motion. At lower wind speeds, firebrands tended to accumulate between the dragon and the blocks. Conversely, at higher wind speeds, the firebrands tended to cluster momentarily before the crossflow region and then move swiftly through the space between the blocks, with some exiting the domain and others accumulating on the leeward edge of the blocks. The dispersion of firebrands was found to be significantly greater in the streamwise direction compared to the spanwise direction due to the higher rms velocity in the former direction.
Recommended Citation
Mankame, Aditya Prakash, "A computational investigation of firebrand dispersion and deposition in areas with cubic structures" (2024). Dissertations. 409.
https://louis.uah.edu/uah-dissertations/409