UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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Item A LARGE EDDY SIMULATION STUDY OF THE EFFECTS OF WIND AND SLOPE ON THE STRUCTURE OF A TURBULENT LINE FIRE(2019) VERMA, SALMAN; Trouve, Arnaud; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to a complex coupling between a large number of physical and chemical processes that happen over a wide range of length and time scales, our current fundamental understanding of wildland fire spread is limited. Wildland fire spread is affected by many parameters, but two of the most important parameters are wind and slope both of which tilt the flame and plume and bring them closer to the unburnt fuel, which, among other things, increases the convective heat transfer and hence the spread rate. The primary objective of this work is to enhance our fundamental understanding of the effects of wind and slope on the structure of a turbulent, buoyant line fire. To meet the aforementioned objective we perform well-resolved Large Eddy Simulations (LES) of a simplified configuration corresponding to a turbulent, buoyant, methane-fueled, stationary, line fire and subjected to wind or slope. Simulations are performed with an LES solver developed by FM Global and called FireFOAM which is based on the OpenFOAM CFD library. For the cases with wind, the transition from the buoyancy-dominated (in which the flame and plume are mostly detached from the downwind surface and have a tilted vertical shape; entrainment is two-sided; downwind surfaces experience convective cooling) to wind-dominated (in which the flame and plume are attached to the downwind surface; entrainment is one-sided; downwind surfaces experience convective heating) regime happens when the Byram’s convection number Nc is ≈1. The flame and plume attachment lengths (defined as the x-wall-distance downwind of the burner within the flame and plume regions, respectively) are found to fluctuate significantly in time. For the cases with slope the transition from the detached regime (equivalent to the buoyancy-dominated regime) to the attached regime (equivalent to the wind-dominated regime) is found to happen between slopes of 16 and 32 degrees. Upslope of the flame zone, the velocity tangent to the surface is found to change from a relatively small negative value (≈ −0.3 m/s) to a relatively large positive value (≈ 2.5 m/s), when the slope is increased from 16 to 32 degrees. The flame attachment length (defined as the tangential-wall-distance upslope of the burner within the flame region) is again found to fluctuate significantly in time. An integral model, capable of describing the effects of cross-wind on the structure of a turbulent, buoyant line fire, is also developed in this work. The model, after some simplifications, suggests that the plume tilt angle is controlled by the Byram’s convection number Nc and the entrainment coefficients α and β. Detailed comparisons are made between the model and LES and show that the model performs well for the cases belonging to the buoyancy-dominated regime (Nc>1) but fails to describe the cases belonging to the wind-dominated regime (Nc<1) because of the absence of a wall attachment sub-model.Item Understanding flame structure in wildfires using Large Eddy Simulations(2014) Verma, Salman; Trouve, Arnaud; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The structure of wildfire flames in the presence of crossflow was analyzed by utilizing suitable non-reacting numerical experiments with low speed flow over a hot isothermal horizontal semi-infinite surface. FireFOAM, a Large Eddy Simulation (LES) based solver developed by FM Global for fire protection engineering applications, was employed for all the calculations. Early-time dynamics of Rayleigh-Taylor Instability (RTI) was first simulated using Direct Numerical Simulations (DNS) so that the solver could be verified against Linear Stability Theory (LST). Then attention was given to late-time dynamics in order understand the different stages (e.g., appearance of secondary instability, generation of larger scales due to interaction between structures) involved in the development of the instability. The onset of thermal vortex instability, in a configuration with low speed flow over a hot isothermal semi-infinite horizontal plate, predicted using DNS was compared with the literature. Spatial evolution of various terms in the streamwise vorticity equation was used to identify the dominant mechanisms responsible for the generation/evolution of vorticity. Streamwise evolution of the instabilities was studied and the effects of the changes in temperature and orientation of the plate on the thermal instabilities were also investigated. Finally, a configuration with low speed flow over a hot isothermal semi-infinite horizontal strip was used to understand the effects of upstream Boundary Layer (BL) height and the length of the strip on both the thin horizontal and larger structures (analogous to Flame Towers (FT) observed in real wildfires and laboratory experiments).Item On The Computation Of Buoyancy Affected Turbulent Wall Flows Using Large Eddy Simulation(2013) Ojofeitimi, Ayodeji; Trouvé, Arnaud; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A high fidelity object-oriented C++ solver was developed in OpenFOAM® for the solution of low Mach number variable density Navier Stokes equations. Employing the Large Eddy Simulation (LES) methodology to compute the turbulent flowfield, the filtered LES equations were subsequently utilized to study buoyancy affected spatially developing boundary layers in natural and mixed convection spatially developing boundary layer flows. For the subgrid scale (SGS) closure, a locally dynamic Smagorinsky SGS model was implemented into OpenFOAM® to enable the backscatter phenomenon intrinsic to transitioning boundary layers. As a precursor to simulating the intricate aero-thermal flowfield of an in-flight aircraft engine pool fire due to a fuel leak, detailed investigations of two canonical problems in the absence of flames were conducted to assess the robustness of the C++ solver and to elucidate the turbulent flow physics; these test cases consisted of a natural convection turbulent boundary layer over an isothermal vertical plate without any forced flow and the mixed convection turbulent boundary layer over an isothermal vertical plate where the effects of a gradually increasing forced flow in the direction opposite to the gravitational vector were assessed. A third canonical case, the mixed convection over an isothermal horizontal plate, was also investigated as an extension of this thesis. For the first two cases, wall-resolved LES computations were compared with experimental data for first and second order turbulent statistics, along with available experimental frequency spectra of temperature and streamwise velocity fluctuations. In an effort to reduce the computational cost, wall-layer modeled LES computations were performed by implementing new wall models into OpenFOAM®. The fidelity of the wall-resolved and wall-layer modeled LES successfully confirmed the ability of the solver in computing high Grashof number transitioning natural and mixed convection spatially developing boundary layers. As it pertains to the third case, while experimental measurements in air of mixed convection over an isothermal horizontal plate is lacking in the literature, the fundamental structure of the boundary layer was qualitatively validated by examining the near-wall vortical flow topology and employing available empirical data. The accuracy of the results acquired for this flow configuration was deemed reliable due to the excellent agreement attained with the prior two test cases. Overall, the level of fidelity illustrated in this thesis has not been previously demonstrated for spatially developing turbulent boundary layers in natural and mixed convection wall flows, especially for LES. Thus, with the establishment of the methodology employed in this work, it can be further utilized as a reliable tool in computing buoyancy affected flame spread problems aboard in-flight aircraft engine fires to shed light upon the complex flow physics inherent to such flows.