UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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Item Physics and Modelling of Compressible Turbulent Boundary Layer(2023) Lee, Hanju; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Key findings from a research study that focuses on understanding the effect of Mach number, Reynolds number and wall temperature on compressible turbulent boundary layers (CTBL) in the hypersonic regime are presented in this dissertation. The study utilizes a comprehensive CTBL database developed using an in-house direct numerical simulation (DNS) code at the CRoCCo laboratory. The database encompasses a range of semi-local Reynolds numbers (800 to 34,000) and Mach numbers up to 12, incorporating wall-cooling. The effects of density and viscosity fluctuations on the total stress balance are identified and used to create a new mean velocity transformation for compressible boundary layers. The role, significance and physical mechanisms connecting density and viscosity fluctuations to the momentum balance and to the viscous, turbulent and total stresses are presented, allowing the creation of generalized formulations. We identify the significant properties that thus-far have been neglected in the derivation of velocity transformations: (1) the Mach-invariance of the near-wall momentum balance for the generalized total stress, and (2) the Mach-invariance of the relative contributions from the generalized viscous and Reynolds stresses to the total stress. The proposed velocity transformation integrates both properties into a single transformation equation and successfully demonstrates a collapsing of all currently considered compressible cases onto the incompressible law of the wall, within the bounds of reported slope and intercept for incompressible data. Based on the physics embedded in the two scaling properties, the success of the newly proposed transformation is attributed to considering the effects of the viscous stress and turbulent stresses as well as mean and fluctuating density viscosity in a single transformation form. The Reynolds number trends of large turbulent structures in compressible turbulent boundary layers are investigated using the pre-multiplied energy spectra based on the density corrected fluctuating streamwise velocity signal. Results demonstrate the existence of friction as well as semi-local Reynolds number trend associated with large-scale structures, similar to trends observable in incompressible turbulent boundary layers (ITBL). In particular, the behavior of turbulence in the inner layer is seen to exhibit dependence based on both definitions of Reynolds numbers. On the contrary, the strength of large turbulent structures is seen to be only dependent on friction Reynolds number. This result directly contrasts with the observation of the near-wall turbulent intensity peak increasing with semi-local Reynolds number. The discrepancy is mended with a suggestion that the large turbulent scales in the log layer of which the strength increases with friction Reynolds number, are modified through the changes in local fluid properties such that the scale interaction near the wall increases as semi-local Reynolds number. In another words, closer to the wall, the CTBL flow behaves like a semi-local Reynolds number flow, while closer to the freestream, it behaves like a friction Reynolds number flow. Furthermore, the present study examines the Reynolds number dependence of the length scale between small and large turbulent scales. The analysis highlights the inadequacy of using a univariable wavelength based on viscous, semi-local or outer length scales to differentiate small and large scales. Based on this, the use of Reynolds number-dependent length scales is recommended. Overall, the study provides valuable insights into the Reynolds number trends of large turbulent structures in CTBL, emphasizing the influence of both semi-local Reynolds number and friction Reynolds number on turbulence characteristics.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).