On The Computation Of Buoyancy Affected Turbulent Wall Flows Using Large Eddy Simulation

Abstract

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.