Umesh Unnikrishnan PhD Defense

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Umesh Unnikrishnan
(Advisors: Prof. Joseph C. Oefelein and Prof. Vigor Yang]

will defend a doctoral thesis entitled,

Subgrid Scale Modeling for Large Eddy Simulation of Supercritical Mixing and Combustion


Friday, November 5 at 2:00 p.m. (EST)

MK325, Bluejeans:



Advances in computing power over the years have broadened the prospects for computational modeling and simulation to substantiate our understanding of turbulent mixing and combustion in aerospace propulsion applications. High-fidelity and predictive modeling and simulation techniques are prerequisite for effective use of numerical simulations in scientific research and development. Large eddy simulation (LES) is a one such powerful technique for turbulent flow research that offers a favorable balance between high accuracy and computational feasibility. While the LES methodology and accompanying subgrid scale (SGS) modeling have been developed and applied over decades, primarily in the context of low pressure, ideal gas conditions, their extension to complex, multi-physics flows encountered in aerospace propulsion requires further refinement. In particular, the application of LES to turbulent flows at high-pressure supercritical flows presents several new modeling challenges and uncertainties. The scope of this dissertation is to investigate the theoretical LES formalism and SGS modeling framework for compressible, multi-species, turbulent mixing and combustion at supercritical pressures. The goal is to establish a refined and consistent framework that accurately accounts for all the necessary physics.

In this dissertation, a consistent theoretical formulation of the filtered governing conservation equations is derived without any prior assumptions or simplifications. The derived formulation reveals the presence of several new subgrid terms that are not considered in the conventional framework. To evaluate the relevance of these terms, direct numerical simulations are performed for canonical non-reacting and reacting mixing layers at supercritical conditions. The complete set of terms in the governing equations are quantified and analyzed using the datasets. Based on the analyses, two new groups of subgrid terms are identified as important quantities to model in the LES framework. The distributions of these terms are examined to obtain physical insights regarding the nature and origin of these quantities. The use of Favre-filtered state variables is customary in LES of variable density flows. The implications with the use of Favre-filtered state variables to compute filtered quantities in the LES, such as the viscous stress, heat flux, density, etc. are rigorously investigated, and the residual terms resulting from such simplified representations are quantified. Parametric analyses are performed as a function of the LES filter resolution to derive an understanding of modeling considerations for practical LES applications that use moderate or coarse resolutions. The performance and accuracies of two state-of-the-art subgrid modeling approaches for the conventional subgrid fluxes are assessed at different filter resolutions. To address the additional modeling requirement identified in this study for the filtered equation of state or equivalently the filtered density, novel subgrid models are proposed and investigated, and good improvements are demonstrated for the representation of the filtered density.




  • Prof. Joseph C. Oefelein – School of Aerospace Engineering (advisor)
  • Prof. Vigor Yang– School of Aerospace Engineering (co-advisor)
  • Prof. Suresh Menon – School of Aerospace Engineering
  • Prof. Timothy C. Lieuwen – School of Aerospace Engineering
  • Dr. Ramanan Sankaran – Oak Ridge National Laboratory



  • Workflow Status:Published
  • Created By:Tatianna Richardson
  • Created:11/01/2021
  • Modified By:Tatianna Richardson
  • Modified:11/01/2021