# Seminars in 2013

### 3D Structured Adaptive Mesh Refinement and Multilevel Preconditioning for Non-Equilibrium Radiation Diffusion

Dr. Bobby Philip

Computing and Computational Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, TN, U.S.A.

Date: Wednesday, October 9, 2013 at 3:30 PM

Abstract: The time dependent non-equilibrium radiation diffusion equations are important for solving the transport of energy through radiation in optically thick regimes and find applications in several fields including astrophysics and inertial confinement fusion. The associated initial boundary value prob- lems that are encountered often exhibit a wide range of scales in space and time and are extremely challenging to solve. To efficiently and accurately simulate these systems we describe our research on combining techniques that will also find use more broadly for long term time integration of non- linear multiphysics systems: implicit time integration for efficient long term time integration of stiff multiphysics systems, local control theory based step size control to minimize the required global number of time steps while con- trolling accuracy, dynamic parallel 3D adaptive mesh refinement (AMR) to minimize memory and computational costs, Jacobian Free Newton-Krylov methods on AMR grids for efficient nonlinear solution, and optimal multi- level preconditioner components that provide level independent solver con- vergence.

### Optimal tangential surface velocity for energetically efficient drag reduction and self-propulsion

Prof. Ratnesh Shukla,

Department of Mechanical Engineering, Indian Institute of Science, Bangalore

Date: 17 October at 3:30 PM

Abstract: An understanding of active and passive flow control strategies is of considerable practical and fundamental interest as it can facilitate drag reduction and efficient propulsion in mechanical systems like underwater robotic and micro air vehicles. The energetic efficiency of a given flow control strategy is a critical parameter that determines its success. Despite its importance however this important parameter has often been overlooked in a vast majority of investigations concerning flow control for drag reduction and propulsion. In this talk we will present a combined theoretical and computational study of an energetically efficient tangential surface velocity based flow control strategy for drag reduction and propulsion. An energy balance on a translating body will be used to define the power loss coefficient; a metric of performance that allows for quantification of energetic efficiency in general flow configurations including drag reduction, self-propulsion and thrust generation. The efficacy of the tangential surface velocity based flow control strategy will be ascertained using the effective reduction in hydrodynamic forces and net power consumption for both bluff and streamlined bodies.

### Combustion instability: fundamental mechanisms and modeling

Santosh Hemchandra,

Assistant Professor, Department of Aerospace Engineering, Indian Institute of Science, Bangalore

Date: Wednesday, 23 October, at 3:30 PM

Abstract: Combustion Instability is a crippling problem which limits the operating envelope of aircrafts engines and gas turbines for power generation. This phenomenon is characterized by high amplitude pressure fluctuations that result from coupling between unsteady heat release processes and the combustor acoustic field. This phenomenon gained relevance as an operability issue following the move towards a lean-premixed combustion strategy - the latter being driven by the need to mitigate excessive emissions of pollutants such as nitrogen oxides (NOX) and soot. Simulating the onset and evolution of this phenomenon in practical systems using traditional CFD based (RANS/LES) approaches is prohibitively expensive. Thus, it is essential for the purpose of designing improved active/passive mitigation strategies for this phenomenon, to develop physically realistic reduced order models that can capture the unsteady dynamics of the combustor acoustic field accurately. This talk will provide an overview of the various fundamental physical processes that combustion instability is a result of. The first part of the talk will focus on coupling mechanisms that result in heat release unsteadiness and how they are influenced by unsteady flame motions and fuel/air ratio inhomogeneity. Next, the role of convective and absolute instabilities in the hydrodynamic field of the combustor in influencing flame steadiness will be discussed. Recent work in our group on the effect of density variation (due to temperature rise across the flame) and flow shear on the nature of flow instabilities will be presented. The talk will conclude with my views on what I think the community should be concentrating on in order to develop a reliable reduced order modeling capability for this problem.

### Local projection stabilization for convection-diffusion and flow problems

Prof. Dr. Lutz Tobiska

Institut for Analysis and Computational Mathematics, Otto von Guericke University Magdeburg, Germany

Date: 30 October, 2013

Abstract: We consider finite element approximations for solving convection-dominated convection-diffusion equations and the Navier-Stokes problem. It is well-known that standard Galerkin approaches are unstable if the mesh is not adapted to the layer behaviour of the solution and/or the finite element spaces approximating velocity and pressure do not satisfy the Babuska-Brezzi condition. We explain the underlying ideas of local projection stabilization and discuss the differences to the streamline-diffusion method.

### Prognosis of an Impending Combustion Instability

Prof. R. I. Sujith,

Dept. of Aerospace Engineering, Indian Institute of Technology Madras

Date: Tuesday, 12 November at 3:30 PM

Abstract: The transition in dynamics from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, combustion instability in confined combustion environments was studied experimentally using two laboratory scale turbulent combustors. We show how combustion noise that appears to have a Gaussian distribution and a broad-band spectrum in frequencies is in fact borne out a deterministic process and is not just the result of a stochastic process. We further show that ‘combustion noise’ is composed of low amplitude chaotic fluctuations that display scale invariance and multifractality that disappears at the onset of combustion instability. Traditional analysis or phenomenological models usually neglect or average out these fluctuations or treat them as a stochastic background. We show how techniques such as the FFT are incapable of describing the complexity of the underlying dynamics and hence are unable to predict the onset of an impending instability. Most importantly, we demonstrate that the irregular fluctuations contain useful information of prognostic value by defining representative measures that can act as early warning signals to impending instability in practical gas turbine combustors.