Applied Math Colloquium
Analysis of Jet Noise and Hypersonic Transition using Momentum Potential Theory
Carefully performed high-fidelity computations, such as Direct Numerical Simulations and Large-Eddy Simulations, capture all pertinent spatio-temporal scales of interest to the aerospace engineer. The extraction of physics insight from the resulting massive databases remains a daunting task. Numerous decomposition techniques have been recently developed or rediscovered to aid in overcoming this challenge. These are generally classified as data-driven or operator-aware. The former are not directly concerned about the underlying governing equations, while the latter take them into account. In this talk, we will examine a technique in this latter category, to decompose, turbulent fluctuations into their hydrodynamic (or vortical), acoustic (irrotational and isentropic) and thermal (irrotational and isobaric) modes. This method, proposed by Doak and designated momentum potential theory (MPT), can be interpreted in a Kovasznay-type disturbance field framework, but has the advantage that it is an exact decomposition, applicable to arbitrary turbulent continuum flows, including those with mean flow gradients. In addition to the decomposition, an accompanying theory based on total fluctuating enthalpy (TFE) describes the inter-modal energy transfer. The power of the method to generate new insight into extensively studied phenomena is demonstrated by considering LES and DNS of jet noise and hypersonic transition respectively. For the former, the hydrodynamic mode manifests the disorganized turbulence field, while the acoustic and thermal modes reveal organized wavepacket forms of much larger length and time scales than those of turbulence. The TFE analysis then explains how energy is transferred from the disorganized hydrodynamic mode to the organized acoustic mode, with primary sources and sinks arising from vortex intrusion and shear, respectively. For hypersonic transition, the approach successfully connects well known results from stability theory to corresponding energy modes. The simulations show that although the hydrodynamic mode is the largest component, the acoustic and thermal modes are both essentially trapped at the conditions of interest, and interact more with each other, consistent with the high sensitivity of transition to wall thermal conditions.
Brief Bio: Dr. Datta V. Gaitonde is the John Glenn Chair and Ohio Research Scholar in the Mechanical and Aerospace Engineering Department. He has diverse research interests, including shock/turbulent interactions, jet noise, hypersonic transition, flow control, bluff body flows, scramjet flowpaths and algorithm development. He has over 200 publications in journals, conferences and books and has delivered numerous national and international presentations. He has served on various advisory panels for national agencies and is a Deputy Editor of the AIAA Journal. He is a recipient of several awards and is a Fellow of the Air Force Research Laboratory, ASME and AIAA.