Mechanisms of Transient Growth and Turbulence Evolution in a Columnar Vortex

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PROPOSAL NO.: CTS-0554165PRINCIPAL INVESTIGATOR: F. HUSSAININSTITUTION: UNIVERSITY OF HOUSTONMECHANISMS OF TRANSIENT GROWTH AND TURBULENCE EVOLUTION IN A COLUMNAR VORTEX This project will focus on the fundamental problem of large-scale/fine scale vorticity coupling through direct numerical simulations (DNS) of ambient turbulence interacting with a single, large-scale coherent vortex - idealized as an isolated columnar vortex. Turbulent flows featuring large-scale vortices (coherent structures, CS) are commonly encountered in engineering applications, e.g. turbines, trailing vortices, jets, wakes and boundary layers. While CS are now well recognized to be responsible for important engineering effects - such as drag, mixing, heat transfer, combustion, and flow noise - turbulence, in turn, strongly affects the evolution of the CS itself by inducing large-amplitude waves in the vortex core, triggering new instabilities in the ambient flow, and enhancing CS decay. The modeling and prediction of CS evolution - involving the intricate coupling between large- and fine-scale turbulence - remains a formidable challenge. A central aspect of the proposed approach is the application of novel spectral DNS techniques that correctly address the proper flow domain boundary conditions. It has been shown that intense, azimuthally-oriented secondary, finer-scale filaments that such vortex breeds can induce large-amplitude waves on the core of the vortex column itself. Transient growth analysis can provide a systematic way to identify specific 'optimal' modes most conducive to vortex instability and transition. Such modes - to be pursued through DNS - can cause 'bypass' transition and sustained turbulence. The failure of existing turbulence models to capture these effects appears to underlie the large discrepancies in predicted and observed vortex decay rates - a quantity of fundamental practical relevance in applications such as the aircraft trailing vortices. This research will have broad educational, scientific, technological, and, ultimately, very significant economic impact. This fundamental problem of vortex/turbulence coupling is relevant to a wide spectrum of applications: ranging from the alleviation of the aircraft wake hazard to the improved design of gas turbines to reducing vehicle/aircraft drag. Significant fuel savings will also result via control of transient growth in trailing vortices and boundary layers. The findings from this research will help minimize spacing of aircraft takeoffs in busy airports, enhancing airport utilization and obviating the need for expensive new runways. Graduate and undergraduate students, in collaboration with NASA/JSC engineers, will work with high-school science teachers and students to give them hands on research experience and to popularize engineering in Houston schools and science fairs via the use of the aircraft wake problem and findings from this research program.