This proposal introduces new methods for modeling and control of transitional and turbulent wall-bounded shear flows. An important attribute of the proposed work is that it builds upon recent research manifesting the significance of uncertainty, such as free-stream turbulence and wall roughness, in channels, pipes, and boundary layers. Drag reduction by sensorless mechanisms is a promising technology, as it represents a much simpler alternative to feedback flow control with wall-mounted arrays of sensors and actuators. Although several numerical and experimental studies indicate that properly designed sensorless strategies yield significant drag reduction, an obstacle to fully utilizing these approaches is the absence of a theoretical framework for their design and optimization. This lack of analytical tools greatly impedes the synthesis of sensorless schemes as well as their extension to different flow regimes. The PI will develop a system-theoretic paradigm for design and optimization of sensorless flow control strategies. The new paradigm is a spatio-temporal analog of the well-known principle of vibrational control, where the system's dynamical properties are altered by introducing zero-mean vibrations into the system's coefficients. The PI's theoretical findings will enable more efficient drag reduction strategies and provide guidelines for design of various surface-based actuation techniques. Since distributed systems are becoming ubiquitous in modern technology, the educational goal of this proposal is to make distributed concepts more pervasive at the undergraduate and early graduate level. The PI will launch a new introductory course that will emphasize classes of systems characterized by their structural properties, practical applications, and physical interpretations. In collaboration with the Science Museum of Minnesota, the PI will organize popular lectures on diversity of flow control strategies in nature. Several video demonstrations will be used to illustrate how observations from the natural world motivate research, engineering design, and technology development. The PI's goal is to provide high-school students with an early exposure to the central role of control engineering in bringing the efficiency of natural fliers and swimmers to man-made systems.
Flow modeling and control is a promising active research area in systems and control theory. The ability to manipulate and control fluid flows is of paramount importance in many applications including transport in pipes, drag reduction for air and water vehicles, and mixing enhancement in chemical reactors and combustion engines. The potential benefits of successful flow control strategies are enormous; they range from economic gains in fuel savings to improved performance of engineering systems involving fluid flows. Since skin-friction drag directly translates into large fuel consumption for airplanes, ships, and submarines, there is a critical demand for development and utilization of advanced theoretical and computational techniques proposed in this work.
The ability to manipulate and control fluid flows is of paramountimportance in many applications including renewable energy generation,transport in pipes, drag reduction for vehicles, and mixing enhancementin chemical reactors and combustion engines. The potential benefits ofsuccessful flow control strategies are enormous; they range fromeconomic gains in fuel savings to improved performance of engineeringsystems involving fluid flows. Since skin-friction drag directlytranslates into large fuel consumption for airplanes, ships, andsubmarines, there is a critical demand for development and utilizationof advanced theoretical and computational techniques proposed in thiswork. This CAREER Award has pioneered a model-based approach to sensor-lessflow control, where the dynamics are impacted by zero-mean oscillations.In contrast to the current practice, which combines physical intuitionwith costly numerical simulations and experiments in an attempt tomitigate transition to turbulence, the developed methodology avoids theneed for expensive numerical simulations and experiments at the earlystages of control design. It also facilitates synthesis of superiorturbulence suppression strategies compared to what was earlier thoughtpossible. All of theoretical predictions, obtained by the developed model-basedapproach, have been verified using high-fidelity simulations of thenonlinear flow dynamics. They showed that a significant positive netefficiency can be achieved with sensor-less control in the form ofstreamwise traveling waves. Furthermore, it was shown that these wavescan even re-laminarize fully turbulent flows. This work demonstratedthat the theory developed for the linearized flow equations withuncertainty has considerable ability to predict full-scale phenomena,and that transition can be inhibited by reducing the tremendoussensitivity of flow dynamics using either active or passive means. The contribution of this CAREER Award goes beyond the problem ofdesigning transpiration-induced streamwise traveling waves. Thedeveloped theory and techniques may also find use in designing periodicgeometries and waveforms for maintaining the laminar flow or dragreduction in vehicles or wind turbines. This CAREER Award suggests thatreducing high flow sensitivity represents a viable approach forcontrolling the onset of turbulence. It also offers a computationallyattractive method for determining the energy amplification of flowssubject to periodic controls.
