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1. Synchronization of Spatiotemporal Irregular Wave Propagation Via Boundary Coupling

   https://doi.org/10.1115/1.4044923  

   Li, Liangliang; Huang, Yu; Xiao, Mingqing (December 2019, Journal of Computational and Nonlinear Dynamics)

   Abstract Wave dynamics reflect a broad spectrum of natural phenomena and are often characterized by wave equation such as in the development of meta-devices used to steer wave propagation. Modeling synchronization of wave dynamics is critical in various applications such as in communications and neuroscience. In this paper, we study the synchronization problem for oscillations governed by wave equation with nonlinear (van der Pol type) boundary conditions through a single boundary coupling. The dynamics of the master system is self-excited and presents sensitive and rapid oscillations. With the only signal received at one end of the boundary, by constructing a mathematical model, we show the existence of a slave system that can be synchronized with the master system via the study of wave reflections on the boundary to recover the actual wave dynamics. The coupling gain, which represents the strength of the connection between the master system and the slave system, has been identified. The obtained result can be also viewed as an observer construction when the measurable output is only on the boundary. Numerical simulations are provided to demonstrate the effectiveness of the theoretical outcomes. 

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2. Optimal transport and control of active drops

   https://doi.org/10.1073/pnas.2121985119  

   Shankar, Suraj; Raju, Vidya; Mahadevan, L. (August 2022, Proceedings of the National Academy of Sciences)

   Understanding the complex patterns in space–time exhibited by active systems has been the subject of much interest in recent times. Complementing this forward problem is the inverse problem of controlling active matter. Here, we use optimal control theory to pose the problem of transporting a slender drop of an active fluid and determine the dynamical profile of the active stresses to move it with minimal viscous dissipation. By parametrizing the position and size of the drop using a low-order description based on lubrication theory, we uncover a natural “gather–move–spread” strategy that leads to an optimal bound on the maximum achievable displacement of the drop relative to its size. In the continuum setting, the competition between passive surface tension and active controls generates richer behavior with futile oscillations and complex drop morphologies that trade internal dissipation against the transport cost to select optimal strategies. Our work combines active hydrodynamics and optimal control in a tractable and interpretable framework and begins to pave the way for the spatiotemporal manipulation of active matter. 

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3. Hierarchical Scheduling Algorithms with Throughput Guarantees and Low Delay

   Swamy, Peruru S; Srinivasan, Aravind; Ganti, Radha K; Jagannathan, Krishna (May 2018, International Symposium on Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks (WiOpt))

   We propose distributed scheduling algorithms that guarantee a constant fraction of the maximum throughput for typical wireless topologies, and have O(1) delay and complexity in the network size. Our algorithms resolve collisions among pairs of conflicting nodes by assigning a master-slave hierarchy. When the master-slave hierarchy is chosen randomly, our algorithm matches the throughput performance of the maximal scheduling policies, with a complexity and delay that do not scale with network size. When the master-slave hierarchy is chosen based on the network topology, the throughput performance of our algorithm is characterized by a parameter of the conflict graph called the master-interference degree. For commonly-used conflict-graph topologies, our results lead to the best known throughput guarantees among the algorithms that have O(1) delay and complexity. Numerical results indicate that our algorithms outperform the existing O(1) complexity algorithms like Q-CSMA. 

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4. Bio-inspired variable-stiffness flaps for hybrid flow control, tuned via reinforcement learning

   https://doi.org/10.1017/jfm.2023.28  

   Nair, Nirmal J.; Goza, Andres (February 2023, Journal of Fluid Mechanics)

   A bio-inspired, passively deployable flap attached to an airfoil by a torsional spring of fixed stiffness can provide significant lift improvements at post-stall angles of attack. In this work, we describe a hybrid active–passive variant to this purely passive flow control paradigm, where the stiffness of the hinge is actively varied in time to yield passive fluid–structure interaction of greater aerodynamic benefit than the fixed-stiffness case. This hybrid active–passive flow control strategy could potentially be implemented using variable-stiffness actuators with less expense compared with actively prescribing the flap motion. The hinge stiffness is varied via a reinforcement-learning-trained closed-loop feedback controller. A physics-based penalty and a long–short-term training strategy for enabling fast training of the hybrid controller are introduced. The hybrid controller is shown to provide lift improvements as high as 136 % and 85 % with respect to the flapless airfoil and the best fixed-stiffness case, respectively. These lift improvements are achieved due to large-amplitude flap oscillations as the stiffness varies over four orders of magnitude, whose interplay with the flow is analysed in detail. 

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5. Studies on Positioning Manipulators Actuated by Solid Media Transmissions

   https://doi.org/10.1109/ICRA.2019.8794356  

   Zhao, Haoran; Liu, Xin; Korpu, Rahul; Heffernan, Michael J.; Becker, Aaron T.; Tsekos, Nikolaos V. (May 2019, EEE International Conference on Robotics and Automation (ICRA 2019, Montreal CA))

   Fluidic transmission mechanisms use fluids to transmit force through conduits. We previously presented a transmission mechanism called solid-media transmission (SMT), which uses conduits filled with spheres and spacers for push-only bidirectional transmission. In this paper, we present new designs of SMT-actuated one-degree-of-freedom (DoF) and two-degree-of-freedom positioning manipulators, and report experiment studies to assess their performance. In these studies, closed-loop position control was performed with a PI controller and/or master-slave control. With braided PTFE tubing, SMT exhibited sub-millimeter accuracy, with a tolerance of 0:05mm for the tested transmission lines with lengths up to 4m. 

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