Mohammed F. Daqaq

This study showcases an experimental deployment of deep reinforcement learning (DRL) for active flow control (AFC) of vortex-induced vibrations (VIV) in a circular cylinder at a high Reynolds number (Re=3000) using rotary actuation. Departing from prior work that relied on low-Reynolds-number numerical simulations, this research demonstrates real-time control in a challenging experimental setting, successfully addressing practical constraints such as actuator delay. When the learning algorithm is provided with state feedback alone (displacement and velocity of the oscillating cylinder), the DRL agent learns a low-frequency rotary control strategy that achieves up to 80% vibration suppression, which leverages the traditional lock-on phenomenon. While this level of suppression is significant, it remains below the performance achieved using high-frequency rotary actuation. The reduction in performance is attributed to actuation delays and can be mitigated by augmenting the learning algorithm with past control actions. This enables the agent to learn a high-frequency rotary control strategy that effectively modifies vortex shedding and achieves over 95% vibration attenuation. These results demonstrate the adaptability of DRL for AFC in real-world experiments and its ability to overcome instrumental limitations such as actuation lag.

Springs inspired by the Kresling origami pattern have recently gained prominence as a foundation for developing functional engineering systems with versatile properties, catering to niche applications across various technological fields. With minimal adjustments to their geometric parameters, the restoring force of such springs can be tuned to exhibit quasi-zero stiffness (QZS) behavior that is ideal for designing low-frequency vibration isolators. In this work, we present the design, fabrication, and performance analysis of a fully functional, ultra-low-frequency torsional vibration isolator based on the Kresling origami spring. Fabricated from single-phase Nylon PA12 using selective laser sintering, the isolator weighs less than 5 grams and combines high static stiffness with low dynamic stiffness, enabling compact, scalable, and cost-effective isolation solutions. Unlike many existing QZS designs that are complex or suited only for large systems, this design is easily manufactured, durable, and well-suited for precision engineering. Quasi-static torsional tests reveal a broad QZS region, while dynamic testing shows a nearly linear response and effective vibration attenuation beginning at frequencies as low as 2.5 Hz, even under large-amplitude excitations. Furthermore, the isolator shows high endurance to prolonged cyclic loading with negligible performance degradation. These findings highlight the potential of origami-inspired structures in creating scalable, high-performance vibration isolators and provide a strong foundation for future research in advanced dynamic applications.

This paper investigates the thermal performance of Kresling origami structures as compact, deployable heat fins for adaptive thermal management applications. These fins are designed to actuate along the surface of interest, enabling space-efficient geometric transformations that influence both convective and conductive heat transfer mechanisms. To evaluate their effectiveness, both numerical and experimental analyses of the conductive and convective behavior of Kresling origami fins are conducted. Experimental measurements are performed in a controlled wind tunnel environment, while numerical results are obtained using a three-dimensional transient model based on the fundamental conservation laws of mass, momentum, and energy. The results indicate that the contracted Kresling fin achieves a 20% higher Nusselt number compared to the expanded configuration at low Reynolds numbers, along with improved temperature uniformity across the surface. These findings demonstrate the potential of Kresling origami as an efficient and reconfigurable thermal management solution, particularly for systems requiring adaptability.

The symmetric monostable Duffing oscillator exhibits a superharmonic resonance of order three when excited harmonically at an excitation frequency that is one third its linear natural frequency. In this letter, it is shown that a certain class of periodic excitations can inherently quench the superharmonic resonance of order three. The Fourier series expansion of such excitations yields a harmonic component at the natural frequency whose magnitude can be properly tuned to completely quench the effect of the superharmonic component. Based on this understanding, the parameters of a piecewise periodic function and the modulus of the cosine Jacobi elliptic function are intentionally designed to passively suppress the superharmonic resonance. Such periodic functions can be used to replace single-frequency harmonic excitations whenever the effects of the superharmonic resonance are to be passively mitigated.