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In this experimental study, we use a data-driven machine learning framework based on genetic programing (GP) to discover model-free control laws (individuals) for suppressing self-excited thermoacoustic oscillations in a prototypical laminar combustor. This GP framework relies on an evolutionary algorithm to make decisions based on natural selection. Starting from an initial generation of individuals, we rank their performance based on a cost function that accounts for the trade-off between the state cost (thermoacoustic amplitude) and the input cost (actuator power). We then breed subsequent generations of individuals via a tournament in which the direct forwarding of elite individuals occurs alongside genetic operations such as mutation, replication, and crossover. We implement this GP control framework in both closed-loop and open-loop forms, followed by benchmarking against conventional open-loop control based on time-periodic forcing. We find that while all three control strategies can achieve similarly large reductions in thermoacoustic amplitude, GP closed-loop control consumes the least actuator power, making it the most efficient. It achieves this efficiency by learning an actuation mechanism that exploits the strong heat-release-rate amplification of the open flame at its preferred mode, even though the GP algorithm has never seen the open flame itself. This study demonstrates the feasibility of using GP to discover new and more efficient model-free individuals for suppressing self-excited thermoacoustic oscillations, providing a promising approach to data-driven feedback control of combustion devices. [ABSTRACT FROM AUTHOR]

This paper studies the existence, regularity, and Hausdorff dimensions of global attractors for a class of Kirchhoff models arising in elastoplastic flow utt-div{|∇u|m-1∇u}-Δut+Δ2u+h(ut)+g(u)=f(x). It proves that under rather mild conditions, the dynamical system associated with above-mentioned models possesses in phase space X a global attractor which has further regularity in Xσ1(↪↪X) and has finite Hausdorff dimension. For application, the fact shows that for the concerned elastoplastic flow the permanent regime (global attractor) can be observed when the excitation starts from any bounded set in phase space, and the dimension of the attractor, that is, the number of degree of freedom of the turbulent phenomenon and thus the level of complexity concerning the flow, is finite. [ABSTRACT FROM AUTHOR]

The paper studies the longtime behavior of solutions to the initial boundary value problem (IBVP) for a class of Kirchhoff models arising in elastoplastic flow utt-div{|∇u|m-1∇u}-Δut+Δ2u+h(ut)+g(u)=f(x). By combining the decomposition idea with the operate technique, it proves that under rather mild conditions, the dynamical system associated with above-mentioned IBVP possesses in different phase spaces a global attractor which is connected, respectively. For application, the fact shows that for the concerned elastoplastic flow the permanent regime (global attractor) can be observed when the excitation starts from any bounded set in phase space. [ABSTRACT FROM AUTHOR]

We numerically investigate the forced synchronization of the self-excited flow behind a plunging airfoil in ground effect at a Reynolds number of Re = 1000. On varying the plunging amplitude and frequency, we find a rich array of nonlinear dynamics, such as a period-1 limit cycle due to natural vortex shedding as well as two-frequency quasiperiodicity on a torus attractor ( T 2 ). For certain non-resonant plunging frequencies without a ground surface, we find that low-dimensional chaos emerges via the Ruelle–Takens–Newhouse route. However, we find that the chaos can be suppressed by introducing a ground surface, inducing a direct transition from T 2 quasiperiodicity to 1:1 phase locking as the plunging amplitude rises over the boundaries of the Arnold tongue. Apart from suppressing chaos, the ground surface also causes the lift and drag coefficients to become less sensitive to the plunging motion itself. Knowledge of the critical plunging conditions required for forced synchronization and chaos could be useful in various engineering applications, such as the design of pico air vehicles. [ABSTRACT FROM AUTHOR]

Understanding the thermal transport in polymer-bonded explosives (PBXs) is critical for enhancing the safety and reliability during PBX design, especially in the absence of effective experimental measurements. In this work, we rigorously investigated the phonon properties of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and polyvinylidene fluoride (PVDF) and calculated the interfacial thermal conductance using an ab initio approach. The diffuse mismatch model and anharmonic inelastic model were adopted to examine the interfacial thermal conductance as a function of temperature for the TATB–PVDF interface. Our calculation results indicate that low-frequency phonon modes and the two-phonon process play dominant roles in the thermal transport at interfaces. In contrast, high-order phonon processes involving three to eight phonons accounted for around 8% of the interfacial thermal conductance at the TATB–PVDF interface. Phonon properties, such as the velocity and degree of phonon density overlap, are discussed for the TATB–PVDF and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX)–PVDF interfaces to estimate the interfacial thermal conductance of PBXs. This study provides a theoretical explanation for the establishment of a research method for PBX thermal transport. [ABSTRACT FROM AUTHOR]

In this article we proposed a modified transmission line model method to analyze non-Ohmic contact. It can differentiate the contact resistance while the current flows into and out of the contact. When the earlier two factors are equal, this method degenerate to traditional method. By this method, we obtained contact parameters for both p-GaN and n-GaN and revealed the validity of this method. To further study the contact characteristics and transportation mechanism between alloyed Ni/Au and p-GaN, we also measured barrier information at different temperature and current. The result indicates the hole passes through the barrier by thermionic emission and the barrier may lowered by image force. [ABSTRACT FROM AUTHOR]

We present a mechanism for the vertical leakage induced current degradation with identification of the properties of the relevant traps in AlGaN/GaN heterostructures on Si. The extent of the current degradation is determined by back-gating sweep measurements in double directions at different sweep rates and temperatures. It is found that the current degradation is only observed at relatively slow sweep rates and high temperatures. Time dependent back-gating measurements further suggest that the current degradation process is related to traps with long time constants. By comparing with the measurement results of samples on sapphire substrates, we confirm that the current degradation is caused by vertical leakage in heterostructures on Si. On the basis of the vertical leakage induced current degradation mechanism and in conjunction with the long-time degradation process, we measure both the trapping and detrapping processes of the relevant trap states to identify their properties. We find that there is a 0.6 eV capture barrier and a 0.67 eV emission barrier for the trap states, indicating that the trap states are of large lattice relaxation. [ABSTRACT FROM AUTHOR]

We report the stimulated emission related anomalous behavior of electrical parameters in GaN-based laser diodes based on both experiments and self-consistent calculations. In particular, the separation of quasi-Fermi levels of electrons and holes across active region shows a drop (or 'pinning') in threshold region, followed by an increase with current. The start and end points correspond to the two kink points of the first derivative of the optical output power-current (P-I) curve. This abnormal change of electrical parameters always satisfies the need of sufficient carriers for stimulated emission in the process of transition to lasing as well as lasing enhancement. [ABSTRACT FROM AUTHOR]

AlN layer was grown as interlayer between undoped GaN and Mg doped Al0.14Ga0.86N/GaN superlattices (SLs) epilayer to modulate the strain distribution between Al0.14Ga0.86N barrier and GaN well layers in SLs sample. Strain relaxation was observed in the SLs sample with AlN interlayer by x-ray diffraction reciprocal space mapping method. The measured hole concentration of SLs sample with AlN interlayer at room temperature was over 1.6×1018 cm-3 but that was only 6.6×1016 cm-3 obtained in SLs sample without AlN interlayer. Variable temperature Hall-effect measurement showed that the acceptor activation energy decreased from 150 to 70 meV after inserting the AlN layer, which indicated that the strain modulation of SLs induced by AlN interlayer was beneficial to the Mg acceptor activation and hole concentration enhancement. [ABSTRACT FROM AUTHOR]

Polarization-resolved edge-emitting electroluminescence of InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) from 395 to 455 nm was measured. Polarization ratio decreased from 3.2 of near-ultraviolet LEDs (395 nm) to 1.9 of blue LEDs (455 nm). Based on TE mode dominant emissions in InGaN/GaN MQWs, compressive strain in well region favors TE mode, indium induced quantum-dot-like behavior leads to an increased TM component. As wavelength increased, indium enhanced quantum-dot-like behavior became obvious and E∥C electroluminescence signal increased thus lower polarization ratio. Electroluminescence spectrum shifts confirmed that quantum dotlike behaviors rather than strain might be dominant in modifying luminescence mode of InGaN/GaN MQWs from near ultraviolet to blue. [ABSTRACT FROM AUTHOR]