Slow Mode Research Articles

Control strategy of load following for ocean thermal energy conversion

Ocean thermal energy conversion (OTEC) provides a feasible solution for sustainable and stable power supply in remote islands. In this paper, a thermodynamic model of the OTEC system is developed to study the dynamic response of power output and superheating degree to manipulated variables. Afterward, a control strategy of load following for the island OTEC system is proposed. Finally, two controllers, MPC and PI, are designed and compared under fast and slow load changes as well as disturbance rejection test. The results indicate that the power output of OTEC system is sensitive to the speed of working-fluid pump, reaching the adjustment range of 14.7 kW when the nominal speed decreases by 30%. In addition, model predictive control (MPC) shows better performance in both rapid and slow load-change modes. Especially, in the rapid load-change mode, the MPC controller could always keep the evaporator outlet superheating in the range of 2–4.5 °C, while the PI controller may introduce no superheat that may cause liquid impact. Both MPC and PI controller can stabilize the power output at the set point under one day's measured disturbance, while MPC controller makes the superheating degree of evaporator outlet within smaller temperature fluctuations (<0.2 °C).

Pore-pressure wave and diffusion induced by fluid-mass sources in partially saturated porous media

The dynamic and quasi-static spatiotemporal responses of pore water induced by fluid-mass sources in unsaturated porous media are determined and compared. An explicit fundamental solution for pore water pressure is derived using the known Green's function of dynamic poroelasticity. Based on the order of the low-frequency approximations for the source impulsive time function, different physical regimes are identified, describing the transition from diffusion to wave propagation. The diffusive regime bears only two diffusions equivalent to diffusive slow P2 and P3 waves. Non-dispersion diffusion and dispersion diffusion regimes, where P1 waves and diffusions coexist, are present. The similarity and diversity of induced pore water pressures owing to diffusive slow waves and fast waves may depend on the fluid loading rate (e.g. fluid injection) in contrast to the critical frequencies of a partially saturated porous medium. The short-term fluctuation in the injection rate may create very similar slow diffusive P2 waves along with a fast-compressional P1 wave by low impulsive frequencies and various slow P2 modes with transmitted radiation to the P1 wave by the comparatively high impulsive frequencies in different regimes. The results indicate that the diffusion and wave processes may generate uniform dynamic and quasi-static responses in partially saturated media for hydraulic tests and induced microseismic analysis.

Analytical solution of thermoelastic attenuation in fine layering for random variations of the Grüneisen ratio

Thermoelastic attenuation of P waves is due to energy conversion to the heat mode, which is diffusive at low frequencies and wave-like at high frequencies, behaving similarly to the Biot slow mode. The conversion is strong in highly heterogeneous media. We consider a layered medium with a random distribution of thermal properties, specifically the Grüneisen ratio, and obtain the phase velocity and quality factor. The relaxation peak of the random medium is wider than those of a periodic sequence of layers and the Zener mechanical model. Indeed, a Cole–Cole fractional model is needed to obtain a good match. These approximations are required to compute wave fields in heterogeneous media. Moreover, the solutions are helpful for studying the physics of thermoelasticity and testing numerical algorithms for wave propagation.

Dynamic stability in hovering flight of insects with different sizes.

Previous works on the flight dynamic stability of insects have focused on relatively large insects. Here, the longitudinal flight dynamic stability of two hovering miniature insects was computed. With the stability properties of the miniature insects from the present work and those of large insects from previous works, we studied the effects of insect size on the stability properties in the full range of insect sizes. The following results were obtained. Although the insects considered have a 30 000-fold difference in mass, their modal structure of flight stability is the same: an unstable oscillatory mode, a stable fast subsidence mode, and a stable slow subsidence mode; because of the unstable mode, the flight is unstable. An approximate analytical expression on the growth rate of the unstable mode as a function of insect mass (m) was derived. It shows that the time to double the initial values of disturbances (t_{d}) is proportional to the 0.17 power of the insect mass (m). That is, as m becomes smaller, t_{d} decreases (i.e., the instability becomes faster). This means that miniature insects need a faster nervous system to control the instability than larger insects. For example, the response time (represented by t_{d}) of a miniature insect, the gall midge (m≈0.05mg), needs to be faster by about 7 times than that of a larger insect, the hawk moth (m≈1600mg).

Observation of chiral and slow plasmons in twisted bilayer graphene.

Moiré superlattices have led to observations of exotic emergent electronic properties such as superconductivity and strong correlated states in small-rotation-angle twisted bilayer graphene (tBLG)1,2. Recently, these findings have inspired the search for new properties in moiré plasmons. Although plasmon propagation in the tBLG basal plane has been studied by near-field nano-imaging techniques3-7, the general electromagnetic character and properties of these plasmons remain elusive. Here we report the direct observation of two new plasmon modes in macroscopic tBLG with a highly ordered moiré superlattice. Using spiral structured nanoribbons of tBLG, we identify signatures of chiral plasmons that arise owing to the uncompensated Berry flux of the electron gas under optical pumping. The salient features of these chiral plasmons are shown through their dependence on optical pumping intensity and electron fillings, in conjunction with distinct resonance splitting and Faraday rotation coinciding with the spectral window of maximal Berry flux. Moreover, we also identify a slow plasmonic mode around 0.4 electronvolts, which stems from the interband transitions between the nested subbands in lattice-relaxed AB-stacked domains. This mode may open up opportunities for strong light-matter interactions within the highly sought after mid-wave infrared spectral window8. Our results unveil the new electromagnetic dynamics of small-angle tBLG and exemplify it as a unique quantum optical platform.

Fluid-Kinetic Variations in the Storm-Time Inner Magnetosphere.

Storm‐time broadband electromagnetic field variations along the interface between the dipolar field of the Earth's inner‐magnetosphere and the stretched fields of the plasma‐sheet are decomposed as a superposition of fluid‐kinetic modes. Using model eigen‐vectors operating on the full set of Van Allen Probes fields measurements it is shown how these variations are composed of a broad spectrum of dispersive Alfvén waves with significant spectral energy densities in the fast and slow modes over scales extending into the kinetic range. These modes occupy volumes in k‐space that define the field variations observed at each spacecraft frame frequency (fsc). They are in aggregate not necessarily planar and often comprise filamentary structures with no distinct propagation direction in the perpendicular plane. Within these volumes the characteristic parallel phase speeds of the fast and Alfvénic modes coincide over a broad range of fsc suggestive of coupling/conversion between modes.

Computation of the Protein Conformational Transition Pathway on Ligand Binding by Linear Response-Driven Molecular Dynamics.

While extremely important for relating the protein structure to its biological function, determination of the protein conformational transition pathway upon ligand binding is made difficult due to the transient nature of intermediates, a large and rugged conformational space, and coupling between protein dynamics and ligand-protein interactions. Existing methods that rely on prior knowledge of the bound (holo) state structure are restrictive. A second concern relates to the correspondence of intermediates obtained to the metastable states on the apo → holo transition pathway. Here, we have taken the protein apo structure and ligand-binding site as only inputs and combined an elastic network model (ENM) representation of the protein Hamiltonian with linear response theory (LRT) for protein-ligand interactions to identify the set of slow normal modes of protein vibrations that have a high overlap with the direction of the protein conformational change. The structural displacement along the chosen direction was performed using excited normal modes molecular dynamics (MDeNM) simulations rather than by the direct use of LRT. Herein, the MDeNM excitation velocity was optimized on-the-fly on the basis of its coupling to protein dynamics and ligand-protein interactions. Thus, a determined set of structures was validated against crystallographic and simulation data on four protein-ligand systems, namely, adenylate kinase-di(adenosine-5')pentaphosphate, ribose binding protein-β-d-ribopyranose, DNA β-glucosyltransferase-uridine-5'-diphosphate, and G-protein α subunit-guanosine-5'-triphosphate, which present important differences in protein conformational heterogeneity, ligand binding mechanism, viz. induced-fit or conformational selection, extent, and nonlinearity in protein conformational changes upon ligand binding, and presence of allosteric effects. The obtained set of intermediates was used as an input to path metadynamics simulations to obtain the free energy profile for the apo → holo transition.

Modeling of biohydrogen production using generalized multi-scale kinetic model: Impacts of fermentation conditions

This paper presents a new multi-scale kinetic model built upon the multi-stage growth Hypothesis for predicting biohydrogen production. The proposed model represents the significant factors affecting biohydrogen production using a sum of first-order kinetic terms with varying dynamics from slow to fast one. The current work investigates 52 case studies of biohydrogen production that show the double first-order kinetic model provides the best modeling fitness (R2 > 0.99). This result suggests two prevalent pathways or microbial groups with distinct dynamics (i.e., fast and slow modes) in biohydrogen production. An increase in temperature (30 °C–43 °C) or substrate concentration (10 g/L to 40 g/L) and the use of simple substrates or mixed cultures can increase the fast-mode dominance up to 100% contribution. Model analysis suggests that the fast mode corresponds to the butyrate production pathway, the growth-associated hydrogen-producing activity, the easily-biodegradable substrates, or the quick hydrogen-producing groups.

Fast equilibration mechanisms in disordered materials mediated by slow liquid dynamics.

The rate at which a nonequilibrium system decreases its free energy is commonly ascribed to molecular relaxation processes, arising from spontaneous rearrangements at the microscopic scale. While equilibration of liquids usually requires density fluctuations at time scales quickly diverging upon cooling, growing experimental evidence indicates the presence of a different, alternative pathway of weaker temperature dependence. Such equilibration processes exhibit a temperature-invariant activation energy, on the order of 100 kJ mol−1. Here, we identify the underlying molecular process responsible for this class of Arrhenius equilibration mechanisms with a slow mode (SAP), universally observed in the liquid dynamics of thin films. The SAP, which we show is intimately connected to high-temperature flow, can efficiently drive melts and glasses toward more stable, less energetic states. Our results show that measurements of liquid dynamics can be used to predict the equilibration rate in the glassy state.

Time scaling of entanglement in integrable scale-invariant theories

In two dimensional isotropic scale invariant theories, the time scaling of the entanglement entropy of a segment is fixed via the conformal symmetry. We consider scale invariance in a more general sense and show that in integrable theories that the scale invariance is anisotropic between time and space, parametrized by $z$, most of the entanglement is carried by the slow modes for $z>1$. At early times entanglement grows linearly due to the contribution of the fast modes, before smoothly entering a slow mode regime where it grows forever with $t^{\frac{1}{1-z}}$. The slow mode regime admits a logarithmic enhancement in bosonic theories. We check our analytical results against numerical simulations in corresponding fermionic and bosonic lattice models finding extremely good agreement. We show that in these non-relativistic theories that the slow modes are dominant, local quantum information is universally scrambled in a stronger way compared to their relativistic counterparts.

Hydrodynamic Theory of the Connected Spectral form Factor

One manifestation of quantum chaos is a random-matrix-like fine-grained energy spectrum. Prior to the inverse level spacing time, random matrix theory predicts a “ramp” of increasing variance in the connected part of the spectral form factor. However, in realistic quantum chaotic systems, the finite-time dynamics of the spectral form factor is much richer, with the pure random matrix ramp appearing only at sufficiently late time. In this article, we present a hydrodynamic theory of the connected spectral form factor prior to the inverse level spacing time. We first derive a general formula for the spectral form factor of a system with almost-conserved sectors in terms of return probabilities and spectral form factors within each sector. Next we argue that the theory of fluctuating hydrodynamics can be adapted from the usual Schwinger-Keldysh contour to the periodic time setting needed for the spectral form factor, and we show explicitly that the general formula is recovered in the case of energy diffusion. We also initiate a study of interaction effects in this modified hydrodynamic framework and show how the Thouless time defined as the time required for the spectral form factor to approach the pure random matrix result is controlled by the slow hydrodynamics modes. We then extend the formalism to Floquet systems, where a ramp is expected but with a different coefficient, and we derive a crossover formula from the Hamiltonian ramp to the Floquet ramp when the Floquet drive is weak. Taken together, these results establish an effective field theory of chaotic spectral correlations which predicts the random matrix ramp at late time and computes corrections to it at earlier times.6 MoreReceived 15 December 2020Revised 12 January 2022Accepted 10 February 2022DOI:https://doi.org/10.1103/PhysRevX.12.021009Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum chaosPhysical Systems1-dimensional spin chainsTechniquesRandom matrix theorySachdev-Ye-Kitaev modelU(N) symmetryCondensed Matter, Materials & Applied PhysicsNonlinear DynamicsFluid Dynamics

Comparison of Maxillary Lateral Incisor Infiltration Pain Using the Dentapen and a Traditional Syringe: A Prospective Randomized Study

IntroductionThe anterior maxillary infiltration is one of the more painful dental injections. The Dentapen is an electronic syringe that uses computer-controlled delivery technology to administer dental local anesthesia at a slow controlled rate. The purpose of this prospective, randomized, single-blind study was to evaluate solution deposition pain of a maxillary lateral incisor infiltration using the Dentapen with the slow flow rate (1.8 mL/162 sec) and ramp-up setting compared with a traditional syringe infiltration at a flow rate of 1.8 mL/60 sec. MethodsOne hundred thirty adults were administered a maxillary lateral incisor infiltration with the Dentapen and a traditional syringe at 2 separate appointments in a single-blind manner. The infiltrations of 2% lidocaine with 1:100,000 epinephrine were given at a rate of 1.8 mL/162 sec with the ramp-up feature for the Dentapen and 1.8 mL/60 sec for the traditional infiltration. The pain of solution deposition was recorded on a visual analogue scale. At the conclusion of the study, subjects selected their preferred injection technique. The data were analyzed statistically using paired t tests, a mixed-effect model, and odds ratio. ResultsThe pain of solution deposition was significantly less for the Dentapen injection than the traditional injection (P < .001). With the Dentapen device, 16% experienced moderate pain, and for the traditional syringe, 39% experienced moderate pain. Overall, 75% of subjects preferred the Dentapen injection over the traditional injection. ConclusionsThe Dentapen, using the slow flow rate and ramp-up mode, significantly reduced the pain of solution deposition for maxillary lateral incisor infiltrations.

Identification of distinct slow mode of reversible adaptation of pancreatic ductal adenocarcinoma to the prolonged acidic pH microenvironment

BackgroundPancreatic ductal adenocarcinoma (PDAC) is the most common pancreatic neoplasm with high metastatic potential and poor clinical outcome. Like other solid tumors, PDAC in the early stages is often asymptomatic, and grows very slowly under a distinct acidic pHe (extracellular pH) microenvironment. However, most previous studies have only reported the fate of cancerous cells upon cursory exposure to acidic pHe conditions. Little is known about how solid tumors—such as the lethal PDAC originating within the pancreatic duct-acinar system that secretes alkaline fluids—evolve to withstand and adapt to the prolonged acidotic microenvironmental stress.MethodsRepresentative PDAC cells were exposed to various biologically relevant periods of extracellular acidity. The time effects of acidic pHe stress were determined with respect to tumor cell proliferation, phenotypic regulation, autophagic control, metabolic plasticity, mitochondrial network dynamics, and metastatic potentials.ResultsUnlike previous short-term analyses, we found that the acidosis-mediated autophagy occurred mainly as an early stress response but not for later adaptation to microenvironmental acidification. Rather, PDAC cells use a distinct and lengthy process of reversible adaptive plasticity centered on the early fast and later slow mitochondrial network dynamics and metabolic adjustment. This regulates their acute responses and chronic adaptations to the acidic pHe microenvironment. A more malignant state with increased migratory and invasive potentials in long-term acidosis-adapted PDAC cells was obtained with key regulatory molecules being closely related to overall patient survival. Finally, the identification of 34 acidic pHe-related genes could be potential targets for the development of diagnosis and treatment against PDAC.ConclusionsOur study offers a novel mechanism of early rapid response and late reversible adaptation of PDAC cells to the stress of extracellular acidosis. The presence of this distinctive yet slow mode of machinery fills an important knowledge gap in how solid tumor cells sense, respond, reprogram, and ultimately adapt to the persistent microenvironmental acidification.

Microscopic Dynamics of Inverse Wormlike Micelles Probed Using X-ray Photon Correlation Spectroscopy.

Wormlike micelles (WLMs) are ubiquitous viscoelastic modifiers that share properties with polymer solutions. While their macroscopic rheology is well-understood, their microscopic dynamics remain difficult to measure because they span a large range of time and length scales. In this work, we demonstrate the use of X-ray photon correlation spectroscopy to interrogate the segmental dynamics of inverse WLM solutions swollen with a rubidium chloride solution. We observe a diffusive scaling of the dynamics and extract a temperature-dependent diffusion coefficient, which we associate with the thermal interactions of the slow segmental dynamics near entanglement points. We probe this relaxation process across the unbranched to branched topological transition and find no microstructural evidence of branch formation in the slow mode. Instead, we observe that the dynamics become more homogeneous and prominent as the temperature is reduced and water content increases.

Unusual divergence of magnetoacoustic beams

Two-dimensional magnetosonic beams directed along a line forming a constant angle θ with the equilibrium straight magnetic field are considered. Perturbations in a plasma are described by the system of ideal magnetohydrodynamic equations. The dynamics of perturbations in a beam are different in the cases of fast and slow modes, and it is determined by θ and equilibrium parameters of a plasma. In particular, a beam divergence may be unusual in the case of parallel propagation (θ = 0). Diffraction is more pronounced in the case of parallel propagation as compared to a flow without magnetic field, and less manifested in the case of perpendicular propagation. The beams propagating oblique to the magnetic field do not reveal diffraction. The dynamics of perturbations in a beam are analytically described in the cases of weak and strong nonlinearity compared to diffraction. Small magnitude perturbations at the axis of a beam in unusual cases propagate slower than that in the plane wave. Involving of thermal conduction leads to the coupling equations describing thermal self-action of a beam, which behaves differently in the ordinary and unusual cases. Self-focusing may occur in the presence of a magnetic field instead of conventional defocusing in gases.

First Results From the SCM Search‐Coil Magnetometer on Parker Solar Probe

AbstractParker Solar Probe is the first mission to probe in situ the innermost heliosphere, revealing an exceptionally dynamic and structured outer solar corona. Its payload includes a search‐coil magnetometer (SCM) that measures up to three components of the fluctuating magnetic field between 3 Hz and 1 MHz. After more than 3 years of operation, the SCM has revealed a multitude of different wave phenomena in the solar wind. Here we present an overview of some of the discoveries made so far. These include oblique and sunward propagating whistler waves that are important for their interaction with energetic electrons, the first observation of the magnetic signature associated with escaping electrons during dust impacts, the first observation of the magnetic field component for slow extraordinary wave modes during type III radio burst events, and more. This study focuses on the major observations to date, including a description of the instrument and lessons learned.

Wall-cooling effects on secondary instabilities of Mack mode disturbances at Mach 6

In hypersonic boundary layers, Mack modes play a crucial role in flow instability, whose secondary instability is a hot research topic. Since hypersonic flight vehicles will probably work under high-stagnation temperature conditions, which significantly affect the aerodynamic heating calculation and aero-thermal protection design of hypersonic vehicles, it is necessary to compare the primary and secondary instabilities in high-stagnation temperature boundary layers and that in the Boeing/AFOSR Mach 6 quiet tunnel (BAM6QT). Herein, wall-cooling is adopted in order not to consider chemical reactions. With the same freestream temperature of 100 K, two Mach 6 boundary layers with the wall temperature of 20 and 600 K, corresponding to the cooled wall condition and the quiet wind tunnel condition, respectively, are chosen to conduct the linear/non-linear stability and the secondary instability analysis. Our results show that the most dangerous Mack mode originates from a fast discrete mode in the present cooled-wall flow and the most dangerous Mack mode is born from the slow discrete modes in BAM6QT boundary layers. Furthermore, when the primary amplitude of Mack mode disturbances is large, the fundamental resonance always dominates the secondary instability, resulting in steady streaky structures that have the largest amplitude in the spectrum. In addition, the present results point out that the distribution of the eigenfunctions of the fundamental modes and subharmonic modes are significantly different under various wall-temperatures. What is more, different ratios of wall temperature to incoming flow temperature have changed the spanwise wave-angle of the secondary disturbances.

Sensing Characteristics of Polarization-Dependent Helical Long-Period Grating-Based Interferometer

We demonstrate an all-fiber Mach-Zehnder interferometer (MZI) by cascading a pair of helical long-period gratings (HLPGs) inscribed in a Panda-type polarization-maintaining fiber (PMF) using a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -laser. By cascading two grating with different lengths, a serial of polarization-dependent MZIs with different free spectral ranges have been demonstrated. The maximum polarization-dependence loss of the MZI was measured to be more than 35 dB. Two orthogonal interference spectra corresponding to fast axis mode and slow axis mode can be observed. The sensing characteristics of the polarization-dependent MZI we investigated experimentally. For liquid level sensing, the maximum wavelength shifts of the fast and slow axis interference dips were measured to be 11.03 and 3.25 nm when the surrounding refractive index is 1.424 and the interference length is 12.0 cm, respectively. The fast and slow axis interference dips have good linearity and opposite response with the temperature and strain change, the measured temperature and strain sensitivities of the fast and slow axis interference dips were 103.43 pm/°C, −31.26 pm/°C and −2.13 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , 1.68 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , respectively. Based on the high polarization extinction ratio, narrow resonant bandwidth, and low insertion loss, the demonstrated MZI has potential application in polarization-dependent multi-parameter sensing or multi-channel polarization filter.

Excitation of ion-acoustic waves by non-linear finite-amplitude standing Alfvén waves

We investigate, using a multi-fluid approach, the main properties of standing ion-acoustic modes driven by non-linear standing Alfvén waves. The standing character of the Alfvénic pump is due to the superposition of two identical circularly polarised counter-propagating waves. We consider parallel propagation along the constant magnetic field and we find that left- and right-handed modes generate via ponderomotive forces the second harmonic of standing ion-acoustic waves. We demonstrate that parametric instabilities are not relevant in the present problem and the secondary ion-acoustic waves attenuate by Landau damping in the absence of any other dissipative process. Kinetic effects are included in our model where ions are considered as particles and electrons as a massless fluid, and hybrid simulations are used to complement the theoretical results. Analytical expressions are obtained for the time evolution of the different physical variables in the absence of Landau damping. From the hybrid simulations we find that the attenuation of the generated ion-acoustic waves follows the theoretical predictions even under the presence of an Alfvénic pump. Due to the non-linear induced ion-acoustic waves the system develops density cavities and an electric field parallel to the magnetic field. Theoretical expressions for this density and electric field fluctuations are derived. The implications of these results in the context of standing slow mode oscillations in coronal loops is discussed.

Performance enhancement of a scanning electron microscope using a deep convolutional neural network

We report noise reduction and image enhancement in scanning electron microscope (SEM) imaging while maintaining a fast scan rate during imaging, using a deep convolutional neural network (D-CNN). SEM images of non-conducting samples without a conducting coating always suffer from charging phenomenon, giving rise to SEM images with low contrast or anomalous contrast and permanent damage to the sample. One of the ways to avoid this effect is to use fast scan mode, which suppresses the charging effect fairly well. Unfortunately, this also introduces noise and gives blurred images. The D-CNN has been used to predict relatively noise-free images as obtained from a slow scan from a noisy, fast scan image. The predicted images from D-CNN have the sharpness of images obtained from a slow scan rate while reducing the charging effect. We show that by using the proposed method it is possible to increase the scanning rate by a factor of about seven with an output of image quality comparable to that of slow scan mode. We present experimental results in support of the proposed method.