Constant Oscillation Amplitude Research Articles

Numerical Analysis of Aeroacoustic Characteristics around a Cylinder under Constant Amplitude Oscillation

The present study numerically investigated a cylinder under oscillating motions at a low Reynolds number. The effects of two oscillation frequencies and amplitudes on the lift drag coefficient, near-field surface pressure fluctuation, and far-field noise were studied. The models were examined at a Mach number of 0.05, corresponding to a Reynolds number of 1.0 × 105. In this paper, the incompressible Navier–Stokes equations (INSE) and linearized perturbed compressible equations (LPCE) were coupled to form a hybrid noise prediction method, which was used to solve the flow field and acoustic radiation field. Based on the simulation results of the acoustic radiation field, the frequency characteristics of the acoustic waves were analyzed by the dynamic modal decomposition (DMD) method. It was observed that when the oscillation amplitude was the same, the variation amplitude and mean value of the lift-drag coefficient increased with the increase in the oscillation frequency. Under the same small oscillation frequency, the oscillation amplitude had little effect on the lift-drag coefficient. However, for the same large oscillation frequency, the variation amplitude of the lift-drag coefficient increased as the oscillation amplitude increased. In addition, both the amplitude and frequency had a significant effect on the directionality of the noise and the intensity of the sound waves. The main energy of the sound field was mainly concentrated on the first and second narrowband frequencies by using the DMD method to analyze the sound pressure level spectrum.

Impact of Controller Saturation on Instability Behavior of Grid-Connected Inverters

For the power-electronics-based power system, a kind of stable operation with constant-amplitude oscillation was often witnessed in the reported instability incidents. In some cases, the root is proved to be negative damping by the theoretical analysis based on the linear model and the impedance-based stability theory. However, the negative damped resonances should diverge. This leaves a void why the analysis result is not in accord with the field measurement. This article aims to provide a potential answer for the disaccord, i.e., the controller saturation which is commonly ignored in the modeling process due to its nonlinearity. The controller saturation is linearized by the describing function and the improved linear model is developed. By applying the generalized Nyquist stability criterion, the impact of the controller saturation on the instability behavior of grid-connected inverters is identified, which reveals the underlying mechanism of the constant-amplitude oscillation. The theoretical expectations are finally validated by the experimental results.

Stability of expanding accretion shocks for an arbitrary equation of state

We present a theoretical stability analysis for an expanding accretion shock that does not involve a rarefaction wave behind it. The dispersion equation that determines the eigenvalues of the problem and the explicit formulae for the corresponding eigenfunction profiles are presented for an arbitrary equation of state and finite-strength shocks. For spherically and cylindrically expanding steady shock waves, we demonstrate the possibility of instability in a literal sense, a power-law growth of shock-front perturbations with time, in the range of$h_c< h<1+2 {\mathcal {M}}_2$, where$h$is the D'yakov-Kontorovich parameter,$h_c$is its critical value corresponding to the onset of the instability and${\mathcal {M}}_2$is the downstream Mach number. Shock divergence is a stabilizing factor and, therefore, instability is found for high angular mode numbers. As the parameter$h$increases from$h_c$to$1+2 {\mathcal {M}}_2$, the instability power index grows from zero to infinity. This result contrasts with the classic theory applicable to planar isolated shocks, which predicts spontaneous acoustic emission associated with constant-amplitude oscillations of the perturbed shock in the range$h_c< h<1+2 {\mathcal {M}}_2$. Examples are given for three different equations of state: ideal gas, van der Waals gas and three-terms constitutive equation for simple metals.

Investigations on vortex-induced vibration of a wind turbine airfoil at a high angle of attack via modal analysis

Vortex-induced vibration of a forced oscillating wind turbine airfoil at 90° angle of attack is numerically investigated with the aid of the dynamic mode decomposition technique. This situation may be encountered during parking or idling operations where the yaw angle is engaged due to a failure of the control system. The airfoil might induce vortex-induced vibration, which accompanies with the “lock-in” phenomenon. In this phenomenon, the shedding frequency will “jump” into the structure natural frequency that could cause limit cycle oscillations. For a series of forced oscillation calculations with constant oscillation amplitude, the airfoil will become unstable before entering the locked-in region and regain stable state before leaving the locked-in region with a frequency ratio from 0.8 to 1.2. Additionally, in this range, a smooth phase shift between the vibration-induced aerodynamic force and the airfoil motion is found, which mainly influences the characteristics of energy exchange on the airfoil surface. Finally, dynamic mode decomposition is employed to identify different flow features in the flow field to illustrate vortex-induced vibration. Dominant pressure modes are obtained, and the mechanism of energy exchange on the airfoil surface is illustrated.

Rancang Bangun Sistem Pengendalian Level Pada Tangki Penyimpanan Menggunakan Degree Of Freedom Analysis Dengan Tunning PID Berdasarkan Metode Cohen-Coon

The storage tank is one of the most important operating units used in the generation process. This operating unit functions as water storage which will later be used as raw material for making steam. To maintain the mass balance system and the energy balance system in the storage tank, a proper control system is needed. One of the variables controlled in this operating unit is implementing a level control system. In a process control system, key knowledge is needed, namely determining the process variable (PV), manipulated variable (MV), and disturbance variable (DV). If it is wrong in determining the three types of variables, then the output from the control system that we provide is not by the value we want (setpoint). Analysis of degrees of freedom (DOF) is used to determine the number of manipulated variables in the storage tank operation unit. To determine the number of variable numbers and the number of equation numbers can be used by looking at the mathematical equation model of the storage unit operation. Meanwhile, to determine the number of variables used in the above equation is to choose the type of variable that affects the process if the variable is changed. PID tunning needs to be done to determine the best proportional, derivative, and integral constant values, which later the output we want is the same as the setpoint value. Cohen Coon is a PID tunning method that can tolerate a constant amplitude oscillation state. Cohen-Coon is trying to improve the oscillation method by using the quarter amplitude decay method. The closed-loop response system, in this method, is made so that the response is in the form of a quarter amplitude decay. Quarter amplitude decay is defined as a transient response whose amplitude in the first period has a ratio of one quarter (1/4).

Aerodynamic damping in vortex resonance from aeroelastic wind tunnel tests on a stack

The paper investigates the negative aerodynamic damping of circular cylinders in vortex-induced vibrations and compares the results of two different types of wind tunnel experiments. Firstly, forced-vibration wind tunnel tests on a rigid sectional model allow measuring the aerodynamic damping directly from the experiments. This is because the aerodynamic damping is related to the phase shift between the force and the imposed motion. They also offer the advantage of a systematic investigation for different velocity ratios and a constant oscillation amplitude. Secondly, aeroelastic experiments on a cantilevered cylinder with distributed elasticity free to oscillate under the wind action are performed. These experiments do not impose the motion from the outside and therefore, the physical nature of the vortex-induced vibration can develop without any further constraint to the motion. However, these tests do not explicitly reveal the relationship between the load and response. Nevertheless, the aerodynamic damping can still be evaluated, if a mathematical model to describe the relationship between load and response is assumed. The cross-check of aerodynamic damping values obtained from the two experimental approaches not only gives evidence of the reliability of the proposed aerodynamic damping model, but also proves the effectiveness of the Vickery&Basu spectral model to predict vortex-induced vibrations.

Stability Assessment of DFIG Subsynchronous Oscillation Based on Energy Dissipation Intensity Analysis

Concerning the subsynchronous oscillation caused by the integration of doubly-fed induction generator (DFIG) to the power grid via the series compensation circuit, a method to assess the stability of subsynchronous oscillation based on the dissipation intensity is proposed in this article. First, the transient energy that DFIG generates in one oscillation period is defined as the dynamic energy of DFIG, and the model of DFIG dynamic energy containing internal control and external networks is constructed. And then, according to the Lyapunov stability theory, the negative gradient of dynamic energy is defined as the dissipation intensity, which reflects the dissipation effect of DFIG on the energy generated during oscillation. On this basis, the effect of the series compensation degree, phase-locked loop, and converter control parameters on subsynchronous oscillation is assessed, and real-time assessment of the stability level of DFIG-integrated power system is realized. Finally, a simulation model is built in RT-LAB to verify the proposed method. Simulation results demonstrate that the stability level of the system can be assessed accurately according to the value of dissipation intensity. When the value of dissipation intensity is positive, system oscillation converges. When the dissipation intensity is zero, the system oscillates with constant-amplitude oscillation. When the value of dissipation intensity is negative, system oscillation diverges.

D’Yakov–Kontorovich instability in planar reactive shocks

The standard D’Yakov and Kontorovich (DK) instability occurs when a planar shock wave is perturbed and then oscillates with constant amplitude in the long-time regime. As a direct result, pressure perturbations generated directly behind the shock propagate downstream as non-evanescent sound waves, an effect known as spontaneous acoustic emission (SAE). To reach the DK regime, the slope of the Rankine–Hugoniot curve in the post-shock state must satisfy certain conditions, which have usually been related to non-ideal equations of state. This study reports that the DK instability and SAE can also occur in shocks moving in perfect gases when exothermic effects occur. In particular, a planar detonation, initially perturbed with a wavelength much larger than the detonation thickness, may exhibit constant-amplitude oscillations when the amount of heat released is positively correlated with the shock strength, a phenomenon that resembles the Rayleigh thermoacoustic instability. The opposite strongly damped oscillation regime is reached when the shock strength and the change in the heat released are negatively correlated. This study employs a linear perturbation model to describe the long-time and transient evolution of the detonation front, which is assumed to be infinitely thin, and the sound and entropy–vorticity fields generated downstream.

Composite adaptive posicast controller for the wing rock phenomenon in a delta-wing aircraft

The nonlinear dynamic phenomenon like wing rock is one of the important issues in the high performance aircraft autopilot design. This phenomenon occurs in the form of constant amplitude oscillations in the roll dynamics, during the flight at high angles of attack (AOAs) and endangers carrying out the mission of an aircraft. In this paper, a composite adaptive posicast controller is designed for the wing rock phenomenon in a delta-wing aircraft with known input delay. The existence of the input delay besides the parametric uncertainties of the system dynamics adds to the complexity of the problem and can cause undesirable troubles in regulation and tracking performance or instability in the control system. Consequently, there is a need for a controller that can provide the stability and desirable regulation and tracking for the system. The proposed control method uses the system state forecasting and the composite model reference adaptive controller in an integrated control structure based on linear quadratic regulator (LQR). Combining the tracking error and the prediction error to form the adaptive laws in the composite model reference adaptive controller improves the characteristics of the system response and provides a better performance compared to the model reference adaptive controller in which the adaptive laws are formed only with the tracking error. Simulation results show the efficiency of the composite adaptive posicast controller in counteracting the system uncertainties in the presence of considerably large input delay cases.

Metapopulation oscillations from satiation of predators

We develop a mathematical model of extinction and coexistence in a generic predator–prey ecosystem composed of two herbivores in asymmetrical competition and a predator of both. With the aim of representing the satiety of predators when preys are overabundant, we introduce for the predation behavior a dependence on prey abundance. Specifically, predation is modeled as growing proportionally to the presence of herbivores at low densities, and saturating when the total metapopulation of prey is sufficiently large. The model predicts the existence of different regimes depending on the parameters considered: survival of a single species, coexistence of two species and extinction of the third one, and coexistence of the three species. But more interestingly, in some regions of parameters space the solutions oscillate in time, both as a transient phenomenon and as persistent oscillations of constant amplitude. The phenomenon is not present for the more idealized linear predation model, suggesting that it can be the source of real ecosystems oscillations.

Experimental investigation of the particle oscillation instability in a single-axis acoustic levitator

Single-axis acoustic levitators are employed in biomedicine, chemistry and physics experiments due to their ability to trap in mid-air objects of a wide range of materials and sizes. Although this type of levitator has been studied for decades, there are effects that are not well understood. One of these effects is the particle oscillation instability, in which the levitating particle starts to oscillate with increasing amplitude until it is ejected out of the levitator. Most of the operations performed with acoustic levitation require high accuracy regarding the positioning of the particle, thus a lack of stability severely hinders the experiments. In this paper, we present an experimental setup that consists of a single-axis levitator, a mechanized stage to control the separation between the emitter and the reflector, a scale to measure the radiation force and a high-speed camera. We experimentally investigate the effect of the distance between the emitter and the reflector on the apparatus resonant frequency and on levitation stability. In accordance with previous theoretical studies, three types of levitation behavior were experimentally identified: stable levitation, oscillation of constant amplitude and unstable oscillation. We also show that the type of levitation behavior can be controlled by changing the distance between the emitter and the reflector.

Waves of desertification in a competitive ecosystem

We study a mathematical model inspired by a common scenario in northern Patagonia consisting of humans and their livestock living together with native wildlife. The main production system in this area is sheep farming, which due to wrong historical management has led to desertification of the habitat, with impact on both native wildlife and livestock. Particularly the largest native herbivore, the guanaco, has reflected in their numbers this alteration induced in the environment. We analyze a mathematical model that captures the main characteristics of the interaction between sheep and guanaco: the hierarchical competition and the advantage granted by humans to the herds, and also incorporates a dynamic for the habitat. Using the metapopulation formalism, the trophic web of two herbivores is extended over a patchy landscape that considers two characteristic times for the dynamics of the resource. Our study stresses the dependence of the metapopulations dynamics on the recovery time of the resource. These results are backed up by a deterministic mean field model which shares some similarities with the stochastic and spatially extended one. We find different regimes depending on the parameters considered: coexistence of both species, survival of a single species and extinction of the other, and extinction of both. Remarkably, in some regions of parameters space we detect the presence of periodical spatio-temporal patterns, with persistent oscillations of constant amplitude. Based on these results, we perform a characterization of the observed scenarios in order to gain insight about the system.

The qPlus sensor, a powerful core for the atomic force microscope.

Atomic force microscopy (AFM) was introduced in 1986 and has since made its way into surface science, nanoscience, chemistry, biology, and material science as an imaging and manipulating tool with a rising number of applications. AFM can be employed in ambient and liquid environments as well as in vacuum and at low and ultralow temperatures. The technique is an offspring of scanning tunneling microscopy (STM), where the tunneling tip of the STM is replaced by using a force sensor with an attached tip. Measuring the tiny chemical forces that act between the tip and the sample is more difficult than measuring the tunneling current in STM. Therefore, even 30 years after the introduction of AFM, progress in instrumentation is substantial. Here, we focus on the core of the AFM, the force sensor with its tip and detection mechanism. Initially, force sensors were mainly micro-machined silicon cantilevers, mainly using optical methods to detect their deflection. The qPlus sensor, originally based on a quartz tuning fork and now custom built from quartz, is self-sensing by utilizing the piezoelectricity of quartz. The qPlus sensor allows us to perform STM and AFM in parallel, and the spatial resolution of its AFM channel has reached the subatomic level, exceeding the resolution of STM. Frequency modulation AFM (FM-AFM), where the frequency of an oscillating cantilever is altered by the gradient of the force that acts between the tip and the sample, has emerged over the years as the method that provides atomic and subatomic spatial resolution as well as force spectroscopy with sub-piconewton sensitivity. FM-AFM is precise; because of all physical observables, time and frequency can be measured by far with the greatest accuracy. By design, FM-AFM clearly separates conservative and dissipative interactions where conservative forces induce a frequency shift and dissipative interactions alter the power needed to maintain a constant oscillation amplitude of the cantilever. As it operates in a noncontact mode, it enables simultaneous AFM and STM measurements. The frequency stability of quartz and the small oscillation amplitudes that are possible with stiff quartz sensors optimize the signal to noise ratio. Here, we discuss the operating principles, the assembly of qPlus sensors, amplifiers, limiting factors, and applications. Applications encompass unprecedented subatomic spatial resolution, the measurement of forces that act in atomic manipulation, imaging and spectroscopy of spin-dependent forces, and atomic resolution of organic molecules, graphite, graphene, and oxides.

Commentary: Intentional Observer Effects on Quantum Randomness: A Bayesian Analysis Reveals Evidence Against Micro-Psychokinesis

In the article [Maier2018] the authors report on a mind-matter experiment comprising an impressive 12.571 subjects in a Micro-Psychokinesis (Micro-PK) task. The main result of that study, testing the aggregate sum of the data against its expectation value, is reported as strong evidence against Micro-PK. Despite this conclusion, the authors move ahead and interpret a post-hoc observed pattern in their data as possible evidence for Micro-PK, albeit of a different kind. The authors put forward the hypothesis that a higher frequency of slow data variations can be observed in their experiment data than in a single set of control data. They propose that Micro-PK-Effects show up not in the aggregate sum, but in fluctuations (i.e. in the time sequence) of their data: “Interestingly, there seems to be a pattern of repeated change.” The authors connect this possible observation with theories of von Lucadou and others (e.g.[Luc2006,Luc2007,Luc2015]), extended by their own thoughts about possible decline effects in Parapsychology experiments. As a smaller remark, it seems to this author that there is a confusion here about decline of the primary effect size and a decline as observed in a cumulative z-score representation of data, as used by the authors. A constant oscillation of an original effect size always leads to a decline of oscillations in a cumulative z-score plot, as more data goes into the z-score calculation. Likewise, this confusion is also evident in Figure 7 of [Maier2018]. The cumulative z-scores in those figures have a constant oscillation amplitude, which is only possible with an oscillating and exponentially growing effect size in the original data.

Formation of electron holes in the long-time evolution of the bump-on-tail instability

AbstractAn Eulerian Vlasov code is applied for the numerical solution of the one-dimensional Vlasov–Poisson system of equations for electrons, and with ions forming an immobile background. We study the non-linear evolution of the bump-on-tail instability in the case when the system length L is greater than the wavelength λ of the unstable mode, with a beam density of 10% of the total density, nb = 0.1. We follow the growth and the saturation of an initially unstable wave perturbation, and the formation of a traveling Bernstein–Greene–Kruskal (BGK) mode, which evolves out of the instability. This first stage is followed by sidebands growing from round-off errors which develop and disrupt the BGK equilibrium. In the excited spectrum, mode coupling is mediated by the oscillating resonant particles and results in the electric energy of the system flowing to the longest wavelengths (inverse cascade), and reaching in the asymptotic state a steady state with constant amplitude oscillation modulated by the persistent oscillation of the trapped particles. Coherent phase-space electron holes are formed, which are localized phase-space regions of reduced density on trapped electron orbits, where the electron density is lower than the surrounding plasma electron density. The distribution function evolves to a shape with stationary inflection points of zero slope, at the phase velocities of the excited waves. The longest wavelengths show oscillations at frequencies below the plasma frequency, with phase velocities higher than that of the injected beam, which can accelerate electrons to energies in excess of the initial beam energy. The present work makes a connection between the formation of electron holes, the existence of inflection points of zero slopes in the electron distribution function at the phase velocities of the dominant waves, and at frequencies below the plasma frequency. A fine resolution grid is used in the Eulerian Vlasov code in the phase space and time to allow an accurate calculation of the time history of the system and of the dynamic and oscillation of the trapped particles in the low-density regions of the phase space, and of those particles at the separatrix regions of the vortex structures which evolve periodically between trapping and untrapping states and which can only be accurately studied using a fine-resolution phase-space grid.

Analysis of focused indirect ultrasound via high-speed spatially localized pressure sensing and its consequences on nucleation

Ultrasonication is frequently used to promote crystallization and chemical reactions, with many papers reporting the effects of ultrasonic parameters. High-speed pressure inducers (aka pinducers) and signal processing are used here to gain insights into the spatial localization of energy dissipation that occurs during ultrasonication, facilitating a design that allows ultrasonication to be spatially localized inside a tube, without requiring that the probe directly contact the solution. The fluid pressure measured inside of a flexible silicone tube pressed against an ultrasonic probe is observed to move between nearly constant amplitude oscillations to variable shape and amplitude waveforms that deviate significantly from being periodic. Ultrasonic power is transferred to the fluid inside the tube, generating a wide frequency range including via bubble oscillations. Discrete Fourier Transform analysis indicates complex interactions within the experimental system, in addition to the energy transferred inside the tubing at the expected ultrasonic source frequency. The total acoustic intensity decays exponentially with distance from the zone, dropping by more than two orders-of-magnitude for each 1cm increase in distance. The pinducer data analyses guide the design of an experimental setup in which crystals nucleate within fluid inside the zone in the tube, right under the ultrasonic probe.

Predicting and assessing rupture in protein gels under oscillatory shear.

Soft materials may break irreversibly upon applying sufficiently large shear oscillations, a process whose physical mechanism remains largely elusive. In this work, the rupture of protein gels made of sodium caseinate under an oscillatory stress is shown to occur in an abrupt, brittle-like manner. Upon increasing the stress amplitude, the build-up of harmonic modes in the strain response can be rescaled for all gel concentrations. This rescaling yields an empirical criterion to predict the rupture point way before the samples are significantly damaged. "Fatigue" experiments under stress oscillations of constant amplitude can be mapped onto the former results, which indicates that rupture is independent of the temporal pathway in which strain and damage accumulate. Finally, using ultrasonic imaging, we measure the local mechanical properties of the gels before, during and after breakdown, showing that the strain field remains perfectly homogeneous up to rupture but suddenly gives way to a solid-fluid phase separation upon breakdown.

Optimized finite-difference (DRP) schemes perform poorly for decaying or growing oscillations

Computational aeroacoustics often use finite difference schemes optimized to require relatively few points per wavelength; such optimized schemes are often called Dispersion Relation Preserving (DRP). Similar techniques are also used outside aeroacoustics. Here the question is posed: what is the equivalent of points per wavelength for growing or decaying waves, and how well are such waves resolved numerically? Such non-constant-amplitude waves are common in aeroacoustics, such as the exponential decay caused by acoustic linings, the O(1/r) decay of an expanding spherical wave, and the decay of high-azimuthal-order modes in the radial direction towards the centre of a cylindrical duct. It is shown that optimized spatial derivatives perform poorly for waves that are not of constant amplitude, under performing maximal-order schemes. An equivalent criterion to points per wavelength is proposed for non-constant-amplitude oscillations, reducing to the standard definition for constant-amplitude oscillations and valid even for pure growth or decay with no oscillation. Using this definition, coherent statements about points per wavelength necessary for a given accuracy can be made for maximal-order schemes applied to non-constant-amplitude oscillations. These features are illustrated through a numerical example of a one-dimensional wave propagating through a damping region.

Optogenetic Control of Calcium Oscillation Waveform Defines NFAT as an Integrator of Calcium Load

It is known that the calcium-dependent transcription factor NFAT initiates transcription in response to pulsatile loads of calcium signal. However, the relative contributions of calcium oscillation frequency, amplitude, and duty cycle to transcriptional activity remain unclear. Here, we engineer HeLa cells to permit optogenetic control of intracellular calcium concentration using programmable LED arrays. This approach allows us to generate calcium oscillations of constant peak amplitude, in which frequency is varied while holding duty cycle constant, or vice versa. Using this setup and mathematical modeling, we show that NFAT transcriptional activity depends more on duty cycle, defined as the proportion of the integrated calcium concentration over the oscillation period, than on frequency alone. This demonstrates that NFAT acts primarily as a signal integrator of cumulative load rather than a frequency-selective decoder. This approach resolves a fundamental question in calcium encoding and demonstrates the value of optogenetics for isolating individual dynamical components of larger signaling behaviors.

Linear friction welding of dissimilar AA6082 and AA2011 aluminum alloys: microstructural characterization and design guidelines

This paper presents the results of an experimental and numerical campaign on Linear Friction Welding of dissimilar AA2011-T8 and AA6082-T6 aluminum alloys. Experimental tests were carried out with constant oscillation amplitude and process time. Varying oscillation frequency, interface pressure, specimen geometry and mutual position were used. Grain size measurements, HV tests and EDX analysis were considered to characterize the microstructure of the joints as a function of the input process parameters. A thermal numerical model was utilized to predict the temperature profiles in the joints during the process. The obtained results allowed the identification of four weld categories: sound joints, “bonding limit” condition and two different unwelded joints. The investigation of the causes of the different joint behavior permitted to obtain a few design guidelines on the LFW of dissimilar alloys with different geometry.