Decrease In Oscillation Frequency Research Articles

Nonlinear free vibration analysis of delaminated composite circular cylindrical shells

This study aims to present an analysis of nonlinear free vibrations of simply supported laminated composite circular cylindrical shells with throughout circumference delamination. Governing equations of motion are derived by applying energy methods; using Galerkin’s method reduced the nonlinear partial differential equations to a system of coupled nonlinear ordinary differential equations, which are subsequently solved using a numerical method. This research examines the effects of delamination on the oscillatory motion of delaminated composite circular cylindrical shells and then the effects of increase in delamination length, shell middle surface radius, number of layers, and orthotropy as changes in material properties on the nonlinearity of these types of shells. The results show that delamination leads to a decrease in frequency of oscillations and displacement. An increase in delamination length, shell middle surface radius, and orthotropy of layers decreases nonlinearity and displacement, whereas an increase in the number of layers increases nonlinearity and displacement. It is also observed that an increase in the circumferential wave number can decrease the effect of delamination.

Effect of column inclination and oscillation on liquid spreading in a trickle bed

Gas–liquid–solid flow in a trickle bed installed on a floating production unit is affected by oscillations imparted by sea waves. The oscillations may affect the spreading of liquid, and in turn, the performance of a reactor. The present work investigates the effect of bed inclination and two types of oscillations (roll and roll+pitch) on the spreading of liquid in a cylindrical trickle bed with a single point liquid injection by using computational fluid dynamics. In the simulations, column tilt varied in a range of 0–15° and three oscillation frequencies (0.1, 0.2 and 0.5Hz) were considered. Initially, a three-phase Eulerian model was validated using the published experimental data for a stationary column. The effect of closure models such as capillary pressure model and Ergun drag model constants on predictions was analyzed for the selection of these closures. In a tilted column, liquid was pushed to flow in the direction of tilt. Increase in tilt angle from 0 to 15° resulted in increase in the flow segregation and ∼20% reduction in pressure drop. Oscillations (both roll and roll+pitch) resulted in more uniform liquid spreading and higher liquid holdup compared to a vertical column. With decrease in oscillation frequency, the index of uniformity in the radial liquid distribution further increased. The higher uniformity and liquid holdup caused by oscillations can be attributed to higher radial velocity of liquid due to the effect of gravity in the radial direction. Compared to the roll type oscillations, the roll+pitch type resulted in higher radial velocity and radial distribution of liquid.

Periodic Oscillation of Liquid Helium Boiling in a Narrow Rectangular Duct

Temperature fluctuations in saturated superfluid He II (at 1.9 K) were measured using small thermistors operated by a lock-in amplifier. The experiments were performed in a rectangular channel ( $$13\times 13\times 200~\hbox {mm}$$ ) with a heater set at the bottom. The temperature oscillated periodically, and a pulsive boiling sound appeared. The two become synchronized, and the oscillating frequencies strongly depend on the helium pressure and the heat flux. When the He pressure decreases, the oscillation frequency decreases. A simple model to predict the cyclic period is presented.

Development of a method for obtaining a section of reduced stiffness on the elastic characteristic of a vehicle tire

This article is devoted to the development of a method for obtaining a section of reduced stiffness on the elastic characteristic of a pneumatic tire of a wheeled vehicle. A way is proposed to reduce the radial stiffness of a tire by obtaining a section with reduced stiffness on the elastic characteristic of pneumatic tires, which, unlike the known ways, can significantly reduce the radial stiffness of a pneumatic tire with-out increasing its static radial deformation and rolling losses. The essence of the method for obtaining a section with reduced stiffness is to reduce the air pressure in the pneumatic tire while increasing its radial deformation using original internal devices located inside the tire. An algorithm has been developed that includes four stages; the elastic characteristic of the pneumatic tire 8.40-15 with a section with reduced rigidity by 4 times is obtained. This ensures a decrease in the natural frequency of vertical oscillations of the sprung mass on the bus by almost 2 times from 3,5 to 1,8 Hz. On the basis of the developed method, the dependences of the length of the reduced stiffness section and the force change in this section on the natural frequency of the sprung masses on the tire realized in this section are determined. For the tire 8.40-15, the dependence of the change in force in the section with a given reduced stiffness on its length was determined, which showed that with increasing length of this section there is a progressive change in the load on the tire in the same section. In addition, the dependences of the pressure change in the tire on its radial deformation in areas of reduced stiffness were obtained for various given frequencies of natural oscillations of the mass of the load on the tire. Structural diagrams of vehicle wheels are presented, in which the tire contains internal structural elements that implement the proposed method for reducing the radial stiffness of a vehicle tire and, in addition, possessing internal hydraulic or air damping of the vertical vibrations of the wheel.

Objective assessment of stiffness in Achilles tendinopathy: a novel approach using the MyotonPRO

ObjectivesThe aim of this study was to establish quantitative values for asymptomatic and symptomatic Achilles tendons.DesignCohort study with a single (cross-sectional) time point of patients diagnosed with unilateral Achilles tendinopathy...

Mode coupling due to the non-uniformly distributed heat release in combustion instabilities

Combustion instabilities in a chamber with circumferentially non-uniform heat release is investigated. A low-order model is developed to predict the combustion instabilities under the effects of a non-uniform heat source. A good agreement was observed between the results of the present model and the previous linear Euler equation method, which indicates that the present model can be used for the prediction of the combustion instabilities with the non-uniform heat source. The analysis shows that mode coupling can be brought by the non-uniformly distributed heat release in azimuthal direction. Results show that the oscillation frequency is less influenced by mode coupling. The growth rate, however, can be significantly affected by the non-uniformity of the heat release. For the instabilities of longitudinal-dominant mode, azimuthal acoustic modes are excited and coupled with the longitudinal mode. If the distribution type of the heat source is sin(jθ), the jth azimuthal mode are the strongest one, while other azimuthal modes are weak. Moreover, with the increase of the duct length, the oscillation frequency decreases and get close to the cutoff value of the azimuthal modes. Mode coupling becomes weak for this condition and the effects of the non-uniformity of the heat source on the combustion instabilities are also reduced.

Numerical simulation of Rayleigh-Bénard convection of nanofluids in rectangular cavities

In order to understand the characteristics of Rayleigh-Benard convection of nanofluids in a rectangular cavity, a series of threedimensional numerical simulations were carried out by using the finite volume method. The working mediums were Al2O3-water and CuO-water nanofluids, and Rayleigh number varied from 1708 to 1.5×105. Results show that the critical Rayleigh number of the Rayleigh-Benard convection onset increases with the increase of the volume fraction of nanoparticles, which hints that nanoparticles make the flow become more stable. Furthermore, the critical Rayleigh number decreases with the increase of the aspect ratio of the cavity. After convection appears, the average Nusselt number at the bottom wall increases, but the flow intensity and the oscillation frequency decrease with the increase of the volume fraction of nanoparticles. With the increase of Rayleigh number, there exist four bifurcation sequences, which are Steady flow (SF) → Periodic oscillatory flow (POF) → Chaotic flow (CF), SF → POF → SF → POF →CF, SF → POF → Multiple periodic oscillatory flow (MPOF) → POF →CF and SF → POF → MPOF →CF.

Sensitivity of hydrothermal wave instability of Marangoni convection to the interfacial heat transfer in long liquid bridges of high Prandtl number fluids

This paper reports the sensitivity of hydrothermal wave (HTW) instability of Marangoni convection to the interfacial heat transfer in liquid bridges (LBs) of high Prandtl number fluids (Pr = 67, 112, and 207) formed under the microgravity environment on the International Space Station. The data for instability are collected for a wide range of AR and for TC = 15 and 20 °C, where AR is the aspect ratio (=height/diameter) of the LB and TC is the cooled disk temperature. A significant decrease in critical oscillation frequency as well as an appreciable decrease in the critical Marangoni number is observed for AR > 1.25. This drastic change of instability mechanisms is associated with the reversal of axial traveling direction of HTWs and roll-structures as reported previously. It is found that this reversal is closely related to the interfacial heat transfer, which is evaluated numerically through accounting for both convective and radiative components. A heat transfer ratio, QI/QH, is introduced as a dimensionless parameter for interfacial heat transfer, where QI and QH are the heat transfer rates at the LB-gas and LB-heated disk interfaces, respectively. It is found that HTWs travel in the same direction as the surface flow for QI/QH > 0 (heat-loss condition) while in the opposite direction for QI/QH < 0 (heat-gain condition) in the present microgravity experiment. It is shown that the heat-transfer condition alters slightly but appreciably the basic temperature and flow field, the alteration that is not accounted for in the previous linear stability analyses for an infinite LB.

Residence time distribution and Peclet number correlation for continuous oscillatory flow reactors

AbstractBACKGROUNDOscillatory baffled reactors have good prospect for use in batch to continuous transition for many industries. For reliable design and scale up it is essential to mathematically describe mixing and flow pattern of oscillatory flow. The aim of this study is to investigate the effects of operating conditions on residence time distribution (RTD) of liquid phase in a continuous oscillatory baffled reactor (COBR) and to propose a correlation that accurately describes the flow pattern.RESULTSAnalysis of results obtained for a wide variety of operating conditions in two geometrically similar reactors show that increase of oscillation frequency and amplitude both decrease axial dispersion, with a more prominent effect of amplitude. Proposed power‐law correlation is in very good agreement with all performed experiments. Peclet numbers predicted by this correlation are in reasonably good agreement with the literature experimental data and is much better than the ones obtained by the commonly used correlation, which contains more fitting parameters.CONCLUSIONResults show that relatively high amplitudes, low frequencies and low to moderate secondary non‐oscillatory flow should be used to ensure effective mixing with a low net flow in COBR. Developed correlation offers a simple, yet reliable way to predict the macroscopic flow pattern of liquid in COBR. © 2017 Society of Chemical Industry

Thermocapillary convection in a differentially heated two-layer annular system with and without rotation

Thermocapillary convection in a differentially heated bilayer annular pool consisting of 5cS silicone oil and HT-70 is investigated by a series of three-dimensional numerical simulations. Both systems with and without rotation are considered. The results indicate that the thermocapillary convection is steady and axisymmetric when the Marangoni number is small. The system rotation suppresses the radial flow in the lower layer. Once the Marangoni number exceeds a threshold value, three-dimensional oscillatory flow occurs in both layers simultaneously. The stability diagram reveals that weak rotation destabilizes the axisymmetric flow, while stronger rotation retards the onset of oscillatory flow. The wave patterns for oscillatory flow appear in the form of blade-like waves propagating outward. The surface oscillation and the interface oscillation always share a common frequency but have about a half-period phase lag. In the rotating pool, single group of curved hydrothermal waves propagates in the azimuthal direction opposite to the pool rotation. As the rotation rate increases, the wave number increases while the oscillation frequency decreases.

Significant reductions in human visual gamma frequency by the gaba reuptake inhibitor tiagabine revealed by robust peak frequency estimation.

The frequency of visual gamma oscillations is determined by both the neuronal excitation–inhibition balance and the time constants of GABAergic processes. The gamma peak frequency has been linked to sensory processing, cognitive function, cortical structure, and may have a genetic contribution. To disentangle the intricate relationship among these factors, accurate and reliable estimates of peak frequency are required. Here, a bootstrapping approach that provides estimates of peak frequency reliability, thereby increasing the robustness of the inferences made on this parameter was developed. The method using both simulated data and real data from two previous pharmacological MEG studies of visual gamma with alcohol and tiagabine was validated. In particular, the study by Muthukumaraswamy et al. [2013a] (Neuropsychopharmacology 38(6):1105–1112), in which GABAergic enhancement by tiagabine had previously demonstrated a null effect on visual gamma oscillations, contrasting with strong evidence from both animal models and very recent human studies was re‐evaluated. After improved peak frequency estimation and additional exclusion of unreliably measured data, it was found that the GABA reuptake inhibitor tiagabine did produce, as predicted, a marked decrease in visual gamma oscillation frequency. This result demonstrates the potential impact of objective approaches to data quality control, and provides additional translational evidence for the mechanisms of GABAergic transmission generating gamma oscillations in humans. Hum Brain Mapp 37:3882–3896, 2016. © 2016 Wiley Periodicals, Inc.

The sympathy of two pendulum clocks: beyond Huygens’ observations

This paper introduces a modern version of the classical Huygens’ experiment on synchronization of pendulum clocks. The version presented here consists of two monumental pendulum clocks—ad hoc designed and fabricated—which are coupled through a wooden structure. It is demonstrated that the coupled clocks exhibit ‘sympathetic’ motion, i.e. the pendula of the clocks oscillate in consonance and in the same direction. Interestingly, when the clocks are synchronized, the common oscillation frequency decreases, i.e. the clocks become slow and inaccurate. In order to rigorously explain these findings, a mathematical model for the coupled clocks is obtained by using well-established physical and mechanical laws and likewise, a theoretical analysis is conducted. Ultimately, the sympathy of two monumental pendulum clocks, interacting via a flexible coupling structure, is experimentally, numerically, and analytically demonstrated.

Modeling and validation of auto-oscillation onset in a constricted tube with application to phonation

An experimental study of the auto-oscillation onset is performed for airflow in a rigid tube containing a constriction downstream from a deformable portion as a function of the constriction degree. A quasi-one dimensional laminar flow model in combination with a reduced order mechanical model (symmetric two mass model) provides a physical fluid–structure interaction model of the deformable portion. Mechanical, geometrical and flow model parameters are chosen to match the experimental setup. Modeled as well as experimental results show that a severe constriction (>80%) at first hinders (≥89%) and eventually inhibits (≥95%) auto-oscillation. Constrictions of different severity occur naturally in voiced speech sound production (phonation) due to articulation. The current study provides quantitative evidence of the role of the vocal tract constriction degree as a control parameter for phonation (voiced speech sound production) since increasing the constriction degree decreases the vocal folds oscillation frequency (decrease by 25%) and increases the minimum pressure needed to initiate oscillation (increase by 80%).

Detection of bifurcations in noisy coupled systems from multiple time series.

We generalize a method of detecting an approaching bifurcation in a time series of a noisy system from the special case of one dynamical variable to multiple dynamical variables. For a system described by a stochastic differential equation consisting of an autonomous deterministic part with one dynamical variable and an additive white noise term, small perturbations away from the system's fixed point will decay slower the closer the system is to a bifurcation. This phenomenon is known as critical slowing down and all such systems exhibit this decay-type behaviour. However, when the deterministic part has multiple coupled dynamical variables, the possible dynamics can be much richer, exhibiting oscillatory and chaotic behaviour. In our generalization to the multi-variable case, we find additional indicators to decay rate, such as frequency of oscillation. In the case of approaching a homoclinic bifurcation, there is no change in decay rate but there is a decrease in frequency of oscillations. The expanded method therefore adds extra tools to help detect and classify approaching bifurcations given multiple time series, where the underlying dynamics are not fully known. Our generalisation also allows bifurcation detection to be applied spatially if one treats each spatial location as a new dynamical variable. One may then determine the unstable spatial mode(s). This is also something that has not been possible with the single variable method. The method is applicable to any set of time series regardless of its origin, but may be particularly useful when anticipating abrupt changes in the multi-dimensional climate system.

Optimization of Abrupt Profile of Germanium in Si/SiGe Heterojunction Bipolar Transistor Specified for Radio Frequency Range Systems

In this paper, we present modeling of Heterojunction Bipolar Transistors (HBT) based on Si / SiGeC very high performance for telecommunications applications dedicated to radio frequency RF. The major objective of our work will be devoted to the influence of the germanium profile on the static and dynamic characteristics of the latter. The optimization of the profile of germanium in the base is a major element in the study of electrical characteristics of the HBT. This optimisation improves the static gain (β) of the transistor, while increasing the germanium profile in the base of 10% to 30%. However, the transition frequency (fT), and the maximum oscillation frequency (fmax) decrease when the profile of germanium is greater than 20%. A numerical modelling approach is investigated using our 2D simulator SIBIDIF, which is based on the Drift-Diffusion Model (DDM). This method solves the continuity equations for electrons and holes and is coupled with the Poissons equation based on the concept of the finite difference mesh.

Spikelets and bursts in axonless retinal AII amacrine cells coupled by gap junctions

AII amacrine cells play a central role in the processing of visual signals in the retina. They do not have axons but possess elaborate, ramified dendrites, through which they receive low-light input from rod bipolar cells. These dendrites also serve as output structures: chemical synapses inhibit the OFF cone-pathway and gap junctions provide input into the ON cone-pathway. Gap junctions also directly couple the AIIs to one another. In slice experiments, the AIIs are found to generate surprisingly small action potentials and can exhibit persistent bursting. In mouse rd1-mutants, which serve as a model for retinal degeneration, the death of the photoreceptors is followed by the appearance of strong rhythmicity in the spiking activity of the ganglion cells, which provide the output of the retina. Using computational modeling we show that the small size of the action potentials, as observed in the soma, is due to the ramified morphology of the AII (Figure ​(Figure1A).1A). In fact, the modeling reveals that the spikes must originate in a single compartment that is electrotonically distant from the soma. Our modeling and subsequent electrophysiology experiments show that the bursting is due to a slow current, most likely an M-type K-conductance [1]. Figure 1 A Confocal image of an AII. Putative spike initiation site is marked by arrow. B Dependence of the burst frequency on M-current kinetics (half-activation voltage and deactivation time). Experimentally, it has been found that the oscillations in rd1-mutant mice are abolished by the gap-junction blocker MFA [2]. Our model suggests that the elimination of the oscillations is not due to the blocking of the gap junctions but rather by the known effects of MFA on the kinetics of the M-type K-current: MFA shifts the activation curve of the M-current to more negative potentials and slows down its deactivation [3]. Consistent with experiments [2] our model shows a decrease in oscillation frequency before the oscillations are abolished with increasing MFA concentration (Figure ​(Figure1B).1B). Moreover, it generates tonic firing if the AIIs are depolarized in the presence of MFA, as has been observed experimentally [4].

Phase velocities of gravity waves in the solar photosphere

Space-time variations of pressure in the solar photosphere are reproduced based on the results of observations in the Fe I line. Local internal gravity waves (IGWs) are isolated by means of proper filtering. A method of determination of the phase velocities of IGWs based on 1D observations is developed. Horizontal and vertical projections of the phase velocities of isolated IGWs with different periods are determined. It is shown that the phase velocity of an IGW decreases significantly with a decrease in oscillation frequency. The horizontal wavelengths of gravity waves with periods ranging from 5 to 60 minutes are commensurable with the granulation scales. The dispersive properties of gravity waves are studied.

Non-minimal Coupling Scalar Field Quasinormal Modes of Schwarzschild-de Sitter Black Hole with a Global Monopole

We use WKB approach and the finite difference method to research non-minimal coupling scalar field quasinormal modes of Schwarzschild-de Sitter black hole with a global monopole. The results show that the small positive constant η makes the scalar field decreasing, and the quasinormal modes of the real part increases but the imaginary parts decreases with the decreasing of the breaking scale ρ. While, when the horizon rh increase, the actual oscillation frequency decreases, while the decay time scale increases. On the other hand, when the horizon rh increases, the oscillation is difficult to stabilize.

Oscillatory dynamics of vasoconstriction and vasodilation identified by time-localized phase coherence

We apply wavelet-based time-localized phase coherence to investigate the relationship between blood flow and skin temperature, and between blood flow and instantaneous heart rate (IHR), during vasoconstriction and vasodilation provoked by local cooling or heating of the skin. A temperature-controlled metal plate (≈10 cm2) placed on the volar side of the left arm was used to provide the heating and cooling. Beneath the plate, the blood flow was measured by laser Doppler flowmetry and the adjacent skin temperature by a thermistor. Two 1 h datasets were collected from each of the ten subjects. In each case a 30 min basal recording was followed by a step change in plate temperature, to either 24 °C or 42 °C. The IHR was derived from simultaneously recorded ECG. We confirm the changes in the energy and frequency of blood flow oscillations during cooling and heating reported earlier. That is, during cooling, there was a significant decrease in the average frequency of myogenic blood flow oscillations (p < 0.05) and the myogenic spectral peak became more prominent. During heating, there was a significant (p < 0.05) general increase in spectral energy, associated with vasodilation, except in the myogenic interval. Weak phase coherence between temperature and blood flow was observed for unperturbed skin, but it increased in all frequency intervals as a result of heating. It was not significantly affected by cooling. We also show that significant (p < 0.05) phase coherence exists between blood flow and IHR in the respiratory and myogenic frequency intervals. Cooling did not affect this phase coherence in any of the frequency intervals, whereas heating enhanced the phase coherence in the respiratory and myogenic intervals. This can be explained by the reduction in vascular resistance produced by heating, a process where myogenic mechanisms play a key role. We conclude that the mechanisms of vasodilation and vasoconstriction, in response to temperature change, are oscillatory in nature and are independent of central sources of variability.

A living cell quartz crystal microbalance biosensor for continuous monitoring of cytotoxic responses of macrophages to single-walled carbon nanotubes

BackgroundNumerous engineered nanomaterials (ENMs) exist and new ENMs are being developed. A challenge to nanotoxicology and environmental health and safety is evaluating toxicity of ENMs before they become widely utilized. Cellular assays remain the predominant test platform yet these methods are limited by using discrete time endpoints and reliance on organic dyes, vulnerable to interference from ENMs. Label-free, continuous, rapid response systems with biologically meaningful endpoints are needed. We have developed a device to detect and monitor in real time responses of living cells to ENMs. The device, a living cell quartz crystal microbalance biosensor (QCMB), uses macrophages adherent to a quartz crystal. The communal response of macrophages to treatments is monitored continuously as changes in crystal oscillation frequency (Δf). We report the ability of this QCMB to distinguish benign from toxic exposures and reveal unique kinetic information about cellular responses to varying doses of single-walled carbon nanotubes (SWCNTs).ResultsWe analyzed macrophage responses to additions of Zymosan A, polystyrene beads (PBs) (benign substances) or SWCNT (3-150 μg/ml) in the QCMB over 18 hrs. In parallel, toxicity was monitored over 24/48 hrs using conventional viability assays and histological stains to detect apoptosis. In the QCMB, a stable unchanging oscillation frequency occurred when cells alone, Zymosan A alone, PBs alone or SWCNTs without cells at the highest dose alone were used. With living cells in the QCMB, when Zymosan A, PBs or SWCNTs were added, a significant decrease in frequency occurred from 1-6 hrs. For SWCNTs, this Δf was dose-dependent. From 6-18 hrs, benign substances or low dose SWCNT (3-30 μg/ml) treatments showed a reversal of the decrease of oscillation frequency, returning to or exceeding pre-treatment levels. Cell recovery was confirmed in conventional assays. The lag time to see the Δf reversal in QCMB plots was linearly SWCNT-dose dependent. Lastly, the frequency never reversed at high dose SWCNT (100-150 μg/ml), and apoptosis/necrosis was documented in conventional 24 and 48 hr-assays.ConclusionThese data suggest that the new QCMB detects and provides unique information about peak, sub-lethal and toxic exposures of living cells to ENMs before they are detected using conventional cell assays.