Intermittent Switching Research Articles

Sudden changes in chaotic attractors and transient basins in a model for rattling in gearboxes

We consider a model for rattling in single-stage gearbox systems with some backlash consisting of two wheels with a sinusoidal driving. The equations of motions are analytically integrated between two impacts of the gear teeth. Just after each impact, a mapping is used to obtain the dynamical variables. We have observed a rich dynamical behavior in such system, by varying its control parameters, and we focus on intermittent switching between laminar oscillations and chaotic bursting, as well as crises, which are sudden changes in the chaotic behavior. The corresponding transient basins in phase space are found to be riddled-like, with a highly interwoven fractal structure.

HIV in IDUs: should we bother?

HIV and drug use continue to pose challenges in tandem*/like a bicycle with seats for two, one riding behind the other*/to the practitioners of public health as well as intervention researchers worldwide. For example, while cities in Australia have recorded a low HIV prevalence in injection drug users (IDUs) (Wodak and Lurie, 1997) and New York has reported declines in HIV incidence among IDUs (Des Jarlais et al., 2000), Kathmandu*/the capital city of Nepal*/has experienced initial containment followed by explosive spread of HIV among IDUs in the last 6 years (Pokharel et al., 2000). These challenges are made tougher by a few hurdles, such as the assumption that the HIV epidemic in heterosexuals is independent from that in IDUs and the intermittent switch from non-injection forms of drug use to injecting under various socio-political influences. Many national HIV/AIDS Control Programmes in developing countries also add to these challenges by having most of their resources spent on breaking the chain of transmission in the population groups who are vulnerable to getting HIV through sex and by having abysmally low support for interventions targeting injection drug users. The mainstay of this approach is formed by the belief that HIV epidemics in IDUs do not contribute significantly to the overall HIV epidemic. Evidence, however, suggests the contrary. Lowndes et al. (2003) identify an interplay of forces which contribute towards conditions favourable for the heterosexual spread of HIV in the Russian Federation, which since 1996 has witnessed a series of large and explosive HIV epidemics among IDUs in different cities. Such a rapid spread of HIV among IDUs reminds us of the HIV epidemic among IDUs in Manipur, a northeastern state of India sharing a border with Myanmar. This response looks back to the past when the epidemic happened in Manipur, as well as taking account of current research evidence to examine critically whether one really needs to bother about HIV epidemics in injectors. Twelve years ago, in the winter of 1989, an outbreak of HIV occurred among a large number of injection heroin users of Manipur (Sarkar et al., 1991). The virus not only infected more than 50% of the local IDUs within 9 months, but it spread into other population groups as well. Some 45% of the non-injecting wives of IDUs (Panda et al., 2000) and 1.2% of the antenatal clinic-attending women (representing female general population) in the state (NACO 1997 /1998) contracted the virus within the next 5 years. A small study from Manipur, conducted in 1997, recruited female sex workers operating from the streets of Imphal, the capital city (there is no brothel-based sex industry in Manipur) and revealed that while 20% of those who had never injected drugs were HIV positive, 57% of female injection drug users involved in sex work had HIV (Panda et al., 2001). About one-third of the female IDUs involved in sex work in this study had male IDUs as their regular sex partners. Lowndes et al. (2003) document a similar progression of HIV among different population groups in the Russian Federation, highlighting different links between IDU and non-IDU populations that may be responsible for generating a wider spread of HIV in countries where epidemic outbreaks first emerged among IDUs. While it took some time for policymakers to see how different pieces of information relating to the progression of HIV in Manipur were parts in one jigsaw puzzle, an early attempt to do so in Russia and elsewhere would certainly pay dividends for HIV prevention. Otherwise, a familiar story of the diffusion of injection drug use in a new area followed by rapid HIV spread among IDUs and subsequent spread to non-IDU population groups may repeat itself. The evidence generated through the randomised controlled trials in African countries on the role of improved STD management in reducing HIV incidence in the early phase of HIV epidemics (Grosskurth et al., 2000) emphasises how STIs could play a * Tel.: /91-332-5910-334; fax: /91-332-3376-290. E-mail address: samiran_panda@rediffmail.com (S. Panda). International Journal of Drug Policy 14 (2003) 83 /85

Noisy heteroclinic networks

The influence of small noise on the dynamics of heteroclinic networks is studied, with a particular focus on noise-induced switching between cycles in the network. Three different types of switching are found, depending on the details of the underlying deterministic dynamics: random switching between the heteroclinic cycles determined by the linear dynamics near one of the saddle points, noise induced stability of a cycle, and intermittent switching between cycles. All three responses are explained by examining the size of the stable and unstable eigenvalues at the equilibria.

Contrast enhanced ultrasound in the liver using low mechanical index--what is the difference?

Echo-enhancing agents (EEA), also known as ultrasound contrast media, have improved the diagnostic potential of gastroenterologic ultrasound within the last years. Initially used to increase color-Doppler signals the potential to use harmonic waves being generated by the use of EEAs was established later. The effect was based upon the distraction of the EEA bubbles and the resulting pseudo-Doppler signal, the exam was carried out with high ultrasound energy and intermittent switching on and off of the ultrasound beam. Using this effect in real-time during continuous scanning is possible since the introduction of new EEAs with a flexible shell. This flexible shell oscillates during the scan process emitting harmonic signals. This new technology opens new possibilities for the detection of liver tumors including the possible characterisation. The detection rate for liver metastases can be increased by app. 40 % with this new technology. Correct sonographic characterisation of liver tumors can be achieved in nearly 100 % more cases.

Observations of hysteresis in flow around two square cylinders in a tandem arrangement

This paper reports an experimental study on the flow characteristics around two square cylinders in a tandem arrangement. The spacing between the centers of the cylinders ranges from 1.5 to 9 widths of the cylinder and the Reynolds number ranges from 2.0×10 3 to 1.6×10 4. Hysteresis with two discontinuous jumps is present for all the Reynolds numbers studied when the spacing is varied in two different ways, one being a progressive increase and the other a progressive decrease. The hysteresis is associated with two different flow patterns, and the discontinuity is attributed to a sudden change of the flow pattern. In the hysteresis regime, only one flow pattern is stable, differing from the bistable phenomenon that involves intermittent switching between two stable flow patterns as reported for higher Reynolds numbers by previous authors. It is found that the hysteresis regime shifts rapidly towards smaller values of spacing as the Reynolds number is increased in the lower range Re=2.0×10 3–5.3×10 3, and then remains nearly unchanged between Re=5.3×10 3–1.6×10 4. The time history and spectra of velocity as well as the flow visualization corresponding to the change of flow pattern in the hysteresis regime are presented and discussed in detail. The effects caused by hysteresis and Reynolds number on drag forces, mean and fluctuating pressures, and Strouhal numbers of the cylinders are examined.

Time-dependent extinction rate and species abundance in a tangled-nature model of biological evolution

We present a model of evolutionary ecology consisting of a web of interacting individuals, a tangle-nature model. The reproduction rate of individuals characterized by their genome depends on the composition of the population in genotype space. Ecological features such as the taxonomy and the macroevolutionary mode of the dynamics are emergent properties. The macrodynamics exhibit intermittent two-mode switching with a gradually decreasing extinction rate. The generated ecologies become gradually better adapted as well as more complex in a collective sense. The form of the species abundance curve compares well with observed functional forms. The model's error threshold can be understood in terms of the characteristics of the two dynamical modes of the system.

EFFECTS OF PHYSICAL ACTIVITY OR EXERCISE ON CARDIOVASCULAR PARAMETERS AND OXIDATIVE STRESS IN RATS

The purpose of this study was to determine if access to physical activity or exercise affects select cardiovascular parameters and oxidative stress biomarkers. Male Sprague-Dawley rats were separated into three groups (n = 24): having no access to physical activity outside of the cage, having access to twice weekly physical activity in a large box, and having regular exercise on a running wheel. Resting blood pressure and heart rate were monitored weekly. At week 20 of the study, blood samples were collected from a total of thirty-six rats, 12 from each group. Glutathione (GSH), oxidized glutathione (GSSG), triglycerides (TG), high density lipoproteins (HDL), and low density lipoproteins (LDL) were measured in whole blood; oxygen radical absorbance capacity (ORAC) was measured in plasma samples. ORAC did not differ among the 3 groups. GSH was lower in the no-physical activity group (p < 0.05) compared to other groups. GSSG:GSH was similar in all groups. TG and LDL were lowest in the exercise (98.4 +11, 19.5+2.1 mgòdl-1) compared to no-physical activity (139+12.4, 27.8+2.5òdl-1) and physical activity (151.3+9.2, 30.3+1.8òdl-1) groups (p < .05). HDL was unexpectedly lower in the exercise group. Over time, SBP declined and was lowest in the exercise group (week 13:SBP = 129.1 mmHg, MAP = 103.8 mmHg, DBP= 90.4 mmHg, p < .05), followed by intermittent switching between the physical activity and no-physical activity and no-physical activity groups, with mean SBP > 140 mmHg. This data shows that running exercise every other day reduces blood pressure. Regular wheel running exercise was associated with healthier levels of blood lipids in comparison to either animals with access twice- weekly physical activity or no regular physical activity. Finally, regular running exercise was associated with increased GSH, implying greater antioxidant protection. However, there was no alteration in antioxidant capacity and no change in redox balance from either physical activity or regular exercise.

Intermittent Switching between Potential Flow and Turbulence in Superfluid Helium at mK Temperatures

Superfluid flow around an oscillating microsphere is investigated at temperatures down to 25 mK. Stable laminar flow below a critical velocity and turbulence at large drives are found to be separated below 0.5 K by an intermediate range of driving forces where the flow is unstable, intermittently switching between laminar and turbulent phases. We have recorded time series of this switching phenomenon and have made a statistical analysis of the switching probability. The mean lifetime of the turbulent phases grows with increasing drive and becomes infinite at a critical value. Stability of the laminar phases above the critical velocity is limited by natural background radioactivity or cosmic rays.

Low-frequency fluctuations in external cavity semiconductor lasers: understanding based on a simple dynamical model

We investigate the dynamical origin of low-frequency fluctuations (LFFs) in external cavity semiconductor lasers by utilizing a simplified, three-dimensional model derived from the infinite-dimensional Lang-Kobayashi (LK) equations. The simplified model preserves the dynamical properties of the external-cavity modes (ECMs) and antimodes which play a fundamental role in the generation of LFFs. This model yields a clear picture of the dynamical origin of the LFFs. Two distinct regions are observed in parameter space, one with the presence of the maximum-gain mode (MGM), and another without it. In particular, we show that, in the absence of noise, LFFs are a consequence of the dynamical interactions among different ECMs and antimodes. When a small amount of noise is present, LFFs result from an intermittent switching of trajectories among different coexisting attractors in the phase space. The presence of double peaks in the distribution of power dropout times, which has been observed recently in experiments, is explained, and a scaling relation is obtained between the average switching time and the noise strength.

The oscillating magnetic microsphere: a tool for investigating vorticity in superconductors and superfluids

A spherical magnet (radius 0.1 mm) is levitating between the superconducting electrodes of a parallel plate capacitor. Small vertical oscillations of the magnet about its equilibrium position can be excited. From the damping of the oscillations (in vacuum) dissipative processes due to flux motion in the superconductors have been measured. For YBCO thin-film electrodes at 4K Q-values are 10 6 implying a surface impedance of the order of 10 −13 Ω . At larger amplitudes nonlinear flux dynamics due to current-dependent pinning energies is observed. For Nb electrodes the Q-values exceed 10 6. When the capacitor is filled with superfluid helium laminar and turbulent drag of the oscillating sphere can be studied. Below 0.5 K we find a range of intermediate driving forces where intermittent switching between potential flow and turbulence occurs. A statistical analysis of the data yields the probability distribution for either direction.

Shell structures with "magic numbers" of spheres in a swirled dish.

Molecular dynamic simulations of a low number N< or = 54 of spheres in a swirled dish yield solid-like shell structures with stable rings. In contrast to known granular media, solidification occurs only at singular values of N: 7, 8, 12, 14, 19, 21, 30, 37, 40. Otherwise, we obtain intermittent switching of particles between rings -- the average switching time scaling exponentially with a control parameter -- or fluid-like disorder. Stable shell structures can be classified by particular geometrical arrangements (one-centered hexagonal, one-centered "quasicircular," three centered, and four centered).

MOTIONS, FORCES AND MODE TRANSITIONS IN VORTEX-INDUCED VIBRATIONS AT LOW MASS-DAMPING

These experiments, involving the transverse oscillations of an elastically mounted rigid cylinder at very low mass and damping, have shown that there exist two distinct types of response in such systems, depending on whether one has a low combined mass-damping parameter (low m*ζ), or a high mass-damping (highm*ζ ). For our low m*ζ, we find three modes of response, which are denoted as an initial amplitude branch, an upper branch and a lower branch. For the classical Feng-type response, at highm*ζ , there exist only two response branches, namely the initial and lower branches. The peak amplitude of these vibrating systems is principally dependent on the mass-damping (m*ζ), whereas the regime of synchronization (measured by the range of velocity U*) is dependent primarily on the mass ratio, m*ζ. At low (m*ζ), the transition between initial and upper response branches involves a hysteresis, which contrasts with the intermittent switching of modes found, using the Hilbert transform, for the transition between upper–lower branches. A 180° jump in phase angle φ is found only when the flow jumps between the upper–lower branches of response. The good collapse of peak-amplitude data, over a wide range of mass ratios (m*=1–20), when plotted against (m*+CA) ζ in the “Griffin” plot, demonstrates that the use of a combined parameter is valid down to at least (m*+CA)ζ ∼0·006. This is two orders of magnitude below the “limit” that had previously been stipulated in the literature, (m*+CA) ζ>0·4. Using the actual oscillating frequency (f) rather than the still-water natural frequency (fN), to form a normalized velocity (U*/f*), also called “true” reduced velocity in recent studies, we find an excellent collapse of data for a set of response amplitude plots, over a wide range of mass ratiosm* . Such a collapse of response plots cannot be predicted a priori, and appears to be the first time such a collapse of data sets has been made in free vibration. The response branches match very well the Williamson–Roshko (Williamson & Roshko 1988) map of vortex wake patterns from forced vibration studies. Visualization of the modes indicates that the initial branch is associated with the 2S mode of vortex formation, while the Lower branch corresponds with the 2P mode. Simultaneous measurements of lift and drag have been made with the displacement, and show a large amplification of maximum, mean and fluctuating forces on the body, which is not unexpected. It is possible to simply estimate the lift force and phase using the displacement amplitude and frequency. This approach is reasonable only for very low m*.

Pressure Fluctuations in Rectangular Cavity Flows

AbstractExperiments are performed to study the unsteadiness of rectangular cavity flows at Mach 0.325, 0.620 and 0.818. Typical characteristics of mean surface pressure distributions show a slight pressure variation near the front face, a local peak surface pressure ahead of the rear corner and a low pressure at immediate downstream of the cavity. Larger peak pressure and pressure variation near the cavity rear face are observed as the length-to-depth ratio increases. Surface pressure fluctuation distribution shows an increase toward the cavity rear face and reaches a peak value. At further downstream locations, the level of surface pressure fluctuation approaches the value of incoming flow. The amplitude of peak surface pressure fluctuation is associated with length-to-depth ratio and reaches the maximum at length-to-depth ratio of 8.60. This is considered due to intermittent switching between open- and closed-cavity flows. Higher moments of surface pressure signal at immediate downstream of the cavity show a similar trend. More negative skewness coefficient and larger deviation of flatness coefficient indicate the presence of more large negative events, which is mainly corresponding to mass removal process (breath-out phase). This unsteady mass flow is more pronounced at higher Mach number.

Global bifurcation structure of chaotic neural networks and its application to traveling salesman problems

This paper studies global bifurcation structure of the chaotic neural networks applied to solve the traveling salesman problem (TSP). The bifurcation analysis clarifies the dynamical basis of the chaotic neuro-dynamics which itinerates a variety of network states associated with possible solutions of TSP and efficiently ‘searches’ for the optimum or near-optimum solutions. By following the detailed merging process of chaotic attractors via crises, we find that the crisis-induced intermittent switches among the ruins of the previous localized chaotic attractors underly the ‘chaotic search’ for TSP solutions. On the basis of the present study, efficiency of the ‘chaotic search’ to optimization problems is discussed and a guideline is provided for tuning the bifurcation parameter value which gives rise to efficient ‘chaotic search’.

Transitions between blocked and zonal flows in a rotating annulus with topography

The mid-latitude atmosphere is dominated by westerly, nearly zonal flow. Occasionally, this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer. Experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the Coriolis force. In the presence of two symmetric ridges at the bottom of the annulus, the resulting flows were nearly zonal at high forcing or blocked at low forcing. Intermittent switching between blocked and zonal patterns occurs because of the jet's interaction with the topography. These results shed further light on previous atmospheric observations and numerical simulations.

Dynamical aspects of chaotic dynamical systems just after band crises

Dynamical aspects of chaotic dynamical systems just after the three band crises are studied by extracting various correlations of dynamical systems. Chaotic systems have both periodic and non-periodic motions of phase points just after the band crises. We decompose the behavior of phase points for dissipative systems into the motions in the three-band attractor and in the repeller. Logistic map and Hénon map are studied as examples of low-dimensional chaotic maps. Driven damped pendulum is taken up as an example of nonlinear ordinary differential equations. Every system exhibits the three-period motion when the local Lyapunov exponent is small and does the non-periodic behavior when the local Lyapunov exponent is large. The generalized power spectrum is used for analyzing the behavior of trajectories. We will analyze the intermittent switching between periodic and non-periodic behaviors by the generalized power spectrum in these dissipative systems. This behavior cannot be found directly only by looking at the thermodynamic functions defined in the frame of the thermodynamic formalism. It is shown that the peculiar behaviors of trajectories in the repeller is clearly extracted.

Solitary-wave conduction in p-type Ge under time-dependent voltage bias.

We present the results of numerical simulations of a drift-diffusion model—including electric-field-dependent generation-recombination processes—for closely compensated p-type Ge at low temperature and under dc+ac and dc+noise voltage biases, with an Ohmic boundary condition. We observe frequency locking and quasiperiodicity under dc+ac bias, but do not find chaotic behavior for a uniform impurity profile. Noise-induced intermittent switching near the onset of solitary-wave conduction is compared to experimentally observed intermittency, type-III intermittency, and on-off intermittency. For a linearly increasing acceptor concentration, we find that the size of the solitary waves diminishes as they advance across the sample

Dynamics of Cognitive Interpretations of a Necker Cube in a Chaos Neural Network

An associative schema model of a Necker cube is presented with the use of a chaos neural network which has two local minima. We consider that two possible cognitive interpretations of the cube correspond to the minima, and dynamics of the transfer schemata is studied by numerical simulations where an intermittent switching between the two interpretations is observed.

Experimental observation of on-off intermittency.

We observe on-off intermittency in a nonlinear electronic circuit tuned near a Hopf bifurcation point. The circuit is driven randomly through the bifurcation point, resulting in intermittent switching between a fixed point (laminar phase) and a limit cycle. The distribution of lengths of laminar phases exhibits -3/2 power law scaling for shorter phases, and an exponential drop for longer phases, due to noise in the system. These results agree with a theoretically predicted distribution. In addition, the crossover from power law to exponential decay obeys the predicted scaling law.

Multi-order power spectrum and its application

A new technique was explored to investigate the time correlations in time series. This is an extension of the ordinary power spectrum which is called the order- q power spectrum. I have extended the order- q power spectrum further and made the multi-order version of it. This technique is able to be applied to various processes which have more than two states. This approach is applied to a chaotic system which exhibits intermittent switching and shows us a surprising result. Appropriate choices for parameters show us various correlations that are not able to be observed in the ordinary power spectra.