Effects Of External Perturbations Research Articles

Dynamic Control of a Large Scale of Pneumatic Multichain Systems

AbstractThe aim of this paper is to propose a general principle able to manage the supervision, coordination, and control problems of a large scale of multichain structures in dynamic and interacting tasks. The main originality lays in its modular feature. The number of chains and of joint per chain are indeed not restricted. The only assumption is that all the chains are linked to a principal element from which the dynamic stability will be computed. This is for example the case for multi‐finger and multi‐legged structures for which respectively the palm and the trunk represent the principal element. The principle consists in controlling the stability of this element and distributing the effort on the other chains to maintain the stability of the whole. To increase the compliant feature, we consider that each joint is pneumatically actuated. Each joint is then dynamically controlled to ensure the asymptotic stability of the local chain it belongs to. A global architecture is presented where each part is detailed. An example is then displayed showing the performances of a two‐legged robot in the standing posture under external perturbation effects. © 2004 Wiley Periodicals, Inc.

Brassinosteroid: A modulator of the developmental window for salt-adaptation in Sorghum bicolor?

Leaf development of salt-treated Sorghum bicolor plants is influenced by treatment with 24-epibrassinolide (EBL), but only during a short period in development. This transient sensitivity coincides with a critical period of reorganization of regulations already identified in Sorghum, during which adaptation (through integration of effects of external perturbation in the new regulation network) is possible. By measuring the effect of EBL on leaf malformations during their unfolding in plants exposed to 150 mM NaCl, it is shown that treatments with brassinosteroids (BR) enable modification of initiation, duration, and intensity of this critical period of reorganization. A critical period is characterized by modification of cell sensitivity to informative substances. Accordingly, it is suggested that BR, at specific concentrations and time in development, may induce changes in cellular sensitivity to many growth regulators. The parallels in the effects of EBL and NaCl suggest that the transient inhibition in leaf elongation is not directly caused by the perturbation (addition of EBL or sublethal NaCl treatment). Rather, it results from interference between an exogenous perturbation and the structure of the regulation network.

On trade-offs in low-input agroecosystems

More fundamental knowledge on critical trade-offs is potentially important to improve the performance of low-input agroecosystems. Productivity, stability and sustainability are expressions of performance, and their trade-offs are a cause of scientific debate. Our results show that articulation and storage within the system play an important role in the trade-offs between productivity and stability, and between productivity and sustainability. Both articulation and storage involve complex structures that have a high maintenance cost, and consequently depress the overall ecosystem productivity. However, they are important to smooth the effect of external perturbations. The inverse relationship between productivity and stability and productivity and sustainability cannot be easily reverted unless external inputs are added. But some structural arrangements in the agroecosystem that produce anomalies and deviations from those inverse trends look promising as a means to reconcile opposite effects in a low-input scheme.

High-resolution spectroscopy of the zero-phonon line of the deep donor EL2 in GaAs

We investigated the zero-phonon line (ZPL) of the deep donor EL2 in GaAs by means of high-resolution absorption spectroscopy with a narrow-band laser. Frequency-selective bleaching (``spectral-hole burning'') experiments and the measurement of the temperature broadening of the ZPL prove an essentially homogeneous broadening of the transition. The observed asymmetry of the line shape is interpreted to be caused by a Fano resonance of the $^{1}\mathrm{T}_{2}$ excited state with the conduction band. A splitting of the $^{1}\mathrm{T}_{2}$ state as the reason for the asymmetry seems unrealistic. The homogeneous broadening of the ZPL prevents the use of spectral-hole burning spectroscopy to study the effect of external perturbations on the ZPL of the EL2.

Angular domains in cometary capture and anisotropy

In the earlier work (1994) of the authors a theoretical formulation was developed for the capture process under the influence of external perturbations. The parameter hyperspace was scanned for physically acceptable values of σ, V 0, U′, and n;α= 2δ being the most probable speed of comets, V 0 the Sun's speed, U′ the perturbative component of velocity and n the number density of comets per pc 3; so that the computed capture rate and anisotropy conform to their observed values. The present paper is an extension of the above said earlier work in the sense that it gives an insight into the cause leading to anisotropy in capture. Different capture conditions obtained here for capture from the antapexial and apexial directions reveal that a larger velocity domain is allowed for capture of comets from the former direction. This disparity in allowed velocity domains for the capture from the two directions is found to explain the observed anisotropy. The observed anisotropy, i.e. the ratio of cometary aphelia in the solar antapex direction to those in solar apex direction is reproduced by Valtonen and Innanen ( Astrophys. J. 255, 307, 1982). In their theoretical formulation of the capture process they used a Maxwellian distribution for cometary speeds and only two directions from which comets approach the solar system, i.e. apexial and antapexial directions. The authors of the present paper have also explained anisotropy (1994) on the basis of the Maxwellian distribution of cometary speeds, but by incorporating the effect of external perturbations on relative velocities of comets with respect to the Sun. Moreover, the present formulation includes the integrated effect of all the directions of approach of comets towards capture from apexial and antapexial directions. This has been achieved by deriving an expression for a parameter (ϵ) after integration over the whole of the geometric as well as velocity space. The total captivity parameter ϵ depends upon the effective relative speed of comets with respect to the Sun. ϵ is split into partial captivity parameters ϵ 1 and ϵ 2 for apexial and antapexial directions, respectively, according to Sun's different relative speeds for the two directions. Here ϵ 1 and ϵ 2 include the integrated contribution from all the directions towards apexial and antapexial captures. It is found that effective relative speed for comets in the former group is always greater than that for comets in the later group. Accordingly, different capture conditions in velocity space have been obtained for the two directions. Such domains in velocity space where these capture conditions are violated (fulfilled) are called here prohibited (allowed) angular domains. It is seen that the capture conditions render a larger allowed domain for capture of antapexial comets than that for capture of apexial comets. Hence the observed anisotropy can be visualized as a consequence of existence of these angular domains in cometary capture. In short, the observed anisotropy is in favour of antapexial capture since the allowed angular domain for antapexial capture is always greater than that for apexial capture.

Calcium and cell cycle progression: possible effects of external perturbations on cell proliferation

Exit from the phase of cellular division appears to be driven by a calcium signal that triggers a cascade of events leading to the completion of mitosis. Here we propose a model that relates the dynamics of cytosolic calcium to progression through mitosis, G1 and G2 phases of the cell cycle. To this end, the assumption has been made that the transient rise ir cytosolic calcium concentration during mitosis is induced by inositol(1,4,5)triphosphate (IP3), which in turn is released at high levels of mitosis-promoting factor (MPF). On this basis, a system of ordinary differential equations is proposed to simulate the evolution of ten cell-cycle-specific molecular species, including cyclins A and B, MPF, IP3, Ca2+, the CaMKII holoenzyme, and the ubiquitination complex. The influence on the cell proliferation capacity exerted by external perturbations, like calcium microinjections, depletion of intracellular calcium stores, electromagnetic fields, or stimulation/inhibition of different calcium currents through the plasma membrane, can be studied by appropriate modulation of the parameters involved in the signal transduction pathway.

Synchronization in the discrete chemical oscillation system

The properties of the coupling between chemical oscillators were studied in the discrete chemical oscillation system which was realized by immersing cation exchange beads loaded with ferroin in the Belousov–Zabotinskii reaction solution. A phase diagram of coupling states was obtained as a function of natural frequencies of oscillators and the distance d between oscillators. The synchronization was found not to be attributed to a simple entrainment of the slower oscillator by the faster one. Various entrainments between oscillators occurred depending on ratios of natural frequencies in the uncoupled state. A chaotic behavior was found at the boundary between stably coupled regions with frequency ratios of n/1 where n is an integer. Furthermore, effects of external perturbation on the coupling were investigated. Irregular oscillations were induced by illumination with a He–Ne laser light, which strongly depended on the phase of oscillator at the beginning of illumination. Such irregular behaviors were localized within the illuminated bead. This indicates that two oscillators are decoupled by illumination.

On the stability of rotational discontinuities: One‐ and two‐dimensional hybrid simulations

Studies of rotational discontinuities (RDs) and their role in affecting the magnetic field rotations at the magnetopause have been generally conducted using one‐dimensional simulations and for RDs in isolation from external perturbations. The conditions at the magnetopause are, however, often turbulent with large Alfvénic and mirror mode fluctuations. Here, previous calculations are extended to two‐dimensions using the hybrid code and the effect of external perturbations on the stability of both isotropic and anisotropic RDs is examined for the first time. As in the one‐dimensional simulations and in agreement with observations at the magnetopause, RDs are found to satisfy the minimum shear condition (rotation angles less than or equal to 180°) and have gradient scale half‐widths of the order of 1–4 ion inertial lengths. In addition, RDs are found to be quite stable even when large amplitude mirror and Alfvén waves impinge upon them. No evidence for two‐dimensional (e.g., surface or shear) instabilities were found. Although simulations show the presence of a coherent wavetrain for some of the RDs, the observations of RDs at the magnetopause have not identified any wavetrain. It is shown that under the turbulent conditions prevalent at the magnetopause, the detection of wavetrain in the data would most likely require multispacecraft measurements.

The effects of trace metals on the apparent water permeability of the shore crab Carcinus maenas (L.) and the brown shrimp Crangon crangon (L.)

The effects of trace metals on the apparent water permeability (AWP) of two decapod crustaceans Carcinus maenas and Crangon crangon, were investigated. One hour exposure to 1 mg l −1 dissolved Hg or Pb significantly decreased the AWP of the shore crab, C. maenas. A similar, but not significant, tendency was observed following exposure to Cd, Cu and Zn, whereas Co had little effect. The AWP of C. maenas exposed to Cd decreased with increasing Cd concentration (0.5, 1, 4 or 8 mg l −1). This decrease was independent of exposure time (1, 5 or 10 days). The influence of Zn (500 μg l −1) on the apparent water permeability of the brown shrimp C. crangon was also investigated. After 24 h exposure to Zn, AWP decreased significantly. This reduction persisted for at least 3 days. After 6 days of exposure, AWP returned to the initial level. Whether trace metal induced changes in the AWP are the result of toxic effects of the metals or represent physiological adaptations minimizing the effects of external perturbations is discussed.

Configurational instabilities at isoelectronic centres in silicon

When an exciton is bound on an isoelectronic centre, one particle (electron or hole) may be severely localised on the centre. In this paper attention is drawn to the importance of the lattice relaxations stimulated by this localised charge density. Two examples are discussed in detail for centres where the exciton binding is produced primarily by the relaxation. It is shown that Jahn-Teller theory applied to the tightly bound particle allows a very precise understanding of the effects of external perturbations on the bound exciton.

Ab initio study of the effect of external perturbations in the dissociation of CH 3Cl

The effect of external perturbations on the dissociation of CH 3Cl was studied by means of ab initio calculations. It was found that the energy profile for this reaction in the gas phase increases monotonically, whereas the presence of either uniform electric fields or solvents changes the energy profile, causing the appearance of a transition state and the increase of the exothermicity of the process. The effect of these uniform electric fields and solvents is explained through analysis of the electron density and density Laplacian plots.

External perturbations on distant planetary orbits and objects in the Kuiper disk

We investigate the potential importance of molecular cloud and stellar perturbations on the orbits of Pluto and more distant (hypothetical) planets up to 500 AU from the Sun. It is found that stellar and molecular cloud-core perturbations are of roughly equal importance. It also is found that the likelihood of substantial perturbations on Pluto is not insignificant, and that numerous substantial stellar and molecular cloud perturbations are likely to have influenced the orbits of any planets beyond ∼200 AU. These perturbations may contribute to a prevalence of moderate eccentricities and inclinations for planets beyond the orbit of Neptune, and may be a characteristic of distant planetary orbits in other solar systems. Given the recent discovery of chaotic behavior in Pluto's orbit (Sussman and Wisdom 1988), the effects of external perturbations on the long-term stability of Pluto's orbit warrant continued study.

Effect of external perturbations on fiber-optic magnetic sensors

The effects of external nonmagnetic perturbations on a fiber-optic magnetic sensor are analyzed theoretically. It is shown that the deleterious effects of these perturbations can be controlled by maintaining the DC magnetic field at prescribed levels. Experiments have been performed with externally applied stresses and accelerations (vibrations) that validate the theory and yield values for the stress and acceleration responsivities of the sensors studied in this investigation. The analysis and techniques given apply equally well to any interferometric sensor using nonlinear transduction mechanisms. >

Explicit solutions of normal form of driven oscillatory systems

We consider an oscillatory dissipative system driven by external sinusoidal perturbations of given amplitude Q and frequency ω. The kinetic equations are transformed to normal form and solved for small Q, near a Hopf bifurcation to oscillations in the autonomous system, for ratios ωn to the autonomous frequency of irrational so that the response of the system is quasiperiodic. The system is assumed to have either two variables or is adequately described by two variables near the bifurcation, and we obtain explicit solutions for this general case. The equations show interesting effects of external perturbations on limit cycles, both stable and unstable. Next we treat a specific model (Brusselator) and show by comparison with results of numerical integration that the theory predicts well the shape of the perturbed limit cycle, its variation with changes in constraints and parameters, and the point of transition from quasiperiodic to periodic response.

The metabolic state of the rat liver in vivo measured by 31P-NMR spectroscopy

Previous 31P nuclear magnetic resonance (NMR) studies have measured the concentrations of phosphates, free Mg 2+, pH and flux through enzyme-catalyzed reactions in a variety of tissues. A surgically-implanted coil has been developed to measure these parameters in the rat liver in vivo, and to assess the effect of external perturbations on the concentrations and physiological environment of phosphorus metabolites in the liver. The sensitive volume and optimal pulse were determined for the coil, which was insulated to exclude signal from surrounding tissues. The metabolic stability of the liver during acquisition of spectra was demonstrated by normal values for [P i], [ATP], [lactate], and [pyruvate] in livers which were freeze-clamped immediately after completion of the NMR experiment. The stability was also confirmed by constant values for intracellular pH (7.2), free [Mg 2+] (0.7 mM), and NMR detectable [ P i ] [ ATP] . The sensitivity of the 31P-NMR spectrum of the liver in vivo to the physiological state of the animals was illustrated by comparing spectra from fed and 48 h fasted rats. The major qualitative differences were an increase in the pyridine nucleotide/adenine nucleotide ratio, and a small, but consistent shift in the frequency of the composite phosphomonoester peak. The spin-lattice relaxation time of each major phosphate resonance was measured in vivo using a modified homospoil saturation recovery pulse sequence; the T 1 of ATP γ-phosphate was 0.17 s. Selective saturation experiments did not detect magnetization transfer between the ATP γ-phosphate and inorganic phosphate.

GaAs, AlAs, and AlxGa1−xAs: Material parameters for use in research and device applications

The AlxGa1−xAs/GaAs heterostructure system is potentially useful material for high-speed digital, high-frequency microwave, and electro-optic device applications. Even though the basic AlxGa1−xAs/GaAs heterostructure concepts are understood at this time, some practical device parameters in this system have been hampered by a lack of definite knowledge of many material parameters. Recently, Blakemore has presented numerical and graphical information about many of the physical and electronic properties of GaAs [J. S. Blakemore, J. Appl. Phys. 53, R123 (1982)]. The purpose of this review is (i) to obtain and clarify all the various material parameters of AlxGa1−xAs alloy from a systematic point of view, and (ii) to present key properties of the material parameters for a variety of research works and device applications. A complete set of material parameters are considered in this review for GaAs, AlAs, and AlxGa1−xAs alloys. The model used is based on an interpolation scheme and, therefore, necessitates known values of the parameters for the related binaries (GaAs and AlAs). The material parameters and properties considered in the present review can be classified into sixteen groups: (1) lattice constant and crystal density, (2) melting point, (3) thermal expansion coefficient, (4) lattice dynamic properties, (5) lattice thermal properties, (6) electronic-band structure, (7) external perturbation effects on the band-gap energy, (8) effective mass, (9) deformation potential, (10) static and high-frequency dielectric constants, (11) magnetic susceptibility, (12) piezoelectric constant, (13) Fröhlich coupling parameter, (14) electron transport properties, (15) optical properties, and (16) photoelastic properties. Of particular interest is the deviation of material parameters from linearity with respect to the AlAs mole fraction x. Some material parameters, such as lattice constant, crystal density, thermal expansion coefficient, dielectric constant, and elastic constant, obey Vegard’s rule well. Other parameters, e.g., electronic-band energy, lattice vibration (phonon) energy, Debye temperature, and impurity ionization energy, exhibit quadratic dependence upon the AlAs mole fraction. However, some kinds of the material parameters, e.g., lattice thermal conductivity, exhibit very strong nonlinearity with respect to x, which arises from the effects of alloy disorder. It is found that the present model provides generally acceptable parameters in good agreement with the existing experimental data. A detailed discussion is also given of the acceptability of such interpolated parameters from an aspect of solid-state physics. Key properties of the material parameters for use in research work and a variety of AlxGa1−xAs/GaAs device applications are also discussed in detail.

Many-body theory of correlation effects in externally perturbed systems

A unified presentation of different schemes for the calculation of the external perturbation effects is given by using the many-body perturbation theory methods. The main attention is focused on the analysis of the total perturbed energy components in terms of different two-body contributions which are classified into the self-consistency and the true correlation effects. A partial or complete summation of the self-consistency contributions is interpreted in terms of appropriately chosen unitary transformations of the single-particle states and the creation and annihilation operators. The advantages of the perturbation scheme based on the coupled Hartree-Fock approach are stressed.

Dynamical instability and external perturbations: Bimolecular collisions in laser fields

The detailed dynamics of F+H2, and to a lesser extent H+H2, in the presence of laser fields are examined in order to establish the relationship between field-free unstable trajectories and the effect of external perturbations. Weak fields are found to alter the dynamics of individual unstable trajectories and, in particular, to induce transitions amongst the unstable trajectory manifold. As a result, the field has little effect, on the average, on gross collision properties. Intense fields substantially alter the qualitative stable/ unstable structure of phase space and hence the reaction probability. An interesting high field limit is also discussed.

Excited-state energies from many-body perturbation theory with the excitation-adapted one-particle potential

Calculation of excited-state energies by many-body perturbation theory is discussed. A Hartree-Fock-type potential suitable for a given excited-state configuration is introduced. Advantages which follow from the excitation-adapted one-particle potential are examined. The theory resembles that for external perturbation effects and amounts to removing or minimizing the contribution of non-diagonal one-particle terms.

Occurrence of Tolman-Schlichting waves in the boundary layer under the effect of external perturbations

Under small external perturbations, the initial stage of the laminar into turbulent flow transition process in boundary layers is the development of natural oscillations, Tolman-Schlichting waves, which are described by the linear theory of hydrodynamic stability. Subsequent nonlinear processes start to appear in a sufficiently narrow band of relative values of the perturbation amplitudes (1–2% of the external flow velocity) and progress quite stormily. Hence, the initial “linear” stage of relatively slow development of perturbations is governing, in a known sense, in the complete transition process. In particular, the location of the “transition point” depends, to a large extent, on the spectrum composition and intensity of the perturbations in the boundary layer, which start to develop according to linear theory laws, resulting in the long run in destruction of the laminar flow mode. In its turn, the initial intensity and spectrum composition of the Tolman-Schlichting waves evidently depend on the corresponding characteristics of the different external perturbations generating these waves. The significant discrepancy in the data of different authors on the transition Reynolds number in the boundary layer on a flat plate [1–4] is probably explained by the difference in the composition of the small perturbing factors (which have not, unfortunately, been fully checked out by far). Moreover, it is impossible to expect that all kinds of external perturbations will be transformed identically into the natural boundary-layer oscillations. The relative role of external perturbations of different nature is apparently not identical in the Tolman-Schlichting wave generation process. However, how the boundary layer reacts to small external perturbations, under what conditions and in what way do external perturbations excite Tolman-Schlichting waves in the boundary layer have practically not been investigated. The importance of these questions in the solution of the problem of the passage to turbulence and in practical applications has been emphasized repeatedly recently [5, 6], Only the first steps towards their solution have been taken at this time [4, 7–10], Out of all the small perturbing factors under the real conditions of the majority of experiments to investigate the flow stability and transition in the case of smooth polished walls, three are apparently most essential, viz.: the turbulence of the external flow, acoustic perturbations, and model vibrations. In principle, all possible mechanisms for converting the energy of these perturbations into Tolman-Schlichting waves can be subdivided into two classes (excluding the nonlinear interactions which are not examined here): 1) distributed wave generation in the boundary layer; and 2) localized wave generation at the leading edge of the streamlined model. Among the first class is both the possibility of the direct transformation of the external flow perturbations into Tolman-Schlichting waves through the boundary-layer boundary, and wave excitation because of the active vibrations of the model wall. Among the second class are all possible mechanisms for the conversion of acoustic or vortical perturbations, as well as the vibrations of the streamlined surface, into Tolman-Schlichting waves, which occurs in the area of the model leading edge.