Showing papers on "Amplitude published in 2002"

Energy detection of a signal with random amplitude

Abstract: Urkowitz (1967) has discussed the detection of a deterministic signal of unknown structure in the presence of band-limited Gaussian noise. That analysis is developed to the case of a signal with random (Rayleigh, Rice, Nakagami, and other) amplitude. For such amplitude the distribution of a decision statistic of an energy detector is retrieved and expressions for detection probability are obtained.

Role of the intramolecular phase in high-harmonic generation

Abstract: We study numerically the generation of high-order harmonics by two-center molecules for arbitrary angles between the molecular axis and the laser polarization axis. For fixed angle, the harmonic spectrum exhibits a minimum at a frequency which is independent of the laser parameters. The amplitude of each harmonic is strongly angle dependent, and a pronounced minimum is found at the same angle where a sudden jump in the harmonic phase occurs. By calculating the spatial dependence of the harmonic amplitudes and phases, we are able to explain these effects in terms of interfering contributions from various regions within the molecule.

Transverse Oscillations in Coronal Loops Observed with TRACE – II. Measurements of Geometric and Physical Parameters

Abstract: We measure geometric and physical parameters oftransverse oscillations in 26 coronal loops, out of the 17 events described in Paper I by Schrijver, Aschwanden, and Title (2002). These events, lasting from 7 to 90 min, have been recorded with the Transition Region and Coronal Explorer (TRACE) in the 171 and 195 A wavelength bands with a characteristic angular resolution of 1", with time cadences of 15–75 seconds. We estimate the unprojected loop (half) length L and orientation of the loop plane, based on a best-fit of a circular geometry. Then we measure the amplitude A(t) of transverse oscillations at the loop position with the largest amplitude. We decompose the time series of the transverse loop motion into an oscillating component A osc(t) and a slowly-varying trend A trend(t). We find oscillation periods in the range of P=2–33 min, transverse amplitudes of A=100–8800 km, loop half lengths of L=37 000–291 000 km, and decay times of t d=3.2–21 min. We estimate a lower limit of the loop densities to be in the range of n loop=0.13–1.7×109 cm−3. The oscillations show (1) strong deviations from periodic pulses, (2) spatially asymmetric oscillation amplitudes along the loops, and (3) nonlinear transverse motions of the centroid of the oscillation amplitude. From these properties we conclude that most of the oscillating loops do not fit the simple model of kink eigen-mode oscillations, but rather manifest flare-induced impulsively generated MHD waves, which propagate forth and back in the loops and decay quickly by wave leakage or damping. In contrast to earlier work we find that the observed damping times are compatible with estimates of wave leakage through the footpoints, for chromospheric density scale heights of ≈400–2400 km. We conclude that transverse oscillations are most likely excited in loops that (1) are located near magnetic nullpoints or separator lines, and (2) are hit by a sufficiently fast exciter. These two conditions may explain the relative rarity of detected loop oscillations. We show that coronal seismology based on measurements of oscillating loop properties is challenging due to the uncertainties in estimating various loop parameters. We find that a more accurate determination of loop densities and magnetic fields, as well as advanced numerical modeling of oscillating loops, are necessary conditions for true coronal seismology.

A phase-based approach to the estimation of the optical flow field using spatial filtering

Abstract: We introduce a new technique for estimating the optical flow field, starting from image sequences. As suggested by Fleet and Jepson (1990), we track contours of constant phase over time, since these are more robust to variations in lighting conditions and deviations from pure translation than contours of constant amplitude. Our phase-based approach proceeds in three stages. First, the image sequence is spatially filtered using a bank of quadrature pairs of Gabor filters, and the temporal phase gradient is computed, yielding estimates of the velocity component in directions orthogonal to the filter pairs' orientations. Second, a component velocity is rejected if the corresponding filter pair's phase information is not linear over a given time span. Third, the remaining component velocities at a single spatial location are combined and a recurrent neural network is used to derive the full velocity. We test our approach on several image sequences, both synthetic and realistic.

Sideband suppression in time-modulated linear arrays by the differential evolution algorithm

Abstract: A novel approach based on the differential evolution (DE) algorithm is proposed to suppress the sideband radiation patterns in time modulated linear antenna arrays. The sideband level of a time modulated linear array can be reduced significantly by rearranging the static excitation amplitudes as well as the switch-on time intervals of each element. The approach is illustrated through a 32-element linear array.

Standing Waves with a Critical Frequency for Nonlinear Schrödinger Equations

Abstract: This paper is concerned with the existence and qualitative property of standing wave solutions \(\) for the nonlinear Schrodinger equation \(\) with E being a critical frequency in the sense that \(\). We show that there exists a standing wave which is trapped in a neighbourhood of isolated minimum points of V and whose amplitude goes to 0 as \(\). Moreover, depending upon the local behaviour of the potential function V(x) near the minimum points, the limiting profile of the standing-wave solutions will be shown to exhibit quite different characteristic features. This is in striking contrast with the non-critical frequency case \(\) which has been extensively studied in recent years.

Normalized amplitude quotient for parametrization of the glottal flow

Abstract: Normalized amplitude quotient (NAQ) is presented as a method to parametrize the glottal closing phase using two amplitude-domain measurements from waveforms estimated by inverse filtering. In this technique, the ratio between the amplitude of the ac flow and the negative peak amplitude of the flow derivative is first computed using the concept of equivalent rectangular pulse, a hypothetical signal located at the instant of the main excitation of the vocal tract. This ratio is then normalized with respect to the length of the fundamental period. Comparison between NAQ and its counterpart among the conventional time-domain parameters, the closing quotient, shows that the proposed parameter is more robust against distortion such as measurement noise that make the extraction of conventional time-based parameters of the glottal flow problematic. Experiments with breathy, normal, and pressed vowels indicate that NAQ is also able to separate the type of phonation effectively.

Capillary forces in tapping mode atomic force microscopy

Abstract: We investigated the influence of the relative humidity on amplitude and phase of the cantilever oscillation while operating an atomic force microscope (AFM) in the tapping mode. If the free oscillation amplitude A0 exceeds a certain critical amplitude Ac, the amplitude- and phase-distance curves show a transition from a regime with a net attractive force between tip and sample to a net repulsive regime. For hydrophilic tip and sample this critical amplitude Ac is found to increase with increasing relative humidity. In contrast, no such dependence was found for hydrophobic samples. Numerical simulations show that this behavior can be explained by assuming the intermittent formation and rupture of a capillary neck in each oscillation cycle of the AFM cantilever.

An approximate exchange-correlation hole density as a functional of the natural orbitals

Abstract: The Fermi and Coulomb holes that can be used to describe the physics of electron correlation are calculated and analysed for a number of typical cases, ranging from prototype dynamical correlation to purely nondynamical correlation. Their behaviour as a function of the position of the reference electron and of the nuclear positions is exhibited. The notion that the hole can be written as the square of a hole amplitude, which is exactly true for the exchange hole, is generalized to the total holes, including the correlation part. An Ansatz is made for an approximate yet accurate expression for the hole amplitude in terms of the natural orbitals. employing the local (at the reference position) values of the natural orbitals and the density. This expression for the hole amplitude leads to an approximate two-electron density matrix that: (a) obeys correct permutation symmetry in the electron coordinates; (b) integrates to the exact one-matrix; and (c) yields exact correlation energies in the limiting cases of...

Basic acoustic properties of microbubbles.

Abstract: Small (encapsulated) gas bubbles in a contrast medium react to an external oscillating pressure field with volume pulsations. Depending on the magnitude of the ultrasound wave, the vibrations will be related either linearly or nonlinearly to the applied acoustic pressure. For low acoustic pressures, the instantaneous radius oscillates linearly in relation to the amplitude of the applied external pressure field. The oscillation of the bubble is governed by parameters such as resonance frequency, damping coefficients, and shell properties. For higher amplitudes of the external field, the pulsation of the bubbles becomes nonlinear. The spectrum of the scattered ultrasound wave also contains higher harmonics of the emitted frequency in addition to the fundamental frequency. The emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation. For encapsulated bubbles exposed to even higher acoustic amplitudes, their scattering effectiveness increases dramatically and becomes transient. The scattered frequency spectrum broadens, containing higher harmonics. This consequence is due to rupture, disappearance, change of gas content, etc. Using these specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis.

Relativistic simulations of rotational core collapse. II. Collapse dynamics and gravitational radiation

Abstract: We have performed hydrodynamic simulations of relativistic rotational supernova core collapse in axisymmetry and have computed the gravitational radiation emitted by such an event. Details of the methodology and of the numerical code have been given in an accompanying paper. We have simulated the evolution of 26 models in both Newtonian and relativistic gravity. Our simulations show that the three different types of rotational supernova core collapse and gravitational waveforms identified in previous Newtonian simulations (regular collapse, multiple bounce collapse, and rapid collapse) are also present in relativistic gravity. However, rotational core collapse with multiple bounces is only possible in a much narrower parameter range in relativistic gravity. The relativistic models cover almost the same range of gravitational wave amplitudes and frequencies as the corresponding Newtonian ones. For a given model, relativistic gravity can cause a large increase of the characteristic signal frequency of up to a factor of five, which may have important consequences for the signal detection. The gravitational wave signals obtained in our study are within the sensitivity range of the first generation laser interferometer detectors if the source is located within the Local Group.

Understanding the Long-Term Spectral Variability of Cygnus X-1 with Burst and Transient Source Experiment and All-Sky Monitor Observations

Abstract: We present a comprehensive analysis of all observations of Cyg X-1 by the Compton Gamma Ray Observatory Burst and Transient Source Experiment (BATSE; 20-300 keV) and by the Rossi X-Ray Timing Explorer all-sky monitor (ASM; 1.5-12 keV) until 2002 June, including approximately 1200 days of simultaneous data. We find a number of correlations between fluxes and hardnesses in different energy bands. In the hard (low) spectral state, there is a negative correlation between the ASM 1.5-12 keV flux and the hardness at any energy. In the soft (high) spectral state, the ASM flux is positively correlated with the ASM hardness but uncorrelated with the BATSE hardness. In both spectral states, the BATSE hardness correlates with the flux above 100 keV, while it shows no correlation with the 20-100 keV flux. At the same time, there is clear correlation between the BATSE fluxes below and above 100 keV. In the hard state, most of the variability can be explained by softening the overall spectrum with a pivot at approximately 50 keV. There is also another, independent variability pattern of lower amplitude where the spectral shape does not change when the luminosity changes. In the soft state, the variability is mostly caused by a variable hard (Comptonized) spectral component of a constant shape superposed on a constant soft blackbody component. These variability patterns are in agreement with the dependencies of the rms variability on the photon energy in the two states. We also study in detail recent soft states from late 2000 until 2002. The last of them has lasted thus far for more than 200 days. Their spectra are generally harder in the 1.5-5 keV band and similar or softer in the 3-12 keV band than the spectra of the 1996 soft state, whereas the rms variability is stronger in all the ASM bands. On the other hand, the 1994 soft state transition observed by BATSE appears very similar to the 1996 one. We interpret the variability patterns in terms of theoretical Comptonization models. In the hard state, the variability appears to be driven mostly by changing flux in seed photons Comptonized in a hot thermal plasma cloud with an approximately constant power supply. In the soft state, the variability is consistent with flares of hybrid, thermal/nonthermal, plasma with variable power above a stable cold disk. The spectral and timing differences between the 1996 and 2000-2002 soft states are explained by a decrease of the color disk temperature. Also, on the basis of broadband pointed observations simultaneous with those of the ASM and BATSE, we find the intrinsic bolometric luminosity increases by a factor of approximately 3-4 from the hard state to the soft one, which supports models of the state transition based on a change of the accretion rate.

Bayesian Analysis of Radio Observations of the Be X-Ray Binary LS I +61°303

Abstract: The binary star LS I +61303 is remarkable for its periodic radio outbursts every 26.5 days. We recently discovered a 4.4 year periodic modulation of the phase and amplitude of these outbursts using the Gregory-Loredo Bayesian algorithm for detecting periodic signals of unknown shape. In this paper we obtain improved estimates of both the orbital period (P1) and modulation period (P2) based on a larger data set and examine the behavior of the spectral index between 2.2 and 8.3 GHz, versus modulation period. The new estimates are P1 =2 6.4960 ± .0028 days and P2 = 1667 ± 8 days and our best estimate of the radio phase of periastron is 0.4. Analysis of the spectral index data indicates that the optical depth in the synchrotron emission region is always << 1a t 8.3 GHz, and can reach values 2.7 at 2.2 GHz. A test of the precessing Be star model of Lipunov and Nazin indicates that it is unlikely to be the correct mechanism to explain the 1667 day periodic modulation.

Quantitative phase‐amplitude microscopy I: optical microscopy

Abstract: In this paper, the application of a new optical microscopy method (quantitative phase-amplitude microscopy) to biological imaging is explored, and the issue of resolution and image quality is examined. The paper begins by presenting a theoretical analysis of the method using the optical transfer function formalism of Streibl (1985). The effect of coherence on the formation of the phase image is explored, and it is shown that the resolution of the method is not compromised over that of a conventional bright-field image. It is shown that the signal-to-noise ratio of the phase recovery, however, does depend on the degree of coherence in the illumination. Streibl (1985) notes that partially coherent image formation is a non-linear process because of the intermingling of amplitude and phase information. The work presented here shows that the quantitative phase-amplitude microscopy method acts to linearize the image formation process, and that the phase and amplitude information is properly described using a transfer function analysis. The theoretical conclusions are tested experimentally using an optical microscope and the theoretical deductions are confirmed. Samples for microscopy influence both the phase and amplitude of the light wave and it is demonstrated that the new phase recovery method can separate the amplitude and phase information, something not possible using traditional phase microscopy. In the case of a coherent wave, knowledge of the phase and amplitude constitutes complete information that can be used to emulate other forms of microscopy. This capacity is demonstrated by recovering the phase of a sample and using the data to emulate a differential interference contrast image.

New dynamical mean-field dynamo theory and closure approach.

Abstract: We develop a new nonlinear mean field dynamo theory that couples field growth to the time evolution of the magnetic helicity and the turbulent electromotive force, e. We show that the difference between kinetic and current helicities emerges naturally as the growth driver when the time derivative of e is coupled into the theory. The solutions predict significant field growth in a kinematic phase and a saturation rate/strength that is magnetic Reynolds number dependent/independent in agreement with numerical simulations. The amplitude of early time oscillations provides a diagnostic for the closure.

Separation of Gravitational-Wave and Cosmic-Shear Contributions to Cosmic Microwave Background Polarization

Abstract: Inflationary gravitational waves (GW) contribute to the curl component in the polarization of the cosmic microwave background (CMB). Cosmic shear—gravitational lensing of the CMB—converts a fraction of the dominant gradient polarization to the curl component. Higher-order correlations can be used to map the cosmic shear and subtract this contribution to the curl. Arcminute resolution will be required to pursue GW amplitudes smaller than those accessible by the Planck surveyor mission. The blurring by lensing of small-scale CMB power leads with this reconstruction technique to a minimum detectable GW amplitude corresponding to an inflation energy near 10^15 GeV.

Persistent Current in Superconducting Nanorings

Abstract: The superconductivity in very thin rings is suppressed by quantum phase slips. As a result, the amplitude of the persistent current oscillations with flux becomes exponentially small, and their shape changes from sawtooth to a sinusoidal one. We reduce the problem of low-energy properties of a superconducting nanoring to that of a quantum particle in a sinusoidal potential and show that the dependence of the current on the flux belongs to a one-parameter family of functions obtained by solving the respective Schrodinger equation with twisted boundary conditions.

Nonlinear surface EMG analysis to detect changes of motor unit conduction velocity and synchronization.

Abstract: Amplitude and frequency content of the surface electromyographic (EMG) signal reflect central and peripheral modifications of the neuromuscular system. Classic surface EMG spectral variables applie...

Tip motion in amplitude modulation (tapping-mode) atomic-force microscopy: Comparison between continuous and point-mass models

Abstract: We discuss the influence of high-order frequency components in the operation of an amplitude modulation atomic-force microscope (AFM) A comparative study of point-mass and continuous models is performed to describe the tip motion The tip–surface interaction force excites high-order frequency components whenever a higher harmonic of the excitation force is close to an eigenmode of the cantilever beam The strength of those components depends on the set point amplitude and the fundamental resonance frequency of the cantilever However, for standard operating conditions with quality factors in the 102–103 range, higher-order components are about three orders of magnitude smaller than the component at the excitation frequency We conclude that point-mass models are suitable to describe the operation of a tapping-mode AFM in air environments

Multiphoton Intrapulse Interference. 1. Control of Multiphoton Processes in Condensed Phases

Abstract: We explore and demonstrate the use of phase-modulated ultrafast laser pulses for controlling nonlinear optical processes in large molecules, proteins, and solid materials. Our experiments illustrate that in condensed phases, when spectra are broad, the spectrum of the nth-order electric field, determined by multiphoton intrapulse interference, plays a major role in controlling multiphoton excitation. These findings determine key parameters (amplitude, period, and symmetry of the phase function) for coherent femtosecond laser control in condensed phases.

Coherent vibrational motion in metal particles: Determination of the vibrational amplitude and excitation mechanism

Abstract: Ultrafast laser excitation of metal particles coherently excites the symmetric breathing mode. This changes the electron density in the particle, which produces a periodic redshift in the position of the plasmon band. In this paper transient absorption data recorded over a range of wavelengths are analyzed to determine the amplitude of the breathing motion for 24.2 nm radius Au particles. The results are compared to a model calculation where the expansion coordinate is treated as a damped harmonic oscillator and the driving force is thermal expansion due to lattice heating (the temperature rise is determined from the energy absorbed by the sample). The only adjustable parameters in these calculations are the dephasing time of the oscillations and the time scale for energy transfer to the solvent. The experimental and calculated vibrational amplitudes are in excellent agreement, which shows that all the absorbed energy goes into expansion. However, the phases of the calculated and experimental traces do not match. The calculations can be brought into almost perfect agreement with the experimental results by including hot-electron pressure effects in the coefficient for thermal expansion of the particles. This contribution is significant in our experiments because laser excitation initially creates a very high electronic temperature. A simple expression for the time dependence of the transient absorption signal is also derived that explicitly accounts for sample polydispersity. In this expression the beat period is related to the mean radius, and the damping time to the mean radius and the width of the size distribution. Thus, time-resolved laser experiments can be used to obtain accurate information about the size distribution of metal particle samples.

Evidence for a Millisecond Pulsar in 4U 1636-53 during a Superburst

Abstract: We report the discovery with the Proportional Counter Array on board the Rossi X-ray Timing Explorer of highly coherent 582 Hz pulsations during the February 22, 2001 (UT) 'superburst' from 4U 1636-53. The pulsations are detected during an 800 s interval spanning the flux maximum of the burst. Within this interval the barycentric oscillation frequency increases in a monotonic fashion from 581.89 to 581.93 Hz. The predicted orbital motion of the neutron star during this interval is consistent with such an increase as long as optical maximum corresponds roughly with superior conjunction of V801 Arae, the optical companion to the neutron star in 4U 1636-53. We show that a range of circular orbits with 90 phi(sub 0) > 0.277 for the neutron star can provide an excellent description of the frequency and phase evolution. The brevity of the observed pulse train with respect to the 3.8 hour orbital period unfortunately does not allow more precise constraints. The average pulse profile is sinusoidal and the time averaged pulsation amplitude, as inferred from the half amplitude of the sinusoid is 1%, smaller than typical for burst oscillations observed in normal thermonuclear bursts. We do not detect any higher harmonics nor the putative subharmonic near 290 Hz. The 90% upper limits on signal amplitude at the subharmonic and first harmonic are 0.1 and 0.06%, respectively. The highly coherent pulsation, with a Q = v(sub 0)/delta-v > 4.5 x 10(exp 5) provides compelling evidence for a rapidly rotating neutron star in 4U 1636-53, and further supports the connection of burst oscillation frequencies with the spin frequencies of neutron stars. Our results provide further evidence that some millisecond pulsars are spun up via accretion in LMXBs. We also discuss the implications of our orbital velocity constraint for the masses of the components of 4U 1636-53.

Pulsed Z-spectroscopic imaging of cross-relaxation parameters in tissues for human MRI: theory and clinical applications.

Abstract: A new method of pulsed Z-spectroscopic imaging is proposed for in vivo visualization and quantification of the parameters describing cross-relaxation between protons with liquid-like and solid-like relaxation properties in tissues. The method is based on analysis of the magnetization transfer (MT) effect as a function of the offset frequency and amplitude of a pulsed off- resonance saturation incorporated in a spoiled gradient-echo MRI pulse sequence. The theoretical concept of the method relies on an approximated analytical model of pulsed MT that provides a simple three-parameter equation for a pulsed steady-state Z-spectrum taken far from resonance. Using this model, the parametric images of cross-relaxation rate constant, content, and T2 of the semisolid proton fraction can be reconstructed from a series of MT-weighted images and a coregistered T1 map. The method was implemented on a 0.5 T clinical MRI scanner, and it provided high-quality 3D parametric maps within an acceptable scanning time. The estimates of cross-relaxation parameters in brain tissues were shown to be quantitatively consistent with the literature data. Clinical examples of the parametric images of human brain pathologies (multiple sclerosis and glioma) demonstrated high tissue contrast and clear visualization of the lesions. Magn Reson Med 47:929–939, 2002. © 2002 Wiley-Liss, Inc.

Solitary waves in the Madelung's fluid: Connection between the nonlinear Schrödinger equation and the Korteweg-de Vries equation

Abstract: An investigation to deepen the connection between the family of nonlinear Schrodinger equations and the one of Korteweg-de Vries equations is carried out within the context of the Madelung's fluid picture. In particular, under suitable hypothesis for the current velocity, it is proven that the cubic nonlinear Schrodinger equation, whose solution is a complex wave function, can be put in correspondence with the standard Korteweg-de Vries equation, is such a way that the soliton solutions of the latter are the squared modulus of the envelope soliton solution of the former. Under suitable physical hypothesis for the current velocity, this correspondence allows us to find envelope soliton solutions of the cubic nonlinear Schrodinger equation, starting from the soliton solutions of the associated Korteweg-de Vries equation. In particular, in the case of constant current velocities, the solitary waves have the amplitude independent of the envelope velocity (which coincides with the constant current velocity). They are bright or dark envelope solitons and have a phase linearly depending both on space and on time coordinates. In the case of an arbitrarily large stationary-profile perturbation of the current velocity, envelope solitons are grey or dark and they relate the velocity u0 with the amplitude; in fact, they exist for a limited range of velocities and have a phase nonlinearly depending on the combined variable x-u0 s (s being a time-like variable). This novel method in solving the nonlinear Schrodinger equation starting from the Korteweg-de Vries equation give new insights and represents an alternative key of reading of the dark/grey envelope solitons based on the fluid language. Moreover, a comparison between the solutions found in the present paper and the ones already known in literature is also presented.

Closed-Form Solutions to the Exact Optimizations of Dynamic Vibration Absorbers (Minimizations of the Maximum Amplitude Magnification Factors)

Abstract: A typical design problem for which the fixed-points method was originally developed is that of minimizing the maximum amplitude magnification factor of a primary system by using a dynamic vibration absorber This is an example of usual cases for which their exact solutions are not obtained by the well-known heuristic approach. In this paper, more natural formulation of this problem is studied, and algebraic closed-form exact solutions to both the optimum tuning ratio and the optimum damping coefficient for this classic problem are derived under assumption of undamped primary system. It is also proven that the minimum amplitude magnification factor, resonance and anti-resonance frequencies are entirely algebraic.

Turbulence mechanism in Klebanoff transition: a quantitative comparison of experiment and direct numerical simulation

Abstract: The mechanism of turbulence development in periodic Klebanoff transition in a boundary layer has been studied experimentally and in a direct numerical simulation (DNS) with controlled disturbance excitation. In order to compare the results quantitatively, the flow parameters were matched in both methods, thus providing complementary data with which the origin of turbulence in the transition process could be explained. Good agreement was found for the development of the amplitude and shape of typical disturbance structures, the Λ-vortices, including the development of ring-like vortices and spikes in the time traces. The origin and the spatial development of random velocity perturbations were measured in the experiment, and are shown together with the evolution of local high-shear layers. Since the DNS is capable of providing the complete velocity and vorticity fields, further conclusions are drawn based on the numerical data. The mechanisms involved in the flow randomization process are presented in detail. It is shown how the random perturbations which initially develop at the spike-positions in the outer part of the boundary layer influence the flow randomization process close to the wall. As an additional effect, the interaction of vortical structures and high-shear layers of different disturbance periods was found to be responsible for accelerating the transition to a fully developed turbulent flow. These interactions lead to a rapid intensification of a high-shear layer very close to the wall that quickly breaks down because of the modulation it experiences through interactions with vortex structures from the outer part of the boundary layer. The final breakdown process will be shown to be dominated by locally appearing vortical structures and shear layers.

Non-Linear Behavior of a Rubber Isolator System Using Fractional Derivatives

Abstract: A non-linear rubber isolator included in a dynamic system is examined where influences of dynamic amplitude and frequency are investigated through measurements and modeling. The frequency dependence of the isolator is modeled by a fractional calculus element while a frictional component accounts for its amplitude dependence. The model works in the time-domain and simulations of harmonic and non-harmonic motion are compared to measurements. Good agreement is obtained in a wide frequency and amplitude range for a freely oscillating one degree of freedom system, with the isolator acting as a coupling between exciting foundation and mass, and for a single isolator showing the typical amplitude dependence known as the Payne effect. The model is found to be superior to the commonly applied Kelvin-Voigt element in modeling the dynamic isolator properties.

Abundance stratification and pulsation in the atmosphere of the roAp star $\boldmath\gamma$ Equulei

Abstract: We present the evidence for abundance stratication in the atmosphere of the rapidly oscillating Ap star Equ. Ca, Cr, Fe, Ba, Si, Na seem to be overabundant in deeper atmospheric layers, but normal to underabundant in the upper layers with a transition in the typical line forming region of 1:5 < log5000 < 0:5. This stratication prole agrees well with diusion theory for Ca and Cr, developed for cool magnetic stars with a weak mass loss of 2:5 10 15 M yr 1 . Pr and Nd from the rare earth elements have an opposite prole. Their abundance is more than 6 dex higher above log5000 8:0 than in the deeper atmospheric layers. We further discuss the implications of abundance stratication in the context of radial velocity amplitudes and phases observed by Kochukhov & Ryabchikova (2001) for a variety of spectral lines and elements using high spectral and time resolved, high S/N observations.

How fins affect the economy and efficiency of human swimming.

Abstract: The aim of the present study was to quantify the improvements in the economy and efficiency of surface swimming brought about by the use of fins over a range of speeds (v) that could be sustained aerobically. At comparable speeds, the energy cost (C) when swimming with fins was about 40 % lower than when swimming without them; when compared at the same metabolic power, the decrease in C allowed an increase in v of about 0.2 ms(-1). Fins only slightly decrease the amplitude of the kick (by about 10 %) but cause a large reduction (about 40 %) in the kick frequency. The decrease in kick frequency leads to a parallel decrease of the internal work rate ((int), about 75 % at comparable speeds) and of the power wasted to impart kinetic energy to the water ((k), about 40 %). These two components of total power expenditure were calculated from video analysis ((int)) and from measurements of Froude efficiency ((k)). Froude efficiency (eta(F)) was calculated by computing the speed of the bending waves moving along the body in a caudal direction (as proposed for the undulating movements of slender fish); eta(F) was found to be 0.70 when swimming with fins and 0.61 when swimming without them. No difference in the power to overcome frictional forces ((d)) was observed between the two conditions at comparable speeds. Mechanical efficiency [(tot)/(Cv), where (tot)=(k)+(int)+(d)] was found to be about 10 % larger when swimming with fins, i.e. 0.13+/-0.02 with and 0.11+/-0.02 without fins (average for all subjects at comparable speeds).