Aeroelastic instability of rectangular cylinders in a heaving mode

This paper deals with wind tunnel experiments on the aeroelastic instability in a heaving mode of two-dimensional rectangular cylinders in a uniform two-dimensional flow. Both the free oscillation method and the forced oscillation method are employed for the experiments. Emphasis is placed on finding the effect of the ratio $\text{c}\text{d}$, which is the ratio of the lengths of the sides of the rectangle, to the aeroelastic instability phenomena in the vicinity of the resonance speed. Emphasis is also placed on finding possible limitations in the application of the quasi-steady aerodynamic theory to the analysis of the aeroelastic characteristics.     

Mechanisms for suppressing the transverse mode coupling instability in a circular accelerator

The chromatic and nonlinear suppression of the transverse mode coupling instability in a circular accelerator is analyzed. Analytical estimates are compared with the results of experiments and numerical simulations. The transverse mode coupling (or fast head—tail) instability is a significant factor that limits the beam intensity in circular accelerators. This instability arises when the bunch current exceeds a threshold value determined by the broadband impedance of the vacuum chamber. Feedback systems are traditionally used to suppress the instability. The experience of working with such systems shows that their efficiency depends strongly on the operating parameters of the amplifier, particularly on the chromaticity and nonlinearity of the magnetic lattice. Thus, both chromatic and nonlinear effects should be taken into account to better understand the physical processes and to increase the feedback efficiency.     

Subharmonic suppression of a transverse Kelvin-Helmholtz instability

A plasma density modulation applied at nearly twice the frequency of an unstable mode was observed to produce amplitude suppression exhibiting strong hysteresis.     

Current-induced transverse spin-wave instability in a thin nanomagnet

We show that an unpolarized electric current incident perpendicular to the plane of a thin ferromagnet can excite a spin-wave instability transverse to the current direction if source and drain contacts are not symmetric. The instability, which is driven by the current-induced "spin-transfer torque," exists for one current direction only.     

Heat propagation due to torsional vibration of shafts

Torsional vibration of rotating shafts can yield substantial temperatures in the shaft due to heat generated from material damping. It has been recently observed that such a situation in electric generators can lead to insulation failures and machine outages. In the study reported here forced torsional vibration is assumed, and both lumped mass and continuous systems are considered. The hysteretic model for material damping is used to yield the heat generation in the elastic deformation range and an elastoplastic material is assumed in the plastic range. The heat conduction equation is solved for a cylindrical shaft with surface cooling. Closed form solutions and expressions for the maximum temperatures and the maximum surface temperatures are obtained and tabulated for design purposes. It is shown that substantial temperatures can develop in shafts undergoing torsional vibration.     

Anomalous energy transfer dynamics due to torsional relaxation in a conjugated polymer

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale-exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.     

On the polarizability of a pair of cylinders in a transverse electric field

A sequential scheme for calculating the polarizability of a pair of parallel cylinders with an arbitrary (but fairly symmetric) form is suggested. An infinite set of algebraic equations is derived for coefficients involved in an expression for the potential. The numerical solution of this set makes it possible to find the polarizability with any degree of accuracy. This method is unrelated to the cylinder shape. The electrostatic properties of the cylinders are described in terms of the multipolar polarizability matrix.     

On the instability of time-domain acoustic boundary element method due to the static mode in interior problems

In the analysis of interior acoustic problems, the time domain boundary element method (TBEM) suffers the monotonically increasing instability when using the direct Kirchhoff integral. This instability is related to the non-oscillatory static acoustic mode describing the constant spatial response in an enclosure. In this work, nonphysical natures of non-oscillatory static mode influencing the instability of TBEM calculation are investigated, and a method for stabilization is studied. In TBEM calculation, the static mode is represented by two non-oscillatory eigenmodes with different eigenvalues. The monotonically increasing instability is caused by the unstable poles of non-oscillatory eigenmodes as well as very small, very low frequency noise of an input signal. Interior problems with impedance boundary condition also exhibit the monotonically increasing instability stemming from its pseudo non-oscillatory static mode due to the lack of dissipation at very low frequencies. Calculation of transient sound fields within rigid and lined boxes provides numerical evidences. It is noted that the stabilization effort by modifying the coefficient matrix based on the spectral decomposition can be used only for correcting the unstable pole. The filtering method based on the eigen-analysis must be additionally used to avoid the remaining instability caused by very low frequency noise of input signal.     

Segmented waves from a spatiotemporal transverse wave instability

We observe traveling waves emitted from Turing spots in the chlorine dioxide-iodine-malonic acid reaction. The newborn waves are continuous, but they break into segments as they propagate, and the propagation of these segments ultimately gives rise to spatiotemporal chaos. We model the wave-breaking process and the motion of the chaotic segments. We find stable segmented spirals as well. We attribute the segmentation to an interaction between front rippling via a transverse instability and front symmetry breaking by a fast-diffusing inhibitor far from the codimension-2 Hopf-Turing bifurcation, and the chaos to a secondary instability of the periodic segmentation.     

Parametric instability due to a low frequency whistler wave

A parametric instability is shown to exist in the presence of a low frequency $\text{(Ω}{}_{\mathrm{ci}}\text{～ ω}{}_0\text{? |Ω}{}_{\mathrm{ce}}\text{|)}$, finite amplitude, right hand circularly polarized wave. An expression for the growth rate and threshold is derived.     

A note on transverse vibrations of a rectangular plate with a free,rectangular, corner cut-out

An approximate solution to the title problem is presented, obtained by using the Rayleigh-Ritz method. The analysis is presented for the case of simply supported and clamped plates. For the case of a rigidly clamped plate results are presented of numerical experiments on minimizing the calculated value of the fundamental frequency coefficient by using Schmidt's approach. An experimental investigation is described on a clamped square plate with a free square, corner cut-out, which has led to the conclusion that the fundamental frequency coeficient remains practically invariant with respect to size when compared with the frequency coefficient of the fully clamped plate. A similar conclusion is arrived at by means of the mathematical model. The problem under consideration is important from a practical viewpoint since cut-outs of the type considered here are quite common in engineering practice.     

Finite,free torsional oscillations of hollow circular cylinders made of a mooney material

Finite, free torsional oscillations of hollow circular cylinders of arbitrary wall thickness and of arbitrary length are investigated. The material of the cylinder is assumed to be homogeneous, isotropic, elastic and incompressible. The governing differential equations of motion are obtained by the use of finite elasticity theory. While the exact solution of the problem requires a rather unrealistic body force distribution in the axial direction, it is shown that, in the solutions corresponding to long tubes and hollow disks, the error introduced by ignoring the body force distribution is negligibly small for large wavelengths.     

Suppression of the Rayleigh-Taylor instability due to self-radiation in a multiablation target

A scheme to suppress the Rayleigh-Taylor instability has been investigated for a direct-drive inertial fusion target. In a high-Z doped-plastic target, two ablation surfaces are formed separately-one driven by thermal radiation and the other driven by electron conduction. The growth of the Rayleigh-Taylor instability is significantly suppressed on the radiation-driven ablation surface inside the target due to the large ablation velocity and long density scale length. A significant reduction of the growth rate was observed in simulations and experiments using a brominated plastic target. A new direct-drive pellet was designed using this scheme.     

Modifications of acoustic modes and coupling due to a leaning wall in a rectangular cavity

Acoustic modes and the coupling characteristics of a rectangular-like cavity with a slight geometrical distortion introduced through a leaning wall are investigated in this paper. A pressure variation index is proposed to quantify the global changes in acoustic modes caused by the inclination of the wall. Effects on the coupling between acoustic modes and structural modes are investigated using coupling coefficients. Numerical results show a simple relationship between the distortion effect and the acoustic wavelength. The effect is most significant when the distortion approaches the half wavelength. Compared with a rectangular enclosure, the existence of the leaning wall gives rise to a much more effective coupling between the structure and the enclosure.     

Critical dynamics near a hard mode instability

Scaling hypothesis and a renormalization group procedure are formulated in the vicinity of the bifurcation point, where the behaviour is governed by inhomogeneous fluctuations. The working of the general ideas is illustrated in a model system in which the number of components of the complex order parameter field goes to infinity.An account of this work was reported at the Eighth International Seminar on Phase Transitions and Critical Phenomena (MECO), Saarbrücken, FRG, March 23–25, 1981