Employing controlled vibrations to predict fluid forces on a cylinder undergoing vortex-induced vibration

In the present study, we measure the fluid forces on a vertical cylinder that is forced to vibrate transversely to a water channel flow, and compare directly to the forces encountered by freely vibrating cylinders, under conditions where we carefully match the amplitude, frequency, and Reynolds number (Re) of the two cases. A key point is that we use precisely the same cylinder and submerged flow configuration for both the free and controlled cases. Where the free vibration exhibits closely sinusoidal motion, the controlled sinusoidal motion yields forces in close agreement with the free vibration case. Although this result might be expected, previous comparisons have not been uniformly close, which highlights the importance of matching the experimental conditions precisely, and of accurately measuring the phase between the force and body motion. For a lightly damped system, which is perhaps the most significant case to analyze, one typically finds that the maximum response amplitude is quite unsteady. One might conventionally expect prediction of forces to be difficult in such cases. However, it is of practical significance that, even in this case, a quasi-steady approximation is effective. This is a significant point because it suggests that controlled vibration measurements for constant amplitude motion might remain applicable to free vibration systems undergoing even transient or intermittent motions.     

Numerical simulation of water entry of twin wedges

The hydrodynamic problem of twin wedges entering water vertically at constant speed is analysed based on the velocity potential theory. The gravity effect on the flow is ignored based on the assumption that the ratio of the entry speed to the acceleration due to gravity is much larger than the time scale of interest. The problem is solved using the complex velocity potential together with the boundary element method through three stages. When the body touches water, the similarity solution is obtained for each wedge in isolation. This is used as the initial solution at the second stage for the time stepping technique for each wedge in a stretched system defined through the ratio of the Cartesian system to the distance the wedge travelled into water. When the disturbed zone of each wedge begins to affect the flow generated by the other wedge, the stretched system is abandoned and the original system is used. At the third stage the full interactions between the two wedges are included. Various results are provided for the wave elevation, pressure distribution and force at different deadrise angles. They are compared with those obtained from a single wedge and the interaction effect is investigated.     

A novel time-division multiplexing fiber Bragg grating sensor interrogator for structural health monitoring

A novel fiber Bragg grating (FBG) sensor system for measurement of strain and temperature is proposed in this paper. The proposed sensor technique is based on time-division multiplexing (TDM). A semiconductor optical amplifier (SOA), connected in a ring cavity, is used to serve as a gain medium and switch. The SOA is driven by a pulse generator, which operates the SOA at different periods of time to select reflected pulses from a particular sensor. The FBG sensors have identical center wavelengths and can be deployed along the same fiber. This technique relieves the spectral bandwidth issue and permits the interrogation of up to 100 FBGs along a fiber, if the reflectivity of the individual sensors is sufficiently low to avoid shadowing effects. This system is particularly suitable for the application in structural health monitoring (SHM) where large numbers of sensors are required in wide measurement ranges.     

Closed form solutions of Euler–Bernoulli beams with singularities

The problem of the integration of the static governing equations of the uniform Euler–Bernoulli beam with discontinuities is studied. In particular, two types of discontinuities have been considered: flexural stiffness and slope discontinuities. Both the above mentioned discontinuities have been modeled as singularities of the flexural stiffness by means of superimposition of suitable distributions (generalized functions) to a uniform one dimensional field. Closed form solutions of governing differential equation, requiring the knowledge of the boundary conditions only, are proposed, and no continuity conditions are enforced at intermediate cross-sections where discontinuities are shown. The continuity conditions are in fact embedded in the flexural stiffness model and are automatically accounted for by the proposed integration procedure. Finally, the proposed closed form solution for the cases of slope discontinuity is compared with the solution of a beam having an internal hinge with rotational spring reproducing the slope discontinuity.     

Tribological properties of epoxy/polyurea composite

The tribological behavior of epoxy/polyurea composite under dry friction was investigated. The worn surface morphologies of epoxy and epoxy/polyurea composite have been analyzed using scanning electron microscopy (SEM), and the functional group of epoxy and epoxy/polyurea composite before and after wear have been analyzed by a Fourier transform infrared spectrometer (FT‐IR). It has been found that epoxy terminal groups, which exist in an epoxy molecule, can react with the end group amine of the polyurea copolymer, generating more H? O bonds, which enhances the deformation capacity. Polyurea particles are dispersed uniformly in epoxy matrix before wear, while plastic deformation and distorted domains occur on the worn surface of epoxy/polyurea composite. This indicates that the wear resistance of epoxy/polyurea composite was greatly improved by the addition of polyurea elastomer, which led to low frictional coefficients and wear mass losses. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of the ICE-PIC method for the dimension reduction of chemical kinetics coupled with transport

A new dimension-reduction method, the Invariant Constrained-equilibrium Edge Pre-Image Curve (ICE-PIC) method, to simplify chemical kinetics has recently been developed by Ren et al. [Z. Ren, S.B. Pope, A. Vladimirsky, J.M. Guckenheimer, J. Chem. Phys. 124 (2006) 114111]. In the present work, the ICE-PIC method is first applied to the homogeneous autoignition of stoichiometric methane/air and its accuracy is shown to compare favorably to those of other methods (QSSA and RCCE). For inhomogeneous systems such as flames, spatial transport by molecular diffusion causes a small perturbation of the composition away from the attracting, low-dimensional, invariant manifold identified by the ICE-PIC method. A “close-parallel” assumption is introduced which allows this perturbation to be determined, and leads to an additional “transport coupling” term in the evolution equation for the reduced variables. For the test case of a steady, one-dimensional, laminar, methane/air flame, it is shown that the inclusion of transport coupling can reduce the dimension-reduction errors by a factor of 100. The ICE-PIC method with eight degrees of freedom (including transport coupling) exhibits comparable accuracy to a quasi-steady state assumption (QSSA) reduced mechanism with 12 degrees of freedom.     

A wide-range experimental and kinetic modeling study of the pyrolysis and oxidation of 2-butyne

To reduce particulate emissions leading to a cleaner environment, it is important to understand how polycyclic-aromatic hydrocarbons (PAHs) and their precursors are formed during combustion. 2-butyne can decompose to propargyl and allyl radicals. These radicals can produce benzene and other PAHs, leading to the formation of soot. In the present study, pyrolysis, oxidation, and laminar flame speed experiments were performed for 2-butyne. The pyrolysis experiments were conducted in a single-pulse shock tube at 2 bar in the temperature range 1000 – 1500 K. Ignition delay times for 2-butyne/‘air’ mixtures were measured in the pressures range 1 – 50 bar, over the temperature range 660 – 1630 K, at equivalence ratios of 0.5, 1.0, and 2.0 using rapid compression machines and shock tubes. Moreover, laminar flame speed (LFS) experiments were performed at ambient temperature, at p = 1 – 3 atm, over an equivalence ratio range of 0.6 – 1.8. A new, detailed chemical kinetic model for 2-butyne has been developed and widely validated against the data measured in this study and those available in the literature. The significant reactions for 2-butyne pyrolysis, ignition, and oxidation are identified and discussed using flux and sensitivity analyses.     

Temperature dependence of the reflectance spectrum of β-(BEDT-TTF)2I3

The temperature dependence of the polarized reflectance spectrum of β-(BEDT-TTF)₂I₃ was measured over the spectral range from 750 cm⁻¹ to 25,000 cm⁻¹. The plasma frequency and the relaxation rate were obtained by analyzing the low-temperature spectra. The absolute value and the anisotropy of the optical conductivity at low temperature agree with those of the dc conductivity within the experimental error. The dimensionless electron-phonon coupling constant was estimated to be 1.6 from the temperature dependence of the relaxation rate.     

Study of the Δ isobar excitation in nuclei with the (3He,t) reaction

Data for the Δ excitation by the (³He,t) reaction at 2.0 GeV on various nuclei are presented together with results on ¹²C at 1.5 and 2.3 GeV. The Δ peak position and width are found to be target mass independent. Comparison with the p(³He,t) Δ⁺⁺ reaction, however, shows a shift of the position in energy of the Δ peak. Implications of these results are discussed.     

Aperiodic crystals: A contradictio in terminis?

Although in the prevailing view a necessary condition for having a crystalline phase is lattice periodicity, it has become clear in the last decades that there are physical systems with many properties of the usual crystalline state but without three-dimensional lattice periodicity. Incommensurate modulated crystals have been known now for some time, and a couple of years ago much excitement was raised by the discovery of quasicrystals, systems with long-range order but with five-fold symmetry axes, which exclude lattice periodicity.A discussion is given of the various generalizations of the concept of lattice periodicity. In fact, these go from ordinary periodic crystal st structures to almost chaotic ones. One of these is the notion of quasiperiodicity. Section two deals with a special type of these quasiperiodic systems, tilings or space fillings with tiles or blocks of a small number of types. In section three the symmetry of quasiperiodic systems is discussed. Here the embedding into a higher-dimensional space is the key concept. Section four deals with N-dimensional crystallographic groups that occur as symmetry groups of quasiperiodic systems, so called superspace groups. In section five the diffraction from quasiperiodic systems is treated, and in section six it is shown that in some cases quasiperiodic structures may be approximated by periodic ones, and that periodic systems sometimes are more conveniently described by quasiperiodic ones. The emphasis in the symmetry discussion is on quasicrystals.This is even more so in the remaining sections. Section seven gives a brief account of the many experimental data, section eight describes what is known about the microscopic structure. Imperfections are even more important for quasiperiodic systems than for periodic ones. They are discussed in section nine.Not only microscopically do quasiperiodic systems have similarities with ordinary crystals, but also macroscopically. The morphological laws may be generalized to quasiperiodic systems, as shown in section ten. The consequences of quasiperiodicity on the physical properties is still to a large extent unclear. Mathematically they differ much from periodic systems. A discussion of a number of results is given in section eleven.