Journal of Engineering Mechanics

Series of plane turbulent negatively buoyant jet (or fountain) experiments were carried out by discharging saline water vertically upward from a rectangular slot into a stagnant fresh water environment. The plane jets were characterized by the formation of fronts that became unstable as the densimetric Froude number increased. When the Froude number was large (F> 10) the maximum penetration Z_m was mainly controlled by the momentum flux M_o and the buoyancy flux B_o, and the measurements confirmed that the asymptotic solution for large Froude numbers is Z_m=C_mb_oF^4/3, where b_o is the slot half-width, and the constant C_m= 2.0. At small Froude numbers, the maximum penetration was less than predicted by this asymptotic solution, and the deviations from this asymptotic solution were correlated using the mass flux Q_o and the buoyancy flux B_o. This correlation suggested that the virtual origin defined by the deviation from the large Froude number solution was located at a distance Z_q=C_qb_oF^2/3, with the constant C_q= 1.12. An attempt was made to explain large discrepancies in previous investigations.

This paper describes a new experimental method using infrared thermography for the evaluation of wind impact at pedestrian level. The developed measurement system consists of a heated plate on which building models stand, an infrared video camera for detecting the surface temperature distributions on the heated plate, and a microcomputer for image processing and presentation. Since the heat loss from the heated plate can be related to air flow speeds by the principle of heat convection, the surface temperature pattern is used as an indicator of the wind impact at the pedestrian level. Results have been derived from thermal images around various buildings both on selected points and over entire affected areas that are quantified by the reduced temperature and the overall impact, respectively. The paper presents details of the experimental setup, operational principles, calibration results, application examples, and further thoughts on this technique. At the current stage, the infrared technique is capable of visualizing surface flow fields through color images, quantifying windy zones in terms of temperature variation, and evaluating the overall wind impact by further calculation.

The flexible interface model is adopted in this paper to analyze the three-dimensional (3D) elastic constitutive relationships, together with the local stress and strain distributions within constituent layers of an imperfectly bonded multilayered medium. Each constituent layer is assumed to be generally anisotropic with 21 independent elastic moduli, and three damage parameters are used to model the imperfect bonding state of each interface. Due to the generality of this model, elastic constitutive relationships for layered media composed of layers of other kinds of elastic symmetry can be readily achieved by algebraic reductions. From the comparison between theoretical predictions and experimental results on artificial bilaminated rock masses composed of isotropic layers, this model is shown to be a reasonable substitute of the conventional perfect bonding constitutive model.

In this paper, a multitangent-plane surface method is suggested for calculating the reliability of complex structures of which the limit-state surfaces are not known explicitly. In this approach, multiple tangent planes are used to approximate the actual limit-state surface. Also radial importance sampling is used in conjunction with probabilistic finite-element (PFE) analysis. Numerical results suggest that the new method can be used to improve the accuracy of solutions obtained from first-order second moment (FOSM) analysis and also require considerably less CPU time when compared with Monte Carlo simulation (MCS) analysis. This method is only suitable for the case of a nonlinear concave or convex limit-state surface.

A theory that encompasses a wide class of elastoplastic models with damage is provided. The coupling between plasticity and damage is based on the concept of effective stress and on the definition of a convex elastic-damage domain and a convex damage domain. Some of the most significant models of elastoplasticity coupled with isotropic and anisotropic damage are derived and analyzed from a unitary point of view.

The first known investigation on the problem of free vibration of symmetric cross-ply laminated plates having elastically restrained edges is considered. The Ritz method is used in combination with a variational formulation and a first-order transverse shear deformation theory. The Ritz functions consist of polynomials that include a “basic function” that imposes the free edge boundary conditions. The elastic edge flexibilities are considered by both the linear elastic rotational and translational supports to act simultaneously. Numerical results are obtained to investigate the effects of elastic restraint stiffness, material properties, and geometric parameters upon the natural frequencies of flexural vibration.

A combination of finite element formulation and wave function expansion is used to investigate the scattering of axisymmetric guided waves by an interphase region with bonding material between two laminated cylinders. The motivation of this study is to develop an effective numerical technique to characterize the material properties and the thickness of the bond region between two cylinders that are laminated and anisotropic. Numerical results of wave scattering are presented for a welded steel pipe with different weldment shapes and for two cross-ply symmetrically laminated composite cylinders that are bonded end to end. It is shown that as the frequency increases the reflection coefficients have resonance peaks while the transmission coefficients have sharp drops at the cutoff frequencies of higher guided modes in the cylinders. The resonance peaks of reflection coefficients become increasingly pronounced as the thickness of the joint increases. Another interesting feature is that the reflection coefficients have minimums at frequencies that depend on the thickness and the material properties of the joint. These features may be useful in quantitative nondestructive evaluation of joint properties.

The paper presents a nonlinear fracture mechanics analysis of mixed-mode fracture and anchor pullout in concrete. The analysis is based on elements with inner softening bands (ISB). A major advantage of the ISB approach is that discrete cracks can be introduced anywhere and in any direction within the finite-element mesh. Calculated loads, deformations, and crack patterns are compared to experimental results. A good correspondence is achieved.

The moment scheme of the finite element method and the method of generalized coordinates are used to construct a multi-degree-of-freedom nonlinear model of a cylindrical shell subjected to two-frequency excitations. This model consists of a system of nonlinear differential equations. The incremental method is then used to find the solution of the equation in the frequency domain, while the Poincaré map, spectral analysis, and Floquet's theory are applied to the stability of the solution at every step of the incremental method. Solutions and discussions are presented to substantiate the suggested algorithm. It is shown that similar results are obtained by using the Poincaré map with numerical integration and Floquet's theory with Fourier's expansion. However, Floquet's theory is a lot less time-consuming and it pinpoints more accurately the moment of loss of stability.

The nonlinear oscillatory water wave propagating over a constant depth channel with a horizontal poroelastic bed of infinite thickness is investigated. The channel flow is treated as potential flow, while the channel bed is considered a poroelastic material. Applying proper boundary conditions, this study uses the Stokes expansion of deep water wave to the second order to handle the problem. The result is compared to the linear wave solution of Huang and Song in order to show the nonlinearity effect, and is also compared to experimental data to verify the correctness of this study. It is found that because the wavelength of the second longitudinal wave inside the porous bed is much shorter than that of the water wave when the bed material is soft, the conventional Stokes expansion of deep water wave based on k₀a is only valid for hard bed material but invalid for soft bed material even though the Ursell parameter is small. An extra constraint for permeable soft bed material other than the Ursell parameter is thus proposed in this study in order to make the Stokes expansion of deep water wave be valid. It is also found that Biot's theory of poroelasticity is proper in simulating hard material bed. The nonlinear effect of water wave with a permeable hard material bed occurs easily even when both $\left|{\mathit{k}}_0\mathit{a}\right|$ and the Ursell parameter are very small. And the major influence of the nonlinearity is only on the water wave itself, not on the porous medium.

An experimental investigation has been conducted to compare and contrast the dynamic load–transfer process in single-chain assemblies of twoand three-dimensional particles. In particular, single-chain assemblies of disks and spheres were subjected to explosive loadings and strain gauges were used to collect the resulting strain pulse information. The data were analyzed to compare the pulse velocity, contact load attenuation, dispersion, and pulse breakup in disks and spheres. The comparison shows that the average pulse velocity in disks is higher than in spheres and the pulse attenuation is lower in the disks. The results also show that there is a characteristic pulse that will propagate through a single chain of disks without significant dispersion. However, the characteristic pulse propagating in assemblies of spheres shows dispersion. For both disks and spheres, when the input pulse is sufficiently long, it undergoes a ringing process, which breaks the long pulse up into smaller signals of size approximately equal to the characteristic length. The larger the input pulse is with respect to the characteristic pulse length, the longer the breakup takes. This breakup is also accompanied by a redistribution of energy, which is easily seen in the frequency domain.

Permanent form periodic waves with zero-average mass flux are generated in a two-dimensional numerical wave tank solving fully nonlinear potential flow equations. An absorbing beach is modeled at the end of the tank in which (1) an external free-surface pressure absorbs energy from high frequency waves; and (2) a pistonlike condition absorbs energy from low-frequency waves. A feedback mechanism adaptively calibrates the beach parameters to absorb the period-averaged energy of incident waves. Wave generation and absorption are validated over constant depth, for tanks and beaches of various lengths, and optimal parameter values are identified for which reflection from the beach is reduced to a few percent. Shoaling of periodic waves is then modeled over a 1:50 slope, up to very close to the breaking point. A quasi-steady state is reached in the tank for which (not previously calculated) characteristics of fully nonlinear shoaling waves are obtained.

A new theory for the analysis of composite beams of open and closed cross section has been developed. The theory takes into account deformation in the plane of the cross section due to anticlastic curvature. Box beams and I-sections containing cross-sectional elements that are angle-ply midplane symmetric laminates have been analyzed with the new theory. It is shown that the inclusion of the anticlastic curvature substantially reduces the predicted buckling loads for these beams. The effect is most pronounced when there are 45° or near 45° layers in the laminates. In addition, box beams with a more optimized lamination scheme are analyzed. It is shown that the anticlastic curvature terms are important when buckling about the weak axis is considered.

An efficient stochastic simulation method is presented for generating sample time histories of multidimensional, spatially correlated, stationary Gaussian random fields. A key feature of the method is the use of filtering functions in directly expressing the conditional mean value of any component of the random field. The general formulation and an updating algorithm for determining the required filtering functions for simulation of any general multidimensional random field are established. Also, emphasis is placed on the efficiencies, which can be achieved when relatively simple restrictions are placed on the cross-spectral densities and the incoherence structure of the random field. These restrictions, which are commonly used in modeling seismic ground motion, are shown to allow the computational effort to be reduced by a factor of more than 100 for a three-dimensional random field. Numerical results are presented to illustrate the application of the direct simulation method to the generation of time histories of seismic ground motion. The method is shown to be both efficient and accurate.

The objectives of control of structures in earthquakes are to prevent injury to occupants and damage to contents and to protect the integrity of the structure. Both objectives can be achieved through base isolation and feedback control, whereby the structure is stabilized relative to the inertial space. Nonlinear on-off control with a dead zone in conjunction with direct feedback has the advantages of economy and simplicity of implementation. Indeed, the actuators operate only when the velocity exceeds certain threshold values and only in response to sensor signals at the same location. An example using two actuators, one on the base and the other at the top of the structure, shows very good performance of the control design.

Scaling exponents originally derived for free air explosions are applied to blast loadings generated by explosions in a room via a model. Using this model, blast loadings on the solid boundaries of a room from one yield can be scaled to those from another yield in the same room. It is shown that as long as the two yields do not differ by more than a factor of several, the scaled blast loading agrees with the calculated or measured loading at the new yield. The agreement in the impulse is within a few percent as long as the point is away from corners. Near corners, the scaled and the calculated impulses differ by as much as 24% because of the highly nonlinear nature of the reflections at corners. Numerical values of the scaling exponents derived from several experiments suggest that the impulse is proportional to the yield 2/3.

Closed-form solutions for stresses and displacements in a polar-anisotropic circular plate subjected to a constant temperature change are obtained. The plate is mounted on a rigid circular shaft with its outer boundary free of any constraints. General anisotropy in the plane of the plate is obtained by aligning the fiber reinforcements at an arbitrary angle to the radial direction. The solutions are presented as functions of the fiber orientations. It has been shown that due to general polar-anisotropy, unlike the polar-orthotropic case, the circumferential displacement is not zero and changes in magnitude with respect to the fiber orientation.