Acta Mechanica Sinica

Design for structural integrity requires an appreciation of where stress singularities can occur in structural configurations. While there is a rich literature devoted to the identification of such singular behavior in solid mechanics, only of late has there been much in the way of corresponding identifications of flow-induced stress singularities in fluid mechanics. These recent asymptotic identifications are for a single incompressible viscous fluid: Here the asymptotic approach is extended to apply to a configuration entailing two such fluids. For this configuration, various specifications leading to power or log singularities are determined. These results demonstrate that flow-induced stress singularities can occur in a structural container at a location where no singularities are identified within solid mechanics alone.

The initial oblique and attacking angles as well as the asymmetrical nose abrasion may lead to bending or even fracture of a projectile, and the penetration efficiency decreases distinctly. The structural stability of a high-speed projectile non-normally penetrating into concrete and the parametric influences involved are analyzed with the mass abrasion taken into account. By considering the symmetrical or asymmetrical nose abrasion as well as the initial oblique and attacking angles, both the axial and the transverse drag forces acting on the projectile are derived. Based on the ideal elastic-plastic yield criterion, an approach is proposed for predicting the limit striking velocity (LSV) that is the highest velocity at which no yielding failure has occurred and the projectile can still maintain its integral structural stability. Furthermore, some particular penetration scenarios are separately discussed in detail. Based on the engineering model for the mass loss and nose-blunting of ogive-nose projectiles established in Part I of this study, the above approach is validated by several high-speed penetration tests. The analysis on parametric influences indicates that the LSV is reduced with an increase in the asymmetrical nose abrasion, the length-diameter-ratio, and the concrete strength, as well as the oblique and attacking angles. Also, the LSV raises with an increase in the initial caliber-radius-head (CRH) and the dimensionless cartridge thickness of a projectile.

In this paper, a design method for an acoustic cloak in the presence of background mean flows is proposed by using topology optimization, which enables the associated fabrication of the cloaking design. The density-based topology optimization method is used to allocate the designated materials, thus providing the structure of the cloak. The optimization problem is efficiently solved with the gradient-based globally convergent method of moving asymptotes, which utilizes the derivative information from the finite element simulation studies of the linearized acoustic potential equation. This paper introduces the whole design method first then numerically demonstrates the corresponding performance, which shall constitute the main contribution of the present work.

Due to the empirical assumptions, the widely-used two-temperature models for hypersonic nonequilibrium flow include considerable uncertainties. To overcome the limitations and shortcomings of two-temperature models, the modified Macheret-Fridman model is developed based on the correction method of the modified Marrone-Treanor model. Some typical test cases are employed to assess the accuracy of the modified and widely-used two-temperature models. Furthermore, the reason for improving the accuracy of modified two-temperature models is analyzed and discussed. This work indicates that the correction method based on the modified Marrone-Treanor model is easily applied and extended to the other widely-used two-temperature models, significantly improving their accuracy. In addition, modeling highly nonequilibrium dissociating flows requires considering three critical respects, i.e., the dissociation rates, the vibration-dissociation coupling effect, and the non-Boltzmann effect. The non-Boltzmann effect reduces the dissociation rates and vibrational energy per dissociation. Comparatively, the dissociation rates have more influence than changing the value of the non-Boltzmann factor for vibrational energy loss per dissociation. Future work can focus on enhancing the accuracy of the dissociation rates to improve the accuracy of widely-used two-temperature models.

In this paper, by the method of collocation, using the cubic β-spline function as the trial function in the time domain and putting zero residuals of the differential equation of motion of the structure at two points of time, the authors obtain an unconditionally stable calculation scheme for the dynamic response of the structure. When a parameter σ in the scheme is within the interval 0.15<σ <0.5 the scheme is absolutely stable. It is shown that the accuracy of the scheme, as may be measured by AD (the decay of the amplitudes), PE (the elongation of periods) and the algorithmic damping ratio, is better than that of traditional methods—the Wilson-σ's method, the Newmark's method and the Houbolt's method. A numerical example is given in which a certain dynamic response problem is solved by the method of this paper and results are compared with that of the traditional methods and the analytic method showing that the accuracy of the method by this paper is superior to the other ones. The computational scheme for the dynamic response of structures by this paper may be regarded as an effective, convenient and accurate method for dynamic response of structures.

This paper deals with the application of Acousto-ultrasonics, in conjunction with Pattern Recognition and Classification techniques, to the identification of residual impact properties of a class of polymeric material, namely, Polyvinylchloride (PVC). PVC specimens of different low-energy repeated impact damage states are processed by Acousto-ultrasonics (AU) to retrieve AU signals in the form of digitalized records. These AU signals are grouped as distinct classes, each pertaining to a known level of repeated impact damage. Describing features of these AU signals are used to build Pattern Recognition (PR) Classifiers. These classifiers are used to identify unknown damage states in other PVC specimens by classifying the retrieved AU signals as belonging to one of the classes. The obtained results indicate that Acousto-ultrasonics in combination with Pattern Recognition and Classification techniques can be used for the quantitative non-destructive identification of damage states in PVC specimens of unknown low-energy repeated impact conditions.

A new concept of pseudo mean wave resistance is introduced to find theoretical mean wave resistances of the precursor soliton generation in two-layer flow over a localized topography at near-resonance in this paper. The pseudo mean wave resistance of the precursor soliton generation of two-layer flow is determined in terms of the AfKdV equation. From the theoretical results it is shown that the theoretical mean wave resistance is equal to the pseudo mean wave resistance times 1/m 1, wherem 1 is the coefficient of the fKdV equation. From the regional distribution of the energy of the precursor soliton generation at the resonant points, it is shown that ratios of the theoretical mean wave resistance and regional mean energy to the total mean energy are invariant constants, i.e. $$\mathop E\limits^ \circ _1 /\mathop E\limits^ \circ :\mathop E\limits^ \circ _2 /\mathop E\limits^ \circ :\mathop E\limits^ \circ _3 /\mathop E\limits^ \circ :< D > /\mathop E\limits^ \circ = (1/2)$$ : (−1/2):1:1, in which $$\mathop E\limits^ \circ _1 ,\mathop E\limits^ \circ _2 $$ and $$\mathop E\limits^ \circ _3 $$ are the mean energy of the generating regions of the precursor solitons, of the depression and of the trailing, wavetrain at the resonant points respectively, $$\mathop E\limits^ \circ $$ and are the total energy of the system and the theoretical mean wave resistance at the resonant points. A prediction of the theoretical mean wave resistance flow over the semicircular topography is carried out in terms of the theoretical results of the present paper. The comparison shows that the theoretical mean wave resistance is in good agreement with the numerical calculation.

The delayed detached-eddy simulation with adaptive coefficient (DDES-AC) method is used to simulate the baseline and leading-edge undulation control of dynamic stall for the reverse flow past a finite-span wing with NACA0012 airfoil. The numerical results of the baseline configuration are compared with available measurements. DDES and DDES-AC perform differently when predicting the primary and secondary dynamic stalls. Overall, DDES-AC performs better owing to the decrease of grey area between the strong shear layer and the fully three-dimensional separated flow. Moreover, the effects of the undulating leading-edge on the forces, lift gradients, and instantaneous flow structures are explored. Compared with the uncontrolled case, the lift gradient in the primary dynamic stall is reduced from 18.4 to 8.5, and the secondary dynamic stall disappears. Therefore, periodic unsteady air-loads are also reduced. Additionally, the control mechanism of the wavy leading edge (WLE) is also investigated by comparison with the straight leading edge (SLE). No sudden breakdown of strong vortices is the main cause for WLE control.