Justification of the Asymptotic Expansion Method for Homogeneous Isotropic Beams by Comparison with De Saint-Venant’s Solutions

The formal asymptotic expansion method is an attractive mean to derive simplified models for problems exhibiting a small parameter, such as the elastic analysis of beam-like structures. Usually this method is rigorously justified using convergence theorems Yu and Hodges, 2004. In this paper it is illustrated how the Saint-Venant’s solution naturally arises from the lowest order terms of an asymptotic expansion of the elastic state for the case of homogeneous isotropic beams. It is also highlighted that the Saint-Venant solutions corresponding to pure traction, bending and torsion involve the solution of the first-order microscopic problems, while for the simple bending problem, the solution of the second-order microscopic problems is needed. The second-order problems provide therefore a way to characterize the transverse shear behavior and the cross-sectional warping of the beam.     

On the Static-Limit Solutions to the Navier—Stokes Equations of Compressible Flow

We consider the zero-velocity stationary problem of the Navier--Stokes equations of compressible isentropic flow describing the distribution of the density r \varrho of a fluid in a spatial domain W ì R^N \Omega \subset {\rm R}^N driven by a time-independent potential external force [(f)\vec] = \triangledown F \vec f = \triangledown F . We study the structure of the set of all solutions to the stationary problem having a prescribed mass m > 0 and a prescribed energy. Cardinality of the solution set depends on m and it is either continuum or at most two. Conditions on m for distinguishing these cases have been found. Uniqueness for the stationary system is also studied.     

On Anisotropic Regularity Criteria for the Solutions to 3D Navier�CStokes Equations

In this short note we consider the 3D Navier–Stokes equations in the whole space, for an incompressible fluid. We provide sufficient conditions for the regularity of strong solutions in terms of certain components of the velocity gradient. Based on the recent results from Kukavica (J Math Phys 48(6):065203, 2007) we show these conditions as anisotropic regularity criteria which partially interpolate results from Kukavica (J Math Phys 48(6):065203, 2007) and older results of similar type from Penel and Pokorny (Appl Math 49(5):483–493, 2004).     

On Global Infinite Energy Solutions¶to the Navier-Stokes Equations¶in Two Dimensions

This paper studies the bidimensional Navier–Stokes equations with large initial data in the homogeneous Besov space . As long as r,q < +∞, global existence and uniqueness of solutions are proved. We also prove that weak–strong uniqueness holds for the d-dimensional equations with data in L ²(? ^d ) for d/r+ 2/q≥ 1.     

On Nonexistence for Stationary Solutions to the Navier–Stokes Equations with a Linear Strain

We consider stationary solutions to the three-dimensional Navier–Stokes equations for viscous incompressible flows in the presence of a linear strain. For certain class of strains we prove a Liouville type theorem under suitable decay conditions on vorticity fields.     

Optimum Young��s Modulus of a Homogeneous Cylinder Energetically Equivalent to a Functionally Graded Cylinder

For a functionally graded (FG) circular cylinder loaded by uniform pressures on the inner and the outer surfaces and Young??s modulus varying in the radial direction, we find lower and upper bounds for Young??s modulus of the energetically equivalent homogeneous cylinder. That is, the strain energies of the FG and the homogeneous cylinders are equal to each other. For a typical power law variation of Young??s modulus in the FG cylinder, it is shown that taking only two series terms, yields good values for bounds of the equivalent modulus. We also study two inverse problems. First, an investigation is made to find the radial variation of Young??s modulus in the FG cylinder, having a constant Poisson??s ratio, that gives the maximum value of the equivalent modulus. Second, the complementary problem of finding the radial variation of Poisson??s ratio in the FG cylinder, having a constant stiffness, that gives the maximum value of the equivalent modulus, is considered. It is found that the spatial variation of the elastic properties, that maximizes the equivalent modulus, depends strongly upon the external loading on the cylinder.     

On the Stability in Weak Topology of the Set of Global Solutions to the Navier–Stokes Equations

Let X be a suitable function space and let ${\mathcal{G} \subset X}$ be the set of divergence free vector fields generating a global, smooth solution to the incompressible, homogeneous three-dimensional Navier–Stokes equations. We prove that a sequence of divergence free vector fields converging in the sense of distributions to an element of ${\mathcal{G}}$ belongs to ${\mathcal{G}}$ if n is large enough, provided the convergence holds “anisotropically” in frequency space. Typically, this excludes self-similar type convergence. Anisotropy appears as an important qualitative feature in the analysis of the Navier–Stokes equations; it is also shown that initial data which do not belong to ${\mathcal{G}}$ (hence which produce a solution blowing up in finite time) cannot have a strong anisotropy in their frequency support.     

Partial Regularity of Weak Solutions to the Navier—Stokes Equations in the Class $ L^{\infty}(0,T;\, L^3(\Omega)^3) $

We show that if v is a weak solution to the Navier—Stokes equations in the class L^￥(0,T; L³(W)³) L^{\infty}(0,T;\, L^3(\Omega)^3) then the set of all possible singular points of v in W \Omega , at every time t₀ ? (0,T) t_0\in(0,T) , is at most finite and we also give the estimate of the number of the singular points.