Hyperbolic Conservation Laws¶with Umbilic Degeneracy

We continue to study hyperbolic systems of conservation laws with umbilic degeneracy. We further extend our compactness framework established earlier to other canonical classes of quadratic flux systems with an isolated umbilic point. With the aid of this compactness framework, we establish the compactness of solution operators and the long-time behavior of entropy solutions in L ^∞ with large initial data, and we prove the convergence of the viscosity method, as well as the Lax-Friedrichs scheme and the Godunov scheme, for a canonical class of nonlinear hyperbolic systems with umbilic degeneracy.     

Stability of the tumbled state in shear flow of a nematic liquid crystal

In shear flow of a nematic liquid crystal with ₃ 0 flow alignment cannot occur. The stability of the stationary in-plane solution, the tumbled state, is investigated using abstract techniques. Employing the existence of an elastic energy a sufficient criterion for stability is formulated. This criterion depends on the in-plane solution which is obtainable as a quadrature that is non-elementary except in special cases. It is shown that the tumbled state is stable and asymptotically stable for some physical configurations. The criterion presented is not a necessary condition for stability and thus only gives a lower bound.     

On patches of uniform vorticity in a plane of irrotational flow

We derive an evolution equation for the motions of patches of vorticity (vortex). Steady state solutions of this equation that include those of Kirchhoff and Moore & Saffman are established. The m-fold symmetric, m3, hypotrochoid is an exact steady solution of this equation when rotation and strain are present. When strain is absent but rotation is present, the m-fold symmetric, m2, hypotrochoid is a perturbation solution with a dispersion relation extending that of Lamb. The case of m=2 is exact and is the Kirchhoff elliptical vortex.     

Divergence-Measure Fields,Sets of Finite Perimeter,and Conservation Laws

Divergence-measure fields in L over sets of finite perimeter are analyzed. A notion of normal traces over boundaries of sets of finite perimeter is introduced, and the Gauss-Green formula over sets of finite perimeter is established for divergence-measure fields in L. The normal trace introduced here over a class of surfaces of finite perimeter is shown to be the weak-star limit of the normal traces introduced in Chen & Frid [6] over the Lipschitz deformation surfaces, which implies their consistency. As a corollary, an extension theorem of divergence-measure fields in L over sets of finite perimeter is also established. Then we apply the theory to the initial-boundary value problem of nonlinear hyperbolic conservation laws over sets of finite perimeter.Acknowledgement We thank WILLIAM P. ZIEMER for helpful discussions, especially on the proof of Propostion 7. GUI-QIANG CHENs research was supported in part by the National Science Foundation Grants.     

Slow motion of a power law liquid drop in another immiscible power law liquid

Summary Previous work on the slow translatory motion of drops, with clean interface, in non-Newtonian media has been briefly reviewed. The velocity and stress variational principles are employed to estimate upper and lower bounds on drag force experienced by a power law fluid sphere moving slowly in another stationary immiscible power law fluid. The upper and lower bounds are reasonably close to each other in the range 1n0.4 but deviate increasingly as the fluid becomes more and more non-Newtonian. The results reported herein contain several prior results as special cases.

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Schleichende, mischungslose Bewegung eines Flüssigkeitstropfens mit Potenzgesetz in einer anderen Flüssigkeit mit Potenzgesetz

Übersicht Zunächst wird ein kurzer Überblick früherer Arbeiten zur schleichenden Translation von Tropfen in nicht-Newtonschen Flüssigkeiten gegeben. Danach werden die Variationsprinzipe für die Spannungen und Geschwindigkeiten angewendet, um obere und untere Schranken für die Widerstandsk raft abzuschätzen für den Fall, daß eine Flüssigkeitskugel mit Potenzgesetz sich schleichend und mischungsfrei in einer anderen, ruhenden Flüssigkeit mit Potenzgesetz bewegt. Obere und untere Schranken liegen befriedigend dicht beieinander für den Exponentenbereich 1n0.4, divergieren aber zunehmend mit wachsender Abweichung vom Newtonschen Verhalten. Mehrere frühere Ergebnisse sind in den hiesigen als Sonderfälle enthalten.

     

New integral estimates for deformations in terms of their nonlinear strains

If u is a bi-Lipschitzian deformation of a bounded Lipschitz domain in ⁿ (n2), we show that the L ^P norm (p1, pn) of a certain nonlinear strain function e(u) associated with u dominates the distance in L ^q (q= np/(n–p) if p if p>n) from u to a suitably chosen rigid motion of ⁿ . This work extends that of F. John, who proved corresponding estimates for p}>1 under the hypothesis that u has uniformly small strain. We also obtain a bound for the oscillation of Du in L ². These estimates are apparently the first to apply with no a priori pointwise hypotheses upon the strain of u. In ³ the integral e(u) ² d³ is dominated by typical hyperelastic energy functionals proposed in the literature for modeling the behavior of rubber; thus the case n=3, p=2 gives the first general bound for the deformations of such materials in terms of the associated nonlinear elastic work.     

Functional and measure-valued solutions of the euler equations for flows of incompressible fluids

We consider the notion of a functional solution of the Euler equations for incompressible fluid flows. We show that a functional solution can be constructed under very weak a priori estimates on approximate solution sequences of the equation; an estimate uniform in L _loc ¹ together with weak consistency with the equation is sufficient to construct a solution. We prove that if we have an estimate uniform in L _loc ² available for the approximate solution sequence, then the structured functional solution just described becomes a measure-valued solution in the sense of DiPerna & Majda. We also show that a functional solution can be obtained from a measure-valued solution. We give an example showing that a much higher concentration of energy than in the case of measure-valued solutions is allowed by the approximation procedure of a functional solution.     

Fluctuating flow in a rotating fluid

Summary Fluctuating flow of a viscous fluid rotating over a disk whose angular velocity oscillates about a nonzero mean is investigated. Initially the disk and the fluid rotate in the same sense with different angular velocities ₁ and ₂ ( ₂> ₁) and at a particular instant of time, the angular velocity of the disk becomes ₁[1+ sin( )]. The problem is solved as an initial boundary value problem and it is found that for small values of the results of analytical and numerical methods are in excellent agreement. The effect of frequency parameter on surface skin frictions has been analysed for various values of angular velocity ratio s and amplitude parameter .

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Fluktuierende Strömung in einer rotierenden Flüssigkeit

Übersicht Untersucht wird die fluktuierende Strömung einer viskosen Flüssigkeit, die über einer Scheibe, deren Winkelgeschwindigkeit um einen von Null verschiedenen Mittelwert schwankt, rotiert. Anfangs drehen sich die Scheibe und die Flüssigkeit gleichsinnig, aber mit verschiedenen Winkelgeschwindigkeiten ₁ und ₂ ( ₂> ₁). Zu einem Anfangszeitpunkt geht die Winkelgeschwindigkeit der Scheibe über in ₁[1+ sin ( )]. Die Aufgabe wird als Anfangs-/Randwertproblem gelöst. Für kleine Werte stimmen die analytischen und numerischen Ergebnisse hervorragend überein. Für verschiedene Werte des Winkelgeschwindigkeitsverhältnisses und des Amplitudenparameters wurde der Einfluß des Frequenzparameters auf die Reibspannungen an der Scheibe untersucht.

     

Global Solution to the Generalized Riemann Problem in the Presence of a Boundary and Contact Discontinuities

This work is a continuation of our previous work. In the present paper we study the global structure stability of the Riemann solution $u=U(\frac{x}{t})$ containing only contact discontinuities for general n×n quasilinear hyperbolic systems of conservation laws in the presence of a boundary. We prove the existence and uniqueness of a global piecewise C ¹ solution containing only contact discontinuities to a class of the generalized Riemann problems for general n×n quasilinear hyperbolic systems of conservation laws in a half space. Our result indicates that this kind of Riemann solution $u=U(\frac{x}{t})$ mentioned above for general n×n quasilinear hyperbolic systems of conservation laws in the presence of a boundary possesses a global nonlinear structure stability. Some applications to quasilinear hyperbolic systems of conservation laws occurring in physics and other disciplines, particularly to the system describing the motion of the relativistic string in Minkowski space R ^1?+?n , are also given.     

Existence of planar flame fronts in convective-diffusive periodic media

We prove the existence of planar travelling wave solutions in a reaction-diffusion-convection equation with combustion nonlinearity and self-adjoint linear part in R ⁿ, n1. The linear part involves diffusion-convection terms and periodic coefficients. These travelling waves have wrinkled flame fronts propagating with constant effective speeds in periodic inhomogeneous media. We use the method of continuation, spectral theory, and the maximum principle. Uniqueness and monotonicity properties of solutions follow from a previous paper. These properties are essential to overcoming the lack of compactness and the degeneracy in the problem.     

Nonlinear Stability for Multidimensional Fourth-Order Shock Fronts

We consider the question of stability for planar wave solutions that arise in multidimensional conservation laws with only fourth-order regularization. Such equations arise, for example, in the study of thin films, for which planar waves correspond to fluid coating a pre-wetted surface. An interesting feature of these equations is that both compressive, and undercompressive, planar waves arise as solutions (compressive or undercompressive with respect to asymptotic behavior relative to the un-regularized hyperbolic system), and numerical investigation by Bertozzi, Münch, and Shearer indicates that undercompressive waves can be nonlinearly stable. Proceeding with pointwise estimates on the Green's function for the linear fourth-order convection–regularization equation that arises upon linearization of the conservation law about the planar wave solution, we establish that under general spectral conditions, such as appear to hold for shock fronts arising in our motivating thin films equations, compressive waves are stable for all dimensions d≧2 and undercompressive waves are stable for dimensions d≧3. (In the special case d=1, compressive waves are stable under a very general spectral condition.) We also consider an alternative spectral criterion (valid, for example, in the case of constant-coefficient regularization), for which we can establish nonlinear stability for compressive waves in dimensions d≧3 and undercompressive waves in dimensions d≧5. The case of stability for undercompressive waves in the thin films equations for the critical dimensions d=1 and d=2 remains an interesting open problem.     

Floquet bundles for scalar parabolic equations

For linear scalar parabolic equations such as on a finite interval 0x, with various boundary conditions, we obtain canonical Floquet solutions u _n (t, x). These solutions are characterized by the property that z(u _n (t, x))=n for all t, where z(·) denotes the zero crossing (lap) number of Matano. The coefficients a(t, x) and b(t, x) are not assumed to be periodic in t, but if they are, the solutions u _n (t, x) reduce to the standard Floquet solutions. Our results may naturally be expressed in the language of linear skew product flows. In this context, we obtain for each N1 an exponential dichotomy between the bundles span {u _n (·,·)} _n ^=1/N and .     

Quasistatic Crack Growth in Nonlinear Elasticity

In this paper, we prove a new existence result for a variational model of crack growth in brittle materials proposed in [19]. We consider the case of n-dimensional nonlinear elasticity, for an arbitrary n1, with a quasiconvex bulk energy and with prescribed boundary deformations and applied loads, both depending on time.     

Well-Posedness for Hyperbolic Systems of Conservation Laws with Large BV Data

We study the Cauchy problem for a strictly hyperbolic n×n system of conservation laws in one space dimension assuming that the initial data has bounded but possibly large total variation. Under a linearized stability condition on the Riemann problems generated by the jumps in we prove existence and uniqueness of a (local in time) BV solution, depending continuously on the initial data in L¹_loc. The last section contains an application to the 3×3 system of gas dynamics.     

The unique limit of the Glimm scheme

We introduce the definitions of a standard Riemann semigroup and of a viscosity solution for a nonlinear hyperbolic system of conservation laws. For a class including general 2×2 systems, it is proved that the solutions obtained by a wavefront tracking algorithm or by the Glimm scheme are precisely the semigroup trajectories. In particular, these solutions are unique and depend Lipschitz continuously on the initial data in the L ¹ norm.     

Error Bounds for a Deterministic Version of the Glimm Scheme

Consider the hyperbolic system of conservation laws . Let u be the unique viscosity solution with initial condition , and let u ^ε be an approximate solution constructed by the Glimm scheme, corresponding to the mesh sizes . With a suitable choice of the sampling sequence, we prove the estimate (Accepted September 13, 1996)     

A stable least-squares finite element method for non-linear hyperbolic problems

A class of stable least-square finite element methods for non-linear hyperbolic problems is developed and some exploratory studies made. The methods are based on modifying the L²-norm of the. residual and a related approximation to the H¹-norm of the residual. The effect of the additional terms in these residual functionals is to introduce a dissipative effect proportional to the solution gradient. This acts to stabilize the solution for non-linear hyperbolic problems which generate shocks. Numerical results for a one-dimensional nozzle and shock tube problem demonstrate the accuracy and stability of the method. Results are for an implicit scheme and calculations for linear, quadratic and cubic elements are given.     

Long-time Stability in Systems of Conservation Laws,Using Relative Entropy/Energy

We study the long-time stability of shock-free solutions of hyperbolic systems of conservation laws, under an arbitrarily large initial disturbance in L ²∩ L ^∞. We use the relative entropy method, a robust tool which allows us to consider rough and large disturbances. We display practical examples in several space dimensions, for scalar equations as well as isentropic gas dynamics. For full gas dynamics, we use a trick from Chen [1], in which the estimate is made in terms of the relative mechanical energy instead of the relative mathematical entropy.     

Finite dimensional aspects of semilinear parabolic equations

We prove that the semiflow generated by _t ,u– _xx ² u=f(x, u), withu(0, t)= u(1,t)=0, onL ²([0, 1]) isC -conjugate to a product of finite dimensional flows.