Special Session 21: Fluid dynamics and PDE

Emergence of peaked singularities in the Euler-Poisson system

Junsik Bae
Korea Advanced Institute of Science and Technology
Korea
Co-Author(s):    
Abstract:
We consider the one-dimensional Euler-Poisson system equipped with the Boltzmann relation and provide the exact asymptotic behavior of the peaked solitary wave solutions near the peak. This enables us to study the cold ion limit of the peaked solitary waves with the sharp range of Holder exponents. Furthermore, we provide numerical evidence for C^1 blow-up solutions to the pressureless Euler-Poisson system, whose blow-up profiles are asymptotically similar to its peaked solitary waves and exhibit a different form of blow-up compared to the Burgers-type (shock-like) blow-up. This is a joint work with Sang-Hyuck Moon(UNIST) and Kwan Woo(SNU).

Suppressing blowup of solutions of the generalized KdV equation

Jerry Bona
University of Illinois at Chicago
USA
Co-Author(s):    Jerry L. Bona and Hongqiu Chen
Abstract:
The generalized Korteweg--de Vries equation $$ u_t + u^pu_x + u_{xxx} \, = \, 0 $$ apparently has solutions that blow up in finite time for $p \geq 4$. The lecture centers around the addition of terms that can counteract this blowup for values of $p > 4$.

The linear BBM-equation on the quarter-plane, revisited: A rigorous novel approach and unexpected phenomena

Andreas Chatziafratis
University of California at Santa Cruz
USA
Co-Author(s):    Jerry L. Bona, Hongqiu Chen, Spyridon Kamvissis
Abstract:
We shall discuss some of our recent findings concerning the rigorous solution and analysis of fully non-homogeneous initial-boundary-value problems for the linearized Benjamin-Bona-Mahony equation on the spatiotemporal quarter-plane. The approach is based on complex-analytic tools and a rigorous implementation of the Fokas unified transform method. Explicit solution formulae for the forced linear problem are thus derived in terms of contour integrals and analyzed for quite general initial values and boundary conditions in classical function spaces. The a posteriori pointwise verification of the closed-form representations brings to the fore a single compatibility condition that must be obeyed by smooth data for a well-defined global solution, thereby indicating a type of boundary-smoothing effect. Subsequent boundary-behaviour analysis allows for a uniqueness theorem to be established, relying also on an energy-type argument. Additional surprising observations (e.g., asymptotic instabilities) will be highlighted. For instance, both for Dirichlet and for Neumann boundary conditions, asymptotic periodicity holds. However, for Robin boundary conditions, we find not only that solutions lack the asymptotic periodicity property, but they in fact display instability, growing in amplitude exponentially in time. This is joint work with J.L. Bona, H. Chen and S. Kamvissis.

Water Wave Models: Bore Propagations

Hongqiu Chen
University of Memphis
USA
Co-Author(s):    
Abstract:
Considered here are two unidirectional water wave models for small amplitude long waves on the surface of an ideal fluid $\begin{equation} \eta_t + \eta_x + \frac34 \alpha (\eta^2)_x - \frac16\beta \eta_{xxt} \, = \, 0, \end{equation}$ and the higher-order model equation $\begin{equation}\begin{split} \eta_t+\eta_x-\gamma_1\beta{\eta}_{xxt}+\gamma_2\beta\eta_{xxx}+\delta_1\beta^2{\eta}_{xxxxt} +\delta_2\beta^2\eta_{xxxxx} \ + \frac34\alpha(\eta^2)_x+ \alpha\beta\Big(\gamma (\eta^2)_{xx}-\frac7{48}\eta_x^2\Big)_x-\frac18\alpha^2(\eta^3)_x=0, \end{split}\end{equation}$ where $\eta=\eta(x,t)$, $x\in\mathbb R$ and $t\geq 0$, is the deviation of the free surface from its rest position at the point corresponding to $x$ at time $t$. $\alpha, \beta $ $\gamma_1, \gamma_2, \delta_1, \delta_2$, $\gamma $ are physical parameters. In this talk, we discuss well-posedness issues when the initial dada is non-localized.

Energy cascade in fluids: from convex integration to mixing

Alexey Cheskidov
Institute for Theoretical Sciences, Westlake University
Peoples Rep of China
Co-Author(s):    
Abstract:
In the past couple of decades, mathematical fluid dynamics has made significant strides with numerous constructions of solutions to fluid equations that exhibit pathological or wild behaviors. In this talk I will overview new developments in discovering not only pathological mathematically, but also physically realistic solutions of fluid equations.

Stability and instability problems of MHD

Mimi Dai
University of Illinois at Chicago
USA
Co-Author(s):    
Abstract:
The stability and instability problems for magnetohydrodynamics (MHD) are challenging due to the coupling nature and intricate interactions of the flow and magnetic field. We consider some particular steady states of MHD and investigate the long term behavior of the perturbed system around such steady states.

Improved $H^1$ Theory for a Higher-Order Water Wave Model

Colette Guillop\`e
Universit\`e Paris-Est Cr\`eteil
France
Co-Author(s):    
Abstract:
This talk is concerned with a class of higher-order models for the unidirectional propagation of small amplitude long waves on the surface of an ideal fluid. In the water waves context, the Boussinesq and Korteweg-de Vries models are proven to be good approximations of the two-dimensional Euler equation in regimes where their derivation is valid. However, the time scale of their validity extends only to about ten wavelengths or so. The second-order models considered here are formally accurate on the order of a hundred wavelengths. We will show an extended version of global well-posedness in $H^1$.

A Hamiltonian Dysthe equation for hydroelastic waves in a compressed ice sheet.

Adilbek Kairzhan
Nazarbayev University
Kazakhstan
Co-Author(s):    Philippe Guyenne, Catherine Sulem
Abstract:
In this talk we consider nonlinear hydroelastic waves along a compressed ice sheet lying on top of a two-dimensional fluid of infinite depth. Based on a Hamiltonian formulation of this problem and by applying techniques from Hamiltonian perturbation theory, a Hamiltonian Dysthe equation is derived for the slowly varying envelope of modulated wavetrains. The derivation is complicated by the presence of cubic resonances. A Birkhoff normal form transformation is introduced to eliminate non-resonant triads while accommodating resonant ones. We also test the newly obtained Dysthe model against direct numerical simulations of the full Euler equations, and very good agreement is observed.

Grounded shallow ice sheets melting as an obstacle problem

Paolo Piersanti
The Chinese University of Hong Kong Shenzhen
Peoples Rep of China
Co-Author(s):    Roger Temam
Abstract:
In this talk, which is the result of a joint work of the speaker with Roger Temam (Indiana University), we will study a model describing the evolution of the thickness of a grounded shallow ice sheet. Since the thickness of the ice sheet is constrained to be nonnegative, the problem under consideration is an obstacle problem. A rigorous modelling exercise shows that this model, which is time-dependent, is governed by a set of variational inequalities that involve nonlinearities in the time derivative and in the elliptic term. In order to establish the existence of solutions for the time-dependent model we recovered, formally, upon completion of the aforementioned modelling exercise, we first depart from a penalized relaxation, and we show - by resorting to a discretization in time - that the corresponding relaxed problem admits at least one solution. Secondly, by means of Dubinskii`s lemma and other new results and new inequalities, we extract compactness for the family of solutions of the relaxed problems and we show that this family of solutions converges to a solution of a doubly nonlinear parabolic variational inequality akin to the one that was recovered formally.