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Free, publiclyaccessible full text available December 1, 2023

We construct a determining form for the 2D RayleighBenard (RB) system in a strip with solid horizontal boundaries, in the cases of noslip and stressfree boundary conditions. The determining form is an ODE in a Banach space of trajectories whose steady states comprise the longtime dynamics of the RB system. In fact, solutions on the global attractor of the RB system can be further identified through the zeros of a scalar equation to which the ODE reduces for each initial trajectory. The twist in this work is that the trajectories are for the velocity field only, which in turn determines the corresponding trajectories of the temperature.Free, publiclyaccessible full text available April 1, 2023

Abstract The Rayleigh–Bénard system with stressfree boundary conditions is shown to have a global attractor in each affine space where velocity has fixed spatial average. The physical problem is shown to be equivalent to one with periodic boundary conditions and certain symmetries. This enables a Gronwall estimate on enstrophy. That estimate is then used to bound the L 2 norm of the temperature gradient on the global attractor, which, in turn, is used to find a bounding region for the attractor in the enstrophy–palinstrophy plane. All final bounds are algebraic in the viscosity and thermal diffusivity, a significant improvement over previously established estimates. The sharpness of the bounds are tested with numerical simulations.

This paper studies a family of generalized surface quasigeostrophic (SQG) equations for an active scalar
on the whole plane whose velocities have been mildly regularized, for instance, logarithmically. The wellposedness of these regularized models in borderline Sobolev regularity have previously been studied by D. Chae and J. Wu when the velocity\begin{document}$ \theta $\end{document} is of lower singularity, i.e.,\begin{document}$ u $\end{document} , where\begin{document}$ u = \nabla^{\perp} \Lambda^{ \beta2}p( \Lambda) \theta $\end{document} is a logarithmic smoothing operator and\begin{document}$ p $\end{document} . We complete this study by considering the more singular regime\begin{document}$ \beta \in [0, 1] $\end{document} . The main tool is the identification of a suitable linearized system that preserves the underlying commutator structure for the original equation. We observe that this structure is ultimately crucial for obtaining continuity of the flow map. In particular, straightforward applications of previous methods for active transport equations fail to capture the more nuanced commutator structure of the equation in this more singular regime. The proposed linearized system nontrivially modifies the flux of the original system in such a way that it coincides with the original flux when evaluated along solutions of themore »\begin{document}$ \beta\in(1, 2) $\end{document} 
Given a velocity field $u(x,t)$, we consider the evolution of a passive tracer $\theta$ governed by $\frac{\partial\theta}{\partial t} + u\cdot\nabla\theta = \Delta\theta + g$ with timeindependent source $g(x)$. When $\u\$ is small in some sense, Batchelor, Howells and Townsend (1959, J.\ Fluid Mech.\ 5:134) predicted that the tracer spectrum scales as $\theta_k^2\proptok^{4}u_k^2$. In this paper we prove that, for random synthetic twodimensional incompressible velocity fields $u(x,t)$ with given energy spectra, this scaling does indeed hold probabilistically, asymptotically almost surely for large $k$ and small $\u\$. We also propose an asymptotic correction factor to the BHT scaling arising from the timedependence of $u$.

We study the computational efficiency of several nudging data assimilation algorithms for the 2D magnetohydrodynamic equations, using varying amounts and types of data. We find that the algorithms work with much less resolution in the data than required by the rigorous estimates in [7]. We also test other abridged nudging algorithms to which the analytic techniques in [7] do not seem to apply. These latter tests indicate, in particular, that velocity data alone is sufficient for synchronization with a chaotic reference solution, while magnetic data alone is not. We demonstrate that a new nonlinear nudging algorithm, which is adaptive in both time and space, synchronizes at a super exponential rate. [7] A. Biswas, J. Hudson, A. Larios and Y. Pei, Continuous data assimilation for the 2D magnetohydrodynamic equations using one component of the velocity and magnetic fields, Asymptot. Anal., 108 (2018), 143.