arXiv subjects

Alfred S. Carasso

Publications and source records attributed to Alfred S. Carasso.

3 recordsLinked to original sources

arXiv2025

Data assimilation in 2D nonlinear coupled sound and heat flow, using a stabilized explicit finite difference scheme marched backward in time

This paper considers the ill-posed data assimilation problem associated with hyperbolic/parabolic systems describing 2D coupled sound and heat flow. Given hypothetical data at time T > 0, that may not correspond to an actual solution of the dissipative system at time T, initial data at time t = 0 are sought that can evolve, through the dissipative system, into a useful approximation to the desired data at time T. That may not always be possible. A stabilized explicit finite difference scheme, marching backward in time, is developed and applied to nonlinear examples in non rectangular regions. Stabilization is achieved by applying a compensating smoothing operator at each time step, to quench the instability. Analysis of convergence is restricted to the transparent case of linear, autonomous, selfadjoint spatial differential operators. However, the actual computational scheme can be applied to more general problems. Data assimilation is illustrated using 512x512 pixel images. Such images are associated with highly irregular non smooth intensity data that severely challenge ill-posed reconstruction procedures. Successful and unsuccessful examples are presented.

math.NA↗

arXiv2025

Linnik point spread functions, time-reversed logarithmic diffusion equations, and blind deconvolution of electron microscope imagery

A non iterative direct blind deconvolution procedure, previously used successfully to sharpen Hubble Space Telescope imagery, is now found useful in sharpening nanoscale scanning electron microscope (SEM) and helium ion microscope (HIM) images. The method is restricted to images $g(x,y)$, whose Fourier transforms $\hat{g}(ξ,η)$ are such that $log~|\hat{g}(ξ,0)|$ is globally monotone decreasing and convex. The method is not applicable to defocus blurs. A point spread function in the form of a Linnik probability density function is postulated, with parameters obtained by least squares fitting the Fourier transform of the preconditioned microscopy image. Deconvolution is implemented in slow motion by marching backward in time, in Fourier space, from $t = 1$ to $t = 0$, in an associated logarithmic diffusion equation. Best results are usually found in a partial deconvolution at time $\bar{t}$, with $0 < \bar{t} < 1$, rather than in total deconvolution at $t=0$. The method requires familarity with microscopy images, as well as interactive search for optimal parameters.

physics.ins-det↗

arXiv2024

Data assimilation in 2D incompressible Navier-Stokes equations, using a stabilized explicit $O(Δt)^2$ leapfrog finite difference scheme run backward in time

For the 2D incompressible Navier-Stokes equations, with given hypothetical non smooth data at time $T > 0 $that may not correspond to an actual solution at time $T$, a previously developed stabilized backward marching explicit leapfrog finite difference scheme is applied to these data, to find initial values at time $t = 0$ that can evolve into useful approximations to the given data at time $T$. That may not always be possible. Similar data assimilation problems, involving other dissipative systems, are of considerable interest in the geophysical sciences, and are commonly solved using computationally intensive methods based on neural networks informed by machine learning. Successful solution of ill-posed time-reversed Navier-Stokes equations is limited by uncertainty estimates, based on logarithmic convexity, that place limits on the value of $T > 0$. In computational experiments involving satellite images of hurricanes and other meteorological phenomena, the present method is shown to produce successful solutions at values of $T > 0$, that are several orders of magnitude larger than would be expected, based on the best-known uncertainty estimates. However, unsuccessful examples are also given. The present self-contained paper outlines the stabilizing technique, based on applying a compensating smoothing operator at each time step, and stresses the important differences between data assimilation, and backward recovery, in ill-posed time reversed problems for dissipative equations. While theorems are stated without proof, the reader is referred to a previous paper, on Navier-Stokes backward recovery, where these proofs can be found.

math.NA↗