arXiv subjects

Michael Schuster

Publications and source records attributed to Michael Schuster.

7 recordsLinked to original sources

arXiv2026

On the Existence of Steady States for Blended Gas Flow with Non-Constant Compressibility Factor on Networks

In this paper, we study hydrogen-natural gas mixtures transported through pipeline networks. The flow is modeled by the isothermal Euler equations with a pressure law involving a non-constant, composition-dependent compressibility factor. For a broad class of such compressibility models, we prove the existence of steady-state solutions on networks containing compressor stations. The analysis is based on an implicit representation of the pressure profiles and a continuity argument that overcomes the discontinuous dependence of the gas composition on the flow direction. Numerical examples illustrate the influence of different compressibility models on the resulting states.

math.NA↗

arXiv2024

Steady State Blended Gas Flow on Networks: Existence and Uniqueness of Solutions

We prove an existence result for the steady state flow of gas mixtures on networks. The basis of the model are the physical principles of the isothermal Euler equation, coupling conditions for the flow and pressure, and the mixing of incoming flow at nodes. The state equation is based on a convex combination of the ideal gas equations of state for natural gas and hydrogen. We analyze mathematical properties of the model allowing us to prove the existence of solutions in particular for tree-shaped networks and networks with exactly one cycle. Numerical examples illustrate the results and explore the applicability of our approach to different network topologies.

math.AP↗

arXiv2020

Probabilistic Constrained Optimization on Flow Networks

Uncertainty often plays an important role in dynamic flow problems. In this paper, we consider both, a stationary and a dynamic flow model with uncertain boundary data on networks. We introduce two different ways how to compute the probability for random boundary data to be feasible, discussing their advantages and disadvantages. In this context, feasible means, that the flow corresponding to the random boundary data meets some box constraints at the network junctions. The first method is the spheric radial decomposition and the second method is a kernel density estimation. In both settings, we consider certain optimization problems and we compute derivatives of the probabilistic constraint using the kernel density estimator. Moreover, we derive necessary optimality conditions for the stationary and the dynamic case. Throughout the paper, we use numerical examples to illustrate our results by comparing them with a classical Monte Carlo approach to compute the desired probability.

math.NA↗

arXiv2020

The Finite-Time Turnpike Phenomenon for Optimal Control Problems: Stabilization by Non-Smooth Tracking Terms

In this paper, problems of optimal control are considered where in the objective function, in addition to the control cost there is a tracking term that measures the distance to a desired stationary state. The tracking term is given by some norm and therefore it is in general not differentiable. In the optimal control problem, the initial state is prescribed. We assume that the system is either exactly controllable in the classical sense or nodal profile controllable. We show that both for systems that are governed by ordinary differential equations and for infinite-dimensional systems, for example for boundary control systems governed by the wave equation, under certain assumptions the optimal system state is steered exactly to the desired state after finite time.

math.OC↗

arXiv2017

Maximal rank subgroups and strong functoriality of the additive eigencone

Let $G$ be a simple connected complex Lie group. The additive eigencone of $G$ is a polyhedral cone containing the set of solutions to the additive eigenvalue problem, a generalization of the Hermitian eigenvalue problem. The additive eigencone is functorial, and for certain subgroups satisfies a stronger functoriality property: the eigencone of the subgroup is determined by the inequalities of the larger eigencone. Belkale and Kumar first studied this property for subgroups invariant under a diagram automorphism of $G$. We study a new class of subgroups arising from centralizers of torus elements that have the strong eigencone functoriality property.

math.RT↗

arXiv2016

Rank reduction of conformal blocks

Let $X$ be a smooth, pointed Riemann surface of genus zero, and $G$ a simple, simply-connected complex algebraic group. Associated to a finite number of weights of $G$ and a level is a vector space called the space of conformal blocks, and a vector bundle of conformal blocks over $\bar{\text{M}}_{0,n}$. We show that, assuming the weights are on a face of the multiplicative eigenvalue polytope, the space of conformal blocks is isomorphic to a product of conformal blocks over groups of lower rank. If the weights are on a degree zero wall, then we also show that there is an isomorphism of conformal blocks bundles, giving an explicit relation between the associated nef divisors. The methods of the proof are geometric, and use the identification of conformal blocks with spaces of generalized theta functions, and the moduli stacks of parahoric bundles recently studied by Balaji and Seshadri.

math.AG↗

arXiv2011

A Spitzer IRAC Imaging Survey for T Dwarf Companions Around M, L, and T Dwarfs: Observations, Results, and Monte Carlo Population Analyses

We report observational techniques, results, and Monte Carlo population analyses from a Spitzer Infrared Array Camera imaging survey for substellar companions to 117 nearby M, L, and T dwarf systems (median distance of 10 pc, mass range of 0.6 to \sim0.05 M\odot). The two-epoch survey achieves typical detection sensitivities to substellar companions of [4.5 μm] \leq 17.2 mag for angular separations between about 7" and 165". Based on common proper motion analysis, we find no evidence for new substellar companions. Using Monte Carlo orbital simulations (assuming random inclination, random eccentricity, and random longitude of pericenter), we conclude that the observational sensitivities translate to an ability to detect 600-1100K brown dwarf companions at semimajor axes greater than ~35 AU, and to detect 500-600K companions at semimajor axes greater than ~60 AU. The simulations also estimate a 600-1100K T dwarf companion fraction of < 3.4% for 35-1200 AU separations, and < 12.4% for the 500-600K companions, for 60-1000 AU separations.

astro-ph.SR↗