752 research outputs found
A Numerical Approach to Stability of Multiclass Queueing Networks
The Multi-class Queueing Network (McQN) arises as a natural multi-class
extension of the traditional (single-class) Jackson network. In a single-class
network subcriticality (i.e. subunitary nominal workload at every station)
entails stability, but this is no longer sufficient when jobs/customers of
different classes (i.e. with different service requirements and/or routing
scheme) visit the same server; therefore, analytical conditions for stability
of McQNs are lacking, in general. In this note we design a numerical
(simulation-based) method for determining the stability region of a McQN, in
terms of arrival rate(s). Our method exploits certain (stochastic) monotonicity
properties enjoyed by the associated Markovian queue-configuration process.
Stochastic monotonicity is a quite common feature of queueing models and can be
easily established in the single-class framework (Jackson networks); recently,
also for a wide class of McQNs, including first-come-first-serve (FCFS)
networks, monotonicity properties have been established. Here, we provide a
minimal set of conditions under which the method performs correctly.
Eventually, we illustrate the use of our numerical method by presenting a set
of numerical experiments, covering both single and multi-class networks
Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation
Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a threeādimensional multipleābasin perspective. We review new paradigms for deep water formation in the highālatitude North Atlantic and the impact of diapycnal mixing on vertical motion in the ocean interior. And we discuss advances in our understanding of the AMOC's stability and its scaling with largeāscale meridional density gradients. Along with reviewing theories for the mean AMOC, we consider models of AMOC variability and discuss what we have learned from theory about the detection and meridional propagation of AMOC anomalies. Simple theoretical models remain a vital and powerful tool for articulating our understanding of the AMOC and identifying the processes that are most critical to represent accurately in the next generation of numerical ocean and climate models
Flow instabilities in circular Couette flow of wormlike micelle solutions with a reentrant flow curve
In this work, we numerically investigate flow instabilities of inertialess
circular Couette flow of dilute wormlike micelle solutions. Using the
reformulated reactive rod model (RRM-R) [Hommel and Graham, JNNFM 295 (2021)
104606], which treats micelles as rigid Brownian rods undergoing reversible
scission and fusion in flow, we study the development and behavior of both
vorticity banding and finger-like instabilities. In particular, we focus on
solutions that exhibit reentrant constitutive curves, in which there exists
some region where the shear stress, , has a multivalued relation to shear
rate, . We find that the radial dependence of the shear stress in
circular Couette flow allows for solutions in which parts of the domain lie in
the region of the flow curve where ,
while others lie in the region where ; this mixed behavior can lead to complex flow instabilities that manifest as
finger-like structures of elongated and anisotropically-oriented micelles. In
3D simulations we find that the initial instability is 2D in origin, and 3D
finger-like structures arise through the axial instability of 2D sheets.
Finally, we show that the RRM-R can capture vorticity banding in narrow-gap
circular Couette flow and that vorticity bands are linearly stable to
perturbations.Comment: 42 pages, 27 figures, 5 supplemental movie
- ā¦