Reduction Rules and ILP Are All You Need: Minimal Directed Feedback Vertex Set

This note describes the development of an exact solver for Minimal Directed Feedback Vertex Set as part of the PACE 2022 competition. The solver is powered largely by aggressively trying to reduce the DFVS problem to a Minimal Cover problem, and applying reduction rules adapted from Vertex Cover literature. The resulting problem is solved as an Integer Linear Program (ILP) using SCIP. The resulting solver performed the second-best in the competition, although a bug at submission time disqualified it. As an additional note, we describe a new vertex cover reduction generalizing the Desk reduction rule.Comment: 11 page

Long-range sediment transport in the world’s oceans by stably stratified turbidity currents

Peer reviewedPublisher PD

Deep-water sediment wave formation: Linear stability analysis of coupled flow/bed interaction

International audienceA linear stability analysis is carried out for the interaction of an erodible sediment bed with a sediment-laden, stratified flow above the bed, such as a turbidity or bottom current. The fluid motion is described by the full, two-dimensional Navier-Stokes equations in the Boussinesq approximation, while erosion is modelled as a diffusive flux of particles from the bed into the fluid. The stability analysis shows the existence of both Tollmien-Schlichting and internal wave modes in the stratified boundary layer. For the internal wave mode, the stratified boundary layer acts as a wave duct, whose height can be determined analytically from the Brunt-Val frequency criterion. Consistent with this criterion, distinct unstable perturbation wavenumber regimes exist for the internal wave mode, which are associated with different numbers of pressure extrema in the wall-normal direction. For representative turbidity current parameters, the analysis predicts unstable wavelengths that are consistent with field observations. As a key condition for instability to occur, the base flow velocity boundary layer needs to be thinner than the corresponding concentration boundary layer. For most of the unstable wavenumber ranges, the phase relations between the sediment bed deformation and the associated wall shear stress and concentration perturbations are such that the sediment waves migrate in the upstream direction, which again is consistent with field observations. © 2011 Cambridge University Press

A System for Cancellation of Two-Level System Noise in Kinetic Inductance Devices

Kinetic Inductance Detectors (KIDs) are showing promise in a variety of low-light applications photometry applications, notably in observing B-mode polarization of the cosmic microwave background. These devices are read out by modulating the inductance of an LC resonator through light, and observing the shift in resonant frequency. Among several contributing sources of noise is Two-Level System noise (TLS noise) that causes low-loss drift in the frequency. Under certain assumptions of the source of the noise, we propose a new dual-resonator design that would allow the TLS noise to be observed independently of the signal, and thus cancelled out. This design comes at a roughly factor-of-2 cost in component size and sensitivity. We designed a manufactured a niobium-on-silicon chip, but encountered issues in that we were unable to observe enough TLS noise to conclusively say that the cancellation works

Report on the Program “Fluid-mediated particle transport in geophysical flows” at the Kavli Institute for Theoretical Physics, UC Santa Barbara, September 23 to December 12, 2013

International audienceThe KITP program held at UC Santa Barbara in the fall of 2013 addressed the dynamics of dispersed particulate flows in the environment. By focusing on the prototypes of Aeolian transport and turbidity currents, it aimed to establish the current state of our understanding of such two-phase flows, to identify key open questions, and to develop collaborative research strategies for addressing these questions. Here we provide a brief summary of the program outcome. Introduction Flows of a continuous fluid phase containing dispersed particles represent a ubiquitous phenomenon, with numerous applications in nature and technology. They can give rise to a great variety of qualitatively distinct flow regimes governed by different balances of inertial, viscous, gravitational and interparticle forces, depending on such aspects as the density ratio between particles and fluid, the nature of the particle-particle interactions, on whether the flows are dilute or concentrated, conservative or nonconservative, and Newtonian or non-Newtonian in nature, to name just a few. Even the narrower field of geophysical particle-laden flows covers a wide variety of phenomena, ranging from Aeolian transport, dust storms and powder snow avalanches to volcanic ash plumes, sediment transport in rivers, estuaries and oceans, and dense pyroclastic and debris flows. While all of the above flows have distinctly different features, they nevertheless share a number of common aspects as well. To advance our capabilities to describe flows of this nature, the community will have to draw heavily on such fundamental research areas as the physics of suspensions and granular flows. The KITP program aimed to review the current state of our understanding of such flows, to identify the key open questions that remain, and to develop collaborative research strategies for addressing these questions via a combination of laboratory experiments, computational investigations and field observations

Potential Flow Interactions With Directional Solidification

The effect of convective melt motion on the growth of morphological instabilities in crystal growth has been the focus of many studies in the past decade. While most of the efforts have been directed towards investigating the linear stability aspects, relatively little attention has been devoted to experimental and numerical studies. In a pure morphological case, when there is no flow, morphological changes in the solid-liquid interface are governed by heat conduction and solute distribution. Under the influence of a convective motion, both heat and solute are redistributed, thereby affecting the intrinsic morphological phenomenon. The overall effect of the convective motion could be either stabilizing or destabilizing. Recent investigations have predicted stabilization by a flow parallel to the interface. In the case of non-parallel flows, e.g., stagnation point flow, Brattkus and Davis have found a new flow-induced morphological instability that occurs at long wavelengths and also consists of waves propagating against the flow. Other studies have addressed the nonlinear aspects (Konstantinos and Brown, Wollkind and Segel)). In contrast to the earlier studies, our present investigation focuses on the effects of the potential flow fields typically encountered in Hele-Shaw cells. Such a Hele-Shaw cell can simulate a gravity-free environment in the sense that buoyancy-driven convection is largely suppressed, and hence negligible. Our interest lies both in analyzing the linear stability of the solidification process in the presence of potential flow fields, as well as in performing high-accuracy nonlinear simulations. Linear stability analysis can be performed for the flow configuration mentioned above. It is observed that a parallel potential flow is stabilizing and gives rise to waves traveling downstream. We have built a highly accurate numerical scheme which is validated at small amplitudes by comparing with the analytically predicted results for the pure morphological case. We have been able to observe nonlinear effects at larger times. Preliminary results for the case when flow is imposed also provide good validation at small amplitudes