Sources and Sinks in Comparability Graphs

We prove that a subset S of vertices of a comparability graph G is a source set if and only if each vertex of S is a source and there is no odd induced path in G between two vertices of S. We also characterize pairs of subsets corresponding to sources and sinks, respectively. Finally, an application to interval graphs is obtained.     

Mobile sink-based data collection in event-driven wireless sensor networks using a modified ant colony optimization

The hotspot problem is one of the primary challenges in the wireless sensor networks (WSNs) because it isolates the sink node from the remaining part of the WSN. A mobile sink (MS)-based data acquisition strategy mitigates the hotspot problem, but the traditional MS-based data gathering approaches do not resolve the issue. However, the conventional techniques follow a fixed order of visits and static traversal of the MS. In this context, this paper uses a modified version of the ant colony optimization strategy for the data collected through a MS to mitigate the hotspot problem in the WSNs while improving the energy efficiency, network lifetime, throughput by reducing the packet loss and delay. In our work, we initially construct a forwarded load spanning tree to estimate the freight of each node in the WSN. Further, we choose RPs and their path simultaneously using the modified ACO algorithm by considering the forward loads, remaining energy, distance, etc. The proposed work also adopts the virtual RP selection strategy void unnecessary data exchanges between the nodes and RPs. Hence, it reduces the burden on relay nodes and optimize the energy usage among the nodes. We compare our approach with the recent ACO-based algorithms, and our approach outperforms them.     

Measurements of velocity fields in finite cylinder arrays with and without tip clearance

The velocity field in a finite cylinder array was investigated experimentally in a water towing tank and an acoustic Doppler velocimeter (ADV). The experimental system consisted of a staggered cylinder array having 14 rows to permit streamwise evolution of the flow. The boundaries were manipulated to create several global flow configurations. Three basic configurations were studied: a globally unidirectional flow, a flow with partial lateral blockage at the inlet and outlet planes, and a flow with the top boundary separated from the cylinders creating a tip clearance. The three components of the velocity vector were measured at various points within the model. Time-averaged results are presented for the different flow configurations. The results provide insight into the development of the flow field in cases of a finite array with complex geometry and boundary effects.     

Experimental Study on Flow Boiling Pressure Drop and Flow Instabilities of Reentrant Micro-channels

Flow boiling pressure drop and flow instabilities of Ω-shaped reentrant copper micro-channels were experimentally explored. Tests were conducted in deionized water and ethanol at inlet subcoolings of 10°C and 40°C, mass fluxes of 125–300 kg/m²·s, and a wide range of heat fluxes and exit vapor qualities. The operational parameters effects, i.e., heat flux, mass flux, inlet subcooling, and coolants, on pressure drop and flow instabilities were systematically explored. The two-phase pressure drop of reentrant micro-channels were found to generally increase monotonically with increasing heat fluxes and exit vapor qualities. Nevertheless, the roles of mass flux and inlet subcooling were dependent on the test coolant.     

Two-phase flow instabilities in a silicon microchannels heat sink

Two-phase flow instabilities are highly undesirable in microchannels-based heat sinks as they can lead to temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations. This work is an experimental study of boiling instabilities in a microchannel silicon heat sink with 40 parallel rectangular microchannels, having a length of 15 mm and a hydraulic diameter of 194 μm. A series of experiments have been carried out to investigate pressure and temperature oscillations during the flow boiling instabilities under uniform heating, using water as a cooling liquid. Thin nickel film thermometers, integrated on the back side of a heat sink with microchannels, were used in order to obtain a better insight related to temperature fluctuations caused by two-phase flow instabilities. Flow regime maps are presented for two inlet water temperatures, showing stable and unstable flow regimes. It was observed that boiling leads to asymmetrical flow distribution within microchannels that result in high temperature non-uniformity and the simultaneously existence of different flow regimes along the transverse direction. Two types of two-phase flow instabilities with appreciable pressure and temperature fluctuations were observed, that depended on the heat to mass flux ratio and inlet water temperature. These were high amplitude/low frequency and low amplitude/high frequency instabilities. High speed camera imaging, performed simultaneously with pressure and temperature measurements, showed that inlet/outlet pressure and the temperature fluctuations existed due to alternation between liquid/two-phase/vapour flows. It was also determined that the inlet water subcooling condition affects the magnitudes of the temperature oscillations in two-phase flow instabilities and flow distribution within the microchannels.     

Fabrication, properties, and applications of porous metals with directional pores

Lotus-type porous metals with aligned long cylindrical pores are fabricated by unidirectional solidification from the melt with a dissolved gas such as hydrogen, nitrogen, or oxygen. The gas atoms can be dissolved into the melt via a pressurized gas atmosphere or thermal decomposition of gaseous compounds. Three types of solidification techniques have been developed: mold casting, continuous zone melting, and continuous casting techniques. The last method is superior from the viewpoint of mass production of lotus metals. The observed anisotropic behaviors of the mechanical properties, sound absorption, and thermal conductivity are inherent to the anisotropic porous structure. In particular, the remarkable anisotropy in the mechanical strength is attributed to the stress concentration around the pores aligned perpendicular to the loading direction. Heat sinks are a promising application of lotus metals due to the high cooling performance with a large heat transfer.     

How often surface diffeomorphisms have infinitely many sinks and hyperbolicity of periodic points near a homoclinic tangency

Here we study an amazing phenomenon discovered by Newhouse [S. Newhouse, Non-density of Axiom A(a) on S², in: Proc. Sympos. Pure Math., vol. 14, Amer. Math. Soc., 1970, pp. 191-202; S. Newhouse, Diffeomorphisms with infinitely many sinks, Topology 13 (1974) 9-18; S. Newhouse, The abundance of wild hyperbolic sets and nonsmooth stable sets of diffeomorphisms, Publ. Math. Inst. Hautes Études Sci. 50 (1979) 101-151]. It turns out that in the space of Cr smooth diffeomorphisms Diffr(M) of a compact surface M there is an open set U such that a Baire generic diffeomorphism f∈U has infinitely many coexisting sinks. In this paper we make a step towards understanding “how often does a surface diffeomorphism have infinitely many sinks.” Our main result roughly says that with probability one for any positive D a surface diffeomorphism has only finitely many localized sinks either of cyclicity bounded by D or those whose period is relatively large compared to its cyclicity. It verifies a particular case of Palis' Conjecture saying that even though diffeomorphisms with infinitely many coexisting sinks are Baire generic, they have probability zero.One of the key points of the proof is an application of Newton Interpolation Polynomials to study the dynamics initiated in [V. Kaloshin, B. Hunt, A stretched exponential bound on the rate of growth of the number of periodic points for prevalent diffeomorphisms I, Ann. of Math., in press, 92 pp.; V. Kaloshin, A stretched exponential bound on the rate of growth of the number of periodic points for prevalent diffeomorphisms II, preprint, 85 pp.].     

Dynamics of a linear oscillator connected to a small strongly non-linear hysteretic absorber

The present investigation deals with the dynamics of a two-degrees-of-freedom system which consists of a main linear oscillator and a strongly non-linear absorber with small mass. The non-linear oscillator has a softening hysteretic characteristic represented by a Bouc-Wen model. The periodic solutions of this system are studied and their calculation is performed through an averaging procedure. The study of non-linear modes and their stability shows, under specific conditions, the existence of localization which is responsible for a passive irreversible energy transfer from the linear oscillator to the non-linear one. The dissipative effect of the non-linearity appears to play an important role in the energy transfer phenomenon and some design criteria can be drawn regarding this parameter among others to optimize this energy transfer. The free transient response is investigated and it is shown that the energy transfer appears when the energy input is sufficient in accordance with the predictions from the non-linear modes. Finally, the steady-state forced response of the system is investigated. When the input of energy is sufficient, the resonant response (close to non-linear modes) experiences localization of the vibrations in the non-linear absorber and jump phenomena.     

Switchable structured bond: A bond graph device for modeling power coupling/decoupling of physical systems

This paper presents a controlled Bond Graph interconnection structure named Switchable Structured Bond, or SS-Bond for short, basically intended for modeling and simulation of ideal switching phenomena (zero transition time) with fixed causality. Serving to model the presence or absence of a power preserving connection between two power ports, these new structures are inspired in the idea yielding the switchable bond (or SB) formalism, but embody some features correcting the shortcomings of the latter. Indeed, when both power ports are connected, both the SB and the SS-Bond behave like a standard power bond, but when the power connection is absent, the SS-Bond fully captures the possible states of the adjacent power ports, unlike the SB, which in many cases leaves undefined the situation of these ports. As SS-Bonds are originally defined to model ideal switching, these possible states are zero-flow or zero-effort for each of the disconnected power ports. These four situations, together with the normally connected state, define the five possible switching modes of an SS-Bond.The SS-Bonds can be internally represented with standard bond graph elements. To keep fixed the causality assignment even under switching, some algebraic constraints are added to the equation set of the switched structure, which in the Bond Graph domain can be represented with residual sinks. A minor modification on the internal implementation of the SS-Bonds allows the formalism for commutation modeling with the non-ideal approach consisting in adding parasitic components to avoid causality changes. Besides some models of electric–electronic circuits, slip-stick friction in a simple mechanical system and abrupt faults in a hydraulic two tank system are used to illustrate the new formalism and its performance in modeling and simulation.