Bubbly Turbulent Drag Reduction Is a Boundary Layer Effect

In turbulent Taylor-Couette flow, the injection of bubbles reduces the overall drag. On the other hand, rough walls enhance the overall drag. In this work, we inject bubbles into turbulent Taylor-Couette flow with rough walls (with a Reynolds number up to 4×105), finding an enhancement of the dimensionless drag as compared to the case without bubbles. The dimensional drag is unchanged. As in the rough-wall case no smooth boundary layers can develop, the results demonstrate that bubbly drag reduction is a pure boundary layer effec

Observer design for a nonlinear two-dimensional pool boiling system

Abstract onl

Boiling Heat Transfer in Battery Electric vehicles

In this paper the feedback stabilisation of a boiling-based cooling scheme is discussed. Application of such cooling schemes in practical setups is greatly limited by the formation of a thermally insulating vapour film on the to-be-cooled device, called burn-out. In this study a first step is made, to check the viability of such cooling systems, already used in high performance electronics, applied to Electric Vehicles (EVs). It can be used for instance for the cooling of high heat flux transistors and for the thermal homogenisation of battery packs. Thereto, the unstable transition to burn-out is stabilised by controlling the pressure inside the boiling chamber, with which boiling (and thus creation of the thermally insulating vapour film) can be stimulated or suppressed. The feedback law used to do this is based on the dominant modes of the temperature field of the thermally conducting element, i.e. the heater, between the device and the boiling liquid. As not all states used in this feedback law can be measured, an observer or "state-estimator" must be implemented in the control strategy. The observer is a copy of the nonlinear boiling model with an additional term to assure convergence of observer to system state. Simulations are performed to demonstrate controller efficiency on the nonlinear cooling device. This puts forth the boiling-based cooling scheme as viable for application in EVs, enabling increased cooling and thermal-homogenisation capacities compared to conventional thermal management methods. The nextstep should be experiments to proof the principle on battery cells/packs and high heat flux transistors

Complexity in Decision Making: The Case of the Rotterdam Harbour Expansion

Decision making about spatial projects is very complex. Decisions to develop the Rotterdam harbour are taken in the context of a network of local, regional, national, European and international actors, both public and private. These decision-making processes exhibit a lot of complexity and the outcomes are of great importance for the development of the harbour. The complexity is the consequence of interactions between actors connected in different arenas, who are all thinking about the same project. This article uses network theory, and the concepts of actors and arenas in particular, to highlight the complexity of decisions and the connections between various separate decisions. It is demonstrated that the outcomes of the decision-making process are a result of the various connections that are being made. The spatial project at the core of this article is a harbour expansion project called Maasvlakte II

On Kink-Dynamics of Stacked-Josephson Junctions

Dynamics of a fluxon in a stack of coupled long Josephson junctions is studied numericallv. Based on the numerical simulations, we show that the dependence of the propagation velocity c on the external bias current γ is determined by the ratio of the critical currents of thc two junctions J

Static and dynamic properties of fluxons in a zig-zag 0-π Josephson junction

We consider a long Josephson junction with alternating 0- and $\pi$-facets with different facet lengths between the 0- and the $\pi$-parts. Depending on the combinations between the 0- and the $\pi$-facet lengths, an antiferromagnetically ordered semifluxons array can be the ground state of the system. Due to the fact that in that case there are two independent ground states, an externally introduced 2$\pi$ fluxon will be splintered or fractionalized. The magnitude of the flux in the fractional fluxons is a function of the difference between the 0 and the $\pi$-facet lengths. Here, we present an analytical calculation of the flux of splintered Josephson fluxons for any combination of 0- and $\pi$-facet lengths. In the presence of an applied bias current, we show numerically that only one of the two fractional fluxons can be moved. We also consider the I–V characteristics of the ground state and the one of a 2$\pi$-fluxon in a zig-zag junction

Static semifluxons in a long Josephson junction with π-discontinuity points

We investigate analytically a long Josephson junction with several $\pi$-discontinuity points characterized by a jump of $\pi$ in the phase difference of the junction. The system is described by a perturbed-combined sine-Gordon equation. Via phase-portrait analysis, it is shown how the existence of static semifluxons localized around the discontinuity points is influenced by the applied bias current. In junctions with more than one corner, there is a minimum-facet-length for semifluxons to be spontaneously generated. A stability analysis is used to obtain the minimum-facet-length for multicorner junctions