Shrinking Cities: Fuzzy Concept or Useful Framework?

Shrinking cities beset with sustained population losses have been the focus of a number of studies in the past decade. Much progress has been made in charting where they are and what cause them, but we are still at a point where more detailed case studies are needed for specificity and local context, keeping commonalities in mind but recognizing the crucial situational differences in how differently cities are situated. Per the observation that the term of shrinkage has been used for cities as diverse as Flint, MI and San Jose, CA, we will critically reflect on the concept of shrinking cities and argue how recognition of heterogeneity must be a starting point for any discussion of both analytical relevance and policy formulation

Simulation of laser cutting

Laser cutting is a thermal separation process widely used in shaping and contour cutting applications. There are, however, gaps in understanding the dynamics of the process, especially issues related to cut quality. This work describes the advances in fundamental physical modelling and process monitoring of laser cutting, as well as time varying processes such as contour cutting. Diagnosis of ripple and dross formation is advanced to observe the melt flow and its separation simultaneously as well as the spatial shape of the cut kerf. The cutting process is described with a spatial three-dimensional Free Boundary Problem for the motion of one phase boundary. In such dissipative dynamical systems a finite dimensional inertial manifold exists which contains the attractor of the system. The existence of a finite dimensional inertial manifold means that the motion of a finite set of degrees of freedom can give a good approximation to the complete solution. Asymptotic methods are used to identify the degrees of freedom, and integral methods are applied to derive their equations of motion. Experimental findings about the morphology of ripple formation guide the modelling approach and motivate the investigation of what is known as the one phase problem. Solving inverse problems and the properties of the thermal boundary layers are discussed. The model reproduces details of the U-shaped ripples evolving at the cut surface. In discussion of what is known as the two phase problem the properties of the melt flow are included. The additional degrees of freedom are the melt film thickness, the mass flow and the temperature at the melt film surface. The onset of evaporation and the increase in capillary forces are the two physical phenomena relevant to the build-up of adherent dross. The dynamic model predicts a modulation frequency for the laser power that leads to almost complete suppression of adherent dross in contour cutting. Heat transport in thin film flow is investigated demonstrating how to control the error of reduced models by spectral methods. To find the properties of the gas flow leading to melt ejection is a fundamental task in cutting. The interaction of the gas flow with the condensed phase is mediated by two quantities, namely the pressure gradient and the shear stress along the liquid surface. Results of a detailed analysis of the momentum boundary layer of the gas flow is compared with numerical calculations using the Euler equations as well as the viscous effects described by the compressible Navier-Stokes equations. Deflection and separation of a supersonic gas jet emanating from a nozzle and propagating into the cut kerf is investigated using Schlieren photography and theoretical analysis. Looking for the different situations present in cutting and trepanning, the formation of horizontal structures in the ripple pattern in the cut is discussed. The effect of design and alignment parameters on nozzle performance in cutting are investigated and two dominant effects are discussed, namely the feedback of the gas flow into the nozzle and deflection of the gas flow away from the cutting front. Discussion of the onset of dross formation is extended to include compressible gas flow in the simulation such that the nozzle pressure enters the calculation of the processing domain for a dross-free cut

Cerebral Oximetry Monitoring in Extremely Preterm Infants

Background: The use of cerebral oximetry monitoring in the care of extremely preterm infants is increasing. However, evidence that its use improves clinical outcomes is lacking. Methods: In this randomized, phase 3 trial conducted at 70 sites in 17 countries, we assigned extremely preterm infants (gestational age, <28 weeks), within 6 hours after birth, to receive treatment guided by cerebral oximetry monitoring for the first 72 hours after birth or to receive usual care. The primary outcome was a composite of death or severe brain injury on cerebral ultrasonography at 36 weeks' postmenstrual age. Serious adverse events that were assessed were death, severe brain injury, bronchopulmonary dysplasia, retinopathy of prematurity, necrotizing enterocolitis, and late-onset sepsis. Results: A total of 1601 infants underwent randomization and 1579 (98.6%) were evaluated for the primary outcome. At 36 weeks' postmenstrual age, death or severe brain injury had occurred in 272 of 772 infants (35.2%) in the cerebral oximetry group, as compared with 274 of 807 infants (34.0%) in the usual-care group (relative risk with cerebral oximetry, 1.03; 95% confidence interval, 0.90 to 1.18; P = 0.64). The incidence of serious adverse events did not differ between the two groups. Conclusions: In extremely preterm infants, treatment guided by cerebral oximetry monitoring for the first 72 hours after birth was not associated with a lower incidence of death or severe brain injury at 36 weeks' postmenstrual age than usual care. (Funded by the Elsass Foundation and others; SafeBoosC-III ClinicalTrials.gov number, NCT03770741.)