Divergence-Conforming Velocity and Vorticity Approximations for Incompressible Fluids Obtained with Minimal Facet Coupling.

We introduce two new lowest order methods, a mixed method, and a hybrid discontinuous Galerkin method, for the approximation of incompressible flows. Both methods use divergence-conforming linear Brezzi-Douglas-Marini space for approximating the velocity and the lowest order Raviart-Thomas space for approximating the vorticity. Our methods are based on the physically correct viscous stress tensor of the fluid, involving the symmetric gradient of velocity (rather than the gradient), provide exactly divergence-free discrete velocity solutions, and optimal error estimates that are also pressure robust. We explain how the methods are constructed using the minimal number of coupling degrees of freedom per facet. The stability analysis of both methods are based on a Korn-like inequality for vector finite elements with continuous normal component. Numerical examples illustrate the theoretical findings and offer comparisons of condition numbers between the two new methods.

Reversing the pump dependence of a laser at an exceptional point.

When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of non-trivial physics in a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.

mFES: A Robust Molecular Finite Element Solver for Electrostatic Energy Computations.

We present a robust method for the calculation of electrostatic potentials of large molecular systems using tetrahedral finite elements (FE). Compared to the finite difference (FD) method using a regular simple cubic grid to solve the Poisson equation, the FE method can reach high accuracy and efficiency using an adaptive grid. Here, the grid points can be adjusted and are placed directly on the molecular surfaces to faithfully model surfaces and volumes. The grid point density decreases rapidly toward the asymptotic boundary to reach very large distances with just a few more grid points. A broad set of tools are applied to make the grid more regular and thus provide a more stable linear equation system, while reducing the number of grid points without compromising accuracy. The latter reduces the number of unknowns significantly and yields shorter solver execution times. The accuracy is further enhanced by using second order polynomials as shape functions. Generating the adaptive grid for a molecular system is expensive, but it pays off, if the same molecular geometry is used several times as is the case for pKA and redox potential computations of many charge variable groups in proteins. Application of the mFES method is also advantageous, if the molecular system is too large to reach sufficient accuracy when computing the electrostatic potential with conventional FD methods. The program mFES is free of charge and available at http://agknapp.chemie.fu-berlin.de/mfes .

Non-touch fittings in hospitals: a possible source of Pseudomonas aeruginosa and Legionella spp.

Non-touch fittings are gradually becoming very common in the bathrooms and toilets of public facilities and restaurants. Hospitals and other healthcare facilities have recently started to install these types of water taps to lower water consumption, thus saving costs, and to prevent healthcare workers from touching the tap, thus promoting hygiene. This study analysed the bacteriological water quality of 38 non-touch water taps in different settings in a 450-bed secondary-care hospital in Upper Austria. Two different tap types were installed: 23 taps were without temperature selection and 15 were with temperature selection (cold and warm). A membrane filtration method was used, and the authors screened for both indicator organisms and Pseudomonas aeruginosa in 100 ml water samples. In 10 non-touch taps without temperature selection, the authors also screened for Legionella spp. in 500 ml water samples. Seventy four percent of the taps without temperature selection and 7% of the taps with temperature selection showed contamination with P. aeruginosa (P<0.001). None of the taps showed contamination with indicator organisms. Detailed analysis of the source of contamination revealed that the magnetic valve and the outlet itself were heavily contaminated, whereas the junction from the central pipe system was free of contamination. All 10 analysed taps showed contamination with Legionella spp. It was concluded that the local contamination of non-touch fittings is a result of the low amount of water that flows through the outlet, the low water pressure and the column of water, which is 'still-standing' and has a temperature of about 35 degrees C, thus providing nearly ideal growth conditions for P. aeruginosa. Additionally, the presence of materials such as rubber, PVC, etc. in the fittings enhances the adhesion of P. aeruginosa and thus the production of biofilms. In conclusion, the authors wish to encourage infection control teams to evaluate the use of non-touch fittings in hospitals, especially when they are installed in risk areas.