The nature and role of the shear layer, which occurs at the level of the average building height in urban canopies, are poorly understood. Velocity data are analyzed to determine the characteristics of the shear layer of the urban canopy, defined as the broad, linear segment of the mean velocity profile in a region of high shear. Particle image velocimetry measurements in a water tunnel were undertaken to resolve velocity profiles for urban canopies of two geometries typical of Los Angeles, California, and New York City, New York, for which the aspect ratios (average building height-to-width ratio) H /w b are 1 and 3, respectively. The shear layers evolve with distance differently: For H /w b ϭ 1 the urban canopy shear layer extends quickly from above the building height to ground level, whereas for H /w b ϭ 3 the urban canopy shear layer remains elevated at the vicinity of the building height, only reaching to a depth of z /H ϳ 0.5 far downstream. Profiles of the mean velocity gradient also differ from each other for urban canopies associated with H /w b of 1 or 3. Values of shear dU/dz increase toward ground level for an urban canopy associated with H /w b ϭ 1. For an urban canopy associated with H /w b ϭ 3, localized peaks of shear dU/dz exist at the building height and at ground level, with values of shear decreasing to zero at building midheight and far above the building height. A consequence of the different forms of the shear layers of the two urban canopies is that the ground-level dispersion coefficient is likely to be greater for urban canopies associated with H /w b ϭ 1 than for those associated with H /w b ϭ 3 because of an increased ventilation and exchange mechanism for cities such as Los Angeles relative to cities such as New York City that possess urban canyons.

Several experiments over the years have shown that the earth's magnetic field is essential for orientation in birds' migration. The most promising explanation for this orientation is the photo-stimulated radical pair (RP) mechanism. In order to define a reference frame for the orientation task radicals must have an intrinsic anisotropy. We show that this kind of anisotropy and consequently the entanglement in the model are not necessary for the proper functioning of the compass. Classically correlated initial conditions for the RP, subjected to a fast decoherence process, are able to provide the anisotropy required. Even a dephasing environment can provide the necessary frame for the compass to work and also implies fast decay of any quantum correlation in the system without damaging the orientation ability. This fact significantly expands the range of applicability of the RP mechanism providing more elements for experimental search.

The possibility of using nanoelectromechanical systems as a simulation tool for quantum many-body effects is explored. It is demonstrated that an array of electrostatically coupled nanoresonators can effectively simulate the Bose-Hubbard model without interactions, corresponding in the single-phonon regime to the Anderson tight-binding model. Employing a density matrix formalism for the system coupled to a bosonic thermal bath, we study the interplay between disorder and thermalization, focusing on the delocalization process. It is found that the phonon population remains localized for a long time at low enough temperatures; with increasing temperatures the localization is rapidly lost due to thermal pumping of excitations into the array, producing in the equilibrium a fully thermalized system. Finally, we consider a possible experimental design to measure the phonon population in the array by means of a superconducting transmon qubit coupled to individual nanoresonators. We also consider the possibility of using the proposed quantum simulator for realizing continuous-time quantum walks.

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