ALBERT

Description: This paper addresses the slender laminar flow resulting from the discharge of a low-Mach-number hot gas jet of radius a and moderately large Reynolds number Rj into a cold atmosphere of the same gas. We give the boundary-layer solution for plane and round jets with very small values of the ambient-to-jet temperature ratio ε accounting for the temperature dependence of the viscosity and conductivity typical of real gases. It is seen that the leading-order description of the jet in the limit ε → 0 exhibits a front-like structure, including a precisely defined separating boundary at which heat conduction and viscous shear stresses vanish in the first approximation, so that the temperature and axial velocity remain unperturbed outside the jet. Separate analyses are given for the jet discharging into a stagnant atmosphere, when the jet boundary is a conductive front, and for the jet discharging into a coflowing stream, when the jet boundary appears as a contact surface. We provide in particular the numerical description of the jet development region corresponding to axial distances of order Rja for buoyant and non-buoyant jets, as well as the self-similar solutions that emerge both in the near field and in the far field. In all cases considered, comparisons with numerical integrations of the boundary-layer problem for moderately small values of ε indicate that these front descriptions give excellent predictions for the temperature and velocity fields in the near-axis region. © 2006 Cambridge University Press.

Description: SummaryA conventional conservation strategy is establishing protected areas to help combat anthropogenic and climate change impacts on tropical ecosystems, but the effectiveness of these measures is often hampered in upland areas by resource conflicts among armed groups, citizens and government institutions. Improved governance and community participation are key to effectively conserving these areas, yet little is known regarding citizen perceptions in such places. Here, a representative protected area in Colombia is used in order to analyse rural and urban citizen perceptions regarding conservation, conflicts with guerrilla groups and nature’s contributions to people (NCPs) around Chingaza National Natural Park. We used on-site, semi-structured in-person surveys, geospatial data and statistics to understand these perceptions and the roles of armed conflict and deforestation. Perceptions on ecosystem degradation were correlated with deforestation and past guerrilla attacks. Age and place of residence were influential pro-conservation factors, while younger respondents were most willing to invest time in conservation activities. Air purification and water supply and quality were the most identified NCPs and citizens differentiated conservation-related recreation activities from natural resource extraction. We suggest that the legacy of past armed conflict still affects conservation strategies and communities living near tropical highland protected areas.

Description: Non-technical summary Sharing information between different countries is key for developing sustainable solutions to environmental change. Coastal wetlands in the Gulf of Mexico are suffering significant environmental and human-related threats. Working across national boundaries, this research project brings together scientists, specialists and local communities from Cuba and the USA. While important advances have been made in strengthening collaborations, important obstacles remain in terms of international policy constraints, different institutional and academic cultures and technology. Overcoming these limitations is essential to formulating a comprehensive understanding of the challenges that coastal socioecological systems are facing now and into the future.

Description: A numerical treatment of the natural convection and passive dispersion in symmetrically interconnected tilted layers embedded in a rock which is subject to a constant vertical temperature gradient is presented. Such a system is a faithful model of configurations commonly found in the geophysical context. There, flow movements and temperature distributions are closely connected to phenomena of interest such as transport of contaminants and diagenesis. The important case of large thermal conductivity of the rock compared with that of the material filling the layer is discussed in order to show the decisive role of the temperature distribution and the geometrical parameters on the convective flow. The present analysis treats two cases, the fluid-filled layer and the saturated porous layer. Convective flows were calculated for small Rayleigh numbers and the resulting velocity fields were included in the analysis of the transport of a passive contaminant that was initially located where layers connect with each other. Transport of contaminants in the isotropic porous layer was studied by using a model which includes hydrodynamic dispersion terms. How far the tracer transports through the layers and the rate the tracer enters into the system were analysed. The influence of the angle of tilt has also been included. The molecular diffusive Péclet number which relates convective to diffusive species transport is closely associated to a considerable transporting rate, and for the porous layer the hydrodynamic dispersion appears to be an important effect to consider. © 2006 Cambridge University Press.

Description: Steady-state distributions of water potential and salt concentration in coastal aquifers are typically modelled by the Henry problem, which consists of a fully coupled system of flow and transport equations. Coupling arises from the dependence of water density on salt concentration. The physical behaviour of the system is fully described by two dimensionless groups: (i) the coupling parameter α which encapsulates the relative importance of buoyancy and viscous forces, and (ii) the Péclet number Pe, which quantifies the relative importance of purely convective and dispersive transport mechanisms. We provide a systematic analytical analysis of the Henry problem for a full range of the Péclet number. For moderate Pe, analytical solutions are obtained through perturbation expansions in α. This allows us to elucidate the onset of density-driven vertical flux components and the dependence of the local hydraulic head gradients on the coupling parameter. The perturbation solution identifies the regions where salt concentration is most pronounced and relates their spatial extent to the development of a convection cell. Next, we compare our solution to a solution of the pseudo-coupled model, wherein flow and transport are coupled only via the boundary conditions. This enables us to isolate the effects caused by density-dependent processes from those induced by external forcings (boundary conditions). For small Pe, we develop a perturbation expansion around the exact solution corresponding to Pe = 0, which sheds new light on the interpretation of processes observed in diffusion experiments with variable-density flows in porous media. The limiting case of infinite Péclet numbers is solved exactly for the pseudo-coupled model and compared to numerical simulations of the fully coupled problem for large Pe. The proposed perturbation approach is applicable to a wide range of variable-density flows in porous media, including seawater intrusion into coastal aquifers and temperature or pressure-driven density flows in deep aquifers. © 2006 Cambridge University Press.

Description: Accurate numerical computations of the onset of thermal convection in wide rotating spherical shells are presented. Low-Prandtl-number (E) fluids, and non-slip boundary conditions are considered. It is shown that at small Ekman numbers (E), and very low σ values, the well-known equatorially trapped patterns of convection are superseded by multicellular outer-equatorially-attached modes. As a result, the convection spreads to higher latitudes affecting the body of the fluid, and increasing the internal viscous dissipation. Then, from E 〈 10-5, the critical Rayleigh number (Rc) fulfils a power-law dependence Rc ∼ E-4/3, as happens for moderate and high Prandtl numbers. However, the critical precession frequency ( ωc ) and the critical azimuthal wavenumber (mc) increase discontinuously, jumping when there is a change of the radial and latitudinal structure of the preferred eigenfunction. In addition, the transition between spiralling columnar (SC), and outer-equatorially-attached (OEA) modes in the (σ, E)-space is studied. The evolution of the instability mechanisms with the parameters prevents multicellular modes being selected from σ ≳0.023. As a result, and in agreement with other authors, the spiralling columnar patterns of convection are already preferred at the Prandtl number of the liquid metals. It is also found that, out of the rapidly rotating limit, the prograde antisymmetric (with respect to the equator) modes of small mc can be preferred at the onset of the primary instability. © 2008 Cambridge University Press.

Description: In this article, we study the influence of solid-to-fluid density ratio m on the type of vortex-induced oscillation of a square section prism placed inside a two-dimensional channel. We assume that the solid body has neither structural damping nor spring restoring force. Accordingly, the prism equation of motion contains only inertia and aerodynamics forces. The problem is considered in the range of Reynolds numbers Re ε [50 200] (based on the prism cross-section height h) and channel widths H = H'/h ε [2.5 10]. We found that, for each Re and H, there is a critical mass ratio mc that separates two different oscillation regimes. For m 〉 mc, the prism oscillation is periodical and contains a single harmonic. For m 〈 mc, the prism oscillation changes completely and assumes an irregular pattern that is characterized by multiple harmonics that appear to belong to a uniform spectrum. The change from one regime to the other is abrupt and we were not able to observe a transitional regime in which the number of response harmonics grew by finite steps. The value of the critical mass ratio grows along with the Reynolds number and the channel width. © 2009 Cambridge University Press.

Description: We are concerned with the behaviour of a two-dimensional jet that issues from a planar orifice, with a top-hat profile. At the orifice the steady flow is modulated by a time-harmonic fluctuation. A suitably defined Reynolds number is assumed to be large throughout. At large streamwise distances from the orifice, the time-averaged flow yields the classical, Bickley, jet with a suitable virtual origin. This decays algebraically whilst, by contrast, the unsteady component decays exponentially with streamwise distance. An asymptotic theory confirms the exponential decay and provides a good agreement with the numerical solution. © 2009 Copyright Cambridge University Press.