A novel supramolecular organic-inorganic adduct containing alpha-Keggin-type [PW12O40](3-) anions and benzo-15-crown-5 molecules

The structure of the title compound, tris(hydroxonium) 12- phosphato-tetracosa- 2-oxo-dodecaoxododecatungsten hexakis( benzo-15-crown-5)±methanol±water (1/1/1), (H3O)3- [PW12O40] 6C14H20O5 CH3OH H2O (where C14H20O5 is benzo-15-crown-5), has been determined at 180 K. [PW12O40]3ÿ anions are typical of -Keggin structures, and the [H3O (C14H20O5)2]+ sandwich-type moieties contain a large number of short O O close contacts, suggesting strong hydrogen bonding within them

Turbulence in Rivers

The study of turbulence has always been a challenge for scientists working on geophysical flows. Turbulent flows are common in nature and have an important role in geophysical disciplines such as river morphology, landscape modeling, atmospheric dynamics and ocean currents. At present, new measurement and observation techniques suitable for fieldwork can be combined with laboratory and theoretical work to advance the understanding of river processes. Nevertheless, despite more than a century of attempts to correctly formalize turbulent flows, much still remains to be done by researchers and engineers working in hydraulics and fluid mechanics. In this contribution we introduce a general framework for the analysis of river turbulence. We revisit some findings and theoretical frameworks and provide a critical analysis of where the study of turbulence is important and how to include detailed information of this in the analysis of fluvial processes. We also provide a perspective of some general aspects that are essential for researchers/ practitioners addressing the subject for the first time. Furthermore, we show some results of interest to scientists and engineers working on river flows

Analysis of the entrainment on lock-exchange density currents

Density or gravity currents are geophysical flows driven by density differences within a fluid which can be caused by temperature differences or the presence of dissolved substances or particles in suspension. Oceanic fronts and turbidity currents in lakes and reservoirs are examples of gravity currents occurring in masses of water. In the governing equations of density currents, namely in the total mass conservation equation, the entrainment rate of the ambient fluid through the upper interface becomes an explicit or source term requiring a closure model. The entrainment rate may be modelled as a function of the buoyancy velocity and a characteristic density, and it is usually evaluated through empirical relations. The aim of the present work is to experimentally investigate the entrainment of ambient fluid into unsteady density currents performed by lock-exchange releases of saline water into a fresh water tank. The experiments were conducted at the Laboratory of Hydraulics of University of Rome “Roma Tre” in a 3.0 m long, 0.20 m wide and 0.30 m deep transparent Perspex flume. Eight lockexchange release tests were performed varying the density of the saline water, the water depth in the water tank and the bed roughness. The experiments were performed by varying one parameter at a time. For smooth bed and for a fixed value of water depth, h = 0.20 m, four different initial densities of the salt-water mixture were analysed: 1015, 1030, 1045 and 1060 kg/m3; for a fixed initial density of 1030 kg/m3, one test was performed by changing the water depth to 0.25 m; the effect of bed roughness was investigated for a fixed water depth and fixed density, 1030 kg/m3, by placing a thick layer of sediments on the bed with three different values of D50: 2.9 mm, 4.6 mm and 24.6 mm, where D50 is the grain size diameter for which 50% of the sediments have smaller diameters. A controlled quantity of dye is added to the saline water in the lock to provide flow visualization and to serve as density tracer. The development of the current is recorded with a 25 Hz CCD camera under controlled light conditions. The resulting video frames are thus converted into gray scale matrices and a calibration procedure establishes a non-linear relation, experimentally determined, between the gray scale values and the quantity of dye in the water. The quantity of dye is converted into salt concentration by assuming a linear relation between quantities, dye and salt, allowing thus the estimation of the 2D instantaneous current density distribution. With the resulting 2D density maps, temporal and spatial evolution of the current height and depthaveraged density and temporal evolution of the front position, front velocity, mass balance and entrainment rate may be assessed and discussed taking into account the differences in the initial density of the saltwater mixture, the water depth and bed roughness. Detailed reconstruction of the current 2D geometry is possible, allowing the identification of such phenomena as instabilities in the upper mixing layer; these are observed in the plots of current height and instantaneous density maps, and may be related with the process of water entrainment. They prove to be quasistationary, or at least varying at a different (lower) time scale than the remainder current, being slightly advected downstream by the mean current velocity

Image analysis technique applied to lock-exchange gravity currents

An image analysis technique is used to estimate the two-dimensional instantaneous density field of unsteady gravity currents produced by full-depth lock-release of saline water. An experiment reproducing a gravity current was performed in a 3.0 m long, 0.20 m wide and 0.30 m deep Perspex flume with horizontal smooth bed and recorded with a 25 Hz CCD video camera under controlled light conditions. Using dye concentration as a tracer, a calibration procedure was established for each pixel in the image relating the amount of dye uniformly distributed in the tank and the greyscale values in the corresponding images. The results are evaluated and corrected by applying the mass conservation principle within the experimental tank. The procedure is a simple way to assess the time-varying density distribution within the gravity current, allowing the investigation of gravity current dynamics and mixing processes

Evaluation of time-space varying density distribution on gravity currents by image analysis technique

Density currents are flows driven by density differences within a fluid which can be due to temperature differences, dissolved substances or particles in suspension. The present work describes a procedure based on an image-analysis technique to experimentally evaluate the time and space varying density distribution on density currents. The experiments were conducted in the Laboratory of Hydraulics of University Roma Tre in a 3.0 m long, 0.20 m wide and 0.30 m deep transparent Perspex flume, with a horizontal smooth bed. Density currents were performed by lock exchange release of saline water and recorded with a 25 Hz CCD camera under controlled light conditions. A controlled quantity of dye is added to the saline water in the lock to provide flow visualization and to work as density tracer. A calibration procedure technique is applied to gray scale matrices, converted from the acquired video frames, by using an experimentally determined non-linear relationship between these and the quantity of dye in the water (Fig. 1). Subsequently, the quantity of dye is converted in salt concentration by assuming a linear relation between both quantities, dye and salt, allowing thus the estimation of the instantaneous current density 2D distribution. With the resulting instantaneous density maps (Fig. 2), temporal and spatial evolution of the current form and structure can be assessed and discussed as well as time varying depth averaged density, front position and front velocity

Kinematics of unsteady gravity currents developing over smooth and rough beds

Gravity currents, from which turbidity currents are an example, occur in reservoirs and lakes with potential hazards on what concerns sedimentation and water quality issues. The dynamics of gravity currents produced by full-depth lock-release of saline water into a fresh water tank are herein investigated. More specifically, the present results concern to the head of gravity currents where main interaction with the ambient fluid is observed. The experiments were conducted in a 3.0 m long Perspex flume of horizontal bed and rectangular cross section of 0.20 x 0.30 m2 and recorded with a 25 Hz CCD video camera. An image analysis technique is used to evaluate time-space varying density distribution of unsteady density currents. The identification of the current head, based on the local vertically-averaged mass of the current, allows the characterization of its length and mass, both varying in time and space. Temporal evolution of these parameters show repeated cycles of stretching and breaking, where mass detachment from the head towards upstream, within the current body, is observed. This feature is related with instabilities signatures observed in the boundary region between current and ambient fluid and help to understand the entrainment phenomenon at the current head

Analysis of lock-exchange gravity currents over smooth and rough beds

Gravity currents produced by full-depth lock-release of saline water into a fresh water tank are studied focusing on the influence of the initial density of the saline mixture in the lock and the bed roughness on gravity current kinematics. Temporal evolution of the current front position and front velocity are analysed and related to different phases of the current. Time–space evolution of current depth-averaged density and current height are assessed as well. Roughness of the channel bed plays an important role in the current kinematics, particularly in decreasing the front velocity due to extra drag at the bed. The analysis of Froude numbers, estimated with the initial and local reduced gravity and established with different length scales of the current, allow for the definition of the important variables and current dynamics of each phase of the current development