Total synthesis of the marine natural product (-)-clavosolide A

[Structure: see text] The total synthesis of the marine metabolite clavosolide A is reported which confirms the structure and absolute configuration of the natural product as the symmetrical diolide glycosylated by permethylated D-xylose moieties, 2.     

Long-Term Evolution of Coal Permeability Under Effective Stresses Gap Between Matrix and Fracture During CO2 Injection

Transport in Porous Media - Understanding the long-term evolution of coal permeability under the influence of gas adsorption-induced multiple processes is crucial for the efficient sequestration of...     

Experimental Measurements of Stress and Chemical Controls on the Evolution of Fracture Permeability

We explore how fracture permeability in confined tight carbonates evolves due to flow of reactive fluids. Core plugs of the Capitan Massive Limestone are saw-cut to form a smooth axial fracture that is subsequently roughened to control the fracture surface topography. Either distilled water or distilled water–ammonium chloride solutions are circulated through these plugs, where fracture roughness, inlet fluid pH, and confining stresses are controlled. Throughout the experiment we measure the fluid flow rate and chemical composition of the effluent fluid. Mass balance, conducted on the effluent fluid mass and on dissolved mineral components, independently constrains the mineral mass removal. We use an idealized lumped parameter model of asperity supported fractures undergoing simultaneous stress corrosion cracking-induced diffusion and free-face dissolution to infer theoretical rates of aperture loss or gain. This model incorporates the roles of confining stress, fracture contact area, and composition and reactivity of the permeating fluid while identifying zones of diffusion-dominated mass transfer within the fracture. These theoretical rates of aperture strain are compared to those inferred from the experimentally determined permeability evolution and permeating fluid mineral mass balance. By measuring in regimes of both increasing and decreasing permeability we quantitatively constrain the transition between fracture-gaping and fracture-closing modes of behavior. We parameterize this transition in permeability evolution by the ratio of mechanically to chemically controlled dissolved mass fluxes. The transition from regimes of closing to regimes of gaping occurs at unity ( $\chi \approx 1$ ) when stress and chemically driven mass fluxes are theoretically equal.     

An adaptive characteristics method for advective-diffusive transport

An adaptive characteristics method is presented for the solution of advective-diffusive groundwater transport problems. The method decomposes the transport processes into advective and diffusive transport components. Advective flows are defined by using a streamtube incrementing procedure, based on the method of characteristics, to define the position of advective front. Diffusive transport orthogonal to the front is represented by an array of propagating streamtube elements, with dimension determined from analytical solution of the one-dimensional diffusion equation. Adaptive time scaling is used to moderate the dimensions and aspect ratios of the advective and diffusive streamtube elements as appropriate to the dominant transport mechanism. Finite differences are used to solve for transport ahead of the advancing front. The distribution of streamtubes are predetermined from a direct boundary element algorithm. Comparison with analytical results for a one-dimensional transport geometry indicates agreement for Peclet numbers between zero and infinity. Solution for transport in two-dimensional domains illustrates excellent agreement for Peclet numbers from zero to 25.     

Textilindustrie

Ohne Zusammenfassung     

Tracer transport in a fractured chalk: X-ray CT characterization and digital-image-based (DIB) simulation

A digital-image-based simulation methodology is applied to evaluate the influence of heterogeneous porosity on the evolution of tracer concentrations in imaged tracer tests. Maps of computed tomography (CT)-number are calibrated relative to average porosity, and then thresholded to define porosity maps. These data are then used to automate the distribution of parameters within a finite element representation of the geometry. The technique is applied to characterize the variability of the porosity, the hydraulic conductivity, and the diffusivity for an artificially fractured chalk core (30 × 5 cm). X-ray CT was used both to characterize the initial condition of the core, and then to concurrently monitor the transport of an NaI tracer within the fracture and into the surrounding matrix. The X-ray CT imaging is used to characterize the heterogeneous rock porosity, based on which the hydraulic conductivity, and diffusivity of the chalk were defined and were directly imported into our newly developed three-dimensional FEMLAB-based multiple physics simulator. Numerical simulations have confirmed the observed tracer transport behaviors: (1) The different tracer-penetration distances imaged in the matrix above and below the horizontal fracture are indicative of a greater tracer mass penetrating into the lower matrix; and (2) Transport in the matrix below the fracture was enhanced. The computer simulated tracer concentration distributions compare favorably with those monitored by X-ray CT.     

Bicyclic oxygen heterocycles from γ,δ-unsaturated alcohols: synthetic targets inspired by blepharocalyxin D

trans-2,8-Dioxabicyclodecanes were prepared in high yield with the creation of up to three stereocenters in a single pot by the acid-mediated reaction of γ,δ-unsaturated alcohols with aldehydes (see scheme, Bn=benzyl). This versatile reaction enables the stereoselective introduction of substituents at the C3, C4, C7, and C9 positions of the bicyclic framework.     

On the modeling of miscible flow in multi-component porous media

An analytical model of miscible flow in multi-component porous media is presented to demonstrate the influence of pore capacitance in extending diffusive tailing. Solute attenuation is represented naturally by accommodating diffusive and convective flux components in macropores amd micropores as elicited by the local solute concentration and velocity fields. A set of twin, coupled differential equations result from the Laplace transform and are solved simultaneously using a differential operator for one-dimensional flow geometry. The solutions in real space are achieved using numeric inversion. In addition, to represent more faithfully the dominant physical processes, this approach enables efficient and stable semi-analytical solution procedure of the coupled system that is significantly more complex than current capacitance type models. Parametric studies are completed to illustrate the ability of the model to represent sharp breakthrough and lengthy tailing, as well as investigating the form of the nested heterogeneity as a result of solute exchange between macropores and micropores. Data from a laboratory column experiment is examined using the present model and satisfactory agreement results.Roman Letters a rate coefficient of internal flow - b velocity ratio (v ₁/v ₂) - h dispersion ratio (D ₂/D ₁) - c ₁ macropore concentration - c ₂ micropore concentration - ¯c ₁ macropore concentration in Laplace space - ¯c ₂ micropore concentration in Laplace space - c ₁ ⁰ macropore concentration at source location - c ₂ ⁰ micropore concentration at source location - D ₁ macropore dispersion coefficient - D ₂ micropore dispersion coefficient - f fraction of pore space occupied by fluid in primary channel - L length of laboratory sample column - K mass exchange rate - t time from initial stage - v ₁ primary flow channel velocity - v ₂ micropore interstitial velocity - x distance from source - y dimensionless distance Greek Letters equivalent Péclet number - dimensionless time, or injected pore volume     

Spontaneous Switching between Permeability Enhancement and Degradation in Fractures in Carbonate: Lumped Parameter Representation of Mechanically- and Chemically-Mediated Dissolution

Principal mechanical and chemical processes contributing to the observed spontaneous switching from net decrease in permeability to net increase in a fracture in carbonate are examined. The evolution of permeability, and related fracture aperture, is represented through a lumped parameter model. The significant processes of pressure solution beneath bridging asperities, transport of dissolved mass to the fracture void, and subsequent precipitation or dissolution within the fracture void enable the principal characteristics of observed behavior to be followed. The evolution of dissolved mass concentration in the pore fluid is followed for arbitrary applied stress, temperature, and pH conditions, with appropriate feedback to the evolution of fracture permeability. Comparisons with experimental measurements in limestone (Polak et al., 2004, Water Resour. Res. Vol. 40, W03502, doi: 10.1029/2003GL017575) show satisfactory agreement for the evolution of fracture aperture and to a lesser degree in calcium concentrations in the effluent pore fluid. Importantly, the spontaneous switching in permeability change, from aperture reducing to aperture increasing, with no change in environmental conditions, is replicated without the need for an ad hoc trigger. Although this switch is accurately replicated, the lumped parameter model is incapable of replicating the rapid observed growth in permeability that directly follows. This inability results from the assumed form of the lumped asperity model, that is incapable of representing the spatially distributed change in aperture that is seen to occur within the fracture. Despite this inconsistency, the model is shown capable of representing the principal behaviors evident in the response.     

Total synthesis of a diastereomer of the marine natural product clavosolide A

The first total synthesis of the reported structure of the sponge metabolite clavosolide A is described using a Prins cyclisation to assemble the tetrahydropyran core followed by manipulation of the side-chain, dimerisation and finally glycosidation.