175 research outputs found

    Solving high-order partial differential equations with indirect radial basis function networks

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    This paper reports a new numerical method based on radial basis function networks (RBFNs) for solving high-order partial differential equations (PDEs). The variables and their derivatives in the governing equations are represented by integrated RBFNs. The use of integration in constructing neural networks allows the straightforward implementation of multiple boundary conditions and the accurate approximation of high-order derivatives. The proposed RBFN method is verified successfully through the solution of thin-plate bending and viscous flow problems which are governed by biharmonic equations. For thermally driven cavity flows, the solutions are obtained up to a high Rayleigh number

    An efficient BEM for numerical solution of the biharmonic boundary value problem

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    This paper presents an efficient BEM for solving biharmonic equations. All boundary values including geometries are approximated by the universal high order radial basis function networks (RBFNs) rather than the usual low order interpolations. Numerical results show that the proposed BEM is considerably superior to the linear/quadratic-BEM in terms of both accuracy and convergence rate

    A stable and accurate control-volume technique based on integrated radial basis function networks for fluid-flow problems

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    Radial basis function networks (RBFNs) have been widely used in solving partial differential equations as they are able to provide fast convergence. Integrated RBFNs have the ability to avoid the problem of reduced convergence-rate caused by differentiation. This paper is concerned with the use of integrated RBFNs in the context of control-volume discretisations for the simulation of fluid-flow problems. Special attention is given to (i) the development of a stable high-order upwind scheme for the convection term and (ii) the development of a local high-order approximation scheme for the diffusion term. Benchmark problems including the lid-driven triangular-cavity flow are employed to validate the present technique. Accurate results at high values of the Reynolds number are obtained using relatively-coarse grids

    Free vibration analysis of laminated composite plates based on FSDT using one-dimensional IRBFN method

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    This paper presents a new effective radial basis function (RBF) collocation technique for the free vibration analysis of laminated composite plates using the first order shear deformation theory (FSDT). The plates, which can be rectangular or non-rectangular, are simply discretised by means of Cartesian grids. Instead of using conventional differentiated RBF networks, one-dimensional integrated RBF networks (1D-IRBFN) are employed on grid lines to approximate the field variables. A number of examples concerning various thickness-to-span ratios, material properties and boundary conditions are considered. Results obtained are compared with the exact solutions and numerical results by other techniques in the literature to investigate the performance of the proposed method

    Asymmetric Palladium-Catalyzed Reactions for the Synthesis of Pyrrolidines and Other Heterocycles.

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    The synthesis of nitrogen-containing heterocycles is important due to the fact that many biologically active molecules contain these motifs. Over the past 10 years, the Wolfe group has developed efficient and highly diastereoselective Pd-catalyzed carboamination reactions for the synthesis of pyrrolidines, ureas, pyrazolidines, isoxazolidines, morpholines and benzodiazepines in good yields. These transformations involve syn-insertion of an alkene into a Pd–N bond of an intermediate Pd-amido species followed by C‒C bond forming reductive elimination. However, enantioselective variants that involve syn-aminopalladation are rare, and only two reports of enantioselective syn-aminopalladation have appeared in the literature. An enantioselective synthesis of 2-(arylmethyl)- and 2-(alkenylmethyl)pyrrolidines via Pd-catalyzed alkene carboamination reactions of N-Boc-pent-4-enylamines and aryl or alkenyl bromides is described. These transformations generate enantiomerically enriched pyrrolidine products with up to 94% ee. The application of this method has been demonstrated for the concise asymmetric synthesis of phenanthroindolizidine alkaloid (‒)-tylophorine. Studies on asymmetric Pd-catalyzed desymmetrization reactions for the formation of cis-2,5-disubstituted pyrrolidines are also described. A new enantioconvergent route for the asymmetric synthesis of (+)-aphanorphine has been accomplished using an asymmetric carboamination/Friedel-Crafts alkylation strategy. Enantioselective Pd-catalyzed carboamination reaction of a racemic aminoalkene derivative provides a 1:1 mixture of enantiomerically enriched diastereomers. This mixture of diastereomers is then converted to one stereoisomer via Friedel-Crafts reaction, which generates a quaternary stereocenter. Three additional steps provide the natural product. Lastly, studies on asymmetric Pd-catalyzed reactions for the enantioselective synthesis of imidazolidin-2-ones, isoxazolidines, pyrazolidines and tetrahydrofurans are described. Promising ligand scaffolds have been identified and promising leads are being followed up by current group members.Ph.D.ChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/86278/1/dmai_1.pd

    A continuum-microscopic method based on IRBFs and control volume scheme for viscoelastic fluid flows

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    A numerical computation of continuum-microscopic model for visco-elastic flows based on the Integrated Radial Basis Function (IRBF) Control Volume and the Stochastic Simulation Techniques (SST) is reported in this paper. The macroscopic flow equations are closed by a stochastic equation for the extra stress at the microscopic level. The former are discretised by a 1D-IRBF-CV method while the latter is integrated with Euler explicit or Predictor-Corrector schemes. Modelling is very efficient as it is based on Cartesian grid, while the integrated RBF approach enhances both the stability of the procedure and the accuracy of the solution. The proposed method is demonstrated with the solution of the start-up Couette flow of the Hookean and FENE dumbbell model fluids

    Imposition of physical parameters in dissipative particle dynamics

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    In the mesoscale simulations by the dissipative particle dynamics (DPD), the motion of a fluid is modelled by a set of particles interacting in a pairwise manner, and it has been shown to be governed by the Navier–Stokes equation, with its physical properties, such as viscosity, Schmidt number, isothermal compressibility, relaxation and inertia time scales, in fact its whole rheology resulted from the choice of the DPD model parameters. In this work, we will explore the response of a DPD fluid with respect to its parameter space, where the model input parameters can be chosen in advance so that (i) the ratio between the relaxation and inertia time scales is fixed; (ii) the isothermal compressibility of water at room temperature is enforced; and (iii) the viscosity and Schmidt number can be specified as inputs. These impositions are possible with some extra degrees of freedom in the weighting functions for the conservative and dissipative forces. Numerical experiments show an improvement in the solution quality over conventional DPD parameters/weighting functions, particularly for the number density distribution and computed stresses

    Investigation of particles size effects in Dissipative Particle Dynamics (DPD) modelling of colloidal suspensions

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    In the Dissipative Particle Dynamics (DPD) simulation of suspension, the fluid (solvent) and colloidal particles are replaced by a set of DPD particles and therefore their relative sizes (as measured by their exclusion zones) can affect the maximal packing fraction of the colloidal particles. In this study, we investigate roles of the conservative, dissipative and random forces in this relative size ratio (colloidal/solvent). We propose a mechanism of adjusting the DPD parameters to properly model the solvent phase (the solvent here is supposed to have the same isothermal compressibility to that of water)

    BEM-RBF approach for viscoelastic flow analysis

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    A new BE-only method is achieved for the numerical solution of viscoelastic flows. A decoupled algorithm is chosen where the fluid is considered as being composed of an artificial Newtonian component and the remaining component is accordingly defined from the original constitutive equation. As a result the problem is viewed as that of solving for the flow of a Newtonian liquid with the non-linear viscoelastic effects acting as a pseudo body force. Thus the general solution is obtained by adding a particular solution to the homogeneous one. The former is obtained by a BEM for the base Newtonian fluid and the latter is obtained analytically by approximating the pseudo body force in terms of suitable radial basis functions (RBFs). Embedded in the approximation of the pseudo body force is the calculation of the polymer stress. This is achieved by solving the constitutive equation using RBF networks (RBFNs). Both the calculations of the particular solution and the polymer stress are therefore meshless and the resultant BEM-RBF method is a BE-only method. The complete elimination of any structured domain discretisation is demonstrated with a number of flow problems involving the Upper Convected Maxwell (UCM) and the Oldroyd-B fluids

    A note on dissipative particle dynamics (DPD) modelling of simple fluids

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    In this paper, we show that a Dissipative Particle Dynamics (DPD) model of a viscous Newtonian fluid may actually produce a linear viscoelastic fluid. We demonstrate that a single set of DPD particles can be used to model a linear viscoelastic fluid with its physical parameters, namely the dynamical viscosity and the relaxation time in its memory kernel, determined from the DPD system at equilibrium. The emphasis of this study is placed on (i) the estimation of the linear viscoelastic effect from the standard parameter choice; and (ii) the investigation of the dependence of the DPD transport properties on the length and time scales, which are introduced from the physical phenomenon under examination. Transverse-current auto-correlation functions (TCAF) in Fourier space are employed to study the effects of the length scale, while analytic expressions of the shear stress in a simple small amplitude oscillatory shear flow are utilised to study the effects of the time scale. A direct mechanism for imposing the particle diffusion time and fluid viscosity in the hydrodynamic limit on the DPD system is also proposed