Browsing by Author "Thacker, William I."

Now showing 1 - 12 of 12
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    Algorithm XXX: QNSTOP—Quasi-Newton Algorithm for Stochastic Optimization
    Amos, Brandon D.; Easterling, David R.; Watson, Layne T.; Thacker, William I.; Castle, Brent S.; Trosset, Michael W. (Department of Computer Science, Virginia Polytechnic Institute & State University, 2014-07-01)
    QNSTOP consists of serial and parallel (OpenMP) Fortran 2003 codes for the quasi-Newton stochastic optimization method of Castle and Trosset. For stochastic problems, convergence theory exists for the particular algorithmic choices and parameter values used in QNSTOP. Both the parallel driver subroutine, which offers several parallel decomposition strategies, and the serial driver subroutine can be used for stochastic optimization or deterministic global optimization, based on an input switch. QNSTOP is particularly effective for “noisy” deterministic problems, using only objective function values. Some performance data for computational systems biology problems is given.
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    Algorithm XXX: SHEPPACK: Modified Shepard Algorithm for Interpolation of Scattered Multivariate Data
    Thacker, William I.; Zhang, Jingwei; Watson, Layne T.; Birch, Jeffrey B.; Iyer, Manjula A.; Berry, Michael W. (Department of Computer Science, Virginia Polytechnic Institute & State University, 2009)
    Scattered data interpolation problems arise in many applications. Shepard’s method for constructing a global interpolant by blending local interpolants using local-support weight functions usually creates reasonable approximations. SHEPPACK is a Fortran 95 package containing five versions of the modified Shepard algorithm: quadratic (Fortran 95 translations of Algorithms 660, 661, and 798), cubic (Fortran 95 translation of Algorithm 791), and linear variations of the original Shepard algorithm. An option to the linear Shepard code is a statistically robust fit, intended to be used when the data is known to contain outliers. SHEPPACK also includes a hybrid robust piecewise linear estimation algorithm RIPPLE (residual initiated polynomial-time piecewise linear estimation) intended for data from piecewise linear functions in arbitrary dimension m. The main goal of SHEPPACK is to provide users with a single consistent package containing most existing polynomial variations of Shepard’s algorithm. The algorithms target data of different dimensions. The linear Shepard algorithm, robust linear Shepard algorithm, and RIPPLE are the only algorithms in the package that are applicable to arbitrary dimensional data.
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    An Annotated Bibliography on Optimization of Programs During Compilation
    Lee, John A. N.; Thacker, William I.; Harrison, T. C. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1978)
    This annotated bibliography covers a collection of articles assembled by the authors in a graduate level course on advanced compilation techniques.
  • Effect of Elevated Mass Center on the Global Stability of a Solid Supported by Elastica Columns
    Thacker, William I.; Wang, Chang Y.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1996)
    Recently we advocated a new stability index, the global critical load, for very elastic structures. This index is extremely useful for flexible structures under large disturbances such as earthquakes. The present note determines this index for a two-dimensional rigid solid supported by two flexible columns. Using the nonlinear elastica equations the buckling and postbuckling problem is solved by a homotopy nonlinear system solver. The present results show the bifurcation curve is quite sensitive to the elevated mass center. The global buckling load is drastically reduced although the critical buckling load of linear stability analysis is the same. An explanation is given through the study of a solid supported by one column.
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    Effect of flexible joints on the stability and large deflections of a triangular frame
    Thacker, William I.; Wang, Chang Y.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 2007)
    An isosceles triangular frame with rotationally resistive joints under a tip load is studied. The large in-plane deformation elastica equations are formulated. Stability analysis shows the frame can buckle symmetrically or asymmetrically. Post-buckling behavior showing limit load and hysteresis are obtained by shooting and homotopy numerical algorithms. The behavior of a frame with rigid joints is studied in detail. The effects of joint spring constant and base length are found.
  • The Global Stability of a Rigid Solid Supported by Elastic Columns
    Thacker, William I.; Wang, Chang Y.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1994)
    A new stability index, the global critical load, is advocated. This index is useful for flexible structures prone to large disturbances such as earthquakes. A symmetric rigid body supported by flexible legs is studied in detail. The nonlinear equilibrium equations are solved and the results show that global stability depends heavily on the height of the mass center and the distance between the legs.
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    Magnetohydrodynamic Flow and Heat Transfer About a Rotating Disk with Suction and Injection at the Disk Surface
    Kumar, Kishore S.; Thacker, William I.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1986)
    This paper studies the effects of a partial magnetic field on the flow and heat transfer about a porous rotating disk. Using modem quasi-Newton and globally convergent homotopy methods, numerical solutions are obtained for a wide range of magnetic field strengths and injection and suction velocities. Results are presented graphically in terms of three nondimensional parameters. There is excellent agreement with previous work and asymptotic formulas.
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    Magnetohydrodynamic Flow Between a Solid Rotating Disk and a Porous Stationary Disk
    Kumar, Kishore S.; Thacker, William I.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1987)
    In this paper we examine the flow of a conducting fluid between a solid rotating disk and a stationary porous disk with uniform section of fluid through the porous disk in the presence of a magnetic fiels. The equations of motion are solved using least change secant update quasi-Newton and modern root finding techniques. The fluid motion depends on the cross-flow Reynolds number, rotation Reynolds number and Hartmann number. The effects of the parameters on the flow field are presented graphically.
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    Magnetohydrodynamic Flow Past a Porous Rotating Disk in a Circular Magnetic Field
    Kumar, Kishore S.; Thacker, William I.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1987)
    This paper studies the effects of a circular magnetic field on the flow of a conducting fluid about a porous rotating disk. Using modern quasi-Newton and globally convergent homotopy methods, numerical solutions are obtained for a wide range of magentic field strengths, suction and injection velocities and Alfven and disk speeds. Results are presented graphically in terms of three nondimensional parameters. There is excellent agreement with previous work and asymptotic formulas.
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    Magnetohydroynamic Free Convection from a Disk Rotating in a Vertical Plane
    Thacker, William I.; Watson, Layne T.; Kumar, Kishore S. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1988)
    The non-axisymmetric motion (produced by a buoyancy induced cross flow) of a fluid in contact with a rotating disk and in the presence of a magnetic field normal to the disk is studied. Using modern quasi- Newton techniques, B-splines, and a Galerkin approximation to the fluid motion equations, numerical solutions are obtained for a wide range of magnetic field strengths and Prandtl numbers (ratio of kinematic viscosity to thermal conductivity). Results are presented both in tabular and graphical form in terms of two non-dimensional parameters. There is excellent agreement with previous work.
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    The Nonlinear Stability of a Heavy Rigid Plate Supported by Flexible Columns
    Thacker, William I.; Wang, Chang Y.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1992)
    A heavy rigid platform is supported by thin elastic legs. The governing equations for large deformations are formulated and solved numerically by homotopy and quasi-Newton methods. Nonlinear phenomena such as non-uniqueness, catastrophe and hysteresis are found. A global critical load for nonlinear stability is introduced.
  • Stability and Postbuckling of a Platform with Flexible Legs Resting on a Slippery Surface
    Thacker, William I.; Wang, Chang Y.; Watson, Layne T. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1997-12-01)
    A rigid platform is supported by thin elastic legs. The legs are able to slide on the ground as they deform. The governing equations for large deformations are formulated and solved numerically by homotopy and quasi-Newton methods. Nonlinear phenomena such as nonuniqueness are found. A global critical load for nonlinear stability is presented.