The Experts below are selected from a list of 309 Experts worldwide ranked by ideXlab platform

Il Han Park - One of the best experts on this subject based on the ideXlab platform.

Michael Yu Wang - One of the best experts on this subject based on the ideXlab platform.

  • Sensitivity Analysis of Topology Optimization Design for Dynamic Characeristics of Anisotropic Materrial Structures
    Journal of Qingdao Technological University, 2020
    Co-Authors: Sen Liang, Michael Yu Wang, Chuijie Yi
    Abstract:

    The new theoretic model of the topology optimization for the anisotropic Material continuum structures with level set function is established. A new approach is employed here to prove the functional Material Derivative formulas of domain integral and boundary integral.The novel sensitivity analysis of the dynamic characteristics for topology optimization of an anisotropic Material structure is presented by employing the Material Derivative method and augmented Lagrangian multipliers method.The evolution of the structural design boundary can be controlled by the artificial velocity which makes the objective function descend.The level set surface of a higher-dimensional function can be moved up and down without changing its topology structures,but the optimization boundaries embedded on level set function can automatically modify the topology structures by the boundaries merging and breaking.The extensively studied 2D examples are employed to demonstrate the validity of the presented methodologies.The conclusions indicate this investigation will provide an important foundation for the advanced development of the topological optimization theory and algorithm of the composite Material structures.

  • A level set based method for topology optimization of continuum structures with stress constraint
    2020
    Co-Authors: Michael Yu Wang
    Abstract:

    This paper presents a stress constrained topology optimization problem and its level set based solution. The volume of Material is minimized, and stress constraints at all the points in a structure are efficiently aggregated into a single equivalent global constraint. Using the Material Derivative and adjoint method we derived the shape sensitivity. The steepest descent optimization algorithm is implemented. Several numerical examples in two dimensions are provided and discussed.

  • topology optimization with pressure load through a level set method
    Computer Methods in Applied Mechanics and Engineering, 2015
    Co-Authors: Michael Yu Wang
    Abstract:

    Abstract The topology optimization problem with pressure load is solved by using a level set method. The free boundary and the pressure boundary of a structure are represented separately as two zero-level sets of two level set functions, and they are independently propagated during the optimization by solving two Hamilton–Jacobi equations. In order to prevent the two boundaries from touching or crossing each other, the design velocities of the two boundaries that amount to the steepest descent directions are modified. The optimization problem of minimum compliance with perimeter regularization is considered. The shape Derivatives of the two boundaries are derived by using the Material Derivative approach and the adjoint method. The finite element analysis is done through an Eulerian method by employing a fixed mesh and an artificial weak Material that represents void. Numerical examples in two dimensions are investigated.

Fulvio Scarano - One of the best experts on this subject based on the ideXlab platform.

  • Dense velocity reconstruction from tomographic PTV with Material Derivatives
    Experiments in Fluids, 2016
    Co-Authors: J.f.g. Schneiders, Fulvio Scarano
    Abstract:

    A method is proposed to reconstruct the instantaneous velocity field from time-resolved volumetric particle tracking velocimetry (PTV, e.g., 3D-PTV, tomographic PTV and Shake-the-Box), employing both the instantaneous velocity and the velocity Material Derivative of the sparse tracer particles. The constraint to the measured temporal Derivative of the PTV particle tracks improves the consistency of the reconstructed velocity field. The method is christened as pouring time into space, as it leverages temporal information to increase the spatial resolution of volumetric PTV measurements. This approach becomes relevant in cases where the spatial resolution is limited by the seeding concentration. The method solves an optimization problem to find the vorticity and velocity fields that minimize a cost function, which includes next to instantaneous velocity, also the velocity Material Derivative. The velocity and its Material Derivative are related through the vorticity transport equation, and the cost function is minimized using the limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) algorithm. The procedure is assessed numerically with a simulated PTV experiment in a turbulent boundary layer from a direct numerical simulation (DNS). The experimental validation considers a tomographic particle image velocimetry (PIV) experiment in a similar turbulent boundary layer and the additional case of a jet flow. The proposed technique (‘vortex-in-cell plus’, VIC+) is compared to tomographic PIV analysis (3D iterative cross-correlation), PTV interpolation methods (linear and adaptive Gaussian windowing) and to vortex-in-cell (VIC) interpolation without the Material Derivative. A visible increase in resolved details in the turbulent structures is obtained with the VIC+ approach, both in numerical simulations and experiments. This results in a more accurate determination of the turbulent stresses distribution in turbulent boundary layer investigations. Data from a jet experiment, where the vortex topology is retrieved with a small number of tracers indicate the potential utilization of VIC+ in low-concentration experiments as for instance occurring in large-scale volumetric PTV measurements.

  • Pouring time into space
    2015
    Co-Authors: J.f.g. Schneiders, Fulvio Scarano, I. Azijli, Richard P. Dwight
    Abstract:

    A method is proposed to reconstruct instantaneous velocity from time-resolved tomographic PTV, employing both instantaneous velocity and velocity Material Derivative. This improves upon current techniques by not only including penalization of velocity divergence, but also requiring consistency with the temporal Derivative of the PTV particle tracks. Hence the procedure is christened as pouring time into space. The aim of the proposed technique is to increase spatial resolution of tomographic PTV in cases where it is limited by the seeding concentration. An inverse problem is solved to find the velocity field that minimizes a cost function including next to instantaneous velocity and its divergence, also the velocity Material Derivative. The velocity and its Material Derivative are related through the vorticity transport equation and the problem is minimized using the L-BFGS algorithm, where gradients are evaluated efficiently using an adjoint implementation of the method. The procedure is assessed numerically using results from a simulated PTV experiment in a turbulent boundary layer from DNS, and experimentally using tomographic PIV measurements in a jet flow. Both the numerical and experimental assessment show that the proposed technique yields improved accuracy of the velocity field in between the measured points over penalization of divergence only, thereby demonstrating that the temporal information available in time-resolved tomographic PTV can be leveraged to increase reconstruction quality of instantaneous velocity.

  • A particle-tracking approach for accurate Material Derivative measurements with tomographic PIV
    Experiments in Fluids, 2013
    Co-Authors: Matteo Novara, Fulvio Scarano
    Abstract:

    The evaluation of the instantaneous 3D pressure field from tomographic PIV data relies on the accurate estimate of the fluid velocity Material Derivative, i.e., the velocity time rate of change following a given fluid element. To date, techniques that reconstruct the fluid parcel trajectory from a time sequence of 3D velocity fields obtained with Tomo-PIV have already been introduced. However, an accurate evaluation of the fluid element acceleration requires trajectory reconstruction over a relatively long observation time, which reduces random errors. On the other hand, simple integration and finite difference techniques suffer from increasing truncation errors when complex trajectories need to be reconstructed over a long time interval. In principle, particle-tracking velocimetry techniques (3D-PTV) enable the accurate reconstruction of single particle trajectories over a long observation time. Nevertheless, PTV can be reliably performed only at limited particle image number density due to errors caused by overlapping particles. The particle image density can be substantially increased by use of tomographic PIV. In the present study, a technique to combine the higher information density of tomographic PIV and the accurate trajectory reconstruction of PTV is proposed (Tomo-3D-PTV). The particle-tracking algorithm is applied to the tracers detected in the 3D domain obtained by tomographic reconstruction. The 3D particle information is highly sparse and intersection of trajectories is virtually impossible. As a result, ambiguities in the particle path identification over subsequent recordings are easily avoided. Polynomial fitting functions are introduced that describe the particle position in time with sequences based on several recordings, leading to the reduction in truncation errors for complex trajectories. Moreover, the polynomial regression approach provides a reduction in the random errors due to the particle position measurement. Finally, the acceleration can be evaluated analytically, which greatly reduces the truncation errors due to finite differences. The approach is first assessed using computer-generated data of an advecting vortex ring. Precision errors in the Material Derivative can be reduced with a factor 2–3. This is achieved when a long sequence is considered (e.g. 15–20 recordings). Similarly, truncation errors typically associated with direct integration and finite differences from the PIV-based technique are almost eliminated. It is shown that the Material Derivative information obtained at the scattered locations in the 3D domain can be reduced to a uniform Cartesian grid by means of a second-order spatial regression with no significant artefact. The technique is applied to a benchmark Tomo-PIV experiment of a transitional jet in water. The results confirm the conclusions obtained with the numerical study. Moreover, it is shown that the evaluation of the instantaneous 3D pressure field can be retrieved with significant reduction in artefacts associated with random and truncation errors.

  • Material Derivative Measurements in High-Speed Flows by Four-Pulse Tomographic PIV
    2013
    Co-Authors: Kyle P. Lynch, Fulvio Scarano
    Abstract:

    A tomographic PIV system is introduced for the instantaneous measurement of the Material Derivative of velocity (VMD). The system is able to operate with very short temporal separation and is therefore suitable for applications in high-speed flows. The method of operation consists of the imaging of a measurement volume using an array of 12 CCD cameras and two double-cavity laser systems. Four independent recordings of particle images are captured by decomposing the system into three separate tomographic PIV systems comprised of four cameras. A discussion is made that compares the present working principle with other methods used to separate the light scattered from multiple pulses, namely by polarization. Various approaches are compared to determine the optimal utilization of four-pulse data to measure the VMD: the Eulerian and Lagrangian schemes are compared with the recently introduced fluid trajectory correlation (FTC) technique from the authors (Lynch and Scarano, 2013). The comparison focuses on the behavior of the schemes with respect to truncation errors and how the error estimates for four-pulse data are modified from those typically applied to image sequence data from a time-resolved PIV experiment. The analysis of synthetic images of a translating vortex clearly shows the envelope of applicability of the different schemes and the structure of the measurement errors introduced by truncation. The 12-camera tomographic system in four-pulse configuration is employed to measure the wake of an axisymmetric truncated base with an afterbody at a Reynolds number of 68,000. The system calibration accuracy and the baseline measurement uncertainty of the velocity are evaluated by performing a test with a negligible time delay between the independent tomographic PIV systems. The comparative performance of the Material Derivative schemes is estimated by appealing to a physical property of the Material Derivative field. The results indicate that a 12-camera system can be employed for Material Derivative evaluation using a variety of estimation schemes. Among these schemes, the FTC technique is found to be the least susceptible to the growth of truncation errors and is thus suitable for measurement at large temporal intervals which are necessary to suppress random errors.

K. K. Choi - One of the best experts on this subject based on the ideXlab platform.

  • Optimization of a hyper-elastic structure with multibody contact using continuum-based shape design sensitivity analysis
    Structural and Multidisciplinary Optimization, 2001
    Co-Authors: Y H. Park, K. K. Choi
    Abstract:

    In this paper, a continuum-based shape design sensitivity formulation is presented for a hyper-elastic structure with multibody frictional contact. A nearly incompressible constraint is treated using the pressure projection method that projects a hydrostatic pressure into a lower order space to avoid a volumetric locking. The variational formulation for multibody frictional contact is developed using a penalty method that regularizes the solution of the variational inequality. The Material Derivative of continuum mechanics is utilized to develop the continuum-based shape design sensitivity analysis for the hyper-elastic constitutive relation and penalized contact formulation. The sensitivity equation is solved at each converged load step using the same tangent stiffness of response analysis due to the path dependency of the sensitivity of the frictional contact problem. A very accurate and efficient sensitivity results are shown through shape optimization of a windshield wiper.

  • Design Sensitivity Analysis of Truss Structures with Elastoplastic Material
    Mechanics of Structures and Machines, 1996
    Co-Authors: Y H. Park, K. K. Choi
    Abstract:

    ABSTRACT A continuum-based design sensitivity analysis (DSA) method is presented for configuration (or layout) design of nonlinear structural systems with rate-independent elastoplastic Material. Configuration design variables are characterized by shape and orientation changes of the structural component. A continuum-based shape DSA method that utilizes the Material Derivative of continuum mechanics is extended to account for effects of shape and orientation variations. The incremental analysis method, with updated Lagrangian formulation, is used to derive the design sensitivity for the nonlinear structural system. To derive the design sensitivity, incremental energy and load forms are utilized. The first variations of energy and load forms and the static response with respect to configuration design variables are described using the Material Derivative. Direct differentiation is utilized to obtain the first variation of the performance measure explicitly in terms of variations of configuration design var...

Ralf Metzler - One of the best experts on this subject based on the ideXlab platform.

  • towards deterministic equations for levy walks the fractional Material Derivative
    Physical Review E, 2003
    Co-Authors: I M Sokolov, Ralf Metzler
    Abstract:

    Levy walks are random processes with an underlying spatiotemporal coupling. This coupling penalizes long jumps, and therefore Levy walks give a proper stochastic description for a particle's motion with broad jump length distribution. We derive a generalized dynamical formulation for Levy walks, in which the fractional equivalent of the Material Derivative occurs. Our approach is expected to be useful for the dynamical formulation of Levy walks in an external force field or in phase space, for which the description in terms of the continuous time random walk or its corresponding generalized master equation are less well suited.

  • Towards deterministic equations for Lévy walks: the fractional Material Derivative.
    Physical review. E Statistical nonlinear and soft matter physics, 2003
    Co-Authors: I M Sokolov, Ralf Metzler
    Abstract:

    Lévy walks are random processes with an underlying spatiotemporal coupling. This coupling penalizes long jumps, and therefore Lévy walks give a proper stochastic description for a particle's motion with broad jump length distribution. We derive a generalized dynamical formulation for Lévy walks, in which the fractional equivalent of the Material Derivative occurs. Our approach is expected to be useful for the dynamical formulation of Lévy walks in an external force field or in phase space, for which the description in terms of the continuous time random walk or its corresponding generalized master equation are less well suited.