Jang Hyuk Kwon

Implementation of k-w Turbulence Models in an Implicit Multigrid Method

Three κ-ω turbulence models using linear and nonlinear eddy-viscosity formulations are implemented in an implicit multigrid method. Detailed techniques of implementation are presented and discussed. Freezing and limiting strategies are applied to improve robustness and convergence of the multigrid method. The Wilcox κ-ω, κ-w shear-stress transport, and Wilcox-Durbin+ (WD+) models are first tested for flat-plate flow, and the results are in good agreement with the empirical correlations. Numerical results for unseparated and separated transonic flows show that the WD+ model using weakly nonlinear eddy viscosity is in better overall agreement with the experimental data. Particularly for the separated flows, the present multigrid diagonalized alternating-direction implicit method provides good convergence without any robustness problems

Preconditioned HLLE Method for Flows at All Mach Numbers

A two-dimensional preconditioned multigrid Navier-Stokes solver has been developed for all Mach number viscous flows. For the computation of all Mach number flows, a local preconditioning technique with a modified reference velocity was used to extend the Harten-Lax-van Leer-Einfeldt (HLLE+) scheme, which has good characteristics for compressible viscous flows. The developed methods used to compute several representative steady problems. Grid refinement and parametric studies are performed to verify the performance. The results show that the preconditioned HLLE+ scheme can be successfully applied to a wide variety of flows and that the implicit multigrid solver provides good convergence for the test problems.

Prediction of Damping Coefficients Using the Unsteady Euler Equations

A prediction method for dynamic damping coefficients using the unsteady Euler equations is presented. Direct unsteady simulation can be used to compute the pitch-damping moment without any geometric approximations when compared to the steady methods using the coning motions. A forced harmonic pitching motion is employed to generate the pitch-damping moments. To compute the pitch- and the roll-damping moments for the basic finner, a dual-time stepping algorithm combined with an implicit multigrid method is applied. The computed coefficients show good agreement with the experimental data. Grid refinement and parametric studies are performed to assess the accuracy of the numerical method. The linearity of the angular rates and the variation with Mach numbers are examined for both pitch- and roll-damping moment coefficients. Through analysis of the pressure distributions at various Mach numbers, the large variations of roll-damping moment coefficient in the transonic region are explained in detail.

On the dissipation mechanism of Godunov-type schemes

Dissipation mechanisms of Godunov-type schemes are presented in the framework of unified representation. The causes of inaccuracy at the contact discontinuity and the dissipation mechanism in the numerical mass flux of the HLLEM scheme are examined first. A vacuum preserving property is defined and the prominent role of the numerical signal speed involved with the rarefaction waves in the expanding region is analyzed. Through a linear perturbation analysis on the odd-even decoupling problem, necessary conditions for designing a shock stable scheme are discussed. As a result, an improved HLLE(HLLE+) scheme is proposed and its dissipation mechanism is analyzed. The diffusivity of the Godunov-type schemes is examined by two-dimensional hypersonic viscous flow.

Development of a Fully Systemized Chimera Methodology for Steady/Unsteady Problems

A new systemized procedure for chimera-domain decomposition is presented. This procedure consists of a new cut-paste algorithm for optimal mesh interface and a two-step search method for donor cell identification. It is fully automated and requires minimal user input. The cut-paste algorithm is based on the advancing front technique in which the fronts are iteratively determined from the initial fronts, which are a collection of fringe points obtained from conventional Chimera hole cutting. The final fronts are determined iteratively in such a way that the overlapping region is minimized. With this method, interpolation points are located away from solid walls where flow gradient is high. We also can reduce the error that may arise from interpolation in stiff gradient regions. Two- and three-dimensional examples are chosen to demonstrate the effectiveness of this new procedure

Periodic Unsteady Flow Analysis Using a Diagonally Implicit Harmonic Balance Method

An implicit harmonic balance method is applied to periodic unsteady flow problems. Diagonal terms of the harmonic source vector are treated implicitly and easily implemented on a diagonalized implicit solver that is very similar to a flow solver. The multi-grid performance and solution convergence of the present implicit harmonic balance method are investigated with a 2-D oscillating airfoil and a 3-D wing with pitching motion. Computational results are compared with results from explicit trials and a dual timestepping method in the time-domain. The present harmonic balance method provides fast and stable convergence characteristics regardless of the number of harmonics. Results show that the present method can largely reduce the computing time for unsteady flow problems, compared to that necessary in the dual time-stepping and explicit harmonic balance methods.

Navier-Stokes Computation of Pitch-Damping Coefficients Using Steady Coning Motions

Ap rediction method for pitch‐damping force and moment coefficients is presented in the unified framework of the unsteady Navier‐Stokes equations with the k‐ω turbulence equations. This approach does not require modification of the governing equations other than the addition of noninertial force terms. The present method is applied to compute the pitch‐damping coefficients using the lunar coning and the zero-spin coning motions. Grid refinement and parametric studies are performed. The computed pitch‐damping coefficients and Magnus moment coefficients are in good agreement with both the parabolized Navier‐Stokes data and the experimental data. The direct unsteady predictions are performed and compared with the steady coning motions. The results show that the present steady and unsteady approaches can be successfully applied to the prediction of the pitch‐damping coefficients.

An Improved HLLE Method for Hypersonic Viscous Flows

Dissipation mechanisms of Godunov-type schemes are presented in the framework of unified representation. We first examined the causes of inaccuracy at the contact discontinuity and the dissipation mechanism in the numerical mass flux of the HLLEM scheme. Through a linear perturbation analysis on the oddeven decoupling problem, we discuss necessary conditions for designing a shock stable scheme. The curing methods for each specified problem are also presented. A new improved HLLE(HLLE+) scheme is proposed and analyzed on its dissipation mechanism. The diffusivity of the Godunov-type schemes is examined by computing two-dimensional hypersonic viscous flow. It is shown that the present method can accurately predict the surface heating rates without grid dependency.

2-D Periodic Unsteady Flow Analysis Using a Partially Implicit Harmonic Balance Method

An efficient solution method for harmonic balance techniques with Fourier transform is presented for periodic unsteady flow problems. The present partially-implicit harmonic balance treats the flux terms implicitly and the harmonic source term is solved explicitly. The convergence of the partially Implicit method is much faster than the explicit Runge-Kutta harmonic balance method. The method does not need to compute the additional flux Jacobian matrix from the implicit harmonic source term. Compared with fully implicit harmonic balance method, this partial approach turns out to have good convergence property. Oscillating flows over NACA0012 airfoil are considered to verify the method and to compare with results of explicit R-K(Runge-Kutta) and dual time stepping methods.

Future Application and Middleware Technology on e-Science

As scientific research increasingly requires the efforts of globally diverse, multidisciplinary teams from different laboratories and organizations, there surfaces a need to facilitate this kind of collaborative work. These large-scale scientific initiatives, where information technology plays an important role, are termed e-Science; e-Science activities employ geographically distributed resources like high performance computing facilities, scientific instruments, databases, and high performance networks. Future Application and Middleware Technology on e-Science presents selected papers from the 2008 Korea e-Science All-Hands-Meeting (AHM 2008). Hosted by the Korea Institute of Science and Technology Information (KISTI), the Korea e-Science AHM was designed to bring together developers and users of e-Science applications and enabling information technologies from international and interdisciplinary research communities. The AHM 2008 conference served as a forum for engineers and scientists to present state-of-the-art research and product/tool developments, and to highlight related activities in all fields of e-Science. The following topics concerning e-Science are covered in this volume: e-Science applications in Physics, Astronomy, and Chemistry e-Science applications in Bio-medicine and Life science e-Science applications in Geo-science with remote sensing e-Science applications in Climates and Earth systems e-Science applications in Engineering (Aerospace, Ship, Automobile, and etc) Grid technologies (Computing, Data, VO, and etc) Collaborative science models and techniques Enabling technologies of Workflows and Web services Resource management and scheduling Problem solving environments Scientific data management Application development environments Robust and transparent middleware Programming paradigms and models Software engineering tools Community development and engagement User outreach, training, and documentation The works presented in this edited volume bring together cross-disciplinary information on e-Science in one cohesive source. This book is suitable for the professional audience composed of industry researchers and practitioners of e-Science. This volume is also suitable for advanced-level students in the field.