• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.35 no.8
• /
• pp.677-684
• /
• 2007

In the Present Study, a 3-D viscous flow solver, based on unstructured hybrid meshses containing tetrahedra, prisms and pyramids, has been developed. A finite-volume discretization scheme is used for solving the compressible Navier-Stokes equations. A cell-vertex median dual volume is used for spatial discretization. The one-equation Spalart-Allmaras turbulence model has been adopted to evaluate the eddy viscosity. Validation were made by computing laminar and turbulent flows around a 3-D wing for steady flows and turbulent flows around an oscillating 3-D wing in harmonic motion for unsteady flows.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.26 no.11
• /
• pp.1521-1530
• /
• 2002

This article proposes a pressure based method for predicting flows at all speeds. The compressible SIMPLE algorithm is extended to unstructured grid framework. Convection terms are discretized using second-order scheme with deferred correction approach. Diffusion term discretization is based on structured grid analogy that can be easily adopted to hybrid unstructured grid solver. This method also uses node centered scheme with edge based data structure for memory and computing time efficiency of arbitrary grid types. Both incompressible and compressible benchmark problems are solved using the above methodology. The demonstration of this method is extended to slip flow problem that has low Reynolds number but compressibility effect. It is shown that the proposed method can improve efficiency in memory usage and computing time without losing any accuracy.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.30 no.8
• /
• pp.37-45
• /
• 2002

A two-dimensional unsteady, inviscid flow solver has been developed for the simulation of airfoil-vortex interactions on unstructured dynamically adapted meshes. The Euler solver is based on a second-order accurate implicit time integration using a point Gauss-Seidel relaxation scheme and a dual time-step subiteration. A vertex-centered, finite-volume discretization is used in conjunction with the Roe's flux-difference splitting. An unsteady solution-adaptive dynamic mesh scheme is used by adding and deleting mesh points to take account of both spatial and temporal variations of the flow field. The effect of vortex interaction on the aeroelastic displacement of an airfoil attached to the idealized two degree-of-freedom spring system is investigated.

• Journal of Radiation Protection and Research
• /
• v.28 no.4
• /
• pp.311-319
• /
• 2003

We developed a grid-based Gaussian plume model to evaluate tracer release data measured at Young Gwang nuclear site in 1996. Downwind distance was divided into every 10m from 0.1km to 20km, and crosswind distance was divided into every 10m centering released point from -5km to 5km. We determined dispersion factors, ${\sigma}_y\;and\;{\sigma}_z$ using Pasquill-Gifford method computed by atmospheric stability. Forecasting ability of the grid-based Gaussian plume model was better at the 3km away from the source than 8km. We confirmed that dispersion band must be modified if receptor is far away from the source, otherwise P-G method is not appropriate to compute diffusion distance and diffusion strength in case of growing distance. So, we developed an empirical equation using linear programming. An objective function was designed to minimize sum of the absolute value between observed and computed values. As a result of application of the modified dispersion equation, prediction ability was improved rather than P-G method.

• Journal of the Society of Naval Architects of Korea
• /
• v.31 no.3
• /
• pp.80-86
• /
• 1994

A numerical method for simulations of inviscid incompressible flow fields around a ship advancing on the free surface is developed. A body fitted coordinate system, generated by numerically solving elliptic type partial differential equations is used to conform the ship and free surface configurations. Three dimensional Euler equations transformed to the non-staggered body fitted coordinate system are discretised by finite difference method. Time and spatial derivatives are discretised by forward and centered differencings, respectively, and artificial dissipations are added to discretised convection terms for improvements of numerical stability. At each time steps, free surface elevations are recomputed to satisfy nonlinear free surface conditions. Poisson equations for pressure field are solved iteratively and the velocity field for next time step is extrapolated. To verify the developed numerical method, flow fields around a Wigley model are simulated(Fn=0.250-0.408) and compared with experimental data to show good agreements.

• Journal of Navigation and Port Research
• /
• v.34 no.5
• /
• pp.367-373
• /
• 2010

When the numerical analysis is carried out, it is necessary to set proper elements as a feature of analysis domains for more accurate simulations. In this study, Distinct Element Method(DEM) is applied, only considering repulsive force and tensile force except for frictional force and resisting force of particle. When the filled particles with initial Quad-tree type is relocated by DEM, a blank space existing among the particles can be minimized because the shape of particle is circular. Finally, it is the effective feature that the centroidal disposion of the particles is similar to an equilateral triangle. Triangular mesh are formed by using the Delaunay triangular technique on these relocated particles, the quality of triangular mesh is more improved by carrying out Laplace interpolations. The compared result of Aspect Ratio before and after the Laplace interpolation is shown that although the quality of triangular mesh made by DEM is good, the later triangular mesh are higher quality than the formers. In this study, although the developed technique takes a longer calculational time than the previous technique to generate triangular mesh, it is considered that the applicable possibility is very high in the generation of finite element mesh about wave analysis and various numerical simulation to need a complex or reappearance of exact topography.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.36 no.3
• /
• pp.211-219
• /
• 2008

A three-dimensional inviscid flow solver has been developed based on unstructured meshes for the investigation of the effect of the external stores on the tail surfaces of a generic fighter aircraft. The numerical method is based on a vertex-centered finite-volume discretization and an implicit point Gauss-Seidel time integration. The calculations were made for a steady flow and the computed results were compared with experimental data to validate the flow solver. An unsteady time-accurate computation of the generic fighter aircraft with external stores at transonic flight conditions showed that the external stores cause unsteady loading on the horizontal tail surface due to the mutual interference between their wake and the horizontal tail surface. It was shown that downward deflection of the trailing edge flap significantly reduces the undesirable interference effect.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.40 no.5
• /
• pp.405-412
• /
• 2012

A flow control on airfoil installed a vortex cell for high efficiency wind turbine blade in stationary and dynamic stall conditions have been numerically investigated by solving the compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with Roe's flux-difference splitting and an implicit time-integration method coupled with dual time step sub-iteration. The computed result for the airfoil in the stationary showed that lift-drag ratio increases due to low pressure by the vortex cell. The oscillating airfoil with the vortex cell showed that the magnitude of hysteresis loop is reduced due to the enhanced vortex in the cell.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.33 no.7
• /
• pp.1-8
• /
• 2005

For study on the unsteady wall interference effect, flows around a circular cylinder in closed test-section wind tunnels have been numerically investigated by solving compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with the Roe's flux-difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The computed results showed that the unsteady pressure gradient over the cylinder is enhanced by the wall interference, and as a result the fluctuations of lift and drag are augmented. The drag is further increased because of the lower base pressure. The vortex shedding frequency is also increased by the wall interference. The pressure on the test section wall shows the harmonics having the shedding frequency contained in the wall effect.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.15 no.6
• /
• pp.2037-2047
• /
• 1991

Strength is not simply a single given value but rather is a statistical one with certain distribution functions. This is because it is affected by many unknown factors such as size, shape, stress distribution, and combined stresses. In this study, a model of loss probability is proposed in view of the fact that one of the fundamental configuration of nature is hexagonal, for example, the shapes of lattice unit, grain, and so on. The model sues the concept of loss of certain element in place of Jayatilaka-Trustrum's length and angle of cracks. Using this model, the loss probability due to each loss of certain elements is obtained. Then, the maximum principal stress is calculated by the finite element method at the centroid of the elements under the tensile load for the 4,095 models of analysis. Finally, the failure probability of the brittle materials is obtained by multiplying the loss probability by the ratio of the maximum principal stress to theoretical tensile strength. Comparison of the result of the Jayatilaka-Trustrum's model and the proposed model shows that the failure probabilities by the two methods are in good agreement. Further, it is shown that the parametric relationship of semi-crack lengths for various degrees of birittleness can be determined. Therefore, the analysis of the failure probability suing the proposed model is shown to be promising as a new method for the study of the failure probability of birttle materials.