Hyungmin Kang

A new approach of a limiting process for multi-dimensional flows

An enhanced Multi-dimensional Limiting Process (e-MLP) is developed for the accurate and efficient computation of multi-dimensional flows based on the Multi-dimensional Limiting Process (MLP). The new limiting process includes a distinguishing step and an enhanced multi-dimensional limiting process. First, the distinguishing step, which is independent of high order interpolation and flux evaluation, is newly introduced. It performs a multi-dimensional search of a discontinuity. The entire computational domain is then divided into continuous, linear discontinuous and nonlinear discontinuous regions. Second, limiting functions are appropriately switched according to the type of each region; in a continuous region, there are no limiting processes and only higher order accurate interpolation is performed. In linear discontinuous and nonlinear discontinuous regions, TVD criterion and MLP limiter are respectively used to remove oscillation. Hence, e-MLP has a number of advantages, as it incorporates useful features of MLP limiter such as multi-dimensional monotonicity and straightforward extensionality to higher order interpolation. It is applicable to local extrema and prevents excessive damping in a linear discontinuous region through application of appropriate limiting criteria. It is efficient because a limiting function is applied only to a discontinuous region. In addition, it is robust against shock instability due to the strict distinction of the computational domain and the use of regional information in a flux scheme as well as a high order interpolation scheme. This new limiting process was applied to numerous test cases including one-dimensional shock/sine wave interaction problem, oblique stationary contact discontinuity, isentropic vortex flow, high speed flow in a blunt body, planar shock/density bubble interaction, shock wave/vortex interaction and, particularly, magnetohydrodynamic (MHD) cloud-shock interaction problems. Through these tests, it was verified that e-MLP substantially enhances the accuracy and efficiency with both continuous and discontinuous multi-dimensional flows.

A Study on CFD Data Compression Using Hybrid Supercompact Wavelets

A hybrid method with supercompact multiwavelets is suggested as an efficient and practical method to compress CFD dataset. Supercompact multiwavelets provide various advantages such as compact support and orthogonality in CFD data compression. The compactness is a crucial condition for approximated representation of CFD data to avoid unnecessary interaction between remotely spaced data across various singularities such as shock and vortices. But the supercompact multiwavelet method has to fit the CFD grid size to a product of integer and power of two,mX2. To resolve this problem, the hybrid method with combination of 3, 2 and 1 dimensional version of wavelets is studied. With the hybrid method, any arbitrary size can be handled without any shrinkage or expansion of the original problem. The presented method allows high data compression ratio for fluid simulation data. Several numerical tests substantiate large data compression ratios for flow field simulation successfully.

Two Dimensional Numerical Study in Gangway of Next Generation High Speed Train For Reduction of Aero-acoustic Noise

As the preceding research for the design of gangway in the next generation high speed train, the aero-acoustic noise at the gangway is calculated. For this purpose, the shape of gangway with mud flaps is assumed as the two-dimensional cavity. Then, 5 gap sizes between mud flaps of gangway are selected and parametric study is performed according to the gap sizes. From this study, the aerodynamic features such as vortex shedding, pressure, etc. are computed. Also, the aero-acoustic properties of tonal noise and overall noise are analyzed at the 3 locations of microphone and the relation between the gap size of mud flap and the noise level is assessed. Through this study, it is shown that the noise characteristics of base and specific models are better than those of other models.

Experimental Study on Flow and Mass Transfer in Rotor-Stator Disk Cavities

An experimental study has been performed in an attempt to seek some possibilities of obtaining the solutions of heat transfer problems related to rotor-stator disk cavity systems of gas turbine engines by analogy with corresponding problems in mass transfer which arises from using naphthalene sublimation technique. Measurements are made to examine the effects of rotational Reynolds number, the flow rate and the gap ratio on the radial pressure distributions. Pressure inversion phenomenon has been verified to exist for the case of shrouded disks with radial clearance, which perform better in terms of heat transfer, too, than those with axial clearance. The stator flow reattachment points are determined directly by the dual sensor pressure probes and compared well with those estimated from the distributions of static pressure and mass transfer coefficients.

Convergence Acceleration for Euler Equation based on SPR

In Computational Fluid Dynamics (CFD), a variety of high order schemes have been extensively applied to an integrated design process, however its capacity still fall short of the full scale integrated design. Full scale integrated design requires a large number of grid points in spatial domain, entailing substantial increase of computing time. However, it is a waste of computational resources to use fine grid in the whole domain. A dense grid is needed only in a rapidly changing region such as shock waves, boundary layers, etc. For that reason, several types of adaptive methods have been implemented to improve efficiency of analysis tool. In these days, numerical techniques based on wavelets methods have emerged as one of revolutionary technologies.

Hybrid Three-Dimensional Supercompact Multiwavelets for Computational Fluid Dynamics

ABSTRACT Supercompact wavelets are extended to hybrid-type applications for practical fluid dynamics data management. Beam and Warmings supercompact multiwavelet method is regarded as one proper tool for representing fluid simulation data because of its compact support. This wavelet compactness significantly reduces storage requirements as well as data processing time, and can prevent unnecessary interaction with closely located data across singularities such as shocks and vortices. For effective domain subdivision, a hybrid method that combines 3-D, 2-D, and 1-D versions of supercompact multiwavelets is developed. The hybrid 3-D type of supercompact multiwavelet shows high data compression ratio in practical fluid simulation data because the mother wavelets are highly effective and encompass subdivision methods. With the present method, efficient data management for datasets of huge size is enabled without losing important features. Several numerical tests substantiate large data compression for flow field simulation.

Research on the Adaptive Wavelet Method for the Enhancement of Computational Efficiency of Three Dimensional Flows

The adaptive wavelet method is studied for the enhancement of computational efficiency of three-dimensional flows. Three-dimensional wavelet decomposition process is introduced based on the previous two-dimensional method. The order of numerical accuracy of an original solver is preserved by applying a proper thresholding technique. The proposed algorithm is applied to the computation of flow field around ONERA-M6 wing in transonic regime. Through the application, it is confirmed that the three-dimensional adaptive wavelet method can reduce the computational time with conserving the numerical accuracy of an original solver.

Efficiency Enhancement in High Order Accurate Euler Computation via AWM

An adaptive wavelet method is proposed to achieve an efficient algorithm with maintaining numerical accuracy of a conventional solver in case of high order accurate Euler computations. Due to wavelet transformation including wavelet decomposition and thresholding, dataset is automatically adapted to local features of a solution. Then, the flux values are calculated only at the adapted dataset, which enables the enhancement of the computational efficiency. For maintaining the higher order accuracy of a conventional solver, threshold value is adjusted to consider the spatial and temporal accuracies. Through this threshold value, the deterioration of the numerical accuracy due to thresholding is obstructed and the wavelet method can present high order accurate solutions. The adaptive wavelet method was successfully applied to high order accurate Euler flow computation. In consequence, computational efficiency was enhanced while maintaining the higher order numerical accuracy of a solution.

Enhancement of the Computational Efficiency of UFP via a MWM

Recently, in Computational Fluid Dynamics (CFD), the computational accuracy and efficiency have been enhanced with the aids of numerical schemes and computing systems. However, for the accurate calculations of flow problems, dense grid points are still needed in the spatial domain. Moreover, the increase of grid points results in the decrement of the size of the cells, which limits the size of time step. Therefore, much computational cost is necessary for accurate calculations of flow problems, especially for unsteady cases.

Data Compression with Hybrid Supercompact Wavelets

Data compression technique is proposed that co mbines 3D, 2D and 1D versions of supercompact. Computation domain is divided into subsets for each dimensional version of multiresolution appl icatio n. Compared to Hartens interpolatory mu ltiresolution, supercompact wavelets require small number of support points, lea d- ing to quite simple and less expensive numerical i m- plementation. The hybrid multiresolution method a l- lows high actual data compression for fluid simulation data since the method is based on highly effective wavelets and subdivision methods. Several numerical tests substantiate large data co mpression ratios for flow field simul ation.