• 한국전산유체공학회지
• /
• 제11권4호
• /
• pp.39-47
• /
• 2006

In this paper, the authors developed a multidimensional interpolation method inside a finite volume cell in the computation of high-order accurate numerical flux such as the fifth order WEND (weighted essentially non-oscillatory) scheme. This numerical method starts from a simple Taylor series expansion in a proper spatial order of accuracy, and the WEND filter is used for the reconstruction of sharp nonlinear waves like shocks in the compressible flow. Two kinds of interpolations are developed: one is for the cell-averaged values of conservative variables divided in one mother cell (Type 1), and the other is for the vertex values in the individual cells (Type 2). The result of the present study can be directly used to the cell refinement as well as the convective flux between finer and coarser cells in the Cartesian adaptive grid system (Type 1) and to the post-processing as well as the viscous flux in the Navier-Stokes equations on any types of structured and unstructured grids (Type 2).

• KSII Transactions on Internet and Information Systems (TIIS)
• /
• 제5권5호
• /
• pp.1049-1068
• /
• 2011

In this paper a simple and effective call admission control (CAC) scheme is proposed for the emerging interleave-division multiple-access (IDMA) systems, supporting a variety of traffic types and offering different quality of service (QoS) requirements and priority levels. The proposed scheme is signal-to-interference-plus-noise ratio (SINR) evolution based CAC (SE-CAC). The key idea behind the scheme is to take advantage of the SINR evolution technique in the process of making admission decisions, which is developed from the effective chip-by-chip (CBC) multi-user detection (MUD) process in IDMA systems. By virtue of this semi-analytical technique, the MUD efficiency can be estimated accurately. Additionally, the computational complexity can be considerably reduced. These features make the scheme highly suitable for IDMA systems, which can combat intra-cell interference efficiently with simple CBC MUD. Analysis and simulation results show that compared to the traditional CAC scheme considering MUD efficiency as a constant, the proposed SE-CAC scheme can guarantee high power efficiency and throughput for multimedia traffic even in heavy load conditions, illustrating the high efficiency of CBC MUD. Furthermore, based on the SINR evolution, the SE-CAC can make accurate estimation of available resource considering the effect of MUD, leading to low outage probability as well as low blocking and dropping probability.

• Wind and Structures
• /
• 제34권1호
• /
• pp.59-72
• /
• 2022

In this work the numerical results of the flow around a 5:1 rectangular cylinder at Reynolds numbers 3 000 and 40 000, zero angle of attack and smooth incoming flow condition are presented. Implicit Large Eddy Simulations (ILES) have been performed with a high-order accurate spatial scheme and an implicit high-order accurate time integration method. The spatial approximation is based on a discontinuous Galerkin (dG) method, while the time integration exploits a linearly-implicit Rosenbrock-type Runge-Kutta scheme. The aim of this work is to show the feasibility of high-fidelity flow simulations with a moderate number of DOFs and large time step sizes. Moreover, the effect of different parameters, i.e., dimension of the computational domain, mesh type, grid resolution, boundary conditions, time step size and polynomial approximation, on the results accuracy is investigated. Our best dG result at Re=3 000 perfectly agrees with a reference DNS obtained using Nek5000 and about 40 times more degrees of freedom. The Re=40 000 computations, which are strongly under-resolved, show a reasonable correspondence with the experimental data of Mannini et al. (2017) and the LES of Zhang and Xu (2020).

• 한국전산유체공학회지
• /
• 제1권1호
• /
• pp.71-80
• /
• 1996

An iterative time marching procedure for solving incompressible turbulent flow has been applied to the flows around a high speed train including cross-wind effects. This procedure solves three-dimensional unsteady incompressible Reynolds-averaged Navier-Stokes equations on a non-orthogonal curvilinear coordinate system using first-order accurate schemes for the time derivatives and third/second-order accurate schemes for the spatial derivatives. Turbulent flows have been modeled by Baldwin-Lomax turbulent model. To validate present procedure, the flow around a high speed train at zero yaw angle was simulated and compared with experimental data. Generally good agreement with experiments was achieved. The flow fields around the high speed train at 9.2°, 16.7°, and 45° of yaw angle were also simulated.

• 한국수자원학회:학술대회논문집
• /
• 한국수자원학회 2005년도 학술발표회 논문집
• /
• pp.409-413
• /
• 2005

Even though the relative importance of length scale of flow system allow us to simplify three dimensional flow problem to one or two dimensional representation, many systems still require three dimensional analysis. The objective of this study is to develop an efficient and accurate finite element model for analyzing and predicting three dimensional flow features in natural rivers and to offend to model spreading of pollutants and transport of sediments in the future. Firstly, three dimensional Reynolds averaged Navier-Stokes equations with the hydrostatic pressure assumption in generalized curvilinear coordinates were combined with the kinematic free-surface condition. Secondly. to simulate realistic high Reynolds number flow, the model employed the Streamline Upwind/Petrov-Galerkin(SU/PG) scheme as a weighting function for the finite element method in conjunction with an appropriate turbulence model(Smagorinsky scheme for the horizontal plain and Mellor-Yamada scheme for the vertical direction). Several tests is performed for the purpose of validation and verification of the developed model. A simple rectangular channel, 5-shaped and U-shaped channel are used for tests and comparisons are made with RMA-10 model. Runs for each case is converged stably without a oscillation and calculated water-surface deformation, longitudinal and transversal velocities, and velocity vector fields are in good agreement with the results of RMA-10 model.

• 한국전산유체공학회지
• /
• 제14권4호
• /
• pp.13-22
• /
• 2009

A two-phase (gas and liquid) flow analysis solver, named CUPID, has been developed for a realistic simulation of transient two-phase flows in light water nuclear reactor components. In the CUPID solver, a two-fluid three-field model is adopted and the governing equations are solved on unstructured grids for flow analyses in complicated geometries. For the numerical solution scheme, the semi-implicit method of the RELAP5 code, which has been proved to be very stable and accurate for most practical applications of nuclear thermal hydraulics, was used with some modifications for an application to unstructured non-staggered grids. This paper is concerned with the effects of interpolation schemes on the simulation of two-phase flows. In order to stabilize a numerical solution and assure a high numerical accuracy, the second-order upwind scheme is implemented into the CUPID code in the present paper. Some numerical tests have been performed with the implemented scheme and the comparison results between the second-order and first-order upwind schemes are introduced in the present paper. The comparison results among the two interpolation schemes and either the exact solutions or the mesh convergence studies showed the reduced numerical diffusion with the second-order scheme.

• 한국수자원학회논문집
• /
• 제44권2호
• /
• pp.145-156
• /
• 2011

본 연구에서는 자연하천의 흐름에서 흔히 발생하는 천이류, 불연속류, 마른하도로의 파의 전파 등을 포함하는 복잡한 흐름을 해석하기 위한 고정확도 2차원 수치모형을 개발하였다. 하상경사항을 효율적으로 처리하기 위해 quasi-steady wave propagation 기법을 적용하여 해당 격자에 대한 생성항의 영향을 효율적으로 반영함으로써 쌍곡선형 적분 보존형의 2차원 천수방정식을 해석하였다. Fractional Step Method를 적용한 유한체적기법의 사용을 위해 HLL Riemann 해법을 이용하여 흐름률을 계산하였고, 시간 및 공간에 대한 2차 정확도를 만족하기 위해 MUSCL 기법을 적용하였다. 2차 정확도의 사용으로 불연속지점에서 발생하는 수치진동은 TVD 기법 적용을통해 제어하였다. 개발된모형은 2차원 제방 붕괴 및 댐하류부에 구조물이 존재하는 경우의댐 붕괴 모의를 통해실측치와의 검증을 실시하였다. 또한 하류부에 역경사가 존재하는 경우의 댐 붕괴 모의를 통해 실측치와 비교함으로써 생성항의 영향에 대한 모형의 적용성을 검증하였다.

• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 2005년도 춘계학술대회 논문집
• /
• pp.1074-1078
• /
• 2005

In precision positioning applications, such as scanning tunneling microscopy and diamond turning machines [1], it is often required that actuators have nanometer resolution in displacement, high stiffness, and fast frequency response. These requirements are met by the use of piezoceramic actuators. A major limitation of piezoceramic actuators, however, is their lack of accuracy due to hysteresis nonlinearity and drift. The maximum error due to hysteresis can be as much as 10-15% of the path covered if the actuators are run in an open-loop fashion. Hence, the accurate control of piezoceramic actuators requires a control strategy that incorporates some form of compensation for the hysteresis. One approach is to develop an accurate model of the hysteresis and the use the inverse as a compensator. The Preisach model has frequently been employed as a nonlinear model for representing the hysteresis, because it encompasses the basic features of the hysteresis phenomena in a conceptually simple and mathematically elegant way. In this paper, a new numerical inversion scheme of the Preisach model is developed with an aim of compensating hysteresis in piezoceramic actuators. The inversion scheme is implemented using the first-order reversal functions and is presented in a recursive form. The inverted model is then incorporated in an open-loop control strategy that regulates the piezoceramic actuator and compensates for hysteretic effects. Experimental results demonstrate satisfactory regulation of the position of the piezoceramic actuator to the desired trajectories.

• 한국전산유체공학회지
• /
• 제10권1호
• /
• pp.8-15
• /
• 2005

Aerodynamic sounds generated by a uniform flow around a two-dimensional circular cylinder at Re=150 are simulated by applying the finite difference lattice Boltzmann method. Thethird-order-accurate up-wind scheme (UTOPIA) is used for the spatial derivatives, and the second-order-accurate Runge-Kutta scheme is applied for the time marching. We have succeed in capturing very small pressure fluctuations with the same frequency of the Karman vortex street compared with the pressure fluctuation around a circular cylinder. The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream due to the Doppler effect in the uniform flow. For the downstream, on the other hand, it is faster. It is also apparent that the amplitude of sound pressure is proportional to r /sup -1/2/,r being the distance from the center of the circular cylinder. To investigate the effect of the lattice dependence, furthermore, 2D computations of the tone noises radiated by a square cylinder and NACA0012 with a blunt trailing edge at high incidence and low Reynolds number are also investigate.

• Journal of Advanced Marine Engineering and Technology
• /
• 제28권5호
• /
• pp.717-727
• /
• 2004

In this study, we simulate the aerodynamic sounds generated by a two-dimensional circular cylinder in a uniform flow are simulated by applying the finite difference lattice Boltzmann method (FDLBM). The third-order-accurate up-wind scheme (UTOPIA) is used for the spatial derivatives. and the second-order-accurate Runge-Kutta scheme is applied for the time marching. The results show that we successively capture very small acoustic pressure fluctuations with the same frequency of the Karman vortex street compared with the Pressure fluctuation around a circular cylinder The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream due to the Doppler effect in the uniform flow For the downstream. on the other hand. it quickly Propagates. It is also apparent that the amplitude of sound Pressure is Proportional to $r^{-1/2}$, r being the distance from the center of the circular cylinder. To investigate the effect of the lattice dependence furthermore a 2D computation of the tone noise radiated by a NACA0012 with a blunt trailing edge at high incidence and low Reynolds number is also investigated.