Se-Myong Chang

Reflection and Penetration of a Shock Wave Interacting with a Starting Vortex

Experimental and numerical studies are conducted to investigate the interaction of a weak shock wave (M s = 1.22) and a strong vortex (M v,max > 1). In terms of shock dynamics, two meaningful physics are observed from the experiment: reflected wave from the vortex edge and transmitted wave penetrating the vortex core. These weak waves are shown in the numerical interpretation to contribute to the emission of acoustic waves in shock-vortex interaction

Compressibility Effect in the Axisymmetric Internal Flow Past a Microgap

In this study, a simplified axisymmetric model is proposed for the problem of compressible internal flow past a microgap. Using numerical and experimental methods, the phenomena of choked flows are observed; these flows are induced by the acceleration of subsonic flows past the narrow cross-section of an annular shape made by a microgap. The relation between mass flow rate and differential pressure is obtained, and by comparing the result with experimental results, the reliability of the numerical results is discussed. The generation of a supersonic jet flow and its diffraction are visualized by performing the numerical analysis of axisymmetric compressible Navier-Stokes equations. This investigation greatly extends the physical understanding of the axisymmetric compressible flow, which has a wide range of engineering applications, e.g., in the case of valves in automotive power systems.

Parametric Study on the Mixing Enhancement of Parallel Supersonic-subsonic Wakes Using Wall Cavities

A series of computational studies on the enhancement of parallel supersonic-subsonic mixing wakes, where M 1 = 1.78 and M 2 = 0.30 at baseline, is conducted and compared with available experimental data. The mixing problem of this parallel flow is characterized by the interaction of “complex” turbulent shear flow with acoustic waves emitted from a wall-mounted cavity. The first aim of the present work is to show a direct comparison between numerical predictions and equivalent experimental data for the baseline case. Hence we make a comparison of eight independent turbulence models, which were selected from the most widely using turbulence models in use in the engineering field, consisting of six eddy–viscosity models and two Reynolds stress models. The Pitot pressure distribution data are in good agreement between computation and experiment, and the results show that Menters SST model gives the best performance. Further, we investigate the effects of primary parameters such as the length-to-depth (L/D) ratio of the cavity, the position of the cavity, and the arrangement of the cavity at the given flow condition. Consequently, this methodology can contribute to finding an optimal guideline for the idea of mixing enhancement in the free turbulent wake using the acoustic wave produced by wall cavities.

Aerodynamic Design of 10 kW-level HAWT Rotor Blades

The procedure for the aerodynamic design of the rotor blades for 10 kW-level HAWT (horizontal axis wind turbine) has been investigated to be practiced systematically. The approximately optimal shape was designed using an inverse method based on the momentum theory and the blade element method. The configuration was tested in the wind tunnel of the Korea Air Force Academy, and the data was compared with those obtained from the real system manufactured from the present design. From this research, the authors established the systematic technolo for wind turbine blades, and set up the technical procedure which can be extended for the future design of middle and large sized wind turbines.ℊ䜀Ѐ㘲〻ጀ䝥湥牡氠䕮杩湥敲楮最

Scattering of Shock into Acoustic Waves in Shock-Vortex Interaction

When a shock interacts with a vortex, various weak waves are radiated causing noise and shock wave scattering. When the vortex is strong, scattering pattern becomes more complex. When the vortex is unstable, vortexlets are brought forth creating diverging acoustics as they interact with the impinging shock. Strong vortex, on the other hand, can foster quadrupole noise source as it impinges into a vortex, like the screech mode of supersonic jet noise. It is described here how we investigated systematically these shock scattering phenomena using shock tube experiments and various computational models.

The Feasibility of Multidimensional CFD Applied to Calandria System in the Moderator of CANDU-6 PHWR Using Commercial and Open-Source Codes

The moderator system of CANDU, a prototype of PHWR (pressurized heavy-water reactor), has been modeled in multidimension for the computation based on CFD (computational fluid dynamics) technique. Three CFD codes are tested in modeled hydrothermal systems of heavy-water reactors. Commercial codes, COMSOL Multiphysics and ANSYS-CFX with OpenFOAM, an open-source code, are introduced for the various simplified and practical problems. All the implemented computational codes are tested for a benchmark problem of STERN laboratory experiment with a precise modeling of tubes, compared with each other as well as the measured data and a porous model based on the experimental correlation of pressure drop. Also the effect of turbulence model is discussed for these low Reynolds number flows. As a result, they are shown to be successful for the analysis of three-dimensional numerical models related to the calandria system of CANDU reactors.

A Study on the Establishment of the Environmental Noise Criteria for Aircraft

Rating scales for environmental noise are varied in their calculation procedure. Among them WECPNL (weighted equivalent continuous perceived noise level) is the rating scale for aircraft noise currently being used in domestic and applied only for aircraft noise. However calculated from is used as a rating scale for not only aircraft noise but also environmental noise. Besides, it is easy to calculate and internationally preferred. It is, therefore, not adequate for the evaluation of residents` exposure. Moreover it is very difficult to measure the aircraft noise by WECPNL due to the complicated calculating procedures if automatic measuring system is not used. Accordingly. this study aims to propose alternative evaluation procedure for the aircraft noise. To achieve this purpose, the data measured by automatic measuring system were gathered and calculated with three evaluation procedures : WECPNL and , and the results calculated from different methods were compared and analyzed.

Unsteady two-dimensional multiphysical simulation of a pressure tube model expanded to contact with the outer concentric tube

For the blind calculation of the International Collaborative Standard Problem (ICSP) experiment on heavy water reactor moderator subcooling requirements, the COMSOL Multiphysics code is used to simulate plastic deformation of a pressure tube (PT) as a result of the interaction of stress and temperature. It is shown that the thermal stress model of COMSOL is compatible to simulate the multiple heat transfers (including the radiation heat transfer and heat conduction) and stress strain in the simplified two-dimensional problem. The benchmark test result for radiation heat transfer is in good agreement with the analytical solution for the concentric configuration of PT and calandria tube (CT). Since the original strain model of COMSOL only considers an elastic deformation with thermal expansion coefficient, the PT/CT contact cannot be predicted in the ICSP. Therefore, the plastic deformation model by the Shewfelt and Godin, widely used in the fuel channel analysis of CANadian Deuterium Uranium (CANDU) reactor, is implemented to the strain equation of COMSOL. The heat-up of PT, the strain rate, and the contact time of the PT/CT are calculated with the boundary conditions (BCs) given for blind calculation of the ICSP experiment. The result shows a sudden expansion of the inner concentric PT within a few milliseconds. This unsteady simulation should be helpful for the conceptual design of experiment as well as for the understanding of multiphysics inside the fuel channels of the CANDU reactor.

Topology Optimization of Incompressible Flow Using P1 Nonconforming Finite Elements

An alternative approach for topology optimization of steady incompressible Navier-Stokes flow problems is presented by using P1 nonconforming finite elements. This study is the extended research of the earlier application of P1 nonconforming elements to topology optimization of Stokes problems. The advantages of the P1 nonconforming elements for topology optimization of incompressible materials based on locking-free property and linear shape functions are investigated if they are also valid in fluid equations with the inertia term. Compared with a mixed finite element formulation, the number of degrees of freedom of P1 nonconforming elements is reduced by using the discrete divergence-free property; the continuity equation of incompressible flow can be imposed by using the penalty method into the momentum equation. The effect of penalty parameters on the solution accuracy and proper bounds will be investigated. While nodes of most quadrilateral nonconforming elements are located at the midpoints of element edges and higher order shape functions are used, the present P1 nonconforming elements have P1, {1, x, y}, shape functions and vertex-wisely defined degrees of freedom. So its implentation is as simple as in the standard bilinear conforming elements. The effectiveness of the proposed formulation is verified by showing examples with various Reynolds numbers.

Rupture Safety Assesment of Bipropellant Propulsion System at High Pressure Testing

The geostationary satellite COMS is going to be launched in 2010, and, in the series of test, there are some high-pressure tests concerning the vessel tank filled with helium gas of hundreds atmospheric pressure. In this paper, authors evaluates risk associated with accidental rupture of the test system. Two possible scenarios are considered: 1) the 310-bar helium tank ruptures at the center of the acoustic chamber, and 2) the 116-bar reduced-pressure helium tank ruptures in the test room shielded by bullet-proof glasses. Using the theory of blast wave propagation and computational simulation, the dynamics of wave reflected in a confined space is investigated for highly complex unsteady flow physics.