Huinam Rhee

Prediction of Fretting Wear Depth for Steam Generator Tubes Based on Various Types of Wear Scars

Calculations of fretting wear depth due to the turbulence excitation around steam generator tubes, for various wear scars, are carried out numerically. Four typical wear topologies, namely, round-, crescent-, flat-, and oblique-shaped wear scars, are adopted to represent the configuration of the wear volume. Oblique wear shows the most severe case for the wear time history, whereas both round- and crescentshaped wears have smaller increasing rates of wear histories than flat- or oblique-shaped wears. It can be estimated that a high cross flow, around the U-bend region of the steam generator tube, significantly enhances the wear phenomena because the basic wear scar, at the contact point between the tube and its support, is flat or oblique. Parametric studies on the inclined angle and radial clearance are also carried out for oblique and crescent wear shapes.

Novel acoustic wave resonance scattering formalism

A novel formalism for the exact isolation of resonances from Rayleigh normal modes (or partial waves) for acoustic wave scattering from submerged fluid or elastic bodies is proposed. The resonance scattering function consisting purely of resonance information is proposed. Both the magnitude and phase of isolated resonances can be correctly obtained by using the proposed formalism while previous works based on classical resonance scattering theory can give only magnitude correctly. The reason previous works could produce correct magnitude information for acoustic wave scattering (no mode conversion) is explained. Plane compressive wave scattering from a variety of submerged bodies is analyzed by utilizing the proposed resonance scattering function and the isolated resonances are compared with previously published studies. The exact π-phase shifts through the resonance and at the anti-resonance caused by the interaction between adjacent resonances, which have never been reported before, show that the propos...

EFFECTS OF SUPPORT STRUCTURE CHANGES ON FLOW-INDUCED VIBRATION CHARACTERISTICS OF STEAM GENERATOR TUBES

Fluid-elastic instability and turbulence-induced vibration of steam generator U-tubes of a nuclear power plant are studied numerically to investigate the effect of design changes of support structures in the upper region of the tubes. Two steam generator models, Model A and Model B, are considered in this study. The main design features of both models are identical except for the conditions of vertical and horizontal support bars. The location and number of vertical and horizontal support bars at the middle of the U-bend region in Model A differs from that of Model B. The stability ratio and the amplitude of turbulence-induced vibration are calculated by a computer program based on the ASME code. The mode shape with a large modal displacement at the upper region of the U-tube is the key parameter related to the fretting wear between the tube and its support structures, such as vertical, horizontal, and diagonal support bars. Therefore, the location and the number of vertical and horizontal support bars have a great influence on the fretting wear mechanism. The variation in the stability ratios for each vibrational mode is compared with respect to Model A and Model B. Even though both models satisfy the design criteria, Model A shows substantial improvements over Model B, particularly in terms of having greater amplitude margins in the turbulence-excited vibration (especially at the inner region of the tube bundle) and better stability ratios for the fluid-elastic instability.

The Effect of Evolution in Artificial Life Learning Behavior

In this paper, we add learning behavior to artificial evolution simulation and evaluate the effect of learning behavior. Each individual establishes its own neural network with its genetic information. Also, we propose a reward function to take reinforcement learning in a complicated and dynamically-determined environment. When the individual-level learning behavior was introduced, evolution of each simulation model got faster and the effectiveness of evolution was significantly improved. But the direction of evolution did not depend on learning and it was possible to affect the forms of evolution through reinforcement learning. This provides the mechanism that can apply the artificial life technique to various fields.

Determination of Principal Axes of a Wineglass Using Acoustic Testing

A general methodology to determine the principal axes of an arbitrary quasi-axi-symmetric resonator oscillating at its low-frequency flexural modal pair has been proposed. For an ideal axi-symmetric case, such a structure has two degenerated modes with identical eigen frequencies and similar modal shapes distinguished by their angular orientation only. However, in reality, due to a small non-uniformity of the structure, the structure has two specific directions corresponding to slightly-different modal properties. This paper describes a simple mathematical method to determine the principal axes using acoustic signals of the structure’s free oscillations in air. An ordinary wineglass was chosen to verify the methodology. An impulse was applied to the wineglass in an arbitrary direction that generates a beating, and the oscillatory data were acquired by a pair of microphones located in a vicinity of the shell rim to estimate its principal axes using the proposed method. The accuracy of this approach was verified by additional tests, in which impacts were applied along the calculated principal axes to show that there is no beating. This method can be applied for initial tuning of an imperfect shell resonator and can be generalized for a fine tuning of a gyroscopic resonator.

Measurement of 3-D Vibrational Motion by Dynamic Photogrammetry Using Least-Square Image Matching for Sub-Pixel Targeting to Improve Accuracy.

This paper deals with an improved methodology to measure three-dimensional dynamic displacements of a structure by digital close-range photogrammetry. A series of stereo images of a vibrating structure installed with targets are taken at specified intervals by using two daily-use cameras. A new methodology is proposed to accurately trace the spatial displacement of each target in three-dimensional space. This method combines the correlation and the least-square image matching so that the sub-pixel targeting can be obtained to increase the measurement accuracy. Collinearity and space resection theory are used to determine the interior and exterior orientation parameters. To verify the proposed method, experiments have been performed to measure displacements of a cantilevered beam excited by an electrodynamic shaker, which is vibrating in a complex configuration with mixed bending and torsional motions simultaneously with multiple frequencies. The results by the present method showed good agreement with the measurement by two laser displacement sensors. The proposed methodology only requires inexpensive daily-use cameras, and can remotely detect the dynamic displacement of a structure vibrating in a complex three-dimensional defection shape up to sub-pixel accuracy. It has abundant potential applications to various fields, e.g., remote vibration monitoring of an inaccessible or dangerous facility.

MODAL TESTING AND MODEL UPDATING OF A REAL SCALE NUCLEAR FUEL ROD

In this paper, modal testing and finite element modeling results to identify the modal parameters of a nuclear fuel rod as well as its cladding tube are discussed. A vertically standing full-size cladding tube and a fuel rod with lead pellets were used in the modal testing. As excessive flow-induced vibration causes a failure in fuel rods, such as fretting wear, the vibration level of fuel rods should be low enough to prevent failure of these components. Because vibration amplitude can be estimated based on the modal parameters, the dynamic characteristics must be determined during the design process. Therefore, finite element models are developed based on the test results. The effect of a lumped mass attached to a cladding tube model was identified during the finite element model optimization process. Unlike a cladding tube model, the density of a fuel rod with pellets cannot be determined in a straightforward manner because pellets do not move in the same phase with the cladding tube motion. The density of a fuel rod with lead pellets was determined by comparing natural frequency ratio between the cladding tube and the rod. Thus, an improved fuel rod finite element model was developed based on the updated cladding tube model and an estimated fuel rod density considering the lead pellets. It is shown that the entire pellet mass does not contribute to the fuel rod dynamics; rather, they are only partially responsible for the fuel rod dynamic behavior.

Dynamic Simulation of the Lunar Landing Using Flexible Multibody Dynamics Model

Flexible multibody dynamic model of the lunar lander was developed in this research. Dynamic stiffness and damping properties of lunar soil and shock absorber are included in the model. Various moon landing simulations were performed for different friction characteristics between the lander and soil, and also for various landing velocities and slope angles of the lunar surface on the landing stability.

Multibody Dynamic Simulation of the Lunar Landing

Three dimensional modeling of the lunar lander and multibody dynamic simulation of the landing on the moon is introduced in this paper. Dynamic soil properties of the moon as well as the Lander structure including shock absorber characteristics are included in the model. Using the developed model various kinds of moon landing simulation can be performed with different characteristics of the friction between the lander and the soil. The effect of the landing velocities and inclination angle of the landing area on the tip-over characteristics during landing can be predicted using the developed model.

Vibration Control Using the Non-Model Based Algorithm

A study on the vibration control of a steel beam and plate using electromagnetic actuator has been performed. The system is assumed time-varying so that the system model is considered unknown, therefore the mathematical model of the control structure is not used in the control algorithm. The algorithm only needs the vibration displacement data in real-time. The control algorithm is based on PID with an adaptive function to the change of dynamic characteristics of the structure. The vibration suppression or excitation was successfully achieved by using a test setup including a cantilever beam, a laser displacement sensor, electromagnet and microcomputer loaded with the developed control program.