Gang-Won Jang

FLUID - STRUCTURE INTERACTION IN A U-TUBE WITH SURFACE ROUGHNESS AND PRESSURE DROP

In this research, the surface roughness affecting the pressure drop in a pipe used as the steam generator of a PWR was studied. Based on the CFD (Computational Fluid Dynamics) technique using a commercial code named ANSYS-FLUENT, a straight pipe was modeled to obtain the Darcy frictional coefficient, changed with a range of various surface roughness ratios as well as Reynolds numbers. The result is validated by the comparison with a Moody chart to set the appropriate size of grids at the wall for the correct consideration of surface roughness. The pressure drop in a full-scale U-shaped pipe is measured with the same code, correlated with the surface roughness ratio. In the next stage, we studied a reduced scale model of a U-shaped heat pipe with experiment and analysis of the investigation into fluid-structure interaction (FSI). The material of the pipe was cut from the real heat pipe of a material named Inconel 690 alloy, now used in steam generators. The accelerations at the fixed stations on the outer surface of the pipe model are measured in the series of time history, and Fourier transformed to the frequency domain. The natural frequency of three leading modes were traced from the FFT data, and compared with the result of a numerical analysis for unsteady, incompressible flow. The corresponding mode shapes and maximum displacement are obtained numerically from the FSI simulation with the coupling of the commercial codes, ANSYS-FLUENT and TRANSIENT_STRUCTURAL. The primary frequencies for the model system consist of three parts: structural vibration, BPF(blade pass frequency) of pump, and fluid-structure interaction.

Non-contact inspection of rail surface and internal defects based on electromagnetic ultrasonic transducers

A non-contact ultrasonic rail integrity inspection technique is discussed in this work as an alternative to concurrent contact-type ultrasonic testing method using piezoelectric transducers, which have some disadvantages due to physical contact between transducers and rails. To this end, transducers which can produce and measure elastic waves in rails without any physical contact were suggested based on electromagnetic ultrasonic transducers. The transducers were designed to selectively produce ultrasonic bulk waves or surface waves according to excitation frequency; it is possible to detect internal and surface defects with same transducer. Simulations using finite element models were performed to determine transducer operating frequencies optimal to defect types. With a series of experiments, it was shown that the proposed method could detect rail internal damages without any physical contact. The effects of air gap between the transducer and a rail and the position of a receiving transducer on the signals were studied to check practical lift-off distance and scanning speed of the method. Non-contact examination of rail surface defects was also carried out using the same transducers only by changing the frequency of excitation current and showed successful results.

Investigation on Guided Wave Dispersion Characteristics for Metal Thin Films

In this study, we investigated the dispersion characteristics of guided waves in thin films. Dispersion curves are essential for understanding not only the behavior of ultrasonic waves, but also the mechanical properties of thin films. Matrix techniques are presented for modeling ultrasonic waves in multilayered structures before being used to calculate the dispersion curves for Al-steel and Al-composite specimens. When compared with the dispersion curves obtained using the commercial program (Disperse), the dispersion curves generated from the transfer matrix method show its validity. These developed methods are used to obtain dispersion curves for Al thin films deposited on a Si substrate. The resulting dispersion curves enable observation of both dispersive and non-dispersive behavior for the guided waves, depending on the thickness of the thin films.

A Modified Formulation for Automatic Synthesis of Planar Linkage Mechanisms

The idea of automatic mechanism synthesis is to find an optimal linkage type as well as its geometric dimensions for a given problem without using any pre-determined linkage type. As the first step towards the automatic mechanism synthesis, the authors proposed the use of unified planar linkage consisting of rigid blocks connected by stiffness-varying zero-length springs and formulated the synthesis problem as the iterative design optimization problem. In this investigation, we extend the automatic mechanism synthesis idea to more realistic large-sized problems by resolving several numerical difficulties observed in the earlier formulation such as instable convergence and many local optima. In particular, the objective and constraint functions in the optimization formulation are newly selected. The rationale for choosing such functions is given and the effectiveness of the proposed problem formulation is verified by designing planar mechanisms of complete paths.Copyright

Vibration Measurement of an Automobile Exhaust System in Operation

In this work, the operational deflection shape(ODS) of an automobile exhaust system is measured by using a recently-developed magnetic sensor. The magnetic sensor is composed of a solenoid and two pairs of permanent magnets generating an antisymmetric magnetic field in the lateral direction inside the solenoid. Lateral movement of a ferromagnetic pipe inside the magnetic field of the suggested sensor induces an electromotive force in the solenoid corresponding to the lateral velocity of the pipe. Due to the simplicity and non-contact characteristics of the magnetic sensor, dynamic behaviors of the structures operating under high temperature such as an exhaust pipe can be efficiently observed. It is shown that the lateral ODS of an exhaust system can be successfully measured by the suggested sensors.