• Journal of the Korea institute for structural maintenance and inspection
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• v.24 no.6
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• pp.146-152
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• 2020

This study was initiated to develop an effective inspection method to detect defects such as corrosion in closed-cell steel members in steel-box girder bridges. The ultrasonic velocity method among various non-destructive method was selected as a rapid and effective method to derive the average propagation velocity in the medium by using the ultrasonic wave velocity method for specimens of different thickness. The regression analysis was performed based on the experimental results, and the results was interpolated to evaluate the prediction accuracy. If the material properties are identical, this ultrasonic velocity method can predict the thickness using the averaged transmitted velocity. In addition, a continuous scanning method moving at 200 mm/s was tested for scanning a wide area of a bridge. The results exhibited that the continuous scanning method was able to effectively scan the different thickness of a bridge.

• Ocean and Polar Research
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• v.44 no.4
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• pp.287-296
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• 2022

High-frequency (HF) radar measures sea surface currents from the radio waves transmitted and received by antenna on land. Since the data quality of HF radar measurements sensitively depend on the radio wave environment around antenna, Antenna Pattern Measurements (APM) plays an important role in evaluating the accuracy of measured surface currents. In this study, APM was performed by selecting the times when the background noise level around antenna was high and low, and radial data were generated by applying the ideal pattern and measured pattern. The measured antenna pattern for each case was verified with the current velocity data collected by drifters. The radial velocity to which the ideal pattern was applied was not affected by the background noise level around antenna. However, the radial velocity obtained with APM in the period of high background noise was significantly lower in quality than the radial velocity in a low noise environment. It is recomended that APM be carried out in consideration of the radio wave environment around antenna, and that the applied result be compared and verified with the current velocity measurements by drifters. If it is difficult to re-measure APM, we suggest using radial velocity in generating total vector with the ideal pattern through comparative verification, rather than poorly measured patterns, for better data quality.

• Journal of Biomedical Engineering Research
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• v.7 no.2
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• pp.139-146
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• 1986

Pulsed ultrasonic Doppler system is a useful diagnostic instrument to measure blood-flow-velocity, velocity profile, and volume-blood-flow. This system is more powerful compare with 2-dimensional B-scan tissue image. A system has been deve- loped and ii being evaluated using TMS 32010 DSP. We use this DSP for the purpose of real-time spectrum analyzer to obtain spectrogram in singlegate pulsed Doppler system and for the serial comb filter to cancel clutter and zero crossing counter to estimate Doppler mean frequency in multigate pulsed Doppler system. The Doppler shift of the backscattered signals is sensed in a phase detector. This Doppler signal corresponds to the mean velocity over a some region in space defined by the ultrasonic beam dimensions, transmitted pulse duration, and transducer ban(iwidth. Multi- gate pulsed Doppler system enable the transcutaneous and simultaneous assessment of the velocities in a number of adjacent sample volumes as a continuous function of time. A multigate pulsed Doppler system processing the information originating from presented.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.03a
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• pp.42-50
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• 2008

To evaluate rock bolt integrity, destructive test such as pull-out test has been commonly carried out. This method is known as time consuming, expensive, and inaccurate procedure. To improve destructive method, non-destructive techniques using transmitted guided ultrasonic waves were suggested. Note for the transmission method, the source for the generation of ultrasonic waves should be installed during the rock bolt construction. The purpose of this study is to investigate the reflection method using reflected guided ultrasonic waves to evaluate the integrity of the rock bolt grouted, and to compare the results evaluated by the reflection and transmission methods. The guided waves are generated by PZT element and received by AE sensor. The measured signals are analyzed by the wavelet transform. The results show that the energy velocities of guided ultrasonic waves increase with the defect ratio in both transmission and reflection method. The reflection method produces the lower velocity in all defect ratio. This research demonstrates that the reflection method may be suitable and easer method for the field tests.

• Structural Engineering and Mechanics
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• v.87 no.2
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• pp.107-116
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• 2023

The three-dimensional Hashin criterion and user subroutine VUMAT were used to simulate the damage in the composite layer, and the secondary stress criterion was used to simulate the interlayer failure of the cohesive element of the bonding layer and the propagation characteristics under the layer. The results showed that when the shear stress wave (shear wave) propagates on the surface of the laminate, the stress wave attenuation along the fiber strength direction is small, and thus producing a large stress profile. When the compressive stress wave (longitudinal wave) is transmitted between the layers, it is reflected immediately instead of being transmitted immediately. This phenomenon occurs only when the energy has accumulated to a certain degree between the layers. The transmission of longitudinal waves is related to the thickness and the layer orientation. Along the symmetry across the thickness direction, the greater is the stress amplitude along the layer direction. Based on the detailed investigation on the impact on various laminated composites carried out in this paper, the propagation characteristics of stress waves, the damage and the destruction of laminates can be explained from the perspective of stress waves and a reasonable layering sequence of the composite can be designed against damage and failure from low velocity impact.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.109-114
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• 2004

This paper presents a theoretical and experimental study of a non-collocated pair of piezopolymer PVDF sensor and piezoceramic PZT actuator, which are bonded on a cantilever beam, in order to suppress unwanted vibration at the tip of the beam. The PZT actuator patch was bonded near the clamped part and the PVDF sensor, which was triangularly shaped, was bonded on the other part of the beam. This is because the triangular PVDF sensor is known that it can detect the tip velocity of a cantilever beam. Because the arrangement of the sensor and actuator pair is not collocated and overlapped each other, the pair can avoid so called 'the in-plane coupling'. The test beam is made of aluminum with the dimension of $200\times20\times2mm$, and the two PZT5H actuators are both $20\times20\times1mm$ and bonded on the beam out-of-phase, and the PVDF sensor is $178mm\times6mm\times52{\mu}m$. Before control, the sensor-actuator frequency response function is confirmed to have a nice phase response without accumulation in a reasonable frequency range of up to 5000 Hz. Both the DVFB and displacement feedback strategies made the error signal from the tip velocity (or displacement) sensor is transmitted to a power amplifier to operate the PZT actuator (secondary source). Both the control methods attenuate the magnitude of the first two resonances in the error spectrum of about 6-7 dB.

• Advances in aircraft and spacecraft science
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• v.3 no.3
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• pp.243-257
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• 2016

The increase of air traffic volume has brought an increasing amount of issues related to carbon and NOx emissions and noise pollution. Aircraft manufacturers are concentrating their efforts to develop technologies to increase aircraft efficiency and consequently to reduce pollutant discharge and noise emission. Ultra High By-Pass Ratio engine concepts provide reduction of fuel consumption and noise emission thanks to a decrease of the jet velocity exhausting from the engine nozzles. In order to keep same thrust, mass flow and therefore section of fan/nacelle diameter should be increased to compensate velocity reduction. Such feature will lead to close-coupled architectures for engine installation under the wing. A strong jet-wing interaction resulting in a change of turbulent mixing in the aeroacoustic field as well as noise enhancement due to reflection phenomena are therefore expected. On the other hand, pressure fluctuations on the wing as well as on the fuselage represent the forcing loads, which stress panels causing vibrations. Some of these vibrations are re-emitted in the aeroacoustic field as vibration noise, some of them are transmitted in the cockpit as interior noise. In the present work, the interaction between a jet and wing or fuselage is reproduced by a flat surface tangential to an incompressible jet at different radial distances from the nozzle axis. The change in the aerodynamic field due to the presence of the rigid plate was studied by hot wire anemometric measurements, which provided a characterization of mean and fluctuating velocity fields in the jet plume. Pressure fluctuations acting on the flat plate were studied by cavity-mounted microphones which provided point-wise measurements in stream-wise and spanwise directions. Statistical description of velocity and wall pressure fields are determined in terms of Fourier-domain quantities. Scaling laws for pressure auto-spectra and coherence functions are also presented.

• Fire Science and Engineering
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• v.24 no.6
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• pp.1-6
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• 2010

A smoldering ground fire can be a probable cause of reignition of surface fire when transmitted from Fermentation layer to Humus layer with temperature higher than that of ignition. Purpose of this paper is to identify experimental methodology on the potential risk of a smoldering ground fire, which is similar to the real surface fuel bed, and its combustion characteristics. The fuel model designed in this study is composed of 3 layers such as Litter layer, Fermentation layer and Humus layer and 8 Thermocouples are set through 3 layer at each boundary and in between to detect the temperature change and duration of smoldering and propagation velocity. As a result, it was observed that ignition conditions in the boundary between L layer and F layer determined transmission and non-transmisstion to F-H layer. In addition, range of critical humidity at which a smoldering ground fire was transmitted in a material layer was 33~44% and when temperature exceeds $350^{\circ}C$, likelihood of transmission of a smoldering ground fire was high. In the research, the experimental model for multi-layer smoldering ground fire is suggested and information about propagation of smoldering fire, possibility of reignition according to moisture content, propagation velocity and temperature change are obtained, Also, the built-up methods were established to help analyze basic characteristics of smoldering ground fire.

• Journal of Institute of Control, Robotics and Systems
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• v.19 no.2
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• pp.113-118
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• 2013

In this paper, spoofing effects on a GNSS receiver were analyzed. The spoofer (spoofing device) was classified to two categories. One is an active spoofer and the other is a passive spoofer. The active spoofer was considered for analysis. For the analysis of spoofing effects on a GNSS receiver, a real-time GNSS spoofing simulator was developed. The simulator was consisted with two parts which are a baseband signal generation part and a RF up-conversion part. The first GNSS baseband signal was generated according to spoofing parameters such as range, range rate, GNSS navigation data, spoofing to GNSS signal ratio, and etc. The generated baseband signal was up-converted to GNSS L1 band. Then the signal transmitted to a GNSS signal. For a perfect spoofing, a spoofer knew an accurate position and velocity of a spoofing target. But, in real world, that is not nearly possible. Although uncertainty of position and velocity of the target was existed, the spoofer was operated as an efficient jammer.

• Geotechnical Engineering
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• v.7 no.1
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• pp.7-14
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• 1991

Determination of dynamic shear modulus of soil was made by measuring directly the velocity of ultrasonic shear waves transmitted through the specimen. The PUNDIT, a generator and detector of ultrasonic waves, has been used to measure the propagation velocity. Forty -six tests of compacted soil at seven different void ratios and seven varying degrees of saturation with four types of materials were made in this study. The primary importance in this study was the investigation of the relations among the para meters which influenced G-modulus, As a results of analysis, the dynamic shear modulus of soil tends to decrease with an increase of void ratio, and also it is affected by soil types. In case of using PUNDIT, the proper range of the specimen length is from 5cm to 8cm. And the degree of saturation doesn't affect the dyn- amic shear modulus of soil.