• Transactions of the Korean Society of Automotive Engineers
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• v.24 no.5
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• pp.520-526
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• 2016

Squeal noise is a typical brake noise that is annoying to both passengers and pedestrians. Its frequency range is fairly wide from 1 kHz to 18 kHz, which can be distressful to people. The brake squeal noise occurs due to various mechanisms, such as the mode coupling of the brake system, self-excited vibration, unstable wear, and others. In this study, several parameters involved in the generation of a squeal noise are investigated experimentally by using a brake noise dynamometer. The speed, caliper pressure, torque, and friction coefficient are measured as functions of time on the dynamometer. The contact pressure and temperature distributions of the disc and the pad are also measured by using a thermal imaging camera and a pressure mapping system. As a result of the simultaneous measurement of the friction coefficient and squeal amplitude as functions of the velocity, it is found that the onset of the squeal may be predicted from the ${\mu}-v$ curve. It is also found that a non-uniform contact pressure causes instability and, in turn, a squeal. Based on the analysis results, design modifications of the pad are suggested for improved noise characteristics.

• Structural Engineering and Mechanics
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• v.31 no.1
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• pp.39-56
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• 2009

Vibration is an undesirable phenomenon in a dynamic system like lightly damped aerospace structures and active vibration control has gradually been employed to suppress vibration. The objective of the current investigation is to introduce an active torsional magneto-rheological (MR) fluid based damper for vibration control of a typical nose landing gear. They offer the adaptability of active control devices without requiring the associated large power sources. A torsional damper is designed and developed based on Bingham plastic shear flow model. The numerical analysis is carried out to estimate the damping coefficient and damping force. The designed damper is fabricated and an experimental setup is also established to characterize the damper and these results are compared with the analytical results. A typical FE model of Nose landing gear is developed to study the effectiveness of the damper. Open loop response analysis has been carried out and response levels are monitored at the piston tip of a nose landing gear for various loading conditions without damper and with MR-damper as semi-active device. The closed-loop full state feedback control scheme by the pole-placement technique is also applied to control the landing gear instability of an aircraft.

• Advances in nano research
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• v.10 no.5
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• pp.481-491
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• 2021

Graphene Nanosheets play an important role in nanosensors due to their proper surface to volume ratio. Therefore, the main purpose of this paper is to consider the nonlinear vibration behavior of graphene nanosheets (GSs) under the influence of electromagnetic fields and electrical current create forces. Considering more realistic assumptions, new equations have been proposed to study the nonlinear vibration behavior of the GSs carrying electrical current and placed in magnetic field. For this purpose, considering the influences of the magnetic tractions created by electrical and eddy currents, new relationships for electromagnetic interaction forces with these nanosheets have been proposed. Nonlinear coupled equations are discretized by Galerkin method, and then solved via Runge-Kutta method. The effect of different parameters such as size effect, electrical current magnitude and magnetic field intensity on the vibration characteristics of GSs is investigated. The results show that the magnetic field increases the linear natural frequency, and decreases the nonlinear natural frequency of the GSs. Excessive increase of the magnetic field causes instability in the GSs.

• International Journal of Fluid Machinery and Systems
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• v.3 no.3
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• pp.235-244
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• 2010

Recent studies on the moment whirl due to leakage flow in the back shroud clearance of hydro-turbine runners or centrifugal pump impellers are summarized. First, destabilizing effect of leakage flow is discussed for lateral vibrations using simplified models. Then it is extended to the case of whirling motion of an overhung rotor and the criterion for the instability is obtained. The fluid moment caused by a leakage clearance flow between a rotating disk and a stationary casing was obtained by model tests under whirling and precession motion of the disk. It is shown that the whirl moment always destabilizes the whirl motion of the overhung rotor while the precession moment destabilizes the precession only when the precession speed is less than half the rotor speed. Then vibration analyses considering both whirl and precession are made by using the hydrodynamic moments determined by the model tests. For larger overhung rotors, the whirl moment is more important and cause whirl instability at all rotor speed. On the other hand, for smaller overhung rotors, the precession moment is more important and cancels the destabilizing effect of the whirl moment.

• International Journal of High-Rise Buildings
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• v.8 no.4
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• pp.303-311
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• 2019

The effectiveness of aerodynamic modification to reduce wind loadings has been widely reported. However, most of previous studies have been investigated dynamic forces and pressure distributions on tall buildings with various unconventional configurations. This study was investigated dynamic characteristics and aerodynamic instability of super-tall buildings with unconventional configurations through extensive aeroelastic model experiments. Seventeen types of supertall building models were considered such as basic and corner modification with corner cut, chamfered, oblique opening, tapered, inversely tapered, bulged, helical with twist angles of $90^{\circ}$, $180^{\circ}$, $270^{\circ}$, $360^{\circ}$ and composite with $360^{\circ}$ helical & corner cut, 4-tapered & $360^{\circ}$ helical & corner cut, setback & corner cut, setback & $45^{\circ}$ rotate. As a result, aerodynamic characteristics of helical models with single modification are superior to those of other models with single modification. However, effect of twist angle for helical model is negligible. Further, the 4-tapered & $360^{\circ}$helical & corner cut model is most effective in reducing the along- and across-wind fluctuating displacement responses in all of experimental models.

• Coupled systems mechanics
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• v.5 no.4
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• pp.315-327
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• 2016

Undergoing large volumetric changes upon incremental environmental stimulation, hydrogels are interesting materials which hold immense potentials for utilization in a wide array of applications in diverse industries. Owing to the large magnitudes of deformation it undergoes, swelling induced instability is a commonly observed sight in all types of gels. In this work, we investigate the instability of photo-thermal sensitive hydrogels, produced by impregnating light absorbing nano-particles into the polymer network of a temperature sensitive hydrogel, such as PNIPAM. Earlier works have shown that by using lights of different intensities, these hydrogels follow different swelling trends. We investigate the possibility of utilizing this fact for remote switching applications. The analysis is built on a thermodynamic framework of inhomogeneous large deformation of hydrogels and implemented via commercial finite element software, ABAQUS. Various examples of swelling induced instabilities, and its corresponding dependence on light intensity, will be investigated. We show that the instabilities that arise have their morphologies dependent on the light intensity.

• Wind and Structures
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• v.37 no.2
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• pp.117-131
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• 2023

Korea Design Standard (KDS) will be updated with two major revisions on the assessment of wind load and performance-based wind design (PBWD). Major changes on the wind load assessment are the wind load factor and basic wind speed. Wind load factor in KDS is reduced from 1.3 to 1, and mean recurrence interval (MRI) for basic wind speed increases from 100 years to 500 years considering the reduction of wind load factor. Additional modification is made including pressure coefficient, torsional moment coefficient and spectrum, and aeroelastic instability. Combined effect of the updates of KDS code on the assessment of wind load is discussed with the case study on the specified sites and building. PBWD is newly added in KDS code to consider the cases with various target performance, vortex-induced vibration, aeroelastic instability, or inelastic behavior. Proposed methods and target performance for PBWD in KDS code are introduced.

• Wind and Structures
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• v.18 no.2
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• pp.103-116
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• 2014

Aeroelasticity is the main source of instability in structures which are subjected to aerodynamic forces. One of the major reasons of instability is the coupling of bending and torsional vibration of the flexible bodies, which is known as flutter. The presented investigation aims to study the aeroelastic stability of composite blades of wind turbine. Geometry, layup, and loading of the turbine blades made of laminated composites were calculated and evaluated. To study the flutter phenomenon of the blades, two numerical and analytical methods were selected. The finite element method (FEM), and JAR-23 standard were used to perform the numerical studies. In the analytical method, two degree freedom flutter and Lagrange's equations were employed to study the flutter phenomena analytically and estimate the flutter speed.

• Journal of the Korean Solar Energy Society
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• v.28 no.1
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• pp.25-32
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• 2008

Aeroelastic phenomena of a wind turbine include stall-induced vibrations and classical flutters. The classical flutter occurs due to coalescence between bending mode and torsion mode. It is typically the aeroelastic instability of an aircraft wing. Different from the classical flutter, the stall-induced vibration is the instability in lead-lag mode due to negative aerodynamic dampings. In the present study, the three degree of freedom aeroelastic model of a wind turbine blade is introduced to characterize and analyze its aeroelastic phenomena. The numerical results show that the aeroelastic stability of flap-lag motion is more unstable than that of flap-pitch motion and the aeroelastic characteristics of lead-lag motion can become unstable as wind speed increases.

• International Journal of Aeronautical and Space Sciences
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• v.13 no.3
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• pp.332-348
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• 2012

The present study deals with the parametric resonance behaviour of laminated composite curved shell panels in a hygrothermal environment using Bolotin's approach. A simple laminated model is developed using first order shear deformation theory (FSDT) for the vibration and dynamic stability analysis of laminated composite shells subjected to hygrothermal conditions. A computer program based on the finite element method (FEM) in a MATLAB environment is developed to perform all necessary computations. Quantitative results are presented to show the effects of curvature, ply-orientations, degree of orthotropy and geometry of laminates on the parametric instability of composite curved shell panels for different temperature and moisture concentrations. The excitation frequencies of laminated composite panels decrease with the increase of temperature and moisture due to reduction of stiffness for all laminates.