• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.10
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• pp.71-79
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• 2006

Swirl injector with multi-stage tangential entry was analyzed to suppress high-frequency combustion instability in Liquid Rocket Engines. In order to analyze the effect of swirl injector as an acoustic absorber, swirl injector was regarded as a quarter-wave resonator and it's damping capacity is verified in atmospheric temperature. It has a finite mode of vibration and natural frequencies which can be tuned to the natural frequencies of a model combustion chamber. The interior air core shape of injector is more stable in the case of using the swirl injector with multi-stage entry than with single-stage entry. Also, when the swirl injector with multi-stage entry is used, tuned-injector length for unstable mode is well agreed with the calculated length. From the experimental data, it is proved that if the interior air core shape of swirl injector is stable, the fine tuned swirl injector can decrease the unstable mode of model chamber effectively and increase the damping rate.

• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.803-809
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• 2005

This paper presents the theoretical model to investigate the effect of Coulomb damping in the sub-structure of a foil bearing. Foil deflection is restricted by friction of bumps. Equivalent viscous damping of the bump foils is derived from the Coulomb friction. Dynamic equation of the bumps is constituted by stiffness and damping terms. This point give the difference from Heshmat's frictionless and simple compliance bump model. The fluid is modeled with the compressible Reynolds equation. A perturbation approach is used to determine the static and dynamic performance of the bearing from the coupled fluid-structural model. The analysis result shows that the static and dynamic performance is enhanced by bump friction. This analysis technique would be extended to development of a high performance bearing.

• Journal of KSNVE
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• v.4 no.2
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• pp.199-208
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• 1994

Two-phase cross-flow exists in many shell-tube heat exchangers such as nuclear steam generators, condensers and reboilers. An understanding of damping and of flow-induced vibration excitation mechanisms in necessary to avoid problems due to excessive tube vibration. In this paper, we present the results of experiments on normal-triangular tube bundles of pitch to tube diameter ratio, p/d, 1.22, 1.32 and 1.47. The bundle were subjected to air-water mixtures to simulate realistic mass fluxes and vapour qualities corresponding to void fractions from 5 to 99%. Damping, fluidelastic instability and turbulence- induced excitation are discussed. The behaivior of damping and two vibration mechanisms are different for intermittent flows from for bubbly flows. The effect of pitch to tube diameter ratio and void fraction is dominant on damping and fluidelastic instability.

• The KSFM Journal of Fluid Machinery
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• v.9 no.1 s.34
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• pp.47-55
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• 2006

This paper presents the theoretical model to investigate the effect of Coulomb damping in the sub-structure of a foil bearing. Foil deflection is restricted by friction of bumps. Equivalent viscous damping of the bump foils is derived from the Coulomb friction. Dynamic equation of the bumps is constituted by stiffness and damping terms. This point give the difference from Heshmat's frictionless and simple compliance bump model. The fluid is modeled with the compressible Reynolds equation. A perturbation approach is used to determine the static and dynamic performance of the bearing from the coupled fluid-structural model. The analysis result shows that the static and dynamic performance is enhanced by bump friction. This analysis technique would be extended to development of a high performance bearing.

• KSTLE International Journal
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• v.4 no.2
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• pp.66-72
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• 2003

This paper presents the hydrodynamic effect by the journal speed, eccentricity and source positions in order to overcome the defects of air bearing such as low stiffness and damping coefficient. Choosing the two row source position of air bearing is different from existing investigations in the side of pressure distribution of air film because of the high speed of journal and the wedge effects by the eccentricity. These optimal choices of the two row source positions enable us to improve the performance of the film reaction force and loading force as making the high-speed spindle. In this paper, The pressure behavior in theory of air film in high speed region of journal according to the eccentricity of journal and the source positions analyzed. The theoretical analysis has been identified by experiments. The results of investigated characteristics may be applied to precision devices like ultra-precision grinding machine and ultra high-speed milling.

• Tribology and Lubricants
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• v.17 no.6
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• pp.476-481
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• 2001

This paper has been presented the hydrodynamic effect by the journal speed, eccentricity and source positions in order to overcome the defects of air bearing such as low stiffness and damping coefficient. Choosing the two row source position of air bearing is different from existed investigations in the side of pressure distribution of air film because of the high speed of journal and the wedge effects by the eccentricity. These optimal chooses of the two row source positions enable us to improve the performance of the film reaction force and loading force as making the high speed spindle. In this paper, The pressure behavior in theory of air film according to the eccentricity of journal and the source positions analyzed. The theoretical analysis have been identified by experiments. The results of investigated characteristics may be applied to precision devices like ultra-precision grinding machine and ultra high speed milling.

• Proceedings of the Korean Magnestics Society Conference
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• 2000.09a
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• pp.546-560
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• 2000

This paper describes the design and evaluation of a noise suppressing hydrophone that is robust to external noise without sacrificing its performance as a receiver. To increase robustness of the receiver to the external noise, first, effects of location of external noise on its performance are analyzed with the finite element method (FEM). Based on the results, geometrical variations are implemented on the structure with additional air pockets and damping layers that work as acoustic shields or scatterers of the noise, and fourteen trial models are developed for the noise suppressing hydrophone structures. The results show that the effect of the external noise is most significant when it is applied to near the mid-side surface of the hydrophone housing. The external noise is isolated most efficiently when two thin damping layers combined with five air pockets are inserted to the circumference of the hydrophone housing. Overall, of the fourteen structural variations of the hydrophone, the best one shows about 87% reduction in the response of the original structure to external noise.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2004.11a
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• pp.143-148
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• 2004

The numerical analysis of the hard disk drive slider is presented. The pressure distribution was calculated using the finite element method. The generalized Reynolds equation was applied in order to include the gas rarefaction effect. The balance of the air bearing force and preload force was considered. The characteristics of the small vibrations near the equilibrium were studied using the perturbation method. Triangular mesh with variable element size was employed to model the two-rail slider. The flying height, pitching angle, rolling angle, stiffness and damping of the two-rail slider were calculated for radial position changing from the inner radius to the outer radius and for a wide range of the slider crown values. It was found that the flying height, pitching angle and rolling angle were increased with radial position while the stiffness and damping coefficients were decreased. The higher values of crown resulted in increased flying height, pitching angle and damping and decreased stiffness.

• Tribology and Lubricants
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• v.23 no.4
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• pp.162-169
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• 2007

This paper deals with friction effects on the performance of double-bumped AFBs. The stiffness and damping coefficients of the double bump vary depending on the external load and its friction coefficient. The double bump can be either in the single or double active region depending on vertical deflection. The equivalent stiffness and damping coefficients of the bump system are derived from the vertical and horizontal deflection of the bump, including the friction effect. A static and dynamic performance analysis is carried out by using the finite difference method and the perturbation technique. The results of the performance analysis for a double-bumped AFB are compared with those obtained for a single-bumped AFB. This paper successfully proves that a double bumped AFB has higher load capacity, stiffness, and damping than a single-bumped AFB in a heavily loaded condition.

• Journal of Ocean Engineering and Technology
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• v.19 no.3
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• pp.11-17
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• 2005

The effect of frequency and amplitude of the OWC (Oscillating Water Column) motion on the nonlinear reaction forces in an air duct are studied experimentally. Experimental owe model is idealized as a simple circular cylinder with an orifice type air duct located at the middle oj the top rid. Reaction forces due to forced heave oscillation are measured and analyzed. By subtracting the effect of inertia forces and restoring forces, pneumatic damping force and added spring force are deduced. The effects of the frequency and amplitude of the heave motion are discussed. Also, the effects of solidity of the duct on the reaction forces are discussed.