• Wind and Structures
• /
• v.17 no.1
• /
• pp.107-121
• /
• 2013

This paper presents an experimental investigation concerning the peak amplitudes of oscillation of a square prism due to Vortex-Induced-Vibrations (VIV) as a function of the mass damping parameter $m^*{\zeta}$(the so called Griffin--plot); $m^*$ and ${\zeta}$ being, respectively, the non-dimensional mass and the mechanical (structural) damping ratio. With this purpose in mind, an electromagnetic actuator has been employed to provide controlled damping. During the experiments the mass--damping parameter was in the range 0.15 < $m^*{\zeta}$ < 2.4. Experiments show that there is a value of $m^*{\zeta}$ below which VIV appears combined with galloping and the prism oscillation increases monotonically with the incoming flow velocity. For $m^*{\zeta}$ >0.3 the present experiments show a well-defined VIV phenomenon and, consequently, a Griffin-plot can be defined.

• Wind and Structures
• /
• v.16 no.4
• /
• pp.341-354
• /
• 2013

This paper presents an experimental investigation concerning the peak amplitudes of oscillation of a square prism due to Vortex-Induced-Vibrations (VIV) as a function of the mass damping parameter $m^*{\zeta}$ (the so called Griffin--plot); $m^*$ and ${\zeta}$ being, respectively, the non-dimensional mass and the mechanical (structural) damping ratio. With this purpose in mind, an electromagnetic actuator has been employed to provide controlled damping. During the experiments the mass--damping parameter was in the range 0.15 < $m^*{\zeta}$ < 2.4. Experiments show that there is a value of $m^*{\zeta}$ below which VIV appears combined with galloping and the prism oscillation increases monotonically with the incoming flow velocity. For $m^*{\zeta}$ >0.3 the present experiments show a well-defined VIV phenomenon and, consequently, a Griffin-plot can be defined.

• Journal of the Korean Society of Combustion
• /
• v.12 no.2
• /
• pp.34-41
• /
• 2007

The dynamic behaviors of counterflow non-premixed flame have been investigated experimentally to study effects of heat losses and Lewis number on edge flame oscillation, which result from the advancing and retreating edge flame motion of outer flame edge at low strain rate flame. For low strain rate flame, lateral conduction heat loss in addition to radiation heat loss could be more remarkable than the others. Oscillatory instabilities appear at fuel Lewis number greater than unity. But excessive lateral conduction heat loss causes edge flame instability even at fuel Lewis number less than unity. The excessive heat loss caused by the smaller burner diameter in which the flame length is an indicator of lateral conduction heat loss extends the region of flame oscillation and accelerates oscillatory instability in comparison to the previous study with the burner diameter of 26mm. Extinction behaviors quite different from the previous study are also addressed.

• Proceedings of the Korea Society for Energy Engineering kosee Conference
• /
• 1999.11a
• /
• pp.149-154
• /
• 1999

This paper describes a passive safety injection system with free piston Stirling pump working withabundant decay heat in the nuclear reactor during the hypothetical accident. The water column in the tube assembly connected from the hot chamber to the cold chamber in the pump oscillates periodically due to thermal volume changes of non-condensable gas in each chamber. The oscillating pressure in the water column is converted into the pumping power with a suction-and-bleed type valve assembly. In this paper a dynamic model describing the frequency of oscillation and pumping pressure is developed. It was found that the pumping pressure is a function of the temperature difference between the chambers. Also, the frequency oscillation depends on the length of the tube with water column.

• Environmental Engineering Research
• /
• v.14 no.3
• /
• pp.158-163
• /
• 2009

It is difficult for a burner to achieve an increase in combustibility and a reduction of NOx emission, simultaneously. The reason is because thermal NOx could be reduced at low temperature, while the combustibility should be decreased. To solve this problem, an externally oscillated staging burner was developed, and experiment was conducted according to effective parameters. The combustibility could be improved through the accelerated transfer of heat, mass and momentum obtained by external oscillation. Also, NO is reduced by the decrease of residence time of burning gas in the local highest-temperature spot, which is decreased by the external oscillation and fuel staging. Experiments on variables were conducted to determine the reference flame, and the flame generating the lowest NO concentration was selected. The conditions of reference flame were oscillation frequency 250 Hz, sound pressure 1 VPP, and air ratio 1.1, and NO and CO concentrations were 1ppm and 20 ppm, respectively.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.4
• /
• pp.997-1008
• /
• 1994

Heat transfer by laminar oscillating flow in a circular pipe has been studied analytically. The general solution with respect to the arbitrary wall boundary condition is obtained by superposing the fluid temperatures with the sinusoidal wall temperature distributions. The fulid temperature distributions are two dimensional, but uniform flow assumption is used to simplify the velocity distribution. The heat transfer characteristics in the thermally developing regions are analyzed by applying the general solution to the two cases of thermal boundary conditions in which the wall temperature and wall heat flux distributions have a square-wave form, respectively. The results show that the length of the thermally developing region becomes larger in proportion to the oscillation frequency at slow oscillation and eventually approaches to the value comparable to the swept distance as the frequency increases. The time and cross-section averaged Nusselt number in the developing region is inversely proportional to the square root of the distance from the position where the wall boundary condition is changed suddenly. In the developed region, Nusselt number is only determined by the oscillation frequency.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.59 no.2
• /
• pp.390-396
• /
• 2010

An 80MHz surface acoustic wave (SAW)-based gyroscope utilizing a progressive wave was developed on a piezoelectric substrate. The developed sensor consists of two SAW oscillators in which one is used for sensing element and has metallic dots in the cavity between input and output IDTs. The other is used for a reference element. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. According to the simulation results, the device was fabricated and then measured on a rate table. When the device was subjected to an angular rotation, oscillation frequency differences between the two oscillators were observed because of the Coriolis force acting on the metallic dots. Depending on the angular rate, the difference of the oscillation frequency was modulated. The obtained sensitivity was approximately 52.35 Hz/deg.s within the angular rate range of 0~1000 deg/s. The performances of devices with three IDT structures for two kinds of piezoelectric substrates were characterized. Good thermal stability was also observed during the evaluation process.

• Nuclear Engineering and Technology
• /
• v.32 no.2
• /
• pp.169-179
• /
• 2000

A detailed numerical analysis of the evolution of thermal stratification in a curved piping system in a nuclear power plant is performed. A finite volume based thermal-hydraulic computer code has been developed employing a body-fitted, non-orthogonal curvilinear coordinate for this purpose. The cell-centered, non-staggered grid arrangement is adopted and the resulting checkerboard pressure oscillation is prevented by the application of momentum interpolation method. The SIMPLE algorithm is employed for the pressure and velocity coupling, and the convection terms are approximated by a higher-order bounded scheme. The thermal-hydraulic computer code developed in the present study has been applied to the analysis of thermal stratification in a curved duct and some of the predicted results are compared with the available experimental data. It is shown that the predicted results agree fairly well with the experimental measurements and the transient formation of thermal stratification in a curved duct is also well predicted.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.47 no.10
• /
• pp.720-727
• /
• 2019

Resonating thermal lags of solid fuel with heat transfer oscillations generated by boundary layer oscillation is the primary mechanism of the occurrence of the LFI (Low Frequency Combustion Instability) in hybrid rocket combustion. This study was experimentally attempted to confirm that how the boundary layer was perturbed and led to the LFI. Special attention was also made on oxidizer swirl injection to investigate the contribution to combustion stabilization. Also the overall behavior of fluctuating boundary layer flow and the occurrence of the LFI was monitored as swirl intensity increased. Fluctuating boundary layer was successfully monitored by the captured image and POD (Proper Orthogonal Decomposition) analysis. In the results, oscillating boundary layer became stabilized as the swirl intensity increases. And the coupling strength between high frequency p', q' diminished and periodical amplification of RI (Rayleigh Index) with similar frequency band of thermal lag was also decreased. Thus, results confirmed that oscillating axial boundary layer triggered by periodic coupling of high frequency p', q' is the primary mechanism to excite thermal resonance with thermal lag characteristics of solid fuel.

• Journal of computational fluids engineering
• /
• v.10 no.4 s.31
• /
• pp.1-11
• /
• 2005

A numerical study of the evaluation of turbulence models for thermal striping phenomenon is performed. The turbulence models chosen in the present study are the two-layer model, the shear stress transport (SST) model and the V2-f model. These three models are applied to the analysis of the triple-jet flow with the same velocity but different temperatures. The unsteady Reynolds-averaged Navier-Stokes (URANS) equation method is used together with the SIMPLEC algorithm. The results of the present study show that the temporal oscillation of temperature is predicted by the SST and V2-f models, and the accuracy of the mean velocity, the turbulent shear stress and the mean temperature is a little dependent on the turbulence model used. In addition, it is shown that both the two-layer and SST models have nearly the same capability predicting the thermal striping, and the amplitude of the temperature fluctuation is predicted best by the V2-f model.