• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.3
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• pp.437-450
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• 1996

A set of stability equations is formulated for natural convection flows adjacent to a vertical isothermal surface melting in cold pure water. It takes account of the nonparallelism of the base flows. The melting rate is regarded as a blowing velocity at the ice surface. The numerical solutions of the linear stability equations which constitute a two-point boundary value problem are accurately obtained for various values of the density extremum parameter $R=(T_m-T_{\infty})/(T_0-T_{\infty})$ in the range $0.3{\leq}R{\leq}0.6$, by using a computer code COLNEW. The blowing effects on the base flow becomes more significant as ambient temperature ($T_{\infty}$) increases to $T_{\infty}=10^{\circ}C$. The maximum decrease of heat transfer rate is about 6.4 percent. The stability results show that the melting at surface causes the critical Grashof number $G^*$ and the maximum frequency of disturbances to decrease. In comparision with the results for the conventional parallel flow model, the nonparallel flow model has a higher critical Grashof number but has lower amplification rates of disturbances than does the parallel flow model. The spatial amplification contours exhibit that the selective frequency $B_0$ of the nonparallel flow model is higher than that of the parallel flow model and that the effects of melting are rather small. The present study also indicates that the selective frequency $B_0$ can be easily predicted by the value of the frequency parameter $B^*$ at $G^*$, which comes from the neutral stability results of the nonparallel flow model.

• The Journal of the Acoustical Society of Korea
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• v.39 no.4
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• pp.371-377
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• 2020

Combustion instability occurred in the combustion test of solid rocket motor with large aspect ration Length/Diameter (L/D) and cylinder-slot grain. As a result of spectral analysis of the pressure perturbation, it was confirmed that the central axis longitudinal frequency was dominant, so that the length of the cylinder part was increased to eliminate the coincidence with acoustic node. In addition, acoustic modal analysis and flow analysis were performed to analyze the cause of instability by unsteady flow structure in solid rocket motors. It was confirmed that the combustion instability is reduced by quantitative comparison of the amplitude and frequencies of the pressure inside the combustion chamber using the grain shape before and after the design change. Finally, a combustion test was performed to verify that the combustion instability was resolved as in the flow analysis.

• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.5
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• pp.90-100
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• 2019

Hypergolic thrusters have been extensively researched and applied to spacecraft propulsion based on their simplicity and high reliability of ignition. Research on the impingement characteristics of $N_2O_4$/amine has been profoundly carried out since the 1960s in advanced countries, especially the United States. Recently, enhancements to advanced hypergolic thrusters using MON/MMH have been planned by NASA to improve compactness and high performance. In this work, technical studies were investigated on the mixing of hypergolic propellant and its combustion instabilities such as reactive separation flow and popping.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.366-375
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• 1991

In this study, vibrational behavior of uniform pipe carrying a moving medium is studied by using a transfer matrix and the displacement function derived from the conventional beam theory. In various boundary conditions, flow velocity and mechanical property change of the variation of natural frequency are investigated. The Coriolis term in the original differential equation of motion has been ignored in the investigation. This method is used to study the variation of natural frequency with flow velocity for clamped-clamped, cantilevered, clamped-pinned, pinned-pinned, free-free straight pipe element. It is shown that clamped-clamped, free-free pipe have the highest natural frequency and critical velocity values while cantilevered pipe have the smallest natural frequency for the same mechanical properties. From the vibration effects of mechanical property variation, it is shown that bending stiffness and pipe length variation has large influence on natural frequency and critical velocity. Since the order of transfer matrix is not changed with boundary conditions of pipe element, this method proposed can be easily applied to personal-computer for vibration analysis of pipe element. Furthermore, this method can be extended to three-dimensional system by using a coordinate transformation for the analysis of piping systems.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.10
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• pp.93-101
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• 2004

A method to calculate stability limits is investigated to predict the characteristics of high-frequency combustion instability in liquid-propellant rocket engine. It is based on the theory of linear stability analysis proposed in previous works and useful to predict combustion stability at the beginning stage of engine development. The system of equations governing reactive flow in combustor has the simplified and linearized forms. The overall equation expressing stability limits is adopted. The procedures to evaluate quantitatively each term included in the equation are proposed. The thermo-chemical properties and flow variables required in the evaluation can be obtained from calculation of thermodynamic equilibrium, CFD results, and experimental test data. Based on the existent data, stability limits are calculated with actual rocket engine (KSR-III rocket engine). The present calculations show the reasonable stability limits in a quantitative manner and the stability characteristics of the engine are discussed. The prediction from linear stability analysis could be serve as the first approximation to the true prediction.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.1
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• pp.1-11
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• 2010

The definition of the speed of sound is reexamined since it is crucial in the numerical analysis of compressible real gas flows. The thermodynamic speed of sound (TSS), $a_{th}$, and the characteristic speed of sound (CSS), $a_{ch}$, are derived using generalized equation of state (EOS). It is found that the real gas EOS, for which pressure is not linearly dependent on density and temperature, results in slightly different TSS and CSS. in this formalism, Roe's approximate Riemann solver was derived again with corrections for real gases. The results show a little difference when the speeds of sound are applied to the Roe's scheme and Advection Upstream Splitting Method (AUSM) scheme, but a numerical instability is observed for a special case using AUSM scheme. It is considered reasonable to use of CSS for the mathematical consistency of the numerical schemes. The approach is applicable to multi-dimensional problems consistently.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.1
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• pp.29-36
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• 2014

Hybrid rocket displays many different low frequency pressure oscillations during combustion. Thermal lag between solid and gas phase is the primary mechanism to trigger low frequency pressure oscillations of around 10Hz, and Helmholtz or $L^*$ mode also produces other types of low frequency oscillations above 10 Hz which is associated with the change in combustion volume. Since the flow characteristics in hybrid rocket is very similar to those in solid rocket combustion, it is not surprising to observe similar pressure oscillation behaviors. Experimental test shows that combustion pressure suddenly turns into to a big amplitude oscillation around 10Hz then followed by returning to an original pressure level after a short period combustion. Further investigations show that this instability is independent of the change in O/F ratio at all. One of the possible candidates is the vortex shedding dynamics over the backward step in the post combustion chamber. It is required to investigate the low frequency oscillation mechanism in the future study.

• Journal of Advanced Navigation Technology
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• v.16 no.2
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• pp.179-189
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• 2012

The aim of this paper is to research the change in the turbulent flow and the AOA(Angle Of Attack) occurred by $40^{\circ}$ crosswind to the direction of runway through the three-dimensional numerical analysis and to predict the take-off and landing flight stability. As a result, the maximum amplitude of AOA variation on runway reached $2^{\circ}$ within 3 second because of the wake formed by the constructions in the vicinity of the airport, and the overall effects appeared as an irregular aperiodic forms. Additionally, it was observed that the layout and shape of the buildings effected on the strength of turbulence directly, and the rapid flow generated between the buildings changed into stronger wake and eventually expected that the flow raises serious take-off and landing flight instability.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.9
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• pp.67-75
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• 2006

A fully nonlinear model accounting for swirling effect has been applied in analyzing the dynamic response for a classical swirl injector. The current work applied highly accurate Boundary Element Methods (BEMs) in assessing its static and dynamic characteristics. On the basis of moving surface treatment method and surface instability study, which are obtained from the previous static characteristics analysis in pressure-swirl injectors, this work was expanded for analyzing the dynamics of a classical swirl injector. The dynamic response through injector components for disturbed inflow condition was investigated. The modified code was validated from comparison with the theoretical result for a typical swirl injector. Clearly the simulated result shows the interesting characteristics of swirl injectors to provide either amplification or damping of the input disturbance through each component. These results give promise in applying the current model to nonlinear dynamic characteristics of swirl injectors.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.1
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• pp.1-9
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• 2018

Lambda wing type Unmanned Combat Aerial Vehicle(UCAV) which adopts Blended Wing Body(BWB) has relatively less drag and more stealth performance than conventional aircraft. However, Pitching moment is rapidly increased at a specific angle of attack affected by leading edge vortex due to leading edge sweep angle. Wind tunnel testing and numerical analysis were carried out with UCAV 1303 configuration on condition of 50 m/s of flow velocity, $-4^{\circ}{\sim}28^{\circ}$ of the range of angle-of-attack. The effect of wing twist for longitudinal stability at the various angles of attack was verified in this study. When negative twist is applied on the wing, Pitch-break was onset at higher angle of attack due to delayed flow separation on outboard of the wing. On the other hand, pitch-break was onset at lower angle of attack and lift-to-drag ratio was increased when positive twist is applied on the wing.