• Kyungpook Mathematical Journal
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• 제46권4호
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• pp.477-488
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• 2006

In this paper, the authors first classify all nonoscillatory solutions of equation (1) $${\Delta}^2|{\Delta}^2{_{y_n}}|^{{\alpha}-1}{\Delta}^2{_{y_n}}+q_n|y_{{\sigma}(n)}|^{{\beta}-1}y_{{\sigma}(n)}=o,\;n{\in}\mathbb{N}$$ into six disjoint classes according to their asymptotic behavior, and then they obtain necessary and sufficient conditions for the existence of solutions in these classes. Examples are inserted to illustrate the results.

• Wind and Structures
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• 제38권1호
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• pp.59-74
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• 2024

Large Eddy Simulation (LES) is used to explore the influence of vibration frequency and amplitude on the aerodynamic performance of a rectangular cylinder with an aspect ratio of B/D=5 (B: breadth; D: depth of cylinder) at a Reynolds number of 22,000 near resonance frequency. In smooth flow conditions, the research employs a sequence of three-dimensional simulations under forced vibration with diverse frequency ratios f_e / f_o = 0.8-1.2 (f_e : oscillation frequency; f_o : Strouhal frequency when the rectangular cylinder is stationary ) and oscillation amplitudes A_h/D = 0.05 - 0.3. The individual influences of f_e / f_o and A_h/D on the characteristics of integrated and distributed aerodynamic forces are the focal points of discussion. For the integrated aerodynamic force, particular emphasis is placed on the analysis of the dependence of velocity-proportional component C₁ and displacement-proportional component C₂ of unsteady aerodynamic force on amplitude and frequency ratio. Near the resonance frequency, the dependencies of C₁ and C₂ on amplitude are stronger than that of frequency ratio. For the distributed aerodynamic force, the increase in frequency and amplitude promotes the position of the main vortex core and reattachment to the leading edge in the streamwise direction. In the spanwise direction, vibration enhances the spanwise correlation of aerodynamic force to weaken the three-dimensional effect of the flow field, and a lower frequency ratio and larger amplitude amplify this effect.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2014년도 제49회 KOSCO SYMPOSIUM 초록집
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• pp.379-381
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• 2014

Influence of fuel concentration gradient was investigated near flame extinction limit in buoyancy-suppressed non-premixed counterflow flame with triple co-flow burner. The use of He curtain flow produced a microgravity level of $10^{-2}-10^{-3}g$ in He-diluted non-premixed counter triple co-flow flame experiments. Flame stability map was presented based on flame extinction and oscillation near extinction limit. The stability map via critical diluent mole fraction with global strain rate was represented by varying outer and inner He-diluted mole fractions. The flame extinction modes could be classified into five: an extinction through the shrinkage of the outmost edge flame forward the flame center with and without self-excitation, respectively ((I) and (II)), an extinction via the rapid expansion of a flame hole while the outmost edge flame is stationary (III), both the outermost and the center edge flames oscillate, and then a donut shaped flame is formed or the flame is entirely extinguished (IV), a shrinkage of the outermost edge flame without self-excitation followed by shrinking or sustain the inner flame (V).

• International Journal of Aeronautical and Space Sciences
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• 제16권2호
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• pp.123-136
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• 2015

The present study numerically investigates the effect of shield on the flow characteristics of Hartmann whistle. The flow characteristics of un-shielded Hartmann whistle are compared with whistles of different shield heights 15 mm, 17 mm, 20 mm, 25 mm and 30 mm. The comparison of Mach number contours and transient velocity vectors of shielded Hartmann whistles with un-shielded ones for the same conditions reveal that the presence of shield causes the exiting jet to stick to the wall of the shield without causing spill-over around the cavity inlet, thus sustaining the shock oscillation as seen in the unshielded Hartmann whistle, which has intense flow/shock oscillation and spill-over around the cavity mouth. The velocity vectors indicate jet regurgitance in shielded whistles showing inflow and outflow phases like un-shielded ones with different regurgitant phases. The sinusoidal variation of mass flow rate at the cavity inlet in un-shielded Hartmann whistle indicates jet regurgitance as the primary operating mode with large flow diversion around the cavity mouth whereas the non-sinusoidal behavior in shielded ones represent that the jet regurgitance is not the dominant operating mode. Thus, this paper sufficiently demonstrates the effect of shield in modifying the flow/shock oscillations in the vicinity of the cavity mouth.

• 대한기계학회논문집B
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• 제20권7호
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• pp.2397-2408
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• 1996

A numerical study was made of the control of transient oscillatory flow modes in Czochralski convection. The reduction of temperature oscillation was achieved by changing the rotation rate of crystal rod, .OMEGA.$_{S}$=.OMEG $A_{S0}$(1+ $A_{S}$sin(2.pi. $f_{S}$/ $t_{p}$t)). The temporal behavior of oscillation flow was scrutinized over broad ranges of two parameters, i.e., the rotation amplitude( $A_{S}$.leq.0.5) and the nondimensional frequency (0.9.leq. $f_{S}$.leq.1.5). The mixed convection parameter was ranged 0.225.leq.Ra/PrR $e^{2}$.leq.0.929, which encompassed the buoyancy-and forced-dominant convection regimes. Computational results revealed that the temperature oscillations could be reduced effectively by a proper adjustment of the control parameters. The uniformity of temperature distribution near the crystal rod was examined. The control of oscillatory flow modes was also made for a realistic, low value of Pr.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2000년도 영호남학술대회 논문집
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• pp.47-50
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• 2000

The optical characteristics of merocyanine dyes have been investigated widely due to their possible application to the high efficiency photo-electric devices. The optical systems are mostly fabricated using vacuum evaporation, casting and Langmuir-Blodgett method and the arrangement and orientation of dye molecules is one of the most important factors in the study on the optical characteristics. In this study, we fabricated the molecular systems through the LB techniques and investigated the optical characteristics of merocyanine dye LB film using the oscillation characteristics of quartz crystal. It was quite interesting behavior that the resistance and frequency shift at the parallel resonance under the UV irradiation.

• Journal of applied mathematics & informatics
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• 제30권1_2호
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• pp.289-304
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• 2012

In this paper, we study asymptotic behaviour of solutions of the following higher order nonlinear dynamic equations $$S_n^{\Delta}(t,x)+{\delta}p(t)f(x(g(t)))=0$$ and $$S_n^{\Delta}(t,x)+{\delta}p(t)f(x(h(t)))=0$$ on an arbitrary time scale $\mathbb{T}$ with sup $\mathbb{T}={\infty}$, where n is a positive integer, ${\delta}=1$ or -1 and $$S_k(t,x)=\{\array x(t),\;if\;k=0,\\a_k(t)S_{{\kappa}-1}^{\Delta}(t),\;if\;1{\leq}k{\leq}n-1,\\a_n(t)[S_{{\kappa}-1}^{\Delta}(t)]^{\alpha},\;if\;k=n,$$ with ${\alpha}$ being a quotient of two odd positive integers and every $a_k$ ($1{\leq}k{\leq}n$) being positive rd-continuous function. We obtain some sufficient conditions for the equivalence of the oscillation of the above equations.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 춘계학술대회논문집
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• pp.21-24
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• 2008

The present study numerically investigates the glottal airflow characteristics as well as acoustic features of phonation fully coupled with dynamic behavior of vocal folds. The vocal folds are described by a low-dimensional body-covered model characterized by bio-mechanical parameters such as glottal width, vocal folds stiffness, and subglottal pressure. The flow in the vocal tract is modeled as an incompressible, axisymmetric form of the Navier-Stokes equations (INS), while the acoustic field is predicted by the linearized perturbed compressible equations (LPCE). The computed result shows that a two-mass model of vocal folds is sufficient to reproduce temporal variations in oral airflow and glottis motion produced by female speakers. It is also found that i) the glottal width has a significant effect on the amplitude of glottal flow, and thus on the amplitude of acoustic wave in the vocal tract, ii) the vocal fold tension is the main control parameter for the fundamental frequency of phonation, iii) the subglottal pressure plays an appreciable role on reproduction of the self-sustained oscillation of vocal folds, and iv) the strength of pulsating airflow and vortical structures are primarily affected by glottal width and subglottal pressure, and are closely related to pitch, loudness, and voice quality. Finally, more comprehensive explanation about the difference between one- and two-mass models is presented with discussion of effectiveness of vocal folds oscillation and voice quality.

• 대한기계학회논문집B
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• 제22권12호
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• pp.1763-1773
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• 1998

In the present experimental study, the flow behavior in a semi-closed two-phase natural circulation loop was examined. Water was used as the working fluid. Heat flux, heater-inlet subcooling, and flow restrictions at the heater-inlet and at the expansion-tank-line were taken as the controlling parameters Six circulation modes were identified by changing heat flux and inlet subcooling conditions ; single-phase continuous circulation, periodic circulation (A), two-phase continuous circulation, and periodic circulations (B), (C), and (D). Among these, the single-phase and two-phase continuous-circulation modes exhibit no significant oscillations and are considered to be stable. Periodic circulation (A) is characterized by the large amplitude two-phase f10w oscillations with the temporal single-phase circulation between them, while periodic circulation (B) featured by the flow oscillations with continuous boiling inside the heater section. Periodic circulation (C) appears to be the manometric oscillation with continuous boiling. Periodic circulation (D) has the longer period than periodic circulation (B) and a substantial amount of liquid flow back and forth through the expansion-tank-line periodically ; this mode is considered the pressure drop oscillation. Parametric study shows that the increases of the inlet- and expansion-tank-line- restrictions and the decrease of inlet subcooling broaden the range of the stable two-phase(continuous circulation) mode.

• 대한기계학회논문집B
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• 제22권3호
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• pp.267-279
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• 1998

A numerical study was made of flow behind a circular cylinder in a uniform flow, where the cylinder was rotationally oscillated in time. The temporal behavior of vortex formation was scrutinized over broad ranges of the two externally specified parameters, i.e., the dimensionless rotary oscillating frequency (.110.leq. $S_{f}$.leq..220) and the maximum angular amplitude of rotation (.theta.$_{max}$=15 deg., 30 deg. and 60 deg.). The Reynolds number (Re= $U_{{\inf}D}$.nu.) was fixed at Re=110. A fractional-step method was utilized to solve the Navier-Stokes equations with a generalized coordinate system. The main emphasis was placed on the initial vortex formations by varying $S_{f}$ and .theta.$_{max}$. Instantaneous streamlines and pressure distributions were displayed to show the vortex formation patterns. The vortex formation modes and relevant phase changes were characterized by measuring the lift coefficient ( $C_{L}$) and the time of negative maximum $C_{L}$( $t_{-C}$$_{Lmax}$) with variable forcing conditions.s.tions.s.s.s.