• Proceedings of the KSME Conference
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• 2000.04b
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• pp.472-478
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• 2000

An experimental study is performed to analyze flow structures behind a local suction/blowing in a flat-plate turbulent boundary layer, The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about $Re_{\theta}=1700$. The effects of local forcing are scrutinized by altering the forcing frequency $(0.011{\leq}f^+{\leq}0.044)$. The forcing amplitude is fixed at $A_0=0.4$. It is found that a small local forcing reduces the skin friction, and this reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the coherent structures. Velocity signals are decomposed into a periodic part and a fluctuating part. An organized spanwise vortical structure is generated by the local forcing. The larger reduction of skin friction for the higher forcing frequencies is attributed to the diminished adverse effect of the secondary vortex. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures.

• Journal of applied mathematics & informatics
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• v.3 no.2
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• pp.169-182
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• 1996

This paper presents the numerical experiment of a dis-cretized loaded cable with periodic forcing. There appeared to be var-ious type of nonlinear oscillations over a wide range of fequencies and amplitudes for the periodic forcing term. The same forcing term can give rise to large or small oscillation by solving initial value problem and observing the solutions after a long time.

• The Pure and Applied Mathematics
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• v.21 no.4
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• pp.281-293
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• 2014

We investigate long-term motions of the cable when cable has different types of periodic forcing term. Various different types of solutions are presented by using the 2nd order Runge-Kutta method under various initial conditions. There appeared to be small- and large-amplitude solutions which have different nodal structure.

• Journal of the Chungcheong Mathematical Society
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• v.33 no.3
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• pp.351-363
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• 2020

We introduce a new concept of Stepanov weighted pseudo almost periodic functions of class r which have been established by recently in [20]. Furthermore, we study the uniqueness and existence of Stepanov weighted pseudo almost periodic mild solutions of partial neutral functional differential equations having the Stepanov pseudo almost periodic forcing terms on finite delay.

• Journal of the Korean Mathematical Society
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• v.60 no.2
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• pp.465-477
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• 2023

In this study, we consider a heat equation with a variable-coefficient linear forcing term and a time-periodic boundary condition. Under some decay and smoothness assumptions on the coefficient, we establish the existence and uniqueness of a time-periodic solution satisfying the boundary condition. Furthermore, possible connections to the closed boundary layer equations were discussed. The difficulty with a perturbed leading order coefficient is demonstrated by a simple example.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.11-16
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• 2002

In this paper, we present two successful results from active controls of flows over a circular cylinder and a sphere for drag reduction. The Reynolds number range considered for the flow over a circular cylinder is 40-3900 based on the free-stream velocity and cylinder diameter, whereas for the flow over a sphere it is $10^{5}$ based on the free-stream velocity and sphere diameter. The successful active control methods are a distributed (spatially periodic) forcing and a high-frequency (time periodic) forcing. With these control methods, the mean drag and lift fluctuations decrease and vortical structures are significantly modified. For example, the time-periodic forcing at a high frequency (larger than 20 times the vortex shedding frequency) produces $50{\%}$ drag reduction for the flow over a sphere at $Re=10^{5}$. The distributed forcing applied to the flow over a circular cylinder results in a significant drag reduction at all the Reynolds numbers investigated.

• Journal of the Korean Mathematical Society
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• v.42 no.2
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• pp.191-202
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• 2005

A condition on forcing term insuring the multiplicity of Dirichlet-periodic solutions of nonlinear dissipative hyperbolic equations is discussed. The nonlinear term is assumed to have coercive growth.

• Journal of the Korean Society of Visualization
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• v.1 no.1
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• pp.64-74
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• 2003

An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. The local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f$^{+}$=0.044, 0.066 and 0.088) with a fixed forcing amplitude (A$^{+}$=0.6) were employed at $Re_{=690. The effect of the forcing angles ($\alpha$=60$^{\circ}$ , 90$^{\circ}$ and 120$^{\circ}$ ) was investigated under the fixed forcing frequency (f$^{+}$=0.088). The PIV results showed that the wall region velocity decreases on imposition of the local forcing. Inspection of phase-averaged velocity profiles revealed that spanwise large-scale vortices were generated in the downstream of the slot and persist further downstream. The highest reduction in skin friction was achieved at highest forcing frequency (f$^{+}$=0.088) and a forcing angle of $\alpha$=120$^{\circ}$. The spatial fraction of the vortices was examined to analyze the skin friction reduction.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.5
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• pp.573-583
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• 1998

Effects of the oscillating incoming flow on vortex shedding and lock-on behind a square cylinder are investigated using numerical simulations at a Reynolds number of 100. Vortex shedding occurred at low forcing frequencies of the incoming flow similar to the natural vortex shedding. As the forcing frequency further increases, the shedding frequency decreases to the half of the forcing freqnency. For a sufficiently large frequency, vortex shedding returns to the natural vortex shedding irrespective of the forcing amplitude. Also, the lock-on region becomes wider with higher forcing amplitudes. The phase diagram between the drag and lift shows a simple periodic behavior in the lock-on region, while a complicated periodic phase relation is observed when there is no lock-on.

• Korean Journal of Mathematics
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• v.23 no.4
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• pp.521-536
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• 2015

We investigate the nonlinear motions of discrete loaded cable with different periodic forcing. We present the numerical evidence of the nonlinear motions of the cable by solving initial value problems and obtaining the motions after a long time. There appeared to be various types of nonlinear oscillations over a wide range of frequencies and amplitudes for the periodic forcing term.