• International Journal of Naval Architecture and Ocean Engineering
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• 제11권2호
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• pp.923-938
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• 2019

Surge motion of top-end platform induced by periodic wave makes marine flexible riser encounter equivalent sheared-oscillatory flow, under which the Vortex-induced Vibration (VIV) response will be more complicated than pure sheared flow or oscillatory flow cases. Based on a time domain force-decomposition model, the VIV response characteristics under sheared-oscillatory flows are investigated numerically in this paper. Firstly, the adopted numerical model is validated well against laboratory experiments under sheared flow and oscillatory flow. Then, 20 sheared-oscillatory flow cases with different oscillation periods and top maximum current velocities are designed and simulated. Under long and short oscillation period cases, the structural response presents several similar features owing to the instantaneous sheared flow profile at each moment, but it also has some different patterns because of the differently varying flow field. Finally, the effects and essential mechanism of oscillation period and top maximum current velocity on VIV response are discussed systematically.

• Journal of Advanced Marine Engineering and Technology
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• 제25권4호
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• pp.817-824
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• 2001

In the present study, flow characteristics of turbulent oscillatory flows in the exit region connected to the square-sectional $180^{\circ}$curved duct was investigated experimentally. The experimental study for air flows was conducted to measure velocity profiles, shear stress distributions by using the Laser Doppler Velocimetry(L.D.V) system with the data acquisition and processing system of Rotating Machinery Resolver(R.M.R) and PHASE software. The results obtained from the experimentation were summarized as follows : The critical Reynolds number for a change from transitional oscillatory flow to turbulent oscillatory flow was about 75,000 in the 90 region of dimensionless axial position (x/Dh) which was considered as a fully developed flow region. In the turbulent oscillatory flow, velocity profiles of the inflow period in the entrance region were gradually developed, but those of the outflow period were not changed nearly. Shear stress distributions of turbulent oscillatory flow was gradually increased as the flow proceeds to downstream.

• 대한기계학회논문집B
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• 제26권10호
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• pp.1378-1386
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• 2002

In the present study, velocity profile and wall shear stress distributions of developing turbulent oscillatory flows in an oscillator connected to straight duct located in exit region of a curved duct was investigated experimentally. The experimental study for air flows was conducted to measure axial velocity profiles, shear stress distributions by using the Laser Doppler Velocimetry(LDV) system with the data acquisition and processing system of Rotating Machinery Resolver(R.M.R) and PHASE software. The results obtained from experimental studies are summarized as follows. The critical Reynolds number for a change from transitional oscillatory flow to turbulent flow was about 7500, in the 60region of dimensionless axial position which was considered as a fully developed flow region. The turbulent oscillatory flow, velocity profiles of the inflow period in the entrance region were gradually developed, but those of the outflow period were not changed nearly. Velocity profiles of inflow and outflow were shown as a symmetric form in a fully developed flow region. The wall shear stress distributions of turbulent oscillatory flow increase rapidly as the flow proceeds to downstream and flow was in good agreement with the theoretically.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1995년도 추계학술발표회논문집(1)
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• pp.475-479
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• 1995

During the hypothetical loss of coolant accident in the nuclear power plant the emergency core cooling water could not penetrate to the reactor core when the steam flow rate from the reactor core exceeds CCFL (Countercurrent flow limitation). The CCFL generated by earlier investigators are developed under the steady gas flow. However the flow instability in the reactor loop could generate oscillatory steam flow, hence their applicability under oscillating flow should be investigated. In this work, an experimental investigation of countercurrent flow in the vertical flow channel has been conducted under oscillatory gas flow. Pulsation of gas under oscillatory flow disturbs the flow pattern significantly and prevents flooding (CCFL) when its minimum value is less than the threshold gas flow rate value.

• 한국전산유체공학회지
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• 제5권3호
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• pp.40-46
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• 2000

Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows from steady to oscillatory convection is investigated for the fluid with Pr=0.1. The unsteady streamfunction-vorticity equation is solved with finite difference method. As Rayleigh number is increased, the steady crescent-shaped flow bifurcates to a time-periodic flow with like-rotating eddies. And afterwards, a transition to an oscillatory multicellular flow with a counter-rotating eddy on the top of the annulus occurs. A transition from steady to an oscillatory flow occurs, but dual solutions and hysteresis phenomena are not observed.

• Korea-Australia Rheology Journal
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• 제19권3호
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• pp.117-123
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• 2007

There has been considerable interest in recent years in field-responsive suspensions, which are of some importance in industry in many different applications. The microstructure of these materials is a significant issue which can be probed by rheological measurements. In this study, measurements were made of a magnetorheological fluid (MRF) under steady and oscillatory shear flow, with and without a magnetic field. Mathematical inversion was used to derive the relaxation time spectrum of the MRF from oscillatory shear data. Experimental evidence is presented of the gel-like properties of this MRF.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2248-2253
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• 2008

A numerical study on oscillatory thermocapillary flow in half-zone has performed to understand the effect of axial rotation. 2d unsteady code is developed to observe the onset of oscillation. 2cs Silicone oil with Prandtl number of 26.5 is used as a working fluid. The critical temperature difference at onset of oscillation is investigated under the different aspect ratios and rotation modes. It is shown that the onset of oscillation is delayed when aspect ratio reduces and rotating speed increases. The oscillatory flow is strongly reduced under top rotation and co-rotation modes, while it is augmented under bottom rotation and counter-rotation modes. It is thought that interaction between return flow and bottom wall is important to explain the oscillatory flow.

• 대한기계학회논문집B
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• 제22권2호
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• pp.139-152
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• 1998

In the present study, the flow characteristics of developing laminar oscillatory flows in a square -sectional 180 deg. curved duct are investigated experimentally. The experimental study using air in a square-sectional 180 deg. curved duct is carried out to measure velocity distributions with a data acquisition and LDV (Laser Doppler Velocimetry) processing system. In this system, Rotating Machinery Resolver (RMR) and PHASE program are used to obtain the results of unsteady flows. The major flow characteristics of developing oscillatory flows are found by analyzing velocity curves, mean velocity profiles, time-averaged velocity distribution of secondary flow, wall shear stress distributions, and entrance lengths. In a lower dimensionless angular frequency, the axial velocity distribution of laminar oscillatory flow in a curved duct shows a convex shape in a central part and axial symmetry. The maximum value of wall shear stress in a lower dimensionless angular frequency is located in an outside wall, but according to increasing the dimensionless angular frequency, the maximum of wall shear stress is moved to inner wall. The entrance lengths of laminar oscillatory flows in a square-sectional 180 deg. curved duct is obtained to 90 deg. of bended angle of duct in this experimental conditions.

• 소음진동
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• 제7권3호
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• pp.489-498
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• 1997

In this paper, chaotic motions of a curved pipe conveying oscillatory flow are theoretically investigated. The nonliear partial differential equation of motion is derived by Newton's method. The transformed nonlinear ordinary differential equation is a type of Hill's equation, which has the external and parametric excitation with a same frequency. Bifurcation curves of chaotic motion of the piping systems are obtained by applying Melnikov's method. Numerical simulations are performed to demonstrate theoretical results and show the strange attractor of the chaotic motion.

• 한국전산유체공학회지
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• 제10권4호통권31호
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• pp.32-38
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• 2005

Present study simulates oscillatory supersonic impinging jet flows using the axisymmetric Navier-Stokes code. To capture the salient features of flow oscillation and overcome the divergence during the initial transient period, several tests have been conducted for the grid and time step sizes. The results also show that the effects of the inlet flow condition at the nozzle exit and turbulence on the oscillatory behavior of supersonic impinging jets are negligible. Frequencies of the surface pressure oscillation obtained by the selected numerical method are in good accord with the measured impinging tones for various cases of nozzle-to-plate distance. Two seemingly different staging behaviors with nozzle-to-plate distance and nozzle pressure variations are found to correlate well if the frequency and distance are normalized by the length of the first shock cell.