• Ocean Systems Engineering
• /
• 제3권3호
• /
• pp.167-180
• /
• 2013

Hydrodynamic characteristics of a bluff cylinder oscillating along transverse direction in steady flow were experimentally investigated at Reynolds number of $2{\times}10^5$. The effects of non-dimensional frequency, oscillating amplitude and Reynolds number on drag force, lift force and phase angle are studied. Vortex shedding mechanics is applied to explain the experimental results. The results show that explicit similarities exist for hydrodynamic characteristics of an oscillating cylinder in high and low Reynolds number within subcritical regime. Consequently, it is reasonable to utilize the test data at low Reynolds number to predict vortex induced vibration of risers in real sea state when the Reynolds numbers are in the same regime.

• 한국항해항만학회지
• /
• 제27권6호
• /
• pp.653-659
• /
• 2003

The main objective of this study is to estimate the hydrodynamic forces and to investigate the nonlinear behaviors of fluid motion around the oscillating body on or below a free surface. We have developed a composite grid method to solve the radiation problems. This method is applied to numerical computation of the radiation forces generated by the oscillating body. The numerical results obtained by the present method are compared with the experimental data and a linear potential theory. As a result, we can confirm the accuracy of the present method. Finally, we have evaluated the effect of viscosity on the hydrodynamic forces acting on the oscillating body.

• 한국초전도ㆍ저온공학회논문지
• /
• 제2권1호
• /
• pp.45-50
• /
• 2000

The present study has been conducted to observe the heat transfer under pulsating pressure and oscillating flow. The experimental apparatus was fabricated to measure the gas temperature, the wall temperature. the pressure and the instantaneous heat flux inside the pulse tube. The measured gas temperature and heat flux must be calibarated to compensate their finite time constant in the oscillating flow conditious. The experiment was performed from 1 Hz to 5 Hz. The phase difference between the instantaneous heat flux and the gas-wall temperature difference was clearly observed. The experimental heat fluxes were compared to the theroretical correlations such as Complex Nusselt Number Model(CNNM) and Variable Coefficient Model(CVM). The heat flux predisted by CNNM was always greater than that of VCM. The experiment confirmed the valisity of the VCM for the instantaneous heat flux under the pulsating pressure and oscillating flow in the warm end of the pulse tube.

• 한국통신학회논문지
• /
• 제10권4호
• /
• pp.193-200
• /
• 1985

정현적으로 진동하는 유전체 경계면으로 부터의 전자파(TE)의 반사와 투과현상을 진동하는 임의의 순간에는 등속운동이라 가정하여 Lorentz변환을 확대 적용하여 이론적으로 해석하였다. 전자파의 경계면에 입사각, 운동매진의 유선율 및 진동속도의 함수로써 반사, 투과계수, 전력반사, 투과율의 평균치를 수치계산하여 검토하였다.

• 대한기계학회논문집B
• /
• 제31권6호
• /
• pp.566-573
• /
• 2007

Flows around two oscillating cylinders in side-by-side arrangement at Re=185 are simulated using an immersed boundary method. The cylinders oscillate vertically in opposite directions in uniform cross-flow. We describe flow patterns, drag and lift forces by varying distance between two cylinders $(1.4{\leq}g{\leq}2.2,\;1.0{\leq}g{\leq}1.8)$ and oscillating frequency ratios $(f_e/f_o=0.8,\;f_e/f_o=1.0\;and\;f_e/f_o =1.2)$. Wake patterns, drag and lift coefficients are affected by both of frequency ratio and gap between two cylinders. Near wakes of each case are classified with the definition of previous studies.

• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2000년도 춘계학술대회논문집
• /
• pp.718-723
• /
• 2000

Dynamic stability of an axially oscillating cantilever beam with a concentrated mass is investigated in this paper. The equations of motion are derived and the derived equations include harmonically oscillating parameters which originate from the motion-induced stiffness variation. Under certain conditions of the frequency and the amplitude of oscillating motion, parametric instabilities may occur. The multiple scale perturbation method is employed to obtain the stability analysis results. It is found that the system stability varies with the magnitude or the location of the concentrated mass. Instability increases as the concentrated mass approaches to the free-end or its magnitude increases.

• 한국전산유체공학회:학술대회논문집
• /
• 한국전산유체공학회 2011년 춘계학술대회논문집
• /
• pp.434-440
• /
• 2011

Oscillating airfoil haw been challenged for the dynamic stalls of airfoil am wind turbines at high angle of attach. Especially, the pressure oscillation has a huge effect on noise generation, structure damage, aerodynamic performance am safety, because the flow has strong unsteadiness at high angle of attack. In this paper, the unsteady aerodynamics coefficients were analyzed for the oscillating airfoil at high angle of attack around two dimensional NACA0012 airfoil. The two dimensional unsteady compressible Navier-Stokes equation with a LES turbulent model was calculated by OHOC (Optimized High-Order Compact) scheme. The flow conditions are Mach number of 0.2 and Reynolds number of $1.2{\times}10^4$. The lift, drag, pressure distribution, etc. are analyzed according to the pitching oscillation. Unsteady velocity field, periodic vortex shedding, the unsteady pressure distribution, and the acoustic fields are analyzed. The effects of these unsteady characteristics in the aerodynamic coefficients are analyzed.

• 한국전산유체공학회지
• /
• 제10권2호
• /
• pp.60-68
• /
• 2005

For study on the unsteady wall interference effect, flows around a forced oscillating airfoil in closed test-section wind tunnels have been numerically investigated by solving compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with the Roe's flux-difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The Spalart-Allmaras one-equation model is employed for the turbulence effect. The computed results of the oscillating airfoil having a thin wake showed that the lift curve slope is increased and the magnitude of hysteresis loop is reduced by the interference effects. Since the vortex around the airfoil is generated and convected downstream faster than the free-air condition, the phase of lift, drag and pitching moment coefficients was shifted. The pressure on the test section wall shows harmonic terms having the oscillating frequency contained in the wail effect.

• 대한기계학회논문집B
• /
• 제29권11호
• /
• pp.1189-1198
• /
• 2005

Direct numerical simulations were performed to investigate the physics of a spatially developing turbulent boundary layer flow subjected to spanwise oscillating electromagnetic forces in the near wall region. A fully implicit fractional step method was employed to simulate the flow. The mean flow properties and the Reynolds stresses were obtained to analyze the near-wall turbulent structure. It is found that skin friction and turbulent kinetic energy can be reduced by the electromagnetic forces. The decrease in production is responsible fur the reduction of turbulent kinetic energy. Instantaneous flow visualization techniques were used to observe the response of streamwise vortices and streak structures to spanwise oscillating forces. The near-wall vortical structures are affected by spanwise oscillating electromagnetic forces. Following the stopping of the electromagnetic force, the flow eventually relaxes back to a two-dimensional equilibrium boundary layer.

• 한국안전학회지
• /
• 제21권2호
• /
• pp.7-14
• /
• 2006

A Problem of low-velocity forced convection in a channel flow with heated wall is of practical importance and widely considered in the design of devices such as heat exchangers, and electronic equipments. Therefore, there is an urgent need for improving heat transfer performance of heated wall in the channel. In the present study, an oscillating vortex generator method is proposed to enhance the heat transfer in a channel. In this method, a rectangular bars are set in the upstream of heated region of the channel. The bars are forced to oscillate normal to the inflow, and then actively and largely generates transverse vortices behind the bars. As a result, this apparatus can enhance the heat transfer rates remarkably. Because of the interaction between the flow and oscillating bars, the variations of the flow and thermal fields become time-dependent state.