• Title/Summary/Keyword: Oscillating flow

Search Result 334, Processing Time 0.019 seconds

Numerical Analysis of Wave Field in OWC Chamber Using VOF Model

  • Liu, Zhen;Hyun, Beom-Soo;Jin, Ji-Yuan
    • Journal of Ocean Engineering and Technology
    • /
    • v.22 no.2
    • /
    • pp.1-6
    • /
    • 2008
  • Recently Oscillating Water Column (OWC) plants have been widely employed in wave energy conversion applications. It is necessary to investigate the chamber and optimize its shape parameters for maximizing air flow and energy conversion due to wave conditions. A 2D numerical wave tank based on a Fluent and VOF model is developed to generate the incident waves and is validated by theoretical solutions. The oscillating water column motion in the chamber predicted by the numerical method is compared with the available experimental data. Several geometric scales of the chamber are calculated to investigate the effect of the shape parameters on the oscillating water column motion and wave energy conversion.

Effect of Internal Fluid Resonance on the Performance of a Floating OWC Device

  • Cho, Il Hyoung
    • Journal of Ocean Engineering and Technology
    • /
    • v.35 no.3
    • /
    • pp.216-228
    • /
    • 2021
  • In the present study, the performance of a floating oscillating water column (OWC) device has been studied in regular waves. The OWC model has the shape of a hollow cylinder. The linear potential theory is assumed, and a matched eigenfunction expansion method(MEEM) is applied for solving the diffraction and radiation problems. The radiation problem involves the radiation of waves by the heaving motion of a floating OWC device and the oscillating pressure in the air chamber. The characteristics of the exciting forces, hydrodynamic forces, flow rate, air pressure in the chamber, and heave motion response are investigated with various system parameters, such as the inner radius, draft of an OWC, and turbine constant. The efficiency of a floating OWC device is estimated in connection with the extracted wave power and capture width. Specifically, the piston-mode resonance in an internal fluid region plays an important role in the performance of a floating OWC device, along with the heave motion resonance. The developed prediction tool will help determine the various design parameters affecting the performance of a floating OWC device in waves.

A Closed Counter-Current Two-Phase Thermosyphon Loop of a Cold Neutron Source in HANARO Research Reactor (하나로 원자로에 설치될 대향 이상 열사이펀 루프에 관한 실험)

  • Hwang, Kwon-Sang;Cho, Man-Soon;Sung, Hyung-Jin
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.24 no.8
    • /
    • pp.1038-1045
    • /
    • 2000
  • An experimental study was carried out to delineate the flow characteristics in a closed countescurrent two-phase thermo syphon with concentric tubes. This is to be installed in the HANARO research reactor as a part of a Cold Neutron Source(CNS). In the present investigation, experiments ata room temperature with Freon-II3 as a moderator were performed. Results show that, based on the magnitude of pressure fluctuation, the flow regimes could be divided into 4 distinct ones in the ($V_f,\;Q_i$) plane, where $V_f$ represents the volume of the charged liquid and $Q_i$ the heat load: a stable flow regime, an oscillatory flow regime, a restablized flow regime and a dryout flow regime. For $V_f$>2.5l, the flow is stable at low $Q_i$. However, as $Q_i$ increases, the flow becomes oscillatory and finally restablizes As $V_f$ increases, the oscillation amplitude decreases, reaching to the restablized flow region at low $Q_i$, and the liquid level in the moderator cell remains high. In the oscillatory flow regimes, for a fixed VI; the oscillating period of time varies with $Q_i$, having a minimum value at a certain value of $Q_i$. The heat load, where the oscillating period of time is minimum, decreases as $V_f$ increases.

Shape Optimization of an Active Micro-Mixer for Improving Mixing Efficiency (혼합 효율 향상을 위한 마이크로 동적 믹서의 형상최적화)

  • Park, Jae-Yong;Kim, Sang-Rak;Lee, Won-Gu;Yoo, Jin-Sik;Kim, Young-Dae;Maeng, Joo-Seung;Han, Seog-Young
    • Transactions of the Korean Society of Machine Tool Engineers
    • /
    • v.16 no.6
    • /
    • pp.146-152
    • /
    • 2007
  • An active micro-mixer, which was composed of an oscillating micro-stirrer in the microchannel to provide rapid, effective mixing at high flow, rates was analyzed. The effects of molecular diffusion and disturbance by the stirrer were considered with regard to two types of mixer models: the simple straight microchannel and microchannel with an oscillating stirrer. Two types of mixer models were studied by analyzing mixing behaviors such as their interaction after the stirrer. The mixing was calculated by Lattice Boltzmann methods using the D2Q9 model. In this study, the time-averaged mixing index formula was used to estimate the mixing performance of time-dependent flow. The mixing indices of the two models compared. From the results, it was found that the mixer with an oscillating stirrer was much more enhanced and stabilized. Therefore, an optimum design for a dynamic micro-mixer with an oscillating stirrer was performed using Taguchi method in order to obtain a robust solution. The design parameters were established as the frequency, the length and the angle of the stirrer and the optimal values were determined to be 2, 0.8D and ${\pm}75^{\circ}$, respectively. It was found that the mixing index of the optimal design increased 80.72% compared with that of the original design.

An Approximate Analytical Method for Hydrodynamic Forces on Oscillating Inner Cylinder in Concentric Annulus (동심원내에서 진동하는 내부 실린더에 작용하는 유체유발력의 근사적 해법)

  • 심우건
    • Journal of KSNVE
    • /
    • v.7 no.5
    • /
    • pp.861-869
    • /
    • 1997
  • An approximate analytical method has been developed for estimating hydrodynamic forces acting on oscillating inner cylinder in concentric annulus. When the rigid inner cylinder executes translational oscillation, fluid inertia and damping forces on the oscillating cylinder are generated by unsteady pressure and viscous skin friction. Considering the dynamic-characteristics of unsteady viscous flow and the added mass coefficient of inviscid fluid, these hydrodynamic forces including viscous effect are dramatically simplified and expressed in terms of oscillatory Reynolds number and the geometry of annular configuration. Thus, the viscous effect on the forces can be estimated very easily compared to an existing theory. The forces are calculated by two models developed for relatively high and low oscillatory Reynolds numbers. The model for low oscillatory Reynolds number is suitable for relatively high ratio of the penetration depth to annular space while the model for high oscillatory Reynolds number is applicable to the case of relatively low ratio. It is found that the transient ratio between two models is approximately 0.2~0.25 and the forcea are expressed in terms of oscillatory Reynolds number, explicity. The present results show good agreements with an existing numerical results, especially for high and low penetration ratios to annular gap.

  • PDF

Reduced Frequency Effects on the Near-Wake of an Oscillating Elliptic Airfoil

  • Chang, Jo-Won;Eun, Hee-Bong
    • Journal of Mechanical Science and Technology
    • /
    • v.17 no.8
    • /
    • pp.1234-1245
    • /
    • 2003
  • An experimental study was carried out to investigate the reduced frequency effect on the near-wake of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched around the center of the chord between -5$^{\circ}$and +25$^{\circ}$angles of attack at an airspeed of 3.4 m/s. The chord Reynolds number and reduced frequencies were 3.3 ${\times}$10$^4$, and 0.1, 0.7, respectively Phase-averaged axial velocity and turbulent intensity profiles are presented to show the reduced frequency effects on the near-wake behind the airfoil oscillating In pitch. Axial velocity defects in the near-wake region have a tendency to increase in response to a reduced frequency during pitch up motion, whereas it tends to decrease during pitch down motion at a positive angle of attack. Turbulent intensity at positive angles of attack during the pitch up motion decreased in response to a reduced frequency, whereas turbulent intensity during the pitch down motion varies considerably with downstream stations. Although the true instantaneous angle of attack compensated for a phase-lag is large, the wake thickness of an oscillating airfoil is not always large because of laminar or turbulent separation.

Computation of Viscous Flows around a Two-dimensional Oscillating Airfoil ( Part 1. without Dynamic Stall ) (진동하는 2차원 날개 단면 주위에 대한 점성 유동장 계산( Part 1. 동적실속이 없는 경우 ))

  • Lee, Pyoung-Kuk;Kim, Hyoung-Tae
    • Journal of the Society of Naval Architects of Korea
    • /
    • v.44 no.1 s.151
    • /
    • pp.8-15
    • /
    • 2007
  • In this paper, numerical calculations are performed to analyze the unsteady flow of NACA airfoil sections. In order to ease the flow computation for the fluid region changing in time, improve the quality of solution and simplify the grid generation for the oscillating foil flow, the computational method adopts a moving and deforming mesh with the multi-block grid topology. The multi-block, structured-unstructured hybrid grid is generated using the commercial meshing software Gridgen V15. The MDM (Moving & Deforming Mesh) and the UDF (User Define function) function of FLUENT 6 are adopted for computing turbulent flows of the foil in pitching motion. Computed unsteady lift and drag forces are compared with experimental data. in general, the characteristics of unsteady lift and drag of the experiments are reproduced well in the numerical analysis.

Effect of Reduced Frequency on the Flow Pattern of Pitch Oscillating Elliptic Airfoil (피치 진동하는 타원형 에어포일의 환산주파수가 날개 주위 유동패턴에 미치는 영향)

  • Lee, Ki-Young;Chung, Hyong-Seok;Sohn, Myong-Hwan
    • Journal of the Korea Institute of Military Science and Technology
    • /
    • v.9 no.4
    • /
    • pp.128-136
    • /
    • 2006
  • The purpose of this paper is to examine the dynamic stall characteristics of an elliptic airfoil when subject to constant pitch motions. In this study, which was motivated by the pressing need for a greater understanding of the reduced frequency$({\kappa})$ effects on flow patterns of elliptic airfoil, the various reduced frequencies were considered. The result confirms that the reduced frequency has a profound effects on the flow patterns. The increase of ${\kappa}$ accelerate the separation bubble bursting process up to ${\kappa}=0.10$, then diminish with further increase in ${\kappa}$. Compared with static condition, the dynamic pitching airfoil delays stall angle approximate $4{\circ}{\sim}5{\circ}$ during pitch-up stroke for ${\kappa}=0.10$. Results from this qualitative analysis provided valuable insight Into the control of dynamics stall.

Experimental Investigation of the flow around an Oscillating Circular Cylinder by Using a PIV System (진동하는 원형주상체 주위의 유동에 관한 PIV를 이용한 실험적 연구)

  • Song Museok;Lee Sang-Dae
    • Journal of the Korean Society for Marine Environment & Energy
    • /
    • v.6 no.1
    • /
    • pp.60-67
    • /
    • 2003
  • Flow around an oscillating circular cylinder was experimentally investigated. With varying Keulegen-Carpenter(KC) number from 10 to 30 the flow field with vortex sheddings and the related hydrodynamic fortes exerting on the cylinder were measured. A newly developed PW(paricle image velocimetry) successfully captured the complex vortical flows varying with the KC number and the flow patterns were 'traverse street', 'single pairing' and 'double pairing' of vortices with increasing KC number, At a certain KC number range the lift force undergoes a transition showing little periodicity due to surrounding complicated shedded vortices.

  • PDF

Comparison of multi-stage explicit methods for numerical computation of the unsteady Navier-Stokes equations (비정상 Navier-Stokes 방정식의 수치해석을 위한 다단계 외재법의 성능 비교)

  • Seo,Yong-Gwon
    • Transactions of the Korean Society of Mechanical Engineers B
    • /
    • v.21 no.2
    • /
    • pp.202-212
    • /
    • 1997
  • In this study, performance of the multi-stage explicit methods for numerical computation of the unsteady Navier-Stokes equations is investigated. Three methods under consideration are 1 st-, 2 nd-, and 4 th-order Runge-Kutta (R-K) methods. Compared in this estimation is stability, accuracy, and CPU time of each method. The computational codes developed are applied to the two-dimensional flow in a square cavity driven by an oscillating lid. It turned out that at Reynolds number 400, the 1 st-order R-K method is the best, while at 3200 the 2 nd-order R-K is recommended. At higher Reynolds numbers, it is conjectured that the 4 th-order R-K method will be the best algorithm among three due to its highest stability.