• 대한조선학회논문집
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• 제40권5호
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• pp.26-35
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• 2003

Propellers operating in a given nonuniform ship wake generate unsteady loads leading to undesirable stern vibration problems. The skew is known to be the most proper and effective geometric parameter to control or reduce the fluctuating forces on the shaft. This paper assumes the skew profile as either a quadratic or a cubic function of the radius and determines the coefficients of the polynomial function by applying the simplex method. The method uses the converted unconstrained algorithm to solve the constrained minimization problem of 6-component shaft excitation forces. The propeller excitation was computed either by applying the two-dimensional gust theory for quick estimation or by the fully three-dimensional unsteady lifting surface theory in time domain for an accurate solution. A sample result demonstrates that the shaft forces can be further reduced through optimization from the original design.

• 연구논문집
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• 통권27호
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• pp.15-34
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• 1997

The 3-dimensional unsteady compressible flows around the high speed train have been simulated for the train entering a tunnel and for passing another train. The simulation method employs the implicit approximation-factorization finite difference algorithm for the inviscid Euler equations in general curvilinear coordinates. A moving grid scheme is applied in order to resolve the train movement relative to the tunnel and the other train. The velo-city and pressure fields and pressure drag are calculated to study the effects of tunnel and the other train. The side directional force which is time dependent is also computed for the passing train. Pressure distribution shows that the compression wave is generated in front of the train noise just after the tunnel entrance and proceeds along the inside of tunnel.

• 항공우주시스템공학회지
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• 제10권1호
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• pp.29-34
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• 2016

Flutter wind-tunnel test is an expensive and complicated process. Also, the test model may has discrepancy in the structural characteristics when compared to those of the real model. "Dry Wind-Tunnel" (DWT) is an innovative testing system which consists of the ground vibration test (GVT) hardware system and software which computationally can be operated and feedback in real-time to yield rapidly the unsteady aerodynamic forces. In this paper, we study on the aerodynamic forces of DWT system to feedback in time domain. The aerodynamic forces in the reduced-frequency domain are approximated by Minimum-state approximation. And we present a state-space equation of the aerodynamic forces. With the two simulation model, we compare the results of the flutter analysis.

• 대한기계학회논문집B
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• 제30권5호
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• pp.480-488
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• 2006

Numerical simulations of flow over two spheres placed in a tandem arrangement are conducted to investigate the effect of the inter-sphere spacing on the flow characteristics. The Reynolds numbers considered are 100, 250, 300 and 425, corresponding to steady axisymmetric, steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric flows, respectively, in the case of a single sphere. For small inter-sphere spacings, the flow past two spheres is more stable than that past a single sphere. For example, with the spacing of the sphere radius, the flow is steady axisymmetric up to Re=300. However, for relatively large spacings, the flow past two spheres becomes unstable and vortex shedding takes place even at Re=250. The drag coefficient of the rear sphere decreases significantly with decreasing inter-sphere spacing due to reduction of the stagnation pressure, thus being smaller than that of the front sphere. Also, the rear sphere shows large fluctuations of the lift force as compared to the front one in the case of unsteady flow.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제27권4호
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• pp.232-244
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• 2023

The unsteady dynamics of the starting flow of a flat plate is studied by using a vortex shedding model. The model describes the body and separated vortex from the trailing edge of the plate by vortex sheets, retaining a singularity at the leading edge. The model is applied to simulate the flow of an accelerated plate for small angles of attack. For numerical computations, we take two representative cases of the translational velocity of a plate: impulsive translation and uniform acceleration. The model successfully demonstrates the formation of wakes shed from the plate. The wake behind the plate is stronger for a larger angle of attack. Predictions for the lifting force from the model are in agreement with results of Navier-Stokes simulations.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.148-153
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• 2001

A computational investigation of the effect of the electromagnetic force(or Lorentz force) on the flow behavior around a circular cylinder, a typical model of bluff bodies, is conducted. Two-dimensional unsteady flow computation for $Re=10^2$ is carried out using a numerical method of finite difference approximation in a curvilinear body-fitted coordinate system by solving the momentum equations including the Lorentz force as a body force. The effect of the spatial variations of the Lorentz forcing region and forcing direction along the cylinder circumference is investigated. The numerical results show that the Lorentz force can effectively suppress the flow separation and oscillation of the lift force of the circular cylinder cross-flow, leading to the reduction of the drag.

• Journal of Mechanical Science and Technology
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• 제16권3호
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• pp.363-375
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• 2002

The effect of the electromagnetic force (or Lorentz force) on the flow behavior around a circular cylinder is investigated by computation. Two-dimensional unsteady flow computation for Re=10$^2$is carried out using a numerical method of finite difference approximation in a curvilinear body-fitted coordinate system by solving the momentum equations including the Lorentz force as a body force. The effect of spatial variations of the Lorentz forcing region and forcing direction along the cylinder circumference is investigated. The numerical results show that the Lorentz force can effectively suppress the flow separation and oscillation of the lift force of circular cylinder cross-flow, leading to reduction of drag.

• 상하수도학회지
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• 제22권6호
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• pp.609-617
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• 2008

Water pipes are supposed to deliver the predetermined demand safely to a certain point in water distribution system. However, pipe burst or crack can be happened due to so many reasons such as the water hammer, natural pipe ageing, external impact force, soil condition, and various environments of pipe installation. In the present study, the reliability model which can calculate the probability of pipe breakage was developed regarding unsteady effect such as water hammer. For the reliability model, reliability function was formulated by Barlow formula. AFDA method was applied to calculate the probability of pipe breakage. It was found that the statistical distribution for internal pressure among the random variables of reliability function has a good agreement with the Gumbel distribution after unsteady analysis was performed. Using the present model, the probability of pipe breakage was quantitatively calculated according to random variables such as the pipe diameter, thickness, allowable stress, and internal pressure. Furthermore, it was found that unsteady effect significantly increases the probability of pipe breakage. If this reliability model is used for the design of water distribution system, safe and economical design can be accomplished. And it also can be effectively used for the management and maintenance of water distribution system.

• International Journal of Fluid Machinery and Systems
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• 제3권1호
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• pp.58-66
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• 2010

A steam control valve is used to control the flow from the steam generator to the steam turbine in thermal and nuclear power plants. During startup and shutdown of the plant, the steam control valve is operated under a partial flow conditions. In such conditions, the valve opening is small and the pressure deference across the valve is large. As a result, the flow downstream of the valve is composed of separated unsteady transonic jets. Such flow patterns often cause undesirable large unsteady fluid force on the valve head and downstream pipe system. In the present study, various flow patterns are investigated in order to understand the characteristics of the unsteady flow around the valve. Experiments are carried out with simplified two-dimensional valve models. Two-dimensional unsteady flow simulations are conducted in order to understand the experimental results in detail. Scale effects on the flow characteristics are also examined. Results show three types of oscillating flow pattern and three types of static flow patterns.

• 대한조선학회논문집
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• 제45권1호
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• pp.29-41
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• 2008

The present paper provides a CFD analysis of diffraction problem for a ship with forward speed using an unsteady RANS simulation method, a WAVIS code. The WAVIS viscous solver adopting a finite volume method has second order accuracy in time and field discretizaions for the RANS equations. A two phase level-set method and a realizable ${\kappa}-{\varepsilon}$ turbulence model are adopted to compute the free surface and to meet the turbulence closure, respectively. To validate the capability of the present numerical methods for the simulation of an unsteady progressive regular wave, computations are performed for three grid sets with refinement ratio of ${\sqrt{2}}$. The main simulation is performed for a DTMB5512 model with a forward speed in a regular head sea condition. Validation of the present numerical method is carried out by comparing the present CFD results with available unsteady experimental data published in the 2005 Tokyo CFD Workshop: resistance, heave force, pitch moment, unsteady free surface elevations and velocity fields.