Ki-Deok Ro

Numerical analysis of a weak shock wave propagating in a medium using lattice boltzmann method (LBM)

This study introduced a lattice Boltzmann computational scheme capable of modeling thermo hydrodynamic flows with simpler equilibrium particle distribution function compared with other models. The equilibrium particle distribution function is the local Maxwelian equilibrium function in this model, with all the constants uniquely determined. The characteristics of the proposed model is verified by calculation of the sound speeds, and the shock tube problem. In the lattice Boltzmann method,a thermal fluid or compressible fluid model simulates the reflection of a weak shock wave colliding with a sharp wedge having various angles θw. Theoretical results using LBM are satisfactory compared with the experimental result or the TVD.

Numerical analysis of unsteady viscous flow through a Weis-Fogh-type ship propulsion mechanism using the advanced vortex method

The velocity and pressure fields of a ships Weis-Fogh type propulsion mechanism are studied in this paper using an advanced vortex method. The wing (NACA0010 airfoil) and channel are approximated by source and vortex panels, and free vortices are introduced away from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from an integral, based on the instantaneous velocity and vorticity distributions in the flow field. Two-dimensional unsteady viscous flow calculations of this propulsion mechanism are shown, and the calculated results agree qualitatively with the measured thrust and drag due to un-modeled large fluctuations in the measured data.

Numerical Analysis of Unsteady Flow in the Weis-Fogh Mechanism by the 3D Discrete Vortex Method With GRAPE3A

The three-dimensional flows in the Weis-Fogh mechanism are studied by flow visualization and numerical simulation by a discrete vortex method. In this mechanism, two wings open, touching their trailing edges (fling), and rotate in opposite directions in the horizontal plane. At the fling stage, the flow separates at the leading edge and the tip of each wing. Then they rotate, and the flow separates also at the trailing edges. The structure of the vortex systems shed from the wings is very complicated and their effect on the forces on the wings have not yet been clarified. Discrete vortex method, especially the vortex stick method, is employed to investigate the vortex structure in the wake of the two wings. The wings are represented by lattice vortices, and the shed vortices are expressed by discrete three-dimensional vortex sticks. in this calculation, the GRAPE3A hardware is used to calculate at high speed the induced velocity of the vortex sticks and the viscous diffusion of fluid is represented by the random walk method. The vortex distributions and the velocity field are calculated. The pressure is estimated by the Bernoulli equation, and the lift and moment on the wing are also obtained.

Numerical Prediction of Acoustic Sounds Occurring by the Flow Around a Circular Cylinder

Acoustic sounds generated by uniform flow around a two-dimensional circular cylinder at Re=150 are simulated by applying the finite difference lattice Boltzmann method. A third-order-accurate up-wind scheme is used for the spartial derivatives. A second-order-accurate RungeKutta scheme is also used for time marching. Very small acoustic pressure fluctuation, with same frequency as that of Karman vortex street, is compared with pressure fluctuation around a circular cylinder. The propagation velocity of acoustic sound shows that acoustic approaching the upstream, due to the Doppler effect in uniform flow, slowly propagates. For the downstream, on the other hand, it quickly propagates. It is also apparent that the size of sound pressure is proportional to the central distance τ-1/2 of the circular cylinder.

Numerical Simulation of Shock Wave Propagation using the Finite Difference Lattice Boltzmann Method

The shock wave process represents an abrupt change in fluid properties, in which finite variations in pressure, temperature, and density occur over the shock thickness which is comparable to the mean free path of the gas molecules involved. This shock wave fluid phenomenon is simulated by using the finite difference lattice Boltzmann method (FDLBM). In this paper, a new model is proposed using the lattice BGK compressible fluid model in FDLBM for the purpose of speeding up the calculation as well as stabilizing the numerical scheme. The numerical results of the proposed model show good agreement with the theoretical predictions.

Performance Improvement of Weis-Fogh Type Ship’s Propulsion Mechanism Using a Wing Restrained by an Elastic Spring

This study was conducted in an attempt to improve the hydrodynamic performance of a Weis-Fogh type ship propulsion mechanism by installing a spring to the wing so that the opening angle of the wing can be changed automatically. With the prototype design, the average thrust coefficient was almost fixed with all velocity ratios; but with the spring type, the thrust coefficient was increased sharply as the velocity ratio increased. The average propulsive efficiency was higher with a bigger opening angle in the prototype but in the spring type design, the one with a smaller spring coefficient had higher efficiency. In the case of velocity ratios over 1.5 where big thrust can be generated, the spring type had more than twice the increase in propulsion efficiency compared with the prototype.

Sailing Characteristics of a Model Ship of Weis-Fogh Type

A model of the propulsion mechanism, I, , was based on a two-dimensional model of the Weis-Fogh mechanism and consisted of one or two wings in a square channel. The sailing and vibration performance characteristics of model ships were tested to compare with each other. we took results as follow. Thrust of propulsion model, and was increased by 31% and 43%, the speed of model ship by 20% and 23%, When compared to model in same condition. The thrust improvement using the elastic spring wing was effective not only on all models but also in the real ship. The maximum amplitude and RMS were largest at the opening angle =15° and smallest at =30° on the vibration of model ship. The thrust of α α propulsion model with opening angle =30°, phase T=0° was large, but the amplitude of vibration was α Δ

The Flow Characteristics Around Airfoil Moving Reciprocally in a Channel

Abstract The Flow characteristics of a ships propulsion mechanism of Weis-Fogh type, in which a airfoil(NACA0010) moves reciprocally in a channel, were investigated by the PIV. Velocity vectors and velocity profiles around the operating and stationary wings were observed at opening angles of α=15° and 30°, velocity ratios of V/U=0.5~1.5 and Reynolds number of Re=0.52×104~1.0×104. As the results the fluid between wing and wall was inhaled in the opening stage and was jet in the closing stage. The wing in the translating stage accelerated the fluid in the channel. And the flow fields of this propulsion mechanism were unsteady and complex, but those were clarified by flow visualization using the PIV. 1. 서 론 소위 Weis-Fogh메커니즘 (1,2) 은 영국의 생물 학자 Weis-Fogh가 Encarsia formosa로 불리는 작은 벌의 정지비행을 관찰함에 의해 발견한 독특하고, 효율이 좋은 양력 발생기구이다. 이 메커니즘의 동작원리를 Fig. 1에 나타낸다. 이 벌은 몸통을 수직으로 유지한 채 날개를 수평면으로 회전운동 시킴에 의해 정지 비행을 행하고 있다. 먼저 날개가 몸통의 등쪽에서 앞전(Leading edge)을 중심으로 닫힌다. 그리고 날개는 뒷전(Trailing edge)을 맞닿은 상태에서 열려, 어느 일정한 열림각(여기서 열림각이란 Fig. 1의 두 번째 그림에서두 날개가 이루는 각의 1/2로 정의한다)을 유지하

The Characteristic Investigation of the Flowfield around Two Circular Cylinders in Various Arrangements Using the PIV

The characteristics of the flowfields around two circular cylinders in various arrangements were investigated by PIV. In tandem arrangement, the Strouhal numbers measured in the rear region of the cylinder of wake side were decreased with the space ratios, the flow between two cylinders was almost stagnated and the closer to upstream cylinder, the larger the width of the stagnated region was. The direction of vortex between two cylinders was opposed each other with the small difference(

Flow past airfoil moving reciprocally in a channel by vortex method

The velocity and pressure fields of a ship’s propulsion mechanism of the Weis-Fogh type, in which a airfoil moves reciprocally in a channel, are studied in this paper using the advanced vortex method. The airfoil and the channel are approximated by a finite number of source and vortex panels, and the free vortices are introduced from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from integrating the equation given by the instantaneous velocity and vorticity fields. Two-dimensional unsteady viscose flows of this propulsion mechanism are numerically clarified, and the calculated results agree well with the experimental ones.