• Journal of Mechanical Science and Technology
• /
• 제16권11호
• /
• pp.1448-1456
• /
• 2002

The present study addresses the flow characteristics involved in the self-induced oscillations of the underexpanded jet impinging upon a cylindrical body. Both experiment and computational analysis are carried out to elucidate the shock motions of the self-induced oscillations and to find the associated major flow factors. The underexpanded sonic jet is made from a nozzle and a cylindrical body is placed downstream to simulate the impinging jet upon an obstacle. The computational analysis using TVD scheme is applied to solve the axisymmetric, unsteady, inviscid governing equations. A Schlieren system is employed to visualize the self-induced oscillations generated in flow field. The data of the shock motions are obtained from a high-speed video system. The detailed characteristics of the Mach disk oscillations and the resulting pressure variations are expatiated using the time dependent data of the Mach disk positions. The mechanisms of the self-induced oscillations are discussed in details based upon the experimental and computational results.

• 한국산학기술학회논문지
• /
• 제14권7호
• /
• pp.3158-3163
• /
• 2013

본 연구에서는 회전속도, 브레이크 압력 제어가 가능한 브레이크 다이나모메타를 제작하여 스퀼 소음 실험을 실시하였다. 브레이크 패드는 각각 $34^{\circ}$, $30^{\circ}$, $26^{\circ}$로 가공하여 패드 각도에 따른 스퀼 발생 조건과 음의 기울기 발생에 따른 스퀼 연관성을 연구 하였다. 각도에 따라 특정한 회전속도와 브레이크 압력에서 스퀼이 발생하였으며, 각도에 상관없이 하모닉 성분을 가지는 특정 주파수 소음을 발견하였다. 햄머링 테스트 및 유한요소해석을 통해서 특정 주파수는 In-plane모드 성격을 갖고 있으며, 패드 각도에 둔감한 모드임을 밝혀냈다.

• 한국전산유체공학회지
• /
• 제9권1호
• /
• pp.1-9
• /
• 2004

Large eddy simulation is used to investigate the compressible flow over a cavity with high aspect ratio. The sub-grid scale stresses are modeled using the dynamic model. The compressible Navier-Stokes equations are solved with the sixth order accurate compact finite difference scheme in the space and the 4th order Runge-Kutta scheme in the time. The buffer Bone techniques are used for non-reflecting boundary conditions. The results show the shear layer oscillation over the cavity. The votical disturbances, the roll-up of vorticity, and impingement and scattering of vorticity at the downstream cavity edge can be seen in the shear layer. Several peaks for the resonant frequencies are found in the spectra of the vertical velocity at the center-line. The most energetic Peak near the downstream edge is different from that at the center part of the cavity The pressure has its minimum value in the vortex core inside the cavity, and becomes very high at the downstream face of the cavity. The variation of the model coefficient predicted by the dynamic model is quite large between 0 and 0.3. The model coefficient increases in the stream-wise evolution of the shear layer and sharply decreases near the wall due to the wall effect.

• 대한기계학회논문집B
• /
• 제28권8호
• /
• pp.887-894
• /
• 2004

A technique of mixing enhancement by using an elliptic jet screech reflector has been examined experimentally in an underexpanded sonic round jet where jet screech tone is generated. Since jet screech is known to enhance jet spreading, a reflector was designed to focus jet screech waves near the nozzle lip at an underexpanded jet. The reflector has an elliptic cross section of which one focus is located near the nozzle lip and the other in the jet screech source region in a plane including the jet axis. In the jet with the elliptic reflector, the mass flow rate showed a significant increase in the jet entrainment when compared to that for the small disk reflector. This was attributed to the increased screech amplitude near the nozzle lip as well as the mode change of the jet. The jet mixing was also increased by the amplified jet screech at two other underexpanded jets, but the jet oscillation mode did not change.

• 대한기계학회논문집B
• /
• 제33권9호
• /
• pp.658-665
• /
• 2009

Lean premixed combustors are used for significant NOx reduction which one of issues in current gas turbine combustor. This study was investigated to estimate the effects of the unmixedness of fuel-air, equivalence ratio on the instability mechanism, NOx emission and combustion oscillation in a lean premixed combustor. The experiments were conducted in a dump combustor at atmospheric pressure conditions using methane as fuel. The swirler angle was $45^{\circ}$, the degrees of fuel-air mixing were 0, 50 and 100 and inlet temperature was 650K. The equivalence ratio was ranging from 0.5 to 0.8. This paper shows that NOx emission was increased when the degree of fuel-air mixing is increased in same equivalence ratio and when equivalence ratio is increased. And the range of the combustion instability was enlarged as a function of increasing of the degree of fuel-air mixing.

• 대한기계학회논문집B
• /
• 제29권10호
• /
• pp.1101-1110
• /
• 2005

In this paper we present the numerical results of the behavior of the horizontal vortex generated by ejecting a liquid vertically upward from an orifice into the bulk fluid above the orifice. The numerical calculation has been performed for the axi-symmetric Navier-Stokes equation. A simple flow-visualization experiment was also conducted to qualitatively verify the numerical solutions. Three cases of the flow configurations studied in this paper are; firstly, the vortex was generated without any background rotation, secondly, the vortex was made under a full background rotation, and thirdly, the vortex was made during the spin-up process such that only the region adjacent to the side wall was set into motion viewed in the inertial frame of reference. It was shown that the swirl flow at the inlet boundary affects considerably the formation and development of the vortex for the second case. In the third case, it was remarkable to see that the vortex cannot penetrate into the region near to the side wall of the tank, because of the strong swirl flow and corresponding high pressure gradient in the region.

• 한국정밀공학회지
• /
• 제26권8호
• /
• pp.55-61
• /
• 2009

Recently, many studies are being conducted to realize high quality polishing technology, but because of high dependence on field experience and insufficient research for ultra-precision polishing technology, it is difficult to establish standardization of polishing conditions. The purpose of this study is to determine high-efficiency superfinishing conditions which are applicable in the field of machining. To achieve this, we have a developed a superfinishing device and conducted a series of polishing experiments for mechanical materials such as SM45C, Brass, Al7075, and Ti, from the perspective of oscillation speed, the rotational speed of the workpiece, contact roller hardness, contact pressure, and feed rate. From the experimental results, it was confirmed that the polishable superfinishing conditions range and efficient feed rate of polishing film can be determined.

• 한국수자원학회:학술대회논문집
• /
• 한국수자원학회 2005년도 학술발표회 논문집
• /
• pp.409-413
• /
• 2005

Even though the relative importance of length scale of flow system allow us to simplify three dimensional flow problem to one or two dimensional representation, many systems still require three dimensional analysis. The objective of this study is to develop an efficient and accurate finite element model for analyzing and predicting three dimensional flow features in natural rivers and to offend to model spreading of pollutants and transport of sediments in the future. Firstly, three dimensional Reynolds averaged Navier-Stokes equations with the hydrostatic pressure assumption in generalized curvilinear coordinates were combined with the kinematic free-surface condition. Secondly. to simulate realistic high Reynolds number flow, the model employed the Streamline Upwind/Petrov-Galerkin(SU/PG) scheme as a weighting function for the finite element method in conjunction with an appropriate turbulence model(Smagorinsky scheme for the horizontal plain and Mellor-Yamada scheme for the vertical direction). Several tests is performed for the purpose of validation and verification of the developed model. A simple rectangular channel, 5-shaped and U-shaped channel are used for tests and comparisons are made with RMA-10 model. Runs for each case is converged stably without a oscillation and calculated water-surface deformation, longitudinal and transversal velocities, and velocity vector fields are in good agreement with the results of RMA-10 model.

• 한국소음진동공학회:학술대회논문집
• /
• 한국소음진동공학회 2002년도 추계학술대회논문집
• /
• pp.255-261
• /
• 2002

A four-degree of freedom model is suggested to understand the basic dynamical behaviors of the normal interaction between two masses of the friction induced normal vibration system. The two masses may be considered as the pad and the disk of the brake. The phase space analysis is performed to understand complicated in-plane dynamics of the non-linear model. Attractors in the phase space are examined for various conditions of the parameters. In certain conditions, the attractor becomes a limit cycle showing the stick-slip phenomena. In this paper, on the basis of the in-plane motion not only the existence of the limit cycle but also the size of the limit cycle is examined o demonstrate the non-linear dynamics that leads the unstable state and then the normal vibration is investigated as the state of the in-plane motion For only one case of the system frequency(two masses with same natural frequencies), the propensity of the normal vibration is discussed in detail. The results show an important fact that it may be not effective when too much damping is present in the only one part of the masses.

• 한국항공우주학회지
• /
• 제36권2호
• /
• pp.179-185
• /
• 2008

분사기형 배플이 설치된 연소기에 대하여 최적의 감쇠를 가지는 배플간극을 모사연소시험을 통하여 최종적으로 검증하였다. 연소실에서 압력섭동의 최대진폭과 연소안정성 여분을 통하여 배플에 의한 감쇠능력을 정량화하였다. 본 연구의 결과가 상온음향시험에서의 결과와 정성적으로 동일함을 파악할 수 있었다. 배플 간극에 있어서도 상온음향시험과 동일한 간극에서 최적의 연소안정성 여분을 가짐을 확인하였다. 이러한 연소장과의 상호작용을 고려한 결과는 배플의 길이 및 간극의 결정에 매우 유용한 자료를 제공하여 실제 연소기 설계 및 조립에 지침으로서 적용될 수 있을 것이다.