• Wind and Structures
• /
• v.5 no.2_3_4
• /
• pp.257-266
• /
• 2002

In the recent years flow around bridges are investigated using computer modeling. Selvam (1998), Selvam and Bosch (1999), Frandsen and McRobie (1999) used finite element procedures. Larsen and Walther (1997) used discrete vorticity procedure. The aeroelastic instability is a major criterion to be checked for long span bridges. If the wind speed experienced by a bridge is greater than the critical wind speed for flutter, then the bridge fails due to aeroelastic instability. Larsen and Walther (1997) computed the critical velocity for flutter using discrete vortex method similar to wind tunnel procedures. In this work, the critical velocity for flutter will be calculated directly (free oscillation procedure) similar to the approaches reported by Selvam et al. (1998). It is expected that the computational time required to compute the critical velocity using this approach may be much shorter than the traditional approach. The computed critical flutter velocity of 69 m/s is in reasonable comparison with wind tunnel measurement. The no flutter and flutter conditions are illustrated using the bridge response in time.

• The KSFM Journal of Fluid Machinery
• /
• v.6 no.2 s.19
• /
• pp.62-69
• /
• 2003

An oil-free turbo-compressor supported by compliant foil bearings which remove oil-contamination by elimination of a conventional ball bearing and oil lubrication systems is presented. Turbo-compressor makes two individual air compressions with two impellers at a operating speed of 39,000 rpm. In this study, the rotordynamic effects caused by aerodynamic instability were investigated with variable mass flow rates. Correlations between frequencies of pressure fluctuation in two diffusers and those of excitation forces on rotor were clearly observed in an aerodynamic unsteady region. Thus, these results show that it is beneficial to design high-speed rotating turbomachinery by considering coupling effect between aerodynamic instability and rotordynamic force.

• International Journal of Ocean System Engineering
• /
• v.2 no.2
• /
• pp.116-138
• /
• 2012

As in the other fields of mechanics, analytical methods represent an important analysis tool in marine hydrodynamics. The analytical approach is interesting for different reasons : it gives reference results for numerical codes verification, it gives physical insight into some complicated problems, it can be used as a simplified predesign tool, etc. This approach is of course limited to some simplified geometries (cylinders, spheres, ...), and only the case of one or more cylinders, truncated or not, will be considered here. Presented methods are basically eigenfunction expansions whose complexity depends on the boundary conditions. The hydrodynamic boundary value problem (BVP) is formulated within the usual assumptions of potential flow and is additionally simplified by the perturbation method. By using this approach, the highly nonlinear problem decomposes into its linear part and the higher order (second, third, ...) corrections. Also, periodicity is assumed so that the time dependence can be factorized i.e. the frequency domain formulation is adopted. As far as free surface flows are concerned, only cases without or with small forward speed are sufficiently simple to be solved semi-analytically. The problem of the floating body advancing in waves with arbitrary forward speed is far more complicated. These remarks are also valid for the general numerical methods where the case of arbitrary forward speed, even linearized, is still too difficult from numerical point of view, and "it is fair to say that there exists at present no general practical numerical method for the wave resistance problem" [9], and even less for the general seakeeping problem. We note also that, in the case of bluff bodies like cylinders, the assumptions of the potential flow are justified only if the forward speed is less than the product of wave amplitude with wave frequency.

• Proceedings of the KSME Conference
• /
• 2007.05b
• /
• pp.2725-2730
• /
• 2007

Turbo-type molecular drag pumps ( MDPs ) are used in the liquid crystal display ( LCD ), semiconductor and other thin film industries. Siegbahn ( disk-type ) molecular drag pumps are used as high-pressure stages in the hybrid-type turbomolecular pumps, where they can operate in the viscous, the transition and the free molecular flow regime. In this study is performed to investigate the pumping characteristics of three-stage disk-type molecular drag pump ( DTDP ) in the molecular transition flow region. The experiments are measured using five vacuum pressure gauges in the positions for rotors of DTDP. The test is performed with nitrogen gas ( $N_2$ ).

• Transactions of the Korean Society of Machine Tool Engineers
• /
• v.12 no.1
• /
• pp.16-23
• /
• 2003

Keeping the layer thickness constant is very essential for improving the shape accuracy in the stereolithography process. The layer thickness is created by recoating process, and also affected by recoating parameters such as blade speed and thickness. The created layer in this process can determine the whole accuracy of the entire parts. The aim of this paper is to improve the accuracy of the layer thickness by adjusting the recoating process parameters. Several experiments with different recoating conditions are Performed to find the optimal recoating parameters that produce the most accurate layer thickness. The effective recoating method is suggested by measuring and analyzing the cured layer thickness.

• Journal of Mechanical Science and Technology
• /
• v.18 no.4
• /
• pp.718-730
• /
• 2004

The solution for the natural convection in internally finned horizontal annuli is obtained by using a numerical simulation of time-dependent and two-dimensional governing equations. The fins existing in annuli influence the flow pattern, temperature distribution and heat transfer rate. The variations of the On configuration suppress or accelerate the free convective effects compared to those of the smooth tubes. The effects of fin configuration, number of fins and ratio of annulus gap width to the inner cylinder radius on the fluid flow and heat transfer in annuli are demonstrated by the distribution of the velocity vector, isotherms and streamlines. The governing equations are solved efficiently by using a parallel implementation. The technique is adopted for reduction of the computation cost. The parallelization is performed with the domain decomposition technique and message passing between sub-domains on the basis of the MPI library. The results from parallel computation reveal in consistency with those of the sequential program. Moreover, the speed-up ratio shows linearity with the number of processor.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2002.04a
• /
• pp.41-42
• /
• 2002

The powered missiles with very high thrust level can make highly underexpanded jet plume downstream of tile exhaust nozzle exit so that strong interactions between the exhaust plume and a free stream occur around the body at transonic or supersonic speeds. The interactions result in extremely complicated flow phenomena, which consist of plume-induced boundary layer separation, strong shear layers, various shock waves, and interactions among these. The flow characteristics are inherent nonlinear and severe unstable during the flight at its normal speed as well as taking-off and landing. Eventually, the induced boundary layer separation and pitching and yawing moments by the interactions cause undesirable effects ell the static stability and control of a missile.

• Proceedings of the KSME Conference
• /
• 2003.04a
• /
• pp.1765-1770
• /
• 2003

This paper describes a study of the spin-up of a free-surface fluid in a rectangular container in which an internal cylindrical obstacle is mounted. Experiments and numerical analysis have been carried out for a variety of obstacle position. Increase in the speed of background rotation and near wall position of cylindrical obstacle results in the complex flow structures. Numerical and experimental results agree well with each other and the Ekman-pumping model is also applied to this flow.

• Atmosphere
• /
• v.23 no.4
• /
• pp.513-517
• /
• 2013

In this study, it is first intended to simulate the vertical profile of atmospheric flow in a short wind tunnel. In order to accomplish it, proper devices are designed properly to reduce freestream flow momentum and it is confirmed from the measured velocity profile using hot-wire anemometer that momentum flux of the tunnel free stream can be reduced and desired atmospheric boundary can be created. Second, experiments are performed to identify influences of a surrounding structure measuring correct wind velocity by an anemometer, which are located nearby due to area limitation in actual airport and correction factors are proposed from experimental results. One of findings is that in order to limit the velocity attenuation due to a nearby structure under 10%, wind velocity measuring equipment should be installed at least 6 times of the structure height away from the structure of concern.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.27 no.5
• /
• pp.628-634
• /
• 2003

We present numerical and experimental results of periodic flows inside a rectangular container under a background rotation. The periodic flows are generated by changing the speed of rotation periodically so that a time-periodic body forces produce the unsteady flows. In numerical computation, a parallel-computation technique with MPI is implemented. Flow visualization and PIV measurement are also performed to obtain velocity fields at the free surface. Through a series of numerical and experimental works, we aim to clarify, if any, the fundamental reasons \ulcornerf discrepancy between the two-dimensional computation and the experimental measurement, which was detected in the previous study for the same flow model. Specifically, we check if the various assumptions prerequisite for the validity of the classical Ekman pumping law are satisfied for periodic flows under a background rotation.