• Ocean Systems Engineering
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• v.3 no.3
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• pp.219-236
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• 2013

The accurate prediction of motion in waves of a marine vehicle is essential to assess the maximum sea state vs. operational requirements. This is particularly true for small crafts, such as Autonomous Surface Vessels (ASV). Two different numerical methods to predict motions of a SWATH-ASV are considered: an inviscid strip theory initially developed at MIT for catamarans and then adapted for SWATHs and new a hybrid strip theory, based on the numerical solution of the radiation forces by an unsteady viscous, non-linear free surface flow solver. Motion predictions obtained by the viscous flow method are critically discussed against those obtained by potential flow strip theory. Effects of viscosity are analyzed by comparison of sectional added mass and damping calculated at different frequencies and for different sections, RAOs and motions response in irregular waves at zero speed. Some relevant conclusions can be drawn from this study: influence of viscosity is definitely non negligible for SWATH vessels like the one presented: amplitude of the pitch and heave motions predicted at the resonance frequency differ of 20% respectively and 50%; in this respect, the hybrid method with fully non-linear, viscous free surface calculation of the radiation forces turns out to be a very valuable tool to improve the accuracy of traditional strip theories, without the burden of long computational times requested by fully viscous time domain three dimensional simulations.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.6
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• pp.1518-1528
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• 1993

The wake-induced vibration and proximity-induced vibration of two interfering circular cylinders in tandem are investigated experimentally, using an elastically supported cylinder and a fixed cylinder in uniform crossflow. Dynamic responses and flow periodicity in wake are measured to investigate the effect of system parameters on aerodynamic instability. The parameters include the free stream wind velocity and the position of two interfering circular cylinders. The oscillating behavior of the amplitude of the elastically supported cylinder is changed by varying the position, the relative gap spacing between two interfering circular cylinders and the reduced velocities. In small gap spacing between the elastically supported cylinder located to upstream and the circular cylinder fixed to downstream, the fluidelastic instability is founded. The vibration amplitude decreases notably as gap spacing between two interfering circular cylinders becomes large. The dynamic behavior at g/D-4.0 is similar to that of the single circular cylinder.

• Journal of Navigation and Port Research
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• v.37 no.6
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• pp.597-601
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• 2013

Recently, many studies about the wave power generation system for the marine structure as the hybrid form in linked with the original features have been made of. Of these, the wave power generation system using oscillating water column(OWC) has function to convert wave energy to electrical energy with original function of the break water structure. In this type of generation system, it is important to make the flow of sea water as much as possible without loss. Output characteristics of wave power generation system depending on entrance section were described in the paper. Also, flow quantity changing with entrance section, velocity of sea water and output of wells turbine were measured by simulating OWC wells turbine model in break water, one of the general marine structure. Finally, entrance section was suggested to enhance the energy conversion efficiency based on the results of simulation.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.30 no.6
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• pp.242-252
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• 2018

It is well known that an Oscillating Water Column Wave Energy Converter (OWC-WEC) is one of the most efficient wave absorber equipment. This device transforms the vertical motion of water column in the air chamber into the air flow velocity and produces electricity from the driving force of turbine as represented by the Wells turbine. Therefore, in order to obtain high electric energy, it is necessary to amplify the water surface vibration by inducing resonance of the piston mode in the water surface fluctuation in the air chamber. In this study, a new type of OWC-WEC with a seawater channel is used, and the wave deformation by the structure, water surface fluctuation in the air chamber, air outflow velocity from the nozzle and seawater flow velocity in the seawater channel are evaluated by numerical analysis in detail. The numerical analysis model uses open CFD code OLAFLOW model based on multi-phase analysis technique of Navier-Stokes solver. To validate model, numerical results and existing experimental results are compared and discussed. It is revealed within the scope of this study that the air flow velocity at nozzle increases as the Ursell number becomes larger, and the air velocity that flows out from the inside of the air chamber is larger than the velocity of incoming air into the air chamber.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.4
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• pp.425-432
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• 2004

The objective of this study is a qualitative comparison between line-integrated OH chemiluminescence(OH$\^$*/) image and its Abel inverted image to investigate the flame structure at different phase of the oscillating pressure field. PIV(Particle Image Velocimetry) measurements were conducted under non-reacting conditions to see the global flow structure and NOx emission was measured to investigate the effect of fuel-air premixing on combustion instability and emission characteristics. Experiments were carried out in an atmospheric pressure, laboratory-scale dump combustor operating on natural gas. Combustion instabilities in present study exhibited a longitudinal mode with a dominant frequency of ∼341.8㎐, which corresponded to a quarter wave mode of combustor. Heat release and pressure waves were in-phase when instability occurred. Results gave an insight about the location where the strong coherence of pressure and heat release existed. Also an additional information on active control to suppress the combustion instabilities was obtained. For lean premixed combustion, strong correlation between OH$\^$*/ and NOx emissions was expected largely due to the exponential dependence of thermal NOx mechanism on flame temperature.

• Journal of the Korean Society of Visualization
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• v.1 no.1
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• pp.64-74
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• 2003

An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. The local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f$^{+}$=0.044, 0.066 and 0.088) with a fixed forcing amplitude (A$^{+}$=0.6) were employed at $Re_{=690. The effect of the forcing angles ($\alpha$=60$^{\circ}$ , 90$^{\circ}$ and 120$^{\circ}$ ) was investigated under the fixed forcing frequency (f$^{+}$=0.088). The PIV results showed that the wall region velocity decreases on imposition of the local forcing. Inspection of phase-averaged velocity profiles revealed that spanwise large-scale vortices were generated in the downstream of the slot and persist further downstream. The highest reduction in skin friction was achieved at highest forcing frequency (f$^{+}$=0.088) and a forcing angle of $\alpha$=120$^{\circ}$. The spatial fraction of the vortices was examined to analyze the skin friction reduction.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.185-188
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• 2002

The objective of present paper is to apply a stereoscopic PIV(Particle Image Velocimetry) techiique for measuring the 3 dimensional flow structure of turbulent wake behind a marine propeller with 5 blades. It is essential to measure 3-components velocity fields for the investigation of complicated near-wake behind the propeller. The out-of-plane velocity component was measured using the particle images captured by two CCD cameras in the angular displacement configuration.400 instantaneous velocity fields were measured for each of few different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}\;and\;54^{\circ}$. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranged from the trailing edge to the region of one propeller diameter(D) downstream. The phase-averaged velocity fields show the viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were formed periodically and the slipstream contraction occurs in the near-wake region. The out-of-plane velocity component has large values at the tip and trailing votices. With going downstream, the axial turbulence intensity and the strength of tip vortices were decreased due to the visous dissipation, turbulence diffusion and blade-to-blade interaction. The blade wake traveling at higher speed with respect to the tip vortex overtakes and interacts with tip vortices formed from the previous blade. Tip vortices are separated from the wake and show oscillating trajectory

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.11
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• pp.2049-2058
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• 1992

In this work, a 6 degree of freedom lumped mass model is constructed for an OHC-type cam valve train analysis, and the model is verified experimentally. Using the verified model, an optimum cam profile is designed to minimize the maximum contact force between cam and follower under the constraints such as cam lift and cam event angle. The designed cam was carefully machined and tested experimentally. As operating the designed cam shaft on the test rig, the valve motion was precisely measured with laser displacement meter and the contact force was indirectly monitored by measuring strain at a certain point of the finger follower. Judging from the model simulation and experiment results, the maximum contact force can be reduced as much as more than 16.7 percent under maintaining the original valve flow area by adopting the optimum cam profile.

• Journal of Mechanical Science and Technology
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• v.18 no.2
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• pp.272-285
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• 2004

The losses at off-design points from a compressor cascade occur due to the deviation from a design incidence angle at the inlet of the cascade. The self-noise from the blade cascade at off-design points comes from a separated boundary layer and vortex sheddings. If the incidence angle to the cascade increases, stalling in blades may occur and the noise level increases significantly. This study applied Large-Eddy Simulations (LES) using deductive and deductive dynamic SGS models to low Mach-number, turbulent flow with each incidence angle to the cascade ranging from -40$^{\circ}$ to +20$^{\circ}$ and compared numerical predictions with measured data. It was observed that the oscillating separation bubbles attached to the suction surface do not modify wake flows dynamically for cases of negative incidence angles. However, an incidence angle greater than 8$^{\circ}$ caused a separated vortex near the leading edge to be shed downstream and created stalling. The computed performance parameters such as drag coefficient and total pressure loss coefficient showed good agreement with experimental results. Noise from the cascade of the compressor is summarized as sound generated by a structure interacting with unsteady, turbulent flows. The hybrid method using acoustic analogy was observed to closely predict the measured overall sound powers and directivity patterns at design and off-design points of blade cascade.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.12
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• pp.1662-1669
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• 2000

Various flame types are observed in a double concentric burner by varying equivalence ratio and flow rates in each tube. Observed flame types include bunsen-type flame, ring-shaped flame, outer lifted flame, inner lifted flame, and oscillatory lifted flame, The doman of existence of various flames is mapped with equivalence ratio and annular jet velocity. Each flame is investigated through direct photography and OH PLIF. As central air velocity increase, the blowout region is diminished and lifted oscillating flames are observed. Inner lifted flames are observed from bunsen flames or rich shaped flames by increasing central air velocity. For inner lifted flames, annular jet velocity, at flame liftoff decreases with increasing central air jet velocity. Axial velocity profile and temperature fie이 using LDV and CRS, respectively, for a typical inner lifted flame are also measured through which the role of tribrachial flame for stabilization in emphasized.