• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.6
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• pp.860-873
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• 1998

In a low speed open-type wind tunnel, a group of parallel wakes downstream of two dimensional grid model consisting of several circular cylinders were experimentally investigated to study the response of the wake flows to the acoustic excitation, in hoping to promote the understanding of the underlying mechanism behind the gross flow change due to artificial excitation. In the unexcited wake flows, the development of the individual wakes behind cylinders was almost uniform for the ratio of the spacing to the cylinder diameter of s/d.geq.1.5. For smaller s/d, however, the jet streams issued through the gaps between the cylinders became biased in one side and the cylinders had wakes of different sizes. At s/d=1.25, the gap flow directions change in time, leading to unstable wake patterns. Further reduction in s/d made this unstable flip-flopping of the jets stable. The most effective excitation frequency was found to be in the Strouhal number range of St=0.5-0.6. This frequency was related to the vortex shedding. At s/d=1.75, the excitation frequency was 2 or 4 times the vortex shedding frequency. When the flow was excited at this frequency, the vortex sheddings were energized, and pairings between neighboring vortices were generated. Also, the merging process between individual wakes was accelerated. The unstable and unbalanced wake patterns at s/d=2.15 were made stable and balanced. The unstable and unbalanced wake patterns at s/d=2.15 were made stable and balanced. For smaller spacing of s/d .leq,1.0, the acoustic excitation became less effective in controlling the flow.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1546-1551
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• 2004

The laser Doppler technique is well-established as a velocity measurement technique of high precision for flow velocity. Recently, the laser Doppler technique has also been used to measure acceleration of fluid particles. Acceleration is interesting from a fluid mechanics point of view, since the Navier Stokes equations, specifically the left-hand-side, are formulated in terms of fluid acceleration. Further, there are several avenues to estimating the dissipation rate using the acceleration. However such measurements place additional demands on the design of the optical system; in particular fringe non-uniformity must be held below about 0.0001 to avoid systematic errors. Relations expressing fringe divergence as a function of the optical parameters of the system have been given in the literature; however, direct use of these formulae to minimize fringe divergence lead either to very large measurement volumes or to extremely high intersection angles. This dilemma can be resolved by using an off-axis receiving arrangement, in which the measurement volume is truncated by a pinhole in front of the detection plane. In the present study an optical design study is performed for optimizing laser Doppler systems for fluid acceleration measurements. This is followed by laboratory validation using a round free jet and a stagnation flow, two flows in which either fluid acceleration has been previously measured or in which the acceleration is known analytically. A 90 degree off-axis receiving angle is used with a pinhole or a slit.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.45 no.4
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• pp.349-357
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• 2017

If an actuation system of the tactical missile is required very fast response time on conditions of short operating time and big loads on the actuator, we would prefer to adopt a small hydraulic system. In this paper, a mathematical model is proposed to analyze and simulate the small hydraulic actuation system. The mathematical model consists of a high pressure vessel model, a pressure regulator model, a hydraulic reservoir model and a actuator model. The suggested model is validated by comparison of simulation results with experimental data. The simulation results show that the mathematical model could be useful for designing a hydraulic actuation system.

• 한국전산유체공학회:학술대회논문집
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• 2005.10a
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• pp.93-97
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• 2005

Air interlacing serves to protect the yarn against damage, strengthens inter-filament compactness or cohesion, and ensures fabric consistency. The air interlacing nozzle is used to introduce intermittent nips to a filament yarn so as to improve its performance in textile processing. This study investigates the effect of interlacing nozzle geometry on the interlacing process. The geometries of interlacing nozzles with multiple air inlets located across the width of a yarn channels are investigated. The basic interlacing nozzle is the yarn channel, with a perpendicular single air inlet in the middle. The yarn channel shapes are cross sections with semicircular or rectangular shapes. This paper presents three doubled sub air inlets with main air inlet and one of them is slightly inclined doubled sub air inlets with main air inlet. The compressed air coming out from the inlet hits the opposing wall of the yarn channel, divides into two branches, flows trough the top side of yarn channel, joins with the compressed air coming out from the sub air inlet and then creates two free jets at both ends of the yarn channel. The compressed air moves in the shape of two opposing directional vortices. The CFD-FASTRAN was used to perform steady simulations of impinging jet flow inside of the interlace nozzles. The vortical structure and the flow pattern such as pressure contour, particle traces, velocity vector plots inside of interlace nozzle geometry are discussed in this paper.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.61-65
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• 2006

A thery was developed to design a high pressure vessel with combined structure of the metal and the composite to withstand the pressure of several tens of thousands psias to reduce the weight of the impulse motor which produces high level of thrust within several tens of seconds. The elastic-plastic stress analyses were carried out to prove the validity of the design theory A combustion chamber of the impulse motor was designed by the design theory, fabricated, and tested by the hydraulic pressure and the static firings. The bursting pressures from the tests were compared to those predicted by tile design theory and the stress analyses and found to be almost the same. It will be possible to design the high pressure vessel with combined structure of the metal and the composite very easily by the proposed design theory.

• Journal of the Korean Society of Propulsion Engineers
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• v.3 no.1
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• pp.1-8
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• 1999

In the present paper an attempt has been made to simulate the secondary injection-primary flow interaction in the conical rocket nozzle and to derive the performance of secondary injection thrust vector control(SITVC) system. Complex three-dimensional flowfield induced by the secondary injection is numerically analyzed by solving unsteady three-dimensional Euler equation with Beam and Warming's implicit approximate factorization method. Emphasized in the present study is the effect of secondary injection such as secondary mass flow rates and the momentum of secondary/primary nozzle flow mass rates upon the gross system performance parameters such as thrust ratio, specific impulse ratio and deflection angle. The results obtained in terms of system performance parameters show that lower secondary mass flow rate is advantageous for to reduce secondary specific impulse loss. It is further found that the nozzle with secondary jet injected downstream and interacting with fast primary flow is preferable for efficient and stable SITVC over the wide range of use with the penalty of side specific impulse loss.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.6 s.261
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• pp.547-557
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• 2007

This study was conducted to investigate the flow and combustion characteristic of the experimental burner which was manufactured for the reflection of the oven and broil burner features. As slot shape, spacing between slots, and slot arrangement of the exit area which emits the mixing gas are different in case of oven burners and broil turners, the purpose of this study is to know the affection of the flame interaction and combustion characteristic according to the change of shape factors such as slot shape, slot arrangement, and slot-to-slot spacing. With no relation of the slot shape, as the spacing between slots became narrow, the occurrence of a lift-flame was delayed. So the combustion was possible in the leaner region, but the appearance of yellow-tip became a little fast. Slit slot port had the broadest operating range among the other slot shapes. Specially, from the side of lift-flame, as the jet that spreads downstream in the longitudinal slot was nearly circular just a few slot lengths away from the orifice, slot-to-slot spacing of the Slit port was closer than the other ports. These results could be expected through the computer numerical method and had a good agreement. As the spacing between slots increased, in case of Slit and Mix port, NOx emission rate was constant or decreased, but the NOx emission of Hole port was increased. CO emission rate of Slit and Hole port was increased as the slot-to-slot spacing was broadened.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.1
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• pp.140-152
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• 1989

The study on the penetration of sprays during the initial phase of injection period, i.e. ignition delay period, in high speed small D.I. diesel engines are strongly affected by such behavior. To investigate the penetration of the sprays injected through single cylinderical orifice, a mathematical model was developed and compared with experimental results. In this model, radial heterogeneity of fuel density in the spray, transiency of injection pressure difference, and spray outrunning phenomenon were considered simultaneously. Experiments on the behaviors of sprays in the high pressure air chamber were conducted at various injection pressure differences and different levels of back air pressure. The behaviors of sprays injected into the chamber through the conventional Bosch injection pump were visualized with side stroboscopic illumination. Comparison of the experimental results with predictions from the mathematical model confirmed the validity of the model. It was also found that during the initial phase of the injection period the penetration of sprays vs. time appeared to have two transition points; one corresponded to disintegration point of liquid fuel jet, the other to the beginning of steady state injection.

• Journal of the Korean Institute of Gas
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• v.24 no.1
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• pp.49-55
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• 2020

Experimental investigations were conducted to understand the multi-staged combustion characteristics of a swirl-stabilized double cone premixed burner nozzle used for industrial gas turbines for power generation. Multi-staged combustion is implemented by injecting the fuel through the existing manifold of the side slots as well as through the apex of the cone with two fuel injection angles which are slanted or axial. NOx and CO emissions, and wall temperature distributions were measured for various fuel distributions and operating conditions. Results show that NOx emissions are decreased when the fuel distribution to the apex is 3% of the total amount of fuel, which is due to more uniform fuel distribution inside the nozzle, hence less hot spots at the flame. NOx emissions are rather increased when the fuel distribution to the apex is 8% of the total amount of fuel for axial fuel injection by occurrence of flash back in premixing zone of burner.

• Journal of the Korean Vacuum Society
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• v.21 no.6
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• pp.312-321
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• 2012

The electric potential of plasma column is measured with the high voltage probe in a pair of atmospheric plasma jets operated by AC-voltage. According to the polarity of voltage applied to the electrodes, the polarity of plasma column potential has the same polarity of applied voltage. The high potential of plasma column at the side of high voltage electrode is decreased linearly along the plasma column to the ground side. Therefore, the plasma column seams to be a kind of resistor whose resistivity is a few 10s $M{\Omega}/m$. In the experiment of double-jets system, the polarity of plasma potential is verified to be the same polarity to the applied voltage. When the different voltage polarities are applied to the electrodes of double plasma jets, the attractive force is acted between two plumes at the merged plasma and the plasma potential is measured to be low as a few 10s V. When the same polarity of voltage is applied to the electrode, the repulsive force is acted and the plasma potential is measured to be high as a several 100s V at the merged plumes. In the exposure of plasma plume on the bio-substrate with the double plasma jets, the electric shock and thermal damage might be proportional to the plasma power which is the multiplication of the plasma potential and the plasma current.