• 한국전산유체공학회:학술대회논문집
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• 2007.10a
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• pp.21-26
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• 2007

This study focuses on the near-field vortical structure and dynamics of coaxial jets. The characteristics of laminar flow and mixing in coaxial jets are investigated using a unsteady flow simulation. In order to analyze the geometic effects on the vortical structure, several cases of different configurations are selected for various values of the velocity ratio of inner jet to outer jet. From the result, it is confirmed that mixing is promoted by the development of vortical structure and the interaction between inner jet and outer jet. This feature is strongly related to the vortex frequency in the shear-layers. The vortex frequency depends on the velocity ratio and the lip thickness of inner nozzle, but the outer pipe length has no effect on the frequency variation.

• Journal of the Korea Institute of Military Science and Technology
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• v.18 no.4
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• pp.415-422
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• 2015

The flow field structures of dual jet-in-cross-flow were examined experimentally for in-lined perforated damage holes configuration using particle image velocimetry. Ensemble averaged in-plane velocity and vorticity data in the jet were determined to study the mean jet structure. Jets are formed by pressure differences between upper and lower airfoil surface. The flow structure of vicinity of the thru holes consist of a vortical structure that wrap around the jets like a horseshoe and develop further downstream through a pair of stream-wise vortices. The shape, size and location of the horseshoe vortex were found to be dependent on the angle of attack. In spite of the existence of battle damage holes, the effect on the control force was insignificant when the damage size was not large enough.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.408-413
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• 2004

Numerical and experimental investigation were car-ried out to clarify the flow structure of underexpanded jet from a square nozzle. The square nozzle rep-resents one of the clustered combustors of a linear aerospike engine. From the numerical results, the three-dimensional shock wave of the underexpanded square jet was found to be composed of two shocks. One is the intercepting shock which corresponds to the shock observed in two-dimensional planar jet. The other is the recompression shock divided into two types. The expansion fans coming from the nozzle edges interact with each other at the comers of the nozzle exit, and overexpanded regions are generated. Therefore one of the two recompression shocks is formed at the comers of the nozzle exit behind the overexpanded regions. As the jet goes downstream, the overexpanded regions grow larger to coalesce at the symmetry planes. Then, the other type of the recompression shock is generated. The three-dimensional shock structure formed by the intercepting shock and the recompression shocks dominates the expansion of the jet boundary. The shock detection algorithm us-ing CFD results was developed to reveal the relation between the shock waves and the jet boundary, and it was found that the cross-sectional jet shape becomes cross-shape. The key features observed in the numerical investigation were verified by the experimental results. The shock structure at the diagonal plane was in good agreement with the experimental schlieren images. Moreover, the cross-sections visualized by the Mie scattering method confirmed that the cross-section of the jet becomes cross-shape.

• Journal of the Korean Society of Safety
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• v.24 no.1
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• pp.14-20
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• 2009

Direct numerical simulations(DNS) were performed for the prediction of transient buoyant jet diffusion flames where the Froude numbers(Fr) are 5 and 160, respectively. The thermodynamic and transport properties were evaluated using CHEMKIN package to enhance the prediction performance of the DNS code. The simulated buoyant jet diffusion flame of Fr=5 and 160 showed the transient, dynamic motion well. It was identified that the buoyant jet flames were flickered periodically, and the simulated flickering frequency of the jet diffusion flame of Fr=5 was 12.5Hz, which was in good agreement with the experimental results. The flamelet structures of the buoyant jet diffusion flames could be well understood by comparing the scalar dissipation rates(SDR) and the heat release rates(HRR) of the flames. It was found that the SDR was strongly coupled with the HRR in the buoyant jet diffusion flames.

• Journal of Mechanical Science and Technology
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• v.17 no.12
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• pp.2107-2115
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• 2003

A supersonic dual coaxial jet has been employed popularly for various industrial purposes, such as gasdynamic laser, supersonic ejector, noise control and enhancement of mixing. Detailed characteristics of supersonic dual coaxial jets issuing from an inner supersonic nozzle and outer sonic nozzles with various ejection angles are experimentally investigated. Three important parameters, such as pressure ratios of the inner and outer nozzles, and outer nozzle ejection angle, are chosen for a better understanding of jet structures in the present study. The results obtained from the present experimental study show that the Mach disk diameter becomes smaller, and the Mach disk moves toward the nozzle exit, and the length of the first shock cell decreases with the pressure ratio of the outer nozzle. It was also found that the highly underexpanded outer jet produces a new oblique shock wave, which makes jet structure much more complicated. On the other hand the outer jet ejection angle affects the structure of the inner jet structure less than the pressure ratio of the outer nozzle, relatively.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.7
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• pp.927-936
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• 1998

In the fully developed pipe flow, when jet is injected in cross to the flow there are complex transition flows caused by interaction of the cross flow and jet. These interactions are studied by means of the flow visualization methods and frequency analysis using a hot-wire anemometer. The velocity range of cross flow of the pipe is 0.3 m/s ~ 1.2 m/s and the corresponding Reynolds number, R$\sub$p/, based on the pipe diameter is 2.25 * 10$\^$3/ ~ 9.02 * 10$\^$3/. The velocity ratio (R), jet velocity/cross flow velocity, is chosen from 2 to 10. A circular cylinder is placed in the pipe instead of jet to observe the vortex shedding from the solid body. To compare the jet and circular cylinder flow, the vortical structure is analyzed in both cases and the structure of vortices and the origin of its formation are investigated, especially. The vortex shedding of the dominant coherent structure is compared between the jet flow and the circular cylinder flow. In the case of the jet flow, the Strouhal numbers are different depending on the existence of the upright vortex as well as the velocity ratio (R).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.3
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• pp.1028-1039
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• 1996

This study was focused on the examination of the flame structure and the combustion characteristics of diffusion flame which was formed the turbulent shear flow of a double coaxial air jet system. The shear flow was formed by the difference velocity of surrounding air jet(U$\_$s/) and center air jet (U$\_$c/). So experimental condition was divided S-type flame (.lambda. > 1) and C-type flame (.lambda. < 1) by velocity ratio .lambda. (=U$\_$s//U$\_$c/). For examination of the flame structure and the combustion characteristics in diffusion flame, coherent structure was observed in flame by schlieren photograph method. We measured fluctuating temperature and ion current simultaneously and accomplished the statistical analysis of its. According to schlieren photograph, the flame was stabilized in the rim of the direction of lower velocity air jet, coherent eddy was produced and developed by higher velocity air jet. The statistical data of fluctuating temperature and ion current was indicated that reaction was dominated by higher velocity air jet. The mixing state of burnt gas and non-burnt gas was distributed the wide area at Z = 100 mm of C-type flame.

• Journal of Ocean Engineering and Technology
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• v.35 no.6
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• pp.403-413
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• 2021

As the size of ships increases, the size and output power of their thrusters also increase. When a large ship berths or unberths, the jet flow produced from its thruster has an adverse effect on the stability of quay walls. In this study, we conducted a numerical analysis to examine the impact of the thruster jet flow of a 30,000 TEU container ship, which is expected to be built in the near future, on the stability of a quay wall. In the numerical simulation, we used the fluid-structure interaction analysis technique of LS-DYNA, which is calculated by the overlapping capability using an arbitrary Lagrangian Eulerian formulation and Euler-Lagrange coupling algorithm with an explicit finite element method. As the ship approached the quay wall and the vertical position of the thruster approached the mound of the quay wall, the jet flow directly affected the foot-protection blocks and armor stones. The movement and separation of the foot-protection blocks and armor stones were confirmed in the area affected directly by the thruster jet flow of the container ship. Therefore, the thruster jet flows of ultra-large ships must be considered when planning and designing ports. In addition, the stability of existing port structures must be evaluated.

• Journal of Advanced Marine Engineering and Technology
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• v.33 no.2
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• pp.259-266
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• 2009

An experimental study was conducted to investigate the effects of forcing amplitude on the flow structure near the nozzle exit of forced jet diffusion flames. The jet was excited up to the blowout occurrence by a considerable large amplitude with a periodic velocity fluctuation at the tube resonating frequency. In the attached flame regime, we disclosed the very interesting result newly that adding of a moderate forcing amplitude caused the jet flame to become longer in spite of being forced. Particular attention is focused on the turnabout mechanism of vortex roll-up around the elongated flame, which has not been reported previously, and on the inner coherent structure of the forced jet in the attached flame regime. From the velocity and flow visualization results, it was ascertained that the surrounding air due to the occurrence of negative velocity parts was suck into the fuel nozzle. To aid in understanding the rotating phenomenon of coherent structure, we present a schematic diagram of the turnabout mechanism of vortex roll-up. The mechanism of vortex turnabout phenomenon can be easily understood by considering the positive and negative velocity amplitudes about the instantaneous velocity of the forcing flow, as shown in this diagram.

• Journal of Korea Water Resources Association
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• v.30 no.5
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• pp.467-475
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• 1997

In this study, breakwater model which has several outlet pipes to discharge heated water is settled in the experimental open channel and velocity distribution of wall jet is measured. Numerical simulation of velocity structure of wall jet using 3-dimensional computer model. Fluent model, is also carried out. The calculated results are verified with the experimental results and the flow characteristics of wall jet are investigated. The length of zone of flow establishment of wall jet is shorter than that of free jet, and the diminution rate of jet centerline longitudinal velocity is larger than that of free jet. Characteristics of buoyant jet and non-buoyant simple jet simulated by Fluent model are compared. Near the outlet pipe, in the region where x/lQ is over 15, this is reversed. Comparison of vertical distribution of longitudinal velocity shows that positive velocity of non-buoyant jet is bigger than that of buoyant jet in the bottom layer and in the upper layer, negative velocity of non-buoyant jet is bigger too. Flow separation in free surface of the buoyant jet occurs in smaller distances from the outlet than the non-buoyant jet. Buoyant jet expands faster than the non-buoyant jet in vertical direction.