• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.5
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• pp.1-9
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• 2013

Variable area nozzle, where both throat and exit area vary, is required for optimal expansion and optimal nozzle shape upon operation of after-burner. Steady-state and transient analyses are carried out for each condition with and without afterburner operation and as a function of the location of the nozzle flap. Effects of that nozzle displacement on flow and thrust characteristics are analyzed from numerical results. With variable area nozzle adopted, the combustion field is variable in time, leading to periodically variable thrust. For off-design conditions, flow separation shows up due to over expansion at the flap tips and shock wave does in the nozzle due to under expansion. The undesirable phenomena can be solved by control of variable area nozzle.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.12
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• pp.1025-1031
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• 2012

The present paper deals with concept design of supersonic inlet based on compressible flow theory and flow control of bleeding in order to guarantee stability of supersonic inlet of ramjet engine in broad range of operating conditions. Shock instability, shock wave-boundary layer interaction and flow separation should be properly controlled to improve performance of the supersonic inlet. Considering shock strength, boundary layer and flow separation, the supersonic inlet is modified from the basic model which is designed under inviscid theory. Consequently, shock is stabilized, and required mass flow rate is obtained. Furthermore, bleeding is applied to the supersonic inlet to maintain performance in off-design conditions. Mass flow condition is adopted for modeling of bleeding effect, and performance of the supersonic inlet is evaluated by changing bleeding locations and numbers.

• Journal of Mechanical Science and Technology
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• v.16 no.10
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• pp.1303-1313
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• 2002

Effects of the reduced frequency of upstream wake on downstream unsteady boundary layer flow were simulated by using a Wavier-Stokes code. The Wavier-Stokes code is based on an unstructured finite volume method and uses a low Reynolds number turbulence model to close the momentum equations. The geometry used in this paper is the MIT flapping foil experimental set-up and the reduced frequency of the upstream wake is varied in the range of 0.91 to 10.86 to study its effect on the unsteady boundary layer flow. Numerical solutions show that they can be divided into two categories. One is so called the low frequency solution, and behaves quite similar to a Stokes layer. Its characteristics is found to be quite similar to those due to either a temporal or spatial wave. The low frequency solutions are observed clearly when the reduced frequency is smaller than 3.26. The other one is the high frequency solution. It is observed for the reduced frequency larger than 7.24. It shows a sudden shift of the phase angle of the unsteady velocity around the edge of the boundary layer. The shift of phase angle is about 180 degree, and leads to separation of the boundary layer flow from corresponding outer flow. The high frequency solution shows the characteristics of a temporal wave whose wave length is half of the upstream frequency. This characteristics of the high frequency solution is found to be caused by the strong interaction between unsteady vortices. This strong interaction also leads to destroy of the upstream wake strips inside the viscous sublayer as well as the buffer layer.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.9
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• pp.615-622
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• 2017

A normal shock wave is initially formed in the shock tube that migrates towards the closed end of the tube, which, in turn, leads to the reflection of shock. Due to the interaction of the reflected shock with the boundary layer, bifurcation of the shock wave takes place. A shock train will be generated after the bifurcated shock wave approaches the contact surface. Until now, only a few studies have been conducted to investigate this shock train phenomenon inside the shock tube. For the present study, a CFD analysis has been performed on a two dimensional axisymmetric model of a shock tube using unsteady, compressible Navier-Stokes equations. In order to investigate the detailed characteristics of the shock train phenomenon, quantitative studies have been performed by varying shock tube length, diameter under fixed diaphragm, and pressure ratio inside a shock tube.

• Progress in Medical Physics
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• v.13 no.3
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• pp.156-162
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• 2002

In this study, we attempted in preparing high precision EEG measuring equipment. To measure EEG in high efficiency, pre-amplifier should get high performance common mode rejection ratio. Also, separation amplifier is essential to eliminate common line noise. So, our study were pointed at elevating the efficiency of eliminating noise, user safety and low noise characteristics. Prepared high precision pre-amplifier for EEG was A/D converted to automatically classify $\alpha$ wave, $\beta$ wave and $\theta$ wave. And converted data were Fast Fourier Transformed with real time DSP (Digital Signal Processing). Clinical demonstrations were carried out with healthy students, aged between 20 to 26 who has no histories of illness. To recognize the efficiency of the EEG, prepared EEG were used with MS equipment in low stimulated state and high stimulated state. Then, we studied at the effect of sensitivity on brain wave. From this study, it is known that our EEG equipment is efficient in sensitivity evaluation and suitable stimulations for each psychological state are required.

• Korean Journal of Veterinary Research
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• v.13 no.1
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• pp.23-30
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• 1973

Thin layer, paper and column chromatography were compared for the separation, detection and quantitation of three kinds of estrogen in urine of dairy cows. While thin layer chromatography utilizing silica gel was better for the detection of estrogens, column chromatography using celite 545 was preferable. Spectrophotometry was compared with fluorometry for determination of estrone, estradiol-17 ${\beta}$ and estriol eluted by paper chromatography and column chromatography. Optical density of three standard estrogens showed almost same curve at maximum absorption wave length of 230 and $282m{\mu}$. However, the former showed a higher peak. In fluorometry, the fluorescence intensity of estrone and estradiol-17 ${\beta}$ were rather strong, when the estrogens were dissolved in sulfuric acid, and showed higher sensitivity than that of the spectrophotometry. However, in the case of estriol was exceptional.

• Journal of the Korean Society of Visualization
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• v.9 no.2
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• pp.61-65
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• 2011

Several previous works on rocket nozzle flows have revealed the existence of the transition from FSS to RSS and the occurrence of asymmetric flow associated with the boundary layer separation, which can cause excessive side-loads of the propulsion system. Thus, it is of practical importance to investigate the asymmetric flow behaviors of the propulsion nozzle and to develop its control method. In the present study, the asymmetric flow control method using a cavity system was applied to supersonic nozzle flow. Time-dependent asymmetric flow was experimentally investigated with the rate of change of the nozzle pressure ratio. The results obtained showed that the cavity system installed on nozzle wall would be helpful in fixing the unsteady motions of the boundary layer separation, consequently reducing the possibility of the occurrence of the asymmetric flow.

• Journal of Ocean Engineering and Technology
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• v.22 no.3
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• pp.24-33
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• 2008

In this study, flow and creation and dissipation of vorticity around rectangular floating breakwater is investigated both experimentally and numerically. The PIV system(Particle image velocimetry) is employed to obtain the velocity field in the vorticity of rectangular structure. The numerical model, combined with ${\kappa}-{\varepsilon}$ turbulence model and the VOF method based on RANS equation, is used to analyze the turbulence structure. In the results of this study, the vorticity is found around conner of rectangular structure at all time domain, and creation and dissipation of vorticity are closely related to wave period. Separation points of phase of vortex due to flow separation for longer period waves are faster then for shorter period waves.

• Structural Engineering and Mechanics
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• v.85 no.1
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• pp.119-133
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• 2023

Impact event is the key factor influencing the operational state of the mechanical equipment. Additionally, nonlinear factors existing in the complex mechanical equipment which are currently attracting more and more attention. Therefore, this paper proposes a novel hybrid-separate identification strategy to solve the force identification problem of the nonlinear structure under impact excitation. The 'hybrid' means that the identification strategy contains both l1-norm (sparse) and l2-norm regularization methods. The 'separate' means that the nonlinear response part only generated by nonlinear force needs to be separated from measured response. First, the state-of-the-art two-step iterative shrinkage/thresholding (TwIST) algorithm and sparse representation with the cubic B-spline function are developed to solve established normalized sparse regularization model to identify the accurate impact force and accurate peak value of the nonlinear force. Then, the identified impact force is substituted into the nonlinear response separation equation to obtain the nonlinear response part. Finally, a reduced transfer equation is established and solved by the classical Tikhonove regularization method to obtain the wave profile (variation trend) of the nonlinear force. Numerical and experimental identification results demonstrate that the novel hybrid-separate strategy can accurately and efficiently obtain the nonlinear force and impact force for the nonlinear structure.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.4B
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• pp.413-419
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• 2008

In this study, we develop analytical solutions for long waves propagating over several types of axis-symmetric topographies where the water depth varies in an arbitrary power of radial distance. The first type is a cylindrical island mounted on a shoal. The second type is a circular island. To get the solution, the methods of separation of variables, Taylor series expansion and Frobenius series are used. Developed analytical solutions are validated by comparing with previously developed analytical solutions. We also investigate various cases with different incident wave periods, radii of the shoal, and the powers of radial distance.