• International Journal of Aeronautical and Space Sciences
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• v.13 no.1
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• pp.43-57
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• 2012

In a modern aircraft, there are many variations in its mass, stiffness, and aerodynamic characteristics. Recently, an analytical approach was proposed, and this approach uses the idea of uncertainty to find out the most critical flight flutter boundary due to the variations in such aerodynamic characteristics. An analytical method that has been suggested to predict robust stability is the mu method. We previously analyzed the robust flutter boundary by using the mu method, and in that study, aerodynamic variations in the Mach number, atmospheric density, and flight speed were taken into consideration. The authors' previous attempt and the results are currently quoted as varying Mach number mu analysis. In the author's previous method, when the initial flight conditions were located far from the nominal flutter boundary, conservative predictions were obtained. However, relationships among those aerodynamic parameters were not applied. Thus, the varying Mach number mu analysis results required validation. Using an optimization approach, the varying Mach number mu analysis was found out to be capable of capturing a reasonable robust flutter boundary, i.e., with a low percentage difference from boundaries that were obtained by optimization. Regarding the optimization approach, a discrete nominal flutter boundary is to be obtained in advance, and based on that boundary, an interpolated function was established. Thus, the optimization approach required more computational effort for a larger number of uncertainty variables. And, this produced results similar to those from the mu method which had lower computational complexity. Thus, during the estimation of robust aeroelastic stability, the mu method was regarded as more efficient than the optimization method was. The mu method predicts reasonable results when an initial condition is located near the nominal flutter boundary, but it does not consider the relationships that are among the aerodynamic parameters, and its predictions are not very accurate when the initial condition is located far from the nominal flutter boundary. In order to provide predictions that are more accurate, the relationships among the uncertainties should also be included in the mu method.

• Journal of Sensor Science and Technology
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• v.32 no.4
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• pp.223-231
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• 2023

An integrated-optical biochemical sensor structure that can perform homogeneous and surface sensing using a 2×2 balanced-bridge Mach-Zehnder interference structure based on the optimized SOI slot optical waveguide was described, and its performance and characteristics were evaluated. Equations for the two output optical powers were derived and examined using the transfer matrices of a 3-dB coupler and phase shifter (channel waveguide). The length of the 3-dB coupler was determined such that the two output optical powers were same using these formulas. In homogeneous sensing, the effect of the refractive index of an analyte in the range of 1.33-1.36 on the two output optical power distributions was numerically derived, and the sensitivity was calculated based on each output and the difference between the two outputs, the former and the latter being 7.5796-19.0305 [au/RIU] and 15.2601-38.1351 [au/RIU], respectively. In the case of surface sensing, the sensitivity range of the refractive index of 1.337 based on each of the two outputs was calculated as -2.2490--3.5854 [au/RIU] and 1.2194-3.8012 [au/RIU], and the sensitivity range of 4.8048-7.0694 [au/RIU] was confirmed based on the difference between the two outputs.

• Journal of Mechanical Science and Technology
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• v.16 no.11
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• pp.1448-1456
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• 2002

The present study addresses the flow characteristics involved in the self-induced oscillations of the underexpanded jet impinging upon a cylindrical body. Both experiment and computational analysis are carried out to elucidate the shock motions of the self-induced oscillations and to find the associated major flow factors. The underexpanded sonic jet is made from a nozzle and a cylindrical body is placed downstream to simulate the impinging jet upon an obstacle. The computational analysis using TVD scheme is applied to solve the axisymmetric, unsteady, inviscid governing equations. A Schlieren system is employed to visualize the self-induced oscillations generated in flow field. The data of the shock motions are obtained from a high-speed video system. The detailed characteristics of the Mach disk oscillations and the resulting pressure variations are expatiated using the time dependent data of the Mach disk positions. The mechanisms of the self-induced oscillations are discussed in details based upon the experimental and computational results.

• Journal of computational fluids engineering
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• v.13 no.4
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• pp.66-71
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• 2008

This research computes the viscous flow field and aerodynamics around the model of a commercial passenger airplane, Boeing 747-400, which cruises in transonic speed. The configuration was realized through the reverse engineering based on the photo scanning measurement. In results, the pressure coefficients at the several wing section on the wing surface of the airplane was described and discussed to obtain the physical meaning. The lift coefficient increased almost linearly up to $17^{\circ}$. Here the maximum lift occurred at $18^{\circ}$ according to the angle of attack. And the minimum drag is expected at $-2^{\circ}$. The maximum lift coefficient occurred at the Mach number 0.89, and the drag coefficient rapidly increased after the Mach number of 0.92. Also shear-stress transport model predicts slightly lower aerodynamic coefficients than other models and Chen's model shows the highest aerodynamic values. The aerodynamic performance of the airplane elements was presented.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.3529-3533
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• 2007

The advantages of the SITVC(secondary injection thrust vector control) technique over mechanical thrust vector systems include a reduction in both the nozzle weight and complexity due to the elimination of the mechanical actuators that are used in conventional vectoring. Computational study is performed to understand the fluidic thrust vectoring control of an axisymmetric nozzle, in which secondary gas injection is made in the divergent section of the nozzle. The nozzle has a design mach number 3. The effect of injection hole number and shape of secondary jet on the mach number distribution of SITVC were investigated. The standard ${\kappa}$ - ${\epsilon}$ turbulence model solved the complex three-dimensional nozzle flows perturbed by the secondary gas jet. The numerical code was validated by experiment. The results showed that the mach number distribution of circular and square nozzle are similar each other. As number of second injection hole increasing, a effect of deflection was decreased.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2862-2867
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• 2007

The steady non-reacted compressible flow field in a symmetric micro-thruster, which is used for the accurate attitude control of a satellite, is analyzed varying the nozzle pressure ratio (NPR) to investigate the plume characteristics. The nozzle throat diameter is 0.06 inch and the area ratio is 56. The recirculation region is found just behind the normal shock at the several NPRs due to the locally adverse pressure gradient along the nozzle centerline when the environmental pressure is atmospheric. This phenomenon, the cause of flow loss, is similar to the flow behind a blunt body. As NPR increases the location of Mach disk, characteristics of the normal shock, moves downstream and its strength increases. The Mach number distribution appears in a wave-type patter after the normal shock because oblique shocks are reflected on the shock boundaries especially when NPRs are very high.

• Journal of the Korean Society of Propulsion Engineers
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• v.8 no.1
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• pp.27-37
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• 2004

The convergence characteristics of preconditioned Euler equations were studied. A perturbation analysis was conducted to understand the behavior of the preconditioned Euler equations. Various speed flows in a two-dimensional channel with a 10％ circular arc in the middle of the channel were calculated. Roe's FDS scheme was used for spatial discretization and the LU-SGS scheme was used for time integration. It is shown that the convergence characteristics of pressure and velocity were maintained regardless of the Mach numbers but that the convergence characteristics of temperature were strongly related to the Mach number and became worse as the Mach number decreased. The perturbation analysis well explained the trend of the convergence characteristics and showed that the convergence characteristics are strongly related with the behavior o( the Preconditioning matrix.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.6
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• pp.586-595
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• 2010

An analytical study based on physical understandings and aero-thermodynamic theories was conducted to observe the performance characteristics and to derive the essential design parameters of dual ramjet(dual-combustion/dual-mode) propulsion for wide Mach number. The performances and operating limitations of the engines with two types combustors, such as constant pressure- and constant area- combustor, over various flight Mach numbers was investigated. Finally, the transition Mach number from ramjet to scramjet was carried out to optimize performance load balancing of ramjet and scramjet.

• Journal of Mechanical Science and Technology
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• v.17 no.11
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• pp.1793-1800
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• 2003

The present study addresses an experimental investigation of the near field flow structures of supersonic, dual, coaxial, free, jet, which is discharged from the coaxial annular nozzle. The secondary stream is made from the annular nozzle of a design Mach number of 1.0 and the primary inner stream from a convergent-divergent nozzle. The objective of the present study is to investigate the interactions between the secondary stream and inner supersonic jets. The resulting flow fields are quantified by pitot impact and static pressure measurements and are visualized by using a shadowgraph optical method. The pressure ratios of the primary jet are varied to obtain over-expanded flows and moderately under-expanded flows at the exit of the coaxial nozzle. The pressure ratio of the secondary annular stream is varied between 1.0 and 4.0. The results show that the secondary annular stream significantly changes the Mach disc diameter and location, and the impact pressure distributions. The effects of the secondary annular stream on the primary supersonic jet flow are strongly dependent on whether the primary jet is under-expanded or over-expanded at the exit of the coaxial nozzle.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.10
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• pp.61-70
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• 2006

A Supersonic air intake model was designed for the high performance ramjet and dual-mode scramjet engine to operate at Mach 4 flight condition. The air intake was tested in the blowdown-type wind tunnel of Kyushu University to identify the internal flow characteristics corresponding to the flight parameters such as the back pressure, angle of attack and angle of yaw. Flow visualization was achieved by the Schlieren and oil flow visualization techniques. The intake performance was analyzed quantitatively based on the surface pressure and total Pressure measurements. The experimental results were compared with the computational fluid dynamics results. The present study exhibits the fundamental but rarely found experimental results of the high Mach number supersonic air intake.