• Title, Summary, Keyword: 팽창파

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Pochhammer 방정식의 분산곡선구성에서 본 비혼합해의 활용 방안

  • 김윤영
    • Journal of the KSME
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    • v.31 no.4
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    • pp.332-340
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    • 1991
  • 본 글에서는 속이 찬 실린더(solid cylinder)에서의 비대칭 탄성파전파 문제를 풀기 위한 해석적 방법의 일부를 소개하고자 한다. 속이찬 실린더에 있어서는 측면벽의 경계조건에 상관없이 평 판에서의 Fourier 시리즈와 유사한 단순해가 존재하지 않는다고 밝혀져 왔다(1). 그러나 최근 발표된 본인의 논문(2)에서 지적된 것처럼, 매우 특별한 측면 경계조건을 갖는 경우에만 정해가 존재한다. 특히 탄성파전파에 관한 한, 이러한 정해는 물리적으로 볼 때 팽창파(dilatational wave)와 전단파(shear wave)가 서로 얽히지 않는 상태에 해당되기 때문에, 소위 "비혼합 해(uncoupled solution)"라 불린다. 이 "비혼합해(uncoupled solution)"의 실제 사용 예를 들면, 상술된 바와 같이 일반적인 측면 경계조건을 갖는 속이 찬 실린더 문제를 풀기 위한 시도함 수(trial function)로 사용될 수 있을 것이다. 주지하는 바와 같이 자유측면벽(traction-free cylindrical wall)을 갖는 속이 찬 실린더는 공학적으로 매우 중요한 구조요소이다. 이 경우에는 측면벽의 경계조건으로 말미암아, 해가 정해의 형태로 존재하지 않는다. 특히 이 구조물에서의 탄성파전파 문제를 다루고자 할 때, 먼저 분산관계식(dispersion relation)을 구한 다음, 이를 이 용해 경계문제를 푸는 것이 상용적으로 사용되는 방법이다. 이 분산 관계식은 파장과 주파수 와의 관계를 나타내는 것으로, 그 복잡성으로 말미암아 이 식을 사용되는 수치해법으로 정확하게 구하는 것은 거의 불가능하다. 따라서, 본 글에서는 특별한 측면벽을 갖는 속이 찬 실린더의 비혼합해를 활용하여 자유측면벽을 갖는 속이 찬 실린더의 분산관계식(pochhammer의 분산관 계식이라 불린다)을 구하는 법을 설명하고자 한다. 이를 위해 비혼합해가 존재하는 측면경계조 건에 대해 먼저 살펴보고자 한다.조 건에 대해 먼저 살펴보고자 한다.

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A Computational Study of Flowfield for a Vent Mixer in Supersonic Flow (초음속 유동장 내 벤트 혼합기에 관한 수치해석 연구)

  • Kim, Chae-Hyoung;Jeung, In-Seuck
    • Journal of the Korean Society of Propulsion Engineers
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    • v.12 no.2
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    • pp.33-39
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    • 2008
  • To improve the mixing efficiency and to reduce the pressure loss, it is needed to develope a new mixing device for supersonic combustion. The vent mixer is introduced as the new supersonic fuel-air mixer. Computational analyses, that include pressure profile, density contour, and streamline tracing, have been carried out. The expansion wave generates at the end of the extended upper wall of the mixer. And it reduces the shock wave from the hole. Incoming air flow through the hole makes several recirculation regions which increase the mixing efficiency, and the separation region at the downward wall expends the boundary layer which reduces the pressure loss.

Experimental Study on the Slanted Portals for Reducing the Micro-pressure Waves in High-speed Train-tunnel System(I) (고속철도 터널에서 경사갱구 입구의 미기압파 저감성능에 관한 연구(I))

  • Kim, Dong-Hyeon;Shin, Min-Ho;Han, Myeong Sik
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.2 no.2
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    • pp.3-10
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    • 2000
  • The compression wave produced when a high-speed train enters a tunnel propagates along the tunnel ahead of the train. The micro pressure wave related to the compression wave is a special physics phenomena created by high-speed train-tunnel interfaces. A among methods for the purpose of reducing the micro pressure wave is to delay the gradient of the compression wave by using aerodynamic structures. The objective of this paper is to determine the optimum slanted portal using the moving model rig. According to the results, the maximum value of micro pressure wave is reduced by 19.2% for the $45^{\circ}$ slanted portal installed at the entrance of the tunnel and reduced by 41.9% for the $45^{\circ}$ slanted portals at the entrance and exit of the tunnel. Also it is reduced by 34.6% for the $30^{\circ}$ slanted portals installed at the entrance and exit of the tunnel.

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A Numerical Simulation on the Process of Diaphragm Opening in Shock Tube Flows (충격파관 유동의 파막과정에 관한 수치 시뮬레이션)

  • Shin, Choon-Sik;Jeong, June-Chang;Suryan, Abhilash;Kim, Heuy-Dong
    • Journal of the Korean Society of Propulsion Engineers
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    • v.13 no.1
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    • pp.27-33
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    • 2009
  • Shock tube flow measurement has been often hampered a finite opening time of diaphragm, but there is no systematic work to investigate its effect on the shock tube flow. In the present study, both the experimental and computational works have been performed on the shock tube flows at low pressure ratios. The computational analysis has been performed using the two-dimensional, unsteady, compressible Navier-Stokes equations, based upon a TVD MUSCL finite difference scheme. It is known that the present computational results reproduce the experimental data with good accuracy and simulate successfully the process of diaphragm opening as a function of time. The concept of an imaginary center is introduced to quantify the non-centered expansion wave due to a finite opening time of diaphragm. The results obtained show that the diaphragm opening time is reduced as the initial pressure ratio of shock tube increases, leading to the effect of a finite opening time of diaphragm to be more remarkable at low pressure ratios.

Expansion of a Fire-Ball and Subsequent Shock-Wave Propagation due to Underwater TNT Explosion (해저에서 TNT 폭발에 의한 파이어볼의 팽창과 이에 따른 충격파 전파)

  • Kwak, Ho-Young;Kang, Ki-Moon;Ko, Il-Gon
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.35 no.7
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    • pp.677-683
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    • 2011
  • Until now, several empirical models for assessing the damage due to TNT explosions have been proposed. A set of analytical solutions for the time-dependent radius of an expanding fire-ball after detonation of TNT was obtained by solving the continuity, Euler (momentum), and energy equations with a "polytrope" assumption at the fire-ball center. The shock waves developed from the rapid expansion of a fire-ball under water were obtained by using the KirkwoodBBethe hypothesis. The calculated period of bubble oscillation and the maximum radius of the bubble resulting from the fire-ball due to a violent underwater TNT explosion were in good agreement with the experimental data.

The Effect of the Variation of Pressure Ratio on the Characteristics of Lateral Forces in an Over-Expanded Nozzle (압력비 변화과정이 과팽창 노즐에서 발생하는 횡력 변동 특성에 미치는 영향)

  • Lee, Jong-Sung;Kim, Heuy-Dong
    • Journal of the Korean Society of Propulsion Engineers
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    • v.14 no.6
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    • pp.38-44
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    • 2010
  • The shock wave and boundary layer interaction patterns in an over-expanded rocket nozzle are associated with the production of undesirable side-forces during the start-up and shut-down processes of the engine. In the present work, a computational study is carried out to investigate the effect of the transient nozzle pressure ratio (NPR) on the flow fields inside the nozzle. The unsteady, compressible, axisymmetric, Navier-Stocks equations with SST k-${\omega}$ turbulence model are solved using a fully implicit finite volume scheme. NPR is varied from 2.0 to 10.0, in order to simulate the start-up and shut-down processes of the rocket engine. It is observed that the interaction patterns and the hysteresis phenomenon strongly depend on the time variation of NPR, leading to significantly different characteristics in the lateral forces.

Theoretical study on compression wave propagating in a sudden reduction duct (급축소관을 전파하는 압축파에 관한 이론적 연구)

  • Kim, Hui-Dong;Kim, Tae-Ho
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.21 no.1
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    • pp.89-98
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    • 1997
  • Compression waves propagating in a high speed railway tunnel impose large pressure fluctuations on the train body or tunnel structures. The pressure fluctuations can cause ear discomfort for the passengers and increase the aerodynamic resistance of trains. As a fundamental research to resolve the pressure wave phenomenon in the tunnel, a steady theory of Chester-Chisnell- Whitham was applied to a simple shock tube with a sudden cross-sectional area reduction to model trains inside the tunnel. The results of the present theoretical analysis were compared with the experiments of the shock tube. The results show that the reflected compression wave from the model becomes stronger as the strength of incident compression wave and the blockage ratio increase. However, the compression wave passing through the model is not strongly dependent on the blockage ratio. The theoretical results are in good agreement with the experiments.

Experimental Study of Micro-Shock Tube Flow (Micro-Shock Tube 유동에 대한 실험적 연구)

  • Park, Jin-Ouk;Kim, Gyu-Wan;Rasel, Md. Alim Iftakhar;Kim, Heuy-Dong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.39 no.5
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    • pp.385-390
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    • 2015
  • The flow characteristics in micro shock tube are investigated experimentally. Studies were carried out using a stainless steel micro shock tube. Shock and expansion wave was measured using 8 pressure sensors. The initial pressure ratio was varied from 4.3 to 30.5, and the diameter of tube was also changed from 3mm to 6mm. Diaphragm conditions were varied using two types of diaphragms. The results obtained show that the shock strength in the tube becomes stronger for an increase in the initial pressure ratio and diameter of tube. For the thinner diaphragm, the highest shock strength was found among varied diaphragm condition. Shock attenuation was highly influenced by the diameter of tube.

A Study on the Flow of POSRV in Reactor Coolant System (원자로 냉각계통의 POSRV 유동에 관한 연구)

  • Kwon, Soon-Bum;Kim, In-Goo;Ahn, Hyung-Joon;Lee, Dong-Won;Baek, Seung-Cheol;Kim, Kyung-Ho
    • Proceedings of the KSME Conference
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    • pp.568-573
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    • 2003
  • When a safety valve equipped in a nuclear power plant opens in an instant by an accident, a moving shock wave propagates downstream the valve, inducing a complicated unsteady flow field. The moving shock wave may exert severe load to the structure. So, to reduce the load acting on the wall of POSRV, a gradual opening of POSRV is adopted in general. In theses connections, a numerical work is performed to investigate the effect of valve opening time on the unsteady flow fields downstream of the valve. Compressible, two-dimensional Navier-Stokes equations are used with the finite volume method. The obtained results show that sharp pressure rise through moving shock tor the case of instant opening is attenuated by employing the gradual opening of valve. It is turned that the flows for the two cases of gradual valve opening time show the similar to that of highly under-expanded one in jet structure having expansion and compression waves and Mach stem. Also, comparing with the results for the two cases of opening time, the shorter the valve opening is, the pressure gradient at the downstream of the valve becomes softly.

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An Experimental Study on Supersonic Jet Issuing from Gas Atomizing Nozzle (I) (가스 미립화용 노즐로부터 방출되는 초음속 분류에 관한 실험적 연구)

  • Kim, Hui-Dong;Lee, Jong-Su
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.2
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    • pp.697-709
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    • 1996
  • Supersonic axisymmetric jets issuing from various kinds of nozzles with a throat diameter of a few millimeters were experimentally investigated. The exit Mach number and Reynolds number based on the throat diameter of nozzle were in the range of 1.0 ~ 5.9 and 8.4$\times$ $10^4$ ~ 2.9$\times$$10^6$, respectively. The nozzle pressure ratio was varied from 5 to 85. Present paper aims to offer fundamental information of the supersonic free-jets, with an emphasis to give data with which the shape of the free-jets can be depicted under a specified condition. Experimental data are summarized to enable an estimation of the shape of the supersonic free-jets. The result shows that the shape of free-jets is dependent on only the nozzle pressure ratio.