• 한국음향학회지
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• 제26권6호
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• pp.293-300
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• 2007

수중운동체가 주행할 때 수중운동체의 표면에는 난류 경계층이 형성되고, 난류 경계층 내 벽면 변동압력은 탄성체인 수중운동체의 표면을 가진시켜 유동 유기 소음을 발생시킨다. 이러한 유체 소음을 감소시키기 위한 하나의 방법으로 수중운동체 표면에 유연재를 부착하여 수중운동체의 유동 유기 방사 소음을 감소시키는 방법이 제안되기도 한다. 본 논문에서는 유연재 부착에 따른 수중에서의 유체소음의 변화 특성을 살펴보기 위하여 유연재가 코팅되지 않은 강판 시편과, 고무 재질의 Neoprene 및 폴리우레탄 재질의 유연재가 부착된 강판시편을 저소음 공동수조에 설치한 후 여러 가지 유속 조건에서 유체소음의 주요 소음원인 난류 경계층 내 벽면 변동압력을 측정하고 그 결과를 비교 분석하였다. 그 결과 유연재를 코팅한 경우에는 유연재로 인하여 난류 경계층의 두께가 두꺼워지는 등 경계층 유동이 변화하지만, 유연재에서의 난류 에너지의 소산으로 인하여 고주파수 대역에서 약 10dB의 벽면 변동압력의 감소 효과를 확인할 수 있었다.

• 한국소음진동공학회논문집
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• 제12권4호
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• pp.280-286
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• 2002

It has been long suspected that the transition region may give rise to local pressure fluctuations and radiated sound that are different from those created by the fully-developed turbulent boundary layer at equivalent Reynolds number. Experimental investigation described in this paper concerns the characteristics of pressure fluctuations at the transition. Flush-mounted microphones and hot wires are used to measure the pressure fluctuations and local flow velocities within the boundary layer in the low noise wind tunnel. From this experiment we could observe the spatial and temporal development process of T-S wave using Wigner-Ville method and find the relations between the characteristic frequency of T-S wave and free stream velocity and the boundary layer thickness based on nondimensional pressure spectra scaled on outer variables.

• 대한기계학회논문집B
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• 제24권7호
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• pp.945-956
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• 2000

The axisymmetric bodies considered in this study have hemispherical and ellipsoidal noses. The near-field pressure fluctuations over each nose model at $Re_D=2.43{\times}10^5$ were investigated in the laminar separation region and developing turbulent boundary layers using a 1/8' pin-holed microphone sensor. The wall pressure fluctuations were also measured in an axisymmetric boundary layer on a cylinder parallel to mean flow at a momentum thickness Reynolds number of 850 and a boundary layer thickness to cylinder radius ratio of 1.88.

• 물과 미래
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• 제27권3호
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• pp.95-105
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• 1994

수직수문하의 경계층 흐름(boundary layer flow)이 경계고정좌표계(Boundary- Fitted Coordinate System)에서 무작위 소용돌이 판 방법(Random Vortex Sheet Method)과 요소내 소용돌이 방법(Vortex-in-Cell Method)을 이용하여 수치계산되었다. 수치해에 의한 수문을 따라 형성된 경계층이 수축률의 실험자료와 비점성이론에 의한 그 결과의 차이를 유발하는 주원인인 것으로 보여진다. 그 동안 주원인일 것으로 믿어왔던 바닥면 경게층의 역할은 수문면의 그 것보다는 적은 것으로 수치계산되었다. 또한 차원해석을 통하여 경계층 흐름에 의한 수축율의 그 차이가 수문 길이의 평방근에 반비례하는 것으로 추정되었으며, 이는 Benjamin(1956)에 의하여 분석된 것과 결국 동일한 것임이 밝혀졌다. 수치모델과 차원해석에 따른 결과는 Benjamin(1956)에 의해 얻어진 수축률의 실허미와 비교하여 만족할 만하였다.

• Wind and Structures
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• 제20권4호
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• pp.527-548
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• 2015

Aeroelastic wind tunnel experiments were conducted for conventional and tapered super-tall building models to investigate the effect of taper on fundamental aeroelastic behaviors in various incident flows. Three incident flows were simulated: a turbulent boundary-layer flow representing urban area; a low-turbulent flow; and a grid-generated flow. Results were summarized focusing on the effect of taper and the effect of incident flows. The suppression of responses by introducing taper was profound in the low-turbulence flow and boundary-layer flow, but in the grid-generated flow, the response becomes larger than that of the square model when the wind is applied normal to the surface. The effects of taper and incident flows were clearly shown on the normalized responses, power spectra, stability diagrams and probability functions.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.69-74
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• 2004

In scramjet engines with sidewall compression inlet, it is well known that a non-uniform flow appears since a separated region is generated near the flow centerline on the body side. The separated region is caused by shock-boundary layer interaction and likely to cause un-start phenomena since the flow in the separated region is subsonic and acts as a communication path between the isolator and the combustor. In the present study, the non-uniform flow characteristics in the scramjet inlet-isolator region are numerically studied in detail. Effect of flow suction from body sidewall surface on the non-uniform flow field numerically examined to clarify the flow mechanism to suppress the un-start transition.

• 대한기계학회논문집B
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• 제22권6호
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• pp.771-787
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• 1998

A Navier-Stokes code with a modified low Reynolds number k-.epsilon. turbulence model was used to study the unsteady transitional boundary layer flow due to rotor-stator interaction. The modification, proposed by Launder, to improve prediction of stagnation flows was incorporated to the low Reynolds number k-.epsilon. turbulence model by Fan-Lakshminarayana-Barnett. Numerical solution is shown to capture well the calmed laminar flow as well as the wake induced transitional strip due to rotor-stator interaction and shows improvement, in terms of onset of transition and its length, over previous Euler/boundary layer solution. The turbulent kinetic energy shows local maximum along the upstream rotor wake in the wake induced transitional strip and this characteristics is observed untill the end of transition. The wake induced strip also shown apparent even in the laminar sublayer as the upstream rotor wake penetrates inside the boundary layer.

• 대한기계학회논문집B
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• 제25권1호
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• pp.96-107
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• 2001

An unsteady numerical simulation was performed to analyze flow structures behind a local suction/blowing in a flat-plate turbulent boundary layer. The local forcing was given to the boundary layer flow by means of sinusoidally oscillating jet. A version of the unsteady $\kappa$-$\xi$-f(sub)u model (Rhee and Sung 2000) was employed. The Reynolds number based on the momentum thickness was about Re(sub)$\theta$=1700. The forcing frequency was varied in the range 0.011$\leq$f(sup)+$\leq$0.044 with a fixed forcing amplitude A(sub)o=0.4. The predicted results were compared and validated with the experimental data. It was shown that the unsteady locally-forced boundary layer flow is predicted well by the $\kappa$-$\xi$-f(sub)u model. The effect of the pitch angle of local forcing on the reduction of skin friction was also examined.

• Journal of Mechanical Science and Technology
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• 제15권3호
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• pp.366-373
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• 2001

A numerical analysis of shock wave/boundary layer interaction in transonic/supersonic axial flow compressor cascade has been performed by using a characteristics upwind Navier-Stokes method with various turbulence models. Two equation turbulence models were applied to transonic/supersonic flows over a NACA 0012 airfoil. The results are superion to those from an algebraic turbulence model. High order TVD schemes predicted shock wave/boundary layer interactions reasonably well. However, the prediction of SWBLI depends more on turbulence models than high order schemes. In a supersonic axial flow cascade at M=1.59 and exit/inlet static pressure ratio of 2.21, k-$\omega$ and Shear Stress Transport (SST) models were numerically stables. However, the k-$\omega$ model predicted thicker shock waves in the flow passage. Losses due to shock/shock and shock/boundary layer interactions in transonic/supersonic compressor flowfields can be higher losses than viscous losses due to flow separation and viscous dissipation.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.143-149
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• 2008

A numerical study was performed for a vane of a compressor with a high-turning angle and meridional divergence. At first, the effect of the suction position was discussed. Then, the optimal suction position was applied to the cascades with the aspect ratio of 2.53 and 0.3, respectively, to get the knowledge of the effect of the endwall boundary layer removal on the secondary flow along the blade height. At last, using the critical principles of the three-dimensional separation, the topological structures of the flow patterns of the body surfaces and the separation configurations were discussed in detail. The results show that the largest reduction of the total loss can be achieved when the suction slot is near the suction side. The topological structure as well as the separation configuration varies due to boundary layer removal, which restrains the flow separation at the corner and delays or depresses the separation on the suction surface. Compared with the original cascade, the cascade with the endwall boundary layer removal has a higher blade loading along the most span. Furthermore the flow loss decreases and distributes uniformly along the span.