• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.37 no.1
• /
• pp.62-68
• /
• 2009

The interaction between wall blowing and oxidizer flow can generate a very complicated flow characteristics in combustion chamber of hybrid rockets. LES analysis was conducted with an in-house CFD code to investigate the features of turbulent flow without chemical reactions. The numerical results reveal that the flow oscillations at a certain frequency exists on the fuel surface, which is analogous to those observed in the solid propellant combustion. However, the observation of oscillating flow at a certain frequency is only limited to a very thin layer adjacent to wall surface and the strength of the oscillation is not strong enough to induce the drastic change in temperature gradient on the surface. The visualization of fluctuating pressure components shows the periodic appearance of relatively high and low pressure regions along the axial direction. This subsequently results in the oscillation of flow at a certain fixed frequency. This implies that the resonance phenomenon would be possible if the external disturbances such as acoustic excitation could be imposed to the oscillating flow in the combustion chamber.

• Proceedings of the KSME Conference
• /
• 2000.11b
• /
• pp.609-614
• /
• 2000

This numerical study investigate resonance frequency of natural convection for steady state, periodic flow and chaotic flow in two-dimensional direct numerical simulations, differentially heated, vertical cavities having aspect ratios near unity. The enclosure cavity has isothermal and time dependent temperature side walls and adiabatic top/bottom walls. The aspect ratio is 1/3, 1/2, 1, 2, and 3 for the varying Rayleigh number. Resonance frequency for AR=1 has decrease as the aspect ratio and the Rayleigh number are increasing.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.36 no.11
• /
• pp.1055-1063
• /
• 2012

Fluctuating wall pressures were measured using an array of 16 piezoelectric transducers beneath a turbulent boundary layer. The coating used in this experiment was an open-cell, urethane-type foam with a porosity of approximately 50 ppi. The ultimate objective of the coating is to provide a mechanical filter to reduce the wall pressure fluctuations. The boundary layer on the flat plate was measured by using a hot wire probe, and the CPM method was used to determine the skin friction coefficient. The wall pressure autospectra and streamwise wavenumber-frequency spectra were compared to assess the attenuation of the wall pressure field by the coating. The coating is shown to attenuate the convective wall pressure energy. However, the relatively rough surface of the coating in this investigation resulted in a higher mean wall shear stress, thicker boundary layer, and higher low-frequency wall pressure spectral levels compared to a smooth wall.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.33 no.7
• /
• pp.1-8
• /
• 2005

For study on the unsteady wall interference effect, flows around a circular cylinder in closed test-section wind tunnels have been numerically investigated by solving compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with the Roe's flux-difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The computed results showed that the unsteady pressure gradient over the cylinder is enhanced by the wall interference, and as a result the fluctuations of lift and drag are augmented. The drag is further increased because of the lower base pressure. The vortex shedding frequency is also increased by the wall interference. The pressure on the test section wall shows the harmonics having the shedding frequency contained in the wall effect.

• Proceedings of the Acoustical Society of Korea Conference
• /
• spring
• /
• pp.443-446
• /
• 2002

해양연구원에서는 최근에 5면에 흡음재를 부착하여 반사파의 영향을 최소화한 무향수조를 제작하였다. 본 논문에서는 사각, 삼각, 해닝, 램프 등 4종류의 임펄스 신호를 이용하여 표면 및 벽면에서의 반사파 특성을 해석하고 있다. 표면 반사파의 경우 프로젝터의 가장 근접한 위치에서의 음압신호는 직접파 및 반사파의 관계가 주파수에 관계없이 일정한 반면 멀리 떨어진 위치에서는 반사파가 더 크게 나타나는 현상이 관찰되었다. 이는 중간에 위치한 하이드로폰의 간섭 현상에 의한 것으로 판단된다. 흠음 벽면의 경우 반사파는 거의 존재하지 않으며 따라서 반사계수는 측정한 주파수 범위$(\~100kHz)$에서 거의 0에 가까운 것을 확인할 수 있었다. 이로서 무향수조의 특성이 만족할만한 수준인 것을 확인하였다.

• Proceedings of the Acoustical Society of Korea Conference
• /
• autumn
• /
• pp.239-242
• /
• 2004

터보팬엔진등에서 발생하는 음장의 특징은 지배적인 음향모드 구조를 가진다는 점이다. 이러한 구조를 파악 해내는 것은 소음원을 찾아내고 그 소음원의 전파를 규명하여 소음을 저감하는데 중요한 기여를 할 수 있다. 음향모드의 검출을 위해서 많은 연구가 이루어져 왔으며 센서의 이용방식에 따라 원음장에서의 측정법, 회전형 갈퀴 (rotating rake)를 이용한 측정법, 덕트내 벽면에 센서를 분포시키는 측정법으로 나눌 수 있다. 이중 벽면에 센서를 위치시키는 방법은 다른 방법에 비해 빠르게 측정결과를 얻을 수 있으며 측정장치가 간단하다는 장점이 있다. 본 연구에서는 기존에 이 방법의 단점으로 여겨져 왔던 많은 수의 센서를 줄이는 새로운 센서 배치에 대해 기술하였으며 측정하고자 하는 주파수에 존재하는 전체 모드의 개수와 비교하여 최소한의 센서 개수를 설명하였다.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.38 no.1
• /
• pp.64-72
• /
• 2010

Due to the interaction between main oxidizer flow and the wall injected flow resulting from the regression process, a specific time characteristics identified in the frequency spectrum of streamwise velocity is generated in the hybrid rocket motor. In order to understand the response of the turbulent flow to two different types of external momentum forcing, LES analysis was conducted without considering the combustion. It turns out that both concentrated and distributed forcings do not lead to the disastrous resonance phenomenon. Energy contents are enhanced due to the added momentum but the peak frequency was not modified in the turbulent flow near the end of the rocket motor. Natural frequency of the flow system should be taken into account to further pursue the instability issue by using external forcing.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.24 no.2
• /
• pp.83-93
• /
• 2020

Dynamic pressure transducer needs to be flush-mounted on hardware due to frequency response characteristics of pressure transmission system. However, it is sometimes necessary to be mounted in recessed configuration due to insufficient space for sensor installation and for protection of sensor from thermal damage. Dynamic response characteristics should be considered due to distortion of original dynamic pressure signal in the pressure transmission system. In this study, small perturbation model and 2nd order reduced model were compared with experiments and a guideline for selecting a frequency response model was suggested.

• Proceedings of the Acoustical Society of Korea Conference
• /
• 1998.06e
• /
• pp.107-110
• /
• 1998

차량의 주요 소음원부터 실내 탑승자의 귀의 위치까지 도달되는 소음의 전달에 대한 전체적인 경로 및 주파수특성에 대한 해석은 차량의 구조-음향적인 특성이 복잡하므로 매우 어렵다. 그러나, 중-고주파수에 대한 대책에 있어서는 흡차음재가 유용함은 이미 알려진 사실이다. 차실 벽면에 사용된 흡/차음재는 소음레벨에 부분적인 기여를 함은 물론이고 음질에도 영향을 미친다. 소음 레벨에 있어서는 수백 Hz 이하의 저주파수 성분이 주요하며, 음질에는 중-고주파수 대역의 소음특성이 큰 영향을 미친다[1]. 본 논문에서는, 실험적으로 측정된 소음원의 특성을 수치해석 모델에 이용하여 소음레벨 저감과 음질개선을 위한 흡/차음재의 개선방향을 모색하였다.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
• /
• 2000.05a
• /
• pp.101-104
• /
• 2000

In this paper. the electric field intensity has been theoretically evaluated by the method of moments in a cavity by a external source. The numerical results show a good agreement with the results of other papers. It is found that the electromagnetic penetration through aperture in the cavity shows peak values at some frequencies dependent largely on the structure of the cavity and aperture.