A Two-dimensional Jet Under Active Excitation by Means of String Vibration

• Main Authors: Wen, Yung-Wei, 溫永煒
• Other Authors: Yu, Ming-Huei
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/85725247493488974856

碩士 === 國立中山大學 === 機械工程研究所 === 85 === 　　A two-dimensional hot jet under excitation by two fine steel strings is investigated experimentally. The jet exit Reynolds number is around 2500 and the density ratio of the jet exit to ambient density is 0.78. Two thin steel strings are placed in each side of the shear layer of the jet exit to actively excite the heated jet at varying frequency and amplitude. By schlieren flow visualization and temperature measurements, the effect of the vibrating string on the jet behavior is explored. The experimental results are presented as follows. When the heated jet is unexcited, its self-excited oscillation frequency is 116Hz (called fundamental frequency, f0). When the two strings vibrate at frequency of 1/3f0, 1/2f0, 2/3f0 and f0, with amplitude of 2∼3mm, either in transverse or streamwise direction, the instability structure of the flow field takes place in advance, the potential core region is shortened, and temperature fluctuation increases more rapidly in the near field. But the value of the downstream temperature fluctuation will be lower than unexcited flow. The preceding phenomenon is more conspicuous especially in case of transverse vibration. However, the different between them is when the hot jet is excited by string vibration in streamwise direction, the vortex structure will decrease even disappear. As a result, its energies cannot transmit downstream. So the jet spreads narrowly. When the hot jet is excited by string vibration in transverse direction, its vortex structures appear more unstable, its energies also transmit further area with vortex pairing form. So the jet spreads widely. When the hot jet is excited by strings with excitation frequency (100Hz) and amplitude smaller than 1mm, the value of the downstream tepmerature fluctuation is smaller than that in unexcited flow. This result is similar to the flow field excited by transverse vibration or streamwise vibration within various harmonic frequencies. Moreover, by analysis of temperature spectral, both of the fundamental and subharmonic modes are suppressed when the exciation frequency is 1/3f0 and 2/3f0. However, when the excitation frequency is 1/3f0 and f0, the spectral energy of the fundamental and subharmonic frequency is increasing rapidly and there always exists the fundamental and subharmonic modes as does in the nature hot jet. As for a hot jet is excited at 90 Hz and 100 Hz, the fundamental and subharmonic modes are suppressed entirely. The flow structure is controlled by the excitation frequency. When the flow field of a hot jet is excited by the different excitation frequency, the main instability waves of the flow structure in a natural hot jet is destroyed, the temperature spectral is entirely controlled by the excitation frequency of the string, and the spectral energy concentrates on the excitation frequency of the string and its integer times.