Evolution of Flow Structures and Vortex Dynamics in the Jet Column Region of a Sharp-edged Orifice Plane Jet

• Main Authors: Yong-chin Lim, 林勇進
• Other Authors: Fei-Bin Hsiao
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/71582903838444536616

碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 96 === The dynamics of coherent structures in jet column region of a sharp-edged orifice plane jet was extensively studied by means of hot-wire technique. The present study was conducted in a sharp-edged orifice plane jet (i.e. 450 beveled edge) and the height of orifice is set to 15mm, therefore, the aspect ratio is 20. The investigation of the flow structures and intrinsic dynamics of vortex interaction are defined in the fixed operating velocity at and the corresponding Reynolds number based on the orifice height is . However, the study of initial flow properties of sharp-edged and right angle orifice plane jet are undertaken within operating velocity of . It is found that jet flow is sensitive to initial conditions, particularly nozzle exit with irregular geometry. Due to the sudden contraction of the orifice jet, the vena contracta effect is formed and plays a crucial role at immediate exit of orifice of sharp-edged and right angle orifice plane jet as well. The vena contracta effect depends upon the jet exit velocity, i.e. the vena contracta effect will be more prominent at lower jet exit velocity. This effect leads the flow to accelerate. Due to the forming of vena contracta at the initial stage, the initially contract inward in momentum thickness, flow entrainment rate and shear layer width is discovered in sharp-edged orifice plane jet. It is found that the vena contracta effect will suppress the flow to spread and delays the development of the coherent structures. In addition, the orifice plane jets perform well in the mixing characteristic than that to the smooth contraction nozzle plane jet. The existence of the shedding frequency due to the separation on the sharp-edged walls is confirmed in study of Fourier and Wavelet power spectra. Meanwhile, the higher formation rate of sharp-edged orifice plane jet is found in flow visualization and spectral analysis. The frequency of the instability waves in the present case are varying as well as decreasing along the downstream direction. Moreover, two initial instability frequencies exist in tandem within the shear flow. It is expected that these two instability waves will compete to each other to extract energy from the mean flow or separation as well. On the other hand, the ‘forking of structures’ in wavelet coefficient indicates to the vortex pairing at former portion of merging process. The location of the vortex formation in present case is locate somewhere near X/H=0.8., and the occurrence of first and second vortex merging are believed at vicinity of X/H=1.2 and X/H=2.3, respectively. No apparent interchange between and at former and latter portion of vortex formation and merging. The presence of the negative Reynolds shear stress or energy production prompts the existence of the unstable boundary layer and oscillation of separation zone, which excites the shear flow under high amplitude. This behavior is presumed as self-oscillation in sharp-edged orifice plane jet. Experimental results show that kinetic energy contributes by mean energy advection term at initial flow development and the fluctuation kinetic energy increases drastically during the vortex formation and interaction.