Re-evaluating mixing length in planar turbulent mixing layer

• Main Authors: Kuan-HuangLi, 李冠篁
• Other Authors: Keh-Chin Chang
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/72475956011308072911

碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 100 === This thesis investigates the velocity distribution of two-dimensional planar turbulent mixing layer by hot-wire anemometry (HWA) and particle image velocimetry (PIV). A turbulent mixing layer is composed of two different flow types within its flow fields, namely a shear layer in the central region and two free streams in each outer high- and low-speed side. Shear layer is formed after the trailing edge of the splitting plate and developing through successively distinct regions, namely the near-field region and fully-developed (self-preserving) region. The cross-type hot-wire anemometry is capable of measuring the temporal velocity information and analysing profile distributions of skewness factor and flatness factor. The positions of flatness extreme values (yFL and yFH) are the farthest positions where the shear turbulence can reach. It can be viewed as the actual sectional range of shear turbulence. Hence the positions of flatness extreme values (yFL and yFH) are the interfaces of central shear turbulence and outer high- and low-speed homogeneous turbulence. Note that analysis of high-order turbulent statistics such as skewness or flatness needs more sampling data, in other words, longer sampling time to achieve statistical stationary. The linear growth rate of regions bounded by ySL and ySH, yFL and yFH are necessary and sufficient conditions and more appropriate for justifying the achievement of self-preserving state. Particles image velocimetry is used to measure planar velocity information of mixing layer. The spatial derivative of planar velocity distribution can be split into vorticity part and straining part. Furthermore, with new decomposition method, namely the triple decomposition method (TDM), the vorticity part is futher split into the shear-induced vorticity and rigid body rotation, while the straining part is split into the shear-induced strain and irrotational strain. The region which the shear-induced vorticity drops below 80 s-1 is defined as the interfaces shear and free-stream regions. After the width between the two interfaces between the shear layer and high-speed as well as low-speed free streams growths linearly, the flow reaches its self-preserving state. After comparing the width defined by positions with yFL, yFH and y0.05, y0.95 by HWA, and the interfaces of shear-induced vorticity (yωL and yωH) and y0.05, y0.95 by PIV, one concludes that the range bounded by extreme flatness factor values can be taken as the actual range of shear turbulence layer. The linear growth rate of lF along stream-wise direction can serve as a criterium to justify whether or not self-preserving state of the flow is achieved.