本研究以鈍形體渦流溢放過程中，存在之三維非定常特性為主軸，並進一步探討自由流紊流擾動對其之影響。實驗在雷諾數為104下進行，針對圓柱、梯型模型，以及梯型模型附加延長平板之流場，使用MEMS熱膜感測器以非侵入方式取得鈍形體模型側向多點之渦流溢放訊號。另外，利用在鈍形體模型上游處加設網格，改變自由流之紊流擾動結構，並藉由連續性小波轉換法分析感測器所得之瞬時資訊，討論其中渦流溢放之非定常擾動行為。
實驗結果發現，當渦流溢放之三維特性發生時，側向訊號間相位差出現2π之跳動，即渦流位錯之現象。而同時伴隨之低頻擾動，則降低渦流溢放頻率與能量振幅之整體相關特性。其中以梯型模型附加延長平板之渦流溢放行為呈現較二維之情況，而圓柱與梯型模型因流場分離情況不同，明顯出現不同之三維非定常結構。然而，自由流紊流擾動明顯可激發圓柱渦流溢放之非定常三維擾動；其次為梯型模型；梯型模型附加延長平板模型則仍然維持流場之二維特性。根據以上之分析比較結果，當鈍形體具有面對流場之垂直平板或後方之延長平板，其渦流溢放現象將分別呈現出不同之三維非定常結構。因此，推測其中三維特性主要決定於鈍形體之幾何外型，其中涉及流動分離與尾流發展等流體運動之物理機制。

Efforts were made to examine the effects of freestream turbulence on three-dimensional unsteadiness of vortex shedding from bluff bodies at Reynolds numbers in the order of 104. The bluff bodies employed for experiment were of circular, trapezoidal and T-shaped cylinders. Instantaneous characteristics of vortex shedding were acquired by an array of MEMS thermal sensors situated spanwisely on the surface of each bluff model. By Wavelet analysis of the signals obtained by each of the MEMS sensors, the instantaneous vortex shedding frequency and its Wavelet modulus were reduced. Thus, one was able to examine the unsteady, three-dimensional characteristics of vortex shedding by comparing the Wavelet results of different sensors.
It is found that among the three cases of bluff models studied, the case of flow over a circular cylinder was affected by the presence of freestream turbulence most significantly. On the other hand, the freestream turbulence effect shows much less impact on the flows over the trapezoidal and T-shaped cylinders. In particular, the two-dimensionality of vortex shedding produced by the T-shaped cylinder is preserved in the presence of artificial freestream turbulence. Above findings signify that the unsteady three-dimensionality of vortex shedding is sensitive to extrinsic effects of bluff bodies, which are concerning with the initial condition of flow separation form a bluff body as well as subsequent vortex formation mechanism.

Apelt, C. J., West, G. S., and Szewczyk, A. A., “The Effects of Wake Splitter plates on the Flow Past a Circular Cylinder in the Range 104<R<5×104,” J. Fluid Mech., Vol. 61, pp. 187-198, 1973.
Bearman, P. W., “Investigation of the Flow Behind a Two-Dimensional Model with a Blunt Trailing Edge and Fitted with Splitter Plates,” J. Fluid Mech., Vol. 21, Part 2, pp. 241-255, 1965.
Bearman, P. W., and Morel, T., “Effect of Free Stream Turbulence on the Flow around Bluff Bodies,” Prog. Aerospace Sci., Vol. 20, pp. 97-123, 1983.
Blevins, R. D., “The Effects of Sound on Vortex Shedding from Cylinders,” J. Fluid Mech., Vol. 161, pp.217-237, 1985.
Bloor, M. S., “The Transition to Turbulence in the Wake of a Circular Cylinder,” J. Fluid Mech., Vol. 19, pp.290-304, 1964.
Braza, M., Faghani, D., and Persillon, H., “Successive Stages and the Role of Natural Vortex Dislocations in Three-Dimensional Wake Transition,” J. Fluid Mech., Vol. 437, pp. 1-41, 2001.
Cimbala, J. M., Nagib, H. M., and Roshko, A., “Large Structure in the Far Wakes of Two-Dimensional Bluff Bodies,” J. Fluid Mech., Vol. 190, pp. 265-298, 1988.
Comte-Bellot, G., and Corrsin, S., “Simple Eulerian Time Correlation of Full- and Narrow-Band Velocity Signals in Grid-Generated ‘Isotropic’ Turbulence,” J. Fluid Mech., Vol. 48, pp. 273-337, 1971.
Fox, T. A., and West, G. S., “On the Use of End Plates with Circular Cylinders,” Exp.Fluids, Vol. 9, pp.237-239, 1990.
Gerich, D., and Eckelmann, H., “Influence of End Plates and Free Ends on the Shedding Frequency of Circular Cylinder,” J. Fluid Mech., Vol. 122, pp.109-121, 1982.
Gerrard, J. H., “Experimental Investigation of Separated Boundary Layer Undergoing Transition to Turbulence,” Phys. Fluids, Vol. 10, pp. 98-100, 1967.
Gerrard, J. H., “The Three-Dimensional Structure of the Wake of a Circular Cylinder,” J. Fluid Mech., Vol. 25, pp. 143-164, 1966a.
Gerrard, J. H., “The Mechanics of the Formation Region of Vortices behind Bluff Bodies,” J. Fluid Mech., Vol. 25, pp. 401-413, 1966b.
Grossman, A., and Morlet, J., “Decomposition of Hardy Functions into Square Integrals Wavelets of Constant Shape,” SIAM J. Math. Anal., Vol. 15, pp. 723-736, 1984.
Hama, F. R., “Three-Dimensional Vortex Pattern Behind a Circular Cylinder,” J. Aero. Sci., Vol. 24, pp. 156-158, 1957.
Hanson, F. B., and Richardson, P. D., “The near-wake of a circular cylinder in cross flow,” J. Basic Eng., Trans. ASME., Vol. 90, pp. 476-484, 1968.
Henderson, R. D., “Nonlinear dynamics and Pattern Formation in Turbulent Wake Transition,” J. Fluid Mech., Vol. 352, pp. 65-112, 1997.
Hunt, J. C. R., “A theory of turbulent flow round two-dimensional bluff bodies,” J. Fluid Mech., Vol.61, Pt. 4, pp. 625-706, 1973.
Laws, E. M., “Flow through screens,” Ann. Rev. Fluid Mech., Vol. 10, pp. 247-266, 1978.
Lisoski, D., “Nominally Two-Dimensional Flow about a Normal Flat Plate,” PhD thesis, California Institute of Technology, 1993.
Mansy, H., Yang, P. M., and Williams, D. R., “Quantitative Measurements of three-Dimensional Structures in the Wake of a Circular Cylinder,” J. Fluid Mech., Vol. 270, pp. 277-296, 1994.
Malard, L., Wisone, W., Strzelecji, A., Gajan, P., and Hebrard, P., “Air Visualizations and Flow Measurements Applied to the Study of a Vortex Flowmeter: Influence of Grid Turbulence and Acoustical Effects,” Proceedings, Flucome' 91, ASME, pp. 689-704, 1991.
Miau, J. J., Yang, C. C., Chou, J. H., and Lee, K. R., “A T-Shaped Vortex Shedder for a Vortex Flowmeter,” J. Flow Meas. and Inst., Vol. 4, No. 4, pp. 259-267, 1993.
Nakamura, Y., and Ohya, Y., “The Effects of Turbulence on the Mean Flow Past Two-Dimensional Rectangular Cylinders,” J. Fluid Mech., Vol. 149, pp. 255-273, 1984.
Najjar, F. M., and Balachandar, S., “Low-Frequency Unsteadiness in the Wake of a Normal Flat Plat,” J. Fluid Mech., Vol. 370, pp. 101-147, 1998.
Norgerg, C., “An Experimental Investigation of the Flow around a Circular Cylinder: Influence of Aspect Ratio,” J. Fluid Mech., Vol. 258, pp. 287-316, 1994.
Norberg, C., “Interaction Between Freestream Turbulence and Vortex Shedding for a Single Tube in Cross-Flow,” J. Wind Eng. and Ind. Aerody., Vol. 23, pp. 501-514, 1986.
Perry, A. E., “Hot-Wire Anemometry,” Oxford University Press, 1982.
Roshko, A., “On the Wake and Drag of Bluff Bodies,” Journal of Aerodynamics, Vol. 22, pp. 124-132, 1955.
Roshko, A., “Perspectives on Bluff Body Aerodynamics,” J. Wind Eng. and Ind. Aerody., Vol. 49, pp. 79-100, 1993.
Stansby, P. K., “The Effects of End Plates on the Base Pressure Coefficients of a Circular Cylinder,” Aeronaut. J., Vol. 78, pp. 36-37, 1974.
Szepessy, S., “On the Spanwise Correlation of Vortex Shedding from a Circular Cylinder at High Reynolds Number,” Phys. Fluids., Vol. 6, pp.2406-2416, 1994.
Szepessy, S., and Bearman, P. W., “Aspect Ratio and End Plate Effects on Vortex Shedding from a Circular Cylinder,” J. Fluid Mech., Vol. 234, pp.191-217, 1992.
Tanner, M., “Reduction of Base Drag,” Prog. Aerospace Sci., Vol. 16, No. 4, pp. 369-384, 1975.
Tennekes, H., and Lumley, J. L., “A first course in turbulence,” MIT Press, pp. 273, 1994.
Tritton, D. J., “Experiments on the Flow Past a Circular Cylinder at low Reynolds Numbers,” J. Fluid Mech., Vol. 16, pp. 547-567, 1959.
Wei, T., and Smith, C. R., “Secondary Vortices in the Wake of Circular Cylinder,” J. Fluid Mech., Vol. 169, pp. 513-553, 1986.
Williamson, C. H. K., “Oblique and Parallel Modes of Vortex Shedding in the Wake of a Circular Cylinder at Low Reynolds Numbers,” J. Fluid Mech., Vol. 206, pp. 579-627, 1989.
Williamson, C. H. K., “The Nature and Forced Formation of Spot-Like Vortex Dislocations in the Transition of a Wake,” J. Fluid Mech., Vol. 243, pp. 393-441, 1992.
Williamson, C. H. K., “Three-Dimensional Wake Transition,” J. Fluid Mech., Vol. 328, pp. 345-407, 1996a.
Williamson, C. H. K., “Three-Dimensional Vortex Dynamics in Bluff Body Wakes,” Exp. Thermal and Fluid Sci., Vol. 12, pp. 150-168, 1996b.
Williamson, C. H. K., “Vortex Dynamics in Cylinder Wake,” Ann. Rev. Fluid Mech., Vol. 28, pp. 477-539, 1996c.
Williamson, C.H.K, “Advances in our understanding of vortex dynamics in bluff body wakes,” J. Wind Eng. Ind. Aerodyn., Vol. 69-71, pp.3-32, 1997.
Wolochuk, M. C., Plesnial, M. W., and Braun, J. E., “The Effects of Turbulence and Unsteadiness on Vortex Shedding From Sharp-Edged Bluff Bodies,” ASME, J. Fluids Engineering, Vol. 118, pp. 18-25, Mar., 1996.
Wu, S. J., Miau, J. J., Hu, C. C., and Chou, J. H., “On Low-Frequency Modulations and Three-Dimensionality in Vortex Shedding Behind a Normal Plate,” J. Fluids Mech., Vol. 526, pp. 526, 2005.
Zdravkovich, M. M., “Flow Around Circular Cylinders,” Vol.1: Fundamentals, Oxford University Press, 1997.
王瑜鈞, “應用MEMS熱膜感測器陣列探討尾流三維特性之初始發展,” 成大航太所碩士論文, 2005.
伍湘杰, “渦流溢放過程低頻調變與三維性之瞬時特性,” 成大航太所博士論文, 2003.
杜榮國, “MEMS熱膜感測器設計製造及應用於探討非定常流動分離現象, ” 成大航太所碩士論文, 2003.
楊啟昌, “延長平板對鈍形體所產生渦流溢放之影響,” 成大航太所碩士論文, 1992.