針對攻角為零度的方柱，固定橫風雷諾數在9200，從方柱迎風面射出一道二維振盪噴流，變化振盪噴流速度對橫風速度比(簡稱為注射比)、振盪噴流頻率對尾流渦漩逸放頻率比(簡稱頻率比)及振盪噴流占空比(duty cycle)，探討方柱迎風面流場特徵。藉由煙霧可視化技術觀察迎風面受振盪噴流之流場特徵。使用熱線風速儀量測迎風面與尾流區之速度特性與頻率特徵，並分析迎風面因振盪噴流與渦旋逸放交互作用引致之相位關係。以PIV技術量化迎風面振盪噴流結構，並與流場可視化之特徵比對。利用壓力掃描器量測方柱之表面壓力，並分析阻力係數。流場可視化的結果顯示，當固定占空比時，在注射比與頻率比的域面上，可以畫分出搖擺噴流與偏折噴流模態。在搖擺噴流模態中，振盪噴流射出後呈現左右搖擺；在偏折噴流模態中，振盪噴流射出後覆蓋於方柱表面並且噴流氣柱呈現上下擺盪。振盪噴流的左右搖擺頻率與上下擺盪的頻率主要是受到方柱迎風面停滯點振盪所支配，而停滯點的振盪特徵則是由尾流渦漩逸放所影響。因此，在迎風面靠近噴流處及尾流渦漩逸放處的頻率特徵相同。時間平均的速度向量與流線顯示，方柱迎風面上方形成一四向鞍點，此四向鞍點使得橫風無法直接衝擊方柱迎風面，因此迎風面的表面壓力係數降低，故方柱的阻力係數降低。另外，隨著占空比的增加，阻力係數隨之增加。

The effects of injection ratio, frequency ratio, and duty cycle on the flow and drag characteristics of a square cylinder with an upstream pulsating jet injection were experimentally studied in wind tunnel. The frequency ratio and duty cycle of the pulsating jets were controlled by the solenoid valves. The evolution process of the characteristic flow patterns was recorded by the flow visualization method. The wake and the pulsating jet instability characteristics were detected simultaneously by using two one-component hot-wire anemometers. The flow field around the upstream surface of the square cylinder was measured by a particle image velocimeter (PIV). The drag force experienced by the square cylinder was obtained by a pressure scanner. Two characteristic flow modes (flapping jet and deflected jet) were observed in the domain of injection ratio and frequency ratio. The pulsating jet presented a left-and-right flapping motion about the symmetry axis in the flapping jet mode. In the deflected jet mode, the jet was deflected toward one side of the upstream surface of the square cylinder with an up-down oscillation of the deflected jet column. The flapping motion and jet column up-down oscillation of the pulsating jets appearing respectively in flapping jet and deflected jet modes were dominated by the oscillation of stagnation point on the upstream surface of square cylinder, which was induced by the vortex shedding in the wake. The characteristic frequencies of the pulsating jets at both flapping jet and deflected jet modes were therefore the same as the vortex shedding frequency. In the time-averaged velocity fields, a four way saddle appeared above the upstream surface of the square cylinder and prevented impingement from the freestream. Therefore, the surface pressure coefficients on the upstream surface of the square cylinder were reduced. This reduction in the surface pressure coefficient decreased the drag force acting on the square cylinder. When the duty cycle was increased from 20% to 50%, the reduction of the drag force induced by pulsating jet decreased.

[1] Nakayama, Y. and Boucher, R. F., Introduction to Fluid Mechanics, Arnold, Great Britain, 1999
[2] Lienhard, J. H., Synopsis of Lift Drag and Vortex Frequency Data for Rigid Circular Cylinders, Research Division Bulletin 300 , Washington State University, 1966.
[3] Huang, R. F., Chen, J. M., and Hsu C. M., “Modulation of surface flow and vortex shedding of a circular cylinder in the subcritical regime by self-excited vibration rod,” Journal of Fluid Mechanics, Vol.555, 2006, pp. 321-352.
[4] Zdravkovich, M. M., “Different modes of vortex shedding: an overview,” Journal of Fluids and Structures, Vol. 10, No. 5, 1996, pp. 427-437.
[5] Roshko, A, “On the wake and drag of bluff bodies,” Journal of Aeronautical Sciences, Vol. 22, No. 2, 1955, pp. 801
[6] Tritton, D. J., “Experiments on the flow past a circular cylinder at low reynolds numbers,” Journal of Fluid Mechanics, Vol. 6, Part 4, 1959, pp. 547-567.
[7] Etkin, B., Kovbaoher, G. K., and Keefe, R. T., “Acoustic radiation froma stationary cylinder in fluid stream (aeolian tones),” The Journal of the Acoustical Society of America, Vol. 29, No. 1, 1957, pp. 30-36.
[8] Weaver, W., “Wind-induced vibrations in antenna members,” Journal of the Engineering Mechanics Division, ASCE, Vol. 87, No. EM1, 1961, pp. 141-165.
[9] Gerrard, J. H., “An experimental investigation of the oscillating lift and drag of a circular cylinder shedding turbulent vortices,” Journal of Fluid Mechanics, Vol. 11, Part 2, 1961, pp. 244-256.
[10] Roshko, A., On the Development of Turbulent Wakes from Vortex Streets, NACA TN 2913, 1953.
[11] In, K. M., Choi, D. H., and Kim, M. U., “Two-dimensional viscous flow past a flat plate,” Fluid Dynamics Research, Vol. 15, No. 1, 1995, pp. 13-24.
[12] Dennis, S. C. R., Qiang, W., Coutanceau, M., and Launay, J. L., “Viscous flow normal to a flat plate at moderate reynolds numbers,” Journal of Fluid Mechaics, Vol. 248, No. 1, 1993, pp. 605-635.
[13] Nakamura, Y., “Vortex shedding from bluff bodies and a universal strouhal number,” Journal of Fluids and Structures, Vol. 10, No. 2, 1996, pp. 159-171
[14] Matsumoto, M., “Vortex shedding of bluff bodies: a review,” Journal of Fluids and Structures, Vol. 13, No. 7-8, 1999, pp. 791-811.
[15] Bearman, P. W. and Trueman, D. M., “An investigation of the flow around rectangular cylinders,” Aeronautical Quarterly, Vol. 23, 1972, pp. 229-237.
[16] Noda, H. and Nakayama, A., “Free-stream turbulence effects on the instantaneous pressure and forces on cylinders of rectangular cross section,” Experiments in Fluids, Vol. 34, No. 3, 2003, pp. 332-344.
[17] Okajima, A., “Strouhal numbers of rectangular cylinders,” Journal of Fluid Mechanics, Vol. 123, 1982, pp. 379-398.
[18] Yang, W. J., Flow Visualization III, Proceedings of the third international symposium on flow visualization, September 6-9, 1983, pp. 381-386.
[19] Huang, R. F., Lin, B. H., and Yen, S. C., “Time-average topological flow patterns and their Influence on vortex shedding of a square cylinder in cross flow at incidence,” Journal of Fluids and Structures, Vol. 26, No. 3, 2010, pp. 406-429.
[20] Luo, S. C., Chew, Y. T., and Ng, Y. T., “Characteristics of square cylinder wake transition flows,” Physics of Fluids, Vol. 15, No. 9, 2003, pp. 2549-2559.
[21] Rockwell, D. O., “Organized fluctuations due to flow past a square cross section cylinder,” Journal of Fluids Engineering, Vol. 99, No. 3, 1977, pp. 511-516.
[22] Kwok, K. C. S., “Effects of turbulence on the pressure distribution around a square cylinder and possibility of reduction,” Journal of Fluids Engineering, Vol. 105, No. 2, 1983, pp. 140-145.
[23] Chen, J. M. and Liu, C. H., “Vortex shedding and surface pressures on a square cylinder at incidence to a uniform air stream,” International Journal of Heat and Fluid Flow, Vol. 20, No. 6, 1999, pp 592-597.
[24] Akansu, Y. E., Fırat, E., “Control of flow around a square prism by slot jet injection from the rear surface,” Experimental Thermal and Fluid Science, Vol. 34, No. 7, 2010, pp. 906-914.
[25] Gu, F. Wang, J. S., Qiao, X. Q., and Huang, Z., “Pressure distribution, fluctuating forces and vortex shedding behavior of circular cylinder with rotatable splitter plates.” Journal of Fluids and Structures, Vol. 28, 2012, pp. 263-278.
[26] Coanda, H. M., “Device for deflecting a stream of elastic fluid projected into an elastic fluid,” United States Patent, No. 2052869, 1936.
[27] Newman, B. G., “The deflection of plane jets by adjacent boundaries – Coanda effect,” in Boundary Layer and Flow Control – Its Principles and Application, Vol. 1, Ed. Lachmann, G. V., Pergamon Press, New York, 1961, pp. 232-264.
[28] Tritton, D. J., Physical Fluid Dynamics, 2nd ed., Oxford University Press, New York, 1988, pp. 150-152
[29] Yamasaki, H. and Honda, S., “A unified approach to hydrodynamic oscillator type flowmeters,” Journal of fluid control, Vol. 13, 1981, pp. 1-17.
[30] Sichlichting, H.. Boundary Layer Theory, 7th ed, Mcgraw-Hill, New York, 1993, pp. 699.
[31] Flagan, R. C. and Seinfeld J. H., Fundamentals of Air Pollution Engineering, Prentice Hall, Englewood Cliffs, New Jersey, 1988, pp.295-307.
[32] 邱柏淳，〈以逆向噴流控制方柱周邊流場〉，國立台灣科技大學機械工程研究所碩士論文，台北，2013。
[33] 楊博淵，〈以迎風面振盪噴流控制方柱流場特性〉，國立台灣科技大學機械工程研究所碩士論文，台北，2015。