近年來由於雲端產業的發展，伺服器的使用需求日漸提高，而伺服器的運作需應付長時間的運作，所以穩定性顯得尤為重要，但伺服器運轉時產生的廢熱會使產品的效能與穩定度降低，其內部狹小的空間導致系統的阻抗較高，一般的風扇無法進行有效的散熱，故需要小型高轉速、高靜壓的風扇才能有效地進行系統散熱。但是風扇在高速運轉下，所衍生的特徵頻率噪音往往所為人垢病的，所以降低風扇特徵頻率的噪音成為本研究的重點；在本研究中針對應用於2U伺服器內的6038軸流式風扇，依據風扇原型之幾何參數建模，並利用ANSYS FLUENT模擬軟體進行數值分析，透過穩態流場之可視化及暫態聲場分析，探討流場與噪音之關聯並找出可能的風扇噪音源，透過加裝亥姆霍茲共振器以期對風扇的整體噪音值以及特徵頻噪音峰值能有所改善，並探討共振器的安裝位置，以頸部及共振腔體長度之和定為機構尺寸限制，讓安裝共振器後的風扇體積不會增加，能夠實際應用於伺服器的狹窄空間內，最後比較整合6038軸流式風扇有/無加裝共振器的效果，而後藉由CNC加工技術製作出風扇與共振器實體，進行風扇性能以及噪音的實測，驗證數值模擬的準確度。
而噪音模擬結果顯示，本研究所設計的共振器對於整體噪音值皆有下降趨勢，其中又以入口處的降噪成效較佳，皆能有效降低目標第二特徵頻率，分別下降15dB和3 dB，而第一特徵頻率分別下降15 dB和16 dB，而整體噪音下降約1.5~2.5 dB。至於實測部分，對於第二特徵頻率設計B可降噪4 dB，同時也使第一特徵頻率降低3-5 dB，而整體噪音值約下降1 dB；實測降噪幅度並沒有模擬值來的顯著，主因是數值模擬的架構省略馬達與結構振動的部分，其次是自行組裝的模型在運轉時會因為扇葉略微偏擺而產生噪音，但是實驗結果與數值模擬所呈現之趨勢一致。綜合本研究之數值模擬與實測驗證結果，建立一套高轉速小型軸流風扇與共震器搭配以達到風扇噪音改良之評估系統，除了提供完整可靠的設計參考外，也提供在不增加風扇整體體積的前提下安裝共振器之位置參考，在不影響風扇實際應用下有效地降地風扇的特徵頻噪音問題。

Recently, the rapid development of cloud industry has generated an increasing demand on the high-performance server; thus, thermal management on the server becomes a crucial challenge to maintain an efficient long-term operation under the limited space. In addition, the compact space leads to a high system impedance, which requires a small high-speed axial flow fan to provide sufficient high-pressure airflow for dissipating the waste heat effectively. Moreover, since the fan is running at high speed, the high-frequency harmonic noises are generated and become the focus of this study. Also, the 6038 axial fan used extensively in the 2U server is selected as the target for noise reduction with the implement of Helmholtz resonator.
At first, the numerical simulation of this fan is carried out by using the well-validated CFD codes Fluent for visualizing the steady flow field to analyze the appropriate installation location for resonators. Besides, the acoustic field and frequency spectrum are attained via the transient CFD simulation. Next, FFT analyzer is applied to measure the noise characteristics of fan inside a semi-anechoic chamber. Thus, a thorough understanding on the acoustic features of this cooling fan is obtained and utilized to design two Helmholtz resonators aiming at the 2nd harmonic frequency. Thereafter, noise reduction of the fan equipped with different resonators at frame corners are evaluated systematically via both CFD and experimental techniques.
Subsequently, the acoustic simulations indicate that all designed resonators result in a noise reduction, especially at the fan inlet side. And, the noise decreases on the 2nd harmonics are 15 dB and 3 dB while the noise reductions on the 1st harmonics are 15 dB and 16 dB, respectively. Also, it is found that the trend and deviation between CFD and test results are well correlated and within an acceptable range. Moreover, the experimental measurements show the 1.5~2.5 dB reduction on the overall sound pressure level by adopting these Helmholtz resonators. In conclusion, the accomplishment of this work establishes a systematic design scheme for the noise reduction of a small axial-flow fan with the implement of Helmholtz resonator installed at the corners of its frame.

[1]https://www.orientalmotor.com.sg/om/technical/cooling-fans/cooling-fan-structure-overview.html
[2]Zeller, W. and Stang, H., “Predetermination of the Axial-Flow Fans,” Heizung-Luftung-Haustchnik, Vol. 9, No. 12, pp. 10-20, 1957.
[3]Zeller, W., “Concerning Mathematical Treatment of Noise Behavior in Fans for Air Conditioning Plants,” VID-Berichte, No. 38, 1959.
[4]Hüebner, G., “Noise Generated by Centrifugal Fans,” Siemens-Zeitschrift, Vol. 8, pp. 145-155, 1959.
[5]Schlichting, H., “Application of the Boundary Layer Theory to Flow Problems of Turbo Machines,” Siemens-Zeitschrift, Vol. 33, No. 7, pp. 74-82, 1959.
[6]Fukano, T. and Takamatsu, Y., “The Effects of Tip Clearance on the Noise of Low Pressure Axial and Mixed Flow Fans,” Journal of Sound and Vibration, Vol. 105, No. 2, pp. 291-308, 1986.
[7]Kamaya, S. and Kanabayashi, S., “A Study on Noise Reduction for Small Axial Flow Fans,” Nippon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan, Vol. 56, No. 531, pp. 3408-3412, 1990.
[8]Horlock, J. H., “Axial Flow Compressors,” Krieger Pub. Co, 1982.
[9]Kamaya, S. and Kanabayashi, S., “Improvement of the Characteristics of the Low-Flow-Rate Region for Axial Flow Fans,” Nippon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan, Vol. 56, No. 532, pp. 3769-3773, 1990.
[10]Quinlan, D. A., “Application of Active Control to Axial Flow Fans,” Noise Control Engineering Journal, Vol. 39 No. 3, pp. 95-101, 1992.
[11]Kawaguchi, K. and Suzuki, M., “Noise Reduction of Automobile Cooling Fans”, Nippon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan, Vol. 58, No. 554, pp. 3108-3114, 1992.
[12]Akaike, S. and Kikuyama, K., “Study of Fan Noise Estimation”, Nippon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan, Vol. 61, No. 589, pp. 3269-3275, 1995.
[13]Fukano, T. and Jang, C. M., “Tip Clearance Noise of Axial-Flow Fans Operating at Design and Off-Design Conditions,” Journal of Sound and Vibration, Vol. 275, No. 3-5, pp. 1027-1050, 2004.
[14]Lighthill, M. J., “On Sound Generation Aerodynamically I. General Theory,” Proceedings of the Royal Society, London, Vol. 211, pp. 564-587, 1952.
[15]Lowson, M. V., “The Sound Field for Singularities in Motion,” Proc. Roy. Soc., London, pp. 559-572, 1965.
[16]Arnone, A., “Viscous Analysis of Three-Dimensional Rotor Flows Using a Multigrid Method,” NASA TM-106266, 1993.
[17]Liu, Q., Qi, D., and Mao, Y., “Numerical Calculation of Centrifugal Fan Noise,” Proceedings of Mechanical Engineers, Part C, Journal of Mechanical Engineering Science, Vol. 220, No. 8, pp. 1167-1177, 2006.
[18]Lu, H. Z., Huang, L., So, R. M. C., and Wang, J., “A Computational Study of the Interaction Noise from a Small Axial-Flow Fan,” The Journal of the Acoustical Society of America, Vol. 122, No. 3, pp.1404-1415, 2007.
[19]蔡明倫，風扇性能評估與設計方法之整合研究，國立台灣科技大學機械工程系博士論文，台北市，2010。
[20]陳佑政，亥姆霍茲共振器應用於風機減噪之實驗與模擬整合研究，國立台灣科技大學機械工程系博士論文，台北市，2019。
[21]吳習，不同類型的離心式風機應用於空氣清淨機開發之數值模擬分析，國立台灣科技大學機械工程系博士論文，台北市，2020。
[22]Neise, W. and Koopman, G. H., “Reduction of Centrifugal Fan Noise by Use of Resonator,” Journal of Sound and Vibration, Vol. 72, No. 2, pp. 297-308, 1980.
[23]Neise, W. and Koopmann, G. H., “The Use of Resonators to Silence Centrifugal Blowers,” Journal of Sound and Vibration, Vol. 82, No. 1, pp. 17-27, 1982.
[24]洪宗揚，後傾式離心風機之噪音研究，國立台灣工業技術學院機械工程系碩士論文，台北市，1996。
[25]呂水煙，前傾式離心風機之噪音研究，國立台灣工業技術學院機械工程系碩士論文，台北市，1997。
[26]吳御銓，λ/4減噪器應用在離心扇之數值與實務整合研究，國立台灣科技大學機械工程系碩士論文，台北市，2014。
[27]Alster, M., “Improved Calculation of Resonant Frequencies of Helmholtz Resonators,” Journal of Sound and Vibration, Vol. 24, No. 1, pp. 63-85, 1972.
[28]Bies, D. A. and Hansen, C. H., Engineering Noise Control: Theory and Practice. E&FN Spon, New York, USA, 1966.
[29]Han, M., Sound Reduction by a Helmholtz Resonator, Thesis, Lehigh University, 2008.
[30]Han, S. S. and Rhim, Y. C., “Noise-Level Reduction of a Slim-Type Optical Disk Drive Using the Idea of a Helmholtz Resonator,” Transactions on Magnetics, Vol. 45, No. 5, pp. 2217-2220, 2009.
[31]陳彥彰，亥姆霍茲共振器應用於管路風機之實驗與模擬整合研究，國立台灣科技大學機械工程系碩士論文，台北市，2014。
[32]嚴文祺，軸流式風機的性能與噪音特性之整合研究，國立台灣科技大學機械工程系碩士論文，台北市，2017。
[33]CNS 8753, “Determination of Sound Power Level of Noises for Fan, Blower, and Compressors,” Chinese National Standard, 2016.
[34]ANSI/AMCA Standard 210-16 “Laboratory Method of Testing Fans for Aerodynamic Performance Rating,” Air Movement and Control Association, Inc., 2016.
[35]白明憲，工程聲學，全華圖書，台北，2005。
[36]Powell, A., “Theory of Vortex Sound,” The Journal of the Acoustical Society of America, Vol. 36, No. 1, pp. 177-195, 1964.
[37]Copley, L. G., “Fundamental Results Concerning Integral Representations in Acoustic Radiation,” The Journal of the Acoustical Society of America, Vol. 44, No. 1, pp. 28-32, 1968.
[38]von Kármán, Theodore, “Aerodynamics,” McGraw-Hill, 1963.
[39]Kondo, L. and Aoki, Y., “Noise Reduction in Turbo Fans for Air Conditioners,” Technical Review-Mitsubishi Heavy Industries, Vol. 26, No. 3, pp. 173-179, 1989.
[40]http://physics.kenyon.edu/EarlyApparatus/Rudolf_Koenig_Apparatus/Helmholtz_Resonator/Helmholtz_Resonator.html
[41]Launder, B. E. and Spalding, D. B., Lectures in Mathematical Models of Turbulence, Academic Press, London, England, 1972.
[42]Smirnov, R., Shi, S., and Celik, I., “Random Flow Generation Technique for Large Eddy Simulations and Particle-Dynamics Modeling,” Journal of Fluids Engineering, Vol. 123, pp. 359-371, 2001.
[43]Uranga, A., Persson, P., Drela, M., and Peraire, J., “Implicit Large Eddy Simulation of Transitional Flows over Airfoils and Wings,” 19th AIAA Computational Fluid Dynamics Conference, San Antonio, Texas, 2009.
[44]Ffowcs Williams, J. E. and Hawkings, D. L., “Sound Generation by Turbulence and Surface in Arbitrary Motion,” Philosophical Transactions of the Royal Society of London, Vol. 264, pp. 321-342, 1969.
[45]Curle, N., “The Influence of Solid Boundaries upon Aerodynamic Sound,” Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 231, pp. 505–514, 1955.
[46]陳文亮，小型冷卻風扇之性能與噪音改善研究，國立台灣科技大學機械工程技術研究所碩士論文，1998年。