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研究生：	陳驊 Chen, Hua
論文名稱：	管口爆炸波與渦漩交互作用及消音探討 Investigation of Blast/Vortex Interaction and Noise Reduction at a Duct Exit
指導教授：	梁勝明 Liang, S. M.
學位類別：	博士 Doctor
系所名稱：	工學院 - 航空太空工程學系 Department of Aeronautics & Astronautics
論文出版年：	2003
畢業學年度：	91
語文別：	英文
論文頁數：	126
中文關鍵詞：	爆炸波、震波、聲波、渦漩、交互作用、視流
外文關鍵詞：	flow visualization, blast wave, shock wave, acoustic wave, vortex, interaction
相關次數：	點閱：161 下載：2
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　　最近幾年，環境保護問題已經成為很重要的問題。尤其是由交通工具排放噪音的問題，更是其中最值得注意的。由內燃機及排氣系統所排放的噪音可以歸類為三種形式：紊流噪音(turbulence noise)、震動噪音(shell noise)及輻射噪音(radiated noise)。在過去，大部分與輻射噪音相關的論文都將重點放在空氣動力問題中主要音源之一的震波與渦漩交互作用。在本研究中，我們考慮由管口排放的平面爆炸波問題，並採用蔣及蘇(Jiang and Shu )(1996)的五階準確度的加權基本不震盪法來探討這個問題。本論文先探討基礎的平面爆炸波與渦漩交互作用來作為了解噪音產生及其基本機制的墊腳石。由目前的結果可以知道弱爆炸波會導致正規反射(regular reflections)而強爆炸波會導致馬赫反射(Mach reflections)。而且由平面爆炸波與渦漩交互作用所產生的聲音與震波與渦漩交互作用所產生的聲音一致，是屬於四極矩(quadrupolar)的性質。由爆炸波與渦漩交互作用所產生的第一道聲音(precursor)在近場有以r-1 衰退的趨勢，而在遠場則是以r-1/2 衰退，這趨勢與對應之震波與渦漩交互作用的結果相吻合，至於第二道聲音(second sound)在遠場似乎以r-1/2 來衰退。在本研究中還發現在平面爆炸波與渦漩交互作用中有第四道聲音(fourth sound)的存在，這是在震波與渦漩交互作用中所沒有的。

　　在本研究中發現產生聲音的數目與爆炸波的強度有關。此外，二次震波與渦漩交互作用在渦量產生過程中扮演一個相當重要的角色。我們也發現產生的渦漩對在爆炸波與渦漩交互作用完成後會週期性地旋轉，並且渦漩對的環流量強度在爆炸波遠離至下游時會維持一定值。

　　一般來說，噪音控制的方法有兩種：一個是主動控制，而另一個是被動控制。對主動控制而言，它需要額外的裝置來抑制噪音並且通常花費較昂貴；對被動控制而言, 修改排氣系統的幾何形狀相當簡單並且也較常使用。在實際應用上，利用修改排氣系統的幾何外型來控制噪音是相當普遍並有效的方法，所以管口平滑轉角在本論文中被採用，並利用平滑角度(δ)及平滑表面半徑(R)來探討這個問題，而且R·δ就是平滑轉角的表面圓弧長度。由計算結果顯示減弱渦漩強度可以成功地降低由爆炸波在轉角繞射所導致的震波與渦漩交互作用所產生的噪音。另外,本研究中發現最大的噪音減低發生在θ = 0º (沿著管道中心線的方向)，而最少的噪音減低則是發生在θ = 90º (沿著垂直管口的壁面方向)。

　　Environment protection issues have become important in recent years. Particularly, noise emitted from automobiles is a serious problem. Noise emitted from an internal combustion engine and an exhaust system can be classified as three types: turbulence noise, shell noise, and radiated noise. In the past, most papers were related to the radiated noise concentrated on shock/vortex interaction, which was one of the major sources of noise and was closely related to some aerodynamic problems. In this study, the problem of a planar blast wave discharged from a duct exit is considered. A numerical approach of a 5th-order WENO scheme of Jiang and Shu (1996) is used to investigate this problem. The fundamental study of planar blast wave/vortex interaction is a stepping-stone to understand the noise generation and the basic mechanism. The present results show that weak blast waves result in regular reflections associated with the blast-wave front while strong blast waves result in Mach reflections. The sound generated by the planar blast/vortex interaction is of quadrupolar nature, as in a shock/vortex interaction. The precursor generated by the blast/vortex interaction tends to decay like r-1 in the near field and r-1/2 in the far field as in the corresponding shock/vortex interaction. As for the second sound, it seems to decay like r-1/2 in the far field. The fourth sound generated by a planar blast/vortex interaction is found to exist in the present study, but not in the case of shock/vortex interaction.

　　It is also found that the number of sound is dependent on the strength of the blast wave. The secondary shock/vortex interaction plays an important role in the vorticity generation process. Moreover, it is found that the induced vortex pair rotates cyclically after the interaction, and the circulation (or strength) of the two vortices is constant when the incident blast wave has moved far downstream.

　　Generally, there are two kinds of methods for noise control, one is active control and the other is passive control. For active control, it needs additional devices to suppress noise and usually costs. For passive control, modifying the exhaust system geometry can be easily applied and is often used. In practical applications, modifying exhaust system geometry is a common and useful method to control noise, so a smoothed corner at the duct exit is adopted and is investigated with different values of two parameters – the smoothing angle (δ) and the radius of the smoothed surface (R), where R·δ is the circular arc length for the smoothed corner. The computed results show that weakening the vortex strength can successfully reduce the noises associated with the shock/vortex interaction due to the blast wave diffraction around the duct corner. Moreover, It is found that the maximum noise reduction occurs at θ = 0º (along the centerline of the duct) and the minimum noise reduction occurs at θ = 90º (along the vertical wall at the duct exit).

CONTENTS

ABSTRACT ……………………………………………………………i
CONTENTS ……………………………………………………………iii
LIST OF TABLES ……………………………………………………vi
LIST OF FIGURES …………………………………………………vii
NOMENCLATURES ……………………………………………………xii

Chapter                                                    
I	INTRODUCTION	1
　　1.1 Motivation and Problem	1
　　1.2 Planar Blast/Vortex Interaction	2
　　1.3 Blast Wave Discharged from an Open-Ended Duct	4
　　1.4 Noise Reduction by Using Smoothed Corner at a Duct Exit	8
　　1.5 Objectives	11
		
II	MATHEMATICAL FORMULATION	12
　　2.1 Compressible Euler Equations 	12
　　2.2 Generalized Coordinates 	14
　　2.3 Vorticity Transport Equation 	15
　　2.4 Source Terms in Lighthill’s Equation	16
　　2.5 Acoustics Defined	17
		
III	NUMERICAL MEHOD	19
　　3.1 Introduction	19
　　3.2 WENO Scheme	21
　　3.3 Applying to Euler Systems	25
　　3.4 Time Discretization	26
　　3.5 Choice of Time Step	26
　　3.5 Boundary Conditions	27
　　　3.6.1 Inviscid Wall Boundary Condition	27
　　　3.6.2 Symmetric Boundary Condition	27
　　　3.6.3 Nonreflecting Boundary Condition	28
		
IV	RESULTS AND DISCUSSIONS	30
　　4.1 Validation of the Numerical Method 	30
　　　4.1.1 Sound Generation by Shock/Vortex Interaction	32
　　　4.1.2 Grid Independence	33
　　　4.1.3 Shock Diffraction at a Ninety-Degree Sharp Convex Corner	33
　　4.2 Planar Blast/Vortex Interaction and Sound Generation	33
　　　4.2.1 Physical Problem	33
　　　4.2.2 Basic Flow Structure of Blast/Vortex Interaction	35
　　　4.2.3 Sound Generation by Blast/Vortex Interaction	36
　　　4.2.4 Summary	40
　　4.3 Planar Blast Waves Discharged form an Open-Ended Duct	41
　　　4.3.1 Physical Problem	41
　　　4.3.2 Flow Structures	42
　　　4.3.3 Sound Generation	44
　　　4.3.4 Vorticity Production	45
　　　4.3.5 Summary	48
　　4.4 Noise Reduction by Using Smoothed Corner at a Duct Exit	49
　　　4.4.1 Physical Problem	49
　　　4.4.2 Flow Structures	50
　　　4.4.3 Vorticity Production	51
　　　4.4.4 Circumferential Pressures	55
　　　4.4.5 Summary	58
		
V	CONCLUSIONS AND SUGGESTIONS 	60
　　5.1 Conclusions	60
　　5.2 Suggestions	63
		
REFERENCES ……………………………………………………………64
APPENDICES………………………………………………………………70
TABLES.……………………………………………………………………73
FIGURES……………………………………………………………………80
VITA
PUBLICATION LIST
                                    

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