由於溫室效應的影響，極端惡劣天氣變得相當頻繁，例如低空風切、颱風、冰或雪。當飛機起飛降落時會不幸遭遇大雨，因此在飛機設計時須考慮這些天氣因素的影響，而大雨對飛機所造成的氣動力損失則是正在進行中的研究主題且需要長遠的進行研究，但是除了本研究團隊近幾年曾經針對大雨對機翼性能分析有過分析之外，近10年來不論是在實驗或是在數值計算方面已鮮少有相關的研究。本研究首先對回顧前人所做的因大雨效應而使得飛機性能減低的研究並使用數值方法做進一步的探討，並使用NACA 64-210二維高升力翼剖面和現有的商用軟體FLUENT，大雨的模擬則是採用FLUENT內的二相流 (Two-Phase Flow)離散相的DPM模組(Discrete Phase Model)和改變表面粗糙度來完成並計算空氣動力特性的改變。
本研究首先進行乾淨翼剖面的驗證工作，並成功模擬出二維高升力翼剖面在大雨下的性能衰減，其衰減程度會隨著降雨量的增加而越大，而失速的情形也有提前發生的現象，研究發現升力係數減少、阻力係數增加的程度與Bezos 實驗結果相近。本研究所得到的量化資料能夠能對航空業上有所助益，長遠來說，可以使得飛機飛行的更安全。

Global warming has led to extreme weather around the world frequently such as low level wind shear, typhoon, ice/snow etc. If aircraft taking-off and landing will unavoidably meet with the heavy rain, then aircraft designer must put these severe weather influences into considerations in the conceptual design phase. Aerodynamic influences due to heavy rain are still the on-going research subject, and needs further investigation. But for the past decade there are neither experimental nor numerical researches about heavy rain except our research team conducted in recent years. In this thesis, we first review some research finding of heavy rain effects on the aerodynamic performance degradation. Secondly, commercial CFD package FLUENT and preprocessing tool Gambit is used as our main analysis tools, and the simulation of rain is accomplished by using Two-Phase Flow approach’s Discrete Phase Model (DPM) and surface roughness provided by FLUENT. 
The results show that this research successfully simulates the aerodynamic investigation of high-lift airfoil under the influence of heavy rain effects, the doubts or errors in the previous numerical and experimental works are also revealed. The degradation rate increases with the rain rate, and the premature stall phenomenon is also discovered. It is expected that the quantitative information gained in this thesis could be useful to the operational airline industry, and greater effort should put in this direction to further improve modern transport aircrafts safety.

Table of Contents
ABSTRACT	I
TABLE OF CONTENTS	IV
LIST OF FIGURES	V
LIST OF TABLES	IX
NOMENCLATURES	XII
CHAPTER 1 INTRODUCTION	1
CHAPTER 2 RESEARCH BACKGROUND	6
2.1 FLOW PHYSICS OF HIGH LIFT AIRFOIL	6
2.2 PHYSICS AND INFLUENCES OF AN AIRFOIL IN RAIN	11
2.3 CHARACTERISTICS OF RAIN	18
CHAPTER 3 NUMERICAL MODELING	21
3.1 GRID GENERATION AND FLOW SOLVER	21
3.2 TURBULENCE MODELING	25
3.3 DISCRETE PHASE MODEL	29
3.4 WALL FUNCTIONS AND SURFACE ROUGHNESS	34
3.5 VERIFICATION	37
CHAPTER 4 RESULTS AND DISCUSSION	41
4.1 PRELIMINARY RESULTS	41
4.2 HIGH-LIFT AIRFOIL UNDER THE HEAVY RAIN	58
CHAPTER 5 CONCLUSIONS	77
REFERENCES	79
 List of Figures
Fig. 1-1 Examples of typical leading edge devices [2]	3
Fig. 1-2 Examples of typical trailing edge devices [2]	3
Fig. 1-3 Typical high-lift airfoil and its effect on lift coefficient [3]	3
Fig. 2-1 Velocity distributions on an airfoil with and without a vortex,
    showing the slat effect [10]	7
Fig. 2-2 Velocity distributions on an airfoil with and without a vortex, 
    simulating the circulation effect [10]	8
Fig. 2 3 Typical three-element airfoil, showing dumping velocity effect
    [10] 	9
Fig. 2-4 Theoretical flow models for the various viscous regions 
    [11] 	10
Fig. 2-5 Idealization of raindrops interacting with a flapped airfoil 
    [16]	14
Fig. 2-6 Observed film flow pattern above a flapped wing [16]	15
Fig. 2-7 Terminal velocity versus droplet diameter	20
Fig. 3-1 Far mesh of NACA 64-210 high-lift airfoil 	22
Fig. 3-2 Medium mesh of NACA 64-210 high-lift airfoil	22
Fig. 3-3 Near mesh of NACA 64-210 high-lift airfoil 	22
Fig. 3-4 The Yplus with NACA 64-210 high-lift airfoil 	23
Fig 3-5 Physics of splashing, momentum, heat, and mass transfer for the 
    Wall-Film	32
Fig. 3-6 Far mesh of NACA 64-210 airfoil 	37
Fig. 3-7 Near mesh of NACA 64-210 airfoil 	38
Fig. 3-8 Wall Yplus at angle of attack 0 degree 	38
Fig. 3-9 Lift coefficients comparison between numerical results and 
    theory 	39
Fig. 3-10 Drag coefficients comparison between numerical results and 
    theory 	40
Fig. 4-1 Lift coefficients for 3 numerical results comparing to 
    experimental data 	46
Fig. 4-2 Drag coefficients for 3 numerical result comparing to 
    experimental data 	47
Fig. 4-3 Lift coefficients for airfoil numerical and experimental results  
    	49
Fig. 4-4 Drag coefficients for airfoil numerical and experimental results 
    	49
Fig.4-5 Lift degradation rate for airfoil at LWC=25 g/m3 for numerical and experimental results 	51
Fig. 4-6 Lift degradation rate for airfoil at LWC=39 g/m3 for numerical 
   and experimental results 	51
Fig.4-7 Drag degradation rate for airfoil at LWC=25 g/m3 for numerical 
   and experimental results 	53
Fig.4-8 Drag degradation rate for airfoil at LWC=39 g/m3 for numerical 
   and experimental results 	53
Fig 4-9 l/d degradation rate for airfoil at LWC=25 g/m3 for numerical and 
   experimental results 	56
Fig 4-10 l/d degradation rate for airfoil at LWC=39 g/m3 for numerical and experimental results 	56
Fig. 4-11 Local view of rain droplets diameter near airfoil	57
Fig. 4-12 Lift coefficients for high-lift airfoil numerical result comparing 
    to experimental data 	58
Fig. 4-13 Drag coefficients for high lift airfoil numerical result comparing 
    to experimental data 	58
Fig. 4-14 Lift coefficients for high-lift airfoil numerical and experimental 
   results 	60
Fig. 4-15 Drag coefficients for high-lift airfoil numerical and 
   experimental results 	60
Fig.4-16 Lift degradation rate for high-lift airfoil at LWC=29 g/m3 for 
   numerical and experimental results 	62
Fig.4-17 Lift degradation rate for high-lift airfoil at LWC=46 g/m3 for 
   numerical and experimental results 	62
Fig. 4-18 CP distribution of slat for 2 rain rate cases and 2 flight attitudes
   	63
Fig. 4-19 CP distribution of main wing for 2 rain rate cases and 2 flight 
    attitudes	63
Fig. 4-20 CP distribution of vane for 2 rain rate cases and 2 flight 
    attitudes	63
Fig. 4-21 CP distribution of flap for 2 rain rate cases and 2 flight attitudes
    	64
Fig.4-22 Drag degradation rate for high-lift airfoil at LWC=29 g/m3 for 
   numerical and experimental results 	65
Fig.4-23 Drag degradation rate for high-lift airfoil at LWC=46 g/m3 for 
   numerical and experimental results 	66
Fig. 4-24 Cavity flow at slat and main wing	69
Fig. 4-25 Pressure and viscous drag degradation rate for slat at 2 rain rate 
   cases	70
Fig. 4-26 Pressure and viscous drag degradation rate for main wing at 2 
   rain rate cases	70
Fig. 4-27 Pressure and viscous drag degradation rate for vane at 2 rain 
   rate cases	70
Fig. 4-28 Pressure and viscous drag degradation rate for flap at 2 rain rate 
   cases	71
Fig. 4-29 l/d degradation rate for high-lift airfoil at LWC=29 g/m3 for 
   numerical and experimental results 	72
Fig. 4-30 l/d degradation rate for high-lift airfoil at LWC=46 g/m3 for 
   numerical and experimental results 	73
Fig. 4-31 Local view of rain droplets diameter near high lift airfoil	73
Fig. 4-32 Different relative static pressure contours with streamlines	74
Fig. 4-33 Different relative velocity magnitude contours with streamlines
    	75
 
List of Tables
Table 4-1 Value of Airfoil's KS and CS for 2 flight attitudes and 2 rain 
    cases 	42
Table 4-2 Value of slat's KS and CS for 3 flight attitudes and 2 rain cases 
    	42
Table 4-3 Value of main wing's KS and CS for 3 flight attitudes and 2 rain 
    cases 	43
Table 4-4 Value of vane's KS and CS for 3 flight attitudes and 2 rain cases 
    	44
Table 4-5 Value of flap's KS and CS for 3 flight attitudes and 2 rain cases  
    	44
Table 4-6 Lift coefficients percentage for airfoil numerical result 
    comparing to experimental data	47
Table 4-7 Drag coefficients percentage for airfoil numerical result 
    comparing to experimental data 	47
Table 4-8 Numerical results for airfoil of lift coefficients degradation 
    percentage for 2 rain rate cases 	50
Table 4-9 Experimental results for airfoil of lift coefficients degradation 
    percentage for 2 rain rate cases 	50
Table 4-10 Numerical results for airfoil of drag coefficients degradation 
    percentage for 2 rain rate cases 	52
Table 4-11 Experimental results for airfoil of drag coefficients 
    degradation percentage for 2 rain rate cases 	52
Table 4-12 Numerical results for airfoil of pressure drag coefficients 
    degradation percentage for 2 rain rate cases	54
Table 4-13 Numerical results for airfoil of viscous drag coefficients 
    degradation percentage for 2 rain rate cases	54
Table 4-14 Numerical results for airfoil of lift to drag (l/d) degradation 
    percentage for 2 rain rate cases 	55
Table 4-15 Experimental results for airfoil of lift to drag (l/d) degradation 
    percentage for 2 rain rate cases 	55
Table 4-16 Lift coefficients percentage for high lift airfoil numerical 
    result comparing to experimental data 	59
Table 4-17 Drag coefficients percentage for high lift airfoil numerical 
    result comparing to experimental data 	59
Table 4-18 Numerical results for high-lift airfoil of lift coefficients 
    degradation percentage for 2 rain rate cases 	61
Table 4-19 Experiment results for high-lift airfoil of lift coefficients 
    degradation percentage for 2 rain rate cases 	61
Table 4-20 Numerical results for high-lift airfoil of drag coefficients 
    degradation percentage for 2 rain rate cases 	64
Table 4-21 Experiment results for high-lift airfoil of drag coefficients 
    degradation percentage for 2 rain rate cases 	65
Table 4-22 Numerical results for slat of pressure drag coefficients
    degradation percentage for 2 rain rate cases	66
Table 4-23 Numerical results for slat of viscous drag coefficients
    degradation percentage for 2 rain rate cases	67
Table 4-24 Numerical results for main wing of pressure drag coefficients
    degradation percentage for 2 rain rate cases	67

Table 4-25 Numerical results for main wing of viscous drag coefficients
    degradation percentage for 2 rain rate cases	67
Table 4-26 Numerical results for vane of pressure drag coefficients
    degradation percentage for 2 rain rate cases	68
Table 4-27 Numerical results for vane of viscous drag coefficients
    degradation percentage for 2 rain rate cases	68
Table 4-28 Numerical results for flap of pressure drag coefficients
    degradation percentage for 2 rain rate cases	68
Table 4-29 Numerical results for flap of viscous drag coefficients
    degradation percentage for 2 rain rate cases	69
Table 4-30 Numerical results for high-lift airfoil of lift to drag (l/d) 
    degradation percentage for 2 rain rate cases 	71
Table 4-31 Experiment results for high-lift airfoil of lift to drag (l/d) 
    degradation percentage for 2 rain rate cases 	72

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