過去的研究結果指出，通過單葉片或雙葉片機械心瓣的流場容易造成溶血及血栓之現象，而發生之可能原因包含流場中之切應力過大，以及葉片的關閉速度過快造成穴蝕的發生，當穴蝕汽泡爆破時，產生之高壓會破壞血球及血小板。然而，三葉片機械心瓣主要是依靠主動脈竇中之渦漩來幫助葉片之關閉，與單葉片或雙葉片機械心瓣依靠反向流推動葉片來關閉不同，因此其葉片的關閉速度較為緩慢，可以減少穴蝕發生的機會。
本研究利用體外脈動流循環模擬系統，在主動脈瓣的位置，分別裝設兩顆不同的人工機械心瓣，雙葉片心瓣St. Jude Medical 27和新研發之三葉片心瓣trileaflet 27進行模擬，並以數位質點影像流速儀量測其流場狀況，配合LES及SGS的方法量化流場中的切應力大小，以評估切應力是否足以破壞血球。此外，因為製造一顆新研發的三葉片機械心瓣相當耗費時間與金錢，因此本研究也利用數值模擬的商用軟體Fluent，模擬這兩顆機械心瓣的流場，並與實驗流場進行驗證，以確認模擬之結果是否正確，期望對於日後三葉片機械心瓣的研發上有所助益。
實驗的結果指出，通過機械心瓣的流場中，最大切應力雖然不足以破壞紅血球，但是仍然有可能會破壞血小板。此外，三葉片機械心瓣的關閉速度明顯慢於雙葉片機械心瓣，可以減少穴蝕發生的機會。數值模擬方面，雖然還有一些需要改進的部份，但流場已大致符合於實驗的結果，表示將來應可利用數值模擬的方式，有效減少三葉片機械心瓣的研發成本。

Previous researchers indicated that the phenomenon of hemolysis and thrombosis would occur in the flow fields across the monoleaflet or bileaflet mechanical heart valves. The probable reasons of causing hemolysis and thrombosis included shear stresses in the flow fields might be large enough to damage red blood cells, and the closing velocity of the leaflet was excessively large to cause cavitation phenomenon. Cavitation bubbles exploded and produced high pressures which would damage red blood cells and platelets. However, the closure mechanism of the trileaflet valve was based on the vortices in the aortic sinus which benefited leaflets to close, and it was apparently different to that of monoleaflet or bileaflet mechanical heart valves which mainly depended on the reverse flow. Therefore, the closing velocity of the trileaflet valve was much slower and the probability of cavitation was also smaller.
A pulsatile mock circulatory loop system that dynamically simulated physiologic circulation was used in this study. A St. Jude Medical 27 mm bileaflet valve and a 27mm new type trileaflet valve were used as test valves positioned in the aortic position. Flow field measurements were made with a digital particle image velocimeter. By applying LES and SGS, turbulent viscous shear stresses were quantified and evaluated whether red blood cells would be damaged or not. Furthermore, because manufacturing a new trileaflet valve would cost a lot of time and money, commercial software Fluent was also applied to run numerical simulations of these two valves in this study. The results of numerical simulations would be valid with the experiments and were expected to be useful for the development of the trileaflet valve in the future.
The results of the experiments showed that although the maximum value of turbulent viscous shear stress was not large enough to cause damage to red blood cells, it still might inflict damage to platelets. Besides, the closing velocity of the trileaflet valve was obviously slower than the St. Jude Medical bileaflet valve, and this would effectively reduce occurrences of cavitation. The results of the numerical simulations showed that the flow fields were similar to that of the experiments even if it should be improved further. The results also indicated that numerical simulations could be applied to reduce the cost for development a new trileaflet valve in the future.

Table of Contents
Table of Contents	I
List of Figures	III
List of Tables	VII
Abbreviations	VIII
Chapter 1 General Introduction	1
1.1 Human Heart and Artificial Heart Valves	1
1.2 Mechanical Heart Valves	2
1.3 Evaluation of Mechanical Heart Valves	3
1.4 The Objectives of this Research	5
Chapter 2 Estimation of Viscous Dissipative Stresses Induced by a Mechanical Heart Valve Using PIV Data	7
2.1 Introduction	7
2.2 Materials and Methods	11
2.3 Results	17
2.4 Discussion	20
2.5 Conclusion	25
Chapter 3 Turbulence Characteristics Downstream of a New Trileaflet Mechanical Heart Valve	26
3.1 Introduction	26
3.2 Materials and Methods	27
3.3 Results	30
3.4 Discussion	33
3.5 Conclusion	36
Chapter 4 Numerical Simulations of the Flow Fields Across a New Trileaflet Mechanical Heart Valve Applying a Fluid-Structure Interaction Method	38
4.1 Introduction	38
4.2 Materials and Methods	40
4.3 Results	43
4.4 Discussion	47
4.5 Conclusion	50
Chapter 5 Conclusions	52
5.1 Conclusions	52
5.2 Future Works	53
References	54
 
List of Figures
Figure 1-1 Human heart illustration. (from: Texas Heart Institute website: http://www.texasheart.org/hic/anatomy/anatomy2.cfm)	64
Figure 1-2 Examples of bioprosthetic heart valves – porcine valve.	64
Figure 1-3 Examples of mechanical heart valves.	65
Figure 1-4 Test mechanical heart valves in this study.	65
Figure 2-1 Schematic diagram of the pulsatile mock circulatory loop system.	66
Figure 2-2 Aortic and left ventricular pressures and flow over a cardiac cycle for St. Jude Medical bileaflet valve. LVP, left ventricular pressure; AOP, aortic pressure; CO, cardiac output.	66
Figure 2-3 Schematic view of DPIV measurements planes.	67
Figure 2-4 Ensemble phase average velocity profile for St. Jude Medical bileaflet valve at each phase.	67
Figure 2-5 Ensemble phase average major principal Reynolds shear stress fields for St. Jude Medical bileaflet valve at each phase.	68
Figure 2-6 Ensemble phase average turbulent dissipation rate fields for St. Jude Medical bileaflet valve at each phase.	68
Figure 2-7 Ensemble phase average Kolmogorov length scale fields for St. Jude Medical bileaflet valve at each phase.	69
Figure 2-8 Ensemble phase average turbulent viscous shear stress fields for St. Jude Medical bileaflet valve at each phase.	69
Figure 3-1 Aortic and left ventricular pressures and flow over a cardiac cycle for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve. LVP, left ventricular pressure; AOP, aortic pressure; CO, cardiac output.	70
Figure 3-2 Schematic view of DPIV measurements planes for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	71
Figure 3-3 Ensemble phase average velocity profile for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	72
Figure 3-4 Ensemble phase average major principal Reynolds shear stress fields for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	73
Figure 3-5 Ensemble phase average Kolmogorov length scale fields for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	74
Figure 3-6 Ensemble phase average turbulent viscous shear stress fields for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	75
Figure 3-7 Ensemble phase average vorticity contour for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	76
Figure 4-1 Schematic diagram of the computational domain. (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	77
Figure 4-2 Inlet and outlet boundary conditions by the aortic flow rate and pressures over a cardiac cycle for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	78
Figure 4-3 Leaflet motions over five cardiac cycles for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	79
Figure 4-4 Aortic flow rate and leaflet motions over the 5th cardiac cycle for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	80
Figure 4-5 Pressure drops between the inlet and the outlet boundaries during the systolic phase over the 5th cardiac cycle for (a) St. Jude Medical (SJM) valve; (b) trileaflet (TRI) valve.	81
Figure 4-6 Contours of velocity magnitude (time = 360ms - 480ms) in the middle plane over the 5th cardiac cycle for SJM valve.	82
Figure 4-7 Contours of velocity magnitude (time = 500ms - 620ms) in the middle plane over the 5th cardiac cycle for SJM valve.	83
Figure 4-8 Contours of velocity magnitude (time = 640ms - 760ms) in the middle plane over the 5th cardiac cycle for SJM valve.	84
Figure 4-9 Contours of velocity magnitude (time = 360ms - 480ms) in the middle plane over the 5th cardiac cycle for TRI valve.	85
Figure 4-10 Contours of velocity magnitude (time = 500ms - 620ms) in the middle plane over the 5th cardiac cycle for TRI valve.	86
Figure 4-11 Contours of velocity magnitude (time = 640ms - 760ms) in the middle plane over the 5th cardiac cycle for TRI valve.	87
Figure 4-12 Contours of vorticity magnitude (time = 360ms - 480ms) in the middle plane over the 5th cardiac cycle for SJM valve.	88
Figure 4-13 Contours of vorticity magnitude (time = 500ms - 620ms) in the middle plane over the 5th cardiac cycle for SJM valve.	89
Figure 4-14 Contours of vorticity magnitude (time = 640ms - 760ms) in the middle plane over the 5th cardiac cycle for SJM valve.	90
Figure 4-15 Contours of vorticity magnitude (time = 360ms - 480ms) in the middle plane over the 5th cardiac cycle for TRI valve.	91
Figure 4-16 Contours of vorticity magnitude (time = 500ms - 620ms) in the middle plane over the 5th cardiac cycle for TRI valve.	92
Figure 4-17 Contours of vorticity magnitude (time = 640ms - 760ms) in the middle plane over the 5th cardiac cycle for TRI valve.	93
Figure 4-18 Velocity profiles at phase A over the 5th cardiac cycle for SJM valve.	94
Figure 4-19 Velocity profiles at phase B over the 5th cardiac cycle for SJM valve.	95
Figure 4-20 Velocity profiles at phase C over the 5th cardiac cycle for SJM valve.	96
Figure 4-21 Velocity profiles at phase D over the 5th cardiac cycle for SJM valve.	97
Figure 4-22 Velocity profiles at phase E over the 5th cardiac cycle for SJM valve.	98
Figure 4-23 Velocity profiles at phase F over the 5th cardiac cycle for SJM valve.	99
Figure 4-24 Velocity profiles at phase A over the 5th cardiac cycle for TRI valve.	100
Figure 4-25 Velocity profiles at phase B over the 5th cardiac cycle for TRI valve.	101
Figure 4-26 Velocity profiles at phase C over the 5th cardiac cycle for TRI valve.	102
Figure 4-27 Velocity profiles at phase D over the 5th cardiac cycle for TRI valve.	103
Figure 4-28 Velocity profiles at phase E over the 5th cardiac cycle for TRI valve.	104
Figure 4-29 Velocity profiles at phase F over the 5th cardiac cycle for TRI valve.	105
 
List of Tables
Table 2-1 Time history of the DPIV measurements.	106
Table 2-2 The maximum values of velocity, vorticity, Reynolds normal stress for x-axis, Reynolds normal stress for z-axis, major principal Reynolds normal stress, Reynolds shear stress and major principal Reynolds shear stress during each phase.	107
Table 2-3 The maximum turbulent kinetic energy, maximum turbulent dissipation rate, minimum Kolmogorov length scale, minimum Kolmogorov time scale and maximum turbulent viscous shear stress during each phase.	108
Table 3-1 Time history of the DPIV measurements.	109
Table 3-2 The maximum values of velocity, vorticity, Reynolds normal stress for x-axis, Reynolds normal stress for z-axis, major principal Reynolds normal stress, Reynolds shear stress and major principal Reynolds shear stress during each phase.	110
Table 3-3 The maximum turbulent kinetic energy, maximum turbulent dissipation rate, minimum Kolmogorov length scale, minimum Kolmogorov time scale and maximum turbulent viscous shear stress during each phase.	111
Table 4-1 Time history and the maximum values of velocity for the SJM valve during each phase.	112
Table 4-2 Time history and the maximum values of velocity for the TRI valve during each phase.	112

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