應用平行化分子動力學模擬法於液滴‐液滴間碰撞力學之研究

Abstract

中文摘要

本論文是利用平行化分子動力學程式(Parallelized cellular molecular dynamics. PCMD)來模擬探討兩個在奈米尺度下由氦(Argon)原子所構成的相同直徑(~10nm) 的液滴在真空環境下以及含背壓環境狀態下的碰撞動力壆行為。由模擬的結果，我們觀察到其動力學行為十分複雜，而且模擬的背壓條件、液滴間的相度速度以及碰撞參數(Impact Parameter) 對液滴碰撞後的行為都有決定性的影響。模擬中觀察到的行為有：液滴彈性碰撞(Bounce)、液滴結合(Direct Coalescence)、液滴變形結合(Stretching Coalescence)、液滴拉伸破裂(Stretching Separation)以及液滴碎裂(Shattering)。我們首次建構了在不同背壓條件下的液滴行為區域圖。並著利用分析,碰撞後的液滴尺寸分布以及碰撞間能量的轉換情形來進一步的解釋複雜的液滴碰撞動力學行為。本論文研究可以區分為以下兩個主要部份： 第一部分，我們發展了一個在Memory-distributed的平行計算器上(例如: PC-cluster 系統)上執行的平行化分子動力學程式(PCMD)，結合Link-cell的結構資料的優點，來快速的搜尋建立Verlet-list。並且於動態區域切割中採用Multi-level Graph-partitioning的技巧來確保每個處理器中負載均衡。設計簡易負載重新分配機制(Simple Threshold Scheme)，當某一工作區域的負載超過設定值時，將重新分割計算區域。以一百萬顆L-J atoms為例，在不同的熱力學狀態（Condensed、Vaporized、Supercritical）下其平行效率分別可以達到57%、 35% 以及 65%。 第二部份中，我們利用第一部份所發展的PCMD程式以及L-J(12-6)的勢能模型，來研究兩個奈米尺度下的液滴於真空環境以及含背壓環境下的的碰撞動力學行為。測試的條件包括，不同的Impact Parameter (0-8 nm)，不同的液滴間相對速度(20-1500 m/s)以及不同的背壓環境(0, 0.055 and 0.55 atm)。觀察到的行為有可以區分為Bounce、Direct Coalescence、Stretching Coalescence、Stretching Separation以及Shattering。真空環境下，相對速度較高時可以容易的觀察到Disruption、Fragmentation及Shattering的行為，而Direct Coalescence和Stretching Coalescence則發在相對速對較低時。而當背壓氣體存在時，Disruption及Fragmentation發生時所需要的相對速度則高於真空狀態下。液滴的Bounce行為則存在於極低的相對速度下，並且Bounce只有在背壓存在時才發生，這與之前的文獻十分吻合。另外，本文中提及Stretching Coalescence的現象則是第一次被發現。針對相對速度、碰撞參數以及背壓對液滴碰撞過程的影響，我們將在本文中利用最大破裂液滴的Rotational energy及Vibration energy的方式分別做詳細的討論。

Abstract Collision dynamics between two nanoscale argon droplets with the same diameter of ~10nm under vacuum and pressurized environment is simulated using a parallelized cellular molecular dynamics (PCMD) simulation code. Simulation results show that the collision dynamics between two droplets can be very complicated, which strongly depends upon the magnitude of the background pressure, the relative inertia (or collision velocity) and impact parameter. These phenomena include bouncing, direct coalescence, stretching coalescence, stretching separation and shattering. Regime maps at different background pressures are constructed for the first time to the best knowledge of the author. Analysis of snapshots of molecular distribution, fragment size distribution, surface tension on droplet surfaces and energy transfer process during collision are used to explain the complicated collision dynamics. The research is divided into two phases, which is described as follows. In the first phase, a PCMD code is developed on memory-distributed parallel machines (e.g., PC-cluster system) by taking advantage of link-cell data structure, which is often used for fast search in constructing the Verlet list. Dynamic spatial domain decomposition using multi-level graph-partitioning technique is employed to enforce the load balancing among processors. A simple threshold scheme (STS), in which workload imbalance is monitored and compared with some threshold value during the runtime, is proposed to decide the proper time for repartitioning the domain. Results show that the parallel efficiency using one million L-J atoms reaches 57%, 35% and 65%, respectively, for condensed, vaporized and supercritical test cases at 64 processors of HP clusters at NCHC. In the second phase, the above developed PCMD code using L-J (12-6) potential is used to study the collision dynamics between two nanoscale droplets under vacuum and pressurized environments. Test conditions will include variations of the impact parameter (0-8 nm), relative velocity of droplets (20-1500 m/s), background gas pressure (0, 0.055 and 0.55 atm; ¥, 2312.3 and 216.9) and the background gas temperature is 216K. Observed phenomena can be categorized as bouncing, direct coalescence, stretching coalescence, stretching separation and shattering. Distributions of these regimes, as a function of relative velocity and impact parameter, are constructed for the first time for different background gas pressures. The simulation results under vacuum condition show that disruption, fragmentation and shattering can be easily observed at higher relative velocities, while direct coalescence can only be found at lower relative velocities. However, with the existence of background gas, disruption and fragmentation can only be observed at higher velocities than those under vacuum conditions. Bouncing at very low velocity (10-30 m/s) can be clearly observed under pressurized environments, which coincides with previous findings. In addition, stretching coalescence is observed for the first time at intermediate relative velocity and impact parameter under pressurized environment. Effects of the relative velocity, impact parameter and ambient pressure to the collision process are discussed in detail using the concept of the separable rotational energy and the vibration energy of the largest cluster during collision.