【摘要】：空化是發(fā)生在液體中復(fù)雜的非定常流動(dòng)物理現(xiàn)象,涉及到多相流、可壓縮以及相間交換等多個(gè)方面,對(duì)水力機(jī)械、水利工程、船舶工程、水下兵器和核工業(yè)等眾多領(lǐng)域均有顯著的影響。為了更加準(zhǔn)確的對(duì)空化性能進(jìn)行預(yù)測(cè)和評(píng)估,很有必要針對(duì)空化初生時(shí)的瞬態(tài)機(jī)理特性進(jìn)行研究,分析空化初生時(shí)的瞬態(tài)特性。本文分別以液氬、水作為微觀(guān)研究對(duì)象,研究液氬、水內(nèi)部單個(gè)空化核生長(zhǎng)過(guò)程中的熱力學(xué)參數(shù)變化。以NACA4412翼形為宏觀(guān)研究對(duì)象,將空化機(jī)理微觀(guān)研究結(jié)論應(yīng)用到宏觀(guān)二維水翼的空化瞬態(tài)特性變化中,對(duì)其進(jìn)行數(shù)值模擬計(jì)算,并運(yùn)用試驗(yàn)研究方法驗(yàn)證瞬態(tài)二維水翼空化的模擬計(jì)算結(jié)果的準(zhǔn)確性。主要內(nèi)容如下:1.查閱相關(guān)資料,介紹了空化、氣泡成核等相關(guān)的理論基礎(chǔ)知識(shí)及國(guó)內(nèi)外最新研究進(jìn)展。并介紹了分子動(dòng)力學(xué)(MD)研究方法,以及相變和氣體狀態(tài)方程的理論基礎(chǔ)知識(shí),敘述了分子動(dòng)力學(xué)(MD)分子力場(chǎng)、邊界條件以及分子動(dòng)力學(xué)(MD)模擬的求解過(guò)程,闡述了在計(jì)算過(guò)程中的各種系綜的定義以及適用類(lèi)型,并詳細(xì)介紹了分子動(dòng)力學(xué)軟件的選擇以及應(yīng)用,同時(shí)采用LAMMPS軟件完成本文第三、四章的模擬計(jì)算工作,在模擬計(jì)算中采用Lennard-Jones(12-6)力場(chǎng),邊界條件采用周期性邊界條件。2.在NVT系綜下,對(duì)存在不同初始尺寸空化核的Lennard-Jones流體進(jìn)行了研究,得到NVT系綜下空化核發(fā)展的演變過(guò)程,并對(duì)其分子勢(shì)能、系統(tǒng)密度、分子徑向分布函數(shù)、系統(tǒng)壓力以及系統(tǒng)總能量等相關(guān)的熱力學(xué)參數(shù)進(jìn)行分析,結(jié)果表明:在空化初生階段,由于流體的局部壓力降低而造成液相分子不斷的進(jìn)入空化核,促使了空化核的生長(zhǎng)。通過(guò)分子勢(shì)能研究發(fā)現(xiàn),空化核位置處的分子勢(shì)能較高,而液相分子勢(shì)能在空化核生長(zhǎng)初期變化較大。在空化核成長(zhǎng)后期,液相分子勢(shì)能相對(duì)較為平穩(wěn)。液相密度和界面密度變化比較明顯,而空泡中心區(qū)域的密度變化甚微。液相區(qū)域徑向分布函數(shù)的峰強(qiáng)度就越大,峰寬度越窄,該區(qū)域分子間距rbin越小。隨著空化核初始尺寸的減小,系統(tǒng)壓力和總能量達(dá)到平衡的時(shí)間越長(zhǎng),平衡后的壓力值和能量值也就越小。而且在空化核生長(zhǎng)初期,系統(tǒng)的總能量和壓力的變化波動(dòng)較大。3.在NPT系綜下,對(duì)存在空化核的水分子計(jì)算域進(jìn)行了研究,得到空化核初始尺寸與系統(tǒng)初始?jí)毫?duì)空化初生的影響關(guān)系,并對(duì)空化核發(fā)展的演變過(guò)程、系統(tǒng)壓力以及系統(tǒng)能量、分子徑向分布函數(shù)等相關(guān)的熱力學(xué)參數(shù)進(jìn)行分析,結(jié)果表明:在系統(tǒng)初始?jí)毫σ欢〞r(shí),空化核尺寸存在臨界值,當(dāng)空化核初始尺寸小于該臨界值時(shí),即使液體受到負(fù)壓的影響,由于分子之間的作用力和氫鍵力束縛空化核不能生長(zhǎng),因此空化不會(huì)發(fā)生�？栈谖⒂^(guān)層面上不僅是壓力和空化核參數(shù)的變化,更重要的原因是系統(tǒng)初始?jí)毫υ斐上到y(tǒng)能量增大,結(jié)構(gòu)失穩(wěn),從而造成空化核的變化。在內(nèi)部空化核尺寸一定時(shí),外部壓力存在一臨界值,當(dāng)外部壓力值大于該臨界值時(shí),計(jì)算域內(nèi)部不會(huì)產(chǎn)生相應(yīng)的負(fù)壓,或者產(chǎn)生負(fù)壓值較小,不足以使系統(tǒng)失穩(wěn),在分子之間的作用力和氫鍵力束縛下空化核不能生長(zhǎng),因此空化不會(huì)發(fā)生。伴隨著空化核的生長(zhǎng),系統(tǒng)能量和分子勢(shì)能總有增大的趨勢(shì),而該增大趨勢(shì)影響了計(jì)算域內(nèi)部結(jié)構(gòu)的穩(wěn)定性,促使空化核迅速增大。4.通過(guò)第四章中納米尺度空化核生長(zhǎng)變化過(guò)程以及目前研究的微米尺度空化核生長(zhǎng)過(guò)程對(duì)Z-G-B空化模型進(jìn)行了優(yōu)化,并在此基礎(chǔ)上選用RNG k-ε湍流模型以及優(yōu)化后的空化模型對(duì)二維水翼空化初生過(guò)程進(jìn)行模擬計(jì)算。在空化初生時(shí),由于不同攻角產(chǎn)生的壓力場(chǎng)略有不同,因此空化核周?chē)后w失穩(wěn)的可能性不同,造成水翼上表面空化核生長(zhǎng)發(fā)育的幾率不同。攻角越大,產(chǎn)生的低壓極值越小,空化核周?chē)后w所受到的負(fù)壓逐漸增大,從而勢(shì)能增大,空化核快速生長(zhǎng)。通過(guò)微觀(guān)空化核研究發(fā)現(xiàn),在不同壓力場(chǎng)作用下,空化核生長(zhǎng)幾率不同,因此在不同攻角影響下,空泡發(fā)展規(guī)模和強(qiáng)度不同。在空化發(fā)展過(guò)程中,因尾部高壓產(chǎn)生的回射流對(duì)空泡的形狀以及脫落有著重要影響�？栈纬珊笃�,低壓和高壓的相互影響直接造成脫落空泡破裂與發(fā)展,因而影響到水流的流態(tài)分布以及旋渦的產(chǎn)生�？栈跎鷷r(shí)只是受到低壓作用的影響,而空化再生則受到低壓作用和紊亂流場(chǎng)的雙重作用。
[Abstract]:Cavitation is a complex non-constant-flow physical phenomenon in the liquid, which involves many aspects such as multi-phase flow, compressible and phase-to-phase exchange, and has a remarkable influence on the hydraulic machinery, hydraulic engineering, ship engineering, underwater weapon and nuclear industry. In order to predict and evaluate the cavitation performance more accurately, it is necessary to study the characteristics of the transient mechanism at the time of cavitation, and to analyze the transient characteristics at the time of cavitation. In this paper, the thermodynamic parameters in the growth of a single cavitation nucleus in liquid argon and water are studied by using liquid argon and water as the micro-research object. In this paper, the micro-study of the cavitation mechanism is applied to the change of the cavitation transient characteristics of the macro-two-dimensional hydrofoil, and the accuracy of the simulation results of the transient two-dimensional hydrofoil cavitation is verified by means of the experimental research method. The main content is as follows: 1. The relevant data, such as cavitation, bubble nucleation, etc., and the latest research progress at home and abroad are introduced. In this paper, the molecular dynamics (MD) method, the theoretical basic knowledge of the phase change and the gas state equation are introduced, the molecular dynamics (MD) molecular fields, boundary conditions and the solution process of the molecular dynamics (MD) simulation are described. The definition and application type of various ensemble in the calculation process are described, and the selection and application of the molecular dynamics software are introduced in detail. At the same time, the simulation calculation of the third and fourth chapters of this paper is accomplished by using the LAMPS software, and the Lennard-Jones (12-6) force field is used in the simulation calculation. The boundary conditions are periodic boundary conditions. In the NVT ensemble, the NVT-Jones fluid with different initial size cavitation cores was studied to obtain the evolution of the development of the cavitation nuclei under the NVT ensemble, and the molecular potential energy, the system density and the molecular radial distribution function were obtained. The thermodynamic parameters, such as the system pressure and the total energy of the system, are analyzed. The results show that, in the initial stage of cavitation, the flow of the liquid phase molecules into the cavitation nucleus due to the decrease of the local pressure of the fluid promotes the growth of the cavitation nucleus. The results show that the molecular potential energy at the position of the cavitation nucleus is high, and the potential energy of the liquid phase changes greatly in the initial stage of the cavitation nucleus. The potential energy of the liquid phase is relatively stable in the later stage of the growth of the cavitation nucleus. The change of the density of the liquid phase and the density of the interface is obvious, and the density of the cavity of the cavity has little change. The larger the peak intensity of the radial distribution function in the liquid phase region, the narrower the peak width, and the smaller the region molecular space rbin. As the initial size of the cavitation nucleus decreases, the longer the system pressure and total energy reach equilibrium, the smaller the pressure and energy values are balanced. and the variation of the total energy and pressure of the system is large in the early stage of the growth of the cavitation nuclei. The influence of the initial size of the cavitation nucleus and the initial pressure of the system on the cavitation is obtained under the NPT ensemble, and the evolution of the development of the cavitation nucleus, the system pressure and the system energy are obtained. The thermodynamic parameters such as the molecular radial distribution function and so on are analyzed. The results show that when the initial pressure of the system is constant, the critical value of the size of the cavitation core, when the initial size of the cavitation nucleus is less than the critical value, even if the liquid is affected by the negative pressure, Cavitation does not occur due to the forces between the molecules and the hydrogen bond forces that bind the cavitation core to be unable to grow. Cavitation is not only the change of pressure and cavitation core parameters at the micro level, but more importantly, the initial pressure of the system causes the system energy to increase and the structure is unstable, thus causing the change of the cavitation core. when the internal cavitation core is of a certain size, the external pressure is a critical value, and when the external pressure value is larger than the critical value, a corresponding negative pressure is not generated in the calculation domain, or the negative pressure value is small, so that the system is unstable, the cavitation nuclei can not grow under the force of the molecules and the hydrogen bond forces, so that the cavitation does not occur. With the growth of the cavitation nucleus, the total energy and molecular potential energy of the system have a tendency to increase, and the increasing trend affects the stability of the internal structure of the computational domain, which causes the cavitation nuclei to increase rapidly. The Z-G-B cavitation model was optimized by the process of nano-scale cavitation nucleus growth in the fourth chapter and the current study of the micro-scale cavitation nuclear growth process. On the basis of this, the RNG k-turbulent flow model and the optimized cavitation model are used to calculate the cavitation primary process of the two-dimensional hydrofoil. When the cavitation is primary, the pressure field generated by the different attack angles is slightly different, so the possibility of liquid instability around the cavitation nucleus is different, and the probability of the development of the cavitation nuclei on the surface of the hydrofoil is different. The higher the attack angle, the smaller the low-pressure extreme value, and the negative pressure of the surrounding liquid of the cavitation nucleus is gradually increased, so that the potential energy is increased and the cavitation nucleus grows rapidly. The results show that under different pressure fields, the probability of cavitation core growth is different, and the development scale and intensity of the cavitation are different under the influence of different attack angle. In the process of cavitation development, the jet-back jet produced by the tail high pressure has an important influence on the shape and the shedding of the cavity. In the late stage of cavitation formation, the interaction of low pressure and high pressure directly causes the drop-off cavitation and development, thus affecting the flow state distribution of the water flow and the generation of the vortex. The cavitation regeneration is only affected by the low pressure, while the cavitation regeneration is the dual function of the low pressure and the disturbance flow field.
【學(xué)位授予單位】：江蘇大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類(lèi)號(hào)】：O35

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