【摘要】：在計(jì)算流體力學(xué)中,起源于格子氣自動(dòng)機(jī)和分子動(dòng)理論的晶格Boltzmann方法,發(fā)展成為一種可靠的新途徑,其高效性、準(zhǔn)確性和魯棒性也被普遍證實(shí)。它不再依賴于宏觀連續(xù)介質(zhì)的Navier-Stokes方程,而從微觀模型出發(fā)模擬復(fù)雜流體的宏觀行為,在研究顆粒流、熱流動(dòng)、微流體等復(fù)雜流體運(yùn)動(dòng)中得到成功的應(yīng)用。特別是在多相流研究中,由于晶格Boltzmann方法不需要邊界積分,避免了傳統(tǒng)計(jì)算流體力學(xué)方法在跟蹤大量分散相界面時(shí)遇到的困難�；跓崃W(xué)原理,課題組提出了具有熱力學(xué)一致性和伽利略不變性的多相流模型,為研究表面潤(rùn)濕現(xiàn)象打下了基礎(chǔ)。表面潤(rùn)濕是一種常見的自然現(xiàn)象;作為體現(xiàn)表面潤(rùn)濕性的特征量,接觸角是氣體、液體和固體表面三相相互作用的結(jié)果,也是研究和應(yīng)用中常用的物理量。雖然晶格Boltzmann方法模擬研究表面潤(rùn)濕已經(jīng)取得了很大成功,但是還沒有一種可靠的算法計(jì)算動(dòng)態(tài)的接觸角;即使是靜態(tài)接觸角也只能采用理想的球冠模型計(jì)算或使用外部工具測(cè)量。首先采用化學(xué)勢(shì)邊界條件,將液滴直接落在基板上,不再需要過(guò)渡區(qū),接觸角可以直接從三項(xiàng)接觸點(diǎn)開始測(cè)量。然后通過(guò)分析接觸角的微觀形態(tài),使用氣液界面和第二排格線的交點(diǎn)確定切線,提出了在模擬過(guò)程中實(shí)時(shí)地計(jì)算動(dòng)態(tài)接觸角的方法。在忽略重力的條件下,對(duì)該方法計(jì)算的接觸角和理論計(jì)算結(jié)果做了系統(tǒng)的比較。在溫度為0.7和0.8時(shí),在30至160度范圍內(nèi)符合的非常好,說(shuō)明該方法的計(jì)算結(jié)果是準(zhǔn)確的。在相同的固體界面上,液滴半徑從20至200個(gè)格子,計(jì)算所得的接觸角保持不變,說(shuō)明該方法的計(jì)算結(jié)果是穩(wěn)定的。在包含重力的情況下,計(jì)算了大小從0.1cm到0.5cm的不同的平面上液滴和懸垂液滴。結(jié)果顯示,隨著尺寸的變大,液滴的形變程度越來(lái)越大,但是當(dāng)前方法計(jì)算得到的液滴接觸角保持不變。這與理論預(yù)期是一致的,也真實(shí)地反映了接觸角的微觀本質(zhì)。進(jìn)一步將液滴放置在親疏水交替排布的固體表面,在表面傾斜程度不太大的時(shí)候,液滴在重力的作用下會(huì)發(fā)生傾斜,從而產(chǎn)生接觸角遲滯現(xiàn)象。使用三種不同親疏水配置的表面計(jì)算接觸角遲滯,結(jié)果顯示,盡管疏水的模式表面比親水表面具有更大的接觸角,而且液滴更不容易失穩(wěn)滑動(dòng),但是接觸角遲滯隨著表面的傾斜增大的趨勢(shì)卻是基本一致的。當(dāng)失穩(wěn)滑動(dòng)以后,液滴會(huì)連續(xù)的滑過(guò)親疏水條帶,前進(jìn)角會(huì)逐漸增加然后突然下降,后退角會(huì)逐漸減小然后突然增加,二者的變化一般不同步,導(dǎo)致接觸角遲滯呈現(xiàn)動(dòng)態(tài)的波動(dòng)狀態(tài)。接觸角遲滯的動(dòng)態(tài)波形與前進(jìn)和后退角的相位有關(guān),而且包含了由于液滴的抖動(dòng)說(shuō)產(chǎn)生的高階波動(dòng)。這些關(guān)于接觸角和遲滯的研究促進(jìn)了深入理解并實(shí)際應(yīng)用潤(rùn)濕現(xiàn)象。顆粒在管道流中的跨流層側(cè)向遷移是奇妙的自然現(xiàn)象,近幾年已發(fā)展成為微流控領(lǐng)域的一個(gè)研究熱點(diǎn)。利用多弛豫晶格Boltzmann方法,我們研究了橢圓顆粒的慣性聚集現(xiàn)象。與經(jīng)典的Segre-Silberberg效應(yīng)類似,橢圓顆粒也表現(xiàn)出側(cè)向遷移和平衡。但是因?yàn)槠浞菆A幾何形狀,橢圓顆粒的運(yùn)動(dòng)中還包括著非勻速扭轉(zhuǎn)和周期性振動(dòng)。我們分別研究了雷諾數(shù)、阻塞比和縱橫比對(duì)橢圓運(yùn)動(dòng)軌跡的影響,發(fā)現(xiàn)在雷諾數(shù)改變對(duì)橢圓平衡位置的影響很小,雷諾數(shù)從3變到300時(shí),平衡位置的變化率只有3%;阻塞比越大的顆粒,其平衡位置越接近靠近管道的中心線,并且顆粒對(duì)流場(chǎng)的影響越強(qiáng)烈;隨著縱橫比的增加,橢圓顆粒的旋轉(zhuǎn)周期變短,而且平衡位置呈現(xiàn)馬鞍形變化,在縱橫比接近0.5時(shí)達(dá)到最低點(diǎn)。這項(xiàng)工作對(duì)于研究具有卵形或橢圓形紅細(xì)胞的鳥類的血液流有積極的促進(jìn)作用。
[Abstract]:In the computational fluid dynamics, the lattice Boltzmann method, which is derived from the lattice gas and the molecular dynamic theory, has been developed into a reliable new way, and its efficiency, accuracy and robustness are also widely confirmed. It is no longer dependent on the Navier-Stokes equations of the macro continuum, and the macroscopic behavior of complex fluid is simulated from the micro-model, and the successful application in the complex fluid movement such as particle flow, thermal flow, micro-fluid and the like is studied. In particular, in the multi-phase flow research, since the lattice Boltzmann method does not need the boundary integration, the difficulties encountered by the traditional CFD method in tracking a large number of dispersed phase interfaces are avoided. Based on the principle of thermodynamics, the research group put forward a multi-phase flow model with thermodynamic consistency and Galileo invariance, which laid the foundation for the study of surface wetting. Surface wetting is a common natural phenomenon; as a feature of surface wettability, the contact angle is the result of three-phase interaction of gas, liquid and solid surface, and also is a common physical quantity in research and application. Although the lattice Boltzmann method has made great success in the simulation of the surface wetting, there is no reliable algorithm to calculate the dynamic contact angle; even the static contact angle can only be measured using an ideal spherical cap model or using an external tool. First, the chemical potential boundary condition is used to drop the liquid drop directly on the substrate, no transition zone is needed, and the contact angle can be measured directly from the three contact points. The method of calculating the dynamic contact angle in real time is presented by analyzing the micro-shape of the contact angle, using the intersection point of the gas-liquid interface and the second-row lattice line to determine the tangent. The contact angle calculated by the method and the theoretical calculation result are compared with the system under the condition of ignoring the gravity. When the temperature is 0. 7 and 0. 8, the results of the method are very good in the range of 30 to 160 degrees, and the calculation result of the method is accurate. On the same solid interface, the droplet radius is from 20 to 200 grids, and the calculated contact angle remains unchanged, indicating that the calculation result of the method is stable. In the case of gravity, the droplets and the suspended droplets in a different plane from 0. 1 cm to 0. 5 cm were calculated. The results show that, with the change of the size, the deformation of the liquid drops is more and more large, but the contact angle of the liquid drop calculated by the current method remains unchanged. This is consistent with the theoretical expectation, and the micro essence of the contact angle is also truly reflected. the liquid drops are further placed on the solid surface which is arranged alternately in the water-friendly water, and when the surface inclination is not too large, the liquid drops can be inclined under the action of gravity, so that the contact angle hysteresis phenomenon is generated. The contact angle hysteresis is calculated using the surface of three different lipophilic water configurations, and the results show that, although the hydrophobic mode surface has a greater contact angle than the hydrophilic surface, and the droplets are less prone to instability, the tendency of the contact angle hysteresis to increase with the inclination of the surface is substantially uniform. After the unstable sliding, the liquid drops can slide through the hydrophobic strip continuously, the advancing angle gradually increases and then suddenly decreases, the back angle gradually decreases and then suddenly increases, the variation of the two is generally not synchronized, resulting in a dynamic fluctuation state of the contact angle hysteresis. The dynamic waveform of the contact angle hysteresis is related to the phase of the forward and backward angles, and contains high-order fluctuations due to the shaking of the droplets. These studies on contact angle and hysteresis have contributed to an in-depth understanding and practical application of the wetting phenomenon. The lateral migration of the cross-flow layer in the pipe flow is a wonderful natural phenomenon, and has developed into a hot spot in the field of micro-flow control in recent years. By using the multi-relaxed lattice Boltzmann method, we have studied the phenomenon of the inertia of the elliptic particles. Similar to the classical Sere-Silberberg effect, elliptical particles also exhibit lateral migration and balance. but because of its non-circular geometry, the motion of the elliptical particles also includes non-uniform torsional and periodic vibrations. The influence of the Reynolds number, the blocking ratio and the aspect ratio on the trajectory of the elliptic motion is studied respectively. It is found that the influence of the change of the Reynolds number on the equilibrium position of the ellipse is very small, the change rate of the equilibrium position is only 3% when the Reynolds number changes from 3 to 300, and the larger the blocking ratio is, The closer the equilibrium position is to the center line of the pipe, the more the effect of the particle convection field is; with the increase of the aspect ratio, the rotation period of the elliptical particles becomes shorter, and the equilibrium position exhibits a saddle-shaped change, reaching the lowest point when the aspect ratio is close to 0.5. This work has a positive effect on the study of the blood flow of birds with ovoid or elliptical red blood cells.
【學(xué)位授予單位】：廣西師范大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：O35

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