本文關(guān)鍵詞： 格子Boltzmann-離散元耦合方法 Bingham流體 Bingham兩相流 自密實混凝土 多孔介質(zhì)　出處：《清華大學(xué)》2014年博士論文　論文類型：學(xué)位論文

【摘要】：堆石混凝土的澆筑過程主要是自密實混凝土在堆石孔隙中的流動填充過程，本質(zhì)上是賓漢姆（Bingham）兩相流在多孔介質(zhì)中運動的過程。本論文建立了耦合的格子Boltzmann-顆粒離散元方法（簡稱LBM-DEM），并將其應(yīng)用于自密實混凝土在多孔介質(zhì)中的流動研究。LBM作為一種新興的計算流體力學(xué)方法，在處理復(fù)雜邊界條件、計算簡單穩(wěn)定、易于并行等方面有著傳統(tǒng)數(shù)值方法無法比擬的優(yōu)勢。而DEM在模擬固體顆粒的摩擦、碰撞、堵塞和遷移等方面物理概念清晰，在巖土力學(xué)和顆粒流等領(lǐng)域也得到了廣泛的應(yīng)用。因此，充分吸收了上述兩種方法特點的LBM-DEM的耦合框架在處理多相流的復(fù)雜流動方面有著明顯的優(yōu)勢。 Bingham流體是一種典型的非牛頓流體，因此需要對常用于牛頓流體的線性LBM方法進行修正。本文采用了Papanastasiou提出的改進指數(shù)模型，并使用了多松弛時間的LBM（簡稱MRT-LBM）對Bingham流體進行數(shù)值模擬和研究。在保持計算穩(wěn)定性的同時，MRT-LBM很好地模擬了非屈服區(qū)，并捕捉到了死區(qū)的存在。另外，通過對Bingham流體中的圓形顆粒曳力系數(shù)進行研究發(fā)現(xiàn)，曳力系數(shù)CD不僅和雷諾數(shù)Re有關(guān)，還與賓漢姆數(shù)Bn有關(guān)，因此，本論文根據(jù)理論推導(dǎo)和數(shù)值計算結(jié)果，給出了Bingham流體中統(tǒng)一的曳力系數(shù)公式。 本論文在上述工作的基礎(chǔ)上建立了Bingham兩相流的LBM-DEM耦合框架，并將其應(yīng)用于Bingham兩相流在二維單孔道和多孔道中的堵塞問題研究。在單孔道流動中，利用通過系數(shù)定量地研究了孔徑比、顆粒固含率和壓力梯度對Bingham兩相流堵塞的影響，，發(fā)現(xiàn)二維條件下堵塞的臨界孔徑比為2，且孔徑比是影響堵塞最關(guān)鍵的因素。隨著顆粒固含率的增加，堵塞的風(fēng)險會有所增加，而壓力梯度對于兩相流的堵塞沒有影響。多孔道的研究發(fā)現(xiàn)，Bingham兩相流的流動是自調(diào)整的，而且窄通道數(shù)（孔徑比小于2）的增加會顯著增加堵塞的風(fēng)險。最后，本論文實現(xiàn)了三維條件下Bingham兩相流的LBM-DEM方法的應(yīng)用。利用該方法，不僅得到多孔介質(zhì)內(nèi)部豐富的流動信息，還有效地模擬出兩相流中的顆粒運動、堵塞等現(xiàn)象。三維定性的研究結(jié)果表明：LBM-DEM耦合方法是一種研究自密實混凝土在堆石孔隙中的流動、堵塞等細(xì)觀力學(xué)過程的有效數(shù)值方法。
[Abstract]:The pouring process of rockfill concrete is mainly the flow filling process of self-compacting concrete in the rockfill pore. In this paper, a coupled lattice Boltzmann- particle discrete element method (LBM-DEMN) is developed and applied to study the flow of self-compacting concrete in porous media. As a new method of computational fluid dynamics, In dealing with complex boundary conditions, simple and stable calculation, easy to parallel and so on, DEM has incomparable advantages over traditional numerical methods, while DEM has clear physical concepts in simulating the friction, collision, blockage and migration of solid particles. It has been widely used in the fields of geotechnical mechanics and particle flow, so the coupling frame of LBM-DEM, which fully absorbs the characteristics of the above two methods, has obvious advantages in dealing with the complex flow of multiphase flow. Bingham fluid is a typical non-Newtonian fluid, so it is necessary to modify the linear LBM method, which is often used in Newtonian fluid. In this paper, an improved exponential model proposed by Papanastasiou is used. The Bingham fluid is numerically simulated and studied by means of multiple relaxation time LBM (MRT-LBM). While maintaining the computational stability, MRT-LBM can well simulate the non-yielding zone and capture the existence of the dead zone. By studying the drag coefficient of circular particles in Bingham fluid, it is found that drag coefficient CD is not only related to Reynolds number re, but also to Bingham number B n. The uniform drag coefficient formula in Bingham fluid is given. Based on the above work, the LBM-DEM coupling framework of Bingham two-phase flow is established and applied to the study of the blockage of Bingham two-phase flow in two-dimensional single-channel and multi-channel. The effects of pore size ratio, particle holdup and pressure gradient on the blockage of Bingham two-phase flow are quantitatively studied by using the coefficient. It is found that the critical pore size ratio is 2, and the pore size ratio is the most important factor affecting the blockage. With the increase of particle solid holdup, the risk of blockage will increase. However, the pressure gradient has no effect on the blockage of two-phase flow. The multi-channel study shows that the flow of Bingham two-phase flow is self-adjusted, and the increase in the number of narrow channels (pore size ratio less than 2) will significantly increase the risk of blockage. In this paper, the application of LBM-DEM method for Bingham two-phase flow under three-dimensional condition is realized. By using this method, not only the abundant flow information in porous media is obtained, but also the movement of particles in two-phase flow is effectively simulated. The results of three-dimensional qualitative study show that the coupling method of the two dimensional LBM-Dem is an effective numerical method for studying the flow and blockage of self-compacting concrete in the pores of rockfill.
【學(xué)位授予單位】：清華大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2014
【分類號】：TV431;TV544

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