956 resultados para Direct Numerical Simulation (Dns)


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本文应用基元反应模型和频散可控耗散格式(DCD)对氢氧爆轰波进行了二维数值模拟.氢氧混合物的化学反应模型考虑了8种组分20个反应方程式.在处理化学反应引起的刚性问题时采用了时间算子分裂的方法.本文首先对爆轰波数值结果和实验结果进行了对照验证,然后对爆轰波在楔面反射由马赫反射向规则反射转变的过程进行了数值分析,得到了反射转变临界角,并和实验结果及理论分析进行了比较,结果是令人满意的;本文还对爆轰波的多波结构进行了初步的数值分析.

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用液固两相流体动力学的方法对离心研磨过程中磨料流的运动特征进行数值模拟,得出磨料流运动的动态结果,并与高速摄影实验结果进行对比,指出计算机数值模拟结果对研磨机的设计和研磨工艺参数的选择具有实际的指导意义。该方法经济简捷,可以代替大量的工艺实验实现对离心研磨技术参数的优化,从而提高生产效率。

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建立了微重力蜡烛火焰的数学模型。计算与分析表明,火焰的形状由空气动力学特征决定,火焰是温度取决于化学反应动力学特征和火焰的热损失。在静止微重力环境中,自然对流的消失使火焰为半球形。辐射热损失对烛火焰温度(颜色)特征的形成有重要贡献,在静止重力环境下,化学反应放热速率受氧气扩散速率控制,辐射热损失的冷却使火焰温度低于正常重力温度值。但当环境气体的流动速度加大时,辐射热损失的影响逐渐减小,蜡烛火焰的温度逐渐接近正常重力蜡烛火焰的温度。当氧浓度较小时,火焰峰值温度小于烟黑形成的阈值温度(1300 K);当氧浓度较大时,火焰温度大于烟黑形成的阈值温度。

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以激光熔凝表面强韧化处理为背景,应用空间弹塑性有限单元和高精度数值算法同时考虑材料组织性能的变潜模拟工件的温度场及残余应力,研究激光熔凝加工中瞬时温度场及残余应力数值模拟,同时考虑相变潜热及相变塑性的影响,用算例验证了模型的正确性,给出了不同时刻温度场分布及残余应力分布。

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通过与相关文献结果的对比,在验证了数值模拟应力波传播可行性的基础上,比较了应力波通过3层花岗岩和夹层为泡沫铝的3层介质后,发现后者应力波幅值的衰减远大于前者,应变能增加为前者的1.6倍,证明了软夹层在研究的速度量级上对能量耗散具有显著作用;在入射波波长为3层介质总厚度1/2的条件下,当泡沫铝的厚度占总厚度约0.2时,得出了3层介质的应变能约为系统总能量的60%,此时入射波波长为泡沫铝厚度的2.5倍,组合介质获得较佳的衰减性能.

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针对渗吸法采油中的毛管力驱动项和流动系数的取值问题,根据地下流体渗流特征,建立了低渗透油藏渗吸法采油数学模型,给出了数值解法.定量分析了渗吸法采油的主要影响因素,通过实例验证,用该方法计算的各项动态指标与矿场实际值符合程度较高.结果表明:基质与裂缝渗透率之比小于O.Ol,油水黏度比小于15,毛管力较大的水湿油层比较适合于渗吸法采油,能够提高水驱采收率2%左右.

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针对断续节理岩体提出了一种随机计算模型.在该模型中,假设结构面的形状为正方形,通过岩体结构面的统计分布函数模拟结构面的空间随机分布.给出了随机节理模型的实现方法,对该随机模型的算法可靠性进行了验证.通过单向加载模拟试验研究了节理岩体破坏强度与节理倾角及节理连通率等因素的关系,并与极限平衡条件推导的理论结果进行了比较,分析了数值模拟结果与极限平衡理论结果的异同性,进而验证了节理随机计算模型的可靠性.同时,研究了节理连通率与岩体等效弹性模量之间的影响关系,给出了二者的影响关系式.

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采用二阶精度频散控制耗散格式(DCD)和8组分20个方程的基元反应模型,对轴对称变截面管道中氢氧爆轰波传播进行数值模拟.结果表明,爆轰波传播至突变截面扩张管道时,由于稀疏波的作用可能会使爆轰波局部熄爆甚至完全熄爆,对于某些敏感度高的反应气体爆轰波可以二次起爆.而在渐变截面扩张管道爆轰波相对不易熄爆.

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提出了一种CH_4/O_2在超声速气流中燃烧的6方程化学反应模型,并且用在超声速高温空气中横向喷射的二维流场进行了数值模拟验证。特别模拟了不同来流条件和壁面条件对燃烧效率的影响及CH_4和空气中的氧气混合燃烧的过程。数值方法采用二阶精度的Harten-Yee隐式TVD格式,计算结果表明6方程反应模型能较好地反映CH_4/O_2的燃烧过程。

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采用三维离散元方法,模拟含节理岩块的单轴受压试验。并针对节理空间分布,给出了解析解与数值计算比较的结果,验证了数值模拟的正确性。当节理正交时,离散元与有限元的计算结果一致。计算结果定量地说明了岩体的各向异性和尺寸效应。

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通过与实验结果的比较验证Fluent软件可以用于超音速流动中侧向喷流干扰特性数值研究.使用Fluent软件对超音速流动中不同喷流压力和攻角下侧向喷流干扰特性进行的数值研究表明,超音速流动中,随喷流压力和负攻角增大,喷流前的高压区明显增大,喷流的控制效果更好.喷流包裹作用的影响范围在90°弹面内.喷流压力增大,喷流包裹作用加强,影响范围增大.

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以激光熔凝表面强韧化处理为背景,应用空间弹塑性有限单元和高精度数值算法同时考虑材料组织性能的变化模拟工件的温度场,主要研究激光熔凝加工中瞬时温度场数值模拟,同时考虑相变潜热的影响,为第二步热应力场及残余应力的数值模拟做准备.用算例验证了模型的正确性,最后给出了激光熔凝加工不同时刻温度场分布.

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借助于Level Set函数,建立了气-液两相的统一控制方程组,并在交错网格中进行离散.用两种格式,即Superbee-TVD格式和5阶WENO格式求解Level Set函数的输运方程,用SIMPLER算法的思想对主流场控制方程的求解方法进行改进.数值实验结果表明,在求解Level Set的控制方程时,5阶WENO方法比Superbee-TVD格式的结果更准确;用改进的数值算法可成功实现对密度比大于1 000/1的气-液两相流界面迁移问题的数值模拟.对几种典型大密度比气-液两相流问题的计算结果与实际问题的物理规律完全一致,验证了该方法的有效性和可靠性.

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The convective instabilities in two or more superposed layers heated from below were studied extensively by many scientists due to several interfacial phenomena in nature and crystal growth application. Most works of them were performed mainly on the instability behaviors induced only by buoyancy force, especially on the oscillatory behavior at onset of convection (see Gershuni et. Al.(1982), Renardy et. Al. (1985,2000), Rasenat et. Al. (1989), and Colinet et. Al.(1994)) . But the unstable situations of multi-layer liquid convection will become more complicated and interesting while considering at the same time the buoyancy effect combined with thermocapillary effect. This is the case in the gravity reduced field or thin liquid layer where the thermocapillary effect is as important as buoyancy effect. The objective of this study was to investigate theoretically the interaction between Rayleigh-Bénard instability and pure Marangoni instability in a two-layer system, and more attention focus on the oscillatory instability both at the onset of convection and with increasing supercriticality. Oscillatory behavious of Rayleigh-Marangoni-Bénard convective instability (R-M-B instability) and flow patterns are presented in the two-layer system of Silicon Oil (10cSt) over Fluorinert (FC70) for a larger various range of two-layer depth ratios (Hr=Hupper/Hdown) from 0.2 to 5.0. Both linear instability analysis and 2D numerical simulation (A=L/H=10) show that the instability of the system depends strongly on the depth ratio of two-layer liquids. The oscillatory instability regime at the onset of R-M-B convection are found theoretically in different regions of layer thickness ratio for different two-layer depth H=12,6,4,3mm. The neutral stability curve of the system displaces to right while we consider the Marangoni effect at the interface in comparison with the Rayleigh-Bénard instability of the system without the Marangoni effect (Ma=0). The numerical results show different regimes of the developing of convection in the two-layer system for different thickness ratios and some differences at the onset of pure Marangoni convection and the onset of Rayleigh-Bénard convections in two-layer liquids. Both traveling wave and standing wave were detected in the oscillatory instability regime due to the competition between Rayleigh-Bénard instability and Marangoni effect. The mechanism of the standing wave formation in the system is presented numerically in this paper. The oscillating standing wave results in the competition of the intermediate Marangoni cell and the Rayleigh convective rolls. In the two-layer system of 47v2 silicone oil over water, a transition form the steady instability to the oscillatory instability of the Rayleigh-Marangoni-Bénard Convection was found numerically above the onset of convection for ε=0.9 and Hr=0.5. We propose that this oscillatory mechanism is possible to explain the experimental observation of Degen et. Al.(1998). Experimental work in comparison with our theoretical findings on the two-layer Rayleigh-Marangoni-Bénard convection with thinner depth for H<6mm will be carried out in the near future, and more attention will be paid to new oscillatory instability regimes possible in the influence of thermocapillary effects on the competition of two-layer liquids