Abstract We use direct numerical simulations (DNS) to investigate the turbulent modulation due to the presence of bubbles in vertical channels flowing downward. The Reynolds number for single-phase flow based on half channel height h * and friction velocity is Re τ = 180. A density and viscosity ratio of ρ d * / ρ c * = 0.01 and μ d * / μ c * = 0.018 is chosen for two void fractions of ϵ = 1.2 % and ϵ = 2.4 % . For each void fraction three different bubble sizes are simulated: D / h = 0.2130, 0.2684 and 0.3382, where D denotes the diameter of the bubbles. Numerical simulations are based on multiple markers Coupled Level-Set/Volume-of-Fluid (CLSVOF) method. To improve the efficiency of this method, a fast pressure-correction method is used in order to enable the simulation to exploit a constant coefficient Poisson equation which can be solved with FFT-based technique. Extensive verification and validation were performed and perfect accuracy and agreement are obtained. In all the simulations performed in this work, the new Poisson solver showed a minimum speedup of 22 times. Accumulation of bubbles in the core region of the channel for all cases is observed, which forms a bubble-free layer in the near-wall region. The presence of bubbles resulted in considerable modification in the mean velocity profile compared to single-phase flow. Another common observation is that all the components of velocity fluctuations in the near-wall region decrease with increasing void fraction and decreasing wall layer thickness. The opposite happens in the core region, where the presence of bubbles favours turbulence. With respect to the bubble size, the wall-normal and spanwise velocity fluctuations decrease in the near-wall region for smaller bubbles, however, the streamwise velocity fluctuations remained almost unaffected. The investigation of turbulent kinetic budgets shows that, unlike single-phase flow, the dissipation terms rises to large values in the core region of the channel. This behaviour is referred to the presence of bubbles and hence enhancement of turbulent kinetic energy in the core region.

中文翻译：

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使用有效的 CLSVOF 方法对湍流气泡向下流动进行直接数值模拟

摘要 我们使用直接数值模拟 (DNS) 来研究由于向下流动的垂直通道中存在气泡而导致的湍流调制。基于半通道高度 h * 和摩擦速度的单相流雷诺数为 Re τ = 180。 ρ d * / ρ c * = 0.01 和 μ d * / μ c * = 0.018 的密度和粘度比为选择两个空隙率 ϵ = 1.2 % 和 ϵ = 2.4 % 。对于每个空隙率，模拟了三种不同的气泡尺寸：D / h = 0.2130、0.2684 和 0.3382，其中 D 表示气泡的直径。数值模拟基于多标记耦合水平集/流体体积 (CLSVOF) 方法。为了提高这种方法的效率，使用快速压力校正方法以使模拟能够利用可以使用基于 FFT 的技术求解的常系数泊松方程。进行了广泛的验证和确认，并获得了完美的准确性和一致性。在这项工作中执行的所有模拟中，新的泊松求解器显示了 22 倍的最小加速。在所有情况下，都观察到通道核心区域中气泡的积累，这在近壁区域形成了无气泡层。与单相流相比，气泡的存在导致平均速度分布发生了相当大的变化。另一个常见的观察结果是近壁区域中速度波动的所有分量随着空隙率的增加和壁层厚度的减小而减小。相反的情况发生在核心区域，其中气泡的存在有利于湍流。对于气泡尺寸，对于较小的气泡，壁面法向和展向速度波动在近壁区域减小，但是，流向速度波动几乎不受影响。湍流动力学预算的研究表明，与单相流不同，耗散项在通道的核心区域上升到很大的值。这种行为是指气泡的存在，从而增强了核心区域的湍流动能。湍流动力学预算的研究表明，与单相流不同，耗散项在通道的核心区域上升到很大的值。这种行为是指气泡的存在，从而增强了核心区域的湍流动能。湍流动力学预算的研究表明，与单相流不同，耗散项在通道的核心区域上升到很大的值。这种行为是指气泡的存在，从而增强了核心区域的湍流动能。