稀疏粗颗粒致水平湍流边界层拟序结构变动的细观机理研究

Turbulence modifications have always been the front and hot issues in the field of multiphase flow research. Several recent experiments in horizontal channel revealed that, even for extremely low mass loading, the presence of particles can also result in a significant turbulence modulation. In order to interpret this unusual phenomenon, the present research will focus on the microscopic mechanism of the interactions between heavy coarse particles and turbulent coherent structures in a horizontal boundary layer. In this project, a direct-forcing immersed boundary method is used to perform fully resolved direct numerical simulations of dilute gas-particle turbulent flow in a horizontal channel. Firstly, the heavy coarse particle behaviour in the near-wall boundary layer will be studied in detail by tracking the particle trajectories and examining the particle distribution. Secondly, the characteristics of particle wake dynamics, as well as the turbulence modifications near the particles, will be investigated at different heights away from the bottom wall, including the inner, buffer, logarithmic and outer regions. Thirdly, the quantitative relationship of "particle parameters - modifications of coherent structures" will be analyzed and established, and then the microcosmic mechanism of coherent structures modification due to coarse particles will be also deeply explored. Finally, as a supplement and validation of the numerical simulation, a very-high-resolution PIV system will be used to measure the gas-phase velocity field near the particle surface. The overall goal of this research is to clarify the physical mechanism of turbulence modification in boundary layer of a dilute gas-particle turbulent flow, and to provide a scientific basis for engineering applications and the two-phase turbulence control.

湍流变动一直都是多相流领域的研究前沿和热点问题。针对近期一系列实验中观察到的稀疏颗粒致壁面湍流的反常变动现象，本项目拟从湍流拟序结构的角度，围绕重质粗颗粒与湍流拟序结构作用的细观机理，对其物理实质进行阐述。本项目拟采用基于有限差分的浸没边界法对低载荷气固两相水平槽道湍流开展全解析直接数值模拟，考察水平湍流边界层中重质粗颗粒的运动特性；详细检查距离壁面不同高度上的颗粒尾迹/尾涡特性及其对附近湍流结构的影响；结合准流向涡特征结构探索重质粗颗粒与湍流拟序结构的微观作用过程；分析不同参数颗粒导致的湍流拟序结构变动规律。在模拟基础上，辅以颗粒附近局部流场的高分辨率PIV测量作为模拟结果的验证和补充。本项目可望在细观层次上增进人们对重质粗颗粒与湍流边界层拟序结构相互作用过程的认识和把握，对于丰富两相流和湍流理论具有重要意义。

两相湍流变动的机理一直是多相流领域的研究热点和难点。一般而言，只有当离散相的体积分数大于10-6时，才会对湍流属性造成显著的改变；然而，近期的一系列实验却发现，对于壁面湍流，即使离散颗粒的体积分数小于10-6，仍能引起明显的湍流变动。针对这一反常现象，本项目从湍流拟序结构的角度，围绕重质粗颗粒与湍流拟序结构作用的细观机理，对其物理实质进行阐述。研究从简单颗粒沉降入手，解析了颗粒尾迹/尾涡的微观结构特性，获取了尾迹/尾涡对临近颗粒的作用过程；在此基础上，对颗粒群的尾迹/尾涡特性开展了整体宏观统计和局部微观结构的研究，阐述了颗粒群局部团聚行为的物理机制；之后，改建了水平槽道循环风洞系统，采用两相PIV/PTV同时测量方法，从宏观统计的角度，获取了湍流边界层中重质粗颗粒的浓度和速度分布特性以及颗粒对壁面附近湍流猝发行为的影响；最后，以直接数值模拟为基础，结合颗粒尾迹/尾涡的微观结构和颗粒的运动特性，对由颗粒导致的近壁面湍流拟序结构变动的物理过程进行了解析，获得了低载荷下壁面湍流变动的微观机理。