微气泡减阻的维持性机理研究

As an inevitable trend for sustainable development of the global economy, energy-saving and emission reduction is one of the key goals for our 13th Five Year Plan as well. The use of microbubbles is one of the most effective measures to achieve the goal as it may significantly reduce the frictional drag for ships. However, the injection of air bubbles will certainly change the flow characters of the boundary layer near the wall due to the significant differences of density and viscosity between air and water. In addition, the parameters such as bubble size, ambient pressure and flow condition will also affect the flow in boundary layer, resulting strong diversity and instability in the two-phase flow. All these make it difficult for people to setup their configuration with a guideline in engineering. Moreover, the weak persistence of bubbles in flow boundary layer leads to weak persistence of drag reduction directly, which critically hinders the promotion of drag reduction method with microbubbles. To solve the problem, drag reduction is studied by theoretical analysis, numerical simulation with immerse boundary method and experimental technology with optical probe, high-speed camera and PIV system. Not only the interaction between bubble and water, bubble and wall will be studied respectively, but also detailed characters of bubbles moving in the boundary layer will be taken into account. Finally, the persistence mechanism of drag reduction with microbubbles will be revealed.

节能减排是全球经济可持续发展的必由之路，也是我国“十三五”规划的建设重点之一。微气泡减阻技术可以减小船舶航行中产生的较大的摩擦阻力，是提高船舶运营的经济效益和环保效益的有效途径之一。然而，由于空气和水的密度和粘性的巨大差异，气泡的摄入必然引起壁面边界层流动特性的显著改变，气泡尺寸、环境压力、来流条件等的变化都会对流动边界层产生影响，导致两相流的发展变化展现出强烈的多向性和不稳定性，使得人们在应用气泡减阻方法时难以找到标准的参考依据。而气泡在壁面边界层中的较差维持性更是直接影响了其减阻的维持性，阻碍了该方法的推广应用。为此，本课题拟以微气泡减阻为研究对象，在理论分析的基础上，采用侵没边界法等先进两相流计算方法建立数值模型，利用近十年来高速发展的介入式激光探针、高速相机等实验设备开展实验研究，分析气泡、水和壁面的相互作用，研究微气泡在壁面流动边界层中的运动特性，探明微气泡减阻的维持性机理。

本研究首先针对S60船型建立了一个简化的绕船体流动计算模型，随后对某一段的平板边界层流动开展了二维通道流动中气泡对近壁面流动的影响进行了数值研究。研究了气泡流与壁面之间的相互作用，考虑了气泡在充分发展的流动中的运动特性。计算结果表明速度梯度是影响壁面剪应力的主要因素，气泡的存在对附近流场的局部速度梯度分布有显著影响。研究还发现，气泡与壁面的距离越小，流动相互作用越强，壁面剪应力扰动越明显。.采用粒子图像测速技术(PIV)研究了微气泡对槽道湍流边界层的影响。结果表明，微气泡的存在改变了边界层中混合流体(微气泡流体)的密度和粘度，在近壁区产生了速度滑移，平均减摩阻力为30.2%。同时发现，微气泡具有浮力和挤压两种效应，它们控制着湍流边界层中混合流体的流动行为。在物理空间边界层相对较低高度处，微气泡的存在使条纹强度降低，结构变得模糊。混合流体(含微气泡流体)的主导阶模能量与湍流总能量之比小于单相流体(不含微气泡流体)的主导阶模能量之比。混合流体的能量含量随POD模数和POD模空间正常高度的增加而衰减。通过对微气泡存在下的条纹和非条纹紊流活动的比较，发现混合流体的条纹结构紊流活动减弱，而非条纹紊流活动增强。