疏水表面减阻的格子Boltzmann方法数值模拟

采用格子Boltzmann方法的多松弛模型和Shan-Chen多相流模型对雷诺数为100的疏水表面方柱绕流进行了数值模拟, 分析了疏水表面接触角和来流含气率对方柱绕流流场的影响. 研究结果表明: 疏水表面接触角一定时, 来流含气率在一定范围内, 疏水表面具有减阻的能力, 超出这一范围时会出现阻力系数、升力系数升高的现象, 同时在方柱近壁面处伴随涡的形成产生了气团脱落; 当来流含气率处于适当水平时, 接触角越大, 绕流物体近壁面处含气率越稳定, 减阻效果越明显. 分析发现疏水表面减阻的关键在于保证近壁面处气层的稳定性, 此时接触角越大, 减阻效果越明显. 本文从含气率角度出发分析疏水表面的减阻现象, 为进一步探索疏水表面减阻机理提出了新的思路.

Numerical investigation on drag reduction with hydrophobic surface by lattice Boltzmann method

In recent years, hydrophobic surface has attracted much attention for its potential applications in flow drag reduction. This article focuses on the drag reduction mechanism of hydrophobic surface by the multi-relaxation-time scheme and the Shan-Chen multiphase model of lattice Boltzmann method. At first, we validate our method through the multiphase cases of wall adhesion effect and the single-phase cases of flow around a square column, showing that the results from our method are in good consistence with those in previous literature. Then, we simulate and analyze the typical problem of flow around a square column with hydrophobic surface while Reynolds number is 100, in order to investigate the influences of contact angle and gas holdup of the inlet flow on drag coefficient and lift coefficient. The simulation results show that for a given contact angle, hydrophobic surface is capable of reducing drag when gas holdup of the inlet flow is in a certain range; otherwise, drag coefficient will increase. With an appropriate gas holdup of the inlet flow, both drag coefficient and lift coefficient will decrease as the contact angle becomes larger. Finally, we compare gas holdup contours and the corresponding streamline patterns under different drag coefficients. Analyses suggest that the increases of drag coefficient and lift coefficient are related to the gas mass shedding near the square column wall where the eddy forms. Increasing the gas holdup of the inflow is properly conducible to reducing the gas mass shedding and also both drag coefficient and lift coefficient greatly if contact angle is too large. However, if the near-wall gas holdup is saturated, it will aggravate the instability of gas holdup and change the near-wall gas holdup a little, which makes drag coefficient increase slightly. When gas holdup of the inlet flow is appropriate, the near-wall gas holdup becomes steadier with a larger contact angle. Through analysis we note that for hydrophobic surface, the key factor of drag reduction is to keep the near-wall gas layer stable, with which the effect of drag reduction becomes better as the contact angle becomes larger. However, the larger the contact angle, the more sensitive to the change of gas holdup both drag coefficient and lift coefficient are, so it is not recommended to adopt the hydrophobic surface with very large contact angle. With the analysis of the gas holdup near hydrophobic surface with different contact angles, in this article we put forward a new approach to the further exploration of the drag reduction mechanism of hydrophobic surface.