Effect of wall heating on turbulent boundary layers with temperature-dependent viscosity
Effect of wall heating on turbulent boundary layers with temperature-dependent viscosity
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DOI：
10.1017/jfm.2013.211
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发表时间：
2013-07-01
影响因子：
3.7
通讯作者：
Zaki, Tamer A.
中科院分区：
文献类型：
--
作者：
Lee, Jin;Jung, Seo Yoon;Zaki, Tamer A.
对等温加热壁面上的湍流边界层进行了直接数值模拟，研究了粘度分层对湍流统计和表面摩擦的影响。采用了水的温度黏度经验关系式。根据自由流温度（30℃），选择了两种壁温（70℃和99℃）。在加热流动中，湍流能量在缓冲层内减小，而在壁面附近增大。缓冲层湍流动能的降低伴随着较小的雷诺剪切应力水平，因此湍流产生较弱。壁面附近湍流能量的增强是由于粘滞分层引起的附加扩散项增强了能量传递。尽管壁面附近的流体粘度较低，但由于近壁面精细运动的增强，耗散也增加了。墙体加热可使表面摩擦系数降低26%。对表面摩擦的不同贡献的评估表明，阻力的减少主要是由于边界层上雷诺数剪切应力的变化。四象限和八象限分析表明，抛射（Q2）和扫掠（Q4）显著减少，这一结果进一步得到了对抛射事件和展向涡的线性随机估计的外部涡结构的检查的支持。
Direct numerical simulations (DNS) of turbulent boundary layers over isothermally heated walls were performed, and the effect of viscosity stratification on the turbulence statistics and skin friction were investigated. An empirical relation for temperature-dependent viscosity for water was adopted. Based on the free-stream temperature (30 degrees C), two wall temperatures (70 degrees C and 99 degrees C) were selected. In the heated flows, the turbulence energy diminishes in the buffer layer, but increases near the wall. The reduction in turbulence kinetic energy in the buffer layer is accompanied by smaller levels of Reynolds shear stresses and, hence, weaker turbulence production. The enhanced turbulence energy near the wall is attributed to enhanced transfer of energy via additional diffusion-like terms due to the viscosity stratification. Despite the lower fluid viscosity near the wall, dissipation is also increased owing to the augmented near-wall fine-scale motion. Wall heating results in reduction in the skin-friction coefficient by up to 26 %. An evaluation of the different contributions to the skin friction demonstrates that drag reduction is primarily due to the changes in the Reynolds shear stresses across the boundary layer. Quadrant and octant analyses showed that ejections (Q2) and sweeps (Q4) are significantly reduced, a result further supported by an examination of outer vortical structures from linear stochastic estimation of the ejection events and spanwise vortices.