Bhagat Apurva, Gijare Harshal, Dongari Nishanth
Department of Mechanical and Aerospace Engineering, Indian Institute of Technology, Hyderabad, Kandi, Medak 502285, India.
Micromachines (Basel). 2019 Feb 12;10(2):118. doi: 10.3390/mi10020118.
The effect of the Knudsen layer in the thermal micro-scale gas flows has been investigated. The effective mean free path model has been implemented in the open source computational fluid dynamics (CFD) code, to extend its applicability up to slip and early transition flow regime. The conventional Navier-Stokes constitutive relations and the first-order non-equilibrium boundary conditions are modified based on the effective mean free path, which depends on the distance from the solid surface. The predictive capability of the standard Maxwell velocity slip-Smoluchwoski temperature jump' and hybrid boundary conditions Langmuir Maxwell velocity slip-Langmuir Smoluchwoski temperature jump' in conjunction with the Knudsen layer formulation has been evaluated in the present work. Simulations are carried out over a nano-/micro-scale backward facing step geometry in which flow experiences adverse pressure gradient, separation and re-attachment. Results are validated against the direct simulation Monte Carlo (DSMC) data, and have shown significant improvement over the existing CFD solvers. Non-equilibrium effects on the velocity and temperature of gas on the surface of the backward facing step channel are studied by varying the flow Knudsen number, inlet flow temperature, and wall temperature. Results show that the modified solver with hybrid Langmuir based boundary conditions gives the best predictions when the Knudsen layer is incorporated, and the standard Maxwell-Smoluchowski can accurately capture momentum and the thermal Knudsen layer when the temperature of the wall is higher than the fluid flow.
研究了克努森层在热微尺度气体流动中的作用。在开源计算流体动力学（CFD）代码中实现了有效平均自由程模型，以将其适用性扩展到滑移和早期过渡流态。基于有效平均自由程对传统的纳维-斯托克斯本构关系和一阶非平衡边界条件进行了修正，有效平均自由程取决于距固体表面的距离。在本工作中，评估了标准的“麦克斯韦速度滑移-斯莫卢霍夫斯基温度跳跃”和混合边界条件“朗缪尔麦克斯韦速度滑移-朗缪尔斯莫卢霍夫斯基温度跳跃”结合克努森层公式的预测能力。在纳米/微尺度后向台阶几何结构上进行了模拟，其中流动经历逆压梯度、分离和重新附着。结果与直接模拟蒙特卡罗（DSMC）数据进行了验证，并且相对于现有的CFD求解器有了显著改进。通过改变流动克努森数、入口流动温度和壁面温度，研究了非平衡对后向台阶通道表面气体速度和温度的影响。结果表明，当纳入克努森层时，基于混合朗缪尔边界条件的修正求解器给出了最佳预测，并且当壁面温度高于流体流动温度时，标准的麦克斯韦-斯莫卢霍夫斯基模型可以准确捕捉动量和热克努森层。
