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热化学非平衡效应对湍流边界层摩阻热流影响研究

THERMOCHEMICAL NONEQUILIBRIUM EFFECTS ON WALL SKIN FRICTION AND WALL HEAT FLUX IN TURBULENT BOUNDARY LAYERS

  • 摘要: 本文针对来流马赫数9.74、单位雷诺数4.9 × 106 m−1、壁温与恢复温度比0.135条件下的湍流边界层, 通过直接数值模拟方法, 研究了量热完全气体和热化学非平衡两种状态下摩阻热流的产生机制. 研究采用壁面摩阻与热流分解方法, 对比了平均壁面摩阻、热流及振动热流的贡献项. 结果表明: 两种状态下平均摩阻均由湍动能生成和黏性耗散项主导, 热化学非平衡影响甚微; 平均热流由热传导、湍流热通量、雷诺应力做功和黏性应力做功四项共同决定, 热化学非平衡效应集中体现在湍流热通量项, 既使部分湍流平动-转动温度通量转化为湍流振动温度通量, 又通过组分湍流输运显著增强其对热流的贡献; 平均壁面振动热流仅存在于非平衡工况, 由振动能源项主导. 进一步在湍动能生成与湍流热通量被积函数峰值位置, 对剪切雷诺应力和法向湍流热通量进行象限分析, 发现湍动能生成由上抛和下扫运动共同主导, 湍流热通量在近壁区由高温流体下扫主导、外区由上抛主导, 上述主导事件的拟序结构均为一对反向旋转的流向涡, 这一涡结构与温度、组分脉动场显著相关.

     

    Abstract: This study investigates the generation mechanisms of skin friction and heat flux in turbulent boundary layers at a freestream Mach number of 9.74, a unit Reynolds number of 4.9 × 106 m−1, and a wall-to-recovery temperature ratio of 0.135, under both calorically perfect gas and thermochemical non-equilibrium conditions using direct numerical simulation. The wall skin friction and heat flux decomposition method is employed to compare the contributions to the mean wall skin friction, mean heat flux, and mean vibrational heat flux. Results show that under both conditions, the mean skin friction is dominated by the turbulent kinetic energy production and viscous dissipation terms, with negligible influence from thermochemical non-equilibrium. The mean heat flux is jointly determined by four terms: heat conduction, turbulent heat flux, work done by Reynolds stress, and work done by viscous stress. The effects of thermochemical non-equilibrium are mainly manifested in the turbulent heat flux term, which not only converts part of the turbulent translational–rotational temperature flux into turbulent vibrational temperature flux, but also significantly enhances its contribution to the heat flux through turbulent transport of species. The mean wall vibrational heat flux exists only in the non-equilibrium case and is dominated by the vibrational energy source term. Furthermore, quadrant analyses of the shear Reynolds stress and wall-normal turbulent heat flux are performed at the peak locations of their respective integrands for turbulent kinetic energy production and turbulent heat flux. It is found that turbulent kinetic energy production is jointly dominated by ejection and sweep events. The turbulent heat flux is dominated by the sweep of high-temperature fluid in the near-wall region and by ejection in the outer region. The coherent structures associated with these dominant events are pairs of counter-rotating streamwise vortices, which are significantly correlated with the temperature and species fluctuation fields.

     

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