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中国物理学会期刊

基于聚类连通法的热化学非平衡湍流边界层拟序结构分析

Coherent Structures in Thermochemical Non-equilibrium Turbulent Boundary Layers Using Clustering Method

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  • 当飞行器在稠密大气高速飞行时,其表面边界层强可压缩湍流与高温非平衡效应耦合,气动环境十分复杂。拟序结构作为反映湍流边界层动力学过程的关键特征,其运动学和动力学特征对壁面摩阻热流的产生具有重要影响。为揭示热化学非平衡效应下湍流边界层的拟序结构特征,并检验Townsend附着涡模型等经典壁湍流理论的适用性,本文采用聚类连通方法,识别并提取了热化学非平衡湍流边界层中的五类拟序结构:强流向速度脉动结构、强平动温度脉动结构、强振动温度脉动结构、雷诺切应力结构以及湍流热通量结构。条件平均结果与结构空间分布表明,这五类结构均具有几何自相似性,符合Townsend附着涡模型的尺度关系。结合壁面摩阻热流的定量分解分析表明,在提取到的各类拟序结构中,速度壁附着结构与雷诺切应力结构对壁面摩阻热流的贡献最为突出。

     

    High-enthalpy turbulent boundary layers are influenced by the coupling between strong compressibility and thermochemical non-equilibrium, which complicates momentum and heat transport. Coherent structures provide a useful description of the organised motions responsible for these transport processes. To characterise coherent structures in a thermochemical non-equilibrium turbulent boundary layer and assess their contributions to wall transport, a clustering method is used to identify and extract spatially connected regions of intense fluctuations. Five classes of structures are considered: structures of intense streamwise velocity fluctuations, transla-tional-rotational temperature fluctuations and vibrational temperature fluc-tuations, together with Reynolds shear stress and turbulent heat flux struc-tures. Their geometrical scales, spatial organisation, conditional averages and wall-attached behaviour are then examined. In addition, quantitative decompositions of wall skin friction and heat flux are used to assess the relative contributions of different structures to near-wall momentum and energy transport. All five classes exhibit multiscale spatial organisation, with their stream-wise and spanwise dimensions increasing approximately linearly with structure height. After normalisation by their respective characteristic di-mensions, the conditionally averaged shapes at different heights show a reasonable collapse, indicating a degree of geometric self-similarity across the velocity fluctuation, temperature fluctuation, Reynolds shear stress and turbulent heat flux structures. These scaling behaviours are consistent with the basic assumptions of Townsend’s attached-eddy model, suggesting that the model retains some applicability to the thermochemical non-equilibrium turbulent boundary layer considered here, despite the influ-ence of high-temperature non-equilibrium effects on the temperature field and on the coupling between internal energy modes. The wall-transport decompositions further show that the contributions to skin friction and heat flux differ substantially among the identified structures. Wall-attached streamwise-velocity structures and Reynolds shear stress structures are most closely associated with regions of strong near-wall momentum and energy transport and make the largest contributions to both quantities. No-tably, the events that dominate turbulence intensities in the logarithmic re-gion are not identical to those that dominate wall skin-friction and heat-flux transport. The former are governed primarily by Q2 ejection events, whereas the latter receive their dominant contribution from Q4 sweep events. This distinction indicates that local turbulence intensity alone is not a direct measure of a coherent structure’s contribution to wall transport, and that the dynamical coupling between the structures and the near-wall region plays an important role in the production of wall skin friction and heat flux. The present analysis characterises the geometry, ejection-sweep behaviour and wall-transport contributions of several classes of coherent structures in a thermochemical non-equilibrium turbulent boundary layer, and provides further insight into the organisation of high-enthalpy wall turbulence and the processes governing wall skin friction and heat transfer.

     

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