Non-equilibrium turbulent boundary layers in high reynolds number flow at incompressible conditions: effects of streamline curvature and three dimensionality

The physics and computational prediction of turbulent boundary layer flow over axisymmetric and three-dimensional bodies are examined. Three cases were considered for which extensive experimental results and companion Reynolds-averaged Navier Stokes (RANS) solutions were obtained and/or available in the open literature. These cases all have Reynolds numbers based upon the freestream velocity and body geometric scale on the order of 105 to 106, which is large for laboratory scales but small compared to the maximum scales observed for full-scale vehicles. Despite significant differences in approach flow fields and geometries for these three cases, some common themes emerged in the findings. All cases involved complications due to pressure gradients combined with streamwise curvature, and all exhibited regions of turbulence reduction due to accelerated flow. These complications led to discrepancies in computed results even in attached flow regions where it is often assumed that RANS models provide reliable predictions. The authors recommend further work on modelling approaches that can capture rapid distortion effects on turbulence transport that can be incorporated into industry-useful frameworks. Two cases involving laterally symmetric, three-dimensional wall-mounted hills with aft-body separation revealed that asymmetric mean flow fields are likely to result. This finding has been observed in experiments conducted in multiple facilities and in computations using multiple solvers and turbulence models. It is concluded that non-unique and asymmetric global flow solutions are fundamental to flow cases with lateral geometric symmetry involving turbulent boundary layer separation. Further work is also needed to accurately predict low-frequency unsteadiness due to geometries that produce non-unique mean flow fields. For such flows, it remains to be definitively determined whether experimentally observed modes of the mean flow are equivalent, or nearly equivalent, to asymmetric mean flow solutions obtained using RANS approaches.

本研究针对轴对称及三维物体表面的湍流边界层流动的物理特性与计算预测方法展开分析。选取了三个研究案例，相关的完整实验结果及配套的雷诺平均纳维-斯托克斯（Reynolds-averaged Navier Stokes, RANS）求解结果均已通过公开文献发布或可公开获取。上述三个案例的雷诺数均基于来流速度与物体几何特征尺度定义，数值介于10^5至10^6量级——该尺度在实验室环境中属于较大规模，但相较于实装运载工具的最大特征尺度仍偏小。尽管三个案例的来流流场与几何构型存在显著差异，但其研究结果仍存在若干共性：所有案例均存在压力梯度与流向曲率耦合引发的流动复杂性，且均出现了因流动加速导致的湍流衰减区域。即使在通常认为RANS模型可提供可靠预测的附面层附着流动区域，上述流动复杂性仍导致了计算结果与实验结果之间的偏差。作者建议进一步开展相关建模方法研究，以精准捕捉湍流输运过程中的快速畸变效应，进而将其集成至工业可用的数值模拟框架中。针对两个带有横向对称三维贴壁山丘且存在后体分离的案例，研究发现其可能产生非对称的平均流场。该结论已在多类实验装置与多套求解器、湍流模型的数值计算中得到验证。综上，对于涉及湍流边界层分离且具备横向几何对称性的流动案例，非唯一且非对称的全局流动解是其本质特征。此外，仍需开展进一步研究以精准预测由非唯一平均流场几何构型引发的低频非定常流动特性。对于此类流动，实验观测到的平均流模态与采用RANS方法得到的非对称平均流解是否等价或近似等价，仍有待通过严谨实验予以确定。