ABSTRACT Temporally developing direct numerical simulations (T-DNS) are performed for free-stream turbulence induced bypass transition flow over a flat-plate under zero-pressure gradient, and results are validated using experimental data and spatially developing DNS (S-DNS) results. The temporal simulations predict the growth of near-wall Klebanoff modes in the pre-transition regime and their subsequent breakdown due to the sinuous-like instability, where the increase in free-stream turbulence intensity and length scale enhanced the instability. A formulation for the domain translation velocity is developed using the momentum integral boundary layer equation, to estimate the translation of the domain along the plate. The formulation was found to be robust for a range of free-stream turbulence intensities, and both the boundary layer growth and free-stream decay compared well with spatial simulations, confirming that the same translation velocity can be used for the entire domain. The optimal domain size for a temporal simulation is dictated by the length scale of the turbulent spots and errors due to streamwise periodicity, and is estimated to be to , where is the boundary layer thickness at the transition onset. The T-DNS prediction of the integral boundary layer parameters, mean flow, second and higher-order statistics, stress budgets, and free-stream decay were comparable to those of S-DNS, even though they were obtained on ten times smaller streamwise domain and numerical grid. However, T-DNS are limited for bypass transtion flows for which leading-edge effects are unimportant, and may have limitations for the accurate predictions of the near-wall Klebanoff modes that extend up to breakdown. Nonetheless, temporal approach is identified to be a viable inexpensive alternative to the spatial approach for canonical test cases for transition flow physics analysis.

中文翻译：

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旁路过渡流时空直接数值模拟的比较

摘要 对零压力梯度下平板上自由流湍流引起的旁路过渡流进行了临时开发的直接数值模拟 (T-DNS)，并使用实验数据和空间开发的 DNS (S-DNS) 结果验证了结果. 时间模拟预测了过渡前状态下近壁 Klebanoff 模式的增长及其随后由于类似蜿蜒的不稳定性而导致的崩溃，其中自由流湍流强度和长度尺度的增加增强了不稳定性。使用动量积分边界层方程开发域平移速度的公式，以估计域沿板的平移。发现该公式对于一系列自由流湍流强度是稳健的，边界层增长和自由流衰减与空间模拟相比都很好，证实了相同的平移速度可用于整个域。时间模拟的最佳域大小由湍流点的长度尺度和由于流向周期性引起的错误决定，估计为 ，其中 是过渡开始时的边界层厚度。T-DNS 对整体边界层参数、平均流量、二阶和更高阶统计量、应力预算和自由流衰减的预测与 S-DNS 的预测相当，即使它们是在小十倍的流域上获得的和数字网格。然而，T-DNS 受限于前沿效应不重要的旁路转换流，并且可能对延伸至击穿的近壁 Klebanoff 模式的准确预测存在限制。尽管如此，对于过渡流物理分析的规范测试用例，时间方法被认为是一种可行的廉价替代方法。