Investigations of the origin of turbulent superstructures in a flat-plate boundary layer with pressure gradient

这一贡献是致力于平板边界层中的湍流的数值研究没有和有流向压力梯度使用长积分区域流向。为了避免湍流超结构发展可能存在的偏差，在积分区域内，湍流应通过层流-湍流转变而产生，这一方法遵循了施莱特等人的成功方法。(2009)。这有一个独特的优点，那就是不符合完整的纳维-斯托克斯方程的人为干扰的量被保持在最小。模拟的目的是达到尽可能大的雷诺数，超过Re_Theta=5000，其中Theta是动量厚度。在逆流向压力梯度的影响下，湍流产生量的最大值与雷诺数的增大类似，向壁面移动。因此，应利用压力梯度来“加速”流向的流动发展，以便更容易观察到大型上层建筑的形成。通过选择合适的压力梯度，可以将靠近壁区和远离壁区的不同机理分开，分别研究和联合研究。须检查和测试用于探测和追踪湍流上层建筑的适当设施。因此，可以将检测到的结构与线性稳定性理论的特征函数和雷诺平均局部流动或过滤流场的线性化N-S方程Re_Theta=Const的结果进行比较。这种比较旨在调查湍流超结构是否可以用局部不稳定性来解释、模拟和预测。根据这一假设，湍流上层建筑可能进一步在上游产生，然后由水流向下游对流，应使用特征跟踪的数据进行评估。目前的直接数值模拟补充了优先方案1881“湍流上层建筑”内的实验研究。它们为检验假说提供了额外的数据和理解，尽管它们限制了比大多数实验中更小的雷诺数。这使我们能够更深入地了解优先方案过程中可能出现的现有假设和新假设的流程和理论研究。一旦方案的结构最终确定，就有可能与所有从事边界层工作的伙伴进行合作，并打算尽快开展合作。在准备提案期间已经进行了材料交换。工作方案中描述了已经确定的合作。

This contribution is devoted to the numerical study of turbulence in flat-plate boundary layers without and with streamwise pressure gradients using long integration domains in streamwise flow direction. To avoid a possible bias of the development of turbulent superstructures turbulence shall be generated via laminar-turbulent transition within the integration domain, following the successful approach of Schlatter et al. (2009). This has the distinctive advantage that the amount of artificial disturbances which do not comply with the full Navier-Stokes equations is kept to a minimum. The simulations are intended to reach as large as possible Reynolds numbers beyond Re_Theta = 5000, where Theta is the momentum thickness. Under the influence of an adverse streamwise pressure gradient the maximum of turbulence production moves away from the wall similar to the increase of the Reynolds number. Therefore, the pressure gradient shall be used to "speed-up" the flow development in streamwise direction such that formation of large-scale superstructures can be observed easier. Different mechanisms in close-to-the-wall and far-from-the-wall areas can be separated from each other by a suitable choice of pressure gradient, studied separately and conjointly. Appropriate means for the detection and tracking of turbulent superstructures shall be inspected and tested. Thus, detected structures can then be compared with eigenfunctions of linear stability theory and results of linearized Navier-Stokes equations for Re_Theta=const which are obtained for the Reynolds-averaged local flow or filtered flow fields. This comparison is intended to investigate whether turbulent superstructures can be explained, modeled and predicted by local instability. The hypothesis, according to which turbulent superstructures might be generated further upstream and then convected downstream by the flow, shall be evaluated using data from feature tracking. The present direct numerical simulations complement experimental investigations within the priority programme 1881 "Turbulent Superstructures". They yield additional data and understanding for examination of hypotheses, despite their limitations to smaller Reynolds numbers than in most experiments. This allows additional insight into the flow and theoretical studies of existing as well as new hypotheses that may appear in the course of the priority programme. Cooperation with all partners who work on boundary layers is possible and intended as soon as the structure of the programme is finalized. An exchange of material during preparation of the proposals has already occurred. Already identified cooperations are described in the work programme.