Numerical Investigations on the Use of Roughness Patterns for Boundary-Layer Flow Control

在目前的建议中，研究边界层流动控制的新方法是基于以特定模式附着在壁上的小鬃毛状毛发结构。本提案的主要目的是获取对技术应用最有用的墙壁模式的知识。正如已经在DFG项目的初步工作中所显示的，“对覆盖有传感器毛发的表面的网格效应的研究，用于选择性检测特征流场模式（巧合传感器）”，细毛发可能会显著影响局部流场。尽管如此，我们能够证明，单个传感器的信号不是不可接受的伪造。只有当第二个传感器被放置在另一个传感器附近时，第二个传感器信号输出的位置相关变化才会出现，这具有新的和令人兴奋的应用方面。我们之前关于壁挂有限长圆柱体单和串联结构的流动的研究结果将用于验证在Navier-Stokes方程中使用体力的新型鬃状结构建模。在对非定常流动条件进行额外验证后，数值方法将用于基于完整的Navier-Stokes方程和建模粗糙度元素的直接数值模拟。个别因素之间的距离将加以修改，使它们的个别贡献以最适当的方式加强。这个想法的一个很容易想象的实现是平行于主流方向的排列，行与行之间有一定的距离，结构的长度或高度一定，这样这些毛发结构对流动的影响与湍流边界层中已知的“细纹”相似。相关问题将在本项目中使用数值调查进行研究。然而，目前的方法并不局限于紊流中的条纹，因为在壁面上控制边界层的其他模式也存在，例如，所谓的“锯齿或疙瘩条纹”，用于引发层流-湍流过渡或延迟流动分离。为了研究这一问题，将通过数值模拟研究相关的粗糙单元模式对平板边界层中层流-湍流过渡的影响。结果将与文献中发现的临界雷诺数数据进行比较。此外，粗糙度模式对层流边界层扰动增长的影响以及对过渡位置的影响也将被量化。

In the present proposal investigations of new means for boundary-layer flow control are intended, based on small bristle-like hair structures attached to a wall in specific patterns. The main purpose of this proposal consists of obtaining knowledge about those wall patterns which are most usefully for technical applications.As already shown in preliminary work performed in DFG grant "Investigation of grid effects on surfaces covered with sensor hairs for selective detection of characteristic flow-field patterns (coincidence sensor)" thin hairs may significantly influence the local flow field. Despite this, we were able to show that the signal of an individual sensor is not inadmissibly falsified by this. Only if a second sensor is placed in the vicinity of another one, position-dependent variations of the second sensor's signal output appear which bear new and exciting application aspects. Results of our previous work on the flow around wall-mounted finite-length cylinders in single and tandem configuration will be used for validation of a new-to-implement modeling of bristle-like structures using body forces in the Navier-Stokes equations. After an additional validation for unsteady flow conditions, the numerical method will be used to perform direct numerical simulations based on the complete Navier-Stokes equations with modeled roughness elements. The distances between individual elements will be modified in such that their individual contributions intensify in the most appropriate way.An easily imaginable realization of this idea is an arrangement in rows parallel to the main flow direction with a certain distance between the rows and a certain length or height of the structures such that these hair structures exert a similar effect on the flow as already-known "riblets" in a turbulent boundary layer. Related questions will be studied in the present project using numerical investigations. However, the present approach is not restricted to riblets in turbulent flow, as other patterns for control of a boundary layer over a wall exist as well, like, e.g., so-called "zig-zag or pimple stripes" for provoking laminar-turbulent transition or delaying flow separation. For investigations of this problem, the impact of relevant roughness-element patterns on laminar-turbulent transition in a flat-plate boundary layer will be studied via numerical simulation. Results will be compared with data for critical Reynolds numbers found in literature. Additionally, the influence of roughness patterns on disturbance growth in a laminar boundary layer and hence on the transitions position will be quantified as well.