Numerical analysis of friction drag above moving surfaces

该子项目的目的是在非定常来流条件下通过展向行进表面波影响湍流边界层，以便通过流动控制，一方面可以减少摩擦阻力，另一方面可以增加壁面剪应力以保持附着流。相对于第一个项目阶段的参数空间的调查所定义的雷诺数的流入，压力梯度，移动表面和表面微观结构的波形扩展的时间相关的扰动流入。非定常扰动如何改变或产生流动结构，从而产生壁面剪应力的修正，这一问题将在本研究项目中进行分析。表面波参数将由子项目3开发的模型预测控制器控制。在实施到数值方法中之后，它将调整表面波参数，使得减小摩擦阻力或减小分离趋势的目标将独立于流场的非定常扰动而保持。对于控制器的发展，主要的流体力学机制将首先确定在加速和减速湍流边界层没有压力梯度的高分辨率大涡模拟。对于具有外部规定的正压梯度的边界层，将进行关于操纵分离趋势的后续研究。与摩擦减阻的研究类似，将进行分析以了解零压力梯度流中发生的机制是否也决定减速流中壁面剪应力的增加。将对光滑和结构化表面进行有和无压力梯度的流场计算，以评估表面结构对移动壁上方壁面剪应力降低的影响。这些结果将得到分项目1的实验研究的补充。

The objective of this subproject is to influence turbulent boundary layers by spanwise travelling surface waves at unsteady inflow conditions such that via flow control on the one hand, the friction drag can be reduced and on the other hand, the wall shear stress can be increased to retain an attached flow. With respect to the first project phase the parameter space of the investigations defined by the Reynolds number of the inflow, the pressure gradient, the wave form of the moving surface and the surface microstructure is extended by time dependent perturbations in the inflow. The question how the unsteady perturbations change or generate flow structures, which produce a modification of the wall shear stress, will be analyzed in this research project. The surface wave parameters will be controlled by a model predictive controller, which will be developed by subproject 3. After the implementation into the numerical method it will tune the surface wave parameters such that the objective to reduce the friction drag or to diminish the separation tendency will be maintained independently of the unsteady perturbation of the flow field. For the development of the controller, the main fluid mechanical mechanisms will be first determined in accelerated and decelerated turbulent boundary layers without pressure gradients by highly resolved large-eddy simulations. The subsequent investigations with respect to the manipulation of the separation tendency will be performed for boundary layers with an externally prescribed positive pressure gradient. Similar to the investigation of the friction drag reduction, an analysis will be conducted to understand whether the mechanisms which occur in zero pressure gradient flow, also determine the increase of the wall shear stress in decelerated flows. The computations of the flow field with and without pressure gradient will be done for smooth and structured surfaces to assess the influence of the surface structure on the reduction of the wall shear stress above the moving wall. The results will be complemented by the experimental investigations of subproject 1.