Numerical analysis of friction drag above moving surfaces

本项目的目的是在非定常入流条件下，通过向展向传播的表面波影响湍流边界层，从而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切应力以保持附流。对于第一个项目阶段，由流入的雷诺数、压力梯度、运动表面的波形和表面微观结构定义的研究参数空间被流入中的时间相关扰动所扩展。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变化，将在本研究项目中进行分析。表面波参数将由一个模型预测控制器控制，该控制器将由子项目3开发。在数值方法中实现后，对表面波参数进行调整，使减小摩擦阻力或减小分离倾向的目标不受流场非定常扰动的影响而保持不变。对于控制器的开发，将首先通过高分辨率大涡模拟确定无压力梯度的加速和减速湍流边界层中的主要流体力学机制。关于分离倾向的操纵的后续研究将对具有外部规定的正压力梯度的边界层进行。与摩擦减阻的研究类似，将进行分析，以了解是否发生在零压力梯度流动中的机制，也决定了减速流动中壁面剪切应力的增加。本文将对光滑表面和结构表面进行有压力梯度和无压力梯度的流场计算，以评估表面结构对动壁面上方壁面剪应力减小的影响。该结果将由子项目1的实验调查补充。

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.