Numerical analysis of friction drag above moving surfaces

这个子项目的目标是在非定常流入条件下，通过沿展向传播的表面波来影响湍流边界层，这样一方面可以通过流动控制来减少摩擦阻力，另一方面可以增加壁面剪应力以保持附着流。对于第一个工程阶段，研究的参数空间由入流的雷诺数、压力梯度、运动表面的波形和表面微结构通过入流的随时间变化的扰动来扩展。在这个研究项目中，将分析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。面波参数将由子项目3开发的模型预测控制器来控制。在应用到数值方法中后，它将调整面波参数，以使减小摩阻或减小分离倾向的目标与流场的非定常扰动无关。对于控制器的开发，首先将通过高分辨率大涡模拟来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。随后将对具有外部规定的正压力梯度的边界层进行关于操纵分离倾向的调查。与摩擦减阻的研究类似，还将分析在零压力梯度流动中发生的机理是否也决定了减速流动中壁面剪应力的增加。分别对光滑表面和结构化表面进行了有压力梯度和无压力梯度的流场计算，以评价表面结构对降低动壁上方壁面剪应力的影响。这一结果将得到分项目1的实验调查的补充。

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.