Numerical analysis of friction drag above moving surfaces

点击翻译按钮获取中文摘要

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.