Browsing by Author "Wu, Yongxiang"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • Thumbnail Image
    ItemOpen Access
    Delay of laminar-turbulent transition by counter-rotating cylindrical roughness elements in a laminar flat plate boundary layer
    (2023) Römer, Tristan M.; Schulz, Kai A.; Wu, Yongxiang; Wenzel, Christoph; Rist, Ulrich
    Delaying laminar-turbulent transition in boundary layers is of great interest since the skin-friction coefficient can be reduced by up to one order of magnitude. In this experimental research, it is shown that counter-rotating cylindrical roughness elements are able to delay transition under realistic flow conditions. Evidence is given by the intermittency, evaluated from hot-film measurements in a laminar water channel. An increase in rotation speed results in a delay of transition of up to 6.5%in the center of the plate. This trend can be explained by the streaks amplified by the rotating cylinders, resulting in a damping of the fluctuation amplitude in the boundary layer. The advantage of this method is that the transition delay can be actively controlled with conventional cylindrical roughness elements.
  • Thumbnail Image
    ItemOpen Access
    Homogenization of streaks in a laminar boundary layer
    (2020) Puckert, Dominik K.; Wu, Yongxiang; Rist, Ulrich
    The present work, based on experimental, numerical and theoretical investigations, introduces a method to homogenize streaks in the laminar boundary layer. The streaks are created by a spanwise array of roughness elements on the surface of a flat plate. A homogenization body in the form of a horizontal bar is added at a downstream location away from the roughness array to homogenize the velocity differences of the streaks in the laminar boundary layer. Measurements are done with hot-film anemometry and supported by numerical simulations and linear stability theory. The streak amplitude can be significantly reduced with the proposed homogenization body. Furthermore, the reduction in spanwise gradients of the mean velocity leads to a significant reduction in the sinuous instability of the streaky flow. The effects of the homogenization body on the displacement thickness and the observation of flow unsteadiness downstream of the homogenization body are discussed. The present work thus proposes and explores a passive technique to control undesired streaks in the laminar boundary layer.