Browsing by Author "Römer, Tristan M."

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Open 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.
Open Access
Experimental investigation on boundary-layer streaks induced by grid-generated free-stream turbulence in a water channel
(2024) Römer, Tristan M.; Kloker, Markus J.; Rist, Ulrich; Wenzel, Christoph
In this study, the influence of various turbulence-grid configurations is analysed on both the induced free-stream turbulence (FST) and the resulting Klebanoff modes/streaks developing in a laminar flat-plate boundary layer downstream in a laminar water channel. All results are based on hot-film and particle image velocimetry measurements as well as visualizations. The grid design and installation has been done according to common grid installation recommendations to ensure homogeneous FST causing meandering Klebanoff modes inside the boundary layer. But it was found that (i) the Klebanoff modes do not meander for all grid configurations, (ii) not all configurations cause Klebanoff modes with the expected temporal and spatial behaviour, and (iii) for some configurations, the spanwise streak spacing is strictly locked to the grid spacing (mesh width). As these observations are unreported in the literature, this study is aimed at a thorough description of the influence of key grid parameters on the FST and the resulting streaks within the boundary layer. The investigation includes the grid parameters typically reported, such as the grid-bar diameters, the associated Reynolds numbers, or the streamwise placement of the grid, but now also the grid-orientation order (horizontal/vertical or vertical/horizontal order of grid bars of the dual-plane grid), the wall-normal position of the horizontal bars relative to the leading edge of the flat plate, and the existence of palpable imperfections in the manufactured grids. The Reynolds-number range covered lies well in the lower band of wind-tunnel experiments. Thus, this study suggests that the reliability and reproducibility of future experimental studies on FST would be greatly improved if they demonstrated homogeneity in the free-stream in both spanwise and wall-normal directions, documented the ongoing meandering and wavelengths of the generated Klebanoff modes and thus (implicitly) documented the spanwise independence of the results in the temporal mean. The latter is a prerequisite for the reliable investigation of FST/isolated-roughness interactions.