06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access DNS of a turbulent boundary layer using inflow conditions derived from 4D-PTV data(2021) Appelbaum, Jason; Ohno, Duncan; Rist, Ulrich; Wenzel, ChristophUnsteady, 3D particle tracking velocimetry (PTV) data are applied as an inlet boundary condition in a direct numerical simulation (DNS). The considered flow case is a zero pressure gradient (ZPG) turbulent boundary layer (TBL) flow over a flat plate. The study investigates the agreement between the experimentally measured flow field and its simulated counterpart with a hybrid 3D inlet region. The DNS field inherits a diminishing contribution from the experimental field within the 3D inlet region, after which it is free to spatially evolve. Since the measurement does not necessarily provide a spectrally complete description of the turbulent field, the spectral recovery of the flow field is analyzed as the TBL evolves. The study summarizes the pre-processing methodology used to bring the experimental data into a form usable by the DNS as well as the numerical method used for simulation. Spectral and mean flow analysis of the DNS results show that turbulent structures with a characteristic length on the order of one average tracer particle nearest neighbor radius r¯NN or greater are well reproduced and stay correlated to the experimental field downstream of the hybrid inlet. For turbulent scales smaller than r¯NN, where experimental data are sparse, a relatively quick redevelopment of previously unresolved turbulent energy is seen. The results of the study indicate applicability of the approach to future DNS studies in which specific upstream or far field boundary conditions (BCs) are required and may provide the utility of decreasing high initialization costs associated with conventional inlet BCs.Item Open Access Homogenization of streaks in a laminar boundary layer(2020) Puckert, Dominik K.; Wu, Yongxiang; Rist, UlrichThe 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.Item Open Access Investigation of Lagrangian Areas of Minimal Stretching (LAMS) in a turbulent boundary layer(2023) Rist, Ulrich; Weinschenk, Matthias; Wenzel, ChristophItem 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, ChristophIn 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.Item 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, UlrichDelaying 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.