Experimental characterization of spatial and temporal dynamics of near-wall structures in a turbulent boundary layer flow

Abstract

The present Masterthesis is focused on the study of wall-bounded turbulent flows, which is of great interest in aerodynamics. The near-wall flow dynamics and the respective wall-shear stress are the most significant contributors to skin-friction drag and an understanding of flow characteristics close to the wall helps in the development of better drag reduction techniques. Direct Numerical Simulations (DNS) studies of near-wall flow behaviour in turbulent boundary layers are available only for low and moderate Reynolds numbers. Therefore, experimental investigation of high Reynolds number turbulent boundary layer flows is necessary to study flow features and coherent structures close to the wall. Measurement and analysis of acceleration and wall-shear stress components in wall-bounded turbulent flows are also open research areas due to the complexity in obtaining these variables through conventional experimental methods. To this end, the advanced Lagrangian Particle Tracking (LPT) technique of Shake-the-Box (STB) [1] has been implemented to study a high Reynolds number zero-pressure-gradient turbulent boundary layer (ZPG-TBL) flow. In conjunction with STB, the data assimilation method FlowFit [2] has been applied to the STB particle track data to obtain the time-resolved 3D velocity gradient tensor and the pressure fields. Several measurement runs of three Reynolds numbers Reτ = 995, 1352, 1762 were recorded using five high-speed cameras at the 1 m wind tunnel test facility at DLR G¨ottingen. A challenging aspect of this evaluation is the accurate estimation of the wall position, for which a new method of estimation and correction based on the flow properties of near-wall particles has been proposed. An in-depth statistical analysis of the obtained experimental STB data has been carried out to study spatial and temporal characteristics of the near-wall flow at Reτ = 1352. The one-point and two point statistics of the velocity, acceleration and wall-shear stress components have been calculated using a spatial binning approach. The 3D two-point correlations of the acceleration components show affiliation to near-wall streamwise vorticity. Space-time correlations are computed at two different wall-normal distances and the convection velocities are calculated for the various components. Reverse flow events occurring in the viscous sublayer are identified and visualized using FlowFit to provide an understanding of their spatial and temporal development.