Investigations of coherent structures in near-wall turbulence and large wall-shear stress events using Shake-The-Box

Abstract

A large portion of aerodynamic drag is produced by turbulent flow structures in the near-wall region which have significant influence on the amplitude and direction of wall-shear stress events acting on the surface. Furthermore, the impact of (very) large-scale structures present in the logarithmic region of a turbulent boundary layer onto the near-wall region has been high-lighted in recent years e.g. by Mathis et al. (2013) and Schlatter and Örlu (2011). Local back-flow close to the wall and thus negative wall-shear stress is a rare phenomenon in zero pressure gradient (ZPG)- turbulent boundary layers (TBL) and is caused by spanwise vorticity [Lenaers et al (2012)], but only limited experimental data are available (e.g. with micropillars [Bruecker 2015]) which cannot directly support investigations on the related role of coherent structures. Thus it is of great importance to enhance the understanding of the physics of near-wall turbulence, especially for the dynamics of extreme wall-shear stress events and related sweep-streak interactions, both from a fundamental and an engineering point of view. Due to strong wall-normal velocity gradients in the immediate wall region of a TBL a Tomo PIV [Elsinga et al. 2006][Scarano 2013] application would fail in delivering the desired quantities in 3D due to the low-pass filtering effect of correlation windows [Raffel et al 2007]. Therefore, the novel Lagrangian particle tracking method Shake-The-Box (STB) [Schanz et al. 2013a, Schanz et al. 2014] has been applied to a ZPG-TBL in order to gain time-resolved and volumetric velocity measurements including both components of the wall-shear stresses.