Near-wall turbulence characterization using 4D-PTV “Shake-The-Box”

Kurzfassung

A near-wall flow characterization including measurements of instantaneous wall-shear stresses of a turbulent boundary layer (TBL) developing along a flat plate with zero pressure gradients has been performed by using two advanced particle based measurement methods. The experiments were conducted in the 1m- wind tunnel facility of Göttingen at U_∞ = 10 m/s and a Reynolds number of Re_θ = 2,770 corresponding to Re_τ = 960. First, high-speed 2C-PIV was performed at two image magnification factors at 2 kHz and 4 kHz frame rates in order to obtain the overall statistical properties of the boundary layer profile together with a large time series of instantaneous 2C velocity vector fields in a streamwise, wall-normal plane. Single pixel line correlation applied to the particle image area close to the wall provided high spatial resolution velocity data down into the viscous sublayer. In a second step, the novel 4D-PTV technique Shake-The-Box (STB) was applied to a time series of particle images acquired with a typical tomographic high-speed camera set-up at 15.873 kHz. The STB measurement domain consists of a wall-bounded volume covering a stream- and spanwise area of 430 x 430 viscous units (l^+= ν/u_τ) and a wall-normal extension of 32 viscous units. A comprehensive set of relatively dense Lagrangian track data was reconstructed from two time resolved sequences of 115,000 time steps each. The data enables an accurate and very high resolution measurement of the mean and rms-velocity profiles averaged in bins sized by a fraction of a viscous unit, of both components of the instantaneous wall shear stress (τ^+_w) and all components of the Reynolds stress tensor. Furthermore, the time-resolved 3D velocity vectors and corresponding gradient tensor have been interpolated onto a regular grid using the time series of irregularly distributed Lagrangian track data. With the present data coherent structures and their dynamics close to the wall can be investigated together with their role for various (rare) wall shear stress events. The STB method is proven capable of coping with strong velocity gradients close to walls and can also be extended to TBL flows with much higher Reynolds numbers.