Effect of Very-Large-Scale Motions on One- and Two-Point Statistics in Turbulent Pipe Flow Investigated by Direct Numerical Simulations

Kurzfassung

Very long coherent regions of energetic streamwise velocity fluctuations, so-called very-large-scale motions (VLSM) play an important role at high Reynolds number wall-bounded turbulence since they carry an substantial fraction of turbulent energy. The footprint of VLSM is visible in the three-dimensional velocity correlation of the streamwise velocity for Re_\tau=1500. Although the maximum energy content of VLSM is located in the outer flow region, they penetrate deep into the buffer layer. In the very vicinity of the wall large local wall-normal velocity fluctuations (velocity spikes), which are rare both and space and time, result in a high wall-normal flatness value. The interaction of the large- and very-large-scale outer flow motions with the near-wall coherent structures --- as velocity spikes --- and their impact on high-order turbulence statistics was recently discussed by Bauer et al (2017). For direct numerical simulations the challenging aspect is to resolve the full range of turbulent scales including VLSM and velocity spikes at Reynolds numbers high enough, and within computational domains large enough, for VLSM to settle. We carried out pipe flow simulations in a domain of length L=42R for Re_\tau = 1500, where Re_\tau=u_\tau R / \nu is the friction Reynolds number based on the friction velocity u_\tau, the pipe radius R and the kinematic viscosity \nu. At the conference the influence of VLSM on turbulent statistics will be presented. Coherent motions will be shown by means of three-dimensional two-point velocity correlations. Furthermore, Reynolds number depended scaling laws for statistical moments of the velocity distribution will be discussed and related to the underlying coherent motions.