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# Article

## Negative streamwise velocities and other rare events near the wall in turbulent flows

Authors: Lenaers, P., Li, Q., Brethouwer, G.B., Schlatter, P., Örlü, R. Conference Published J. Phys.: Conf. Ser. 318   022013 2011

### Abstract

Negative streamwise velocities, extreme wall-normal velocites and high flatness values for the wall-normal fluctuations near the wall are investigated for turbulent channel flow simulations at a series of Reynolds numbers up to $Re_\tau = 1000$ in this paper. Probability density functions of the wall-shear stress and velocity components are presented, as well as joint probability density functions of the velocity components and the pressure. Backflow occurs more often ($0.06\%$ at $Re_\tau = 1000$) and further away from the wall into the buffer layer for rising Reynolds number. An oblique vortex outside the viscous sublayer is found to cause this backflow. Extreme $v$ events occur also more often for rising Reynolds number. Positive and negative velocity spikes appear in pairs, located on the two edges of a strong streamwise vortex: the negative spike occurring in a high speed streak indicating a sweeping motion, while the positive spike is located between a high and low speed streak. These extreme $v$ events cause high flatness values near the wall ($F(v) = 43$ at $Re_\tau = 1000$).

http://dx.doi.org/10.1088/1742-6596/318/2/022013