Experimental Evidence of Near-Wall Reverse Flow Events in a Zero Pressure Gradient Turbulent Boundary Layer

TL;DR

This paper provides experimental evidence of rare near-wall reverse flow events in a zero pressure gradient turbulent boundary layer, confirming their size, frequency, and characteristics predicted by numerical simulations.

Contribution

First experimental observation of near-wall reverse flow events in a zero pressure gradient turbulent boundary layer at high Reynolds numbers.

Findings

Reverse flow events occur with a probability of 0.01% to 0.1%.

Reverse flow bubble size is about 30-50 wall units long and 5 wall units high.

Experimental results agree with direct numerical simulation predictions.

Abstract

This study reports on experimentally observed near-wall reverse flow events in a fully developed flat plate boundary layer at zero pressure gradient with Reynolds numbers between $Re_\tau = 1000$ and $Re_\tau = 2700$. The reverse flow events are captured using high magnification particle image velocimetry sequences with record lengths varying from 50,000 to 126,000 samples. Time resolved particle image sequences allow singular reverse flow events to be followed over several time steps whereas long records of nearly statistically independent samples provide a variety of single snapshots at a higher spatial resolution. The probability of occurrence lies in the range of 0.01% to 0.1% which matches predictions made with direct numerical simulations (DNS). The self-similar size of the reverse flow bubble is about 30-50 wall units in length and 5 wall units in height which also agrees well to DNS data provided by Lenaers et al. (ETC13, Journal of Physics: Conference Series 318 (2011) 022013).