Experimental evidence of near-wall reverse flow events in a zero pressure gradient turbulent boundary layer

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 τ = 1000 and Re τ = 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

Backflow events under the effect of secondary flow of Prandtl's first kind

“…These studies confirmed the indications in earlier works [19][20][21] regarding the presence of backflow events in wall-bounded turbulence, despite the fact that they had not been detected in certain measurement campaigns [22,23]. The more recent experimental study by Willert et al [24], who performed particle image velocimetry (PIV) measurements in zero-pressure-gradient (ZPG) turbulent boundary layers (TBLs) at Re τ = 1000 and 2700, confirmed the findings by Lenaers et al [17] and Yao, Huang, and Xu [18]. Brücker [25] used micropillar sensors to measure the topology of the wall-shear stress vector in a ZPG TBL at Re τ 940, and identified a probability of finding backflow events of around 0.05%, aligned with the results by Lenaers et al [17] at approximately the same Re.…”

Energy transfer structures associated with large-scale motions in a turbulent boundary layer

“…Figure 1 shows the wall-normal profiles of both the mean streamwise velocityŪ + (y + ) and the streamwise velocity fluctuating intensity σ + u (y + ), which were measured by a 1-D laser Doppler anemometry (LDA) (Dantec Flow Explorer, Dantec, Denmark) with a probing diameter of 1.0 mm. Existing results of σ + u (y + ) obtained by Sillero et al (2013) (via DNS) and Osaka, Kameda & Mochizuki (1998), Hultmark et al (2013) and Willert et al (2018) (via experimental measurement) at similar Re τ are superimposed in figure 1(b) for a comparison. Owing to the insufficient resolution of the LDA, the inner peak is slightly underestimated, but σ + u (y + ) in the buffer region and above agrees with those of previous studies within the measurement uncertainty.…”

“…Dashed lines in panel (a) represent the linear law and the logarithmic law, respectively. Dashed lines, dotted lines and dash-dotted lines in panel (b) represent the experimental results of Osaka et al (1998), Hultmark et al (2013) and Willert et al (2018), respectively. and E3.…”

Extreme wall shear stress events in turbulent pipe flows: spatial characteristics of coherent motions

“…Although intuition and results from early experiments might dictate the non-existence of backflow (BF) events in wall-bounded flows (Eckelmann 1974), recent experimental (Brücker 2015;Willert et al 2018) and DNS investigations have shown evidence of regions with negative streamwise velocity within the viscous sublayer (Jalalabadi & Sung 2018). Other studies have even found reverse flow events at the buffer region (Lenaers et al 2012;Cardesa et al 2019).…”