The observation of large-scale coherent structures in turbulent boundary layers has sparked great experimental and
numerical interest. While most studies have focussed on channel flows and zero pressure gradient (ZPG) boundary layer
flows, our understanding of wall turbulence under the influence of a streamwise pressure gradient is still quite limited
due to the lack of sufficiently high Reynolds number data and large facilities to reach an equilibrium state [1], where
theoretical scaling laws can be relevant. The length of these structures (7-14δ [2]) requires a large field of view and a
high spatial resolution to measure all relevant spatial scales.

To resolve and characterize the structures in an adverse pressure gradient (APG) boundary layer, a set-up in the LML
boundary layer wind tunnel was built using 16 sCMOS cameras (a consortium involving 4 teams under the framework
of EuHIT) in order to perform large scale turbulent boundary layer measurements with appropriate spatial resolution
[3]. The length of the 2D2C PIV measurement domain was 3.5m long and 0.25m high to ensure the
possibility of capturing very large-scale structures with lengths more than 10δ. A total of 30000 images were recorded
for two free-stream velocities of 5m/s and 9 m/s (corresponding to Reθ≈10600 and Reθ≈17700 at station 4 respectively)
[4]. The topology and dynamics of the large-scale turbulent structures under an APG will be presented. As
a similar experiment with a large field of view (1.16m long and 0.3m high) was earlier conducted on a ZPG turbulent
boundary layer at LML, the influence of an APG on these structures will be shown.

References
[1] L. Castillo and W. K. George. Similarity analysis for turbulent boundary layer with pressure gradient: outer flow. AIAA journal,
39.1:41-47, 2001
[2] N. Hutchins and I. Marusic. Evidence of very long meandering features in the logarithmic region of turbulent boundary layers. J
Fluid Mech, 579:1-28, 2007
[3] Cuvier et al. Under preparation.
[4] Database of the large field APG experiments: https://turbase.cineca.it/turbase/default/#/view_dataset/25