ABSTRACT
Adverse-pressure-gradient turbulent boundary layer flow was inspected at Reynolds number based on momentum thickness, 
, using particle image velocimetry in a refractive-index-matching flume. Proper orthogonal decomposition was used to quantify the effect of large-scale motions on the Reynolds stresses at the onset of separation and within the separated flow. Results show that approximately of the Reynolds shear stress, , is due to large-scale motions containing of the turbulence kinetic energy at the tested Reynolds number. The decomposed velocity field revealed that only the first of the modes is sufficient to recover of the turbulence kinetic energy. In this partition, the large-scale motion contribution to the streamwise component of the Reynolds normal stress, , is about and continues to grow with flow separation. In addition, the large- and small-scale motions equally contributed to the vertical component of the Reynolds normal stress, , and the contribution of the large-scale motions increased as the flow separated. Overall, results emphasise the significant impact of the large-scale motions on the Reynolds stresses in the separated flow, which may impact flow control strategies.
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Funding
This research was funded in part by the Division of Chemical, Bioengineering, Environmental, and Transport Systems NSF-CBET-ONR:1512393: International Collaboration: The role of initial conditions on LSMs/VLSMs in turbulent boundary layers.
https://orcid.org/0000-0003-4172-493X
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