On the large- and small-scale motions in a separated, turbulent-boundary-layer flow

ABSTRACT

Adverse-pressure-gradient turbulent boundary layer flow was inspected at Reynolds number based on momentum thickness, $R{e}_{\theta }\approx 1200$, 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 $90\mathrm{\%}$ of the Reynolds shear stress, $\overline{uv}$, is due to large-scale motions containing $65\mathrm{\%}$ of the turbulence kinetic energy at the tested Reynolds number. The decomposed velocity field revealed that only the first $\sim 10\mathrm{\%}$ of the modes is sufficient to recover $65\mathrm{\%}$ of the turbulence kinetic energy. In this partition, the large-scale motion contribution to the streamwise component of the Reynolds normal stress, $\overline{u^2}$, is about $70\mathrm{\%}$ 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, $\overline{v^2}$, 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.

KEYWORDS:

• Large-scale motions
• turbulent flow
• separation
• POD
• PIV

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Suranga Dharmarathne http://orcid.org/0000-0003-4172-493X

Ali M. Hamed http://orcid.org/0000-0002-1373-2409

Burak Aksak http://orcid.org/0000-0002-1295-6515

Leonardo P. Chamorro http://orcid.org/0000-0002-5199-424X

Murat Tutkun http://orcid.org/0000-0003-0702-5051

Ali Doosttalab http://orcid.org/0000-0002-5571-2641

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.