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Boundary layer stabilization using free-stream vortices

Published online by Cambridge University Press:  30 December 2014

L. Siconolfi ,
S. Camarri  and
J. H. M. Fransson
L. Siconolfi
Affiliation:
Dipartimento di Ingegneria Civile ed Industriale, Università di Pisa, I-561 26 Pisa, Italy
S. Camarri
Affiliation:
Dipartimento di Ingegneria Civile ed Industriale, Università di Pisa, I-561 26 Pisa, Italy
J. H. M. Fransson*
Affiliation:
Linné Flow Centre, KTH-Royal Institute of Technology, SE-100 44 Stockholm, Sweden
*
†Email address for correspondence: jens.fransson@mech.kth.se
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Abstract

In this numerical investigation we explore the possibility of applying free-stream vortices as a passive flow control method for delaying the transition to turbulence. The work is motivated by previous experimental studies demonstrating that stable streamwise boundary layer (BL) streaks can attenuate both two- and three-dimensional disturbances inside the BL, leading to transition delay, with the implication of reducing skin-friction drag. To date, successful control has been obtained using physical BL modulators mounted on the surface in order to generate stable streaks. However, surface mounted BL modulators are doomed to failure when the BL is subject to free-stream turbulence (FST), since a destructive interaction between the two is inevitable. In order to tackle free-stream disturbances, such as FST, a smooth surface is desired, which has motivated us to seek new methods to induce streamwise streaks inside the BL. A first step, in a systematic order, is taken in the present paper to prove the control idea of generating free-stream vortices for the attenuation of ordinary Tollmien–Schlichting waves inside the BL. In this proof-of-concept study we show that, by applying a spanwise array of counter-rotating free-stream vortices, inducing streamwise BL streaks further downstream, it is possible to alter the BL stability characteristics to such a degree that transition delay may be accomplished. For the demonstration we use direct numerical simulations along with stability analysis.

JFM classification

Boundary Layers: Boundary layer control Flow Control: Drag reduction Instability: Transition to turbulence
Type
Rapids
Information
Journal of Fluid Mechanics , Volume 764 , 10 February 2015 , R2
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Copyright
© 2014 Cambridge University Press 

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