Journal of Fluid Mechanics

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Contributions of the wall boundary layer to the formation of the counter-rotating vortex pair in transverse jets

FABRICE SCHLEGELa1 c1, DAEHYUN WEEa2, YOUSSEF M. MARZOUKa3 and AHMED F. GHONIEMa1

a1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

a2 Department of Environmental Science and Engineering, Ewha Womans University, Seoul 120-750, Republic of Korea

a3 Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract

Using high-resolution 3-D vortex simulations, this study seeks a mechanistic understanding of vorticity dynamics in transverse jets at a finite Reynolds number. A full no-slip boundary condition, rigorously formulated in terms of vorticity generation along the channel wall, captures unsteady interactions between the wall boundary layer and the jet – in particular, the separation of the wall boundary layer and its transport into the interior. For comparison, we also implement a reduced boundary condition that suppresses the separation of the wall boundary layer away from the jet nozzle. By contrasting results obtained with these two boundary conditions, we characterize near-field vortical structures formed as the wall boundary layer separates on the backside of the jet. Using various Eulerian and Lagrangian diagnostics, it is demonstrated that several near-wall vortical structures are formed as the wall boundary layer separates. The counter-rotating vortex pair, manifested by the presence of vortices aligned with the jet trajectory, is initiated closer to the jet exit. Moreover tornado-like wall-normal vortices originate from the separation of spanwise vorticity in the wall boundary layer at the side of the jet and from the entrainment of streamwise wall vortices in the recirculation zone on the lee side. These tornado-like vortices are absent in the case where separation is suppressed. Tornado-like vortices merge with counter-rotating vorticity originating in the jet shear layer, significantly increasing wall-normal circulation and causing deeper jet penetration into the crossflow stream.

(Received November 29 2010)

(Revised November 29 2010)

(Accepted January 28 2011)

(Online publication April 08 2011)

Key words:

  • jets;
  • vortex flows

Correspondence:

c1 Email address for correspondence: schlegel@mit.edu

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