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Title: Evolution of quasistreamwise vortex tubes and wall streaks in a bubble-laden turbulent boundary layer over a flat plate

Description: The video shows the temporal evolution of the vortical structures and wall streaks in a spatially developing turbulent boundary layer over a flat plate for both the single-phase and bubble-laden cases computed from the direct numerical simulation (DNS) data of Ferrante and Elghobashi [1] and Ferrante [2]. In the animation the x-axis denotes the streamwise direction, and the gray surface with the black grid denotes the flat plate. The video consists of four parts. The first three parts show the evolution in time and space of the three-dimensional vortical structures, identified via the λ2 method of Jeong and Hussain [3]. The last part of the video shows the evolution of the wall streaks, i.e., the instantaneous fluid streamwise velocity contours in the xy-plane parallel to the flat plate, located at z+=0.58.

The first part of the video shows the three-dimensional top and front views of the λ2 (= −4) isosurface for the single-phase flow (cyan surfaces). The vortical structures appear as tubes elongated in a quasistreamwise direction, which sometimes resembles asymmetric horseshoes or hairpins.

In the second part of the video, the top and front views of the vortical structures (beige surfaces) are shown for the bubble-laden case (bubble diameter in wall units db+=2.4) at an average volume fraction φv=0.02. The gravitational acceleration is perpendicular to and oriented towards the plate ("plate-on-bottom"). As for the single-phase flow, the vortical structures appear as quasistreamwise tubes. However, in the bubble-laden case, the structures are slightly less frequent than those for the single-phase case, as shown in the third part of the video, where the side views of the tubes are shown simultaneously for both the bubble-laden (beige surfaces) and single-phase (cyan surfaces) cases.

In the bubble-laden flow, the generated positive velocity divergence [1,2] induces a wall-normal fluid velocity that displaces the vortical structures away from the wall. The displacement of the vortical structures increases the spanwise gaps between the high-speed streaks (area in red) which are associated with the sweep events, and to reduce the streamwise velocity in these streaks (the probability of occurrence of green and blue areas becomes larger, whereas that of red areas becomes smaller) in comparison to that of the single-phase flow as shown in the last part of the video. Consequently, the skin friction is reduced by up to 14% for plate-on-bottom (case shown in this video) and 20% for plate-on-top (not shown) for φv=0.02.

Credits: A. Ferrante, S. Elghobashi, P. Adams, M. Valenciano, and D. Longmire

References: [1] Ferrante, A. and S. Elghobashi, "On the physical mechanisms of drag reduction in a spatially developing turbulent boundary layer laden with microbubbles," J. Fluid Mech. 503, 345 (2004).
[2] Ferrante, A. "Reduction of skin-friction in a microbubble-laden spatially developing turbulent boundary layer over a flat plate," Ph.D. thesis, University of California, Irvine, 2004.
[3] Jeong, J. and F. Hussain, "On the identification of a vortex," J. Fluid Mech. 285, 69 (1995).

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Contributed By: Tausif Billah

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