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On the physical mechanisms of drag reduction in a spatially developing turbulent boundary layer laden with microbubbles. (English) Zbl 1116.76368
Summary: The objective of this paper is to explain, in as much detail as possible, the physical mechanisms responsible for the reduction of skin friction in a microbubble-laden spatially developing turbulent boundary layer over a flat plate, for \(Re_{\theta} = 1430\). Our DNS results with microbubble volume fraction ranging from \(\phi_v = 0.001\) to 0.02 show that the presence of bubbles results in a local positive divergence of the fluid velocity, \({\pmb\nabla} \,{\mathbf \cdot}\,{\mathbf U} > 0\), creating a positive mean velocity normal to (and away from) the wall which, in turn, reduces the mean streamwise velocity and displaces the quasi-streamwise longitudinal vortical structures away from the wall. This displacement has two main effects: (i) it increases the spanwise gaps between the wall streaks associated with the sweep events and reduces the streamwise velocity in these streaks, thus reducing the skin friction by up to 20.2% for \(\phi_v = 0.02\); and (ii) it moves the location of peak Reynolds stress production away from the wall to a zone of a smaller transverse gradient of the mean streamwise velocity (i.e. smaller mean shear), thus reducing the production rate of turbulence kinetic energy and enstrophy.

MSC:
76F40 Turbulent boundary layers
76T10 Liquid-gas two-phase flows, bubbly flows
76F65 Direct numerical and large eddy simulation of turbulence
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