Microbubble drag reduction in liquid turbulent boundary layers

Journal Article · Tue Dec 01 04:00:00 EST 1992 · Applied Mechanics Reviews

DOI:https://doi.org/10.1115/1.3119751· OSTI ID:433509

Merkle, C L; Deutsch, S ^[1]

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1. Pennsylvania State Univ., University Park, PA (United States)

The interactions between a dense cloud of small bubbles and a liquid turbulent boundary layer are reviewed on the basis of available experimental observations to understand and quantify their capability for reducing skin friction. Gas bubbles are generally introduced into the boundary layer by injection through a porous surface or by electrolysis. After injection, the bubbles stay near the wall in boundary-layer-like fashion giving rise to strong gradients in both velocity and gas concentration. In general, the magnitude of the skin friction reduction increases as the volume of bubbles in the boundary layer is increased until a maximum skin friction reduction of typically 80-90% of the undisturbed skin friction level is reached. The volumetric gas flow required for this maximum is nominally equal to the volume flow of the liquid in the boundary layer. Bubble size estimates indicate that in most microbubble experiments the bubbles have been intermediate in size between the inner and outer scales of the undisturbed boundary layer. Additional studies with other nondimensional bubble sizes would be useful. However, the bubble size is most likely controlled by the injection process, and considerably different conditions would be required to change this ratio appreciably. The trajectories of the bubble clouds are primarily determined by the random effects of turbulence and bubble-bubble interactions. The effects of buoyancy represent a weaker effect. The trajectories are unlike the deterministic trajectory of an individual bubble in a time-averaged boundary layer. Bubbles are most effective in high speed boundary layers and, for the bubble sizes tested to date, produce an effect that persists for some on hundred boundary layer thicknesses. Modeling suggests that microbubbles reduce skin friction by increasing the turbulence Reynolds number in the buffer layer in a manner similar to polymers.

• Sponsoring Organization: USDOE
• OSTI ID: 433509
• Journal Information: Applied Mechanics Reviews, Journal Name: Applied Mechanics Reviews Journal Issue: 12 Vol. 45; ISSN 0003-6900; ISSN AMREAD
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
66 PHYSICS
BOUNDARY LAYERS
BUBBLES
DRAG
FRICTION
REYNOLDS NUMBER
TURBULENT FLOW