If you stand on a bridge and watch the current flow past pylons below, you’ll see disturbances marking the wakes. Dragging a rod – or an oar – at a high enough speed through the water creates something similar: a wavy cavity in the fluid surface that surfs along behind the rod. The faster you pull the rod, the harder you’ll have to work, until that wake becomes so turbulent that it begins entraining air bubbles, like the tiny ones seen above. Once entrainment starts, the drag coefficient drops somewhat, presumably due to changes in the pressure distribution around the rod. The characteristics of air entrainment change with object size as well. Larger rods can entrain air through the cavity and not just in the wake. (Image and research credit: V. Ageorges et al.)
Entraining Bubbles
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One response to “Entraining Bubbles”
What about the barrier surface or i mean the surface of the rod, or maybe because the rod is wet, then the surface of that wetness, as it enters the water again, does the entire surface that was in contact with air briefly, drag some, maybe minor, or molecular even, drag air in? Im guessing it does, the body of water being stationary and the surface of the rod moving at some velocity, does not give sufficient time for air molecules to be displaced?
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