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I Fluid Bearing vs Bernoulli

  1. Apr 2, 2016 #1
    Fluid or sleeve bearings use an off center shaft location to generate a hydrodynamic wedge in direction of rotation. This converging fluid causes higher pressure and can support a load applied normal to shaft axis during rotation. This action appears to be similar to the hydrodynamic support of a water skier, high speed boats, airplanes, etc.

    But this seems to me a violation of the Bernoulli Principal which clearly demonstrates a reduction in pressure with converging flow (where the flow velocity is caused to speed up). I am under the impression that fluid inertia causes hydrodynamic bearing or water skier support and even airplane wing support, but we certainly have fluid inertia in Bernouli flow -- and it seems that both are converging flow situations.

    Could it be that in the bearing, skier, and wing we don't really have converging flow but instead flow "redirection" which due to fluid mass & inertia, the action yields pressure or force normal to the flow??

    My question is if "redirection" is present and not converging flow, can someone provide a simple explanation?? -- to me it sure sounds like converging flow --
  2. jcsd
  3. Apr 2, 2016 #2
    The behavior of bearings is dominated by viscous flow of the lubricating fluid, and inertial effects usually associated with the Bernoulli equation are totally negligible. So the Bernoulli equation does not even apply to bearings. Try estimating the pressure variations in a bearing using the Bernoulli equation. The variations you calculate will be insignificant compared to the pressure variations caused by viscous effects in a bearing.

  4. Apr 2, 2016 #3
    Thanks for the reply. I had trouble with Bernoulli effects when in college and now find myself in the same kind of trouble. I understand how a static head in a converging pipe can yield a reduced pressure at (or at least after) the restricted outlet where fluid flow out. But I don't grasp how if this flow now pases into and through an expanding duct (divergent flow & remember it is water, not air) it's pressure increases??

    My actual problem relates to a scoop similar to air scoop on an airline fuselage where it seems to be accepted theory that a "dynamic" pressure develops within the air flow as it passes into and through the scoop. Except I am interested in incompressible fluid as with water for example -- The following example is an attempt to pose my question --

    .... If we were to drag a funnel through stagnant water with wide diameter leading and narrow outlet trailing, I understand per "dynamic pressure" effects within the captured water there is a pressure increase which I believe is kinetic energy getting converted to mechanical force (actually to the work required to overcome the angular force of this pressure x effective area) -- Now if water is actually flowing through the funnel do we still have this "dynamic pressure" due to motion of the funnel through the water?? How does this "dynamic" pressure relate to what we might call "drag" (as with a flat plate being dragged through the water with flat surface perpendicular to motion) -- do they end up the same?? (considering conical angle effect involved, is the effective area the area of conical "entrance" or max area of the funnel, less outlet area)??
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