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Mechanics of Fluids

Mechanics of Fluids

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The diagram illustrates an orifice in one side <strong>of</strong> an open reservoir containing<br />

a liquid. The reservoir and the free surface are so large in comparison<br />

with the orifice that the velocity <strong>of</strong> the fluid at the free surface is negligibly<br />

small. The liquid issues from the orifice as a free jet, that is, a jet unimpeded<br />

by other liquid, and therefore under the influence <strong>of</strong> gravity.<br />

Fluid approaching the orifice converges towards it. Because an instantaneous<br />

change <strong>of</strong> direction is impossible, the streamlines continue to converge<br />

beyond the orifice until they become parallel at the section cc. Parallel flow<br />

is attained at only a short distance (about half the diameter if the orifice is<br />

circular) from the orifice. The jet may diverge again beyond section cc, so<br />

this is then the section <strong>of</strong> minimum area. It is termed the vena contracta<br />

(Latin: contracted vein).<br />

At low velocities some curvature <strong>of</strong> the streamlines results from the downward<br />

deflection <strong>of</strong> the jet by gravity and the vena contracta may be ill defined.<br />

The curvature, however, is generally negligible close to the orifice. When a<br />

jet <strong>of</strong> liquid is discharged vertically downwards, gravity causes further acceleration<br />

<strong>of</strong> the liquid and so, by the principle <strong>of</strong> continuity ϱAu = constant,<br />

a further reduction <strong>of</strong> the cross-sectional area; the vena contracta is then<br />

defined as the section at which marked contraction from the orifice ceases.<br />

Since the streamlines are parallel and, we assume, sensibly straight at<br />

the vena contracta, the pressure in the jet there is uniform. (A non-uniform<br />

pressure over the section would cause accelerations perpendicular to the axis,<br />

and thus curved or non-parallel streamlines.) The pressure in the jet at the<br />

vena contracta therefore equals that <strong>of</strong> the fluid – usually the atmosphere –<br />

surrounding the jet, any small difference due to surface tension being ignored.<br />

The vena contracta is the only section <strong>of</strong> the jet at which the pressure is<br />

completely known.<br />

If the flow is steady and frictional effects are negligible Bernoulli’s equation<br />

may be applied between two points on a particular streamline. Taking a<br />

horizontal plane through the centre <strong>of</strong> the orifice as the datum level and<br />

considering the points (1) and (2) in Fig. 3.15, we have<br />

p1<br />

ϱg + u2 1<br />

2g + z1 = patm<br />

ϱg + u2 2<br />

+ 0<br />

2g<br />

We suppose that the reservoir is sufficiently large and the point (1) sufficiently<br />

far from the orifice for the velocity u1 to be negligible. Subject to this proviso<br />

Simple applications <strong>of</strong> Bernoulli’s equation 113<br />

Fig. 3.15

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