Page 51 - Fluid mechanics, heat transfer, and mass transfer

FLUID FLOW
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Drag coefficients for spheres, disks, and cylinders.
FIGURE 2.7

liquid metals are driven through a magnetic field in & Vortex shedding is caused when air flows past a blunt
order to generate electricity. structure. The air flow past the object creates alter-
& The study of magnetohydrodynamics is also moti- nating low-pressure vortices on the downwind side of
vated by its widespread application to the description the object. The object will tend to move toward the
of space (within the solar system) and astrophysical low-pressure zone.
plasmas (beyond the solar system). & Eventually, if the frequency of vortex shedding
. What is creeping flow? Under what circumstances it matches the resonant frequency of the structure, the
occurs? Give examples of systems in which creeping structure will begin to resonate and the movement of
flow is likely to be present. the structure can become self-sustaining.
& Fluid flow at very small values of Reynolds number, & As Reynolds number (for flow over spherical bodies)
that is, at very low velocities relative to a solid object, increases above 20, vortices form in the wake of the
in general a very viscous flow, is called creeping flow sphere.
or Stokes flow. & For Reynolds numbers in the range of 100–200,
& Basic assumption for creeping flow: The inertia instabilities in flow break up vortices. This phenom-
terms are negligible in the momentum equation if enon is called vortex shedding.
Re 1. . What are the undesirable effects of vortex shedding?
& Flow over immersed bodies, usually small particles. & It causes severe vibration and mechanical failure of
& Creeping flow can occur in lubrication systems in- cylindrical elements such as heat exchanger tubes,
volving narrow and variable passages, for example, columns, stacks, transmission lines, suspended pip-
flow through porous media such as ground water ing, and so on.
movement. . What is Von K arm an vortex street? Explain.
. Above what Reynolds numbers, for flow past a sphere, & AVon K arm an vortex street is a repeating pattern of
flow separation takes place? swirling vortices caused by the unsteady separation
& N Re > 20. of flow over bluff bodies. Avortex street will only be
observed over a given range of Reynolds numbers,
. What is vortex shedding?
typically above a limiting N Re value of about 90. The
& Vortex shedding is an unsteady flow that takes place
range of N Re values will vary with the size and shape
at special flow velocities (according to the size and
of the body from which the eddies are being shed,
shape of the cylindrical body). In this flow, vortices
as well as with the kinematic viscosity of the fluid.
are created at the back of the body and periodically
The governing parameter, the Reynolds number, is
from both sides of the body.