Page 44 - Fluid mechanics, heat transfer, and mass transfer

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FLUID FLOW

2.1 Flow Phenomena 21 & Recirculating eddies are formed during flow through
2.2 Water Hammer 30 weirs or where the main flow runs up against a wall or
2.3 Compressible Fluids 32 obstruction and part of the flow moves back up-
stream. The flow is often erratic.
. What is turbulence? Explain.
& Turbulence is irregular and seemingly random
(chaotic) movement of flowing fluid elements in all
directions, the net flow being unidirectional. Turbu-
2.1 FLOW PHENOMENA
lence manifests itself at high flow velocities at
Reynolds numbers above 10,000.
. How is Reynolds number useful in interpreting flow
& The origin of turbulence is rooted in the instability of
conditions?
& Reynolds number serves as a criterion to shear flows. It is also derived from buoyancy-driven
describe laminar, transitional, and turbulent flow flows.
conditions. & Turbulence is rotational and three-dimensional
➢ N Re > 4000, flow will be turbulent with inertial motion.
forces dominating. & Large-scale turbulent motion is roughly independent
➢ N Re < 2100, flow will be laminar in nature with of viscosity. In other words, at high Reynolds num-
viscous forces dominating the flow. bers, viscous forces, which contribute to flow stabil-
ity, are insignificant compared to inertial forces that
. What are eddies? Where do you find these?
contribute to flow instability.
& An eddy is the swirling of a fluid and the reverse
& Turbulence is associated with high levels of vorticity
current created when the fluid flows past an obstacle.
fluctuation. Smaller scales are generated by the
The moving fluid creates a space devoid of down-
vortex stretching mechanism.
stream flowing fluid on the downstream side of the
& Turbulence is highly dissipative and requires a source
object. Fluid behind the obstacle flows into the void
creating a swirl of fluid on each edge of the obstacle, of energy to maintain it.
followed by a short reverse flow of fluid behind & Rapid mixing involved in turbulence increases mo-
the obstacle flowing upstream, toward the back of mentum, heat, and mass transfer processes.
the obstacle. & Turbulence can be generated by contact between two
& Eddies are areas of still or reverse (upstream) moving layers of fluid moving at different velocities or by a
fluid. They are formed, for example, in rivers when flowing stream in contact with a solid boundary.
water comes up against an obstacle (most commonly & Turbulence can arise when a jet of fluid from an
rocks) and has to pass around it, creating a void in the orifice flows into a mass of fluid.
flow directly downstream. They are also formed in & In turbulent flow at a given place and time, large
any flowing fluid under turbulent flow conditions in a eddies continually form and break up into smaller
conduit such as a pipe. eddies and finally disappear. Eddies can be as small

Fluid Mechanics, Heat Transfer, and Mass Transfer Chemical Engineering Practice, By K. S. N. Raju
Copyright Ó 2011 John Wiley & Sons, Inc.

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