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Unformatted text preview: may be occasional
natural disturbances which damp out quickly (Fig. 6.1a). If transition is occurring, there
will be sharp bursts of turbulent fluctuation (Fig. 6.1b) as the increasing Reynolds number causes a breakdown or instability of laminar motion. At sufficiently large Re, the
flow will fluctuate continually (Fig. 6.1c) and is termed fully turbulent. The fluctuations, typically ranging from 1 to 20 percent of the average velocity, are not strictly
periodic but are random and encompass a continuous range, or spectrum, of frequencies. In a typical wind-tunnel flow at high Re, the turbulent frequency ranges from 1
to 10,000 Hz, and the wavelength ranges from about 0.01 to 400 cm.
The accepted transition Reynolds number for flow in a circular pipe is Red,crit 2300. For flow
through a 5-cm-diameter pipe, at what velocity will this occur at 20°C for (a) airflow and (b) water flow? Solution
Almost all pipe-flow formulas are based on the average velocity V Q/A, not centerline or any
other point velocity. Thus transition is specified at Vd...
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- Spring '08