Chapt06

61 if the flow is laminar there may be occasional

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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. EXAMPLE 6.1 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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