Chapt10

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Unformatted text preview: ather sensitive to small effects, such as plate thickness and sidewall boundary-layer growth. Small heads (H 75 mm) and small slot widths (b 30 cm) are not recommended. See Refs. 19 and 20 for further details. The V notch, in Table 10.2c, is intrinsically interesting in that its overfall has only one length scale, H — there is no separate “width.” The discharge will thus be proportional to 3 H5/2, rather than a power of 2 . Application of Bernoulli’s equation to the triangular opening, in the spirit of Eq. (10.52), leads to the following ideal flow rate for a V notch: V notch: Qideal 8 2 15 tan 2 g1/2H5/2 (10.59) where is the total included angle of the notch. The actual measured flow is about 40 percent less than this, due to contraction similar to a thin-plate orifice. In terms of an experimental discharge coefficient, the recommended formula is QV notch Cd tan 2 g1/2H5/2 Cd 0.44 for 20° 100° (10.60) for heads H 50 mm. For smaller heads, both Reynolds-number and Weber-number effects may be imp...
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