CAVITATION 205
surface is ‘smooth’ and well constructed. (Elder reported values of
absolute roughness of about 1 mm for the 5.5 m diameter Appalachia
tunnel, which is probably the smoothest concrete surface obtainable
without steel trowelling of the finished surface.)
A value of cof around 0.2 is sometimes considered (Falvey, 1990)
when assessing the critical velocity on ‘smooth’ concrete surfaces; another
possibility is to express cas a function of the Darcy–Weisbach friction factor
(see eq. 8.4) of the surface (e.g. c 4 (Cassidy and Elder, 1984)).
The value of pvin equation (4.14) is a function of atmospheric pres-
sure and temperature (pv10 m H 2 Op 0 for 100 °C; pv6.5 m for 90 °C
and 0.5 m for 30 °C). Although it is generally assumed that the onset of
cavitation occurs when ppv(0 for normal water temperatures, i.e. at
10 m below p 0 / g), the presence of dissolved gases and/or particles in sus-
pension can cause cavitation at higher pressures; thus it is advisable to
Fig. 4.4 Incipient cavitation number for sloped protrusions (Cassidy and
Elder, 1984)