oa Civil Engineer in South Africa - Discrepancy of the Colebrook-White equation in the extreme rough turbulent zone



Following some experimental observations in the Crocodile River near Brits, it is shown in this paper that the Colebrook-White equation loses its validity in the extreme rough turbulent zone where Manning's exceeds about 0,05. Consequently, it is recommended that the Colebrooke-White equation should not be applied in this zone () and should also be applied very cautiously in the rough turbulent zone () where flow calculations are frequently done in practice (eg backwater calculations in natural canals or rivers). Since Manning's equation may be used in both the rough and extreme rough turbulent zones, it is recommended that this equation should be applied in these zones. This implies that - if Darcy-Weisbach's or Chézy's flow equations are used in these flow regions - the friction coefficients ƒ or should not be obtained from Colebrook-White's equation but should be derived directly from Manning's by means of or (in metric form). A number of important research requirements with regard to the Colebrook-White equation, are identified in this paper.

In hierdie verhandeling word daar na aanleiding van praktiese waarnemings in die Krokodilrivier naby Brits, getoon dat die Colebrook-White-vergelyking nie in die ekstreem ruwe turbulente stromingsgebied waar die Manning -waarde ongeveer, 0,05 oorskry, geldig is nie. Gevolglik word aanbeveel dat die Colebrook-White-vergelyking nie in hierdie stromingsgebied () aangewend behoort te word nie, terwyl dit ook met omsigtigheid in die ruwe turbulente stromingsgebied () waar vloeiberekeninge dikwels in die praktyk uitgevoer word, toegepas moet word (soos bv in die geval van natuurlike waterleidings). Aangesien die Manning-vergelyking beide die ruwe en die ekstreem ruwe stromingsgebiede bevredigend dek, behoort hierdie vergelyking in genoemde stromingsgebiede aangewend te word. Dit impliseer dat - indien Darcy-Weisbach of Chézy se vloeivergelykings in hierdie stromingsgebiede aangewend word - die wrywingskoëffisiënte ƒ of nie mbv die Colebrook-White-vergelyking vereken moet word nie, maar dat hulle regstreeks mbv die volgende metriese vergelykings vanaf die Manning -waarde afgelei moet word : of . 'n Aantal belangrike navorsingsbehoeftes tov die Colebrook-White-vergelyking word in hierdie verhandeling aangestip.


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