اثر زبری تاج و بدنه بر ضریب تخلیه و شرایط هیدرولیکی جریان در سرریز های تاج دایره ای با استفاده از مدل فیزیکی

Identifying the parameters affecting discharge coefficient of circular crest weir in an optimum application is very important. Since circular weir crest may be constructed by different materials with different roughness. So in this research influence of roughness on discharge coefficient has been investigated. Without present of roughness, the result of the experimental test on weirs with five different curvature radii ( from ۲.۵ to ۷.۵ cm) indicated that the most effective parameter on discharge coefficient was the ratio of upstream total head to crest radius. Also discharge coefficient increased with increasing this ratio. On the other hand variation of weir height to upstream total head ratio had no significant effect on the discharge coefficient. The effect of four different sizes of roughness on discharge coefficient, which had been overlooked in previous researches, was also evaluated. Results of the tests showed that with increasing relative roughness, discharge coefficient reduced and lower relative roughness, had the higher influence on reducing  the discharge coefficient. Two mathematical relationships based on the affected parameters on discharge coefficient were presented. The first relation that is for the conditions imposed without roughness, discharge coefficient is just a function of upstream total head to crest radius ratio. Applying a correction factor, which is an exponential function of the relative roughness, the final relationship for calculating the coefficient of discharge including the effects of roughness, was obtained.