Characteristics of Flow Over Normal and Oblique Weirs With Semicircular Crests

Abstract

In this investigation, the characteristics of free flow over normal and oblique weirs with semicircular crests are studied experimentally. For this purpose, forty eight weir models were constructed and tested. The first twelve models were normal weirs in which the crest radius was varied three times; 5cm, 7.5cm and 10cm. For each crest radius, the weir height was varied four times; 35cm, 30cm, 25cm and 20cm. The remaining models were oblique weirs. The oblique angle was varied three times; 60, 45 and 30. In weirs of the same oblique angle, the crest radius and weir height were varied similarly to those of normal weirs.The experimental results showed that for normal weirs, the discharge coefficient (C dw) increases with the increase of head to crest height ratio (h/P) for the same height of weir. In case of oblique weirs, it was found that (C dw) decreases with the increase of (h/P) values and weirs of small oblique angle () give high values of (C dw).For normal weirs, the discharge magnification factor (Q NC /Q NS) and performance increase as values of (h/P) increase. Normal weirs of semicircular crests perform better than those of sharp crested weirs for all values of weir height and crest radius tested in this study. While, for oblique weirs the discharge magnification factor (Q OB /Q NS) and performance increased with the decrease of (h/P) values. As (h/P) value approaches zero, the discharge magnification factor approaches the length magnification of the weir. Weirs of small oblique angles give high discharge magnification factor and high performance.A simple procedure was applied for the hydraulic design of oblique weirs. The design method yields the final dimensions of a weir and predicts its head-discharge curve for the whole range of operation. .-. .) C dw () h/P (.) C dw () h/P () C dw .() Q NC /Q NS () h/P (. .) Q OB /Q NC () h/P () h/P (. .-. Al_Rafidain engineering Vol.13 No.1 2005 50 NOTATION The following symbols are used in this paper : C d = dimensionless discharge coefficient, d C. g dw C , g = acceleration due to gravity, h = upstream head above weir crest, L = crest length of oblique weir, P = height of weir above channel bed, Q OB = discharge passing over oblique weir of semicircular crest, Q NC = discharge passing over normal weir of semicircular crest, Q NS = discharge passing over normal sharp-crested weir, q = discharge per unit width of the channel, R = radius of weir crest, W = channel width, = angle which the weir makes with channel axis (oblique angle),and 1 to 4 = signify function of. INTRODUCTION The increasing need for stored water requires intensive efforts from hydraulic engineers to develop rapid methods for weir design and construction. A weir structure is an essential feature of many hydraulic structures such as dams, barrages, canal drops or falls, regulators, etc. An oblique weir usually passes greater discharge at given operating head than a corresponding normal weir (weir perpendicular to the channel occupying the same lateral width). The advantage of oblique weir over normal weir appears clearly when the lateral width of the channel is limited.The performance of oblique sharp-crested weir was studied by Aichel [1] and Ibraheem et al. [2]. While, laybrinth sharp-crested weirs of different shapes were studied extensively by many investigators, to name few, Hay and Taylor [3,4] , Darvas [5] , Al-Hasson [6] and Ibraheem [7] .Characteristics of flow over normal weirs having round crests were studied by Al-Tabatabie et al. [8] and Abid Ali [9]. From the available literature, no work has been published on the hydraulic characteristics of flow over oblique weirs of semicircular crests. The present investigation attempts to do that under free flow conditions for the purpose of obtaining a general equation for the discharge coefficient and design criteria for these types of weirs.