Self-aerated bounded flows in special hydraulic structures. Part 1. A short review on design concepts and sizing procedures of aerators

Abstract

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.In a number of hydraulic structures, significant fluiddynamical differences may be observed between flows which are well aerated and flows which are not ([16]). These differences are not just a matter of scientific speculation. Effective or ineffective aeration may be in fact responsible for inducing a variety of flow regime transitions, some of which may result desirable in particular situations and undesirable in other cases ([16]). When flows are bounded the venting system actually rules the behaviour of the entire system. Despite the importance of aeration for the performance of many hydraulic structures, design methods and procedures, in force of the great complexity of the involved physical phenomena, may be still nowadays quite simplified and experimental tests on large scale physical models are to be considered unavoidable to properly size the air supply system ([15], [21]). In the first part of the paper, structural layout and flow patterns which may be observed in bottom outlets, chute spillways, baffled weirs and leaping weirs are described. Flow regime transitions occurring in these structures are shown to be ruled to a great extent from the sizing of the aerators, giving evidence that such elements are unavoidable whenever civil structures and mechanical equipment are prone to cavitation risk. Stemming from these premises, in the second part of the paper it is stressed that common design procedures of air vents are based upon the hypothesis that the flow of air through vents may be treated as that of an incompressible fluid. It is brought to light however that this procedure contrasts, not so infrequently, with many experimental results collected from various researchers over more than 50 years ([6], [15], [22]). A compressible flow formulation is therefore reckoned to be necessary to predict the main flow characteristics of air through ducts of variable size, length and roughness.