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 (). 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 (). 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 (, ).
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 (, , ). 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.