Waterhammer Effects In The Case Of Air Release
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In the case of unsteady flow in pipes, the main problems seem to be the modelling of headlosses, especially when air release phenomena occur. A number of approaches are possible, but in this paper only one-dimensional and one-phase models have been investigated, in order to check their adequacy in reproducing real data, as they require much less computational effort and are therefore much faster and easy to apply. Laboratory experiments have been performed at the Hydraulic Laboratory of the Politecnico di Milano, Italy, and simple models have been implemented in a computer code and then applied. The results are reported and commented; it appears that to reach some achievements, some other points have to be discarded and to improve the overall solution it is probably necessary to use two-dimensional two-phase models. Keywords: waterhammer, column separation, air release, headlosses. 1 Introduction There are a number of examples of benefits derived by the presence of unsteady flow, but in most of the cases this phenomenon has destructive characteristics. This is why the subject has been studied since the XIX Century. However, still many uncertainties are present when the model of a complex system is built. One of the main problems is related to the use of steady flow formulas to compute the headlosses. Generally speaking, smoothing and phase shift are present even when no air release occurs , implying the problems are tied to the unsteady flow phenomena; the reasons for such inadequacy have to be found  in the different velocity profiles between those in unsteady flow and those correspondingly (i.e. with the same discharge) in steady flow. In unsteady flow the gradient of the velocity close to the pipe wall is steeper than in steady flow and therefore the stresses are higher, and finally so are the resistances .
waterhammer, column separation, air release, headlosses