The aim of this research is to assess the maximum transient pressures in a rapidly filling water pipeline containing entrapped air. A three-dimensional computational fluid dynamics model is used to simulate the rapid filling process and to describe the maximum transient pressures for a given range of initial conditions (i.e. air pocket size and initial pressure differences). Results are compared with experimental data. The air pocket size that leads to the highest maximum pressure (critical air pocket size) has been obtained for each initial pressure difference. Two types of behaviours have been identified at the maximum compression of the air pocket: air compression preserving a clear air–water interface and total air compression with the collapse of the air–water interface. The former typically occurs for small air pockets combined with low initial pressure differences, whereas the latter occurs for large air pocket sizes with high initial pressure differences.

The aim of this paper is to investigate the complex nature of the transient energy dissipation by using a computational fluid dynamics (CFD) model with a high spatial and temporal resolution together with one-dimensional (1D) models incorporating different unsteady friction (UF) formulations. The analysis focuses on a transient event in a single pipe system with a smooth-wall turbulent flow (with an initial Reynolds number equal to 7,638) generated by an instantaneous valve closure. For the considered flow condition, the numerical experiments point out the importance of the used UF model with regard to the wall shear stress simulation. In such a context, it is shown that the convolution-based UF models better describe the pressure signal than the instantaneous acceleration–based ones because they take into account a set of previous time steps. This is due to the fact that, similarly to the laminar regime, to simulate the characteristics of low turbulent transients the flow time history plays a crucial role.

The current paper aims at the experimental and numerical analysis of the cavitating pipe flow during the occurrence of hydraulic transients in a quasi-horizontal straight copper pipe rig. Transient events were simulated by the quasi-instantaneous closure of a pneumatically actuated ball valve located at the downstream end of the pipe. A hydraulic transient model has been developed for describing cavitating pipe flow by means of two approaches – the discrete vapour cavity model (DVCM) and the discrete gas cavity model (DGCM). Firstly, the model has been calibrated by using transient data without cavitation. Numerical results have been compared with collected data and a good agreement has been observed as long as the unsteady friction losses are considered. Secondly, DVCM and DGCM have been used to describe cavitating flows. Results of both models have been compared, and the DGCM model has shown to better describe transient events with cavitation

The aim of this paper is the experimental characterization of a ball valve under steady and unsteady conditions, based on experimental tests carried out in a pressurized pipe system assembled at Instituto Superior Técnico. Data analysis showed that the response of the valve under steady-state conditions depends not only on the valve geometry and closure percentage, but also on the flow regime (i.e., Reynolds number); the comparison with values published in literature confirmed that typical valve head loss curves presented in manuals and textbooks refer to turbulent flows. The behavior under unsteady conditions was also analyzed based on transient pressure head measurements showing that the valve effective closure time varies between 4 and 10% of the total time of the maneuver. Two mathematical functions (hyperbolic and sigmoidal) are proposed to describe the discharge variation as a function of the total time of the maneuver and of the initial steady-state discharge; the most appropriate function depends on the simplifications imposed on the hydraulic transient simulator. A computational fluid dynamics (CFD) model was used to support the observed experimental behavior and to highlight the effect of the pipe length on the discharge variation.