Author(s): Ming Zhao
Linked Author(s):
Keywords: Flow decomposition; Unsteady pipe flows; Waterhammer; Transients; Quasi-two-dimensional model; Turbulence model
Abstract: Existing turbulence models for pipe flows are derived and tested for steady or slowly varying unsteady flows, where the velocity profile is nearly logarithmic. In this paper, a turbulence model for transient pipe flows (i.e., waterhammer) based on flow decomposition is formulated, implemented and evaluated against experimental data for a wide range of Reynolds number and ratios of wave to diffusion time scales. Both the mean and turbulent flow fields are decomposed into a steady part and an unsteady part. The decomposition is motivated by the fact that while the total velocity field is far from logarithmic and contains inflection points and regions of flow reversal; the steady part of the velocity profile is identically logarithmic and the unsteady part of the velocity profile is initially uniform and becomes nearly logarithmic as the ratio of the wave to diffusion time scale increases. Therefore, steady flow-based turbulence models are well suited for each part of the decomposed flow field. Comparison of the pressure head results of the current model with those of existing (i.e., non-decomposed) model and laboratory experimental data shows that the proposed model is superior to existing models and that the improvement in results becomes significant at the ratio of the wave to diffusion time scale increases. The good agreement between model and data support the decomposition method and the hypothesis that the correlations between the steady and unsteady turbulent fluctuations are negligible for waterhammer time scales.
Year: 2001