Numerical Simulation of Precessing Vortex Rope in Draft Tube

Introduction

Precessing Vortex Rope is the result of swirling flow instability in the draft tube at a part load operating zone. It could be at the origin of the pressure oscillation in the draft tube, even vibration in the whole installation. This effect becomes more dangerous if the frequency of mentioned pressure oscillation approaches to the natural frequency of structure, where the vortex rope risk of resonance increases. Under part load conditions flow instability occurs due to high swirling flow. The angular and axial velocity profiles, are the origins of this instability.

Doctoral student : Alireza Zobeiri

Vortex rope simulation

Numerical simulation of part load vortex rope is investigated. The goal of the present study is to develop a computational methodology, which can provide detailed description of this time dependent phenomenon. Analysis of pressure fluctuations due to precessing vortex rope is performed in the cone of draft tube. The inception of a precessing vortex rope is although discussed. Physical mechanisms of precessing vortex rope is summarized with the help of numerical results. Vortex rope in a Francis turbine ppears due to satisfying three conditions,at first, high swirl number, secondly, adverse pressure along the swirl axis and finally a conical geometry. A numerical simulation is performed to follow a vortex rope formation in draft tube. The mentioned investigations are performed in the frame work of FLINDT, Eureka Research Project n° 1625.

Pressure Fluctuations