An in-depth numerical analysis of transient flow field in a Francis turbine during shutdown
Journal article, 2021

Power production from intermittent renewable energy resources, such as solar and wind, has increased in the past few decades, leading researchers and engineers to establish techniques to preserve a stable electrical grid. Consequently, hydraulic turbines are being used more frequently in transient operating modes to regulate the grid. The present work provides a comprehensive numerical study on the transient flow field of a high-head Francis turbine model throughout the shutdown sequence. The computations were performed using OpenFOAM, utilizing the SST-SAS turbulence model. A Laplacian smoothing scheme is employed to conduct the mesh deformation of the guide vane domain. The time-averaged draft tube velocity field at the steady Best Efficiency Point (BEP) is validated against experimental data. Then different aspects of the transient flow field in the shutdown sequence are carefully assessed and explained for the first time. Short-Time Fourier Transform (STFT) analysis is carried out on the fluctuating part of the static pressure and force signals. High-amplitude low-frequency oscillations, due to the formation of a Rotating Vortex Rope (RVR) were observed during a specific period of the shutdown sequence. Thereafter, at deep part load conditions, the RVR vanishes and, a wide range of stochastic frequencies are identified at minimum load. A signal coherence analysis was accomplished to distinguish the deterministic and stochastic frequencies. The variation of the velocity field in the draft tube is described in detail with the help of velocity triangles. An in-depth explanation of the formation and variation of vortical structures during the whole sequence is presented. The physical mechanism of formation and destruction of the RVR is thoroughly explained.

Transient operation


Deterministic and stochastic frequencies

Shutdown sequence

Rotating vortex rope (RVR)

Francis turbine


Saeed Salehi

Chalmers, Mechanics and Maritime Sciences (M2), Fluid Dynamics

Håkan Nilsson

Chalmers, Mechanics and Maritime Sciences (M2), Fluid Dynamics

Eric Lillberg


Nicolas Edh

Forsmarks Kraftgrupp AB

Renewable Energy

0960-1481 (ISSN) 18790682 (eISSN)

Vol. 179 2322-2347

Flow in turbines during new operating procedures

Svenskt Vattenkraftcentrum, 2019-10-01 -- 2021-08-31.

Energiforsk AB, 2019-10-01 -- 2020-12-31.

Swedish Energy Agency, 2019-10-01 -- 2021-08-31.

Subject Categories

Energy Engineering

Fluid Mechanics and Acoustics

Areas of Advance



C3SE (Chalmers Centre for Computational Science and Engineering)



Related datasets

DOI: 10.1016/j.renene.2021.07.107

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