| dc.description.abstract | The growing awareness of climate change and the related environmental impacts
is driving a progressive transformation in the energy market, with an
increasing relevance of the role of renewable energy sources. However, due to
their intermittent and variable nature, challenges arise towards their integration
into the current electrical grid. In this scenario, hydropower can be used
to stabilize the system but this calls for an increased flexibility demanded to
the power plants and all their components, especially the turbine.
Francis turbines are widely diffused in Norway, Italy and Europe in general,
therefore there is significant interest in the understanding of how to operate
these types of machines in today’s and future energy scenario. Prior
research at the Waterpower Laboratory (NTNU, Trondheim), mainly within
the HydroCen framework, explored various aspects of structural and fluid
dynamic phenomena across different turbine geometries. These researches
provided a comprehensive overview of dynamic loads, stresses, deflections,
and vibratory behaviour of the explored turbine systems.
The primary objective of this master thesis is the characterization of the hydrodynamic
damping of hydrofoils in a Circular Blade Cascade (CBC), consisting
of eight of them in a radial symmetrical configuration. This specific
setup has been devised to extend previous studies conducted on hydrofoils in
non-cylindrical symmetry layouts. This study relies exclusively on numerical
simulations and is a fundamental complement to the upcoming experimental
campaign.
Firstly modal acoustic analysis has been carried out to assess natural frequencies
and mode shapes of the structure. Fluid presence was found to
lower natural frequencies compared to those in air, with minimal impact on
eigenvectors. Subsequently, reverse one-way coupled simulations were performed
for various speeds, focusing on the first mode shape of the first two
nodal diameters. Special attention was given to speeds close to lock-in. In
this situation, a match occurs between the natural frequency of the structure
and the vortex shedding frequency, potentially leading to catastrophic
resonance effects. Using the “Aerodynamic Damping” tool in Ansys CFX®,
the damping work done by the fluid on the hydrofoils was calculated and
then, following a “modal work approach”, the hydrodynamic damping value
was derived.
The trend of the results aligns with previous experimental and numerical
studies conducted in the Waterpower laboratory, demonstrating the almost
independence of hydrodynamic damping from velocity pre-lock-in and a linear
dependence post-lock-in. Not all the numerical values are, instead, convincing
and consistent with previous studies due to the complex evaluation
of some parameters.
In this context, the obtained values and trends will serve as a valuable benchmark
for comparing future experimental results. | |
