Time-accurate Turbulence Modeling of Swirling Flow for Hydropower Application
Licentiate thesis, 2014
Hydraulic turbomachines have played a prominent role in the procurement of renewable
energy for more than a century. Embedded in the context of general technological progress,
their design for efficiency and reliability has reached an outstanding level of quality. At
the design point, water turbines generally operate with little swirl entering the draft tube
and no flow separations, but at off-design, at both high- and low-load, the flow leaving
the turbine has a large swirling component.
The present work describes the turbulence modeling of a wide range of physical
mechanisms that produce pressure pulsations in swirling flows. The available knowledge
about these pulsations are still far from complete. If the swirl exceeds a certain level, the
flow patterns associated with the swirl dominated vortex motions vacillate. A key feature
of strongly swirling flows is vortex breakdown. The vortex breakdown is an abrupt change
in the core of a slender vortex and typically develops downstream into a recirculatory
“bubble” or a helical pattern. The swirl motion and the helical pattern has for long been
of interest to scientists and engineers who have constantly strived in reproducing the
naturally occurring phenomena and take advantage of their performance enhancing effects
thermal and mass transport applications. The swirl effects are usually seen as either the
desired result of design or unavoidable, possibly unforeseen, side effects which comprise a
forced vortex core centered around its axis of rotation. The core is due to viscous forces,
increases in size with successive increases in viscosity and varies over widely dissimilar
length and time scales depending on the physical context.
The pulsations and their impact on the efficiency and hydraulic structures of water
turbines depend on the flow rate, the velocity distribution after the runner, the shape
of the draft tube, and the dynamic response of the whole hydraulic structure. The high
level of unsteadiness in the flow field necessitates the utilization of advanced turbulence
treatment to predict the small-scale structures.
Time-accurate Reynolds-averaged Navier-Stokes (URANS) models are primarily useful
for capturing large-scale flow structures, while the details of the small-scale turbulence
eddies are filtered out in the averaging process. In many cases also the large-scale structures
are damped by the URANS modeling, which is formulated to model all the turbulence.
The quality of the results is thus very dependent on the underlying turbulence model.
Better approaches should be used to handle the anisotropic and highly dynamic character
of turbulent swirling flows. An extended series of turbulence models are scrutinized in this
work while the main focus is on hybrid URANS-LES and LES methods. Detached-eddy
simulation (DES) is a promising hybrid URANS-LES strategy capable of simulating
internal flows dominated by large-scale detached eddies at practical Reynolds numbers.
The method aims at entrusting the boundary layers with URANS while the detached
eddies in separated regions or outside the boundary layers are resolved using LES. DES predictions of massively separated flows, for which the technique was originally designed,
are typically superior to those achieved using URANS models, especially in terms of the
three-dimensional and time-dependent features of the flow. Scale-adaptive simulation
(SAS) is another hybrid URANS-LES method which is based on detecting the unsteadiness
according to the velocity gradients in the flow field. The present work gives a thorough
comparison between the different levels of unsteady turbulence modeling, applied to
swirling flow and the rotor-stator interaction.