Time-accurate Turbulence Modeling of Swirling Flow for Hydropower Application

Licentiatavhandling, 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.