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• W2351293367 abstract "A series of numerical investigations is undertaken using a wide range of turbulence mod-els including conventional and non-conventional URANS models, hybrid URANS-LESmethods and LES to capture a large variety of physical mechanisms that produce pres-sure pulsations in the swirling flows. The available knowledge about these pulsations,which are usual in hydropower, are still far from complete. When the swirl is moder-ately low, a stable on-axis structure generates in the pipe. If the swirl exceeds a certainlevel, the flow patterns associated with the swirl dominated vortex motions vacillate. Akey feature of strongly swirling flows is vortex breakdown. The vortex breakdown is anabrupt change in the core of a slender vortex and typically develops downstream into arecirculatory “bubble” or a helical pattern. The swirl effects are usually seen as either thedesired result of design or unavoidable, possibly unforeseen, side effects which comprise aforced vortex core centered around its axis of rotation. The vortex breakdown is an invis-cid process and the pulsations caused by the vortex breakdown and their impact on theefficiency and hydraulic structures of water turbines depend on the flow rate, the velocitydistribution after the runner, the shape of the draft tube, and the dynamic response ofthe whole hydraulic structure. The high level of unsteadiness in the flow field necessitatesthe utilization of appropriate turbulence treatments to predict the complexity of the flowstructures.Time-accurate Reynolds-averaged Navier-Stokes (URANS) models are primarily use-ful for capturing large-scale flow structures, while the details of the small-scale turbu-lence eddies are filtered out in the averaging process. In many cases also the large-scalestructures are damped by the URANS modeling which is formulated to model all theturbulence. The swirling flows in a pipe are dominated by large-scale detached eddies,therefore the URANS models should be capable of predicting the flow fields. The qual-ity of the URANS results is very dependent on the underlying turbulence model. Theknowledge about URANS is limited to the simplest (most robust) linear eddy-viscositymodels which are available in the proprietary codes. The inability of the conventionallinear eddy-viscosity models available in a CFD code should thus not be generalizedto the URANS method alone. The conventional linear eddy-viscosity model provides adirect link between the turbulent stress tensor and the mean strain rate, forcing themto be directly in phase, which is wrong. In the highly swirling flows, the curvature ofthe streamlines should be taken into account for a better predicting of the flow fields.Reynolds Stress Models (RSM) have the potential to significantly improve the flow pre-dictions by resolving anisotropy and incorporating more sensitivity and receptivity ofthe underlying instabilities and unsteadiness. Since they are difficult to use they arenot widely used in industry. Most of the RSMs are not robust for highly swirling flowsbecause of instability in the rapid part of the pressure-strain term in the transport equa-tion. The Explicit Algebraic Reynolds Stress Models (EARSMs) are simplified RSMsthat are much more numerically and computationally robust and have been found tobe comparable to standard two-equation models in computational effort. The EARSMsassume that the Reynolds stress tensor can be expressed in the strain and vorticity ratetensors.A more advanced approach, also called the second generation URANS method, is thehybrid URANS-LES method which is capable of capturing the high level of unsteadinessand handling the anisotropic and highly dynamic character of turbulent swirling flows.An extended series of turbulence models is scrutinized in this work while the main focusis on the Detached-eddy simulation (DES) method. The DES method is a promising hy-brid URANS-LES strategy capable of simulating internal flows dominated by large-scaledetached eddies at practical Reynolds numbers. Another hybrid URANS-LES methodis scale-adaptive simulation (SAS). This method is based on detecting the unsteadinessaccording to the velocity gradients in the flow field. This method gives better resultsthan LES in a highly swirling flow in a pipe using a relatively coarse resolution." @default.
• W2351293367 created "2016-06-24" @default.
• W2351293367 creator A5032091349 @default.
• W2351293367 date "2016-01-01" @default.
• W2351293367 modified "2023-09-23" @default.
• W2351293367 title "Turbulence-resolving Simulations of Swirling Flows" @default.
• W2351293367 hasPublicationYear "2016" @default.
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