8 results found
Steiros K, Bempedelis N, Ding L, 2021, Recirculation regions in wakes with base bleed, PHYSICAL REVIEW FLUIDS, Vol: 6, ISSN: 2469-990X
It is well known that perforation of a flat plate reduces its drag when exposed to a flow. However, studies have shown an opposite effect in the case of cylinders. Such a counterintuitive result can have significant consequences on the momentum modelling often used for wind turbine performance predictions, where increased porosity is intrinsically linked to lower drag. Here, a study of the drag of various types of porous cylinders, bars and plates under steady laminar inflow is presented. It is shown that, for most cases, the drag decreases with increased porosity. Only special types of perforations can increase the drag on both cylinders and bars, either by enhancing the effect of the rear half of the models or by organizing the wake structures. These rare occurrences are not relevant to wind turbine modelling, which indicates that current momentum models exhibit the qualitatively correct behaviour.
Ayati AA, Steiros K, Miller MA, et al., 2019, A double-multiple streamtube model for vertical axis wind turbines of arbitrary rotor loading, WIND ENERGY SCIENCE, Vol: 4, Pages: 653-662, ISSN: 2366-7443
Steiros K, Hultmark M, 2018, Drag on flat plates of arbitrary porosity, Journal of Fluid Mechanics, Vol: 853, ISSN: 0022-1120
A new model for the drag force on a two-dimensional flat plate of arbitrary porosity, oriented normal to the free stream, is introduced. The model is an extension of that introduced by Koo & James (J. Fluid Mech., vol. 60(3), 1973, pp. 513–538), where the performance at low porosities is improved by including a base-suction term. The additional drag due to the base suction is calculated implicitly using momentum theory, which makes the model self-contained. The model predictions exhibit convincing agreement with experimental observations over a wide range of porosities, including the solid case, as long as shedding is absent or suppressed.
Steiros K, 2017, Transient torque in stirred tanks, Journal of Fluid Mechanics, Vol: 831, Pages: 554-578, ISSN: 0022-1120
The transient dynamics of stirred tanks whose impeller speed undergoes smooth or step changes is investigated. First, a low-order model is developed, linking the impeller torque with the ‘extent’ of the solid-body rotation in the tank, derived from an angular momentum balance in a control volume around the impeller. Utilisation of this model enables the prediction of the torque ‘spike’ appearing after an impulsive change of the shaft speed, and of the torque evolution during a quasi-steady transition. For the case of a small impulsive change in the shaft speed, a characteristic spin-up time is also proposed. Torque measurements performed in an unbaffled stirred tank show considerable agreement with the theoretical predictions.
Steiros K, Bruce PJK, Buxton ORH, et al., 2017, Effect of blade modifications on the torque and flow field of radial impellers in stirred tanks, PHYSICAL REVIEW FLUIDS, Vol: 2, ISSN: 2469-990X
We perform both high- and low-speed particle image velocimetry and torque measurements to characterize eight radial impeller types in an unbaffled stirred tank. The blade types consist of a set of regular flat blades, used as a baseline, regular blades of increased thickness, perforated blades, and fractal blades. We find a qualitative correlation between the blades' torque coefficient and both vortex coherence and turbulent kinetic energy, possibly explaining the torque differences of the tested impellers. Furthermore, we find that the proposed modifications increase the bulk turbulence levels and mass flow rates while at the same time reducing the shaft torque, showing promise for applications. Finally, we attempt a comparison between fractal and perforated geometries using data from this study and the literature.
Steiros K, Bruce PJK, Buxton ORH, et al., 2017, Power consumption and form drag of regular and fractal-shaped turbines in a stirred tank, AIChE Journal, Vol: 63, Pages: 843-843, ISSN: 0001-1541
Previous wind-tunnel measurements have shown that fractal-shaped plates have increased drag compared to square plates of the same area. In this study, the power consumption and drag of turbines with fractal and rectangular blades in a stirred tank are measured. Power number decreases from rectangular to fractal impellers by over 10%, increasingly so with fractal iteration number. Our results suggest that this decrease is not caused by the wake interaction of the blades, nor solely by the wake interaction with the walls either. Pressure measurements on the blades’ surface show that fractal blades have lower drag than the rectangular ones, opposite to the wind tunnel experiment results. All tested blades’ center of pressure radius increases with Re, while their drag coefficient decreases, a possible effect of the solid body rotation expansion with Re. Spectral analysis of the pressure signal reveals two peaks possibly connected to the blades’ roll vortices.
Steiros K, Bruce, Buxton O, et al., 2016, Flow Field Characteristics and Energy Injection in a Tank Stirred by Regular and Fractal Blade Impellers, Proceedings of the 5th International Conference on Jets, Wakes and Separated Flows (ICJWSF2015), Editors: Segalini, Publisher: Springer, Pages: 363-369, ISBN: 978-3-319-30602-5
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