| 1. |
- Ivanell, Stefan, et al.
(författare)
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Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry
- 2015
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Ingår i: WAKE CONFERENCE 2015. - : IOP Publishing.
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Konferensbidrag (refereegranskat)abstract
- Results from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes.
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| 2. |
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| 3. |
- Kleusberg, Elektra, et al.
(författare)
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Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods
- 2016
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Ingår i: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016). - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- The wake structure behind a wind turbine, generated by the spectral element code Nek5000, is compared with that from the finite volume code EllipSys3D. The wind turbine blades are modeled using the actuator line method. We conduct the comparison on two different setups. One is based on an idealized rotor approximation with constant circulation imposed along the blades corresponding to Glauert's optimal operating condition, and the other is the Tjffireborg wind turbine. The focus lies on analyzing the differences in the wake structures entailed by the different codes and corresponding setups. The comparisons show good agreement for the defining parameters of the wake such as the wake expansion, helix pitch and circulation of the helical vortices. Differences can be related to the lower numerical dissipation in Nek5000 and to the domain differences at the rotor center. At comparable resolution Nek5000 yields more accurate results. It is observed that in the spectral element method the helical vortices, both at the tip and root of the actuator lines, retain their initial swirl velocity distribution for a longer distance in the near wake. This results in a lower vortex core growth and larger maximum vorticity along the wake. Additionally, it is observed that the break down process of the spiral tip vortices is significantly different between the two methods, with vortex merging occurring immediately after the onset of instability in the finite volume code, while Nek5000 simulations exhibit a 2-3 radii period of vortex pairing before merging.
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| 4. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
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LES of the Flow Around a Generic Wheel in a Wheelhouse
- 2010
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Ingår i: ASME Conference Proceedings. 3rd Joint US-European Fluids Engineering Summer Meeting, FEDSM 2010 Collocated with 8th International Conference on Nanochannels, Microchannels, and Minichannels, Montreal, 1-5 August 2010. - 0888-8116. - 9780791849484 ; 1:PARTS A, B AND C, s. 2681-2692
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Konferensbidrag (refereegranskat)abstract
- The flow around generic wheels in wheel housings used in previous experimental investigations is studied using large eddy simulations. A comparison is given here of the results of the simulations with existing experimental data and previous qualitative results of RANS simulations. Both instantaneous and time averaged flows are described, showing agreement with previous knowledge and adding new insight inflow physics. Two different widths of the wheel housing are used in the simulations, and their influence on the flows is studied. The present work shows that the width of the wheel housing has an influence onflows on both the inside and the outside of the wheel.
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| 5. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
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Numerical Investigation of the Flow Around a Simplified Wheel in a Wheelhouse
- 2011
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Ingår i: Journal of Fluids Engineering, Transactions of the ASME. - : ASME International. - 1528-901X .- 0098-2202. ; 133:11, s. 111001-
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Tidskriftsartikel (refereegranskat)abstract
- The flow around generic wheels in wheel housings used in previous experimental investigations is studied using large eddy simulations (LES). A comparison is given here of the results of the simulations with existing experimental data and previous results of RANS simulations. Both instantaneous and time-averaged flows are described, showing agreement with previous knowledge and adding new insight in flow physics. Two different widths of the wheel housing are used in the simulations, and their influence on the flows is studied. The present work shows that the width of the wheel housing has an influence on flows on both the inside and the outside of the wheelhouse.
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| 6. |
- Krajnovic, Sinisa, 1970, et al.
(författare)
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Numerical investigation of the influence of side winds on a simplified car at various yaw angles
- 2010
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Ingår i: ASME Conference Proceedings. 3rd Joint US-European Fluids Engineering Summer Meeting, FEDSM 2010 Collocated with 8th International Conference on Nanochannels, Microchannels, and Minichannels, Montreal, 1-5 August 2010. - 0888-8116. - 9780791849484 ; 1:PARTS A, B AND C, s. 2693-2704
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Konferensbidrag (refereegranskat)abstract
- The flow around a generic passenger car under the influence of crosswind was predicted using large eddy simulation (LES). The Reynolds number based on the incoming velocity the car's length, L used was Re = 9 × 10 5. Yaw angles of crosswind of 10°, 20° and 30° were studied and the LES results were compared with the experimental observations and previous Reynolds-averaged Naviers-Stokes (RANS) and detached eddy simulations (DES). The present LES were found to predict flows in better agreement with the experimental observations than previous RANS and DES. This shows that LES is better suited than RANS or DES for moderate Reynolds number flows around scale-model car in crosswinds which are inherently unsteady with regions of massive separations.
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| 7. |
- Mikkelsen, R. F., et al.
(författare)
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Rotor aerodynamic power limits at low tip speed ratio using CFD
- 2014
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Ingår i: SCIENCE OF MAKING TORQUE FROM WIND 2014 (TORQUE 2014). - : IOP Publishing.
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Konferensbidrag (refereegranskat)abstract
- When investigating limits of rotor aerodynamic models, the Betz limit serves as a solid marker of an upper limit which no model should be able to exceed. A century ago Joukowsky (1912) proposed a rotor aerodynamic model utilizing a rotating actuator disc with a constant circulation. This model has since then been the subject of much controversy as it predicts a power performance that for all tip speed ratios exceeds the Betz limit and which goes to infinity when the tip speed ratio goes to zero. Recently, it was demonstrated that the Joukowsky model is fully consistent with the inviscid Euler equations and that the apparent inconsistency partly can be explained by the lack of viscous effects (Sprensen and van Kuik [4]). However, even including a term to account for the effect of viscosity at small tip speed ratios, the model still predicts a power yield that exceeds the Betz limit. In the present work we study in detail, using a CFD actuator line model, the flow behavior for rotors at small tip speed ratios. It is shown that the excessive swirl appearing towards the rotor center at small tip speed ratios generates vortex breakdown, causing a recirculating zone in the wake that limits the power yield of the rotor. The appearance of vortex breakdown has a similar effect on the flow behavior as the vortex ring state that usually appears at higher tip speed ratios. Limits to where vortex breakdown might occur with tip speed ratio and rotor loading as parameter are investigated and presented in the paper. The limits found correspond to well-known criterion for vortex breakdown onset for swirling flows in general. By applying a criterion for vortex breakdown in combination with the general momentum theory, the power performance always stays below the Betz limit.
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| 8. |
- Sarlak, H., et al.
(författare)
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Aerodynamic behaviour of NREL S826 airfoil at Re=100,000
- 2014
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 524:1
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents wind tunnel measurements of the NREL S826 airfoil at Reynolds number Re 100,000 for angles of attack in a range of -10° to 25° the corresponding Large Eddy Simulation (LES) for selected angles of attack. The measurements have been performed at the low speed wind tunnel located at Fluid Mechanics laboratory of the Technical University of Denmark (DTU). Lift coefficient is obtained from the forge gauge measurements while the drag is measured according to the integration of the wake profiles downstream of the airfoil. The pressure distribution is measured by a set of pressure taps on the airfoil surface. The lift and drag polars are obtained from the LES computations using DTU's inhouse CFD solver, EllipSys3D, and good agreement is found between the measurement and the simulations. At high angles of attack, the numerical computations tend to over-predict the lift coefficients, however, there is a better agreement between the drag measurements and computations. It is concluded that LES computations are able to capture the lift and drag polars as well as the pressure distribution around the airfoil with an acceptable accuracy.
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