| 1. |
- Kleine, Vitor G., 1986-, et al.
(författare)
-
Tip-vortex instabilities of two in-line wind turbines
- 2019
-
Ingår i: Journal of Physics: Conference Series. - : Institute of Physics (IOP). - 1742-6588 .- 1742-6596. ; , s. 012015-
-
Konferensbidrag (refereegranskat)abstract
- The hydrodynamic stability of a vortex system behind two in-line wind turbines operating at low tip-speed ratios is investigated using the actuator-line method in conjunction with the spectral-element flow solver Nek5000. To this end, a simplified setup with two identical wind turbine geometries rotating at the same tip-speed ratio is simulated and compared with a single turbine wake. Using the rotating frame of reference, a steady solution is obtained, which serves as a base state to study the growth mechanisms of induced perturbations to the system. It is shown that, already in the steady state, the tip vortices of the two turbines interact with each other, exhibiting the so-called overtaking phenomenon. Hereby, the tip vortices of the upstream turbine overtake those of the downstream turbine repeatedly. By applying targeted harmonic excitations at the upstream turbine's blade tips a variety of modes are excited and grow with downstream distance. Dynamic mode decomposition of this perturbed flow field showed that the unstable out-of-phase mode is dominant, both with and without the presence of the second turbine. The perturbations of the upstream turbine's helical vortex system led to the destabilization of the tip vortices shed by the downstream turbine. Two distinct mechanisms were observed: for certain frequencies the downstream turbine's vortices oscillate in phase with the vortex system of the upstream turbine while for other frequencies a clear out-of-phase behaviour is observed. Further, short-wave instabilities were shown to grow in the numerical simulations, similar to existing experimental studies [1].
|
|
| 2. |
- Kleusberg, Elektra, et al.
(författare)
-
Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods
- 2016
-
Ingår i: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016). - : Institute of Physics (IOP).
-
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.
|
|
| 3. |
- Kleusberg, Elektra, et al.
(författare)
-
High-Order Numerical Simulations of Wind Turbine Wakes
- 2017
-
Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing. - 1742-6588 .- 1742-6596. ; 854:1
-
Tidskriftsartikel (refereegranskat)abstract
- Previous attempts to describe the structure of wind turbine wakes and their mutual interaction were mostly limited to large-eddy and Reynolds-averaged Navier-Stokes simulations using finite-volume solvers. We employ the higher-order spectral-element code Nek5000 to study the influence of numerical aspects on the prediction of the wind turbine wake structure and the wake interaction between two turbines. The spectral-element method enables an accurate representation of the vortical structures, with lower numerical dissipation than the more commonly used finite-volume codes. The wind-turbine blades are modeled as body forces using the actuator-line method (ACL) in the incompressible Navier-Stokes equations. Both tower and nacelle are represented with appropriate body forces. An inflow boundary condition is used which emulates homogeneous isotropic turbulence of wind-tunnel flows. We validate the implementation with results from experimental campaigns undertaken at the Norwegian University of Science and Technology (NTNU Blind Tests), investigate parametric influences and compare computational aspects with existing numerical simulations. In general the results show good agreement between the experiments and the numerical simulations both for a single-turbine setup as well as a two-turbine setup where the turbines are offset in the spanwise direction. A shift in the wake center caused by the tower wake is detected similar to experiments. The additional velocity deficit caused by the tower agrees well with the experimental data. The wake is captured well by Nek5000 in comparison with experiments both for the single wind turbine and in the two-turbine setup. The blade loading however shows large discrepancies for the high-turbulence, two-turbine case. While the experiments predicted higher thrust for the downstream turbine than for the upstream turbine, the opposite case was observed in Nek5000.
|
|
| 4. |
- Kleusberg, Elektra, 1988-, et al.
(författare)
-
High-order numerical simulations of wind turbine wakes
- 2017
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Previous attempts to describe the structure of wind turbine wakes and their mutual interaction were mostly limited to large-eddy and Reynolds-averaged Navier-Stokes simulations using finite volume solvers. We employ the higher-order spectral element code Nek5000 to study the influence of numerical aspects on the prediction of the wind turbine wake structure and the wake interaction between two turbines. The spectral element method enables an accurate representation of the vortical structures, with much lower numerical dissipation than the more commonly used finite volume codes. The blades are modeled as body forces using the actuator line method (ACL) in the incompressible Navier-Stokes equations. Both tower and nacelle are represented with appropriate body forces. An inflow boundary condition is used which emulates the homogeneous isotropic turbulence of wind tunnel flows. We validate the implementation with results from experimental campaigns undertaken at the Norwegian University of Science and Technology, investigate parametric influences and compare computational aspects with the existing finite volume codes. The results show good agreement between the experiments and the numerical simulations.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Kleusberg, Elektra, et al.
(författare)
-
Parametric dependencies of the yawed wind-turbine wake development
- 2020
-
Ingår i: Wind Energy. - : John Wiley and Sons Ltd. - 1095-4244 .- 1099-1824. ; 23:6, s. 1367-1380
-
Tidskriftsartikel (refereegranskat)abstract
- Yaw misalignment is currently being treated as one of the most promising methods for optimizing the power of wind farms. Therefore, detailed knowledge of the impact of yaw on the wake development is necessary for a range of operating conditions. This study numerically investigates the wake development behind a single yawed wind turbine operating at different tip-speed ratios and yaw angles using the actuator-line method in the spectral-element code Nek5000. It is shown that depending on the tip-speed ratio, the blade loading varies along the azimuth, resulting in a wake that is asymmetric in both the horizontal and vertical directions. Large tip-speed ratios as well as large yaw angles are shown to decrease the vertical asymmetry of the yaw-induced counter-rotating vortex pair. Both parameters have the effect that they increase the spanwise force induced by yaw relative to the wake rotation. However, while the strength of the counter-rotating vortex pair in the far wake increases with yaw angle, it is shown to decrease with the tip-speed ratio. The vertical shift in the wake center is found to be highly dependent on the yaw angle and the tip-speed ratio. These detailed insights into the yawed wake are important when optimizing potential downstream turbines.
|
|
| 8. |
|
|
| 9. |
- Kleusberg, Elektra, 1988-, et al.
(författare)
-
Parametric study of the actuator line method in high-order codes
- 2017
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The high accuracy of spectral element methods combined with low computationalcost and a high level of parallelization, makes them appealing for large-scale investigations of wind turbines and wind turbine interaction using the state-of-the-art actuator line method by Sørensen & Shen (2002). While the spectral element code Nek5000 has already been used for wind turbine simulations by e.g. Peet et al. (2013), Chatterjee & Peet (2015), Chatterjee & Peet (2016) and Kleusberg et al. (2016) for investigations of wind turbine wakes and wake interaction, the implications of the actuator line method in a high-order code and the effect of the involved parameters have not been studied in detail. This paper investigates the constant circulation turbine in the fixed and rotatingframe of reference. In the rotating frame of reference several wake parameters previously discussed e.g. by Ivanell et al. (2009) and Sarmast (2013) are revisitedand analyzed. Further, parametric studies are conducted in the fixed frame ofreference to investigate an observed instablility related to the spectral element width. The instablity is not a property of the spectral element discretization as it is also observed in other research using finite volume techniques. However, the decreased numerical dissipation and the non-equidistant grid used in spectral element methods leads to amplification of the instability. The parameters are investigated on a reduced two-dimensional test case and the conclusions transfered to the full actuator line setup. It is established that a Gaussian width of approximately five times the average grid spacing is necessary to reduce the effect of the instability related to the spectral element width when investigating sensitive flow cases. A force projection method proposed by Pinelli et al. (2010) is investigated as an alternative to the typically used Gaussian kernel. Finally, the influence of this instability is investigated when perturbations are applied tothe flow. Both small-scale perturbations that are introduced at the blade tips and low inflow turbulence which is imposed as an inlet condition are investigated.It is shown that when perturbations are introduced to the flow the large-scale wake behavior in the rotating and fixed frame of reference are similar and a Gaussian width which is 2.4 times the averaged grid spacing is sufficient.
|
|
| 10. |
|
|
