| 1. |
- Al Sam, A., et al.
(författare)
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An Investigation of Wind Farm Power Production for Various Atmospheric Boundary Layer Heights
- 2017
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Ingår i: Journal of Energy Resources Technology. - : ASME International. - 0195-0738 .- 1528-8994. ; 139:5
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Tidskriftsartikel (refereegranskat)abstract
- The dependency of the atmospheric boundary layer (ABL) characteristics on the ABL's height is investigated by using large eddy simulations (LES). The impacts of ABL's height on the wind turbine (WT) power production are also investigated by simulating two subsequent wind turbines using the actuator line method (ALM). The results show that, for the same driving pressure forces and aerodynamic roughness height, the wind velocity is higher at deeper ABL, while the wind shear and the wind veer are not affected by the depth. Moreover, the turbulence intensity, kinetic energy, and kinematic shear stress increase with the ABL's height. Higher power production and power coefficient are obtained from turbines operating at deeper ABL.
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| 2. |
- Al Sam, Ali, et al.
(författare)
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Evaluation of sub grid scale and local wall models in Large-eddy simulations of separated flow
- 2015
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Ingår i: 2nd Symposium on OpenFOAM® in Wind Energy. - : EDP Sciences. ; 5
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Konferensbidrag (refereegranskat)abstract
- The performance of the Sub Grid Scale models is studied by simulating a separated flow over a wavy channel. The first and second order statistical moments of the resolved velocities obtained by using Large-Eddy simulations at different mesh resolutions are compared with Direct Numerical Simulations data. The effectiveness of modeling the wall stresses by using local log-law is then tested on a relatively coarse grid. The results exhibit a good agreement between highly-resolved Large Eddy Simulations and Direct Numerical Simulations data regardless the Sub Grid Scale models. However, the agreement is less satisfactory with relatively coarse grid without using any wall models and the differences between Sub Grid Scale models are distinguishable. Using local wall model returned the basic flow topology and reduced significantly the differences between the coarse meshed Large-Eddy Simulations and Direct Numerical Simulations data. The results show that the ability of local wall model to predict the separation zone depends strongly on its implementation way.
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| 3. |
- Alsam, A., et al.
(författare)
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Wind-Wave Interaction Effects on a Wind Farm Power Production
- 2017
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Ingår i: Journal of Energy Resources Technology. - : ASME International. - 0195-0738 .- 1528-8994. ; 139:5
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Tidskriftsartikel (refereegranskat)abstract
- In the current study, the effects of the nonlocally generated long sea surface waves (swells) on the power production of a 2 × 2 wind farm are investigated by using large-eddy simulations (LES) and actuator-line method (ALM). The short sea waves are modeled as a roughness height, while the wave-induced stress accounting for swell effects is added as an external source term to the momentum equations. The results show that the marine atmospheric boundary layers (MABLs) obtained in this study have similar characteristics as the MABLs observed during the swell conditions by many other studies. The current results indicate also that swells have significant impacts on the MABL. As a consequence of these changes in the MABL, swells moving faster than the wind and aligned with the local wind direction increase the power extraction rate.
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| 4. |
- Christle, David J., et al.
(författare)
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Isolated Spin Qubits in SiC with a High-Fidelity Infrared Spin-to-Photon Interface
- 2017
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Ingår i: Physical Review X. - : AMER PHYSICAL SOC. - 2160-3308. ; 7:2
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Tidskriftsartikel (refereegranskat)abstract
- The divacancies in SiC are a family of paramagnetic defects that show promise for quantum communication technologies due to their long-lived electron spin coherence and their optical addressability at near-telecom wavelengths. Nonetheless, a high-fidelity spin-photon interface, which is a crucial prerequisite for such technologies, has not yet been demonstrated. Here, we demonstrate that such an interface exists in isolated divacancies in epitaxial films of 3C-SiC and 4H-SiC. Our data show that divacancies in 4H-SiC have minimal undesirable spin mixing, and that the optical linewidths in our current sample are already similar to those of recent remote entanglement demonstrations in other systems. Moreover, we find that 3C-SiC divacancies have a millisecond Hahn-echo spin coherence time, which is among the longest measured in a naturally isotopic solid. The presence of defects with these properties in a commercial semiconductor that can be heteroepitaxially grown as a thin film on Si shows promise for future quantum networks based on SiC defects.
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| 5. |
- Falk, Abram L., et al.
(författare)
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Optical Polarization of Nuclear Spins in Silicon Carbide
- 2015
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 114:24, s. 247603-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate optically pumped dynamic nuclear polarization of Si-29 nuclear spins that are strongly coupled to paramagnetic color centers in 4H- and 6H-SiC. The 99% +/- 1% degree of polarization that we observe at room temperature corresponds to an effective nuclear temperature of 5 mu K. By combining ab initio theory with the experimental identification of the color centers optically excited states, we quantitatively model how the polarization derives from hyperfine-mediated level anticrossings. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging.
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| 6. |
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| 7. |
- Ivády, Viktor, et al.
(författare)
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High-Fidelity Bidirectional Nuclear Qubit Initialization in SiC
- 2016
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 117:22
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic nuclear polarization (DNP) is an attractive method for initializing nuclear spins that are strongly coupled to optically active electron spins because it functions at room temperature and does not require strong magnetic fields. In this Letter, we theoretically demonstrate that DNP, with near-unity polarization efficiency, can be generally realized in weakly coupled electron spin-nuclear spin systems. Furthermore, we theoretically and experimentally show that the nuclear spin polarization can be reversed by magnetic field variations as small as 0.8 Gauss. This mechanism offers new avenues for DNP-based sensors and radio-frequency free control of nuclear qubits.
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| 8. |
- Ivády, Viktor, et al.
(författare)
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Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : AMER PHYSICAL SOC. - 1098-0121 .- 1550-235X. ; 92:11, s. 115206-
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defect electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process.
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| 9. |
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| 10. |
- Nilsson, Johan, et al.
(författare)
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Passive load control in backward-facing step flow by using chevrons
- 2016
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Ingår i: Proceedings of the 5th International Conference on Jets, Wakes and Separated Flows, ICJWSF 2015. - Cham : Springer International Publishing. - 9783319306001 ; 185, s. 223-239
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Konferensbidrag (refereegranskat)abstract
- The ability of chevrons at the top edge of a backward-facing step to reduce downstream surface pressure fluctuations is investigated numerically. Three different chevron configurations are compared against a baseline case without chevrons. Low frequency reduction in the surface pressure fluctuations is observed for two of the configurations. The chevrons do not appear to have a significant effect on the flow as the mean reattachment length for all configurations is nearly constant and there is only a small increase in streamwise turbulence for one configuration with the other configurations unchanged.
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