| 1. |
- Szász, Robert-Zoltán, et al.
(författare)
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Algorithm for automatic quantification of flashback and flash forward events from high-speed chemiluminescence recordings
- 2016
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Ingår i: Springer Proceedings in Physics. - Cham : Springer Science+Business Media B.V.. - 9783319306001 - 9783319306025 ; , s. 519-526
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Konferensbidrag (refereegranskat)abstract
- Three methods are employed to identify and quantify flashback and flash forward events based on chemiluminescence recordings of swirling flames. The approaches differ in the procedure to determine the instantaneous flame position. The results revealed that the most robust method is to determine a threshold relative to the instantaneous maximum intensity. Analysis of the complete dataset indicated that flashback events are significantly slower than flash forward events.
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| 2. |
- 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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| 3. |
- 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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| 4. |
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| 5. |
- Al Sam, Ali, et al.
(författare)
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The Influence of Sea Waves on Offshore Wind Turbine Aerodynamics
- 2015
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Ingår i: Journal of Energy Resources Technology. - : ASME International. - 1528-8994 .- 0195-0738. ; 137:5
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Tidskriftsartikel (refereegranskat)abstract
- The impacts of swells on the atmospheric boundary layer (ABL) flows and by this on the standalone offshore wind turbine (WT) performance are investigated by using large eddy simulations (LES) and actuator-line techniques. At high swell to wind speed ratio, the swell-induced stress reduces the total wind stress resulting in higher wind velocity, less wind shear, and lower turbulence intensity level. These effects increase by increasing swell to wind speed ratio (C/U) and/or swell steepness. Moreover, for the same hub-height wind speed (U-hub), the presence of swells increases the turbine power extraction rate by about 3% and 8.4% for C/U-hub = 1.53 and 2.17, respectively.
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| 6. |
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| 7. |
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| 8. |
- Grosshans, Holger, et al.
(författare)
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Computational sensitivity study of spray dispersion and mixing on the fuel properties in a gas turbine combustor
- 2017
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Ingår i: Fluid Dynamics Research. - : Institute of Physics Publishing (IOPP). - 0169-5983 .- 1873-7005. ; 49:2
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Tidskriftsartikel (refereegranskat)abstract
- A swirl stabilized gas turbine burner has been simulated in order to assess the effects of the fuel properties on spray dispersion and fuel-air mixing. The properties under consideration include fuel surface tension, viscosity and density. The turbulence of the gas phase is modeled applying the methodology of large eddy simulation whereas the dispersed liquid phase is described by Lagrangian particle tracking. The exchange of mass, momentum and energy between the two phases is accounted for by two-way coupling. Bag and stripping breakup regimes are considered for secondary droplet breakup, using the Reitz-Diwakar and the Taylor analogy breakup models. Moreover, a model for droplet evaporation is included. The results reveal a high sensitivity of the spray structure to variations of all investigated parameters. In particular, a decrease in the surface tension or the fuel viscosity, or an increase in the fuel density, lead to less stable liquid structures. As a consequence, smaller droplets are generated and the overall spray surface area increases, leading to faster evaporation and mixing. Furthermore, with the trajectories of the small droplets being strongly influenced by aerodynamic forces (and less by their own inertia), the spray is more affected by the turbulent structures of the gaseous phase and the spray dispersion is enhanced.
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| 9. |
- Grosshans, Holger, et al.
(författare)
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Enhanced liquid-gas mixing due to pulsating injection
- 2015
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Ingår i: Computers & Fluids. - : Elsevier BV. - 0045-7930 .- 1879-0747. ; 107, s. 196-204
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers the effects of intermittent injection of a liquid jet or spray on the initial break-up and mixing of one fluid with the surrounding ambient fluid. The aim of the analysis is to describe the physical process and indicate the mechanisms that control the mixing under different flow conditions (time-dependent injection and its frequency relative to the time scales of the flow) and fluid properties (density ratio), Schmidt number for a single phase case which is studied for comparison, or the Weber number for the two-phase cases. The computations use Large Eddy Simulation (LES) to account for turbulence, and either Volume Of Fluid (VOF) for the initial break-up or Lagrangian Particle Tracking (LPT) with droplet break-up model in the case of liquid droplets injected into the ambient gas. The results show that, depending on the physical properties of the liquid and ambient gas, the initial break-up and turbulent mixing can be enhanced substantially with intermittent injection. The numerical modeling is validated by recovering key results of experimental and analytical works. It can be observed that a main effect during the mixing is the suction of ambient fluid at the tail of the injected liquid, which depends on the fluid properties. Increased injection frequency shows to increase the mixing significantly during the initial transient phase.
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| 10. |
- Grosshans, Holger, et al.
(författare)
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Influence of the helicopter configuration on its electrostatic charging
- 2017
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Ingår i: 23rd AIAA Computational Fluid Dynamics Conference, 2017. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624105067
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Konferensbidrag (refereegranskat)abstract
- A helicopter flying through an atmosphere containing particulates may accumulate high electrostatic charges which can challenge its operational safety. In this paper we report on the in uence of the helicopter configuration on its electrification. Our study is based on a recently developed numerical approach according to which the turbulent air flow around the rotorcraft is estimated via large eddy simulations while the particulate flow is computed via Lagrangian particle tracking. Also, this approach incorporates a model for the triboelectric charge transfer during particle-helicopter collisions that is brie y described herein. The configurations that we examined in our study include rotor systems of two different sizes equipped with two, three or four blades. Our results reveal that a helicopter with fewer blades accumulates less electric current even though the charge on each individual blade is higher. Further, the location of the charge build-up on the rotor disk depends strongly on the number of blades. Also, according to our computations, a reduction of the rotor size leads to a reduction of its electrification, if all other parameters are kept constant.
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