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Research Papers: Alternative Energy Sources

J. Energy Resour. Technol. 2017;139(5):051201-051201-5. doi:10.1115/1.4035553.

Control of heat transfer is important in wind power systems. In cold climate, the problems of icing and de-icing of the turbine blades need to be handled by efficient heat transfer technologies. Heat-generating components like electric generator, gear box, and frequency converters usually need cooling under operation by various cooling solutions such as air cooling, liquid cooling, and evaporative cooling. This paper reviews heat transfer problems in wind energy systems and presents some existing solutions to manage the thermal issues, and also discusses the challenges and new ideas on finding improved methods to control the heat transfer or cooling. Advanced liquid and evaporative cooling methods are suggested. Also, the need for improved ice sensors is discussed particularly for the ice accretion on the turbine blades.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051202-051202-12. doi:10.1115/1.4035754.

Diffuser-augmented wind turbines (DAWTs) can significantly increase the performance of the rotor. Multirotor systems (MRSs) have a lot of merits such as significant saving mass and overall cost of the wind turbine system. A MRS is defined as containing more than one rotor in a single structure. In the present research, DAWTs are studied in a MRS. In wind tunnel experiments, the power output and aerodynamics of two and three DAWTs placed in close vicinity, in side-by-side arrangements, have been investigated, along with circular disks and conventional wind turbines in the same configurations as the MRS. Results show a significant increase of up to 12% in total power output of the MRS with DAWTs compared to the sum of the stand-alone same turbines. The results can be explained by observing the bluff body flow phenomena in the wake interference around the multiple circular disks. Those flow phenomena are due to the accelerated gap flows and those biasing in the flow direction caused by the vortex interactions in the gap.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051203-051203-16. doi:10.1115/1.4035753.

A free-vortex-wake aeroelastic framework evaluates the impact of blade elasticity on the near-wake formation and its linear stability for onshore and offshore configurations of the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine. Numerical results show that motion of the flexible rotor further destabilizes its tip-vortices through earlier onset of mutual inductance relative to the rigid rotor results for onshore and offshore turbines. The near-wake growth rate is demonstrated to depend on the azimuthal position of the rotor for all cases considered, which appears to not have been reported previously for wake stability analyses in the rotorcraft literature.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051204-051204-8. doi:10.1115/1.4036047.

Feasibility of increasing lift and decreasing drag by drilling narrow span-wide channels near the leading edge of NACA 4412 airfoils is investigated. It is proposed to drill two-segment slots that allow some of the incoming air to flow through them and then exit from the bottom surface of the airfoil. Such slots can result in an increased local pressure and thereby higher lift. Length, width, inlet angle, and exit angle of slots are varied to determine optimum configurations. Aerodynamic performance at different angles of attack (AoAs) and the chord-based Reynolds number of 1.6 × 106 is investigated. It is concluded that longer and narrower slots with exit streams more aligned with the air flowing below the airfoil can result in a higher lift. Also, in order to keep the slotted airfoils beneficial for AoAs greater than zero, it is proposed to (a) slightly lower the slot position with respect to the original design and (b) tilt up the first-leg by a few degrees. For the best design case considered, an average improvement of 8% is observed for lift coefficient over the entire range of AoA (with the maximum increase of 15% for AoA = 0), without any significant drag penalty.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051205-051205-10. doi:10.1115/1.4036048.

In this work, experimental measurements are made to study wind turbines over complex terrains and in presence of the atmospheric boundary layer. Thrust and power coefficients for single and multiple turbines are measured when introducing sinusoidal hills and spires inducing an artificial atmospheric boundary layer. Additionally, wake interaction effects are studied, and inflow velocity profiles are characterized using hot-wire anemometry. The results indicate that the introduced hills have a positive impact on the wind-turbine performance and that wake-interaction effects are significantly reduced during turbulent inflow conditions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051206-051206-9. doi:10.1115/1.4036050.

Turbulent air flow over an NACA 4412 airfoil is investigated computationally. To overcome the near-wall inaccuracies of higher order turbulence models such as large Eddy simulation (LES) and detached Eddy simulation (DES), it is proposed to couple DES with algebraic stress model (ASM). Angles of attack (AoA) of 0 and 14 deg are studied for an airfoil subjected to flow with Re = 1.6 × 106. Distribution of the pressure coefficient at airfoil surface and the chordwise velocity component at four locations near the trailing edge are determined. Results of the baseline DES and hybrid DES–ASM models are compared against published data. It is demonstrated that the proposed hybrid model can slightly improve the flow predictions made by the DES model. Findings of this research can be used for the improvement of the near-wall flow predictions for wind turbine applications.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051207-051207-19. doi:10.1115/1.4036051.

The advancement of wind energy as an alternative source to hydrocarbons depends heavily on research activities in turbulence modeling and experimentation. The velocity deficit behind wind turbines affects the power output and efficiency of a wind farm. Being able to simulate the wake dynamics of a wind turbine effectively can result in optimum spacing, longer wind turbine life, and shorter payback on the wind farm investment. Two-equation turbulence closure models, such as k–ε and k–ω, are used extensively to predict wind turbine performance and velocity deficit profiles. The application of the Reynolds stress model (RSM) turbulence closure method has been limited to few studies where the rotor is modeled as an actuator disk (AD). The computational cost associated with RSM has made it challenging for simulations where the rotor is discretized directly; however, with advances in computer speed and power coupled with parallel computing architecture, RSM may be a better turbulence closure option. In this research, wind tunnel experiments were conducted, using hot-wire anemometry, to measure the velocity deficit profiles at different wake locations behind a small-scale, three-bladed, horizontal-axis wind turbine (HAWT). Experiments were also performed with two and three HAWTs in series to evaluate the change in velocity deficit and turbulence intensity (TI). High-speed imaging with an oil-based mist captured the vortices produced at the blade tips and showed the vortices dissipated approximately three rotor diameters downstream. Computational fluid dynamics (CFD) simulations were performed to predict the velocity deficit at wake locations matching the experiments. The Reynolds stress model was applied to a fully discretized rotor with a tower and nacelle included in the simulation. A steady-state moving reference frame (MRF) model was created with the computational domain subdivided into rotating and stationary domains. The MRF results were used as an initial condition for time-accurate rigid body motion (RBM) simulations. The RBM CFD simulations showed excellent agreement with experimental measurements for velocity deficit after properly accounting for experimental boundary effects. Isosurfaces of the Q-criterion highlighted the vortices produced at the blade tips and were consistent with high-speed images.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051208-051208-7. doi:10.1115/1.4036052.

Development of high bending stresses due to a sudden gust of wind is a significant cause for the failure of wind turbine blades. Self-healing provides a fool proof safety measure against catastrophic failure by healing the damages autonomously, as they originate. In this study, biomimetic, vascular channel type of self-healing was implemented in glass fiber reinforced polymer matrix composite that is used in wind turbine blades. Microscale borosilicate tubes are used to supply the healing agent to the epoxy type of thermoset polymer matrix, and the healing was very effective. However, 25% decrease in tensile strength and 9% decrease in three-point bending flexural strength were imminent with the inclusion of a single layer of vascular vessels in the composite material. Three-point bending tests were performed before and after self-healing of flat specimens to find the extent of recovery of flexural strength on using vascular channel type of self-healing. An average recovery of flexural strength of 84.52% was obtained using a single layer of vascular vessels on the tensile stress side of three-point bending. Breakage and bleeding of the healing agent within the composite specimens during three-point bending tests were observed in real-time. Based on the encouraging findings, the above self-healing feature was successfully implemented in a prototype wind turbine.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051209-051209-8. doi:10.1115/1.4036177.

The objectives of this study are to reconstruct a turbulence model of both the large Eddy simulation (LES) and the Reynolds-averaged Navier–Stokes simulation (RANS) which can predict wind synopsis in various thermally stratified turbulent boundary layers over any obstacles. Hence, the direct numerical simulation (DNS) of various thermally stratified turbulent boundary layers with/without forward-step, two-dimensional block, or two-dimensional hill is carried out in order to obtain detailed turbulent statistics for the construction of a database for the evaluation of a turbulence model. Also, DNS clearly reveals the characteristics of various thermally stratified turbulent boundary layers with/without forward-step, two-dimensional block, or two-dimensional hill. The turbulence models employed in LES and RANS are evaluated using the DNS database we obtained. In the LES, an evaluated turbulence model gives proper predictions, but the quantitative agreement of Reynolds shear stress with DNS results is difficult to predict. On the other hand, the nonlinear eddy diffusivity turbulence models for Reynolds stress and turbulent heat flux are also evaluated using DNS results of various thermally stratified turbulent boundary layers over a forward-step in which the turbulence models are evaluated using an a priori method. Although the evaluated models do not make it easy to properly predict the Reynolds shear stresses in all cases, the turbulent heat fluxes can be qualitatively predicted by the nonlinear eddy diffusivity for a heat turbulence model. Therefore, the turbulence models of LES and RANS should be improved in order to adequately predict various thermally stratified turbulent boundary layers over an obstacle.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051210-051210-6. doi:10.1115/1.4036250.

In the current experiments, two identical wind turbine models were placed in uniform flow conditions in a water flume. The initial flow in the flume was subject to a very low turbulence level, limiting the influence of external disturbances on the development of the inherent wake instability. Both rotors are three-bladed and designed using blade element/lifting line (BE/LL) optimum theory at a tip-speed ratio, λ, of 5 with a constant design lift coefficient along the span, CL = 0.8. Measurements of the rotor characteristics were conducted by strain sensors installed in the rotor mounting. The resulting power capacity has been studied and analyzed at different rotor positions and a range of tip-speed ratios from 2 to 8, and a simple algebraic relationship between the velocity deficit in the wake of the front turbine and the power of the second turbine was found, when both rotors have the coaxial position.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2017;139(5):051211-051211-9. doi:10.1115/1.4036329.

Wind turbines operating in cold climate are susceptible to icing events. In order to gain a better understanding of the blade icing, the water droplets local collection efficiency affected by different factors was investigated. First, the water droplets conservation equations which are based on the fluent user-defined scalar (UDS) were introduced. Second, the Eulerian method was validated. Two test cases indicate that the developed method is effective. Then, the local collection efficiency on the S809 airfoil was studied. Results show that as the angle of attack (AOA) increases, the water droplets impingement region moves toward the airfoil lower surface and the maximum local collection efficiency decreases. The local collection efficiency and the impingement region increase with the water droplets diameter and the air flow velocity but decrease with the airfoil chord length. Finally, the local collection efficiency affected by the three-dimensional (3D) effect was studied. Results show that the maximum local collection efficiency in the blade tip region decreases up to 96.29% due to the 3D effect.

Commentary by Dr. Valentin Fuster

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