Flow control of a wind-turbine airfoil with a leading-edge spherical dimple
11 Mar 2022-International Journal of Green Energy-pp 1-19
TL;DR: In this article , the authors incorporated a spherical dimple on the NACA (National Advisory Committee for Aeronautics) 4415 leading edge as a passive flow control device and compared the aerodynamic performances with the plain NACA 4415 airfoil.
Abstract: The flow separation occurred at an early angle of attack (AOA) in airfoil directs the researchers to focus on the methods of flow controlling. The present study incorporated a spherical dimple on the NACA (National Advisory Committee for Aeronautics) 4415 leading edge as a passive flow control device and compared the aerodynamic performances with the plain NACA 4415 airfoil. The dimple diameter (d) was varied from 1% to 6% of the chord length (0.01C-0.06C) to generate four numbers of the modified airfoil. A chord-based Reynolds number (Re) of 2 × 105 was selected for the present study. Shear-Stress Transport (SST) k-ω turbulence model with SIMPLE (semi-implicit method for pressure linked equations) scheme was chosen to solve the present problem computationally in ANSYS FLUENT 14.0. The results showed that the modification helped in delaying stall by 6° at the expense of 0.8% maximum lift coefficient with a dimple diameter of 0.01C. The modified airfoils experienced a primary low-velocity circular zone near the trailing edge and a secondary circulation zone at the dimple edges. In contrast, only one larger circulation zone was present near the plain airfoil trailing edge. The smallest dimple (d = 0.01C) showed a maximum lift enhancement ratio and lift to drag enhancement ratio of 14.8% and 7.86%, respectively.
TL;DR: In this article , the local flexible membrane (LFM) was employed on the suction surface of a Clark-Y airfoil for aerodynamic phenomena including the formation of a laminar separation bubble (LSB) and transition to turbulence.
Abstract: Impact of the local flexible membrane (LFM) on aerodynamic phenomena including the formation of a laminar separation bubble (LSB) and transition to turbulence was experimentally investigated over the suction surface of a Clark-Y airfoil first time in literature. The experiments such as aerodynamic force measurement, smoke-wire flow visualization and hot-film tests were carried out at the free-stream velocity of U∞ = 3.2 m/s, U∞ = 6.4 m/s, U∞ = 9.6 m/s, U∞ = 12.8 m/s, and Reynolds number based upon on the chord length was Rec = 3.5 × 104, Rec = 7.0 × 104, Rec = 1.05 × 105 and Rec = 1.4 × 105, respectively. The experimental angle of attack was set at 0° = α ≤ 20°. In detailed intermittency analysis by the hot-film sensor over the uncontrolled airfoil, it was seen that the LSB and transition to turbulence formed close to the trailing edge at a lower angle of attack, and it moved towards the leading edge when increasing the angle of attack simultaneously. Employing LFM on the suction surface obviously affected the progress of these flow phenomena. In the results of smoke-wire flow visualization, either the size of the laminar separation bubble (LSB) was reduced or its presence was suppressed at lower incidences. The aerodynamic force measurement results also supported those behaviors. In particular, at lower incidences, the negative effects of LSB were mitigated, resulting in the presence of a more stable lift curve. Additionally, it was clearly observed that utilizing LFM ensured positive effects, especially at the pre- and the post-stall regions in terms of fewer fluctuations at the CL curve, meaning that less aerodynamic vibration and noise on wind/hydro turbine could be obtained.
TL;DR: In this article , the authors incorporated protrusions in the leading edge of the NACA 4415 airfoil as a passive flow control measure on the wind turbine blades for a horizontal axis wind turbine (HAWT) to investigate its performance.
Abstract: In the present study, protrusions in the leading edge of the NACA 4415 airfoil were incorporated as a passive flow control measure on the wind turbine blades. Three airfoil models: unmodified leading-edge (ULE), spherical leading-edge protrusion (SLEP), and triangular leading-edge protrusion (TLEP), were investigated experimentally. Thereafter, CFD investigations were carried out using ANSYS 14.0 simulation tool to observe the flow characteristics around the airfoils. Further, LEP-based passive design was applied on a horizontal axis wind turbine (HAWT) to investigate its performance. An amplitude (A) of 1% of the chord length (C) was considered as the height of protrusion, while a distance of 0.25C was maintained between the protrusions to fabricate the experimental models. The study was performed at a low Reynolds number (Re) of 1.5 × 105 for a wide angle of attack (α) of 0°–20°. The experimental results demonstrate that the SLEP model has a higher lift coefficient at α ≥ 18°, whereas the TLEP model performs poorly when compared with the ULE model. The instantaneous lift coefficient plot indicates that the SLEP model generates a more stable force at a higher angle of attack. The computational analysis reveals that a larger primary circulation (extended till 0.2C) is observed in the ULE model at a post-stall angle of attack (α = 18°), indicating early flow separation. Whereas, in the SLEP and TLEP models, it is extended to 0.70C and 0.54C, respectively, indicating the best flow controlling measures achieved by using the SLEP model. The investigations of HAWT rotors with LEP revealed that SLEP HAWT exhibit 8.2% more power coefficient than ULE HAWT.
TL;DR: In this article , the authors analyzed the effect of cooling air technology on the overall propelling performance with or without adjusting cycle parameters and concluded that the improvement of cycle parameters to reduce exergy destruction should be considered when introducing CCA technology.
Abstract: The cooled cooling air technology (CCA technology) shows expected performance in solving the growing thermal challenge for advanced aero engines by reducing the temperature of cooling air. The effect of CCA technology on the overall propelling performance with or without adjusting cycle parameters is controversial. Based on this, both the energy and exergy methods have been adopted to elaborate the specific mechanisms of the above energy utilization discrepancy. As a result, the scheme of CCA technology without optimizing cycle parameters has lower propelling work and efficiency with the total exergy destruction increasing 0.5~2%. Oppositely, as for the scheme of CCA with meliorated cycle parameters, the propelling efficiency improved by around 2~4% with total exergy destruction reduced by 1~3.5%. By analyzing the distribution of exergy destruction, the avoidable and unavoidable exergy destruction caused by the combustion chamber, compressors, and turbines accounts for the largest proportion, which indicates that more attention needs to be paid in the future. During the whole flight mission, the percentage of exergy destruction is much higher in supersonic, subsonic cruise, combat, and escape conditions. In conclusion, the improvement of cycle parameters to reduce the exergy destruction should be considered when introducing CCA technology.
TL;DR: In this paper, two new two-equation eddy-viscosity turbulence models are presented, which combine different elements of existing models that are considered superior to their alternatives.
Abstract: Two new two-equation eddy-viscosity turbulence models will be presented. They combine different elements of existing models that are considered superior to their alternatives. The first model, referred to as the baseline (BSL) model, utilizes the original k-ω model of Wilcox in the inner region of the boundary layer and switches to the standard k-e model in the outer region and in free shear flows. It has a performance similar to the Wilcox model, but avoids that model's strong freestream sensitivity
TL;DR: In this paper, the authors show that the addition of leading-edge tubercles to a scale model of an idealized humpback whale flipper delays the stall angle by approximately 40%, while increasing lift and decreasing drag.
Abstract: The humpback whale (Megaptera novaeangliae) is exceptional among the baleen whales in its ability to undertake acrobatic underwater maneuvers to catch prey. In order to execute these banking and turning maneuvers, humpback whales utilize extremely mobile flippers. The humpback whale flipper is unique because of the presence of large protuberances or tubercles located on the leading edge which gives this surface a scalloped appearance. We show, through wind tunnel measurements, that the addition of leading-edge tubercles to a scale model of an idealized humpback whale flipper delays the stall angle by approximately 40%, while increasing lift and decreasing drag.
TL;DR: In this paper, the authors measured lift, drag, and pitching moments of airfoils with leading-edge sinusoidal protuberances in a water tunnel and compared with those of a baseline 63 4 -021 airfoil.
Abstract: Lift, drag, and pitching moments of airfoils with leading-edge sinusoidal protuberances were measured in a water tunnel and compared with those of a baseline 63 4 -021 airfoil. The amplitude of the leading-edge protuberances ranged from 2.5 to 12% of the mean chord length; the spanwise wavelengths were 25 and 50% of the mean chord length. These ranges correspond to the morphology found on the leading edge of humpback whales' flippers. Flow visualization using tufts was also performed to examine the separation characteristics of the airfoils. For angles of attack less than the baseline stall angle, lift reduction and drag increase were observed for the modified foils. Above this angle, lift of the modified foils was up to 50% greater than the baseline foil with little or no drag penalty. The amplitude of the protuberances had a distinct effect on the performance of the airfoils, whereas the wavelength had little. Flow visualization indicated separated flow originating primarily from the troughs and attached flow on the peaks of the protuberances at angles beyond the stall angle of the baseline foil.
TL;DR: In this paper, the influence of sinusoidal leading-edge protrusions on the performance of two NACA airfoils with different aerodynamic characteristics was investigated and it was found that reducing the tubercle amplitude leads to a higher maximum lift coefficient and larger stall angle.
Abstract: An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a certain point at which further reduction in wavelength has a negative impact on performance. The results also suggest that tubercles act in a manner similar to conventional vortex generators.
TL;DR: In this paper, the authors applied the CIRA RANS flow solver by employing a large set of turbulence models, to typical aerodynamic applications for which certified experimental data are available in literature.
Abstract: In a numerical simulation the choice of a turbulence model must be a compromise between physical modelling and computational cost. The CIRA RANS flow solver has been applied, by employing a large set of turbulence models, to typical aerodynamic applications for which certified experimental data are available in literature. The transonic flows over an airfoil and a wing placed in a wind tunnel, both characterized by a strong shock-boundary layer interaction with an induced separation, and the high lift flow around a multi component airfoil are taken into consideration. An evaluation, in terms of accuracy and numerical behaviour, of some common turbulence models ranging from one-equation to high order, using the same code and numerics, is presented. Satisfactory and consistent results have been achieved; the more sophisticated the turbulence model the more accurate the simulation is. The SST Menter κ – ω turbulence model has shown, for the applications investigated, the best compromise between the physical capabilities and the numerical stiffness.