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The behaviors of unsteady flow structures and corresponding hydrodynamics for a pitching hydrofoil are investigated numerically and theoretically in the present paper. The aims are to derive the total lift by finite-domain impulse theory for sub-cavitating flow (σ = 8.0) and cavitating flow (σ = 3.0), and to quantify the distinct impact of individual vortex structures on the transient lift to appreciate the interplay among cavitation, flow structures, and vortex dynamics. The motion of the hydrofoil is set to pitch up clockwise with an almost constant rate from 0° to 15° and then back to 0°, for the Reynolds number, 7.5 × 105, and the frequency, 0.2 Hz, respectively. The results reveal that the presence of cavities delays the migration of the laminar separation bubble (LSB) from the trailing edge (TE) to the leading edge (LE), consequently postponing the hysteresis in the inflection of lift coefficients. The eventual stall under the sub-cavitation regime is the result of LSB bursting. While the instabilities within the leading-edge LSB induce the convection of cavitation-dominated vortices under the cavitation regime instead. Having validated the lift coefficients on the hydrofoil through the finite-domain impulse theory using the standard force expression, the Lamb vector integral emerges as the main contribution to the generation of unsteady lift. Moreover, the typical vortices’ contributions to the transient lift during dynamic stall are accurately quantified. The analysis indicates that the clockwise leading-edge vortex (−LEV) contributes positively, while the counterclockwise trailing-edge vortex (+TEV) contributes negatively. The negative influence becomes particularly pronounced after reaching the peak of total lift, as the shedding of the concentrated wake vortex precipitates a sharp decline due to a predominant negative lift contribution from the TEV region. Generally, the vortices’ contribution is relatively modest in sub-cavitating flow, but it is notably more significant in the context of incipient cavitating flow. [ABSTRACT FROM AUTHOR]

The aerodynamic and aeroacoustic performance of a low-aspect-ratio ( AR = 0.2 ) pitching foil during dynamic stall are investigated numerically with focus on the effects of trailing edge serrations. A hybrid method coupling an immersed boundary method for incompressible flows with the Ffowcs Williams–Hawkings acoustic analogy is employed. Large eddy simulation and turbulent boundary layer equation wall model are also employed to capture the turbulent effects. A modified NACA0012 foil with a rectangular trailing edge flap attached to the trailing edge (baseline case) undergoing pitching motion is considered. Trailing edge serrations are applied to the trailing edge flap and their effects on the aerodynamic and aeroacoustic performance of the oscillating airfoil are considered by varying the wave amplitude ( 2 h ∗ = 0.05 , 0.1 , and 0.2) at a Reynolds number of 100,000 and a Mach number of 0.05. It is found that the reduction of the sound pressure level at the dimensionless frequency band S t b ∈ [ 1.25 , 4 ] can be over 4 dB with the presence of the trailing edge serrations ( 2 h ∗ = 0.1 ), while the aerodynamic performance and its fluctuations are not significantly altered except a reduction around 10% in the negative moment coefficient and it fluctuations. This is due to the reduction of the average spanwise coherence function and the average surface pressure with respect to that of the baseline case, suggesting the reduction of the spanwise coherence and the noise source may result in the noise reduction. Analysis of the topology of the near wake coherent structure for 2 h ∗ = 0.1 reveals that the alignment of the streamwise-oriented vortex with the serration edge may reduce the surface pressure fluctuation. [ABSTRACT FROM AUTHOR]

Dynamic stall is a serious phenomenon that restricts aeronautical vehicles' maneuverability. It happens on the blades as their angle of attack increase, especially on the blade of wind turbines. In this paper, two airfoils are investigated in actual conditions. To alleviate dynamic stall the geometries of the airfoils are modified by drooping and rounded leading-edge tip method and also combination of both. To study the effect of the modifications, the unsteady flow fields around the pitching airfoils, numerically simulated using URANS. Results illustrates, in addition to alleviate dynamic stall, the methods enhance aerodynamic characteristics by reducing drag force and preventing sudden jump of lift force, especially at the maximum angle of attack. Finally, a detailed investigation is conducted on the flow behavior around the airfoil to discover the supporting physics behind the improvements made by the present methods. Which revealed that these two passive methods, aim to prevent the formation of leading-edge vortex on the airfoil which finally delays the dynamic stall. [ABSTRACT FROM AUTHOR]

Abstract This study investigates the effects of positive horizontal acceleration of the freestream velocity on a pitch‐oscillating VR‐12 airfoil using computational fluid dynamics. The shear stress transport k–ω model, coupled with a low‐Reynolds number correction, was employed for Re

Abstract There is a growing demand to improve the performance of vertical axis wind turbines to facilitate their commercialization for application in urban areas. This study utilizes a 3D numerical analysis to examine the influence of different vortices generated on turbine efficiency with straight and J‐type blades. The numerical simulation of this study employs the Reynolds‐Averaged Navier–Stokes equations and ‎sliding ‎mesh techniques ‎to more accurately model the rotational motion of blades about the turbine axis in relation to the ‎wind. Comparing the output ‎torque and the flow field at different span‐wise sections, the J‐type blades achieve better ‎performance at mid‐spans where the effect of stall vortices is dominant. Conversely, the lower ‎performance of J‐type blades is seen at tip spans due to stronger tip vortices. Investigations also ‎reveal the criticality of the downwind region on the overall performance at high tip speed ratios. It is observed that by ‎increasing the height, the tip vortices are limited to the tip sections, and stall vortices expand further ‎along the blade. At TSR = 1, the improvement by J‐type blades rises from 10% at a height of 0.8 m to 44% ‎at 3 m. The growth in height at lower wind speeds becomes more beneficial. Compared to the straight blades, the self‐starting ‎generated torque by J‐type blades for heights of 0.8, 1.2, and 1.6 m, are improved by 15.6%, ‎‎26.9%, and 34.7%, respectively. Overall, it is concluded that by increasing the blade height, the superiority ‎of the J‐type blade becomes more noticeable as the blade mainly contributes to suppressing the stall ‎vortices effect where the tip vortices effect is not presented. ‎

ABSTRACT Three unsteady aerodynamic tools at different levels of fidelity and computational cost were used to investigate the unsteady aerodynamic behavior of a delta kite applied to airborne wind energy. The first tool is an in‐house unsteady panel method that is fast but delivers low to mid fidelity predictions. The second tool uses the open‐source CFD code SU2 to solve the unsteady Reynolds‐averaged Navier–Stokes equations with the k−ω SST turbulence model. At an intermediate level of fidelity, a semiempirical dynamic stall model that combines the panel method with a phenomenological dynamic stall module is proposed. The latter has free parameters that are fine‐tuned with CFD results from the second tool. The research on the dynamic stall model has been inspired by two flight test campaigns suggesting dynamic stall phenomena possibly driven by the periodic variation of the angle of attack (aerodynamic pitching motion) during crosswind maneuvers. The recorded inflow along the flight path was prescribed in the three aerodynamic tools. As expected, the price to pay for the low computational cost of the panel method is its inability to capture the dynamic stall phenomenon. The results from unsteady CFD qualitatively matched the experimental data identifying a leading‐edge vortex that forms and detaches cyclically during the pitching motion. Using RANS data, the semiempirical tool was fined‐tuned to reproduce the dynamic stall behavior, becoming an accurate and fast aerodynamic tool for coupling with any kite flight simulator. Further discussions on the effects of kite aerostructural deflections are included.

Abstract In view of the fact that dynamic stall models in the wind energy industry such as ONERA model, Beddoes–Leishman model, and Snel model are mostly semi‐empirical models, and the determination of empirical time constants has a great influence on the model accuracy. To optimize the time constant in dynamic stall model and improve the prediction accuracy of unsteady aerodynamics, the influence of boundary layer and pressure lag on the unsteady performance of the S809 airfoil under 2D flow conditions is explored using a simple semi‐empirical dynamic stall engineering model. The proposed model consists of four first‐order differential equations accounting for attached flow and dynamic separation flow of trailing edge based on the Theodorsen theory. A validation is carried out by the wind tunnel experiment in the Key Laboratory of Wind and Solar Energy Utilization Technology of the Ministry of Education at Inner Mongolia University of Technology. The main conclusions are as follows. The time constants for lag in pressure and boundary layer both have a great influence on the unsteady lift coefficient. When the mean angle of attack is relatively small and the airflow is between the attached flow and the separated flow, appropriately reducing the time constant can make the prediction results closer to the experimental values. When the mean angle of attack is relatively large and the airflow is in condition of fully separated flow, the time constant value can be appropriately increased. The influence of pressure lag and boundary layer lag on the unsteady drag coefficient is not significant.

This study investigates the effects of positive horizontal acceleration of the freestream velocity on a pitch‐oscillating VR‐12 airfoil using computational fluid dynamics. The shear stress transport k–ω model, coupled with a low‐Reynolds number correction, was employed for Re <105 during dynamic stall. The flow equations were solved in two‐dimensional, incompressible form using the finite volume method. The study examined various parameters, including positive acceleration values of the inflow and the angle of attack of the airfoil, to determine their impact on lift and drag coefficients, as well as the Cl/Cd ratio. Additionally, the maximum lift coefficient was analyzed under different inflow and airfoil motion conditions. The results indicate that aerodynamic force coefficients and the Cl/Cd ratio are influenced by both the attack angle and the acceleration of the inflow. Furthermore, inflow acceleration affects the onset of dynamic stall conditions. Generally, inflow acceleration modifies the lift coefficient of the airfoil during the upstroke, while having minimal effect on the drag coefficient, except near dynamic stall points. The findings also suggest that, for a specific airfoil, the sequence of factors with the greatest influence on lift force generation before static stall occurs is as follows: asymmetric airfoil oscillation, symmetrical airfoil oscillation, accelerated inflow, constant velocity inflow, and static airfoil. [ABSTRACT FROM AUTHOR]

There is a growing demand to improve the performance of vertical axis wind turbines to facilitate their commercialization for application in urban areas. This study utilizes a 3D numerical analysis to examine the influence of different vortices generated on turbine efficiency with straight and J‐type blades. The numerical simulation of this study employs the Reynolds‐Averaged Navier–Stokes equations and ‎sliding ‎mesh techniques ‎to more accurately model the rotational motion of blades about the turbine axis in relation to the ‎wind. Comparing the output ‎torque and the flow field at different span‐wise sections, the J‐type blades achieve better ‎performance at mid‐spans where the effect of stall vortices is dominant. Conversely, the lower ‎performance of J‐type blades is seen at tip spans due to stronger tip vortices. Investigations also ‎reveal the criticality of the downwind region on the overall performance at high tip speed ratios. It is observed that by ‎increasing the height, the tip vortices are limited to the tip sections, and stall vortices expand further ‎along the blade. At TSR = 1, the improvement by J‐type blades rises from 10% at a height of 0.8 m to 44% ‎at 3 m. The growth in height at lower wind speeds becomes more beneficial. Compared to the straight blades, the self‐starting ‎generated torque by J‐type blades for heights of 0.8, 1.2, and 1.6 m, are improved by 15.6%, ‎‎26.9%, and 34.7%, respectively. Overall, it is concluded that by increasing the blade height, the superiority ‎of the J‐type blade becomes more noticeable as the blade mainly contributes to suppressing the stall ‎vortices effect where the tip vortices effect is not presented. ‎ [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Applied Mechanics is the property of Chinese Journal of Applied Mechanics Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Three unsteady aerodynamic tools at different levels of fidelity and computational cost were used to investigate the unsteady aerodynamic behavior of a delta kite applied to airborne wind energy. The first tool is an in‐house unsteady panel method that is fast but delivers low to mid fidelity predictions. The second tool uses the open‐source CFD code SU2 to solve the unsteady Reynolds‐averaged Navier–Stokes equations with the k−ω$$ k-\omega $$ SST turbulence model. At an intermediate level of fidelity, a semiempirical dynamic stall model that combines the panel method with a phenomenological dynamic stall module is proposed. The latter has free parameters that are fine‐tuned with CFD results from the second tool. The research on the dynamic stall model has been inspired by two flight test campaigns suggesting dynamic stall phenomena possibly driven by the periodic variation of the angle of attack (aerodynamic pitching motion) during crosswind maneuvers. The recorded inflow along the flight path was prescribed in the three aerodynamic tools. As expected, the price to pay for the low computational cost of the panel method is its inability to capture the dynamic stall phenomenon. The results from unsteady CFD qualitatively matched the experimental data identifying a leading‐edge vortex that forms and detaches cyclically during the pitching motion. Using RANS data, the semiempirical tool was fined‐tuned to reproduce the dynamic stall behavior, becoming an accurate and fast aerodynamic tool for coupling with any kite flight simulator. Further discussions on the effects of kite aerostructural deflections are included. [ABSTRACT FROM AUTHOR]

In view of the fact that dynamic stall models in the wind energy industry such as ONERA model, Beddoes–Leishman model, and Snel model are mostly semi‐empirical models, and the determination of empirical time constants has a great influence on the model accuracy. To optimize the time constant in dynamic stall model and improve the prediction accuracy of unsteady aerodynamics, the influence of boundary layer and pressure lag on the unsteady performance of the S809 airfoil under 2D flow conditions is explored using a simple semi‐empirical dynamic stall engineering model. The proposed model consists of four first‐order differential equations accounting for attached flow and dynamic separation flow of trailing edge based on the Theodorsen theory. A validation is carried out by the wind tunnel experiment in the Key Laboratory of Wind and Solar Energy Utilization Technology of the Ministry of Education at Inner Mongolia University of Technology. The main conclusions are as follows. The time constants for lag in pressure and boundary layer both have a great influence on the unsteady lift coefficient. When the mean angle of attack is relatively small and the airflow is between the attached flow and the separated flow, appropriately reducing the time constant can make the prediction results closer to the experimental values. When the mean angle of attack is relatively large and the airflow is in condition of fully separated flow, the time constant value can be appropriately increased. The influence of pressure lag and boundary layer lag on the unsteady drag coefficient is not significant. [ABSTRACT FROM AUTHOR]

The blade profile selection is paramount for the efficient operation of straight Darrieus wind turbines in terms of torque and power generation. In this work, we have used the Kline-Fogleman Airfoil (KFA) design for the wind turbine blades. The concept of KFA design aims to cause flow separation, vortex formation, and reattachment establishment before the trailing edge. Thus, geometric tests on have been performed on the baseline airfoil NACA0015 as one of the best profiles for operating a straight Darrius wind turbine. A twodimensional Computational Fluid Dynamic (CFD) model using the twoequation Shear Stress Transport k-ω (SST k-ω) turbulent model was developed in ANSYS/FLUENT software to assess the aerodynamic efficiency of the modified airfoil. Two designs (KFA-2 and KFA-4) were tested initially in the static case. The effects of the opening step angle and its curvature diameter were studied for an angle of attack's range of -20° to +20°. The rounded KFA-4 design with an opening step angle of 93.6° led to a significant improvement in the lift-to-drag ratio thus, aerodynamic efficiency. Finally, the straight KFA-4 design with the opening step angle of 93.6° revealed a the most advantageous effects on the operation of a straight Darrieus wind turbine for a Tip Speed Ratio less than 1.6 (TSR<1.6). It allowed a noticeable reduction of the dead zone and TSR corresponding to the nominal power, thus consequently improving the starting torque and delaying torque stall. [ABSTRACT FROM AUTHOR]

Copyright of Transactions of Nanjing University of Aeronautics & Astronautics is the property of Editorial Department of Journal of Nanjing University of Aeronautics & Astronautics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The blade profile selection is paramount for the efficient operation of straight Darrieus wind turbines in terms of torque and power generation. In this work, we have used the Kline-Fogleman Airfoil (KFA) design for the wind turbine blades. The concept of KFA design aims to cause flow separation, vortex formation, and reattachment establishment before the trailing edge. Thus, geometric tests on have been performed on the baseline airfoil NACA0015 as one of the best profiles for operating a straight Darrius wind turbine. A two-dimensional Computational Fluid Dynamic (CFD) model using the two-equation Shear Stress Transport k-ω (SST k-ω) turbulent model was developed in ANSYS/FLUENT software to assess the aerodynamic efficiency of the modified airfoil. Two designs (KFA-2 and KFA-4) were tested initially in the static case. The effects of the opening step angle and its curvature diameter were studied for an angle of attack’s range of -20° to +20°. The rounded KFA-4 design with an opening step angle of 93.6° led to a significant improvement in the lift-to-drag ratio thus, aerodynamic efficiency. Finally, the straight KFA-4 design with the opening step angle of 93.6° revealed a the most advantageous effects on the operation of a straight Darrieus wind turbine for a Tip Speed Ratio less than 1.6 (TSR

The nonlinear post-flutter aeroelastic behavior of a classical pitch-and-plunge airfoil model in low-speed wind tunnel bench tests is reported in this study for a range of airflow speeds where stable oscillations are observed. An experimental airfoil prototype is designed, characterized and evaluated. Time domain data of the airfoil motion as well as other pertinent frequency and bifurcation characteristics are presented for different values of airflow speed, starting at the critical linear flutter speed of the airfoil model and increasing up to the sudden manifestation of violent unstable oscillations (when the test is interrupted for the safety of the structural apparatus). Stable post-flutter nonlinear oscillations, mainly attributed to the dynamic stall phenomenon and in a lesser degree to hardening structural effects, are observed for a range of airflow speeds starting at the neutral stability boundary of the aeroelastic system. The amplitudes of oscillation increase with increasing airflow speed and settle onto a limit-cycle. The coupled frequency of oscillation is dominated by the plunge degree-of-freedom and also increases with increasing airflow speed. The observed critical airfoil cut-in speed of limit-cycle onset is about 8.1 m/s (reduced speed of 5.1), and the observed cut-out speed of unstable response is about 9.5 m/s (reduced speed of 6.0). This work contributes with the literature of Aeroelasticity by presenting the realization, evaluation, and wind tunnel test data of a pitch-and-plunge airfoil model undergoing nonlinear post-flutter oscillations that may be useful to support other studies for verification purposes of eventual numerical simulations of similar aeroelastic systems. [ABSTRACT FROM AUTHOR]

This study investigated unsteady pressure measurements on a pitching OA309 airfoil at a Mach number of 0.1 using a fast-responding pressure-sensitive paint (fast PSP). Two commonly used data acquisition methods applicable to fast PSPs, namely the real-time intensity-based method and the single-shot lifetime-based method, were separately used to obtain the pressure distributions on the upper surface at a reduced pitching frequency (k = πfc/U∞) of 0.074. The signal-to-noise ratio, influences of model motion, and temperature-induced errors associated with the two methods were compared to explore the advantages and disadvantages of the methods. The real-time intensity-based method outperformed the single-shot lifetime-based method in pressure measurements on moving models with very low speeds. Flow separation and reattachment were identified according to the temporal- and spatial-resolved pressure fields obtained through the real-time intensity-based method; finally, the effects of the pitching amplitude and the leading-edge vortex generators were studied. The results showed that flow separation was postponed as the pitching amplitude increased, while flow reattachment occurred earlier on the airfoil equipped with leading-edge vortex generators. [ABSTRACT FROM AUTHOR]

This paper studies a rotor dynamic stall control method using an alternating current dielectric barrier discharge (AC DBD) plasma actuator through numerical simulation methods. The flow field evolution during a dynamic stalling process under the excitation of AC DBD plasma discharge is analyzed using the two-dimensional Reynolds time-averaged (RANS) method. The impact of the AC DBD plasma discharge on the flow field is then simulated using the phenomenological method. The influence of the position and intensity of the plasma excitation on the static stall characteristics of the NACA0012 airfoil is also studied. Deformed mesh and dynamic mesh techniques are used to simulate an aerodynamic environment with variable incoming flow and variable angles of attraction on a rotor airfoil. The application of AC DBD plasma excitation for controlling mild and deep dynamic stalls of rotor blades is investigated. The obtained results show that the AC DBD plasma excitation accelerated the evolution and shedding of dynamic stall vortices and facilitated the reattachment of airflow. The application of plasma excitation allowed for significantly increasing the static stall angle of the airfoil and improving the lift coefficient. In addition, the intensity of the plasma excitation is a key factor affecting the control. Moreover, the application of AC DBD plasma excitation for rotor dynamic stalls allowed for reducing the size of the dynamic stall vortex, which helped mitigate the aerodynamic hysteresis effect caused by the dynamic stall and accelerated the recovery from aerodynamic forces. [ABSTRACT FROM AUTHOR]

Computational simulations of three-dimensional flow around a NACA 0018 wing with an aspect ratio of A R = 5 were carried out by using the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations with the Shear-Stress Transport turbulence model closure. Simulations were performed to capture aerodynamic stall hysteresis by using the developed pseudo-transient continuation (PTC) method based on a dual-time step approach in CFD OpenFOAM code. The flow was characterized by incompressible Mach number M = 0.12 and moderate Reynolds number R e = 0.67 × 10 6 . The results obtained indicate the presence of noticeable aerodynamic hysteresis in the static dependencies of the force and moment coefficients, as well as the manifestation of bi-stable flow separation patterns, accompanied by the development of asymmetry in the stall zone. The URANS simulation results are in good agreement with the experimental data obtained for the NACA 0018 finite-aspect-ratio wing in the low-speed wind tunnel under the same test conditions. A new phenomenological bifurcation model of aerodynamic stall hysteresis under static and dynamic conditions is formulated and is proven to be able to closely match the experimental data. [ABSTRACT FROM AUTHOR]

Abstract Vortex generators (VGs) have been widely applied to wind turbines thanks to their potential to increase aerodynamic performance. Due to the complex inflow perceived by a rotor and the proneness to flow separation, VGs on wind turbines usually experience highly unsteady flow. While there are models that exist to simulate the steady effects of VGs, we lack a fast and efficient tool to model the unsteady performance of airfoils equipped with VGs. This paper adopts an unsteady double‐wake panel model with viscous–inviscid interaction developed to simulate a vertical axis turbine in dynamic stall, adding the capability of predicting the dynamic aerodynamic performance of VG‐equipped airfoils. The results of a series of steady and unsteady cases of an airfoil with different VG configurations in various pitch motions in free and forced transition are verified against experimental data. Results show that the double wake model offers results with sufficient accuracy compared with experimental data to claim the model's validity in a preliminary evaluation of an airfoil's capability to prevent stall with VGs. A few limitations, including the accuracy in prediction the transition location, separation, and reattachment, have been identified for future development.

Vortex generators (VGs) have been widely applied to wind turbines thanks to their potential to increase aerodynamic performance. Due to the complex inflow perceived by a rotor and the proneness to flow separation, VGs on wind turbines usually experience highly unsteady flow. While there are models that exist to simulate the steady effects of VGs, we lack a fast and efficient tool to model the unsteady performance of airfoils equipped with VGs. This paper adopts an unsteady double‐wake panel model with viscous–inviscid interaction developed to simulate a vertical axis turbine in dynamic stall, adding the capability of predicting the dynamic aerodynamic performance of VG‐equipped airfoils. The results of a series of steady and unsteady cases of an airfoil with different VG configurations in various pitch motions in free and forced transition are verified against experimental data. Results show that the double wake model offers results with sufficient accuracy compared with experimental data to claim the model's validity in a preliminary evaluation of an airfoil's capability to prevent stall with VGs. A few limitations, including the accuracy in prediction the transition location, separation, and reattachment, have been identified for future development. [ABSTRACT FROM AUTHOR]

The dynamic stall is a common phenomenon in horizontal and vertical axis wind, reducing system efficiency. In order to enhance the aerodynamic performance (L/D) of a NACA0012 airfoil under the deep dynamic stall at Reynolds number of 1.35 × 10 5 , computational intelligence algorithms were utilized to find the best operational parameters of a continuous blowing jet. The airfoil undergoes a sinusoidal motion between − 5 and 25, and the rotation center is around a quarter of its chord. Unsteady Navier–Stokes equation (URANS) was used with k - ω SST turbulence model. Two types of computational intelligence algorithms, including neural networks and genetic algorithms, were coupled for this optimization. The average lift to drag ratio (L/D) in an oscillation period was considered as the objective function. The blowing jet parameters, which included location, opening length, velocity magnitude and angle of jet, were selected as design variables. Two neural networks have been utilized to find a relation between design variables and the mean lift and drag coefficients over a period to reduce the computational cost of the optimization. The optimization algorithm converged after almost 115 simulations. The ANNs in the last simulation were able to predict the input data with 92% and 93% regression coefficients for average values of drag and lift coefficient in terms of the operational parameters of the jet, respectively. The optimized jet enhanced the mean aerodynamic performance by reducing the drag coefficient and increasing the lift coefficient during a period of oscillation. For the optimal case, this parameter reached the value of 11.727 or 4.717 times the uncontrolled case. The most impact of the jet is in the downward movement. Significant improvement in aerodynamic performance was observed for the optimal blowing jet, which is due to the lack of formation leading edge vortex (LEV), dynamic stall vortex (DSV) and trailing edge vortex (TEV). The results indicated that about 2–5% of the chord is the best location for jet. This location is near the place where the leading edge vortex is formed. Aerodynamic performance improved better when the jet angle was in the range of 55°–70°. Although the jet momentum coefficient was not maximized, jet-opening length and blowing velocity magnitude converged to their maximum values quickly. [ABSTRACT FROM AUTHOR]

Copyright of Engineering Mechanics / Gongcheng Lixue is the property of Engineering Mechanics Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The air and structural loads of a 5-ton class light helicopter (LH) rotor in a 2.24 g pull-up maneuver are investigated using a coupling between the computational structural dynamics (CSD) and computational fluid dynamics (CFD) methods. The LH rotor is characterized by a five-bladed system with elastomeric bearings and inter-bladed dampers. The periodic trim solution along with the converged CFD/CSD delta airloads obtained in steady-level flight (advance ratio of 0.287) are used to perform the transient CSD maneuver analysis. The resulting vehicle attitude angles and velocity profiles of the aircraft are then prescribed in the quasi-static (QS) CFD maneuver analysis. It is demonstrated that the present QS approach provides an effective means for the maneuver loads' analysis. The important flow behaviors such as BVI (blade–vortex interaction)-induced oscillations and the negative pitching moment peaks met in maneuver flight are captured nicely with the proposed method. Either the vortex trajectories or the surface pressure distributions are examined to identify the sources of the oscillations. A loose CFD/CSD coupling (LC) is used to predict the blade elastic motions, structural moments, and pitch link loads at the specified maneuver revolution of the rotor and also to correlate these with the transient CSD-based predictions. A reasonable correlation is obtained. The LC results show more pronounced 5P (five per revolution) oscillations on the structural response than those of the CSD-based methods. [ABSTRACT FROM AUTHOR]

The laminar flow separation and separation-induced transition at low Reynolds number are complex, and have great difficulty in numerical simulation. Based on fully-connected back-propagation neural network, a data-driven model of intermittency at low Reynolds number was established. The input parameters of the data-driven model to reflect transition process and predict intermittency were selected through optimization design. By modifying the k-ω SST two equation turbulence model with a data-driven transition equation, the flow field evolution and unsteady aerodynamic characteristics of a two-dimensional airfoil under dynamic stall were solved. Results show that the data-driven transition equation combined with two equation turbulence model has the generalization ability for the angle of attack, and clearly reflects the typical flow conditions such as the growth and shedding of the leading-edge vortex and the reattachment of the flow under dynamic stall. The relative error of unsteady aerodynamic lift in dynamic stall between the data-driven transition model and the SST-γ three equation model is lower than 12%.

Dynamic mode decomposition (DMD) technology is used to analyze the control of dynamic stall on a pitching S809 airfoil using an off-surface rod. The unsteady flows around the original and the controlled airfoil are simulated by using the SST k-ω turbulence model. With the introduction of the off-surface rod, the hysteresis effect of the dynamic stall process of the original airfoil is considerably reduced, and the clockwise sub-loop of the pitching moment coefficient is eliminated. The improvement of the dynamic stall process is beneficial to the safe and high-efficiency operations of wind turbine. The coherent structure of the unsteady flow fields are decoupled by the DMD method compiled by an in-house code in MATLAB. Results reveal that the hysteresis effect is dominated by mode 2 with a pitching frequency and mode 3 with twice the pitching frequency. The global energy of the two modes is reduced by the off-surface rod, which alleviates the hysteresis effect for the original airfoil. [ABSTRACT FROM AUTHOR]

The dynamic stall of the airfoil can adversely affect the performance of helicopter rotor blades, jet engine compressor blades, wind turbine blades, and so on. Therefore, the flow control on the dynamic stall is necessary. This paper conducted an experimental study on the multiobjective flow control effect of the dynamic stall on a NACA0012 airfoil using a passive vortex generator (VG). The control effect of the VG on the dynamic stall is discussed, and the control mechanism is explained. Within the scope of this paper, the VG could significantly increase the maximum normal force coefficient (Cn) by 0.223, reduce the peak negative moment coefficient (Cm) by 0.049, and delay the stall angle of attack of Cn and Cm by Δα=1.8° and 2.9°, respectively. These multiobjective control effects were attributed to the control of the complex vortices on the suction surface. The VG delays stall by suppressing the formation of the shear layer vortex, and enhances the aerodynamic performance during dynamic stall by energizing the dynamic stall vortex (DSV). The effects of the VG on the convection speed and strength of the DSV are additionally investigated from a space-time perspective. Within the reduced-frequency range studied in this paper, the DSV convection speed controlled by VG was smaller than that of the baseline airfoil, and the VG could significantly enhance the strength of the DSV. Dynamic stall phenomena are widely found in nature (wings of flying birds, fins of swimming fish, and so on) and in engineering (bionic flutter wings, helicopter rotors, wind turbine blades, and so on). It is widely believed that the DSV is the main flow structure dominating the dynamic stall. For helicopter rotor blades and large wind turbine blades, dynamic stall is prone to occur in the blade tip under complex realistic conditions. The vortex lift during dynamic stall will significantly raise the blade load, and the motion of the DSV will cause a sharp change in the blade moment, which may damage the blade structure and control system. Therefore, the dynamic stall needs to be controlled. In this paper, passive VGs were used to control the dynamic stall on an airfoil, and ideal multiobjective control effects were obtained: reducing the negative peak moment, increasing the maximum lift coefficient, delaying the stall, and reducing the hysteresis effect. This work is a guideline for the dynamic stall control of helicopter rotor and wind turbine blades. [ABSTRACT FROM AUTHOR]

This work proposes a multifidelity modeling approach to mitigate adverse characteristics of airfoil dynamic stall through aerodynamic shape optimization (ASO). Cokriging regression (CKR) is used to efficiently determine an optimum airfoil shape by combining data from high-fidelity (HF) and low-fidelity (LF) computational fluid dynamics simulations. The HF dynamic stall response is modeled using the unsteady Reynolds-averaged Navier–Stokes equations and Menter’s SST turbulence model, whereas the LF model is developed by simplifying the HF model with a coarser discretization and relaxed convergence criteria. The CKR model, constructed using various infill criteria to model the objective and constraint functions with six PARSEC parameters, is utilized to find the optimal design. The results show that the optimal shape from CKR delays the dynamic stall angle over 3° while reducing the peak values of the aerodynamic coefficients compared to the baseline airfoil (NACA 0012). Comparing the optimized shapes from the CKR and a HF Kriging regression (HF-KR) shows a similar delay in dynamic stall angle; however, the CKR optimum provides a better design for the current problem formulation while requiring 39% less computational time than the HF-KR approach. This work presents a new multifidelity modeling approach to saving the computational burden of dynamic stall mitigation through ASO. The approach used in this work is general and can be applied for other unsteady aerodynamic applications and optimization. [ABSTRACT FROM AUTHOR]

Numerous studies have been conducted to investigate effect of blade geometry of vertical axis wind turbine performance. Most of the evaluations have focused on the airfoil series and airfoil geometry parameters such as thickness and camber of the airfoil. Few studies have examined the effect of other blade geometry parameters on the vertical axis wind turbine performance. In the present study, the effect of geometric change in leading-edge radius (LER) of a vertical axis wind turbine performance has been numerically studied. Hence, modified NACA 0021 airfoil profiles were created using the geometric method (CST). Then, the flow behavior around a Darrieus vertical axis wind turbine was simulated under the influence of the reduction and set-up coefficients of the leading-edge radius at a constant wind speed of 9 m/s and a tip speed ratio of 1.5 to 3.5 using the computational fluid dynamics. Additionally, the effects of the examined parameter (leading-edge radius) on fluid flow and aerodynamic performance coefficients, including the coefficients of power and torque, were investigated. The results indicated that the leading-edge radius affected the near wake flow of the turbine, and the optimization of leading-edge radius parameter controls the dynamic stall and reduces the formation of a vortex. Finally, the optimization of LER revealed that at 20% reduction in the LER the performance of the turbine at tip speed ratio of 1.5 was increased by more than 50%. This reinforces the self-starting capability of a Darrieus wind turbine.

Abstract Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades, helicopter rotors, etc. So the dynamic stall physics of the NACA0012 airfoil under a large Reynolds number of Re = 1.5 × 106 was studied using experimental and numerical methods. The reduced frequency range was k = 0.035 – 0.1. The unsteady flow field in dynamic stall was studied in detail by using the transient pressure measurement and the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes (URANS) equation. And the time-frequency characteristics of the dynamic stall were studied using the wavelet analysis. The study showed that the aerodynamic performance during the dynamic stall was dominated by the shear layer vortex (SLV) and the leading edge vortex (LEV), and the phase difference between the SLV and the LEV was the key factor in the existence of the bimodal characteristics of the aerodynamic force/moment. There was a significant linear correlation between the negative peak of the vortex-induced C p and the C n in the reduced frequency range studied in this paper. During the convection of the near-wall LEV to the trailing edge, the high-frequency features firstly decay, and the multi-scale structures of the LEV become more significant as the reduced frequency gradually increases.

This paper studies a rotor dynamic stall control method using an alternating current dielectric barrier discharge (AC DBD) plasma actuator through numerical simulation methods. The flow field evolution during a dynamic stalling process under the excitation of AC DBD plasma discharge is analyzed using the two-dimensional Reynolds time-averaged (RANS) method. The impact of the AC DBD plasma discharge on the flow field is then simulated using the phenomenological method. The influence of the position and intensity of the plasma excitation on the static stall characteristics of the NACA0012 airfoil is also studied. Deformed mesh and dynamic mesh techniques are used to simulate an aerodynamic environment with variable incoming flow and variable angles of attraction on a rotor airfoil. The application of AC DBD plasma excitation for controlling mild and deep dynamic stalls of rotor blades is investigated. The obtained results show that the AC DBD plasma excitation accelerated the evolution and shedding of dynamic stall vortices and facilitated the reattachment of airflow. The application of plasma excitation allowed for significantly increasing the static stall angle of the airfoil and improving the lift coefficient. In addition, the intensity of the plasma excitation is a key factor affecting the control. Moreover, the application of AC DBD plasma excitation for rotor dynamic stalls allowed for reducing the size of the dynamic stall vortex, which helped mitigate the aerodynamic hysteresis effect caused by the dynamic stall and accelerated the recovery from aerodynamic forces.

Over the last few decades, researchers have been focusing on determining the critical attack angle at which dynamic stall occurs. This angle is usually determined by solving the Navier-Stokes equations, which include viscosity, pressure, gravity, and acceleration terms. However, Navier-Stokes equations are quite complex to solve and consequently difficult to simulate, thus the simulation is not accurate enough. Therefore, this article predicts the critical attack angle for the first time using Euler equations devoid of viscous terms. One of the key advantages of Euler equations is their ability to capture the vortices and predict stall dynamics. The Euler equations are thus integrated and the resulting equations are discretized using the finite volume method. A first-order upwind-based method is used to calculate the convective fluxes at the cell boundaries in the finite volume approach. A NACA 0012 airfoil is chosen for this study at various attack angles with a Mach number of 0.3. Based on the justification of Crocco's theorem, the Euler equations successfully act as Navier-Stokes equations. The vortex patterns are found to behave independently of the artificial dissipation. All the vortices are successfully predicted using the inviscid governing equations. The numerical results obtained are validated by other published experimental and numerical data.

Numerous studies have been conducted to investigate effect of blade geometry of vertical axis wind turbine performance. Most of the evaluations have focused on the airfoil series and airfoil geometry parameters such as thickness and camber of the airfoil. Few studies have examined the effect of other blade geometry parameters on the vertical axis wind turbine performance. In the present study, the effect of geometric change in leading-edge radius (LER) of a vertical axis wind turbine performance has been numerically studied. Hence, modified NACA 0021 airfoil profiles were created using the geometric method (CST). Then, the flow behavior around a Darrieus vertical axis wind turbine was simulated under the influence of the reduction and set-up coefficients of the leading-edge radius at a constant wind speed of 9 m/s and a tip speed ratio of 1.5 to 3.5 using the computational fluid dynamics. Additionally, the effects of the examined parameter (leading-edge radius) on fluid flow and aerodynamic performance coefficients, including the coefficients of power and torque, were investigated. The results indicated that the leading-edge radius affected the near wake flow of the turbine, and the optimization of leading-edge radius parameter controls the dynamic stall and reduces the formation of a vortex. Finally, the optimization of LER revealed that at 20% reduction in the LER the performance of the turbine at tip speed ratio of 1.5 was increased by more than 50%. This reinforces the self-starting capability of a Darrieus wind turbine. [ABSTRACT FROM AUTHOR]

Simulation of the aerodynamic stall phenomenon in both quasi-static and dynamic conditions requires expensive computational resources. The computations become even more costly for static stall hysteresis using an unsteady solver with very slow variation of angle of attack at low reduced frequencies. In an explicit time-marching solver that satisfies the low Courant number condition, that is, C F L ≤ 1 , the computational cost for such simulations becomes prohibitive, especially at higher Reynolds numbers due to the presence of thin-stretched cells with large aspect ratio in the boundary layer. In this paper, a segregated solver method such as the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) is modified as a dual pseudo-time marching method so that the unsteady problem at each time step is reformulated as a steady state problem. The resulting system of equations in the discretized finite volume formulation is then reduced to zero or near-zero residuals using available convergence acceleration methods such as local time stepping, multi-grid acceleration and residual smoothing. The performance and accuracy of the implemented algorithm was tested for three different airfoils at low to moderate Reynolds numbers in both incompressible and compressible flow conditions covering both attached and separated flow regimes. The results obtained are in close agreement with the published experimental and computational results for both quasi-static and dynamic stall and have demonstrated significant savings in computational time. [ABSTRACT FROM AUTHOR]

The prediction of the unsteady aerodynamic load on an airfoil has frequently used the Leishman-Beddoes (L-B) model. Meanwhile, flow control techniques could change the flow around the blade, increase the effectiveness of wind energy conversion, and reduce the wind load. However, there have been a limited number of investigations examining this semi-empirical model taking into account the effects of such aerodynamic add-ons. As a result, this study intends to extend and modify the L-B model so that it can predict the unsteady aerodynamic characteristics of a wind turbine airfoil equipped with a Gurney flap. According to the unsteady aerodynamic changes of the Gurney flap during the oscillation period, the prediction module was proposed by the circulation changes, as well as characteristics of vortex generation and shedding traveling on the airfoil surface. These predicting results were preliminarily confirmed through a wind tunnel experiment, which illustrated that this approach has high practical potential. Nevertheless, the aerodynamic coefficients slightly deviated in the deep stall region. Adopting such a wind turbine optimization technology might improve the prediction and evaluation of the aerodynamic characteristics of a flapped airfoil under varying turbulent flow conditions. [ABSTRACT FROM AUTHOR]

In this research, the improvement of wind turbine blade undergoing pitching motion by means of identifying optimized aerofoil profile has been studied as a crucial part of enhancing effectiveness of wind energy turbine system. Here, the projected air is considered as unidirectional with flow speed range of Re 5*105 to 106. The effect of different pitching aerofoil blade profiles ranging from NACA 0012, S-809, and SD 7062 has been investigated in addition to the aerodynamic characteristics, entropy generation in the flow and exergy which are also proposed as a criterion for selecting optimum design for the oscillating aerofoil blade. Finally, increment in Re shows increase in the entropy generation rate and decrease in exergy efficiency. Consequently, aerofoil profile shows prominent variations in exergy with SD 7062 experiencing least entropy generation rate and exergy efficiency of around 93% due to its streamlined profile as compared to other profiles in this study. Meanwhile, NACA 0012 experiences minimum exergy efficiency of around 39% for Re 106. [ABSTRACT FROM AUTHOR]

This paper investigates the hysteresis characteristics of force coefficients of an oscillating hydrofoil in a near-free surface cavitation flow field by utilizing unsteady numerical simulation methods. The study primarily focuses on analyzing the effects of dynamic stall conditions, reduced frequency, cavitation number, immersion depth, and Froude number on the hysteresis curve. By comparing the vortex distribution, volume fraction, velocity streamlines, and fluctuating pressure coefficient in the flow field, the authors also examine the reasons for the differences in the hysteresis curve at the same angle of attack under different conditions. The results suggest that cavitation significantly impacts the fluctuation of the hysteresis curve, mainly due to the shedding and collapse of the cavity on the hydrofoil pressure surface, which results in pressure fluctuations at the trailing edge. This issue can be addressed by reducing the stall angle of attack, Froude number, and increasing the reduced frequency, cavitation number, and immersion depth to slow down cavitation in the flow field, thus reducing the fluctuation of the hysteresis loop. Furthermore, the structure of the cavitation flow field under different conditions is clearly distinguished, and the hysteresis loop experiences obvious fluctuations when there is obvious vortex separation in the flow field and many small cavities remaining above the pressure surface. As the immersion depth decreases and the corresponding Froude number increases, the effect of the free surface becomes stronger, leading to an increase in the free surface wave amplitude. This effect causes the hydrofoil pressure surface to gradually evolve into super-cavitation, and the fluctuation of the hysteresis curve tends to be stable. [ABSTRACT FROM AUTHOR]

This paper provides 2D Computational Fluid Dynamics (CFD) investigations, using OpenFOAM package, of the unsteady separated fully turbulent flows past a NACA 0015 airfoil undergoing sinusoidal pitching motion about its quarterchord axis in deep stall regime at a reduced frequency of 0.1, a free stream Mach number of 0.278, and at a Reynolds number, based on the airfoil chord length, 𝑐, of 1.95 × 106. First, eighteen 2D steady-state computations coupled with the 𝑘 − 𝜔 SST model were carried out at various angles of attack to investigate the static stall. Then, the 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations of the flow around the oscillating airfoil about its quarterchord axis were carried out. Three eddy viscosity turbulence models, namely the Spalart-Allmaras, Launder-Sharma 𝑘 − 𝜀, and 𝑘 − 𝜔 SST were considered for turbulence closure. The results are compared with the experimental data where the boundary layer has been tripped at the airfoil’s leading-edge. The findings suggest that the 𝑘 − 𝜔 SST performs best among the other two models to predict the unsteady aerodynamic forces and the main flow features characteristic of the deep stall regime. The influence of moving the pitching axis downstream at mid chord was also investigated using URANS simulations coupled with the 𝑘 − 𝜔 SST model. It was found that this induces higher peaks in the nose-down pitching moment and delays the stall onset. However, the qualitative behavior of the unsteady flow in post-stall remains unchanged. The details of the flow development associated with dynamic stall were discussed [ABSTRACT FROM AUTHOR]