Dynamic-stall Vortex Research Articles

Direct numerical simulations of an airfoil undergoing dynamic stall at different background disturbance levels

Thin airfoil dynamic stall at moderate Reynolds numbers is typically linked to the sudden bursting of a small laminar separation bubble close to the leading edge. Given the strong sensitivity of laminar separation bubbles to external disturbances, the onset of dynamic stall on a NACA0009 airfoil section subject to different levels of low-amplitude free stream disturbances is investigated using direct numerical simulations. The flow is practically indistinguishable from clean inflow simulations in the literature for turbulence intensities at the leading edge of ${Tu} = 0.02\,\%$ . At slightly higher turbulence intensities of ${Tu} = 0.05\,\%$ , the bursting process is found to be considerably less smooth and strong coherent vortex shedding from the laminar separation bubble is observed prior to the formation of the dynamic stall vortex (DSV). This phenomenon is considered in more detail by analysing its appearance in an ensemble of simulations comprising statistically independent realisations of the flow, thus proving its statistical relevance. In order to extract the transient dynamics of the vortex shedding, the classical proper orthogonal decomposition method is generalised to include time in the energy measure and applied to the time-resolved simulation data of incipient dynamic stall. Using this technique, the dominant transient spatiotemporally correlated features are distilled and the wave train of the vortex shedding prior to the emergence of the main DSV is reconstructed from the flow data exhibiting dynamics of large-scale coherent growth and decay within the turbulent boundary layer.

Numerical Investigation on the Evolution Process of Different Vortex Structures and Distributed Blowing Control for Dynamic Stall Suppression of Rotor Airfoils

The influencing characteristic for the evolution mechanism of a dynamic stall vortex structure and distributed blowing control on rotor airfoils was investigated. Based on the moving-embedded grid method, the finite volume scheme, and Roe’s FDS scheme, a simulation method for the unsteady flow field of a pitch-oscillating airfoil was established. The flow field of the NACA63-218 airfoil was calculated using Reynolds-averaged Navier–Stokes equations. The evolution processes of different vortex structures during dynamic stall and the principal controlled vortex mechanism affecting aerodynamic nonlinearity were analyzed based on the pressure contours Cp and Q of the flow field structure and the spatiotemporal evolution characteristics of the wall pressure distribution. The research indicated that dynamic stall vortices (DSVs) and shear layer vortices (SLVs) were the major sources of the increase in aerodynamic coefficients and the onset of nonlinear hysteresis. Building upon these findings, the concept of distributed blowing control for DSVs and shear layer vortices (SLVs) was introduced. A comparative analysis was conducted to assess the control effectiveness of dynamic stall with different blowing locations and blowing coefficients. The results indicated that distributed blowing control effectively inhibited the formation of DSVs and reduced the intensity of SLVs. This led to a significant decrease in the peak values of the drag and pitch moment coefficients and the disappearance of secondary peaks in the aerodynamic coefficients. Furthermore, an optimal blowing coefficient existed. When the suction coefficient Cμ exceeded 0.03, the effectiveness of the blowing control no longer showed a significant improvement. Finally, with a specific focus on the crucial motion parameters in dynamic stall, the characteristics of dynamic stall controlled by air blowing were investigated. The results showed that distributed air blowing control significantly reduced the peak pitching moment coefficient and drag coefficient. The peak pitching moment coefficient was reduced by 72%, the peak drag coefficient was reduced by 70%, and the lift coefficient hysteresis loop area decreased by 46%. Distributed blowing jet control effectively suppressed the dynamic stall characteristics of the airfoil, making the unsteady load changes gentler.

Dynamic stall characteristics of unswept pitching wing

An experimental investigation of dynamic stall on upswept pitching wing was conducted using Particle Image Velocimetry (PIV) and Pressure Sensitive Paint (PSP) techniques. The wing had an aspect ratio AR = 4 and a NACA 0012 airfoil section profile assembled with a round-tip. The wing motion of interest in this study is pitching at a certain angle of attack range and the wing was robustly designed and machined out of Aluminum to minimize elasticity and tip flapping motions. The freestream Mach number, M∞ = 0.1, and the Reynolds number based on chord length, Rec = 2 × 105, were considered as flow parameters for observing dynamic stall phenomena and for comparison with available numerical simulations from literature. The pitching motion of the wing is sinusoidal with a reduced frequency k = πfc/U∞ = 0.1, and the minimum and maximum angle of attack of the pitching motion were 4° and 22°, respectively. Details of flow structures during pitching at various phase angles were captured using planar two-dimensional (2D) PIV at three spans, x/c = 0.5, 0.75, and 1.0 and the pressure imprint on the surface of the wing was captured by a single shot lifetime method using fast porous pressure-sensitive paint. The initial results show the formation of a separation bubble at a small incident angle and the emergence of a dynamic stall vortex (DSV) after the bursting of the separation bubble. The DSV propagates towards the trailing edge of the wing with its integral scale varying across the three spans of the wing revealing three-dimensionality. The PSP results show a uniform build of suction pressure at low angle of attack which later intensifies in a non-uniform manner due to a circulatory motion of the flow during pitching. After the end of the upstroke phase, the flow starts to reattach to the surface of the wing while the DSV grows and moves away from the surface as it travels downstream along the chord until sheds beyond the trailing edge. The PSP results also capture the tip unloading period during the pitching motion.

Effects of the Reynolds number and reduced frequency on the aerodynamic performance and dynamic stall behaviors of a vertical axis wind turbine

Dynamic stall is the main reason for the low power efficiency of vertical axis wind turbines (VAWTs) at low tip speed ratios, where the utilizable wind kinetic energy is also high. However, the determining factors behind VAWT dynamic stall have not been adequately investigated. Therefore, we carefully investigated VAWT dynamic stall under the different Reynolds numbers (Re) and reduced frequencies (k). The unsteady flow characteristics are identified using transitional URANS simulations. Although the blade undergoes significant vortex movements during each revolution, the output power is primarily determined by the aerodynamic responses close to the onset of dynamic stall. Increasing Re and k can impact dynamic stall behaviors significantly and also improve VAWT performance effectively. As both Re and k increase, the onset of dynamic stall is progressively postponed to a higher angle of attack, effectively suppressing the separated flow. Dynamic stall behaviors are therefore changed from abrupt stall to moderate stall with attenuated laminar separation bubble (LSB) bursting and dynamic stall vortex (DSV) shedding. An increase in Re hardens the LSB and prevents the DSV formation due to strong inertial forces, particularly at low Re. On the other hand, increasing k slows the flow-field transitions and delays the onset of LSB bursting and DSV shedding, particularly at high k.

Flow Control around the UAS-S45 Pitching Airfoil Using a Dynamically Morphing Leading Edge (DMLE): A Numerical Study

This paper investigates the effect of the Dynamically Morphing Leading Edge (DMLE) on the flow structure and the behavior of dynamic stall vortices around a pitching UAS-S45 airfoil with the objective of controlling the dynamic stall. An unsteady parametrization framework was developed to model the time-varying motion of the leading edge. This scheme was then integrated within the Ansys-Fluent numerical solver by developing a User-Defined-Function (UDF), with the aim to dynamically deflect the airfoil boundaries, and to control the dynamic mesh used to morph and to further adapt it. The dynamic and sliding mesh techniques were used to simulate the unsteady flow around the sinusoidally pitching UAS-S45 airfoil. While the γ-Reθ turbulence model adequately captured the flow structures of dynamic airfoils associated with leading-edge vortex formations for a wide range of Reynolds numbers, two broader studies are here considered. Firstly, (i) an oscillating airfoil with the DMLE is investigated; the pitching-oscillation motion of an airfoil and its parameters are defined, such as the droop nose amplitude (AD) and the pitch angle at which the leading-edge morphing starts (MST). The effects of the AD and the MST on the aerodynamic performance was studied, and three different amplitude cases are considered. Secondly, (ii) the DMLE of an airfoil motion at stall angles of attack was investigated. In this case, the airfoil was set at stall angles of attack rather than oscillating it. This study will provide the transient lift and drag at different deflection frequencies of 0.5 Hz, 1 Hz, 2 Hz, 5 Hz, and 10 Hz. The results showed that the lift coefficient for the airfoil increased by 20.15%, while a 16.58% delay in the dynamic stall angle was obtained for an oscillating airfoil with DMLE with AD = 0.01 and MST = 14.75°, as compared to the reference airfoil. Similarly, the lift coefficients for two other cases, where AD = 0.05 and AD = 0.0075, increased by 10.67% and 11.46%, respectively, compared to the reference airfoil. Furthermore, it was shown that the downward deflection of the leading edge increased the stall angle of attack and the nose-down pitching moment. Finally, it was concluded that the new radius of curvature of the DMLE airfoil minimized the streamwise adverse pressure gradient and prevented significant flow separation by delaying the Dynamic Stall Vortex (DSV) occurrence.

Pitch–Plunge Equivalence in Dynamic Stall of Ramp Motion Airfoils

Large-eddy simulations (LES) are employed to investigate the pitch–plunge equivalence of an SD7003 airfoil undergoing constant ramp motions at Reynolds number . The equivalence is constructed based on the geometric effective angle of attack according to the quasi-steady thin-airfoil theory. Two rates of descent (or pitch up) are analyzed for different Mach numbers in order to investigate the effects of compressibility on the evolution of the dynamic stall vortex (DSV). During the onset of the DSV and its transport along the airfoil surface, remarkable similarities are found between pitch and plunge in terms of flow topology, aerodynamic loads, and signatures of wall pressure and friction coefficients. However, these flow similarities cease at high-load conditions as the DSV becomes more susceptible to the peculiarities of the airfoil motion, manifested here by different trailing-edge vortices. Employing a correction for the rotation-induced apparent camber effect present in the pitching case, which results from the quasi-steady thin-airfoil theory, improves the agreement between pitch and plunge. However, it is not sufficient to assimilate their disparate trailing-edge systems. Results also demonstrate that the limit angle at which pitch–plunge equivalence remains valid decreases for higher Mach numbers.

Numerical investigation of a pitching airfoil undergoing dynamic stall using Delayed Detached Eddy Simulations

This paper examines the detailed flow physics and predictive capabilities of Delayed Detached Eddy Simulations (DDES) related to the dynamic stall onset process during a pitching motion at a transitional Reynolds number of 200,000 and Mach number of 0.10. A comparison of the detailed flow physics of the onset of dynamic stall process between DDES and Large Eddy Simulations (LES) is first presented in this paper. State-of-the-art transition models will be tested in terms of their capabilities in capturing complex flow physics. The overall flow physics is captured in detail with a mesh composed of roughly 16 million grid points. A leading-edge separation region and formation of a dynamic stall vortex is examined in detail. The onset of lift stall was predicted slightly earlier using DDES with all turbulence transition models studied, and the peak lift coefficient was also slightly underpredicted compared to LES. However, DDES captures the important phenomena that are presented in LES including the formation and collapse of a Laminar Separation Bubble (LSB) and the subsequent formation of the Dynamic Stall Vortex (DSV). Coder’s SA Amplification Factor Transport (AFT) transition model is found to yield the most accurate results when compared with LES results and shows promising results for future use in more high-fidelity case studies. Langtry and Menter’s SST correlation-based transition model, its SA counterpart developed by Medida and Baeder, and a fully turbulent SA turbulent model are also investigated.

Numerical simulation of dynamic stall flow control using a multi-dielectric barrier discharge plasma actuation strategy

To alleviate the deterioration in wind turbine performance caused by dynamic stall, the flow control of a pitching NACA0012 airfoil is investigated through numerical simulation of an alternating current dielectric barrier discharge (AC-DBD) plasma actuator at a Reynolds number Re = 135 000. To avoid the harmonic oscillations of aerodynamic force caused by unsteady DBD actuation, this work focuses on improving the control potential for steady actuation. The control mechanisms of actuators at various positions are investigated using five groups of actuators mounted at 0%, 3%, 10%, 45%, and 80% chord lengths c above the upper surface of an airfoil. The actuator at 80%c performs more efficiently in terms of lift enhancement in the initial upstroke and the final downstroke. The actuator at 0%c suppresses the growth of the leading-edge vortex and maintains the suction of the dynamic stall vortex (DSV). After the shedding of the DSV, it suppresses the secondary separation to delay the onset of dynamic stall. At the flow reattachment stage, the actuators at 3%c and 10%c accelerate the boundary layer reattachment by momentum injection. From these results, a multi-DBD control strategy is proposed. The scheme selects the optimal actuator in operation at a certain stage of dynamic stall and takes advantage of actuators at different positions to enhance the average and maximum aerodynamic force, delay the onset of dynamic stall, accelerate flow reattachment, and avoid excessive energy consumption.

Analysis of the onset and evolution of a dynamic stall vortex on a periodic plunging aerofoil

The onset and evolution of the dynamic stall vortex (DSV) are analysed by means of large eddy simulations of an SD7003 aerofoil undergoing periodic plunging motion in a transitional Reynolds number flow ($Re =6\times 10^{4}$). Interactions between upstream propagating Kelvin–Helmholtz instabilities and a shear layer formed at the leading edge trigger flow separation. The former appear to be related to acoustic waves scattered at the trailing edge due to initial vortex shedding. Two freestream Mach numbers ($M_{\infty }=0.1$and$0.4$) are employed to examine the flow differences due to compressibility variations. The existence of a common timing for the acoustic perturbations in both flows suggests a possible Mach number invariance for the birth of the Kelvin–Helmholtz instability. Increasing compressibility, however, induces earlier spanwise fluctuations, higher flow three-dimensionality and a weaker and more diffuse DSV, which is formed further downstream of the leading edge and has lower residency time. In order to better characterize the onset of the DSV, two empirical criteria are assessed: the leading edge suction parameter and the chord-normal shear layer height. Results demonstrate a higher robustness of the latter with respect to Mach number variations. Modal decomposition, performed with both the classical dynamic mode decomposition (DMD) and its multi-resolution variant (mrDMD), highlights key trends and demonstrates the capacity of the mrDMD to extract physically meaningful flow structures related to the stall onset. Such detailed characterization of the shear layer can be used for a systematic exploration of flow control strategies for unsteady aerofoils.

Trailing Edge Flap Effects on Dynamic Stall Vortex and Unsteady Aerodynamic Forces on a Pitching Airfoil

This study focuses on the effect of the upward deflection of trailing edge flap (TEF) on the strength and trajectory of dynamic stall vortex (DSV) around a pitching airfoil by means of numerical simulations based on unsteady Reynolds-averaged Navier-Stokes (URANS). The effect of the upward deflection of the TEF on the unsteady aerodynamic forces due to DSV is analyzed. The numerical simulation method for large mesh deformation is constructed. Radial basis function- (RBF-) based mesh deformation algorithm, as well as Laplacian and optimization-based mesh smoothing algorithm, is adopted to ensure the mesh quality in flow field simulations. The results reveal that the upward deflection of the TEF can reduce the peaks of drag and pitching moment coefficients. Although the maximum lift coefficient of the airfoil is slightly reduced, its maximum drag and pitching moment coefficients are significantly reduced by up to 34.8% and 31.8%, respectively. The vorticity transport behavior in a planar control region during the DSV formation and detachment is analyzed. It is found that the TEF can change the development process of the DSV. The upward deflection of the TEF reduces the vorticity flux from the leading edge shear layer, which causes the circulation of the DSV and the translational velocity of the vortex center to decline. The peaks of the unsteady aerodynamic forces on the airfoil induced by the DSV are reduced. The upward deflection of the TEF plays the role of alleviating the pitching moment load. The longer TEF can result in a better control effect. The bigger the upward deflection angle of the TEF, the better its control effect.

Critical indicators of dynamic stall vortex

During a dynamic stall process, various flow structures leave their pressure footprints on a wing surface, through which it is possible to understand the development of different flow stages and predict the critical flow events. Based on a classical airfoil ramp-up pitching motion, proper orthogonal decomposition analysis of pressure on the suction surface is carried out in this study. Accordingly, the surface pressure evolution during dynamic stall is summarized into three basic physical schemas, based on which several real-time critical indicators for predicting the flow events related to the dynamic stall vortex (DSV) are constructed. These critical indicators include the spatial distribution coefficient of pressure (SDCP), the high-order central moment of pressure (HCMP), the location of peak pressure (LPP) and the modulated location of peak pressure (MLPP). These indicators can predict the formation of laminar separation bubbles, DSV initiation, DSV centre position and the detachment of DSV. Therefore, the real-time whole-life monitoring of DSV has been realized. Moreover, the effectiveness of these indicators has also been confirmed under different parameters, and testing using wind tunnel experimental data proves their noise robustness. Studies show that the SDCP and HCMP may also be effective even if only two transducers are used. Finally, a modification method of SDCP based on Z-score standardized pressure is proposed. It is found that the modified SDCP can effectively reduce sensitivity to kinematic parameters and the Reynolds number. The critical indicators in this study can be used as a reference for the standardized mathematical and physical description of DSV evolution, thus laying a foundation for constructing a universal theoretical framework of dynamic stall.

Dynamic Stall Characteristics of Pitching Swept Finite-Aspect-Ratio Wings

An experimental investigation regarding the dynamic stall of various swept wing models with pitching motion was performed to analyze the effect of sweep on the dynamic stall. The experiments were performed on a wing with a NACA0012 airfoil section with an aspect ratio of AR = 4. The experimental study was conducted for chord-based Reynolds number Rec =2×105 and freestream Mach number Ma=0.1. First, a ‘particle image velocimetry’ (PIV) experiment was performed on the wing with three sweep angles, Λ=0o, 15o, and 30o, to obtain the flow structure at several wing spans. The results obtained at a reduced frequency showed that a laminar separation bubble forms at the leading edge of the wing during upward motion. As the upward pitching motion continues, a separation burst occurs and shifts towards the wing trailing edge. As the wing starts to pitch downward, the growing dynamic stall vortex (DSV) vortex sheds from the wing’s trailing edge. With the increasing sweep angle of the wing, the stall angle is delayed during the dynamic motion of the wing, and the presence of DSV shifts toward the wingtip. During the second stage, a ‘turbo pressure-sensitive paint’ (PSP) technique was deployed to obtain the phase average of the surface pressure patterns of the DSV at a reduced frequency, k=0.1. The phase average of pressure shows a distinct pressure map for two sweep angles, Λ=0o, 30o, and demonstrates a similar trend to that presented in the published computational studies and the experimental data obtained from the current PIV campaign.

Onset of dynamic stall of tubercled wings

In the present study, numerical simulation of the flow field of the tubercled NACA0021 (National Advisory Committee for Aeronautics) wing has been conducted with the large eddy simulation model. Then, proper orthogonal decomposition (POD) analysis has been carried out at trough and peak sections. According to the spatial distributions and temporal coefficients in the case of αA=5°, the physical backgrounds of corresponding POD modes could be identified, including flow attachment, separation, and the dynamic stall vortex (DSV). The onsets of local dynamic stall could be indicated with the extremum values of the DSV mode coefficients, which stay almost synchronized at trough and peak. Actually, the DSV is originally generated at trough and then moves to peak due to the spanwise convection induced by streamwise vorticity. The generation of the secondary vortex at trough is triggered with the streamwise pressure gradient, the development of which pinches off the feeding vorticity of the DSV and results in the detachment of the DSV. Eventually, the influence of pitching amplitude has also been discussed. The strength of the DSV at peak is increased with a larger pitching amplitude, which could be interpreted with the feeding sheet connected with the leading edge.

Physics of Dynamic Stall Vortex During Pitching Oscillation of Dynamic Airfoil

Dynamic stall, which has a significant effect on the aerodynamic performances of dynamic airfoils, is closely related to the physics of the dynamic stall vortex (DSV). The physics of the DSV on the NACA 0012 airfoil was experimentally studied using unsteady pressure measurements with high time accuracy. The experimental Reynolds number was Re = 1.5 × 106, and the reduced frequency was k = 0.069. The propagation of the unsteady pressure field during the dynamic stall process was analyzed in detail. The motion characteristics of the DSV were examined, including its near-wall development characteristics and near-wall evolution velocity. Moreover, the frequency characteristics of the near-wall DSV were studied using wavelet analysis combined with proper orthogonal decomposition (POD) technology. In addition, the effects of the mean angle of attack (AoA) and the amplitude on the DSV motion and frequency characteristics were examined in detail. The effects of the mean AoA on the near-wall DSV strength and the propagation velocity were linear, while the effects of amplitude were nonlinear. The mean AoA and amplitude had a significant influence on the frequency of the leading-edge vortex (LEV) at the initial stage of the DSV development (x/c = 0.10–0.20). This work allows the DSV physics to be understood more deeply.Graphic abstract

Large-Eddy Simulation of Unsteady Pitching Aerofoil using a Oneequation Subgrid Scale (SGS) Model based on Dynamic Procedure

Large-Eddy Simulations (LES) for an oscillating NACA0012 aerofoil at Reynolds number 1.35 × 105 have been performed to investigate the capability of a new approach of a One-equation SGS model. The phenomena of the oscillating aerofoil at higher amplitude angle (𝛼𝑎𝑚𝑝) is believed to produce deep dynamic stall in which strong non-equilibrium turbulence could also visible. Hence, a new approach of a one-equation SGS model is proposed where the coefficient of production term in the kSGS transport equation is defined dynamically. From the simulations, the leading-edge vortex (LEV), dynamic stall vortex (DSV), shedding mechanism and trailing edge vortex (TEV) are captured in this study.

On the influence of blade thickness-to-chord ratio on dynamic stall phenomenon in H-type Darrieus wind rotors

The present work examines the influence of blade thickness-to-chord ratio (t/c) on dynamic stall phenomenon in an H-type Darrieus wind rotor. A 2D incompressible numerical study is conducted on a single-bladed rotor at a tip speed ratio (TSR) of 2 with a motivation to understand the intricate flow physics around the blade by employing a transitional shear stress transport (TSST) model. Five different t/c ratios (9%, 12%, 15%, 18% and 21%) have been studied for symmetrical NACA airfoils. It is found that in case of thinner airfoils (t/c = 9% and 12%), the formation of the dynamic stall vortex (DSV) is preceded by the formation and bursting of leading edge (LE) laminar separation bubble (LSB) leading to a more abrupt LE type stall. For t/c = 15%, a mixed type stall is observed with LSBs distributed over the airfoil resulting in two coherent vortex structures that subsequently merge to form the DSV. A trailing edge (TE) type stall was observed in case of thicker airfoils (t/c = 18% and 21%). The largest peak in the lift and drag coefficients (Cl, Cd) values of 1.87 and 1.27, respectively are obtained with the thinnest airfoil. However, the largest peak in the moment coefficient (Cm) value is achieved with t/c = 12%. Further, due to the DSV separation at the lift stall point, a further increase of 6.3%, 7.1%, 1.5%, 8.94% and 11.8% in Cd values is observed for increasing order of t/c. Overall, the study sheds light on the capability of TSST model in capturing the dynamic stall phenomenon efficiently.

Dynamic stall control using inflatable leading edge

The inflatable leading edge (ILE) as a dynamic stall control concept for helicopter rotor blades was investigated numerically on a dynamically pitching airfoil. A fluid–structure interaction (FSI) numerical method for the elastic membrane structure was constructed based on unsteady Reynolds-averaged Navier–Stokes (URANS) equations and mass spring damper (MSD) structural dynamic model. The numerical results indicate that the ILE can change the radius of curvature of the airfoil leading edge, which could reduce the streamwise adverse pressure gradient and suppress the formation of dynamic stall vortex (DSV). Although the maximum lift coefficient of the airfoil is reduced by 8.2%, the maximum drag and pitching moment coefficients of the airfoil are reduced by up to 50.1% and 55.3%, respectively.

The effects of leading-edge tubercles on dynamic stall

The effects of leading-edge tubercles, based on the flippers of humpback whales, on the flow field around an airfoil undergoing large-amplitude dynamic changes in the angle of attack have been studied experimentally. Airfoils were pitched from an initial angle of attack of to at constant pitch rates with a chord Reynolds number of 12 000. Velocity and vorticity fields around a standard NACA 0012 airfoil and NACA 0012 modified with leading-edge tubercles were quantified using molecular tagging velocimetry. Vortex dynamics were characterized by tracking the location, core radius and circulation. The resulting velocity fields showed that the dynamics of the formation and separation of the leading-edge vortex were fundamentally different between the straight leading-edge airfoil and the tubercled airfoil. The tubercled airfoil also showed spanwise variation in dynamics of the dynamic stall vortex (DSV) formation. The characteristics of the DSV, specifically the circulation and proximity of the DSV to the airfoil suction surface, are known to have an impact on lift during dynamic pitching. The results showed that the DSV was stronger and remained closer to the airfoil longer for the modified airfoil. The baseline DSV convected away from the airfoil faster than the DSV on the tubercled airfoil once it began to separate from the airfoil. Shear-layer vortices, which form during dynamic stall near the mid-cord region, appeared to affect the convective behaviour of the DSV. The results suggest that the leading-edge tubercles observed on Humpback whale flippers act as passive flow-control mechanisms to control or delay dynamic stall.

Inflatable Leading Edge-Based Dynamic Stall Control considering Fluid-Structure Interaction

The inflatable leading edge (ILE) is explored as a dynamic stall control concept. A fluid-structure interaction (FSI) numerical method for the elastic membrane structure is constructed based on unsteady Reynolds-averaged Navier-Stokes (URANS) and a mass-spring-damper (MSD) structural dynamic model. Radial basis function- (RBF-) based mesh deformation algorithm and Laplacian and optimization-based mesh smoothing algorithm are adopted in flowfield simulations to achieve the pitching oscillation of the airfoil and to ensure the mesh quality. An airfoil is considered at a freestream Mach number of 0.3 and chord-based Reynolds number of 3.92×106. The airfoil is pitched about its quarter-chord axis at a sinusoidal motion. The numerical results indicate that the ILE can change the radius of curvature of the airfoil leading edge, which could reduce the streamwise adverse pressure gradient and suppress the formation of dynamic stall vortex (DSV). Although the maximum lift coefficient of the airfoil is slightly reduced during the control process, the maximum drag and pitching moment coefficients of the airfoil are greatly reduced by up to 66% and 75.2%, respectively. The relative position of the ILE has a significant influence on its control effect. The control laws of inflation and deflation also affect the control ability of the ILE.

Effects of Compressibility on Dynamic-Stall Onset Using Large-Eddy Simulation

Large-eddy simulations of the onset of dynamic stall for a NACA 0012 airfoil, pitching at a constant rate, are performed for a chord Reynolds number of . Using four simulations, spanning , the effects of compressibility are examined for constant Reynolds number. The interplay between the laminar separation bubble (LSB), separation within the turbulent boundary layer, and compressibility effects is elucidated. It is shown that increased compressibility results in earlier dynamic-stall onset related to bursting of the leading-edge LSB. Because of this earlier stall onset, which occurs at the leading edge, the role of the turbulent boundary-layer separation in dynamic stall is diminished. It is hypothesized that fundamental changes in the transition mechanism within the LSB, due to compressibility, are the cause of earlier leading-edge breakdown. At a region of supersonic flow develops above the leading-edge LSB. A unique system of compression and expansion structures is observed and documented. It is shown that this system couples with the inherent unsteadiness of the LSB, resulting in a brief period of high-amplitude coherent flow oscillations, before the development of the dynamic-stall vortex (DSV). At this condition, an elliptical shape of the DSV is observed due to the presence of supersonic flow above and below the developing vortex.