Phase-averaged Flow Fields Research Articles

Vortex dynamics and heat transfer behind self-oscillating inverted flags of various lengths in channel flow

The influence of an inverted flag’s length-to-channel-width ratio (C* = L/W) on its oscillating behavior in a channel flow and the resultant vortex dynamics and heat transfer are determined experimentally. Three systems with C* values of 0.125, 0.250, and 0.375 were chosen for comparison. The interaction of highly unsteady flow with the inverted flag is measured with time-resolved particle image velocimetry. Variations in the underlying flow physics are discussed in terms of the statistical flow quantities, flag displacement, phase-averaged flow field, and vortex dynamics. The results show that the increase in C* shifts the occurrence of the flapping regime at high dimensionless bending stiffness. With the flag in the flapping region, three distinct vortex dynamics—the von Kármán vortex street, the G mode, and the singular mode—are identified at C* values of 0.375, 0.250, and 0.125, respectively. Finally, the heat transfer enhancement from the self-oscillating inverted flag is measured to serve as complementary information to quantify the cause-and-effect relationship between vortex dynamics and wall heat transfer. The increase in C* strongly promotes wall heat removal because disruption of the boundary layer by the energetic vortices is substantially intensified. Among all systems, wall heat transfer removal is most efficient at the intermediate C* value of 0.250.

Unsteady behavior of a sweeping impinging jet: Time-resolved particle image velocimetry measurements

The unsteady behavior of a sweeping impinging jet is measured experimentally using time-resolved particle image velocimetry. For the configuration with a jet-to-wall spacing ratio L/dh = 8, an approximately linear increase in the sweeping frequency is observed with Reynolds numbers (Re) between 2.7 × 103 and 9.3 × 103, especially at high Reynolds numbers. The saturation of the sweeping jet, at which the maximum deflection angle is reached, occurs at Re = 6.7 × 103. Special focus is then placed on the spatial and temporal variations of unsteady flow fields at two Reynolds numbers Re = 4.0 × 103 and 9.3 × 103. The unsteady behavior in the near-exit region is first compared. At a higher Reynolds number, the jet in the near-exit region remains at the maximum deflection angle for a longer time during one oscillation cycle with more concentrated jet momentum, resulting in a faster switching process. In the near-wall region at Re = 4.0 × 103, the sweeping impinging jet exerts a large region of influence due to the oscillation motion of the impact region along the wall. The time-averaged velocity components and velocity fluctuating components near the wall show double peaks on both outer sides and a minimum in the middle of the near-wall region. At Re = 9.3 × 103, due to the rapid and intensive sweeping process, the jet column breaks in the near-exit region, resulting in a weak flow in the middle of the near-wall region. Accordingly, the profiles of the time-averaged velocity components and velocity fluctuating components show higher double-peak values but an even lower minimum value. Finally, the state-of-the-art dynamic mode decomposition method is used to capture the main flow behavior of distinct frequencies at these two selected Reynolds numbers. Bounded by the breaking location of the jet column, the flow with a superharmonic oscillation frequency in the middle of the near-wall region disappears at Re = 9 × 103. Therefore, the energetic flow patterns are found to be more evenly distributed in the near-exit region than in near-wall region. The phase correlation between the captured flow patterns is determined by projecting the phase-averaged flow fields onto the most energetic modes.

A POD-Based Procedure for the Split of Unsteady Losses of an LPT Cascade

A time-resolved particle image velocimetry (TR-PIV) system has been employed to investigate the unsteady propagation of upstream wakes in a low-pressure turbine cascade. Data are obtained in the steady state condition and for two passing wake reduced frequencies. The study is focused on the identification and split of the different dynamics responsible for deterministic and random oscillations, thus loss generation by means of a new procedure based on proper orthogonal decomposition (POD). The paper takes advantage of the properties of POD that reduce the data set to a low number of modes that represent the most energetic dynamics of the system. It is clearly shown that the phase averaged flow field can be represented by a few number of POD modes related to the wake passing event for the unsteady cases. Proper orthogonal decomposition is also able to capture flow features affecting the instantaneous flow field not directly related to the wake passage (i.e., the vortex shedding phenomenon induced by the intermittent separation developing between adjacent wakes), which are smeared out in the phase averaged results. A procedure exploiting the biorthogonality condition of the POD modes, and the related temporal coefficients, has been developed for the quantification of the contribution due to the different POD modes to the overall turbulence kinetic energy production, or, equivalently, the mean flow energy dissipation rate. Results into the paper clearly show that losses due to wake migration, boundary layer and vortex shedding related phenomena can be distinguished and separately quantified for the different tested conditions.

Experimental investigation of high-incidence delta-wing flow control

The possibility of extending the flight envelope for configurations with slender delta-shaped wings is investigated in this study by means of active flow control through pulsating jets from slot pairs distributed along the leading edge. The experiments comprise stereoscopic particle image velocimetry as well as force and moment measurements on a half-delta wing model. The analysis focuses on three high-incidence regimes: pre-stall, stall, and post-stall. This study also compares different perturbation methods: blowing with spatially constant and variable parameters, frequency and phase. At an incidence of 45 $$^\circ$$ , the unison pulsed blowing facilitates the most significant flow transformation. Here, the separated shear layer reattaches on the wing’s suction side, thus increasing the lift. Phase-averaged flow field measurements describe, in this particular case, the underlying physics of the flow–disturbance interaction.

Near-field interaction of an inclined jet with a crossflow: LIF visualization and TR-PIV measurement

Experiments are conducted in a water tunnel to investigate the near-field interaction of an inclined jet with a crossflow over a flat plate. The tunnel contains a jet emerging from a round pipe inclined at a 30° angle to the streamwise direction of the crossflow. The flow structures induced by the inclined jet are examined with laser-induced fluorescence (LIF) visualization and time-resolved particle image velocimetry (TR-PIV). The behaviors of the near-field flow are compared at four different jet-to-cross-flow velocity ratios (VRs): 0.25, 0.5, 0.75, and 1.0. It is found that the inclined configuration significantly weakens the interaction between the jet and the crossflow, especially at lower VRs. As such, at VR ≤0.5 (typically at VR = 0.25), the inclined jet-in-crossflow (JICF) behaves differently from a highly unsteady normal JICF at low VRs. The flow patterns are relatively simple and only weakly unsteady. A counter-rotating vortex pair (CRVP) is well observed. As VR increases up to 1.0, the inclined JICF fully detaches from the flat plate and shows the classical topology of the normal JICF at high VRs. Both CRVP and shear layer vortices are well captured in this highly unsteady flow regime. The different flow structures together with the interaction between the inclined jet and the crossflow near the jet exit are found to have a strong impact on the distribution of jet-shear layer, jet trajectory, and the jet influence on the crossflow, especially on the near-wall region. Proper orthogonal decomposition is performed on the TR-PIV results to extract the dominant fluctuating modes and reconstruct phase-averaged flow fields. It is found that the highly unsteady flow regime at VR = 1.0 is very unstable, varying between two flow patterns with different fluctuating frequencies at downstream of the jet column. The jet flow near to the exit is also found to be remarkably unsteady due to the interaction between the emerging jet and the crossflow.

Large eddy simulation of propeller wake instabilities

The wake of a five-bladed marine propeller at design operating condition is studied using large eddy simulation (LES). The mean loads and phase-averaged flow field show good agreement with experiments. Phase-averaged and azimuthal-averaged flow fields are analysed in detail to examine the mechanisms of wake instability. The propeller wake consisting of tip and hub vortices undergoes streamtube contraction, which is followed by the onset of instabilities as evident from the oscillations of the tip vortices. Simulation results reveal a mutual-induction mechanism of instability where, instead of the tip vortices interacting among themselves, they interact with the smaller vortices generated by the roll-up of the blade trailing edge wake in the near wake. It is argued that although the mutual-inductance mode is the dominant mode of instability in propellers, the actual mechanism depends on the propeller geometry and the operating conditions. The axial evolution of the propeller wake from near to far field is discussed. Once the propeller wake becomes unstable, the coherent vortical structures break up and evolve into the far wake, composed of a fluid mass swirling around an oscillating hub vortex. The hub vortex remains coherent over the length of the computational domain.

Vortex dynamics for flow over a circular cylinder in proximity to a wall

The dynamics of vortical structures in flow over a circular cylinder in the vicinity of a flat plate is investigated using particle image velocimetry (PIV). The cylinder is placed above the flat plate with its axis parallel to the wall and normal to the flow direction. The Reynolds number $Re_{D}$ based on the cylinder diameter $D$ is 1072 and the gap $G$ between the cylinder and the flat plate is varied from gap-to-diameter ratio $G/D=0$ to $G/D=3.0$. The flow statistics and vortex dynamics are strongly dependent on the gap ratio $G/D$. Statistics show that as the cylinder comes close to the wall ($G/D\leqslant 2.0$), the cylinder wake becomes more and more asymmetric and a boundary layer separation is induced on the flat plate downstream of the cylinder. The wake vortex shedding frequency increases with decreasing $G/D$ until a critical gap ratio (about $G/D=0.25$) below which the vortex shedding is irregular. The deflection of the gap flow away from the wall and its following interaction with the upper shear layer may be the cause of the higher shedding frequency. The vortex dynamics is investigated based on the phase-averaged flow field and virtual dye visualization in the instantaneous PIV velocity field. It is revealed that when the cylinder is close to the wall ($G/D=2.0$), the cylinder wake vortices can periodically induce secondary spanwise vortices near the wall. As the cylinder approaches the wall ($G/D=1.0$) the secondary vortex can directly interact with the lower wake vortex, and a further approaching of the cylinder ($G/D=0.5$) can result in more complex interactions among the secondary vortex, the lower wake vortex and the upper wake vortex. The breakdown of vortices into filamentary debris during vortex interactions is clearly revealed by the coloured virtual dye visualizations. For $G/D<0.25$, the lower shear layer is strongly inhibited and only the upper shear layer can shed vortices. Investigation of the vortex formation, evolution and interaction in the flow promotes the understanding of the flow physics for different gap ratios.

Large-eddy simulation of a pulsed jet into a supersonic crossflow

Abstract Numerical investigation of a pulsed jet injected into a supersonic crossflow was carried out using large-eddy simulation. The corresponding steady jet injection was also calculated for comparison with the pulsed jet injection and for validation against experimental data. Various fundamental mechanisms dictating the intricate flow characteristics, such as multi-scale vortical structures, complex shock systems, mixing properties and turbulence behaviors, have been studied. As the pulsed jet issuing transversely into the supersonic crossflow, the salient shock systems and vortical structures in terms of the instantaneous and phase-averaged flow field are analyzed. The large-scale jet shear vortices are formed along the interface of jet and crossflow due to the pulsation effect, which can promote the engulfing of the crossflow fluid and the mixing of the jet and crossflow fluids. Further we investigate turbulence properties of the jet and crossflow interaction and scalar statistics of the injectant mass fraction to reveal jet penetration enhancement and mixing property improvement for the pulsed jet. Results obtained in this study provide physical insight into the understanding the mechanisms for the interaction of pulsed jet and supersonic crossflow.

Three-Dimensional Flow Field Investigations of Flapping Wing Aerodynamics

Three-dimensional unsteady flow fields of a flapping, low-aspect-ratio wing have been investigated by means of highly resolved tomographic particle image velocimetry (Tomo-PIV) measurements and computational fluid dynamics (CFD). Furthermore, force measurements have been carried out. Tomo-PIV was applied to the flow above a flat plate wing during the downstroke. High spatial resolution and large volume thickness could be achieved by using sensitive sCMOS cameras and a traversing setup. The CFD calculations covered the complete period of motion. The analysis of the vortex-dominated flow fields provides a deeper understanding of vortex interaction and three-dimensionality of low Reynolds number ( and ) flows. Two different Strouhal numbers ( and ) are considered and their effects on the development of a leading edge and tip vortex are discussed. The PIV results show instantaneous flow fields after a leading edge separation that are dominated by small-scale turbulent vortex structures. The presented CFD approach is able to predict these vortices by using highly resolved meshes. Coarser grids compare well with the phase-averaged experimental flow fields, which feature multiple large-scale leading edge vortices developing during the downstroke. Turbulent effects decrease for the lower Reynolds number. Force and moment hystereses as well as large-scale leading edge vortice circulation, calculated from the PIV results, increase with increasing Strouhal number. Vortex breakdown of the wing tip vortex can be observed during the downstroke in the experimental data.

Tomographic PIV behind a prosthetic heart valve

The instantaneous three-dimensional velocity field past a bioprosthetic heart valve was measured using tomographic particle image velocimetry. Two digital cameras were used together with a mirror setup to record PIV images from four different angles. Measurements were conducted in a transparent silicone phantom with a simplified geometry of the aortic root. The refraction indices of the silicone phantom and the working fluid were matched to minimize optical distortion from the flow field to the cameras. The silicone phantom of the aorta was integrated in a flow loop driven by a piston pump. Measurements were conducted for steady and pulsatile flow conditions. Results of the instantaneous, ensemble and phase-averaged flow field are presented. The three-dimensional velocity field reveals a flow topology, which can be related to features of the aortic valve prosthesis.

Benchmark CFD validation data for surface combatant 5415 in PMM maneuvers – Part II: Phase-averaged stereoscopic PIV flow field measurements

Part II of this two-part paper presents computational fluid dynamics (CFD) benchmark local flow field measurements for a surface combatant in dynamic planar-motion-mechanism (PMM) maneuvers. Phase-averaged stereoscopic particle-image-velocimetry (PIV) measurements are performed for pure sway and pure yaw maneuvers. The geometry is David Taylor Model Basin (DTMB) model 5512, a 3.048m geosim of DTMB model 5415. The experiments are done in a 100m×3m×3m towing tank. The measurement system is a custom-designed towing tank maneuvering test flow-mapping system, which features a PMM for captive model testing with an integrated stereoscopic PIV. Statistical convergence error is defined and monitored by using the confidence intervals of the mean and variance. The quality of data is also assessed by following standard uncertainty assessment procedures. The present experimental data are cross-referenced with computational simulations for identification of the major vortices in the flow. The core locations and trajectories of the major vortices are traced and the flow variables at the core locations are analyzed. The tests of both Parts I and II are sufficiently documented and detailed to be useful as experimental benchmark data for validation of CFD codes. Part I presents the force/moment/motion measurements.

Dynamic stall process on a finite span model and its control via synthetic jet actuators

An experimental study of the process by which dynamic stall occurs on a finite span S809 airfoil was conducted at the Center for Flow Physics and Control at Rensselaer Polytechnic Institute. Understanding the flow field around a dynamically pitching airfoil helped in controlling the dynamic stall process through active flow control via synthetic jet actuators. The three component, two dimensional flow fields were measured with a stereoscopic particle image velocimetry system. This study demonstrated that, through the introduction of periodic momentum near the leading edge of this model, the evolution of the dynamic stall vortex, which forms and convects downstream under dynamic conditions, could be delayed or suppressed in favor of the preservation of a trailing edge vortex that arises due to trailing edge separation and recirculation in the time averaged sense. This process seems to be the result of changing how the flow field transitions from trailing edge separation to a fully separated flow. In a phase-averaged sense, absent of flow control, this process is defined by the creation of a phase averaged leading edge recirculation region, which interacts with the trailing edge separation. Through the introduction of momentum near the leading edge, this process can be altered, such that the phase averaged trailing edge separation region is the dominant structure present in the flow. Additionally, a cursory investigation into the instantaneous flow fields was conducted, and a comparison between the phase averaged flow field and instantaneous fields demonstrated that while similar effects can be observed, there is a significant difference in the flow field observed in the instantaneous fields versus the phase averaged sense. This would imply that a different method of analyzing dynamic stall from PIV measurements may be necessary.

Benchmark CFD validation data for surface combatant 5415 in PMM maneuvers – Part I: Force/moment/motion measurements

Part I of this two-part paper presents benchmark CFD validation force/moment/motion measurements for surface combatant 5415 in planar-motion-mechanism (PMM) maneuvers. The experiments are conducted in the IIHR towing tank as part of an international collaboration for development/application of uncertainty analysis procedures and assessment of scale effects and facility biases. Stationarity and normality tests and statistical convergence errors, single and multiple run methods for obtaining the maneuvering mathematical model hydrodynamic derivatives, and the effects of heave, pitch and roll motions and fixed/free mount conditions are assessed. Assessment of the methods for obtaining the hydrodynamic derivatives is based on reconstructions of the measured force and moment time histories. Part 2 presents phase-averaged stereo PIV flow field measurements. The data is used as a test case at the SIMMAN 2008 and 2014 workshops on verification and validation of ship maneuvering simulation methods.

Flow field studies on a micro-air-vehicle-scale cycloidal rotor in forward flight

This paper examines the flow physics and principles of force production on a cycloidal rotor (cyclorotor) in forward flight. The cyclorotor considered here consists of two blades rotating about a horizontal axis, with cyclic pitch angle variation about the blade quarter-chord. The flow field at the rotor mid-span is analyzed using smoke flow visualization and particle image velocimeV are compared with flow fields predicted using 2D CFD and time-averaged force measurements acquired in an open-jet wind tunnel at three advance ratios. It is shown that the experimental flow field is nearly two dimensional at μ = 0.73 allowing for qualitative comparisons to be made with CFD. The incoming flow velocity decreases in magnitude as the flow passes through the retreating (upper) half of the rotor and is attributed to power extraction by the blades. A significant increase in flow velocity is observed across the advancing (lower) half of the rotor. The aerodynamic analysis demonstrates that the blades accelerate the flow through the lower aft region of the rotor, where they operate in a high dynamic pressure environment. This is consistent with CFD-predicted values of instantaneous aerodynamic forces which reveal that the aft section of the rotor is the primary region of force production. Phase-averaged flow field measurements showed two blade wakes in the flow, formed by each of the two blades. Analysis of the blades at several azimuthal positions revealed two significant blade-wake interactions. The locations of these blade-wake interactions are correlated with force peaks in the CFD-predicted instantaneous blade forces and highlight their importance to the generation of lift and propulsive force of the cyclorotor.

An Investigation into In-Cylinder Tumble Flow Characteristics with Variable Valve Lift in a Gasoline Engine

In this paper, the investigation into in-cylinder tumble flow characteristics with reduced Maximum Valve Lifts (MVL) is presented. The experimental work was conducted in a modified four-valve Spark-Ignition (SI) test engine, with optical accesses for measuring in-cylinder air motion in the vertical direction. Three different MVL of 6.8 mm, 4.0 mm and 1.7 mm were tested and Particle Image Velocimetry (PIV) was employed for those measurements. Measurement results were analysed by examining the tumble flow field, the tumble ratio variation and the fluctuating kinetic energy distribution. Meanwhile, a numerical analysis method for detecting the vortex centre was developed. From results of the vortex centre distribution, the cyclic variation of the in-cylinder flow was explored. The phase-averaged flow fields show that higher MVLs could produce stronger vertical flows which turn more toward to the piston top and finally are possible to form big scale tumble flow structure. Although lower MVLs create a higher tumble ratio when the piston is close to the Bottom Dead Centre (BDC), higher MVLs substantially produce higher tumble ratios when the piston is moving close to the Top Dead Centre (TDC). In terms of kinetic energy, lower MVLs result in higher values including higher total kinetic energy and higher fluctuating energy. Finally, the vortex centres results demonstrate lower MVLs could enhance cycle-to-cycle variation due to the weakened tumble vortex.

Drag reduction of square cylinders with cut-corners at the front edges

Flow around square cylinders with cut-corners at the front edges is investigated using particle image velocimetry. It is found that drag reduction can be achieved for the tested cut-corner dimensions. The mechanism for the drag reduction is explored on the statistical and structural aspects of the flow. After cutting the corners, the fluctuation intensity of the wake is weakened, the length of the recirculation region behind the square cylinder is increased, while the width of the wake decreases. It is found that the drag coefficient is proportional to the minimum wake width, and the Strouhal number St is inversely proportional to the minimum wake width. It is revealed that the reduced wake width is due to the suppressed separation over the side surfaces for the cylinders with cut-corners at the front edges. On the structural aspect, the phase-averaged flow field and the modes from proper orthogonal decomposition both indicate a decrease in the wake vortex size. A statistical analysis of instantaneous vortices based on Oseen vortex fit reveals that not only the size of the vortex is reduced, but also the strength is weakened.

Effect of metachronal phasing on the pumping efficiency of oscillating plate arrays

A programmable oscillating plate array was constructed in order to study the detailed hydrodynamics of external pumping by a series of oscillating plates at Reynolds numbers on the order of 10. The array was modeled after the geometry and kinematics found in the nymphal mayfly (Ephemeroptera) Centroptilum triangulifer, and consisted of five plates, each of which could be actuated independently for stroke and pitch. Scaled tests were performed at a Reynolds number, Re = fL g 2 /ν = 18, with a single stroke kinematic pattern modeled after the living animal. In mayflies, and in many other oscillating plate systems, an antiplectic metachronal wave is used with a phase delay of approximately 90°, which corresponds to a travelling wave that moves from posterior to anterior with a wavelength of approximately four plates. In order to better understand possible reasons for why the animal system might favor the observed phase lag, ensemble-correlation stereo PIV measurements were made to reconstruct the unsteady three-dimensional phase averaged flow field at a resolution that allowed a uniform and converged estimate of the net pumped flux and the total energy dissipation within and around the vicinity of the gill array. The results indicate that the baseline case offered an optimal spot in the mass flux of fluid pumped through the array per unit energy expended, while also providing a great deal of flexibility in modifying the stroke amplitude without interference effects from adjacent gills.

Flow past a cylinder with a flexible splitter plate: A complementary experimental–numerical investigation and a new FSI test case (FSI-PfS-1a)

Objectives: The objective of the present paper is to provide a challenging and well-defined validation test case for fluid-structure interaction (FSI) in turbulent flow to close a gap in the literature. The following list of requirements are taken into account during the definition and setup phase. First, the test case should be geometrically simple which is realized by a classical cylinder flow configuration extended by a flexible structure attached to the backside of the cylinder. Second, clearly defined operating and boundary conditions are a must and put into practice by a constant inflow velocity and channel walls. The latter are also evaluated against a periodic setup relying on a subset of the computational domain. Third, the material model should be widely used. Although a rubber plate is chosen as the flexible structure, it is demonstrated by additional structural tests that a classical St. Venant-Kirchhoff material model is sufficient to describe the material behavior appropriately. Fourth, the flow should be in the turbulent regime. Choosing water as the working fluid and a medium-size water channel, the resulting Reynolds number of Re=30,470 guarantees a sub-critical cylinder flow with transition taking place in the separated shear layers. Fifth, the test case results should be underpinned by a detailed validation process.Methods: For this purpose complementary numerical and experimental investigations were carried out. Based on optical contactless measuring techniques (particle-image velocimetry and laser distance sensor) the phase-averaged flow field and the structural deformations were determined. These data were compared with corresponding numerical predictions relying on large-eddy simulations and a recently developed semi-implicit predictor-corrector FSI coupling scheme.Outcome: Both results were found to be in close agreement showing a quasi-periodic oscillating flexible structure in the first swiveling FSI mode with a corresponding Strouhal number of about StFSI=0.11.

입자영상유속계를 이용한 자항상태 모형선의 프로펠러 후류 계측

A two-dimensional particle image velocimetry (2D PIV) system in a towing tank is employed to measure a wake field of a very large crude oil carrier model with rotating propeller in self propulsion condition, to identify characteristics of wake of a propeller working behind a ship. Phase-averaged and time-averaged flow fields are measured for a horizontal plane. Scale ratio of the model ship is 1/100 and Froude number is 0.142. By phase-averaging technique, trajectories of tip vortex and hub vortex are identified and characteristic secondary vortex distribution is observed in the hub vortex region. Propeller wake on the starboard side is more accelerated than that on the port side, due to the difference of inflow of propeller blades. The hub vortex trajectory tends to face the port side. With the fluctuation part of the phase-averaged velocity field, turbulent kinetic energy (TKE) is also derived. In the center of tip vortex and hub vortex region, high TKE concentration is observed. In addition, a time-averaged vector field is also measured and compared with phase-averaged vector field.

Experimental investigation of a trailing edge L-shaped tab on a pitching airfoil in deep dynamic stall conditions

An L-shaped tab was tested at the trailing edge of an oscillating airfoil to evaluate its effects on blades aerodynamic performance. The tests were conducted on a NACA 23012 pitching airfoil in deep dynamic stall conditions with the L-shaped tab fixed in two different positions. When deployed the tab is attached to the airfoil upper surface so that the end prong protrudes at the airfoil trailing edge. In retracted position the tab features an angle of 9.1° with the airfoil upper surface, since its prong tip touches the airfoil trailing edge. The airloads time histories during a pitching cycle were evaluated by pressure measurements carried out on the airfoil midspan contour. The phase-averaged flow field at the trailing edge region was investigated by means of particle image velocimetry to evaluate the detailed flow physics involved in the use of the device. The experimental results indicate that the use of such a pivoting L-shaped tab can introduce similar effects to those that can be obtained by the use of an active Gurney flap. Thus, the L-shaped tab can be considered an attractive device due to its easier integration on helicopter blades.