Wake Of Airfoil Research Articles

Experimental study of wake steadiness of an airfoil in pitch–hold–return motion

PurposeThe purpose of this paper is to analyze experimentally subsonic wake of a supercritical airfoil undergoing a pitch–hold–return motion. The focus of the investigation has been narrowed to concentrate on the steadiness of the flow field in the wake of the airfoil and the role of reduced frequency, amplitude and the hold phase duration.Design/methodology/approachAll experiments were conducted in a low sub-sonic closed-circuit wind tunnel, at a Reynolds number of approximately 600,000. The model was a supercritical airfoil having 10% thickness and wall-to-wall in ground test facilities. To calculate the velocity distribution in the wake of the airfoil, total and static pressures were recorded at a distance of one chord far from the trailing edge, using pressure devices. The reduced frequency was set at 0.012, 0.03 and the motion pivot was selected at c/4.FindingsAnalysis of the steadiness of the wake flow field ascertains that an increase in reduced frequency leads to further flow time lag in the hold phase whereas decreases the time that the wake remains steady after the start of the return portion. Also, the roles of amplitude and stall condition are examined.Practical implicationsExamination of a pitch–hold–return motion is substantial in assessment of aerodynamics of maneuvers with a rapid increase in angle of attack. Moreover, study of aerodynamic behavior of downstream flow field and its steadiness in the wake of the airfoil is vital in drag reduction and control of flapping wings, dynamic stability and control of aircrafts.Originality/valueIn the present study, to discuss the steadiness of the flow field behind the airfoil some statistical methods and concept of histogram using an automatic algorithm were used and a specific criterion to characterize the steadiness of flow field was achieved.

Parallel vortex body interaction enabled by active flow control

An experimental investigation has been conducted to demonstrate the utility of active flow control as a disturbance generator for vortex body interaction studies. The technique is used to explore the flow physics of parallel vortex body interaction between two NACA 0012 airfoils in series. Experiments were carried out at a chord-based Reynolds number of 740,000 relative to the first airfoil. Active flow control in the form of nanosecond pulse-driven dielectric barrier discharge plasma actuation, originating close to the leading edge, was used to produce vortex shedding from the upstream (disturbance) airfoil at various frequencies ($$0.038 \le F^+ \le 0.762$$). These vortices were characterized, showing reduced circulation and diameter with increasing frequency, before examining the downstream wake-airfoil interactions. Time-resolved pressure and phase-locked PIV measurements were taken on the downstream (target) airfoil for multiple angles of attack. For $$F^+ \le 0.5$$, the target airfoil is subject to strong oscillations from the wake of the disturbance airfoil that lead to large fluctuations in lift and pitching moment. However, a further increase in $$F^+$$ reattaches the flow over the disturbance airfoil and no major vortex body interactions are observed on the target. Governing parameters for this type of vortex body interaction are explored, and differences between isolated and non-isolated encounters as well as the presence of a viscous response are examined. Finally, means to alleviate loads caused by the incident vortex are explored.

Mode and regime identification for a static NACA0012 airfoil at transitional Reynolds numbers

This work examines the flow structure modes in the boundary layer and in the wake of a NACA0012 airfoil in static conditions at transitional chord-based Reynolds numbers (Rec), for small angles of attack (α). A laminar mode, with a laminar separation of the boundary layer and laminar Kármán streets in the wake, was first observed for Rec< 61400 andα= 0°. For 77 000 < Rec< 118600, which corresponds to a regime between laminar and transitional mode called subcritical mode, the boundary layer exhibited a long separation bubble reattached close to the trailing edge, and the wake showed a turbulent Kármán street. Finally, for higher Recandα, a critical transition mode consisted of a long bubble followed by a turbulent separation, and a less structured vortex street in the wake of the airfoil.

Prediction of Far Wake of a Two-dimensional Airfoil using the Large Eddy Simulation

Prediction of Far Wake of a Two-dimensional Airfoil using the Large Eddy Simulation

Modal Analysis of Fluid Flows: Applications and Outlook

We present applications of modal analysis techniques to study, model, and control canonical aerodynamic flows. To illustrate how modal analysis techniques can provide physical insights in a complementary manner, we selected four fundamental examples of cylinder wakes, wall-bounded flows, airfoil wakes, and cavity flows. We also offer brief discussions on the outlook for modal analysis techniques, in light of rapid developments in data science.

EXPERIMENTAL STUDY OF THE EFFECT OF THE WAKE GENERATED BY OSCILLATING GURNEY FLAP

The study of the turbulent near wake of an airfoil provided with flow control devices shows the flow configuration in such conditions and the starting vortices mechanism, while, the far wake offers information about the general performance of the model. The objective of the present work is to study the develop of the fluid-dynamic structures found in the NACA 4412 airfoil wake, as well as the development of the same structures when flow con-trol techniques are applied by means of a oscillating Gurney Flap place in the lower surface of the wing model, close to the trailing edge. Tests were per-formed at a given Reynolds numbers and the flow control system was set at different frequencies. In order to study the effect of the control mechanism on the wake, hot wire anemometry techniques were used. Two components of the velocity vector were measured - longitudinal and vertical - by means of a vertical array of three sensors acquiring simulta-neously. The intention is to quantify the flow general configuration at the airfoil wake for different test conditions. The velocity vectors will be analyzed, together with the turbulence intensity and integral scales.

Proper Orthogonal Decomposition Analysis of Flow Characteristics of an Airfoil with Leading Edge Protuberances

A numerical investigation of the aerodynamic performances and flow mechanisms of an airfoil with leading-edge protuberances is presented within stall region at a Reynolds number of . In detail, a large-eddy simulation has been conducted and then validated through quantitative comparisons with previous experimental and numerical investigations. Based on the numerical results, a proper orthogonal decomposition (POD) analysis has been carried out. The flow control mechanisms within stall regime and physical backgrounds of corresponding POD modes have been uncovered from the view of flow energy. It has been found that the improved aerodynamic characteristics of modified airfoil might be attributed to different flow mechanisms at peaks and troughs, including spanwise momentum transfer process and flow transition within laminar separation bubble. In addition, the fluctuating characteristics of airfoil flow field have also been investigated. Consequently, it has been found that the Karman vortices dominate the rear wake of the smooth airfoil and the first four POD modes are closely related with the shedding process. In contrast, the corresponding vortex shedding process is suppressed by the modified airfoil according to the results of POD analysis.

An information-theoretic study of fish swimming in the wake of a pitching airfoil

Abstract Swimming in schools affords several advantages for fish, including enhanced ability to escape from predators, searching for food, and finding correct migratory routes. However, the role of hydrodynamics in coordinated swimming is still not fully understood due to a lack of data-driven approaches to disentangle causes from effects. In an effort to elucidate the mechanisms underlying fish schooling, we propose an empirical study that integrates information theory and experimental biology. We studied the interactions between an actively pitching airfoil and a fish swimming in a flow. The pitching frequency of the airfoil was varied randomly over time, eliciting an information-rich interaction between the airfoil and the fish. Within an information-theoretic framework, we examined the information content of fish tail beating and information transfer from the airfoil to the fish. The proposed framework may help improve our understanding of the role of hydrodynamics in fish swimming, thereby supporting hypothesis-driven studies on the hydrodynamic advantages of fish schooling.

Effect of Piezo-Embedded Inverted Flag in Free Shear Layer Wake

The use of flexible inverted piezo embedded Polyvinylidene Difluoride (PVDF) as a simultaneous energy harvester and as a wake sensor is explored. The oscillation amplitude (characterized by voltage output) and oscillation frequency of the piezo-embedded PDVF was quantified in the wake of a 2D NACA 0012 model and SD7003 model at a Reynolds number of 100,000 and 67,000, respectively. The performance of the sensor was also quantified in the freestream without the presence of the wing. In order to quantify the sensor response to angle of attack and downstream distance, the amplitude and frequency of oscillations were recorded in the wing wake. Increase in angle of attack of the wing resulted in increase in oscillation frequency and amplitude of the PVDF. The results also indicated that the inverted flag configuration performed better in the wake under unsteady conditions when compared to freestream conditions. The results from Particle Image Velocimetry indicated that the wake signature was not affected by the presence of the PVDF in the wake. The root mean square voltage contours in the wake of SD7003 airfoil show remarkable free shear layer wake features such as upper and lower surface stratification and downwash angle which shows the sensitivity of the sensor to the unsteadiness in the wake. The capability of this device to act as a potential energy harvester and as a sensor has serious implications in extending the mission capabilities of small UAVs.

Experimental and Numerical Study of the Unsteady Wake of a Supercritical Airfoil in a Compressible Flow

Experimental investigations were carried out to study the wake profile of a supercritical airfoil at Mach numbers of 0.4 and 0.6 in a pitching motion. Both static and dynamic tests were conducted in a tri-sonic wind tunnel. Flow field inside the wake was measured by hot wire anemometry at downstream distances of 0.25 and 0.5 times the chord length from trailing edge. All data were taken at mean incidence angle of 3°; the amplitude of oscillation was 3° and the oscillation frequencies were 3 and 6 Hz. Moreover, numerical study was applied for the same airfoil under similar experimental test conditions; finally, wake profiles obtained from both numerical and experimental methods were compared.

Dual-position excitation technique in flow control over an airfoil at low speeds

PurposeThe purpose of this paper is to control flow separation over a NACA 4415 airfoil by applying unsteady forces to the separated shear layers using dielectric barrier discharge (DBD) plasma actuators. This novel flow control method is studied under conditions which the airfoil angle of attack is 18°, and Reynolds number based on chord length is 5.5 × 105.Design/methodology/approachLarge eddy simulation of the turbulent flow is used to capture vortical structures through the airfoil wake. Power spectral density analysis of the baseline flow indicates dominant natural frequencies associated with “shear layer mode” and “wake mode.” The wake mode frequency is used simultaneously to excite separated shear layers at both the upper surface and the trailing edge of the airfoil (dual-position excitation), and it is also used singly to excite the upper surface shear layer (single-position excitation).FindingsBased on the results, actuations manipulate the shear layers instabilities and change the wake patterns considerably. It is revealed that in the single-position excitation case, the vortices shed from the upper surface shear layer are more coherent than the dual-position excitation case. The maximum value of lift coefficient and lift-to-drag ratio is achieved, respectively, by single-position excitation as well as dual-position excitation.Originality/valueThe paper contributes to the understanding and progress of DBD plasma actuators for flow control applications. Further, this research could be a beneficial solution for the promising design of advanced low speed flying vehicles.

Numerical investigation of aerodynamic and acoustic characteristics of bionic airfoils inspired by bird wing

Airfoil is the basic element of fluid machinery and aircraft, and the noise generated from that is an important research aspect. Aiming to reduce the aerodynamic noise around the airfoil, this study proposes an airfoil inspired by the long-eared owl wing and another airfoil coupled with the bionic airfoil profile, leading edge waves, and trailing edge serrations. Numerical simulations dependent on the large eddy simulation method coupled with the wall-adapting local eddy-viscosity model and the Ffowcs Williams and Hawkings equation are conducted to compare the aerodynamic and acoustic characteristics of two types of bionic airfoils at low Reynolds number condition. The simulations reveal the dipole characteristic of acoustic source and sound pressure level distribution at various frequencies. Two types of bionic airfoils show lower noise compared with the conventional NACA 0012 airfoil with a similar relative thickness of 12%. Compared with the bionic airfoil, the average value of sound pressure level at the monitoring points around the bionic coupling airfoil is decreased by 9.94 dB, meanwhile the lift-to-drag ratio also keep higher. The bionic coupling airfoil exerts a suppression of sound pressure fluctuation on the airfoil surfaces, which result from that the range and size of separation vortices are reduced and the distance between vortices and airfoil surface are increased. The tube-shaped vortices in the wake of airfoil are effectively restrained and split into small scale vortices, which are important to cause less aerodynamic noise around the bionic coupling airfoil. Consequently, a novel bionic coupling airfoil is developed with the excellent aerodynamic and acoustic performance.

Experimental study of stall control over an airfoil with dual excitation of separated shear layers

This paper investigates the stall control over an airfoil employing dual excitation of separated shear layers with dielectric barrier discharge plasma actuators. This novel approach is studied experimentally on the NACA 0015 airfoil at the angle of attack and Reynolds number of 14° and 0.3×106, respectively. To analyze the baseline flow, Time-averaged pressure distributions on the airfoil surfaces are evaluated with 40 pressure taps. Further, the dominant natural frequencies associated with flow structures in the airfoil wake are examined utilizing a hot-wire anemometer. Two plasma actuators, one on the suction side just upstream the separation point and the other on the pressure side at the trailing edge, are exploited to control the baseline flow. Based on the active actuators, three controlled cases are considered in the present study. In the first case, the frequency of natural vortex shedding and its different harmonics are employed to excite the suction side separated shear layer. In the second case, plasma actuation is applied to the shear layer at the trailing edge. Further, in the third case, both of the actuators are utilized to excite the separated shear layers on the suction side and the trailing edge, simultaneously. The results of the pressure coefficient distributions, as well as the aerodynamic coefficients, are compared for all cases. Moreover, the actuators energy consumption is evaluated for the controlled cases and a new efficiency parameter is presented. Accordingly, the first case prevents the stall phenomenon only at the wake mode frequency and its first super-harmonic. In addition, using the plasma actuator at the trailing edge solely is an ineffective flow separation control strategy. However, dual excitation is promising for a full stall control in a wide range of excitation frequencies. Analyses of the lift-to-drag ratio and the efficiency parameter reveal that the dual excitation case is the most efficient stall control technique.

Flow separation control over an airfoil using dual excitation of DBD plasma actuators

This paper investigates flow separation control over an airfoil using dual excitation of DBD plasma actuators as a novel approach. Large eddy simulation is adopted to capture vortical structures within the airfoil wake. Power spectral density and dynamic mode decomposition analyses have been utilized to identify local and global oscillatory behavior associated with flow structures of the wake. Three controlled cases are considered in the present study. In the first case, the wake mode frequency (i.e., the frequency of natural vortex shedding) is used to excite separated shear layers at both the upper surface and the trailing edge of the airfoil, simultaneously. In the second case, the excitation frequency of the trailing edge actuator is increased to sixth multiples of the wake mode, while the other actuator excites the flow same as the first case. In the third case, excitation with frequency of the wake mode is only applied to the upper surface separated shear layer and no excitation is exerted at the trailing edge. According to the results, actuations manipulate the shear layers instabilities and modify the wake patterns remarkably. In all controlled cases, breakdown of the upper surface shear layer takes place closer to the separation point and also less strong vortices shed from the trailing edge compared to the baseline flow. It is revealed that the coherency of vortices within the wake region is not quite the same for all cases and the least coherency of vortical structures is observed in the first case. The maximum values of lift-to-drag ratio and lift coefficient is achieved in the first and second cases, respectively.

High-order delay detached-eddy simulations of cylindrical separated vortex/vortex induced noise based on transition model and acoustic analogy

The numerical prediction of transition from laminar to turbulent flow has proven to be an arduous challenge to computational fluid dynamics (CFD). Few approaches can provide routine accurate results within the cost limitations of engineering applications. In the present paper described is the application of a -Re transition model in combination with the delay detached eddy simulation (DDES) and Ffowcs Williams and Hawkings (FW-H) acoustic analogy method to cylinder vortex/vortex induced noise at a subcritical Reynolds number. In the process of numerical simulation, a traditional DDES based on the full-turbulence model SST is carried out for comparison and a 7th-order weighted compact nonlinear scheme (WCNS-E8T7) is adopted to ensure that the physical models are not affected by numerical dissipation or dispersion. In the first case, single cylinder cross-flow at ReD =4.3104 and Ma=0.21, is considered as a benchmarking problem for validating turbulence models and aerodynamic noise prediction methods. Its aerodynamic coefficients, St, CL and CD at root-mean-square (rms) and averaged values are measured by Szepessy and Bearman, while an acoustic measurement was recently made at Ecole Centrale de Lyon. The traditional DDES only based on SST model (SST-DDES) delays the instability of the shear layer on the sides of the cylinder, which leads to the recirculation zone in mean flow to grow and the induced drag to increase. Moreover, the vortex shedding frequency predicted by SST-DDES is larger than the actual value, which makes the whole sound pressure level (SPL) spectrum move toward high frequency region. However, combining the -Re transition model, the DDES (called Tran-DDES in the present article) can give the results in good agreement with the experimental data. In the second case considered is an airfoil in the wake of the cylinder. The flow condition is similar to that in the first case and the experimental results are also obtained at Ecole Centrale de Lyon. The issue of SST-DDES in recirculation zone in mean flow is weakened, which relates to the interaction between the airfoil and cylinder wake, the prediction of mean flow by SST-DDES is similar to that by the Tran-DDES. But in terms of the rms values of turbulent fluctuation components and SPL, the predictions by Tran-DDES are still better than those by SST-DDES.

Floquet stability analysis in the wake of a NACA0015 airfoil at post-stall angles of attack

We perform a Floquet stability analysis in the wake of a NACA0015 airfoil, with four angles of attack, α=20°, 17.5°, 15°, and 12.5°, considered. The central aim is to predict the secondary instabilities at the fixed moderate angles of attack, which are sufficiently large for massive separation from the airfoil, while at the same time allowing the formation and travelling of surface vortex on the airfoil, differentiating from the cases with very large angles of attack. We divide the angles of attack into two groups. We report that mode C is the first unstable mode for both groups, and which is also the only unstable mode for the small angles of attack, i.e., α=15° and α=12.5°, while for the high angles of attack, i.e., α=20° and α=17.5°, four unstable modes are observed. They are mode A, mode quasi-periodic, mode SL, and mode SS. The modes SL and SS are both subharmonic but with different wave numbers. We conjecture that these two subharmonic modes are resulted from the splitting of mode C. A comparison in the base flow topologies between the two groups shows that the different instability behaviors are probably due to their different flow patterns of the base flows. Three-dimensional direct numerical simulations (DNSs) have also been employed to study the physical realizability of the dominant unstable modes. A good consistency between the Floquet analysis and the 3D DNS results is achieved, indicating that the dominance of the linear instability is responsible for such a three-dimensional flow as the Reynolds number is not far from the critical value. Moreover, we find that the critical Reynolds numbers for the onset of three-dimensional instability fall into the range of 159.7–234.2 by defining this new Reynolds number according to the width of the flow wake. These values are very close to that of a bluff body. Furthermore, we note that the corresponding Strouhal numbers are around 0.17 for all the angles of attack, implying the relevance of three-dimensional instabilities to the wake dynamics or more specifically the fluctuation in the wake.

Proper-Orthogonal-Decomposition Study of Turbulent Near Wake of S805 Airfoil in Deep Stall

A proper-orthogonal-decomposition analysis was performed on the turbulent flow in the near-wake region of an S805 airfoil in deep stall at an angle of attack of 30 deg. The flow was measured using tomographic particle image velocimetry at a Reynolds number of 4600. Instantaneous turbulence structures, which were significant contributors to the first two proper-orthogonal-decomposition modes, were studied. These structures included the large-scale Kármán vortex and small-scale shear-layer vortices that were interacting with the Kármán vortex. An interesting correspondence was observed between the rotational vectors in proper-orthogonal-decomposition modes 4 and 5 and the locations of the leading-edge shear-layer vortices in the flowfields, which were major contributors to these two modes. Similarity was found in the autospectral functions computed from the velocity fields, which were significant contributors to the first five proper-orthogonal-decomposition modes. The wavelengths corresponding to the peak values of autospectral functions could be related to the size of the induced flows by the Kármán vortex and the streamwise spacing of the shear-layer vortices.

NACA0012翼型後流の変動流速をトリガーとしたプラズマアクチュエータのFeedback駆動による後縁剥離制御

NACA0012翼型後流の変動流速をトリガーとしたプラズマアクチュエータのFeedback駆動による後縁剥離制御

Experimental and numerical investigation into the wake of a supercritical airfoil in a compressible unsteady flow

Experimental investigations were carried out to study the wake characteristics of a pitching supercritical airfoil at Mach number of 0.6. Flow field inside the wake was measured by a hot-wire anemometry at downstream distances from trailing edge of 0.25 and 0.5 times the chord length. All data were taken at mean incidence angles of 0 and 3°. The amplitudes of oscillation were 1 and 3° while the oscillation frequencies were 3 and 6 Hz. Output signals acquired from sensors were analyzed besides the effects of such parameters as frequency and oscillation amplitude. Moreover, a comprehensive numerical study was carried out for the same airfoil under similar experimental test conditions; then, the results of numerical simulations were analyzed and compared with those of experimental tests. Results of the present research could be summarized as: observation of hysteresis and how it is affected by frequency and amplitude variations, observation of increasing turbulence intensity by root mean square investigation and also increasing signal energy by means of power spectral density diagram for those sensors lied inside the wake, and finally, study of correlation between wake’s interior sensors and exterior ones.

On the mechanism of trailing vortex wandering

The mechanism of trailing vortex wandering has long been debated and is often attributed to either wind-tunnel effects or an instability. Using particle image velocimetry data obtained in the wake of a NACA0012 airfoil, we remove the effect of wandering from the measured velocity field and, through a triple decomposition, recover the coherent wandering motion. Based on this wandering motion, the most energetic structures are computed using the proper orthogonal decomposition (POD) and exhibit a helical mode with an azimuthal wavenumber of $|m|=1$ whose kinetic energy grows monotonically in the downstream direction. To investigate the nature of the vortex wandering, we perform a spatial stability analysis of a matched Batchelor vortex. The primary stability mode is found to be marginally stable and nearly identical in both size and structure to the leading POD mode. The strikingly similar structure, coupled with the measured energy growth, supports the proposition that the vortex wandering is the result of an instability. We conclude that the cause of the wandering is the non-zero radial velocity of the $|m|=1$ mode on the vortex centreline, which acts to transversely displace the trailing vortex, as observed in experiments. However, the marginal nature of the stability mode prevents a definitive conclusion regarding the specific type of instability.