Pulsed Blowing Research Articles

Active Flow Control of Supersonic Jet Using Streamwise Pulsed Blowing

The present study investigates a novel control strategy to create a sweeping oscillatory jet using fluidic actuation principle. A supersonic underexpanded primary jet issued into a suddenly expanding duct is oscillated by giving cyclic pulsed secondary jet blowing in the top and bottom step regions of the suddenly expanding duct. The current study employs the bistable nature of jet expansion seen in a suddenly expanding duct for the jet oscillation. The cyclic perturbations (pulsed secondary jet blowing) given in the streamwise direction switch the direction of the asymmetric jet expansion of the underexpanded primary supersonic jet to the top and bottom wall alternatively, thereby creating an oscillatory supersonic jet. It is found that the switching of the direction of the primary jet to any one wall is caused by the Coanda effect produced by the secondary jet issued in the corresponding wall side. The results show that the primary jet quickly responds to the secondary jet perturbations and that the supersonic primary jet oscillation frequency nearly matches secondary jet perturbation frequency. A parametric study to investigate the jet oscillation characteristics with various secondary jet oscillation frequencies and mass flow rate ratios are also being carried out in the present study. It is found that the jet oscillation frequency increases with increase in the secondary jet pulsation frequency. The jet deflection is found to increase with increase in the secondary jet to primary jet mass flow ratio (blowing ratio) up to a critical limit, after which the increment in jet deflection ceases due to the restrictions imposed by the duct geometry.

Coandă-Type Flow Actuation for Load Alleviation

Circulation control has the potential to improve current gust and maneuver load alleviation schemes, with the ultimate goal of reducing wing structure weight and saving fuel. A Coandă-type flow actuator is integrated on the pressure side of a two-dimensional airfoil model, and its load alleviation performance and dynamic behavior are determined experimentally. Quasi-steady force and wake measurements are conducted in a wind tunnel to quantify the effect of the actuator on lift and drag. Lift decrements of are reached for steady blowing, exceeding typical gust-induced wing loads in magnitude. For pulsed blowing, only moderate mass flow reductions of up to 25% are achieved, caused by discontinuous flow turning around the Coandă surface. Unsteady force and pressure measurements characterize the dynamic lift response to the actuation. The total onset time of the lift response, normalized by the ratio of chord length to freestream velocity, is for actuator activation and for actuator deactivation. These values are below typical onset times for gust loads (). The present results show a high-lift reduction potential and fast dynamic behavior of the Coandă-type flow actuator, making it a viable candidate for fast and effective load alleviation.

Suction and Pulsed Blowing Applied to Local Separation Downstream of Slat Cutout

An experimental study on the application of active flow control (AFC) to a .4 scale model of a swept wing in a landing configuration was conducted. The wing is fitted with an ultrahigh bypass ratio (UHBR) engine nacelle. The highly efficient UHBR engines are characterized by a large diameter that interferes with the flow around the wing, especially with slats deflected, degrading its performance. An innovative AFC device, creating steady suction and pulsed blowing (PB), was installed in the leading-edge region of the wing, above the nacelle, and its performance was experimentally evaluated. The effects of the suction and PB mechanisms were examined individually and simultaneously using relevant normalized parameters to pave the way to a full-scale wind-tunnel test. It was shown that the AFC actuators increased lift by up to 3%, redirected the flow to the desired downstream direction, and reduced the size of the separation zone created due to the implementation of the UHBR nacelle. The next step is validating the small-scale results of this study in a full-scale wind-tunnel tests that would be the last step before the technology is flight test ready.

Microjet Configuration Sensitivities for Active Flow Control on Multi-Element High-Lift Systems

Various characteristics of small surface-normal jets (microjets) located on a flap’s pressure-side near the trailing edge are investigated as an active aerodynamic load control technology for multi-element high-lift systems. Two-dimensional computational studies are carried out to investigate the sensitivity of microjet aerodynamic effectiveness to configuration. Initially, the effects of microjet location and width are explored. The study shows that lift enhancement decreases as the microjet is moved forward from the trailing edge, and that wider microjets are more effective in lift enhancement and drag reduction, whereas thinner microjets are more efficient. Microjet exit velocity profile effect on the integrated forces is also studied, and the effects are shown to be insignificant. To gain preliminary insight of the microjet system requirements, several plenum configurations are studied, and a preliminary analysis of the power requirements to supply the air mass flow is presented. Next, the study is extended to a geometry with moderate trailing edge separation to demonstrate microjet’s separation mitigation capability. To gain better understanding of the system requirements, pulsed versus continuous microjets are examined, and it is found that pulsed blowing causes a breakdown of the favorable effect of the microjet on the trailing edge flow and therefore reduces the circulation control effectiveness of the microjet.

Manipulation of three-dimensional asymmetries of a turbulent wake for drag reduction

Abstract

A comparative experimental analysis of two unsteady flow control methods in a highly loaded compressor cascade

A comparative experimental analysis is performed on pulsed suction (PS) and pulsed blowing (PB) used to control flow separations in a highly loaded compressor cascade. The effectiveness of the control methods is assessed via oil-flow visualization and steady and unsteady pressure measurements. Firstly, the control effect of the PS is evaluated by comparing the conventional steady continuous suction (SCS). A more efficient control effect is achieved by the PS compared to the SCS. Additionally, in order to further explore the potentials of the PS and PB and gain some insight into their controlling mechanisms, some important excitation parameters including excitation location, momentum coefficient and frequency are comparatively investigated in detail. It is found that the PS and PB are both able to improve the cascade performance by effectively suppressing the passage vortex. With the excitation location moving downstream, the almost opposite change trends for the PS and PB on the total pressure loss and energy efficiency are shown. The PS has an advantage over the PB in improving the cascade performance at the same average excitation momentum. But there is a slighter change of the losses for the PB cases at different excitation frequencies relative to the PS ones, indicating that the PB is more insensitive to the excitation frequency. Based on the optimal excitation parameters, the total pressure loss coefficients for the PS and PB are reduced by 11.3% and 10.3%, respectively. Furthermore, the effectiveness of the PS and PB is also corroborated at a larger incidence angle.

The suppression of flying-wing roll oscillations with open and closed-loop spanwise blowing

Uncommanded self-induced roll oscillations cause significant challenges in flight control and are ubiquitous in high-attack-angle flights in the flying-wing configuration. Spanwise blowing slots were designed in this article to suppress these roll oscillations with nonzero mean angles of a 53° swept flying-wing model. Three flow control strategies were proposed and studied successively. The open-loop symmetric blowing successfully suppressed oscillations below the nominal angle of attack of 24° before losing its effectiveness. Subsequently, adaptive continuous blowing using the proportion-integral-differential algorithm was performed with symmetric pre-blowing to eliminate the ineffective region with a small blowing coefficient. Using this strategy, roll oscillations under all tested nominal angles of attack were successfully addressed as the nonzero trim angles were eliminated and the oscillations were suppressed. However, the air consumption needed to maintain the model balance was too large, which necessitated the development of pulsed blowing. Static force measurements showed that the blowing-induced rolling moment had a linear relationship with the duty cycle of blowing. The results from the particle-image-velocimetry tests revealed that the mechanism of the linear relationship was the increased duration of coherent vortices over a period at longer duty cycles. Based on the static experimental results, the model was stabilized via proportion-integral-differential adaptive blowing, which utilized the duty cycle as the control variable. Compared to the second strategy, the last one reduced the required mean blowing momentum coefficient by two orders of magnitude while maintaining a great control effect.

Pulsed Blowing Interacting with a Leading-Edge Vortex

Manipulation of vortex instabilities for aerodynamic performance increase is of great interest in numerous aeronautical applications. With increasing angle of attack, the leading-edge vortex of a semi-slender delta wing becomes unsteady and eventually collapses, endangering the flight stability. Hence, active flow control by pulsed blowing stabilizes the vortex system, enlarging the flight envelope for such wing configurations. The most beneficial outcome is the reattachment of the separated shear layer during post-stall, contributing to a lift increase of more than 50%. In contrast to high power consuming brute-force actuation, manipulating the flow instabilities offers a more efficient alternative for mean flow field control, which has direct repercussions on the aerodynamic characteristics. However, the flow mechanisms involving jet–vortex and vortex–vortex interactions and the disturbance convection through the flow field are little understood. This paper reports on the unsteady flow field above a generic half delta wing model with a 65 ° sweep angle and its response to periodic blowing. Numerical and experimental results are presented and discussed in a synergistic manner.

Control of Upswept Afterbody Vortices Using Continuous and Pulsed Blowing

An experimental study was performed in a water tunnel to evaluate the effects of continuous and pulsed blowing jets on the counter-rotating vortices generated by the afterbody of a slanted base cylinder. Drag reductions from continuous blowing through circular jets were found to vary significantly with direction and location, and approached 7% when blowing outboard from upstream locations on the upswept face. However, for all circular jets tested, the external power required was larger than the power saved due to the drag reduction. Jet vortices restricted shear layer development, leading to smaller afterbody vortex cores further from the surface. A high-aspect-ratio jet flap, ejecting nearly parallel to the freestream, achieved drag reductions close to 9%, equating to the net energy savings of almost 3% for the best case. Jet vortices shortened the shear layer, resulting in vortices with reduced circulation, which were displaced away from the upswept face. Pulsing the jet flap resulted in improved drag reductions and energy savings (up to around 6%) compared with the equivalent continuous blowing case at the same time-averaged jet momentum coefficient. Pulsed blowing caused an increase in vortex separation and meandering, whereas the circulation was reduced by up to 10% of that for continuous blowing.

Adaptive control of the dynamics of a fully turbulent bimodal wake using real-time PIV

In this study we focus on the control of the dynamics of 3D turbulent wake downstream a square-back Ahmed body ($Re_H=3.9\times10^5$). The peculiar dynamics of such a wake are first characterized through the trajectories of the pressure barycenter over the rear part of the model as well as the recirculation barycenter in the wake. In particular it is shown that these dynamics allow the definition of three different states: the two so-called reflectional symmetry-breaking (RSB) modes and the transient symmetric (TS) mode. It was shown recently that the time-fluctuations of the pressure barycenter could be characterized as a weak chaotic system with a well-defined attractor. We show that the dynamics of the bimodal wake can then be forced into a stable asymmetric or symmetric state in open loop control, using tangential continuous or pulsed blowing in three different regions along the upper edge of the rear part of the model. Finally, a simple closed-loop opposition control, based on real-time identification of the wake barycenter in the PIV fields, is used to force the chaotic dynamics of the wake into a regular oscillatory motion at a well-controlled frequency. Depending on the actuation parameters, the wake dynamics can also be switched from bimodal to a new multimodal behavior. We show that this new mode also exhibits a peculiar dynamics with an up-down instead of left-right chaotic oscillations. Interestingly, the recirculation area (size of the recirculation bubble) is much more reduced for the closed-loop experiments when the jets are pulsed rather than continuous. For the pulsed jets, the reduction is also increased when the proper frequency is chosen.

The Effect of Steady and Pulsed Air Jet Vortex Generator Blowing on an Airfoil Section Model Undergoing Sinusoidal Pitching

Experimental results are reported on the assessment of steady and pulsed air jet vortex generators (AJVGs) for the suppression of dynamic stall on a sinusoidal pitching RAE9645 airfoil model. Tests at Rec of 1 million, at reduced pitching frequencies between 0.01 and 0.10 were performed with and without steady and pulsed AJVG blowing. The effect of jet momentum coefficient (0.0003< Cμ< 0.0046), jet duty cycle (0.25< DC< 1) and jet pulsing frequency (0.29< F+< 2.93) were investigated. Pulsed air jet blowing with F+ in the range 0.5–1.0 and with a duty cycle in the range 0.4–0.5, was found to be the most effective to achieve full suppression of dynamic stall vortex formation.

Numerical studies of active flow control on wing tip extension

PurposeIn the European project AFLoNext, active flow control (AFC) measures were adopted in the wing tip extension leading edge to suppress flow separation. It is expected that the designed wing tip extension may improve aerodynamic efficiency by about 2 per cent in terms of fuel consumption and emissions. As the leading edge of the wing tip is not protected with high-lift device, flow separation occurs earlier than over the inboard wing in the take-off/landing configuration. The aim of this study is the adoption of AFC to delay wing tip stall and to improve lift-to-drag ratio.Design/methodology/approachSeveral actuator locations and AFC strategies were tested with computational fluid dynamics. The first approach was “standard” one with physical modeling of the actuators, and the second one was focused on the volume forcing method. The actuators location and the forcing plane close to separation line of the reference configuration were chose to enhance the flow with steady and pulsed jet blowing. Dependence of the lift-to-drag benefit with respect to injected mass flow is investigated.FindingsThe mechanism of flow separation onset is identified as the interaction of slat-end and wing tip vortices. These vortices moving toward each other with increasing angle of attack (AoA) interact and cause the flow separation. AFC is applied to control the slat-end vortex and the inboard movement of the wing tip vortex to suppress their interaction. The separation onset has been postponed by about 2° of AoA; the value of ift-to-drag (L/D) was improved up to 22 per cent for the most beneficial cases.Practical implicationsThe AFC using the steady or pulsed blowing (PB) was proved to be an effective tool for delaying the flow separation. Although better values of L/D have been reached using steady blowing, it is also shown that PB case with a duty cycle of 0.5 needs only one half of the mass flow.Originality/valueTwo approaches of different levels of complexity are studied and compared. The first is based on physical modeling of actuator cavities, while the second relies on volume forcing method which does not require detailed actuator modeling. Both approaches give consistent results.

A comparison of different unsteady flow control techniques in a highly loaded compressor cascade

A numerical research is applied to investigate the effect of controlling the flow separation in a certain highly loaded compressor cascade using different unsteady flow control techniques. Firstly, unsteady pulsed suction as a new novel unsteady flow control technique was proposed and compared to steady constant suction in the control of flow separation. A more exciting effect of controlling the flow separation and enhancing the aerodynamic performance for unsteady pulsed suction was obtained compared to steady constant suction with the same time-averaged suction flow rate. Simultaneously, with the view to further exploring the potential of unsteady flow control technique, unsteady pulsed suction, unsteady pulsed blowing, and unsteady synthetic jet (three unsteady flow control techniques) are analyzed comparatively in detail by the related unsteady aerodynamic parameters such as excitation location, frequency, and amplitude. The results show that unsteady pulsed suction shows greater advantage than unsteady pulsed blowing and unsteady synthetic jet in controlling the flow separation. Unsteady pulsed suction and unsteady synthetic jet have a wider range of excitation location obtaining positive effects than unsteady pulsed blowing. The ranges of excitation frequency and excitation amplitude for unsteady pulsed suction gaining favorable effects are both much wider than that of unsteady pulsed blowing and unsteady synthetic jet. The optimum frequencies of unsteady pulsed suction, unsteady pulsed blowing, and unsteady synthetic jet are found to be different, but these optimum frequencies are all an integer multiple of the natural frequency of vortex shedding. The total pressure loss coefficient is reduced by 16.98%, 16.55%, and 17.38%, respectively, when excitation location, frequency, and amplitude are all their own optimal values for unsteady pulsed suction, unsteady pulsed blowing, and unsteady synthetic jet. The optimum result of unsteady synthetic jet only slightly outperforms that of unsteady pulsed suction and unsteady pulsed blowing. But unfortunately, there is no advantage from the standpoint of overall efficiency for the optimum result of unsteady synthetic jet because the slight improvement has to require a greater power consumption than the unsteady pulsed suction and unsteady pulsed blowing methods.

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.

Simulations of an aircraft with constant and pulsed blowing flow control at the engine/wing junction

In high-lift and especially landing conditions, the vortices emerging from the engine strake and pylon as well as from the inboard and outboard slat tips of civil aircraft configurations can be the cause of the so-called nacelle wake flow separation. This phenomenon might be responsible for the wing stall and sudden lift loss; the effect is expected to increase with the coming up of enlarged nacelles and larger slat cutouts typical of Ultra High Bypass Ratio turbofan engines. In the framework of the European project AFLoNext, DLR and ONERA performed numerical studies focused on a realistic aircraft in high-lift configuration. Structured and unstructured grids were generated to perform Reynolds Averaged Navier–Stokes computations aimed at analyzing the baseline flow without control and determining relevant location and settings for active flow control systems. Constant blowing devices with different slot sizes, types and injection velocities were evaluated with RANS simulations over complete lift polars. The gains in CLmax were quantified; they range from 1 to 3% for actuator capacities compatible with manufacturer requirements, the nacelle wake separation appears at angles of attack one to two degrees higher than without control and the lift levels in post-stall conditions are significantly improved. Finally, unsteady RANS computations were carried out to investigate the potential of a pulsed blowing device. The gains in CLmax and flow separation containment obtained with the latter are similar to those of constant blowing devices which yet exhibit greater momentum coefficient and mass flow rate values.

Detached-eddy simulation of flow past a pitching NACA 0015 airfoil with pulsed actuation

This study conducted Detached-Eddy Simulations (DES) of actively controlled flow over a 0.35 m chord NACA0015 airfoil at an incidence angle of 11° and a chord Reynolds number of 1 million. The uncontrolled, natural flow were simulated by using DES-type methods, with a comparison of different underlying RANS models and subgrid-scale stress models in LES mode. Results from these computations were compared with experimental observations, enabling their reliable assessment through the detailed investigation of the Reynolds stresses as well as the separation and reattachment. It was found that among all five DES-type methods, only the Spalart–Allmaras-based Improved Delayed DES (IDDES-SA) captured the separation point as measured in the experiments. The classical vortex-shedding and the shear-layer flapping modes for airfoil flows with shallow separation were also extracted from the IDDES-SA results by using Dynamic Mode Decomposition. With better understanding of unsteady flow features, effective control practices were illustrated. The pulsed blowing actuation was adopted according to the experimental settings. The flow-field around jet orifices was resolved, instead of the use of actuation boundary conditions. It was found that for all actuation frequencies investigated in this article, the saturation of lift-to-drag ratio was yielded already at intermediate momentum coefficients, two-fifths of the actuation amplitude used in the experiment; the optimal actuation frequency was 0.6 among all cases, consistent with the dominant frequency of the baseline flow. The suppression of separation bubble led to a maximum lift-to-drag ratio enhancement of 194%, as well as a sharp decrease of all Reynolds stress components. To further reduce the strong unsteadiness introduced by excitation, a phase shift among jet orifices was applied and evaluated. The pulsed blowing with the jet orifices in an anti-phased manner was found to be very effective to damp the fluctuations of the lift around its mean value, though it led to a small decrease in mean lift.

Blowing momentum and duty cycle effect on aerodynamic performance of flap by pulsed blowing

Control surface, which is often located in the trailing edge of wings, is important in the attitude control of an aircraft. However, the efficiency of the control surface declines severely under the high deflect angle of the control surface because of the flow separation. To improve the efficiency of control surface, this study discusses a flow-control technique aimed at suppressing the flow separation by pulsed blowing at the leading edge of the control surface. Results indicated that flow separation over the control surface can be suppressed by pulsed blowing, and the maximum average lift coefficient of the control surface can be 95% times higher than that of without blowing when average blowing momentum coefficient is 0.03 relative to that of without blowing. Finally, this study shows that the average blowing momentum coefficient and non-dimensional frequency of pulsed blowing are two of the key parameters of the pulsed blowing control technique. Otherwise, duty cycle also has influence on the effect of pulsed blowing. Numerical simulation is used in this study.

Parameters effect of pulsed-blowing over control surface

Control surface, which is often located in the trailing edge of wings, is important in the attitude control of an aircraft. However, the efficiency of the control surface declines severely under the high deflect angle of the control surface because of the flow separation. To improve the efficiency of control surface, this study discusses a novel flow control technique aimed at suppressing the flow separation by pulsed blowing at the leading edge of the control surface. Results indicated that flow separation over the control surface can be suppressed by pulsed blowing, and the maximum average pitching moment coefficient of the control surface can be increased by nearly 90% when average blowing momentum coefficient is 0.03 relative to that of without blowing. Moreover, the lift coefficient of the control surface can be 95% times higher than that of without blowing, and the drag coefficient of the control surface can be reduced by 43% compared with that of without blowing. Finally, this study shows that the average blowing momentum coefficient and non-dimensional frequency of pulsed blowing are two of the key parameters of the pulsed blowing control technique. Both experimental and numerical simulations are used in this study. The experiment is completed in D-4 wind tunnel of Beihang University under conditions of Re 0.8×106.

Flow Control Applied to the Rounded Front Edges of a Square Prism

An experimental study of a 2D square prism with two rounded front edges is described. The purpose of the study, performed at roughly 10% full scale Reynolds number is to reduce the aerodynamic drag by means of Active Flow Control (AFC) of boundary layer separation. Highly efficient suction and pulsed blowing (SaOB) actuators array is installed inside a circular cylinder that forms the upper-front curved edge of the model, simulating one of the side-front-edges of a cab. In addition and for simplicity, steady suction is applied at the other rounded corner, both with corner radius to cab width ratio of 0.127. Both oscillatory blowing and steady suction are aimed to inhibit the separation from the front edges. The experimental results show complete suppression of separation, significant decrease in the form-drag, narrowing of the wake and a corresponding increase in the base pressure with the application of AFC. Overall system efficiency is also considered. Future studies will focus on higher Reynolds number...

Vortex-Flow Manipulation on a Generic Delta-Wing Configuration

The manipulation of characteristic vortex-flow instabilities, which occur in the flow around delta wings, is of great interest for increasing the performance of such configurations. This work presents experimental results of vortex-flow manipulation on a generic delta-wing configuration using pulsed blowing slot actuators along the leading edge. The frequencies of the examined blowing cases are chosen on the basis of unsteady characteristics that occur in the shear-layer roll-up during vortex formation and vortex breakdown. The results of hot-wire measurements substantiate the receptivity of the shear layer for the pulsed excitation until into the vortex core. The flowfield response to this periodic forcing is shown by the displacement of the vortex bursting location in a chordwise direction, thus satisfying the aim of enhancing the aerodynamic performance.