Suppression Of Flow Separation Research Articles

Experimental investigation of synthetic jet control of wing rock for a flying wing aircraft

Flying wing aircraft easily experience wing rock due to the lack of lateral-directional stability, which causes serious challenges to flight control and safety. Thus, it is necessary to reduce the wing rock amplitude or reduce the mean roll angle by additional control. For a flying wing model with a 65° leading-edge sweep, we propose a strategy using an array of synthetic jet actuators to control the wing rock. The control effect and mechanism are studied by attitude measurement and particle image velocimetry measurement in a wind tunnel; the results confirm that the synthetic jet can effectively change the trim position of the wing rock. The control effect is affected by the angle of attack, Reynolds number, actuation position, actuation voltage, and frequency. In general, downstream actuators perform better at low angles of attack, while upstream actuators perform better at high angles of attack; the actuators positioned at the downward rolling side have a better effect than those positioned at the upward side. Furthermore, continuously variable control of the trim position can be achieved by changing the actuation voltage or modulation frequency, which provides a base for attitude manipulation by using active flow control instead of a mechanical control surface. Quantitative analysis of the flow field indicates that the leading-edge vortex on the upward side provides a rolling moment, while the recirculation zone on the downward side also contributes to the wing rock. This is a dynamic process, causing the flying wing to balance at a nonzero mean roll angle. The synthetic jet positioned at the downward rolling side can transport high-momentum fluids to the near-wall region, thereby suppressing flow separation and reducing the size of the recirculation zone. This enhances the lift on the control side and thus reduces the mean roll angle of the wing rock.

원심 펌프의 임펠러에 대한 혹등고래 결절 형상 적용 및 최적화

In this study, bio-mimetic tubercle shapes of humpback whale are applied to the impeller of a commercial centrifugal pump based on RANS simulations and design optimization. In specific, a few geometric parameters of tubercles are employed as the design variables of the trailing and leading edges of the impeller. Design optimization is performed based on a sampling approach and the radial-basis artificial neural network. As the objective function, a combination of the turbulent kinetic energy (TKE) and pump efficiency is used based on the effects of tubercle shapes found in previous studies. Pump performance and flow fields are compared between the optimized and original impeller designs. The results show that the optimized tubercle design at the impeller trailing edge affects the vortical structures significantly. Tubercles generate counter-rotating vortices in wake region of the impeller, which leads to vortex breakdown and a decrease of turbulent fluctuation by about 25%. Also, a trade-off between the TKE and pump efficiency is observed. In contrast, the optimized tubercle design at the impeller leading edge results in relatively smaller (about 3%) reduction of the TKE. While the leading edge tubercles suppress flow separation at the suction side significantly, the effect is localized near the leading edge. For both leading and trailing edge optimization, the changes of the pump efficiency and head are found to be relatively minor.

Flow Reattachment on a NACA 0025 Airfoil Using an Array of Microblowers

An array of commercially available synthetic jet actuators was developed for controlling flow separation on a NACA 0025 airfoil. The array is more compact in comparison to standard synthetic jet actuators previously studied for separation control. Wind tunnel experiments were conducted at chord Reynolds number and . Control of separation was applied by the array through forcing at the wake and shear layer frequencies ( and , respectively). Both effectively suppress flow separation, increase lift, and decrease drag. The dynamics of the reattached flow show different feature for the two forcing schemes. The results demonstrate that this array has comparable control effectiveness on separation to other synthetic jet actuators.

A parametric study on control authority and vorticity transport in a compressor airfoil with plasma actuation at low Reynolds number

In order to obtain the general criterion of control authority for suppressing flow separation in the compressor airfoil at low Reynolds number by nanosecond pulsed dielectric barrier discharge plasma actuation with different actuation parameters, a parametric study on the actuation power and actuation frequency is performed using a large eddy simulation. A non-dimensional actuation power is proposed to reveal the relationship between the actuation power and the characteristic power of the baseline flow field, in order to provide a reference for obtaining a balance between energy consumption and flow control. With different actuation powers, the mechanism behind the evolution process of vortex structures induced by the plasma actuation is revealed through a decomposition of the vorticity according to the vorticity transport equation and the analysis of the non-dimensional circulation according to the Q criterion. It is found that the evolution shows a relatively consistent tendency and can be divided into three stages corresponding to different disturbance processes induced by Kelvin–Helmholtz instabilities. Finally, a criterion for the effective actuation frequency based on the continuous generation of induced vortex structures is established considering different actuation powers and simplified to specific parameters within the flow field.

Experimental investigation of dynamic stall flow control using a microsecond-pulsed plasma actuator

To alleviate the performance deterioration caused by dynamic stall of a wind turbine airfoil, the flow control by a microsecond-pulsed dielectric barrier discharge (MP-DBD) actuator on the dynamic stall of a periodically pitching NACA0012 airfoil was investigated experimentally. Unsteady pressure measurements with high temporal accuracy were employed in this study, and the unsteady characteristics of the boundary layer were investigated by wavelet packet analysis and the moving root mean square method based on the acquired pressure. The experimental Mach number was 0.2, and the chord-based Reynolds number was 870 000. The dimensionless actuation frequencies F + were chosen to be 0.5, 1, 2, and 3, respectively. For the light dynamic regime, the MP-DBD plasma actuator plays the role of suppressing flow separation from the trial edge and accelerating the flow reattachment due to the high-momentum freestream flow being entrained into the boundary layer. Meanwhile, actuation effects were promoted with the increasing dimensionless actuation frequency F +. The control effects of the deep dynamic stall were to delay the onset and reduce the strength of the dynamic stall vortex due to the accumulating vorticity near the leading edge being removed by the induced coherent vortex structures. The laminar fluctuation and Kelvin–Helmholtz (K–H) instabilities of transition and relaminarization were also mitigated by the MP-DBD actuation, and the alleviated K-H rolls led to the delay of the transition onset and earlier laminar reattachment, which improved the hysteresis effect of the dynamic stall. For the controlled cases of F + = 2, and F + = 3, the laminar fluctuation was replaced by relatively low frequency band disturbances corresponding to the harmonic responses of the MP-DBD actuation frequency.

Aerodynamics and fluid–structure interaction of an airfoil with actively controlled flexible leeward surface

Piezoelectric macro-fibre composite (MFC) actuators are employed onto the flexible leeward surface of an airfoil for active control. Time-resolved aerodynamic forces, membrane deformations and flow fields are synchronously measured at low Reynolds number (Re = 6 × 104). Mean aerodynamics show that the actively controlled airfoil can achieve lift-enhancement and drag-reduction simultaneously in the angle of attack range of 10° ≤ α ≤ 14°, where the rigid airfoil encounters stall. The maximum increments of lift and lift-to-drag ratio are 27.1 % and 126 % at the reduced actuation frequency of ${f^ + } = 3.52$ . The unsteady coupling features are further analysed at α = 12°, where the maximum lift-enhancement occurs. It is newly discovered that the membrane vibrations and flow fields are locked into half of the actuation frequency when ${f^ + } > 3$ . The shift of the dominant vibration mode from bending to inclining is the reason for the novel ‘half-frequency lock-in’ phenomenon. To the fluid–structure interaction, there are three characteristic frequencies for the actively controlled airfoil: $S{t_1} = 0.5{f^ + }$ , $S{t_2} = {f^ + }$ , and $S{t_3} = 1.5{f^ + }$ . Here, St1 and its harmonics (St2, St3) are coupled with the natural frequencies of the leading-edge shear layer, resulting in the generation of multi-scale flow structures and suppression of flow separation. The lift presents comparable dominant frequencies between St1 and St3, which means the instantaneous lift is determined by the flow structures of St1 and St3. The local membrane bulge and dent affect the instantaneous swirl strength of flow structures near the maximum vibration amplitude location, which is the main reason for the variation of instantaneous lift.

Experimental investigation of the dynamic characteristics of the flow generated by a sliding dielectric barrier discharge in the flat plate boundary layer flow

The sliding dielectric barrier discharge (SL-DBD) has attracted attention due to its ability to suppress flow separation. This paper investigated the effect of SL-DBD on the flow field in the flat plate boundary layer by time-resolved particle image velocimetry. We obtained the finite-time Lyapunov exponent field and the Lagrangian coherent structures of the flow field through the velocity field. The results show the effect of SL-DBD has spatial differences, and SL-DBD will produce a “strong–weak–strong” spatial effect on the flow field. The directionality of the flow structure induced by SL-DBD is the main reason for the difference. SL-DBD will induce a large number of vortex structures in the local downstream area of the electrode. The vortex structure enhances the mixing and squeezing effects between the upper and lower flow fields. The upper flow field moves the lower flow field downward by about 0.1 mm through the squeezing effect. In addition, at the downstream region adjacent to the electrode, the oblique momentum injection of the SL-DBD is the dominant effect. At one electrode distance from the electrode, the SL-DBD induces a large number of vortex structures. However, when the distance from the electrode is twice the electrode spacing, the number of vortex structures decreases, and the structure becomes larger. The results show spatial differences in the perturbation of the flow field by SL-DBD, which cannot be ignored when SL-DBD is used to suppress flow separation.

Experimental study on airfoil flow separation control via an air-supplement plasma synthetic jet

An air-supplement plasma synthetic jet (PSJ) actuator increases the air supplemental volume in the recovery stage and improves the jet energy by attaching a check valve to the chamber of a conventional actuator. To explore the flow control effect and mechanism of the air-supplement actuator, via particle image velocimetry experiments in a low-speed wind tunnel, the flow field and boundary layer characteristics of a two-dimensional airfoil surface under different actuation states were compared for different attack angles and jet orifices. The experimental results show that, compared with the conventional actuation state, the jet energy of the air-supplement PSJ is higher and the indirect mixing effect of the counter-vortex sequence produced by the jet-mainstream interaction is stronger. Furthermore, the boundary layer mixing effect is better, which can further suppress flow separation and improve the critical flow separation attack angle. Moreover, increasing the jet momentum coefficient can enhance the flow control effect. The findings of this study could provide guidance for the flow control application of air-supplement PSJs.

Parametric research and aerodynamic performance analysis of wind turbine airfoil with added flap

A self-popped up flap is added to the airfoil (S809) suction surface to improve aerodynamic performance under large angle of attack (AOA) inspired by the slightly popped up feathers on the trailing edge of a bird’s wing. The response surface methodology (RSM) optimization of H, D, and θ is conducted. The lift–drag ratio of an airfoil is taken as the optimization response target, and the Box–Behnken design is adopted to design the experiment scheme for H, D, and θ. Multivariate quadratic polynomials are used to carry out equation regression analysis on the combined results of 17 sample schemes, and the mathematical surrogate model between the flap structure parameters and the airfoil lift–drag ratio and the optimal design parameter combination of the flap structure are obtained. The clean airfoil and the airfoil with optimal flap are compared and analyzed from the static and dynamic aerodynamic characteristics by numerical simulation. The calculation results show that the optimal flap obtained by RSM increases the pressure difference between the suction and the pressure surfaces at large AOA, suppresses flow separation on the suction surface, and delays the stall AOA. The airfoil with optimal flap leads to a smaller separation vortex and wake vortex, therefore delaying the dynamic stall effect.

Sliding discharge plasma jet actuators for circular-cylinder wake modification

A sliding discharge (SD) plasma actuator designed for the control of a circular cylinder wake is examined experimentally in this paper. This kind of discharge demonstrates a thicker and higher maximum speed wall jet than a Dielectric Barrier Discharge (DBD). The plasma actuator mounted strategically on the rear part of the cylinder model can induce either a downward or upward jet into the flow around the circular cylinder by simply adjusting the electrodes’ electrical circuits. Experiments were performed in a low-speed and low-turbulence wind tunnel at Reynolds numbers between 7000 and 24,000 based on the diameter of the circular cylinder. Wake measurements by particle image velocimetry (PIV) showed that both the mean velocity and the turbulence level in the cylinder wake were modified under the plasma actuation. Reducing or increasing the cylinder drag force estimated from the velocity field could be realized by changing plasma actuation directions. They showed that up to 30% drag reduction and 24% drag increase were obtained with the downstream and upstream actuation respectively at the continuous plasma blowing. The efficiency of flow control was found to be about 1.8% for drag reduction. This study suggests that an appropriate arrangement of an SD actuator can practically suppress flow separation or enhance flow mixing.

Effect of flow structure frequency on flow separation control using dielectric barrier discharge actuator

A better understanding of the mechanism of flow separation suppression by a dielectric barrier discharge is essential for flow control. This paper investigates the mechanism of improving the aerodynamic performance of the airfoil by dielectric barrier discharge when the Reynolds number is in the range of 6 × 104–4 × 105. The results show that the disturbance of the gas discharge to the flow field will form a new flow structure. The fluctuating frequency of the new flow structure determines the ability of the plasma actuator to suppress flow separation. This investigation improves and develops the mechanism of plasma flow control.

Numerical studies on aerodynamic forces and flow control regimes of square cylinder with four surface ribs

Façade ribs attached to building surface are effective in mitigating wind-induced effects. In this paper, LES approach is adopted to explore the impact of four surface ribs on aerodynamic forces and flow structure around a square cylinder with width D. ANSYS Fluent is used to carry out the simulations at Re=2.2 × 104, the standard sub-grid scale (SGS) model is used. Simulation results indicate that small vortices formed near ribs can remarkably suppress flow separation, and reduce the shear layer's curvature and periodic flapping. Facade ribs accelerate the vortex shedding process, and clearly reduce the wake vortices’ intensity. The change of the flow field clearly mitigates surface pressure with increase in rib extension depth dr, resulting in a clear reduction in wind forces. When the depth dr is greater than 0.06D, the variation trends of drag coefficient C¯D and lift coefficient C’L are relatively complex with change of rib location br. This is due to the significant changes of flow patterns with different rib configurations. The maximum reduction rates of C¯D and C’L are 61% and 82%. It is found that extension of rib dr from around 0.08D to 0.1D with distance to the corner br from about 0.16D to 0.2D may show the best performance in reducing wind load.

Mechanics of drag reduction of an axisymmetric body of revolution with shallow dimples

In this article, the mechanics of drag reduction on an axisymmetric body of revolution by shallow dimples is presented by using the high-fidelity Reynolds Stress Modeling based simulations. Experimental results of drag evolution from published literature at different Reynolds numbers are used to validate the model predictions. The numerical predictions show good agreement with the experimental results. It is observed that the drag of the body is reduced by a maximum of [Formula: see text] with such shape modification (for the depth to diameter ratio of [Formula: see text] and coverage ratio of [Formula: see text]). This arises due to the reduced level of turbulence, flow stabilization and suppression of flow separation in the boundary layer of the body. From the analysis of turbulence states in the anisotopic invariant map (AIM) for the case of the dimpled body, we show that the turbulence reaches an axisymmetric limit in the layers close to the surface of the body. There is also a reduced misalignment between the mean flow direction and principal axis of the Reynolds stress tensor, which results in such drag reduction. The dimple depth to diameter(s/d) and coverage ratio (a/A) are also varied to evaluate its effect on drag evolution. An empirical correlation for the drag coefficient is also developed by using polynomial regression in terms of the different shape and flow parameters, for example, s/d ratio, a/A ratio, and Reynolds shear stress.

Active Flow Control of Helicopter Rotor Based on Coflow Jet

When a helicopter rotor undergoes flow separation, the drag of the rotor increases substantially, as does the power demand, which seriously affects the aerodynamic performance and flight safety of the helicopter. Therefore, it is crucial to research how to suppress the flow separation of rotor blades. An active control technique based on a coflow jet (CFJ) at the rotor blade tip was employed in this study to suppress the flow separation over the rotor. The mechanisms and behavior were investigated. The rotor flow field was numerically simulated by solving the Reynolds-averaged Navier–Stokes equations with the finite volume method. The turbulence model was k − ω SST, and the rotor motion was simulated using the overset mesh technique. After applying CFJ control, the airflow from the injection slot at the leading edge of the rotor increased the energy of the mainstream in the near-wall area, which enhanced its ability to resist the adverse pressure gradient. A flow separation was effectively suppressed, both on the advancing and retreating sides, which improved the aerodynamic performance of the rotor. During the whole rotation period, the thrust coefficient of the rotor increased by up to 5.6%, the moment coefficient decreased by as much as 26.8%, and the equivalent lift-to-drag ratio increased by up to 44.0%. Moreover, the effects of the CFJ parameters on the flow separation suppression of the rotor are researched. These results may provide a foundation for the development of aerodynamic performance improvement for helicopter rotor based on a CFJ.

Active flow control in an S-shaped duct at Mach 0.4 using sweeping jet actuators

An experimental study was conducted on the flow control capability of sweeping jet actuators (SJAs) in an aggressive offset S-shaped duct with an inlet Mach number of 0.4. The SJAs were placed in two different spacing arrangements to suppress flow separation in the duct and flow distortion at the aerodynamic interface plane (AIP). Multiple measurements were taken, including surface oil flow visualization, steady pressure on the wall surface, five-hole probe pressure, and velocity at the AIP. For the baseline case without control, the surface oil flow visualization and pressure measurement results clearly reflected the three-dimensional flow separation characteristics in the duct. In the controlled cases, the streamwise surface pressure coefficient distribution showed that SJAs can effectively suppress the flow separation. At a spacing ratio of 8.13 and an excitation pressure of 250 kPa, the inflection point of the pressure coefficient Cp was completely eliminated. As a result, the absolute pressure recovery at the upper region of the AIP increased by nearly 5%, and the mean value and RMSE of the cross-flow velocity declined by 54.9% and 48.1%, respectively.

Characteristics and suppression of vibration in cross-flow turbine with a cavity

Abstract Cross-flow turbine is widely used for small hydropower in run-of-river type stations because of its excellent partial load characteristics, easy manufacturing, and low cost. The Banki-Michell turbine is the origin, and various studies have focused on improving the turbine performance. The authors have developed a new cross-flow turbine with a cavity and a guide wall from the outer nozzle wall tip to suppress flow separation on the guide wall and make the casing smaller. However, little researches are focusing on vibration, and it is also essential to get knowledge for suppressing vibration and noise from the turbine aiming to install at higher head sites. Our previous studies showed that high-pressure fluctuation occurs near the outer nozzle tip, and the cavity suppresses the pressure fluctuations, but the suppression mechanism is not fully understood. Therefore, this study experimentally investigates the pressure fluctuation generation mechanism and the cavity’s effect on suppressing the vibration. Furthermore, it aims to suppress the vibration by modifying the nozzle tip shape. The vibration level was measured on a lab-scale turbine and CFD analysis to reveal the generation of pressure fluctuation. As a result, the predominant frequency of the vibration corresponds to the blade passing frequency and the pressure fluctuation at the nozzle tip. The cavity and nozzle tip shape show a significant influence on pressure and vibration characteristics and turbine performance.

Aeroacoustic interaction between owl-inspired trailing-edge fringes and leading-edge serrations

The silent flight achieved by owls is attributed to their unique wing morphologies, characterized by leading-edge (LE) serrations, trailing-edge (TE) fringes, and a velvet-like surface. The specific morphological effects of LE serrations and TE fringes on aeroacoustic performance have been widely studied, but the LE–TE aeroacoustic interaction remains poorly understood. This paper describes a simulation-based study of the aeroacoustic characteristics of owl-inspired TE fringes and their interplay with LE serrations by combining large-eddy simulations of unsteady near-field flow structures with the Ffowcs Williams–Hawkings equation for sound radiation. Using owl-inspired LE serrated and TE fringed wing models, it is verified that TE fringes enable a pronounced high-frequency sound reduction at angles of attack (AoAs) of 5–15° while achieving comparable aerodynamic performance to a clean model. The near-field vortex dynamics, pressure distributions, and velocity spectra reveal that TE fringes suppress flow separation and vortex shedding in the vicinity of the TE, consequently reducing local velocity fluctuations and far-field overall sound pressure levels. Furthermore, the combination of TE fringes and LE serrations enables a remarkable reduction in overall sound pressure levels at all AoAs, and their aeroacoustic interplay is responsible for stabilizing velocity fluctuations over the suction surface, which suppress both low- and high-frequency sound. Our results demonstrate that TE fringes are a robust sound reduction device in resolving the trade-off between aerodynamic force production and sound reduction, while LE serrations and TE fringes complement one another as an effective noise-reducing biomimetic design.

Effect of grooving on the surface of an object on aerodynamic characteristics

The strong pressure drag induced by the separation of the flow from the surface of the object is a major factor that greatly reduces the energy efficiency of various fluid mechanical elements. In this study, the effect of suppressing flow separation by applying shallow grooves that are regularly arranged on the surface of the object was verified by CFD analysis and wind tunnel experiments. Using a CFD analysis model in which shallow grooves are machined at equal intervals in the axial direction on the surface of a cylinder with a diameter of 42.67 mm, we pursued groove shape parameters that maximize the drag reduction effect. Based on this result, grooving was carried out in the span direction on the back surface of the wing model, and a remarkable drag reduction effect at a high angle of attack was confirmed by a wind tunnel experiment.

Enhancing turbulence: a potential strategy for drag reduction based on ground transportation systems (GTS) model

PurposeThe purpose of this paper is to investigate the mechanism and performance of a potential strategy, which is to enhance turbulence to carry out drag reduction for heavy trucks.Design/methodology/approachEnhancing turbulence deflector (ETD) was placed on the roof surface of an ground transportation system (GTS) to investigate the drag reduction mechanism of enhancing turbulence. Transition shear-stress transport improved delay detach eddy simulation model was adopted to simulate the unsteady small-scale flow around the ETD.FindingsBy optimizing the three influencing factors, diameter, streamwise length and streamwise position, the optimized ETD has achieved a maximum drag reduction of 7.04%. The analysis of flow field results shows that enhancing turbulence can effectively suppress flow separation and reduce the negative pressure intensity in the wake region of GTS.Originality/valueThe present work provides another potential possibility for the improvement of the aerodynamic performance of heavy trucks.

Energy extraction of oscillating foil with an active deflecting trailing-edge flap

The method of oscillating-foil energy extraction can be used to extract kinetic energy from the surrounding flow by a combined pitching and heaving motion of the foil. In order to improve the efficiency of energy extraction, a slotted foil with an active deflecting trailing-edge flap—inspired by the structure of the tail edge of bird wings—is designed. In this study, the unsteady Reynolds-averaged Navier–Stokes equation is solved to investigate the energy extraction performance of an oscillating foil at a Reynolds number of 5.0 × 105. In the numerical simulation, the dynamic overset mesh technology is used in order to ensure the accuracy and convergence of numerical solution. The effect of the deflecting motion of trailing-edge flaps on the efficiency of energy extraction is studied at a range of oscillating frequencies. In addition, the flow control mechanisms of slots on the oscillating foil are revealed by comparing the flow fields of the slotted foil and the NACA0015 foil. The result shows that active deflecting trailing-edge flaps can improve the efficiency of energy extraction over a wide range of oscillation frequencies. The active deflection of trailing-edge flaps increases the energy extraction efficiency of oscillating foils by 21.1% relative to conventional foils under a specific operating condition of oscillating frequency f* = 0.18. A detailed analysis of the flow fields indicates that the slot on the foil can suppress flow separation, while it has a negative effect on the attachment of leading-edge vortices. The deflecting trailing-edge flap enhances the heaving force. Therefore, the energy output and the efficiency of the oscillating foil are enhanced especially at the operating conditions of the oscillating foil without leading-edge vortex shedding.