Flow Separation Control Research Articles

Experimental Analysis of Flow Separation Control by a Dielectric Barrier Discharge Plasma Actuator in Burst-in-Burst Actuation Mode

This study investigated the effectiveness of a dielectric barrier discharge (DBD) plasma actuator operating in burst-in-burst (BIB) mode for flow separation control on a NACA 0015 airfoil. Time-resolved particle image velocimetry measurements were conducted at a Reynolds number of 66,000 and 13° angle of attack. Various BIB signal configurations were tested, with actuation periods of 70 ms and 150 ms, non-actuation periods ranging from 5 ms to 50 ms, and burst frequencies of 300 Hz and 600 Hz. Proper orthogonal decomposition was applied to analyze the flow field dynamics. The results showed that BIB actuation maintained flow attachment with reduced power consumption compared with continuous burst actuation. However, the effectiveness was highly sensitive to the BIB parameters, with some configurations failing to achieve consistent reattachment and becoming unstable. This study reveals complex interactions between actuation vortices and separation processes, highlighting both the potential and challenges of intermittent plasma actuation for efficient flow control.

Triglobal resolvent-analysis-based control of separated flows around low-aspect-ratio wings

We perform direct numerical simulations of actively controlled laminar separated wakes around low-aspect-ratio wings with two primary goals: (i) reducing the size of the separation bubble and (ii) attenuating the wing tip vortex. Instead of preventing separation, we modify the three-dimensional (3-D) dynamics to exploit wake vortices for aerodynamic enhancements. A direct wake modification is considered using optimal harmonic forcing modes from triglobal resolvent analysis. For this study, we consider wings at angles of attack of $14^\circ$ and $22^\circ$ , taper ratios $0.27$ and $1$ , and leading edge sweep angles of $0^\circ$ and $30^\circ$ , at a mean-chord-based Reynolds number of $600$ . The wakes behind these wings exhibit 3-D reversed-flow bubble and large-scale vortical structures. For tapered swept wings, the diversity of wake vortices increases substantially, posing a challenge for flow control. To achieve the first control objective for an untapered unswept wing, root-based actuation at the shedding frequency is introduced to reduce the reversed-flow bubble size by taking advantage of the wake vortices to significantly enhance the aerodynamic performance of the wing. For both untapered and tapered swept wings, root-based actuation modifies the stalled flow, reduces the reversed-flow region and enhances aerodynamic performance by increasing the root contribution to lift. For the goal of controlling the tip vortex, we demonstrate the effectiveness of actuation with high-frequency perturbations near the tip. This study shows how insights from resolvent analysis for unsteady actuation can enable global modification of 3-D separated wakes and achieve improved aerodynamics of wings.

Increasing the Economic Efficiency of Marine Power Plants Using Waste Heat Boilers with Controlled Flow Separation

Abstract Heat recovery of exhaust gases from main and auxiliary marine diesel engines is an effective way to improve the technical and economic parameters of marine power plants. Improvements in engine efficiency necessitate an increase in the weight-size parameters of the waste heat boilers, which makes it difficult to recover heat. Intensification of the heat transfer process is considered to be an effective way to reduce these indicators. By utilising mathematical modelling, this paper shows the effectiveness of using profiled heating surfaces of waste heat boilers for this purpose. The use of elliptical heating surfaces with a mechanism of controlled flow separation, in the form of a triangular notch, is proposed. This will reduce surface drag and increase the overall thermal-hydraulic efficiency of the heat transfer processes. It is shown that the use of such surfaces in waste heat boilers makes it possible to increase the efficiency of marine power plants in tankers with a deadweight of about 45,500 tons up to 1.5% absolute and container ships with a deadweight of about 122,000 tons up to 2.5% absolute.

Numerical and experimental investigations of flow separation control through a linear compressor cascade

Modern large-scale gas turbines are equipped with high-pressure ratio compressors to increase engine work and its overall efficiency. Flow separation and energy losses are also two interrelated phenomenon associated with changes in compressor loading level and performance. This paper examines therefore the control of flow separation using a passive-control technique. An arced divergent-convergent slot grooved from the blade pressure side to its suction side was adopted to control flow separation, reducing the losses through a linear compressor cascade. The spanwise location of the slot was selected based on CFD simulations where the corner separation was predicted. The slot height in the spanwise direction was selected to be 8% of the blade height at the end-wall side. The present work was performed experimentally and numerically at an inlet Reynolds number,Rec=ρV∞C/μ=2.98×105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${R}_{ec}=\\rho {V}_{\\infty }C/\\mu =2.98\ imes {10}^{5}$$\\end{document}, covering a wide range of incidence angles from +6∘to-6∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$+ 6^{ \\circ } {\ ext{ to}} - 6^{ \\circ }$$\\end{document}. The experimental work was carried out using a linear cascade test section consisting of six NACA 65-009 blade profiles integrated into a low-speed wind tunnel. A five-hole pressure probe system was used to obtain main flow parameters. Numerically, four turbulence models, including Spalart–Allmaras (S–A) model, Realizable (R k-ε) model, Shear-Stress Transport (SST k-ω) model, and Reynolds Stress model (RSM) were tested to predict the velocity and pressure fields. Good agreement between the experimental measurements and the numerical results, which were obtained using the RSM turbulence model in terms of velocity profiles and total pressure downstream of blades. It was observed also that the use of the arced-slotted blades for positive incident angles was more effective in reducing the separation than the negative and zero incident angles, approaching a maximum value of 33% for 6° with enhanced blade loading reaching 17.6%. It is to be concluded that, the use of arced slotted blade improves the compressor performance specially for positive incident angles.

The effect of shortfin mako shark skin at the reattachment of a separated turbulent boundary layer

This smooth flat experimental study investigates the capability of mako shark scales to control flow separation when placed downstream of the onset of turbulent boundary layer separation and within the reattachment region. The objective of the study is to validate the hypothesis that the shark scales' bristling and recoiling would prevent the flow separation on the flank region (the fastest flow region) of the shark. A rotating cylinder was used to induce an adverse pressure gradient over a flat plate to produce a region of separated flow where the shark skin specimen was mounted. Two types of mako shark scales (flank (B2) and between flank and dorsal fin (B1)) were positioned in the preferred flow direction on a flat plate. The B2 scales are slender, 200μm tall, and can bristle up to 50°. In contrast, B1 scales are wider, shorter, and can bristle at 30°. The bristling angle and shape are the main mechanisms by which the scales act to inhibit flow from moving upstream near the wall. Thus, the difference in the bristling angles and structures of the scales is attributed to the fact that the B2 scales function in a thicker boundary layer (behind the shark's gills) where they must bristle sufficiently high into the boundary layer to control the flow separation, and because the adverse pressure gradient in this region is higher where flow separation is more likely. The scales are placed in the reattachment region to elucidate their ability to control and reattach an already separated turbulent flow. The results show that B2 scales placed in the reattachment region reduce the size of the turbulent separation bubble and decrease the turbulent kinetic energy, while B1 scales have the opposite effect.

Experimental closed-loop flow separation control: Data- and phenomenological-driven approaches

Flow control aims at modifying a natural flow state to reach an other flow state considered as advantageous. In this paper, active feedback flow separation control is investigated with two different closed-loop control strategies, involving a reference signal tracking architecture. Firstly, a data-driven control law, leading to a linear (integral) controller is employed. Secondly, a phenomenological/model-driven approach, leading to a non-linear positive (integral) control strategy is investigated. While the former benefits of a tuning simplicity, the latter prevents undesirable effects and formally guarantees closed-loop stability. Both control approaches were validated through wind tunnel experiments of flow separation control over a movable NACA 4412 plain flap. These control laws were designed with respect to hot-film measurements, performed over the flap for different deflection angles. Both control approaches proved efficient in avoiding flow separation. The main contribution of this work stands in providing practitioners, simple but yet efficient control design methods for the flow separation phenomena. Equivalently important, a complete validation campaign data-set is also provided.

Understanding Low-Speed Streaks and Their Function and Control through Movable Shark Scales Acting as a Passive Separation Control Mechanism.

The passive bristling mechanism of the scales on the shortfin mako shark (Isurus oxyrinchus) is hypothesized to play a crucial role in controlling flow separation. In the hypothesized mechanism, the scales are triggered in response to patches of reversed flow at the onset of separation occurring in the low-speed streaks that form in a turbulent boundary layer. The two goals of this investigation were as follows: (1) to measure the reversing flow occurring within the low-speed streaks in a separating turbulent boundary layer; (2) to understand the passive flow control mechanism of movable shark skin scales that inhibit reversing flow within the low-speed streaks. Experiments were conducted using digital particle image velocimetry (DPIV). DPIV was used to analyze the flow in a turbulent boundary layer subjected to an adverse pressure gradient formation over both a smooth flat plate and a flat plate on which shark skin specimens were affixed. The experimental analysis of the flow over the smooth flat plate corroborated the findings of previous direct numerical simulation studies, which indicated that the average spanwise spacing of the low-speed streaks increases in the presence of adverse pressure gradients upstream of the point of separation. However, the characteristics of the flow over the shark skin specimen more closely resemble that of a zero-pressure gradient turbulent boundary layer. A comparative analysis of the width and velocity of the reversed streaks between flat plate and shark skin cases reveals that the mean spanwise spacing decreases, and thus, the number of streaks increases over the shark skin. Additionally, the reversed streaks observed over shark scales are thinner and the highest negative velocity within the streaks falls within the range required to bristle the scales.

Use of machine learning to optimize actuator configuration on an airfoil

Machine learning was used to optimize the geometric arrangement of a pair of unsteady actuators on flow separation over an efficient low Reynolds number airfoil in post-tall conditions. Large eddy simulation was used to validate the results. Two actuators: one with blowing and the other with suction openings were installed on the top surface of an airfoil at low Reynolds number of 60,000. An SD7003 airfoil at a post stall angle of attack of 13° was utilized. The boundary layer flow of the top surface was manipulated by the actuators to control flow separation. The influence of several actuator parameters: frequency, energy input, opening area, location and orientation angle were considered in an optimization of the dual actuator configuration. A genetic algorithm-based optimization was implemented to find the most effective configuration of this coupling. Since the optimization process is time-consuming, machine learning was used to train artificial neural networks to be coupled with genetic algorithm to reduce the computational cost. The artificial neural networks and their training was constantly upgraded during the optimization cycle. Results for the optimal case indicated an increase in lift coefficient and the objective function in comparison to uncontrolled case by factors of 1.88 and 3.33 respectively. We also found a reduction in drag coefficient. It was also found that using a pair of actuators was more efficient than using a single actuator.

Control on Flow Separation over a Cylinder by a Ferrofluid Film Adsorbed by a Magnet

Flow separation can lead to increased resistance and vibration generation, which is a difficult problem that cannot be ignored in engineering. In this paper, we propose a method of controlling flow separation by adsorbing ferrofluid onto the surface of a magnetized cylinder, taking the common flow around a cylinder as an example. Parametric effects of the ferrofluid film, including its viscosity and thickness, on the flow behavior were investigated in terms of the vortex shedding process, velocity distribution, dominant frequency, pressure distribution, and the flow motion inside the ferrofluid film. The results indicate that the ferrofluid film can suppress the generation of flow separation and achieve effective control, which is mainly caused by wall slip and the internal movement of the ferrofluid film. Furthermore, the flow separation control effect of ferrofluid thin films with different parameters varies, with low-viscosity ferrofluid exhibiting a superior control effect.

Experiments for control of boundary layer transition by plasma actuators

Plasma flow control technology is a new type of active flow control technology, and this paper carries out certain research on the basic principle of plasma flow control technology and its application on the wing model. The paper introduces the theory of plasma discharge and analyzes how plasma can control the flow field. A parallel dielectric barrier discharge plasma exciter is then designed and tested by using the Particle Image Velocimetry (PIV) technique to investigate the spatial flow field distributions of single-stage and three-stage actuators, and the mechanism of jet generation by the actuator is analyzed. Finally, the plasma actuator was utilized to control flow separation on a super-zero-boundary airfoil, and the oil droplet interferometric method was used to study the actuator’s role in controlling boundary layer transition. The experiment revealed that increasing the actuator discharge voltage can further reduce wall drag and more effectively delay the transition. Under the given conditions of a head-on angle of 6° and the incoming wind speed of 20 m/s, a two-stage actuator’s continuous discharge parameter was set to an output voltage of 12 kV and an output frequency of 4.7 kHz. The results showed that the transition position can be delayed by 13%, indicating the effectiveness of the actuator’s jet effect in enhancing the stability of the boundary layer flow.

Enhancing Flow Separation Control Using Hybrid Passive and Active Actuators in a Matrix Configuration

Efficient control of flow separation holds significant economic promise. This study investigates flow separation mitigation using an experimental platform featuring a combination of passive and active actuators arranged in a matrix configuration. The platform consists of 5 × 6 hybrid actuator units, each integrating a height-adjustable vortex generator and a micro-jet actuator. Inspired by the distributed pattern of V-shaped scales on shark skin, these actuator units are strategically deployed in a matrix configuration to reduce flow separation on a backward-facing ramp. Distributed pressure taps encircling the hybrid actuators monitor the flow state. Parametric analyses examine the effect of different control strategies. By adopting appropriate passive and active actuation patterns, effective pressure recovery on the ramp surface can be achieved. The most significant flow control outcome occurs when the actuators operate under combined active and passive excitation, harnessing the benefits of both control strategies. Particle image velocimetry (PIV) results confirm a notable reduction in flow separation under the best-controlled case. These findings suggest a promising future for flow control devices employing combined passive and active actuation in matrix-like configurations.

Sweeping JET actuator for the control of flow separation on a hump airfoil

Flow control can effectively improve lift and delay flow separation. Sweeping jet (SWJ) actuators have attracted increasing interest for as active flow control actuators. In this paper, the characteristics of the SWJ actuator were studied through large-eddy simulation and experiments. It was discovered that the oscillation frequency of the SWJ actuator is directly proportional to the mass flow rate, and its working principle was determined. Subsequently, SWJ actuators with [Formula: see text], 1.9%, and 6.6% were applied to flow control on the hump airfoil. It was observed that with an increase in momentum coefficient, the SWJ actuator was able to push the shear layer toward the hump airfoil surface, leading to a reduction in the size of the separation vortex. At [Formula: see text], it was achieved that the reattachment point of the separation vortex shifted upstream from [Formula: see text] to [Formula: see text], and the separation vortex size essentially disappeared. During the investigation of the flow control mechanism of the SWJ actuator, it was found that the SWJ actuator periodically generates a separation vortex V1, which has entrainment effects, resulting in the formation of an external jet on the airfoil surface, effectively inhibiting flow separation.

Virtual leading-edge aero-shaping of an airfoil using a dielectric barrier discharge plasma actuator

Recently, it has been demonstrated that dielectric barrier discharge plasma actuators, which can be capable of generating a quasi-steady wall jet under the effect of a sinusoidal power supply, are well suitable for controlling the flow around the airfoil at a low Reynolds number. To uncover the controlling mechanism of plasma actuators, flow separation control over an SC (2)-0714 supercritical airfoil using an asymmetrical plasma actuator arranged at the leading edge of the airfoil, is studied at Reynolds number of 7.8 × 104. The stall angle of attack is delayed from 9° to 13°, and the maximum lift coefficient is increased by about 27%. In addition to the momentum injection to the boundary layer, which is one of the control mechanisms and can be achieved by the induced spanwise vortex of the plasma actuator, the closed recirculation region, which is similar to superimposing a hump on the leading edge of the airfoil, is another control mechanism, leading to an increase in the effective camber of the airfoil. The localized recirculation zone, which is created by the plasma actuator and modifies the leading-edge aero-shaping and results in the enhancement of the lift coefficient, is first observed, and this effect of the plasma actuator is called the virtual leading-edge aero-shaping.

Delayed Detached Eddy Simulation of Flow Separation Control Using Fluidic Oscillator

The investigation of flow separation control over a NACA 0015 airfoil model using a fluidic oscillator (FO) is conducted through delayed detached eddy simulation. First, the flowfield within and outside an FO operating in quiescent air is resolved simultaneously. The oscillation of the jet flow induced by the FO is attributed to the cyclic expansion and contraction of the recirculation bubbles located near the two Coanda surfaces in the mixing chamber. Significantly, the predicted jet oscillation frequency closely matches the experimental data, validating the accuracy of our findings. Next, the FO is integrated into an airfoil model to suppress the flow separation. The airfoil is under deep stall conditions, with angles of attack of 20 and 17° and a Reynolds number of Re=4.8×105. The key driving force for flow reattachment is the spanwise vortices induced by the oscillating jet, which substantially enhance the mixing between the separated flow and the external high-momentum flow. Consequently, the aerodynamic performance of the airfoil is notably improved. Additionally, both spectral and dynamic mode decomposition analyses indicate that the flow, under the influence of the FO forcing, becomes more orderly and well-organized and is effectively locked into the forcing frequency.

Dynamics and control of separated flow over small-scale surface deformations with slip

Dynamics and control of separated flow over small-scale surface deformations with slip

Effect of protuberances on the aerodynamic performance of a wind turbine blade – a review

ABSTRACT This paper thoroughly reviews the influence of protuberances on the performance of a wind-turbine blade in terms of aerodynamic characteristics, power generation capability, and noise emission. It is observed that the protuberances are significantly useful in improving the aerodynamic performance of wind turbine blades, especially in the post-stall regime and during fluctuating wind conditions, such as wind gusts. The protuberance amplitude was found to be the dominant parameter to influence the blade performance. The amplitude and wavelength were tested in the range of 2–12%c and 2–50%c, respectively. The highest wind turbine performance was observed at small amplitude and large wavelength in the pre-stall regime and at large amplitude and small wavelength in the post-stall regime. Incorporating leading edge protuberances may increase lift and power coefficients by 50 and 55%, respectively, and reduce noise emission by nearly 16 dB. With the increase in the angle of attack, the instability noise was found to change its form. The broadband noise forms at a lower angle of attack (~2°) which converts to the tonal noise at the angle of attack, 6.5°. The protuberances substantially alter the boundary layer flow, subsequently modifying the pressure and velocity field over the suction surface of the blade. A pair of vortices forms across a protuberance, which exchanges momentum with the boundary layer flow and helps delay/eliminate the flow separation. This flow separation control is responsible for the benefits as mentioned above.

Numerical Simulation of the Transient Flow around the Combined Morphing Leading-Edge and Trailing-Edge Airfoil.

An integrated approach to active flow control is proposed by finding both the drooping leading edge and the morphing trailing edge for flow management. This strategy aims to manage flow separation control by utilizing the synergistic effects of both control mechanisms, which we call the combined morphing leading edge and trailing edge (CoMpLETE) technique. This design is inspired by a bionic porpoise nose and the flap movements of the cetacean species. The motion of this mechanism achieves a continuous, wave-like, variable airfoil camber. The dynamic motion of the airfoil's upper and lower surface coordinates in response to unsteady conditions is achieved by combining the thickness-to-chord (t/c) distribution with the time-dependent camber line equation. A parameterization model was constructed to mimic the motion around the morphing airfoil at various deflection amplitudes at the stall angle of attack and morphing actuation start times. The mean properties and qualitative trends of the flow phenomena are captured by the transition SST (shear stress transport) model. The effectiveness of the dynamically morphing airfoil as a flow control approach is evaluated by obtaining flow field data, such as velocity streamlines, vorticity contours, and aerodynamic forces. Different cases are investigated for the CoMpLETE morphing airfoil, which evaluates the airfoil's parameters, such as its morphing location, deflection amplitude, and morphing starting time. The morphing airfoil's performance is analyzed to provide further insights into the dynamic lift and drag force variations at pre-defined deflection frequencies of 0.5 Hz, 1 Hz, and 2 Hz. The findings demonstrate that adjusting the airfoil camber reduces streamwise adverse pressure gradients, thus preventing significant flow separation. Although the trailing-edge deflection and its location along the chord influence the generation and separation of the leading-edge vortex (LEV), these results show that the combined effect of the morphing leading edge and trailing edge has the potential to mitigate flow separation. The morphing airfoil successfully contributes to the flow reattachment and significantly increases the maximum lift coefficient (cl,max)). This work also broadens its focus to investigate the aerodynamic effects of a dynamically morphing leading and trailing edge, which seamlessly transitions along the side edges. The aerodynamic performance analysis is investigated across varying morphing frequencies, amplitudes, and actuation times.

Control of separated flow at low Reynolds number around NACA0012 airfoil by boundary layer suction

The separated flow at low Reynolds number around the NACA0012 airfoil is numerically studied by large-eddy simulation. Strategies of boundary layer suction to control flow separation are investigated. A method of using two-zone suctions, near the leading edge and near the trailing edge, are calculated. Based on verification with direct numerical simulation (DNS) and experimental data, the results of the lift and the drag, the vortices, and the strength of near-field pressure fluctuations, are checked. The results show that the two-zone suctions can supress flow transition and separation, thereby increase the lift and reduce the drag. The shedding of vortices is weakened, and the near-field pressure fluctuations are attenuated. For comparison with the two-zone suctions, the strategies of suction near the leading edge only and suction near the trailing edge only are also studied. It is found that suction near the leading edge only may suppress transition and delay separation when the suction zone is large enough, but the flow property deteriorates due to shedding vortices in the wake. The suction near the trailing edge only may improve the flow performance by reducing the size of the vortices in the rear section of the airfoil and in the wake region, but it has little effect on the separation bubble and transition.

Position-dependence straight-wing experiments of artificial coverts in flow separation control at a high Reynolds number

Position-dependence straight-wing experiments of artificial coverts in flow separation control at a high Reynolds number

Ramped Vane Control of a Compression Corner-Induced Flow Separation

Ramped Vane Control of a Compression Corner-Induced Flow Separation