Flow Separation Control Research Articles

On the drag reduction of an inclined wing via microstructures with the immersed boundary-lattice Boltzmann flux solver

Flow separation control has a wide application prospect in drag reduction for industry. This paper numerically studies the effect of microstructures on flow separation and drag reduction. Simple morphological microstructures, derived from the tilted shark scales, are attached to the wing at an angle of attack. The spacing and height of microstructures are made dimensionless by using the microstructure width and half of the wing width, respectively, that is, d̃m=dm/dAB and h̃m=hm/(H/2). The angle of attack is set to 10°. It is found that microstructures can reduce the motion amplitude of shed vortices, thereby suppressing flow separation and reducing drag. Both the planar and curved microstructures have excellent drag reduction performance. The microstructure spacing d̃m and tilt angle θ should not be too large or too small; otherwise, it will weaken the drag reduction ability. Cases d̃m=1.51, θ=20°, and θ=30° exhibit excellent drag reduction performance. The microstructure has the characteristic for being small, yet it needs to reach a certain height h̃m to effectively reduce drag. The case h̃m=0.667 is the most superior choice. Based on the proposed microstructure shape and spacing, the drag reduction performance of microstructures can reach more than 28%. Meanwhile, the drag reduction performance of microstructures increases with the improvement of the attachment proportion pm, and case pm≥50% is suggested for significant drag reduction performance. Finally, we discuss the drag reduction performance of microstructures on the wing at different angles of attack and find that microstructures can achieve good drag reduction, provided that the pressure drag caused by the flow separation is a significant proportion of the total drag and the flow separation occurs within the controllable range of microstructures.

Computational and Experimental Modeling in Magnetoplasma Aerodynamics and High-Speed Gas and Plasma Flows (A Review)

This paper provides an overview of modern research on magnetoplasma methods of influencing gas-dynamic and plasma flows. The main physical mechanisms that control the interaction of plasma discharges with gaseous moving media are indicated. The ways of organizing pulsed energy input, characteristic of plasma aerodynamics, are briefly described: linearly stabilized discharge, magnetoplasma compressor, capillary discharge, laser-microwave action, electron beam action, nanosecond surface barrier discharges, pulsed spark discharges, and nanosecond optical discharges. A description of the physical mechanism of heating the gas-plasma flow at high values of electric fields, which are realized in high-current and nanosecond (ultrafast heating) electric discharges, is performed. Methods for magnetoplasma control of the configuration and gas-dynamic characteristics of shock waves arising in front of promising and advanced aircraft (AA) are described. Approaches to the control of quasi-stationary separated flows, laminar–turbulent transitions, and static and dynamic separation of the boundary layer (for large PA angles of attack) are presented.

Comparison and modification of turbulence models for active flow separation control over a flat surface

The present work studied various models for predicting turbulence in the problem of injecting a fluid microjet into the boundary layer of a turbulent flow. For this purpose, the one-equation Spalart–Allmaras (SA), two-equation k–ε and k–ω, multi-equation transition k-kL–ω, transition shear stress transport (SST), and Reynolds stress models were used for solving the steady microjet into the turbulent boundary layer, and their results are compared with experimental results. Comparing the results indicated that the steady solution methods performed sufficiently we for this problem. Furthermore, it was found that the four-equation transition SST model was the most accurate method for predicting turbulence in this problem. This model predicted the velocity along the x-axis in near- and far-jet locations with about 1% and 5% average errors, respectively. It also outperformed the other methods in predicting Reynolds stresses, especially at the center (nearly 5% error). Moreover, the modified four-equation transition SST model has improved the system's performance in predicting the studied parameters by utilizing Sørensen correlations in predicting Reθt (the transition momentum thickness Reynolds number), Flength (an empirical correlation that controls the length of the transition region), and Reθc (the critical Reynolds number where the intermittency first starts to increase in the boundary layer).

Unsteady flow control mechanisms of a bio-inspired flexible flap with the fluid–structure interaction

Recently, the development of bio-inspired aircrafts has broad application prospects. However, the flow separation in the boundary layer of the bio-inspired wing under low Reynolds number becomes a great challenge for the design of a novel bio-inspired aircraft. It is worth noting that birds in nature can easily control flow separation, thanks to the flap-like flexible plumes attached to their wing surfaces. In this paper, the unsteady flow control of the flexible flap is studied by the immersed boundary-lattice Boltzmann-finite element method (IB-LB-FEM). The mechanism of flow separation on the airfoil surface at a bio-inspired large angle of attack (AOA) is suggested. The effects of the flexible flap position and its material properties on the unsteady flow control of the airfoil at large AOA are systematically discussed. The deformation law of the flexible flap with fluid–structure interaction (FSI) is revealed, and its influence on unsteady aerodynamics of the airfoil is discussed. The results show that with the increase in the AOA, the aerodynamic characteristics of the airfoil change with time from “periodic state” to “chaotic state” to “quasi-periodic state,” which is closely related to the unsteady flow separation on the airfoil upper surface. The new induced vortex is formed at the end of the flexible flap because of the FSI, which enhances or weakens the strength of vortices on the airfoil surface, affecting the aerodynamics of the airfoil. The flow control mechanism of the flexible flap proposed in this paper will provide a new design idea for the novel bio-inspired aircraft.

Effects of Frequency and Amplitude of Local Dynamic Hump and Inlet Turbulence Intensity on Flow Separation Control in Low-Pressure Turbine

Effects of Frequency and Amplitude of Local Dynamic Hump and Inlet Turbulence Intensity on Flow Separation Control in Low-Pressure Turbine

Study on flow separation and suction control of a compressor tandem cascade

To control the boundary layer separation on the front blade suction surface in a compressor tandem cascade at a high incidence angle, three suction schemes at different axial positions were introduced and the cascade performance improvement and separation flow control effects were investigated. Based on the experimental and numerical simulations, the aerodynamic performance of the prototype tandem cascade reaches the peak at the incidence angle of 5°. Continue to increase the incidence angle, the loss increases sharply, and the static pressure rise coefficient and diffusion factor begin to decrease. With the boundary layer suction on the suction surface of the front blade, fluid energy in the upstream boundary layer of the suction slot is increased, and the adverse pressure gradient downstream is decreased. By suction, the development and separation of the boundary layer are inhibited, and the loss in the tandem cascade is significantly reduced. The flow control effect of the suction scheme at the onset location of boundary layer separation is the best, and the flow loss can be reduced by about 34.9% with a suction flow rate of 1% at the incidence angle of 5°. The suction schemes have good flow control effects in a wide working range and broaden the available range of incidence angles of the tandem cascade. This study provides a reference for the design of tandem blades considering flow control and has a positive significance for improving compressor efficiency and pressure ratio.

Control of flow separation from a curved ramp using a steady-blowing jet

This work experimentally investigates flow separation control over a two-dimensional curved ramp using a spanwise steady-blowing slit jet. The flow separation and reattachment are first characterized in the absence of control for the momentum-thickness-based Reynolds number Reθ=2000–15 000. Under control, the separation bubble sizes or pressure coefficient Cp,end at the end of the ramp exhibits a dependence on Reθ, blowing jet angle α, and jet Reynolds number Red. It is found that, for a given α, the relationship Cp,end=f1 (Reθ, α, Red) may be reduced to Cp,end=f2(Cm), where Cm=Uj/U∞ is the jet velocity ratio. That is, given the same Cm, Cp,end remains unchanged, irrespective of different Reθ or Red. It is further found that flow separation from the ramp may be completely suppressed given a small α, i.e., α≤55°, but not when α is large, i.e., >85°. The corresponding mechanisms behind are discussed in detail.

Large-eddy simulation of a planar offset-jet with heat transfer: The effects of ventilation

Several industrial and engineering applications employ ventilated offset-jets such as flow separation control devices, upper surface blowing used in short take-off and landing aircraft, drying processes, and fuel injection systems. The design and efficient operation of these devices relies on controlling the turbulence levels, heat transfer rates, and the locations of flow reattachment points. Therefore, an understanding of the flow and thermal characteristics of ventilated offset-jets helps in the design and operation of these devices. The present study aims to address this by performing large-eddy simulations (LES) of a planar turbulent offset-jet for different velocities of the ventilated-jet. To understand and quantify the effects of ventilation on the offset-jet, a range of velocity-ratios of 0, 0.1, 0.14, and 0.18 are considered for a primary offset-jet Reynolds number of 14,000. First, a grid-sensitivity study is performed to establish the convergence of the solver and the LES index of quality of resolution is obtained to ascertain the quality of the mesh. Thereafter, the mean and second-order statistics of the streamwise component of velocity are compared with reference data from the literature to validate the numerical solver. The effects of ventilation on the offset-jet are studied using the decay of streamwise velocity, jet-spread, the evolution of pressure coefficient, friction coefficient, the mean Nusselt number, and the unsteady characteristics. As the velocity-ratio increases, the suction pressure in the initial region of the domain decreases, which causes the jet to spread away from the bottom wall. Further the peak magnitudes of the mean Nusselt number and coefficient of pressure decrease exponentially with an increase in the velocity-ratio. The peak magnitude of the mean Nusselt number for the unventilated-jet case – the offset-jet without the ventilated-jet – is the largest and its value for the ventilated-jet cases decreases as the velocity-ratio is increased. It is concluded that in the initial region, the ventilated-jet has a profound effect on the offset-jet flow and as the flow develops into a wall-jet the effects of ventilation vanish.

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.

Endwall-pulsed blowing of different excitation models to control flow separation on a highly-loaded compressor cascade

Abstract Endwall-pulsed blowing (EPB) is studied for three different excitation waveforms to improve the aerodynamic performance of highly loaded compressors. Some important excitation parameters include the excitation frequency and momentum coefficient, which were analyzed in detail. The results of the EPB are compared with the endwall steady blowing (ESB) case. For EPBs with the three excitation waveforms (Waveforms sine, triangle and trapezoid), excitation frequencies that are equal to an integral multiple of the natural frequency of the vortex shedding are optimal and provide better performances than the ESB with the same time-mean momentum coefficient. Moreover, the EPBs of the three excitation waveforms have significant differences in their aerodynamic performance improvements. The optimal case is achieved by the EPB with Waveform triangle and provides a total pressure loss coefficient with a reduction of 25.64%.

Numerical investigation on high-speed jet actuation for transient control of flow separation

Flow separation around a lifting body was controlled using a high-speed jet generated via impulsive actuation, and important flow features associated with the transient dynamics were numerically investigated. A jet flow was impulsively applied to the VR-12 airfoil using a boundary condition modeled as a COMPACT module in the wind tunnel experiments. A delayed detached-eddy simulation based on the Spalart–Allmaras turbulence model was conducted. Computational results indicated that the aerodynamic characteristics quickly varied in the early stage after jet initiation, and that the impact of the actuation on the flow behavior was gradually reduced through a larger time scale than the freestream convection. A detailed investigation was performed to assess relevant flow features, and two distinct flow characteristics associated with the transient dynamics were identified: reattachment and recirculation. Reattachment in the early stage after actuation yielded a high suction peak near the leading edge and rapidly enhanced the lift force. Recirculation in the later stage affected gradual variations in the aerodynamic forces and moment to the baseline flow via the generation of a low-pressure region on the suction side.

Cluster-based control for net drag reduction of the fluidic pinball

We propose a Cluster-Based Control (CBC) strategy for model-free feedback drag reduction with multiple actuators and full-state feedback. CBC consists of three steps. First, the input of the feedback law is clustered from unforced flow data. Second, the feedback law is interpolated with actuation commands associated with the cluster centroids. Thus, centroids and these actuation commands facilitate a low-dimensional parameterization of the feedback law. Third, the centroid-based actuation commands are optimized, e.g., with a downhill simplex method. This framework generalizes the feature-based CBC from Nair et al. [“Cluster-based feedback control of turbulent post-stall separated flows,” J. Fluid Mech. 875, 345–375 (2019)] in three aspects. First, the control law input is the velocity field. Second, the control law output commands multiple actuators here. Third, a reformulation of the downhill simplex method allows parallelizing the simulations, thus accelerating the computation threefold. Full-state CBC is demonstrated on a multiple-input configuration, the so-called fluidic pinball in three flow regimes, including symmetric periodic at Re = 30, asymmetric periodic at Re = 100, and chaotic vortex shedding at Re = 150. The net drag reductions for the three cases amount to 33.06%, 24.15%, and 12.23%, respectively. CBC shows distinct advantages for robustness control at different flow conditions. The full-state CBC further reveals the evolution of the control flow associated with the centroids, which contributes to the physical interpretation of the feedback control process.

Comparison of flow structures for vane and rod type vortex generators on a wind turbine airfoil

PurposeThe purpose of this paper is to investigate and compare the effects of rod and vane-type vortex generators for wind turbine applications. In large wind turbine rotors, an attached flow at all sections along the span direction is difficult to achieve which leads to an increase in aerodynamic losses, noise generation, and fatigue stress. Therefore, flow control strategies such as vortex generators (VGs) are beneficial to improve performance.Design/methodology/approachThe benefits of the application of rod-type vortex generators (RVGs) to control flow separation on a wind turbine airfoil are assessed numerically using computational fluid dynamics (CFD). The validation of the computational model is conducted against the experimental data available for the DU96-W-180 wind turbine airfoil equipped with 44 RVGs. In addition, a revised wind tunnel angle of attack (AoA) calibration procedure (based on CFD) is proposed that is applicable for separated flows. A comparison of the RVGs to the conventional vane-type vortex generators (VVGs) is presented for inflow velocity of 30 m/s and AoA leading to significant flow separation. A parametric evaluation of the geometric characteristics of both types of VGs is conducted to quantify the generated streamwise vortices.FindingsThe comparison of the induced flow structures and aerodynamic efficiency enhancements proves that RVGs may be used as an alternative to the more conventional VVGs applied on wind turbine blades for boundary layer separation control.Originality/valueA new type of VG (rod) has been investigated and compared against conventional VG (vanes) for wind turbine applications.

Flow separation control in a two-airfoil system by trailing edge modification and active flow control

Flow separation control in a two-airfoil system by trailing edge modification and active flow control

Flow separation control in a three-element airfoil system at moderate Reynolds number

Flow separation control in a three-element airfoil system at moderate Reynolds number

Investigation of flow separation control over airfoil using nanosecond pulsed plasma actuator

A detailed investigation of flow separation control over a stalled NACA 0015 airfoil model using pulsed nanosecond dielectric barrier discharge (NS-DBD) plasma actuator is conducted through combined large eddy simulation (LES), dynamic mode decomposition (DMD) and experimental particle image velocimetry (PIV) measurement possibly for the first time. Airfoil flow at chord-based Reynolds number (Re) of Re = 6.3 × 104 is considered. It is found that the interplay of the residual heat due to the nanosecond plasma discharge with external flow causes the generation of vortices through the baroclinic and vortex dilation mechanisms and finally promotes the formation of large-scale vortical structures that dominate the early-stage evolution of the flow control process. One of the most important findings of this work is that the flow control mechanisms are not universal but depend on the forcing frequencies of plasma actuation, and are categorized into three different types. Meanwhile, three-dimensional DMD analysis of LES datasets is performed and the output of modal analysis helps provide more physical insights into flow control and determine the optimum forcing frequency. Depending on the value of forcing frequency, the global or local resonance of the forced flow with the periodic plasma actuation occurs. The plasma forcing at any frequencies in the range 1≤F+≤20 produces pronounced flow control authority except for that at F+=20. Moreover, the optimum forcing frequency is identified and its relationship with the inherent characteristic frequencies of the baseline flow is established based on DMD analysis. It is found that the most favorable control effect is achieved when the plasma excitation frequency is high and very close to the characteristic frequency of the separated shear layer in the unforced baseline flow.

Wind-tunnel tests of synthetic jet control effects on airfoil flow separation

To find out the comprehensive principles of synthetic jet control effects on airfoil stall characteristics, fundamental contrastive wind-tunnel tests were conducted systematically. By using six-component balance, Particle Image Velocimetry (PIV) technology and boundary layer probe, the measurements of model aerodynamic forces, the whole velocity field over airfoil and velocity profiles in the boundary layer were conducted, respectively. Based upon the experimentally parametric analyses of synthetic jet control on airfoil, it is concluded that the incline angle of synthetic jet has an important impact on both the flow in boundary layer and aerodynamic forces of airfoil. A tangential jet inject energy and accelerate the velocity of flow in inner boundary layer thus has a better control effects on delay stall of airfoil when the momentum coefficient of jet is relatively large, and the normal jet helps to enlarge the thickness of boundary layer which is proved to have better control effects on flow separation control and improving the aerodynamic characteristics of airfoil when the momentum coefficient of a jet is relatively small.

Vortex separation cascades in simulations of the planar flow past an impulsively started cylinder up to

Direct numerical simulations of the flow past an impulsively started cylinder at high Reynolds numbers (25k–100k) reveal an intriguing portrait of unsteady separation. Vorticity generation and vortex shedding entails a cascade of separation events on the cylinder surface that are reminiscent of Kelvin–Helmholtz instabilities. Primary vortices roll up along the cylinder surface as a result of instabilities of the initially attached vortex sheets, followed by vortex eruptions, creation of secondary vorticity and formation of dipole structures that are subsequently ejected from the surface of the cylinder. We analyse the vortical structures and their relationship to the forces experienced by the cylinder. This striking cascade of vortex instabilities may serve as reference for reduced-order models of flow separation and as guide for flow control of separated flows at high Reynolds numbers.

The effect of vortex induced vibrating cylinders on airfoil aerodynamics

Two-dimensional fluid-structure coupling dynamic models of single- and double-cylinder with airfoil are established using fluid-structures dynamics methods. The accuracy of the airfoil and vortex-induced vibration simulation models is verified by comparing them with available experimental data. Coupled simulations are used to study the vortex-induced vibration of two cylinders under uniform incoming flow, and the influence of two cylinders at different positions of the airfoil leading edge on their aerodynamic forces. Vorticity and streamline diagrams provide evidence of the effects of single cylinder and double cylinders on the airfoil when placed at different angles of attack. The vibration response of the cylinder is also observed. The results show that at high angles of attack the flow separation of the airfoil can be effectively controlled by positioning small cylinders in the proximity of the airfoil's leading edge. A small oscillating cylinder can contribute to control the flow around the airfoil, better than fixed cylinder. Indeed, when the airfoil is within stall condition, a single vibrating cylinder can control flow separation and improve airfoil lift-to-drag ratio, better than when no cylinder or double cylinders are considered. For increasing angle of attack post stall condition, the ability of a single oscillating cylinder to control the flow around airfoils decreases gradually, while the one of double cylinders increases.

Nozzle Flow Separation Phenomena and Control for different conditions

A detailed study of separated nozzle flows has been conducted. For a subscale, non-axisymmetric, two-dimensional, convergent divergent nozzle, schlieren flow visualization was acquired along with measurements of force, moment, and pressure as part of an extensive static performance evaluation. Additionally, two-dimensional numerical simulations were performed using the computational fluid dynamics package PAB3D together with algebraic Reynold’s stress modelling and two-equation turbulence closure. This study's experimental findings show that shock-induced boundary layer separation, which was classified into two distinct flow regimes: three-dimensional separation with partial reattachment and entirely detached two-dimensional separation, dominated off design over expanded nozzle flow. The impact of variable shock generation and reflections in various nozzle types on the two primary separation modes, namely Free and Restricted Shock Separation (FSS & RSS), is investigated. The flow separation problem in rocket nozzles has been an unwelcome phenomenon for engineers ever since the birth of the space era. Naturally, the engineers were given the job of bringing things under control. But it proved to be a difficult endeavor; despite the fact that many people were able to explain the physics underlying this occurrence, it is still not completely understood today.