Flow Separation Control Research Articles

Delay or control of flow separation for enhanced aerodynamic performance using an effective morphed surface

Delay or control of flow separation for enhanced aerodynamic performance using an effective morphed surface

Numerical and experimental study on flow separation control of airfoils with various leading-edge tubercles

In recent years, leading-edge tubercles of humpback whales have received increasing attention as a passive stall control mechanism. The control mechanism and flow field structure of a reference (ref) and two modified airfoils (mod-1 and mod-2) are investigated numerically and experimentally at a Reynolds number of Re = 5 × 105 in this paper. The results demonstrate that the lift coefficient of the modified airfoils is enhanced to a certain extent after these airfoils stall. In addition, the stall processes of mod-2 are smooth and stable over the observed sudden decrease in ref. Counter-rotation vortex pairs (CRVPs), which improves momentum exchange in the trailing edge boundary layer on the suction side, according to streamline slices and iso-surfaces of the vorticity. Moreover, interaction between streamwise vortices is the main reason for the improved hydrodynamic characteristics at high angles of attack (AOAs). Furthermore, when smoke visualization is carried out in a low-Re wind tunnel, mod-1 and mod-2 exhibit similar characteristics, vortex pairs are periodic and symmetric along the wingspan direction at α = 5°, and vortex pairs begin to interact and influence flow separation at α = 15°. Finally, numerical analysis of the characteristics of tubercle turbines is conducted. The findings reveal that the energy efficiency can be enhanced within a narrow range of tip speed ratios (TSRs).

Interaction of synthetic jets with a massively separated three-dimensional flow field

The interaction of an array of finite span synthetic jet actuators with a massively separated flow was explored experimentally over a cantilevered, swept, and tapered model having a deflected control surface. The interaction of one synthetic jet with the local flow led to a flow field associated with formation and advection of vortex rings that bent into the flow, resulting in flow reattachment in their trajectory. Under specific conditions, when multiple jets were actuated, their interaction was either constructive or distractive, depending on the separation severity and spanwise flow magnitude. This work improves control of three-dimensional flow separation using distributed actuators.

Influence of different blowing parameters on flow control on an airfoil

A numerical study on flow separation control is conducted for an airfoil with a blowing jet. In this regard, the effect of different parameters of air blowing on stall controlling and flow structure over NACA 0012 is followed. RANS equations were employed in conjunction k-ω-SST turbulent model. To validate the numerical results, they are compared with reported experiments, and good agreement is observed. In this paper, the effects of different parameters like blowing location, angle of jets, and local jet velocity are investigated in various cases. Blowing location in the range of 0.1-0.9 of chord length from the leading edge, local jet velocities of 0.1, 0.3, and 0.5 of free stream velocity, and angles of 30o, 60o, and 90o are studied. Results reveal that blowing near airfoil ending increases the ratio of lift/drag coefficients. Furthermore, blowing near the angle of 30o prove a positive effect on aerodynamic characteristics. Using a jet velocity equal to the half of free stream velocity shows a favorable effect on blowing. Blowing the flow never revealed to weaken vortices, and therefore it is not recommended to use blowing jet.

Numerical Investigation of Plasma Actuator Effects on Flow Control Over a Three-Dimensional Airfoil With a Sinusoidal Leading Edge

Abstract The impetus of the present bio-inspired work is to investigate the impact of simultaneously using wavy leading-edge (WLE) airfoils in combination with curved multidielectric barrier discharge (DBD) plasma actuators as hybrid passive and active flow control mechanisms, respectively. A precise distinction of the produced frequency and noise signals, altogether with the acoustic effect of using WLE and DBD plasma actuators, is herein analyzed with precision. Two specific DBD plasma actuators are designed to actuate at x/C = 3% and x/C = 30% on a NACA 634-021 airfoil with sinusoidal WLE that bears a wavelength of 25% and an amplitude of 5% of the mean chord length and straight-leading-edge (SLE). A large eddy simulation (LES) turbulence model was used. This includes the dynamic control of unsteady flow separation, the three-dimensional vortical structure and induced trains of vortices, the aerodynamic forces, the velocity variation, and also the spanwise flow. The momentum transfer between the main flow and boundary layer was improved by the DBDs-induced vortices train and formed streamwise counter-rotating pair-of-vortices over the tubercle. Also, both the continuous wavelet transform (CWT) and fast Fourier transform (FFT) methods were used to investigate the induced plasma flow spectral content for the WLE and SLE geometries. We witnessed an optimized flow control, by using DBD plasma actuators with the WLE airfoil, that resulted in less massive flow separation, faster turbulent transition, and a robust earlier flow reattachment. This modification was beneficial in increasing the efficiency and decreasing the noise for low Reynolds number operational conditions.

Duffing–van der Pol nonlinear reduced-order model for explaining the phenomena and mechanism in pulsed jet flow separation control

A two-equation nonlinear reduced-order model is established to describe the interaction between external periodic excitation from a pulsed jet and unsteady flow separation. This model, which is based on a 2D Navier–Stokes equation, the Stuart vortex row model, and the Stuart–Landau model, can approximately describe the typical phenomena occurring in pulsed jet flow separation control. These phenomena include the frequency-dependent, threshold, and lock-on effects, which are related to the frequency, intensity, and phase of excitation. Two indices, namely, the maximum Lyapunov exponent and the entrainment degree, are introduced to identify the mechanism of flow control via external periodic excitation. These indices are used to reflect the order degree and momentum transfer of the flow field. A comparison of the results of the model and those of a numerical simulation in a curved diffuser shows that the nonlinear reduced-order model is effective for qualitatively featuring the behavior of pulsed jet flow separation control. Moreover, the mechanism of pulsed jet flow separation control is explained via the model. Three aspects (i.e., flow instability, flow field ordering, and momentum transfer) are assumed to function together, resulting in an efficient flow control performance.

Flow Separation Control on NACA0015 Airfoil Using Synchronized Jet Actuator

In this study, a novel fluidic jet actuator is designed to control flow separation on a NACA0015 airfoil at various angles of attack. The U-shaped jet actuator has two rectangular slots implemented near the leading edge of the airfoil. It is driven by a piston mechanism and operates at three excitation frequencies. Depending on the motion of the mechanism, a synchronized jet flow is generated by blowing and suction at the dual exits of the actuator slots. The experimental studies are carried out in a subsonic wind tunnel. The unsteady 2D Computational Fluid Dynamics simulations are performed by Detached Eddy Simulation with the SST k-ω turbulence model where measured jet velocities at the exits of the actuator slots are imposed as boundary conditions to mimic motion of the piston. The results at the on-mode and off-mode of the actuator are evaluated in terms of surface pressure coefficient distributions on the airfoil and averaged aerodynamic force coefficients. At low angles of attack, there is an adequate match between numerical and experimental results for the base flow without any control. At higher angles of attack, flow separation becomes considerably dominant and stall prevention by active flow control is detected especially at high excitation frequencies‎.

A numerical investigation on flow past skewed vortex generators ahead of a backward facing ramp

Flow past a backward facing ramp (BFR) with rectangular vane-type vortex generators (VGs) located upstream has been studied numerically using OpenFOAM based steady-state RANS simulations. In particular, single and multiple pair(s) of boundary layer height VGs are skewed at 10° and 30° to study their effects on the flow separation behaviour at Re=3×106. Streamwise and cross-stream results show that single VGs produce counter-rotating streamwise vortices with increasingly different vortex-core strengths and vortical interactions when skewness angle increases. At 30° however, co-rotating vortices are formed instead with significantly heightened vortical interaction levels, leading to asymmetric flow separation and reattachment behaviour. In particular, the use of multiple VGs under the same condition further accentuate these behaviour and results in significant changes to the wall shear stress distribution. Clarifications on how the flow separation region is distorted by the symmetric/asymmetric streamwise vortices based on velocity component analysis are also provided. Lastly, trajectories of the streamwise vortices and vortex-core characteristics support the notion that the streamwise vortices behave significantly more non-linearly at 30° skewness angle here, and that skewing the present VGs such that they produce co-rotating vortices instead of counter-rotating ones leads to very different flow separation control characteristics.

Control of leading-edge separation on bioinspired airfoil with fluttering coverts.

In this work, the aerodynamic role of the artificial covert feathers (i.e., coverts) on an airfoil is experimentally studied in a wind tunnel to investigate the flow control effect on the leading-edge separation. We apply flexible featherlike devices on a high-angle-of-attack airfoil. We use a hot-wire anemometer to measure the velocity profiles and turbulent fluctuations in the downstream wake flow. As a baseline of flow separation, a two-dimensional NACA 0018 airfoil model is set at the angle of attack of 15 ° at the chord-based Reynolds number of 1.0×10^{5}, causing strong leading-edge and trailing-edge shear layers and a low-speed wake flow area in between as large as 0.35 chord length. When deployed on the upper wing surface, the flexible coverts adaptively flutter under the influence of the local unsteady airflow. Hot-wire measurement results show that the leading-edge coverts effectively suppress the flow separation and reduce the size of the wake flow area. The change of power spectral density shows that the predominant peaks as the fundamental and harmonic frequencies are both attenuated due to the suppression of unsteady motions of the shear layers. On the other hand, the fluttering coverts at the trailing edge modify the trailing-edge shear layer by redistributing the turbulent kinetic energy to the high-frequency components. By simultaneous double-point measurement, we find that the leading-edge and trailing-edge shear layers are drawn closer to each other, and the two shear layers show an increased peak in the coherence spectrum. Further multiscale wavelet analysis shows that the perturbations at the 60% chord length increase the large-scale amplitude modulation of small-scale turbulence and therefore they stabilize the leading-edge and trailing-edge shear layers. Meanwhile, the flow intermittency outside of the wake flow area is attenuated as well. The effective flow control effects in the present work are in good agreement with the previous direct observations of bird flight in literature that the coverts on the upper wing surface play an important role in flow separation control during high-angle-of-attack flight. These findings advance the understanding of aerodynamic contribution of the covers on bird wings and reveal the engineering potential of bioinspired coverts for flow separation control of aircrafts and unmanned air vehicles.

Numerical investigation of double sided plasma vortex generator in separation control

Control of flow separation is a great issue to deal with a moving body to ensure its proper aerodynamic characteristics. To achieve this, various methods including active and passive control are suggested depends upon the flow characteristics and the surface in which control is necessary. To make the better use of both active and passive method of flow control this article proposed a new type of double sided plasma actuator (DSPVG) to overcome the drag penalty of conventional vortex generators (VGs) that commonly used in controlling flow and to use actively control. In this regard, the effectiveness of DSPVG has been numerically and experimentally investigated in a separated flow region of a 20° diffuser of an open type tunnel. DSPVG is placed at the upstream of separation location normal to the surface as like as conventional VG except zero angle with flow direction. Both numerical and experimental results of DSPVG are compared with that of VG and baseline flow and better agreements are found. Moreover, DSPVG has shown better separation suppression ability than conventional VGs due to its dual vortices. It is found that DSPVG significantly delay the separation. A freestream flow of 4 m s−1 is used for experiments and numerical computations.

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

Separation Control by Streamwise Vortices in Flow around Automobile Front Bumper

Separation control by plasma actuator is performed in flow around car front bumper to improve electric economy by reducing drag. The evaluation of flow separation control was performed using numerical simulation by OpenFOAM. A front bumper corner model was made in order to reproduce the actual model used in the wind tunnel experiments. Plasma actuator was installed in two different ways, common flow up type (CFU) and common flow down type (CFD) on the surface of front bumper corner model. The effect of separation control in PA both installations was analyzed in the distributions of streamwise velocity u in the various X-Y planes. The vorticity distribution in Y-Z plane showed the same tendency as the experimental results that the streamwise vortices created by the induced flow setting decreased as the flow downstream. From the results of Q-criterion, CFD case archived the development of streamwise vortices for the downstream direction, more than the case of in CFU. It provides insight into the understanding of the phenomenon of the separation control effect. In the evaluation using momentum thickness, CFD had the greatest separation control effect in the central area.

Growth Process of Vortex Structure Formed by a Sweeping jet in Cross Flow near a Wall

The Fluidic Oscillator can permanently generate a high-velocity oscillating jet (Sweeping jet) and give a large fluctuation to the flow field without driving parts. Therefore, it is attracting attention as a new active flow control device. However, the flow characteristics that directly contribute to flow separation control have not been sufficiently clarified, and their optimization and practical application have not progressed. Therefore, in order to clarify the details of the flow characteristics that contribute to the flow separation control, it is necessary to clarify the flow structure generated by the sweeping jet that blows into the main flow from the wall surface．In this study, we clarify the growth process of the vortex structure generated by the interaction of the sweeping jet, which blows out from the wall at 30 degrees to the main flow, by numerical simulation. It is conducted to understand the complex and large-scale flow structure and its generation process generated by a sweeping jet in the main flow. The vortex structure forms one flow tube at z/d < 4.5 and splits into two at z/d > 4.5. Shear layer vortices with the same frequency as the oscillation frequency are generated in the stream tube, and after splitting into two, it is generated alternately to the left and right. After that, at z/d > 6.0, the shear layer vortex begins to collapse due to wall interference, and at z/d > 7.5 it transforms into discrete vortices.

主流と高速振動ジェットが作り出す三次元渦構造

A jet in crossflow formed by the interaction between the jet and a main flow is a typical unsteady flow. Jet in crossflow have been studied using experimental and numerical approaches. In particular, a sweeping jet in crossflow has recently attracted attention from the viewpoint of active flow separation control. A sweeping jet, which is an oscillating jet with high frequency and large amplitude, energy can be injected widely into the flow separation area. However, the flow fields formed by the interaction between the sweeping jet and the main flow and their flow mechanisms are poorly understood, it has not been put to practical use. Furthermore, the flow structure produced by the interaction between a sweeping jet and a main flow with different behaviors is very complex and difficult to understand. Therefore, it is extremely important to elucidate the flow structure generated by the interaction between a sweeping jet and a main flow. In this study, the detailed vortex structure generated by the interaction between the sweeping jet and the main flow was quantitatively examined using stereo PIV. The vortex structure behaves differently depending on the velocity ratio. The larger velocity ratio, a sweeping jet extends to the wake and develops near the wall.

Three-dimensional Vortex Structure Formed by a Sweeping jet and Main flow

A sweeping jet produced by a fluidic oscillator has recently been examined from the viewpoint of flow separation control. A sweeping jet, which is an oscillating jet with high frequency and large amplitude, is generated by the internal structure of the oscillator, which eliminates the need for a drive unit. Sweeping jets have attracted attention as an active flow control device. Studies have proposed active flow control techniques for an airfoil based on sweeping jets. Most of these studies focused on the characteristics of the dynamic forces acting on the airfoil. However, the flow fields formed by the interaction between the sweeping jet and the main flow and their flow mechanisms are poorly understood, especially for the case where a sweeping jet is ejected into a main flow when a flow field is used to control flow separation. In this study, the detailed vortex structure generated by the interaction between the sweeping jet and the main flow was quantitatively examined using stereo PIV. The stream tube generated by the interaction between the sweeping jet and the main flow consists of two vortex tubes near the outlet of the sweeping jet at a location of 2.0d to 6.0d. At the wake over 10.0d, a pair of vortex tubes gradually separates.

Drag decomposition of laminar channel flows developing over convergent–divergent riblets

Convergent–divergent (C–D) riblets are known to induce large-scale secondary flow motion in laminar flow that may point to their potential for flow separation control and heat transfer enhancement. In this paper, a fully-developed pressure-driven laminar channel flow with C–D riblets is studied numerically. The effects of Reynolds number, riblet wavelength and riblet cross-sectional shape on the secondary flow and drag characteristics are investigated. An exact expression for the drag coefficient of this flow is derived based on the balance of kinetic energy principle for a rough-wall channel. Furthermore, the triple decomposition technique is used to identify the contribution to drag production from the mean velocity field, the small- and large-scale dispersive flow field. Our results show that the normalized drag increment starts to rise when the secondary flow begins to alter the velocity field across the span as the Reynolds number based on the channel height (ReH) increases above 100, a behavior which is very different from that of a laminar channel flow developing over homogeneous roughness in which the normalized drag increment is found to remain constant up to a much greater Reynolds number due to an absence of secondary flow. It is found that the drag produced by C–D riblets is predominantly caused by the wall-normal gradient of streamwise velocity. In addition, while the drag increment is predominantly caused by the mean and small-scale dispersive velocity at ReH≤100, the contribution from the large-scale dispersive velocity field increases rapidly with the Reynolds number and becomes dominant as ReH exceeds 400. It is also observed that the secondary flow scales with the riblet wavelength Λ, and its swirling strength peaks around Λ=1.25H, which is accompanied by a drag maxima at ReH≥400. Finally, among the three C–D riblet cross-sectional shape (rectangular, triangular and sinusoidal) examined here, the triangular riblet pattern is found to produce a secondary motion with a similar strength to the other two shapes but with less drag penalty.

Control of flow separation over a circular cylinder using synthetic jet

The development of methods of active separation flow control is of great applied importance for many technical and engineering applications. Understanding the conditions for the flow separation from the surface of a bluff body is essential for the design of aircrafts, cars, hydro and gas turbines, bridges and buildings. Drag, acoustic noise, vibrations and active flow mixing depend drastically on the parameters of the vortex separation process. We investigated the possibility of reducing the longitudinal length of a reverse-flow region using the method of «synthetic jet» active separation flow control. The experiment was carried out on a compact straight-through wind channel with a 1-m long test section of a cross-section of 125x125 mm. The jet was placed at the rear stagnation point of a circular cylinder. The Reynolds number, based on the cylinder diameter and the free-stream velocity, was 5000 and the von Kármán street shedding frequency without the synthetic jet was equal to 64.8 Hz. For the first time, for such a set of parameters, we applied high speed PIV to demonstrate that the injection of the synthetic jet into the cylinder wake region leads to a significant reduction in the longitudinal length of the reverse-flow region.

Control of the turbulent wake flow behind a circular cylinder by asymmetric sectoral hydrophobic coatings

We demonstrate that sectoral coating by a hydrophobic fluoropolymer is an effective method for controlling flow separation and the turbulent wake behind a cylinder in high Reynolds number flows (Re = 2.2 × 105). Time-resolved particle image velocimetry measurements show that the shape of the wake and trajectory of large-scale vortex structures are inclined due to delayed flow separation on one side of the cylinder. Near-wall, high-resolution visualization reveals that this effect is related to micro-bubbles traveling along the coated surface. The properties of the coatings and bubble presence did not deteriorate, even after many hours of continuous facility operation.

Quantifying Hydraulic Roughness From Field Data: Can Dune Morphology Tell the Whole Story?

AbstractHydraulic roughness is a fundamental property in river research, as it directly affects water levels, flow strength and the associated sediment transport rates. However, quantification of roughness is challenging, as it is not directly measurable in the field. In lowland rivers, bedforms are a major source of hydraulic roughness. Decades of research have focused on dunes to allow parameterization of roughness, with relatively little focus on field verification. This study aims to establish the predictive capacity of current roughness predictors, and to identify reasons for the unexplained part of the variance in roughness. We quantified hydraulic roughness based on the Darcy‐Weisbach friction factor (f) calculated from hydraulic field data of a 78 km‐long trajectory of the river Rhine and river Waal in the Netherlands. This is compared to predicted roughness values based on dune geometry, and to the spatial trends in the local topographic leeside angle, both inferred from bathymetric field data. Results from both approaches show the same general trend and magnitude of roughness values (0.019 &lt; f &lt; 0.069). Roughness inferred from dune geometry explains at best 31% of the variance. Efforts to explain the remaining variance from statistics of the local topographic leeside angles, which supposedly control flow separation, were unsuccessful. Unexpectedly, multi‐kilometer depth oscillations explain 34% of the total roughness variations. We suggest that flow divergence associated with depth increase causes energy loss, which is reflected in an elevated hydraulic roughness. Multi‐kilometer depth variations occur in many rivers worldwide, which implies a cause of flow resistance that needs further study.

Ice shape modulation with nanosecond pulsed surface dielectric barrier discharge plasma actuator towards flight safety

Ice accretion on aircraft wings seriously damage the aerodynamic performance and flight safety of an aircraft. To deal with this issue, a leading-edge ice shape modulation method using nanosecond pulsed surface dielectric barrier discharge (nSDBD) is proposed, featuring the merit of less energy consumption when compared to full-wing de-icing. A set of wind tunnel experiments are conducted for verification purposes. Results show that the dangerous continuous ice can be modulated into periodically segmented ice pieces by nSDBD, forming a wavy leading edge, and the energy consumption of ice shape modulation is less than 50% of that of complete deicing. In the case of NACA0012 airfoil, ice shape modulation can improve the lift noticeably at each angle of attack (0° to 22°), delay the stall angle by 2°, and reduce the drag coefficient by more than 50% compared to full ice condition. The duration of ice shape modulation with plasma actuators lasts for about 4 min to 6 min. For flow separation control, the coherent streamwise vortices generated by the wavy leading edge are able to enhance the mixing between the boundary layer and the freestream, resulting in an increase of the ability to resist the adverse pressure gradient above the airfoil. Since both ice shape modulation and flow separation control benefit the aerodynamic performance of icing airfoils, the flight safety is improved.