Turbulent Separation Control Research Articles

Numerical analysis of novel wavy wall based control of turbulent boundary layer separation

In this work, a comprehensive insight into the new passive flow control method, i.e. a two-dimensional streamwise waviness (recently proposed by Dróżdż et al. [1]) is provided using Large Eddy Simulations. This approach was demonstrated to be effective in postponing turbulent boundary layer separation at high Re, which is out of reach for other commonly known passive flow control strategies. Although the effectiveness of the wavy wall in delaying flow separation has been confirmed, the physical mechanisms standing behind the skin friction enhancement are not fully understood which opens a space for future work. Of particular interest is an insight into the fluid flow in the near-wall region just above the waviness and this paper is aimed at doing so. To separate the effect of the global adverse pressure gradient, the results were confronted with the ones obtained under zero pressure gradient conditions for the identical wavy wall geometry and the same Reτ=2500. Also additional simulations of the turbulent boundary layer building up on the flat wall were conducted to have an additional reference for the flow fields obtained above the wavy wall under zero and adverse pressure gradients respectively. A detailed inspection of the streamwise velocity and its fluctuation profiles indicated that the contribution of the global adverse pressure gradient to the flow statistics begins when the Rotta-Clauser pressure gradient parameter β≥3. The paper confirms an enhancement of the wall shear stress τw downstream the corrugation, which is the necessary condition to postpone the separation. Interestingly, the effect of increased τw was not observed for the case with the same waviness and without pressure gradient.

Turbulent Boundary Layer Separation Control Using Magnetohydrodynamic Plasma Actuator

The pulse electric arc discharge in an external magnetic field is studied as a vortex generator in the subsonic boundary layer. A pulsed Ampere force induces a hairpin vortex near the wall; its structure depends on the relative direction of arc propagation and external flow velocity. The data presented in this article were obtained from parametric studies of vortex characteristics and their effects on the boundary layer profile at various actuator momentum coefficients (Cμ=1−30) and vortex sizes relative to the boundary layer thickness (D/δ=0.5−1.2). Also, the control of turbulent boundary layer separation on a bump at a flow velocity up to 50 m/s was attempted. An average shift of the separation line by 15% of the bump height was obtained at a flow velocity of 50 m/s and a total momentum coefficient of 0.6%.

Proper Orthogonal Decomposition Analysis of Coherent Structure in a Turbulent Flow after a Micro-vortex Generator

A micro-vortex generator (MVG) is a common device used for turbulent flow separation control. In this study, in order to better understand the flow control mechanism using an MVG, we analyzed the flow data set obtained by implicit large eddy simulation with fifth order weighted essentially nonoscillatory scheme spatial discretization at Ma = 2.5 and Reθ = 5760 in detail by using the proper orthogonal decomposition (POD) method. The POD method was applied with 120 snapshots using both the velocity (u, v, w) and Liutex vector (Lx, Ly, Lz) directly as inputs to analyze the vortex structure behind the MVG. According to the results, the first six POD modes were suitable for explaining the vortex structures behind the MVG. In addition, for the first time, the data loaded directly from the Liutex vector instead of velocity were organized in a snapshot matrix and the correlation was determined between the absolute strength of rotation (Liutex) and cumulative rotation strength (Liutex magnitude). Vortex pairing and merging of co-rotating vortices were found in both data input cases with the velocity and Liutex. When the modes were ranked by energy, the first six modes were paired and the streamwise structures were dominant in modes 1–2. When the modes were ranked by Liutex strength, the first two modes mixed by streamwise and spanwise vortices were not paired, but modes 3–4 and 5–6 were paired, and they were all spanwise-structure dominant. Comparisons also showed that the spanwise vortices had higher frequencies.

Effective use of the streamwise waviness in the control of turbulent separation

It is commonly known that for a sufficiently high Reynolds number, for flow over a rough surface and other previously examined wall topologies, turbulent boundary layer separation generally occurs earlier, and the separation bubble is noticeably larger due to a greater momentum deficit caused by wall irregularities. The recently discovered amplitude modulation effect has provided an improved understanding of the momentum transport that occurs from the outer to the inner part of the boundary layer and its subsequent effect on skin friction. At sufficiently large Reynolds numbers, large-scale motion first causes an increase and then decrease in the flow velocity near the wall. This results in an increased net convection velocity of small structures in the streamwise direction, and consequently an increase in wall shear stress. In this work, we demonstrate that a wavy wall, with a streamwise waviness with carefully selected amplitude and period, can effectively enhance the amplitude modulation effect and ensure an increase in the wall shear stress, thereby postponing turbulent separation. The experimental results are presented in normalized general form using suitable flow scaling. The research goal is to find values of the amplitude, period, and length (always with the total number of periods) of the wavy wall for which the highest rise in skin friction is gained at a fixed point downstream of the wavy wall. The most effective wavy wall geometry investigated ensured a 13% increase in skin friction (relative to the value at zero-pressure-gradient inlet flow) at the location where flow separation occurred on an unmodified surface (i.e. on a flat plate).

Mechanisms for turbulent separation control using plasma actuator at Reynolds number of 1.6 × 106

We have conducted large-eddy simulations of turbulent separated flows over a NACA0015 airfoil with control by a plasma actuator. The Reynolds number based on the chord length is 1 600 000, and the angle of attack is 20.1°. At this angle of attack, the flow around the airfoil is fully separated. The effects of the location and operating conditions of the plasma actuator on the separation control are investigated. The plasma actuator is set at the leading edge, the turbulent reattachment point, or near the turbulent separation point. The nondimensional burst frequency (F+) is set to 1, 4, or 100. These frequencies are determined based on the dominant frequencies of the turbulent separated flow field of the no control case. A continuous actuation case has also been conducted. The location of the actuator where it most effectively suppresses the separation is the one closest to the turbulent separation point. In the burst mode case, the nondimensional burst frequency of unity is most effective in terms of the increase in the lift. To clarify the effective control mechanism, five objectives for turbulent separation control are compared. The results show that it is difficult to suppress the turbulent separation using the same strategies as in laminar separation control. The effective mechanism for turbulent separation control by burst actuation is found to be inducing the pairing of large-scale vortices near the airfoil surface. This large-scale vortex pairing induces freestream momentum into the boundary layer, leading to separation suppression. In addition, three other control effects can be achieved by varying the operating settings of the plasma actuator. The drag is slightly improved by reducing the length of the laminar separation bubble through high-frequency actuation from the leading edge.

Vortex Ring Formation Characteristics in Synthetic Jet due to Changes of Excitation Frequency in the ½-Ball Cavity Actuator

A jet flow that contains vortex ring has a large energy compared to a regular jet. As one of the causes of the aerodynamic drag to the vehicle, the flow separation that occurs behind the bluff body must be controlled, so that aerodynamic drag can be significantly reduced. This study is a basic work on the development of turbulent flow separation control for aerodynamic purpose, especially in the design of the vehicle body. The main objective of this study is to analyze the performance of the synthetic jet (SJA) as one of flow control tool to reduce separation area. To get the maximum performance of the synthetic jet actuator, the research starts by characterizing the actuator. Characterization of ½ ball-shaped cavity is done with excitation frequency changes and orifice diameter of 3, 5 and 8 mm. The study was conducted using computational and experimental methods. The experimental data was obtained by testing synthetic jet actuator with providing sinusoidal signal to drive the membrane and at the orifice end a hotwire probe that is set and plugged into a CTA (Constant Temperature Anemometry) to obtain the speed velocity of the exhaust jet. Computational methods used a commercial CFD software (FLUENT 6.3) with a Reynolds Stress Model as a model of turbulence. Each of these calculations or measurements was conducted under the same conditions. The research result is displayed in frequency testing curve to get the maximum velocity of the jet stream. The results are further indicative of the synthetic jet actuator capability to generate vortex rings. In the experimental results, the determination of ring vortex formation taken from the calculation of the flow velocity, while the CFD simulations, the formation of vortex rings can be seen from the visualization of the flow contour. Vortex ring formed from this ½ -ball cavity, occurred at 3 mm and 5 mm orifice diameter, while the 8 mm orifice diameter cavity cannot form a ring vortex.

Turbulent boundary layer separation control using plasma actuator at Reynolds number 2000000

An experimental investigation was conducted to evaluate the effect of symmetrical plasma actuators on turbulent boundary layer separation control at high Reynolds number. Compared with the traditional control method of plasma actuator, the whole test model was made of aluminum and acted as a covered electrode of the symmetrical plasma actuator. The experimental study of plasma actuators’ effect on surrounding air, a canonical zero-pressure gradient turbulent boundary, was carried out using particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) in the 0.75m×0.75m low speed wind tunnel to reveal the symmetrical plasma actuator characterization in an external flow. A half model of wing-body configuration was experimentally investigated in the ∅ 3.2m low speed wind tunnel with a six-component strain gauge balance and PIV. The results show that the turbulent boundary layer separation of wing can be obviously suppressed and the maximum lift coefficient is improved at high Reynolds number with the symmetrical plasma actuator. It turns out that the maximum lift coefficient increased by approximately 8.98% and the stall angle of attack was delayed by approximately 2° at Reynolds number 2×106. The effective mechanism for the turbulent separation control by the symmetrical plasma actuators is to induce the vortex near the wing surface which could create the relatively large-scale disturbance and promote momentum mixing between low speed flow and main flow regions.

Investigation of coherent structures generated by acoustic tube in turbulent flow separation control

An acoustic tube was designed in order to control the turbulent flow separation downstream of a backward-facing step. The Reynolds number based on the free-stream velocity and the step height was Reh = 2.0 × 104. As an active flow control device, the acoustic tube generated periodic pressure perturbations at a frequency of fa = 100 Hz, which was close to the most amplified frequency of the shedding instability of the turbulent shear layer. Spanwise vortices rolled up due to the perturbations. 2D–2C particle image velocimetry was used to measure separated shear layer and the reattachment area downstream of the BFS. The flow control results show that the acoustic tube can suppress recirculation regions behind the step and reduce the reattachment length by 43.7 %. The roll-up and pairing processes of the vortices lead to an increase in the total Reynolds shear stress. The coherent structures are extracted by proper orthogonal decomposition and represented by two pairs of modes, of which the coherence is analyzed by the corresponding coefficients. Both the primary and secondary series of vortices are reconstructed as traveling waves with the fundamental frequency fa and the overtone frequency 2fa, respectively.

S0550204 高レイノルズ数におけるDBDプラズマアクチュエータを用いた翼周り乱流剥離制御のLES([S055-02]プラズマアクチュエータ(2))

We have conducted large-eddy simulations of the turbulent separation control by the DBD plasma actuator over NACA0015 airfoil. Reynolds number based on the chord length is 1,600,000 and the angle of attack is 20.1 degs. The location and operating conditions of the plasma actuator are changed as the numerical parameter. It is clarified that the effective mechanism for the turbulent-separation control by the burst mode is to induce the pairing of the large-scale vortex shed from the turbulent-separation shear-layer. The large-scale vortex results in the not only the momentum induction from freestream to the boundary layer but also the lift improvement through its convection. In addition to this control mechanism, various control effects can be achieved depending on the settings of the DBD plasma actuator.

Reynolds-Averaged Navier-Stokes Computations of Synthetic Jet Flows Using Deforming Meshes

Realistic computations of flows induced by synthetic jets in multiple flow conditions are studied with an unsteady preconditioned Reynolds-averaged Navier–Stokes solver with deforming meshes. Deforming meshes suitable for structured grid systems are used to simulate the motion of diaphragms driven at a given frequency.With deforming meshes, a boundary-conditionmodel is not necessary for synthetic jet calculations. To obtain time-accurate solutions with a grid system under deformation, the geometric conservation law is employed. Numerical calculations of three cases of the Langley Research Center Workshop Computational Fluid Dynamics Validation of Synthetic Jets and Turbulent Separation Control workshop are carried out, and the computational results are compared with the experimental data. The results indicate the effectiveness of the current method in synthetic jet flow computations.

A DES Procedure Applied to a Wall-Mounted Hump

This paper describes a detached-eddy simulation (DES) for the flow over a wall-mounted hump. The Reynolds number based on the hump chord isRec=9.36×105with an in-let Mach number of 0.1. Solutions of the three-dimensional Reynolds-averaged Navier-Stokes (RANS) procedure are obtained using the Wilcoxk−ωequations. The DES results are obtained using the model presented by Bush and Mani and are compared with RANS solutions and experimental data from NASA's 2004 Computational Fluid Dynamics Validation on Synthetic Jets and Turbulent Separation Control Workshop. The DES procedure exhibited a three-dimensional flow structure in the wake, with a 13.65% shorter mean separation region compared to RANS and a mean reattachment length that is in good agreement with experimental measurements. DES predictions of the pressure coefficient in the separation region also exhibit good agreement with experiment and are more accurate than RANS predictions.

Transient growth of coherent streaks for control of turbulent flow separation

In this paper, we summarise our recent results on turbulent flow separation control using transient growth of large-scale coherent streaks. According to linear stability analysis, the optimal perturbations (i.e., disturbances experiencing the largest transient energy growth) sustained by a turbulent boundary layer are large-scale streamwise uniform coherent vortices leading to streaks, the lift-up effect being responsible for their growth. A first experimental study confirms that using arrays of suitably shaped cylindrical roughness elements, streaks can be artificially forced in a flat plate turbulent boundary layer at a Reynolds number based on the displacement thickness of 1,000. Interacting with the mean flow at leading order, these streaks induce a strong controlled spanwise modification and that their amplitude transiently grows in space. Eventually, streaks are forced on the roof of a generic car model (Ahmed body, see Ahmed et al., 1984) to test their ability to suppress the separation around the rear-end.

Large-Eddy Simulation of Plasma-Based Turbulent Boundary-Layer Separation Control

A large-eddy simulation (LES) was carried out in order to numerically describe the use of plasma-based actuation for the control of turbulent boundary-layer separation. The configuration consisted of a flat plate section over which the boundary layer developed, followed by a curved convex rearward-facing ramp, corresponding to an experimental arrangement. A single dielectric-barrier-discharge (DBD) plasma actuator was then employed to reduce the extent of the separated flow region. Solutions were obtained to the Navier-Stokes equations, that were augmented by source terms used to represent plasma-induced body forces imparted by the actuator on the fluid. A simple phenomenological model provided the electric field generated by the plasma, resulting in the body forces. The numerical method utilized a high-fidelity time-implicit scheme, employing domain decomposition in order to perform calculations on a parallel computing platform. Simulations were first conducted for an isolated actuator in the absence of external flow, which were used to optimize the choice of parameters inherent in the plasma model. Subsequently, actuation was applied to the plate development section without the downstream ramp section. Following these computations, the complete plate/ramp configuration was simulated. In all cases, both continuous and pulsed operation of the actuator was considered. Comparisons are made with available experimental data, and with baseline flows where no control was enforced. The large separated flow region characterizing the baseline flow for the plate/ramp configuration was well captured by the simulation. When continuous control was applied, separation was almost entirely eliminated. Pulsing operation of the actuator using a 40% duty cycle was not as eective, but did result in a considerable reduction of the recirculating zone. The control cases compared reasonably well with experimental measurements. Although deficiencies in the plasma model were apparent in the near-wall region, it appeared to be adequate for use with LES in the exploration of plasma-based control for turbulent wall-bounded flows.

Strategies for control of transitional and turbulent flows using plasma-based actuators

An exploratory numerical study of the control of transitional and turbulent separated flows by means of dielectric-barrier-discharge (DBD) actuators is presented. The flow fields are simulated employing a high-fidelity Navier–Stokes solver augmented with a phenomenological model representing the plasma-induced body forces imparted by the actuator on the fluid. Several applications are considered, including interaction of an actuator with a laminar boundary layer, suppression of wing stall, control of boundary layer transition on a plate, control of laminar separation over a ramp, and turbulent separation over a wall-mounted hump. Effective suppression of stall over a NACA 0015 airfoil at moderate Reynolds numbers is demonstrated using either co-flow or counter-flow actuators pulsed at a sufficiently high frequency. By contrast, continuous actuation is found to provide little control of separation. For a laminar boundary layer developing along a flat plate, a counter-flow DBD actuator is shown to provide an effective on-demand tripping device. This property is exploited for the suppression of laminar separation over a ramp. Control of turbulent boundary-layer separation over a wall-mounted hump suggests that once the flow is turbulent, control effectiveness is only achieved for higher actuator strengths with implications for the scalability of DBD devices to higher freestream velocities.

Turbulent Boundary-Layer Separation Control with Single Dielectric Barrier Discharge Plasma Actuators

An experimental study was conducted to determine the effect of single dielectric barrier discharge plasma actuators on turbulent boundary-layer separation control. Two-component particle image velocimetry and laser Doppler velocimetry measurements showed the effect of plasma actuators on ambient air, a canonical zero-pressure gradient turbulent boundary layer, and a two-dimensional turbulent boundary-layer separation from a convex ramp section. Different actuator configurations and control strategies were implemented. Spanwise actuators were operated in both steady and unsteady modes. Plasma streamwise vortex generators were also used to enhance boundary-layer mixing. Flow visualization using a high-speed camera captured the temporal aspects of the separation control process and helped discern the physical mechanisms associated with each actuation strategy. The steady spanwise actuation produced a wall jet effect that augmented near-wall momentum and reattached the separated boundary layer. The streamwise oriented actuators also showed effective control authority by creating counter-rotating vortices within the boundary layer that promote mixing of high and low momentum fluid.

Flow Control Predictions Using Unsteady Reynolds-Averaged Navier-Stokes Modeling: A Parametric Study

T HE effective control of flow separation promises substantial performance improvements for a wide variety of air vehicles. Although the methods are well known, there is very little by way of theory or numerical models that can adequately predict lift enhancements, drag reduction, etc. An attemptwasmade to address this problem by conducting a computational fluid dynamics (CFD) validation workshop for synthetic jets and turbulent separation control [1], where one casewas dedicated to predicting the nominally two-dimensional flow over a hump. The baseline (uncontrolled) case was considered in addition to control by means of steady suction [2] and zero-net-mass flux (oscillatory) blowing [3]. The workshop determined that CFD with steady or unsteady Reynolds-averaged Navier–Stokes (RANS or URANS) consistently overpredicted the reattachment location, regardless of turbulence model or method. Within the separation bubble, most computations predicted velocity profiles well but considerably underpredicted the magnitude of turbulent shear stresses. Large-eddy simulations and other costly methods appear capable of overcoming this deficiency, but the focus of the current study is on the more affordable RANS and URANS methodologies. See, for example, Krishnan et al. [4], Morgan et al. [5], and Saric et al. [6]. Although these individual test cases were challenging to CFD codes, only a single test case was considered for both steady suction and zero-net-mass flux blowing. During the course of the experimental investigation, however, steady and unsteady surface pressures were acquired for a wide variety of control parameters, including Reynolds number, suction flow rate, and frequency and blowing amplitude in the zero-net-mass flux case. Significant variations, or changes, in control authority were observed depending upon the control input or prevailing Reynolds numbers. For example, varying the suction flow rate at different Reynolds numbers indicated an increase in the rate of drag reduction; in contrast, no Reynolds number effect could be detected with regard to zero-net-mass flux blowing. In addition, for the oscillatory case, the flow was seen to be highly dependent on control frequency and peak blowing amplitude. Different, sometimes counteracting, mechanisms dominated the separated flowfield during different parts of the control cycle. Comparisons of these changes, or trends, with numerical results will clearly facilitate more precise evaluation of CFD’s value for predictive purposes. This is because the uncontrolled, or baseline, state is usually known and hence the correct prediction of changes to the baseline state as a result of a particular control strategy would be of great value for assessing different concepts or downselection. To address this need, Rumsey and Greenblatt [7] undertook a detailed parametric study and this Note presents a summary of the main findings.

Improving Aircraft Endurance Through Turbulent Separation Control by Pulsed Blowing

Boundary-layer unsteady blowing is one of the most advanced solutions for reducing aircraft parasite drags and flow separation at high angles of attack. It allows high lift along with low drag to be achieved and, because endurance is one of the most important performance parameters for certain types of aircraft, such as unmanned aerial vehicles, clearly the CL 3/2 /CD ratio has to be maximized. The main goal of the present investigation is to explore possible ways to obtain efficient turbulent boundary-layer control and, at the same time, to consider the practical problems connected to the installation of the device in a real wing. This study seeks mainly to verify the effectiveness of active control via pulsed blowing as a tool to delay boundary-layer separation. Numerical simulations and wind-tunnel experimental investigations on a wing model equipped with instruments are presented and the results discussed. An extensive numerical and experimental investigation on the parameters that can affect flow separation has been carried out. As a result of this analysis, it seems that tubing length produces the most significant effect on the resonant frequencies of a pulsed-blowing system, whereas slot exit and cavity volume mainly affect the resonant peak amplitude. The best actuation frequency varies, depending on the type of aerodynamic performance to be optimized (efficiency, lift, or endurance).

Large-Eddy Simulation of Separation Control for Flow over a Wall-Mounted Hump

This work describes an implicit large-eddy simulation (LES) for active control of flow over a wall-mounted hump. Both steady suction and oscillatory blowing and suction are compared to baseline conditions with no flow control. This simulation models an experiment conducted by NASA which was one of the test cases in their 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop. A previous LES of the baseline case with the current scheme demonstrated significantly better agreement with the experimental flow physics than RANS in the separated region downstream of the hump. The current work concludes that this also holds for the cases using active control. The LES is accomplished using an implicit parallel flow solver that is based on an approximately-factored time-integration method using fourth-order spatial compact-dierence formulations and a high-order filtering strategy. To properly resolve the flow for this LES, the Reynolds number of 9.36 ◊ 10 5 used in the experiment was reduced to 2.0 ◊ 10 5 . Eects of lowering the Reynolds number were previously investigated using RANS. Flow features of the active control cases are compared with the baseline case, each other, and experimental data. The active control LES cases matches experimental data significantly better in the recirculation region than RANS. The baseline and steady suction LES separation reattachment lengths are within 2% and 4%, respectively, of the experimental locations. These simulations achieve very good agreement with the experimental surface pressure coecient, skin friction coefficient, mean velocity profiles, Reynolds stresses, and flow reattachment location. Because of the lower Reynolds number, the oscillatory blowing and suction flow control only has about 25% of the eectiveness of that observed in the experiment. The LES separation reattachment length is 10% longer than its experimental counterpart. Other flow quantities display favorable agreement with experimental data. Results demonstrate that both steady suction and oscillatory blowing and suction can be properly simulated by LES and eectively reduce the size of the separated flow region.

Three-Dimensional Computation of a Synthetic Jet in Quiescent Air

An unsteady Reynolds-averaged Navier-Stokes solver, WIND, is employed to capture the flowfield features of a synthetic jet pulsing into the quiescent air, case 1 of NASA Langley Research Center Workshop: CFD validation of synthetic jets and turbulent separation control, held at Williamsburg, Virginia in March 2004. Both 2-D and 3-D simulations of this case have been performed by employing the appropriate boundary conditions at diaphragm of the actuator. It should be noted that the computations reported in this paper strictly follow the guidelines of the workshop in terms of all the parameters and modeling issues such as grid resolution, time step, boundary conditions, turbulence modeling, etc. Furthermore, to authors' knowledge, this is the only paper among the workshop participants reporting the 3-D simulations for this case. Comparisons are made with the experimental data; fair agreement is obtained. It is shown that the unsteady Reynolds-averaged Navier-Stokes simulations are capable of predicting the overall features of the oscillatory synthetic jet flowfield; however, noticeable discrepancies remain between the computations and the experimental data.

Isolated Synthetic Jet in Crossflow: Experimental Protocols for a Validation Dataset

An overview of the data acquisition, reduction, and uncertainty estimates of experimental measurements made of the flowfield created by the interaction of an isolated synthetic jet and a turbulent boundary layer is presented. The experimental measurements were undertaken to serve as the second of three computational fluid dynamics validation databases for active flow control. The validation databases were first presented at the NASA Langley Research Center Workshop on CFD Validation of Synthetic Jets and Turbulent Separation Control (CFDVAL2004) in March 2004. Detailed measurements were made to document the boundary conditions for the flow and also for the phase-averaged flowfield itself. Three component laser-Doppler velocimetry, 2-D particle image velocimetry, and stereo particle image velocimetry were used to document the phase-averaged velocity field and the turbulent stresses.