Koichi Okada

Mechanisms for laminar separated-flow control using dielectric-barrier-discharge plasma actuator at low Reynolds number

Large-eddy simulations have been conducted to investigate the mechanisms of separated-flow control using a dielectric barrier discharge plasma actuator at a low Reynolds number. In the present study, the mechanisms are classified according to the means of momentum injection to the boundary layer. The separated flow around the NACA 0015 airfoil at a Reynolds number of 63 000 is used as the base flow for separation control. Both normal and burst mode actuations are adopted in separation control. The burst frequency non-dimensionalized by the freestream velocity and the chord length (F+) is varied from 0.25 to 25, and we discuss the control mechanism through the comparison of the aerodynamic performance and controlled flow-fields in each normal and burst case. Lift and drag coefficients are significantly improved for the cases of F+ = 1, 5, and 15 due to flow reattachment associated with a laminar-separation bubble. Frequency and linear stability analyses indicate that the F+ = 5 and 15 cases effectively exc...

LES of transient flows controlled by DBD plasma actuator over a stalled airfoil

Large-eddy simulations (LES) are employed to understand the flow field over a NACA 0015 airfoil controlled by a dielectric barrier discharge (DBD) plasma actuator. The Suzen body force model is utilised to introduce the effect of the DBD plasma actuator. The Reynolds number is fixed at 63,000. Transient processes arising due to non-dimensional excitation frequencies of one and six are discussed. The time required to establish flow authority is between four and six characteristic times, independent of the excitation frequency. If the separation is suppressed, the initial flow conditions do not affect the quasi-steady state, and the lift coefficient of the higher frequency case converges very quickly. The transient states can be categorised into following three stages: (1) the lift and drag decreasing stage, (2) the lift recovery stage, and (3) the lift and drag converging stage. The development of vortices and their influence on control is delineated. The simulations show that in the initial transient state, separation of flow suppression is closely related to the development spanwise vortices while during the later, quasi-steady state, three-dimensional vortices become more important.

Computational Study on Effect of Synthetic Jet Design Parameters

Effects of amplitude and frequency of synthetic jet on the characteristics of induced jet are investigated. To estimate effects of the parameters, flow inside the synthetic jet cavity and orifice and the outer flow is simultaneously simulated using large-eddy simulation (LES). Comparison of the present LES result with the experimental data shows that three-dimensional LES of the flow inside the cavity is essential for accurate estimation of the velocity and velocity fluctuation of the synthetic jet. Comparison of the present results under various flow conditions shows that amplitude and frequency can control profiles of time-averaged vertical velocity and fluctuation of the vertical velocity as well as damping rate of the induced velocity and fluctuation.

Computational Analysis of Vortex Structures Induced by a Synthetic Jet to Control Separated Flows

The vortex structure of a separated flow over a backward-facing step controlled by a synthetic jet is investigated by using an implicit large-eddy simulation with a high-order compact difference scheme. The computation results show that mixing in the shear layer is not enhanced, when the flow is controlled at the normalized frequency of 2.0 based on the height of backward-facing step. In this case the separation length is similar to that in the case without flow control because weak and short periodic vortices are induced by the synthetic jet, and they weakly interact with the shear layer and diffuse in the recirculation region. On the other hand, the separation length becomes 20% shorter when the flow is controlled at F+h = 0.2 than that in the case without flow control. Strong two-dimensional vortices generated from the synthetic jet interact with the shear layer, which increases the periodic component of the Reynolds stress within that layer. These vortices are deformed into three-dimensional structure...

Plasma Flow Control Simulation of an Airfoil of Wind Turbine at an Intermediate Reynolds Number

The flow over a National Renewable Energy Laboratory S825 airfoil was simulated for a chord Reynolds number of 7.5×105 and an angle of attack of 22.1 deg. These conditions approximately matched a blade element condition of 75% radius of 42-m-diameter wind turbine operating 2.5 rpm under a free-stream of 10 m/s. Computed flow of the uncontrolled case characterized massive separation from near the leading edge due to high angle of attack. With the active flow control by a dielectric barrier discharge plasma actuator, separation was reduced and the lift-to-drag ratio increased from 2.25 to 6.52. Impacts of the plasma actuator on the shear layer near the leading edge were discussed. Direct momentum addition provided by the case setup of plasma actuator considered in current study seemed to be a dominant factor to prevent the separation of shear layer near the leading edge rather than influence of small disturbances induced by the plasma actuator operated in a burst modulation. However, due to the high angle of attack and the thick airfoil, the control authority of the plasma actuator with the setup (i.e. the operating condition and number of plasma actuators installed on the wing surface) considered was insufficient to completely suppress the separation over the NREL S825 airfoil.Copyright

Plasma-Actuator Burst-Mode Frequency Effects on Leading-Edge Flow-Separation Control at Reynolds Number 2.6×105

This paper investigates the control of leading-edge flow separation over an airfoil using a dielectric-barrier discharge plasma actuator. A chord-based Reynolds number and an angle of attack are 2....

Effects of Burst Frequency and Momentum Coefficient of DBD Actuator on Control of Deep-stall Flow around NACA0015 at Rec=2.6x10^{5}

Current study investigates effects of a burst frequency (F) and a momentum coefficient (cμ) of a single dielectric barrier discharge(DBD) actuator on control of deep-stall flow over NACA0015 at a chord Reynolds number of 2.6×10 using large-eddy simulations. The DBD actuator is installed at the leading edge that is near the laminar separation point of the uncontrolled case. The DBD actuator-based flow control with the burst modulation effectively suppresses the leading edge separation and improves the aerodynamic performance. Better aerodynamic performance and standard deviation of lift are obtained by the cases of F=6 and 50 compared to the case of F=1 due to the suppression of separation. Although within the range of the momentum coefficient considered the increase in the momentum coefficient seems to enhance the aerodynamic performance, the manipulating frequency of burst actuation (F) is more efficient and realistic for the operation of DBD plasma actuator in practical engineering problems.

Significance of Three-dimensional Unsteady Flows inside the Cavity on Separated-flow Control around an NACA0015 using a Synthetic Jet

The simulation of a separation control using a synthetic jet around an NACA0015 airfoil at Reynolds number 63,000 is conducted by a large-eddy simulation (LES) with a compact difference scheme. The synthetic jet is installed at a leading edge and actuated with nondimensional frequencies F = 1.0 and 6.0, which is numerically modeled by a threedimensional deforming cavity: “Cavity model” and a two-dimensional boundary condition on the airfoil: “ Bc model”. The aerodynamic coefficients of the controlled flows are similarly recovered from those of the separated flow using both of the Cavity and Bc model. However, the time-averaged values and flow fields are significantly different in two models, and the use of Bc model on the three-dimensional analysis is not proper. In the case with F = 6, a turbulent transition near the leading edge occurs much earlier with the Cavity model than the Bc model. This result indicates that the spanwise disturbance from the cavity to the separated shear layer should be carefully considered when three-dimensional unsteady analysis is conducted by LES.

Computational Study of Frequency and Amplitude Effects on Separation Flow Control With the Synthetic Jet

In order to investigate the frequency and amplitude effects of the synthetic jet on the flow field, numerical simulation is carried out. Even though the final objective of this study is to understand mechanism of separation control for various objects, streamline and bluff bodies, the configuration of backward-facing step is chosen as the first step because of the simplicity. Three-dimensional Navier-Stokes equations are solved. Implicit large eddy simulation using high-order compact difference scheme is applied. The present analysis is addressed on the frequency characteristics of the synthetic jet for understanding frequency characteristics and flow-filed. Three cases are selected, No-control, F+ h = 0.2 and F+ h = 2.0, where non-dimensional frequency F+ h is normalized with the height of backward-facing step and the free stream velocity. The present computation shows that at F+ h = 2.0, separation length is 20 percent shorter than the No-control case. Strong two-dimensional vortices generated from the synthetic jet interact with the shear layer, which results in the increase of the Reynolds stress in the shear layer region. These vortices are deformed into three-dimensional structures, which make Reynolds stress stronger in the recirculation region. At F+ h = 2.0, size of the separation length is almost same as the No-control case because the mixing between the synthetic jet and the shear layer is not enhanced. Weak and short periodic vortices induced from the synthetic jet do not interacts with the shear layer very much and diffuse in the recirculation region.© 2009 ASME

Computational Study of the Synthetic Jet on Separated Flow Over a Backward-Facing Step

Frequency effects of the synthetic jet on the flow field over a backward facing step are investigated using numerical analysis. Three-dimensional Navier-Stokes equations are solved. Implicit large-eddy simulation using high-order compact difference scheme is conducted. The present analysis is addressed on the frequency characteristics of the synthetic jet for understanding frequency characteristics and flow filed. Three cases are analyzed; the case computing flow over backward facing step without control, the case computing flow with synthetic jet control at F + h =0.2, and the case computing flow with synthetic jet control at F + h =2.0, where nondimensional frequency F + h is normalized with the height of backward-facing step and the freestream velocity. The present computation shows that separation length in the case of the flow controlled at F + h =0.2 is 20 percent shorter than the case without control. Strong two-dimensional vortices generated from the synthetic jet interact with the shear layer, which results in the increase of the Reynolds stress in the shear layer region. These vortices are deformed into three-dimensional structures, which make Reynolds stress stronger in the recirculation region. Size of the separation length in the case of the flow controlled at F + h =2.0 is almost the same as the case without control because the mixing between the synthetic jet and the shear layer is not enhanced. Weak and short periodic vortices induced from the synthetic jet do not interacts with the shear layer very much and diffuse in the recirculation region.