Kengo Asada

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...

Computational Analysis of Unsteady Flow-fleld Induced by Plasma Actuator in Burst Mode

In order to clarify the important phenomenon for the burst mode effect of the plasma actuator on the control of leading edge separation over an airfoil, The flow-field controlled by plasma actuator over the NACA0015 airfoil is analyzed using Large Eddy Simulation. While the flow has the large bubble near the airfoil leading edge in normal mode, the bubble is shorter than normal mode in burst mode. In addition, observing the process of controlling the previously separated flow, it is clarified that the free stream directional strong vortices are induced by the span directional vortex generated by the plasma actuator in burst mode, and it transfers momentum from the free stream to the boundary layer.

Airfoil Flow Experiment on the Duty Cycle of DBD Plasma Actuator

The parameters of DBD (Dielectric Barrier Discharge) plasma actuator with burst wave (duty cycle) are investigated by the low speed wind tunnel experiment for airfoil. In this paper, influence of burst frequency f, input voltage sine wave frequency fbase and burst ratio BR on the stall control are discussed. The experiments are conducted with the conditions Rec=44,000, 63,000. The actuator is applied to NACA0015 airfoil and then flow-fields around the airfoil are visualized by the smoke wire method and pressure around its surface is measured by the multipoint steady pressure measurement. The results show that within the present experimental conditions, the higher f and fbase are more effective in the separation control and the smaller BR has the stronger separation control capability in spite of less input energy. In present conditions, the optimum dimensionless burst wave frequency F is 9.1. This result shows the same tendency as the result of Sidorenko, et al.

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.

An Effective Three-Dimensional Layout of Actuation Body Force for Separation Control

We conducted large eddy simulations of the control of separated flow over an airfoil using body forces and discuss the role of a three-dimensional vortex structure in separation control. Two types of cases are examined: (1) the body force is distributed in a spanwise uniform layout and (2) the body force is distributed in a spanwise intermittent layout, with three-dimensional vortices being expected to be generated in the latter cases. The flow fields in the latter cases have a shorter separation bubble than those in the former cases although the total momentum of the body force in the latter cases is the same as or half of the former cases. In the flow fields of the latter type, the three-dimensional vortices, which are not observed in the former cases, are generated by the body force downstream of the body force distributed. Thus, three-dimensional vortices are considered to be effective in controlling the separated flow.

Burst Frequency Effect of DBD Plasma Actuator on the Control of Separated Flow Over an Airfoil

The flow fields around NACA0015 airfoil, separation over which is controlled by a DBD plasma actuator are simulated by implicit large eddy simulation with compact difference scheme, and the burst frequency effect of DBD plasma actuator on the control of separated flow over the airfoil is discussed. The Reynolds number based on chord length is set to 63,000 and the angle of attack is set to 14 [deg]. The DBD plasma actuator is installed at the 0 % and 5 % chord length from the leading edge, and actuated in burst mode. For the burst mode, the nondimensional burst frequency is set to 1 and 6. Through the present analysis, the two mechanisms of separation control are discussed. The first mechanism enhance the vortex shedding from the separation shear layer and avoid the massive separation from the leading edge on burst mode with nondimensional burst frequency of 1. The second mechanism improves the airfoil performance by suppressing the separation region on the burst mode with nondimensional burst frequency of 6. In addition, it is clarified that the first mechanism is more sensitive to the location of the DBD plasma actuator than the second mechanism and it is associated with large fluctuation of lift.

Comparative study of co-flow and counter blowing DBD plasma actuators for separated flow over an airfoil

A comparative study of co-flow and counter-blowing dielectric barrier discharge plasma actuator for separation control is conducted. These actuators are applied with normal mode and burst mode, where normal mode represents the actuation with continuous alternative current (AC) input and burst mode represents the actuation with the AC input switched on and off periodically. They are used for controlling the separated flow around NACA0015 airfoil at low Reynolds number Rec = 6.3 × 10. Pressure measurement and particle image velocimetry are conducted. In this study, four cases are conducted changing blowing direction, co-flow or counter-blowing, and actuation mode, normal or burst. Comparison among four cases shows that the dominant factor for suppressing separation with burst actuation is promoting transition, regardless of blowing direction. It also shows that the dominant factor of co-flow normal actuation is direct momentum addition. Counter-blowing normal actuation cannot suppress separation with any input voltage. Focusing on the minimum input voltage for suppressing separation, effectiveness for each attached case is compared and it is revealed that burst actuation more or less includes the effect of direct momentum addition.

Effective Mechanisms for Turbulent-separation Control by DBD Plasma Actuator around NACA0015 at Reynolds Number 1,600,000

We have conducted large-eddy simulations of turbulent separation flow with control by the DBD plasma actuator over the NACA0015 airfoil. The Reynolds number based on the chord length is 1,600,000 and the angle of attack is 20.11 degs. 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 separation control are investigated. The most effective location of the actuator to suppress the separation is the vicinity of the turbulent-separation point (second separation). In the burst mode cases, the most effective non-dimensional burst frequency to improve the lift coefficient is unity. The effective mechanism for the turbulent-separation control by the burst mode is to induce the pairing of the large-scale vortices near the airfoil surface. This large-scale vortex results in not only the momentum induction from the freestream to the boundary layer but also the lift improvement by its convection. In addition, several control effects can be achieved depending on the settings of the DBD plasma actuator. The slight drag improvement can be obtained with shortening the laminar separation bubble through the high frequency actuation from the leading edge.

LES on Turbulent Separated Flow around NACA0015 at Reynolds Number 1,600,000 toward Active Flow Control

Large-eddy simulation of a separated flow over NACA0015 at Reynolds number 1,600,000 at angle of attack 20.11 deg. is conducted to clarify the features of turbulent separated flow at high Reynolds number. The total number of grid point is approximately one billion, and a high order scheme is used in this computation. The LES result agrees with the experimental result in terms of the locations of the laminar-separation, turbulent reattachment, and the turbulent separation, and of the surface pressure distribution. The laminar-separation bubble is formed near the leading edge with turbulent transition. Then turbulent boundary layer develops over the airfoil surface and the flow is separated as a turbulent flow. The time-frequency analysis indicates that there are two characteristic frequencies: 1)Strouhal number St = 100 at the turbulent reattachment point, 2)St = 4 at the turbulent separation point. These frequencies are expected as effective excitation frequencies to control the separated flow considered.

Computational Study of Aerodynamic Characteristics of an Airfoil With DBD Plasma Actuator

The relation between aerodynamic characteristics and the effectiveness of separation control with the DBD plasma actuator over the airfoil are discussed. The flow-fields around the NACA0015 airfoil are simulated with implicit large-eddy simulation using compact difference scheme. The normal mode generates moderately separated region over the airfoil and gains lift by negative pressure at the vortex center. The burst mode with nondimensional burst frequency of 1 enhances the vortex shedding from the separation shear layer and avoid the massive separation from the leading edge. However, the lift coefficient oscillate very much, in this case. The burst mode with nondimensional burst frequency of 6 improves the airfoil performance by suppressing the separation region. These facts indicate that the unsteady aerodynamic characteristics must be discussed when the effectiveness of separation control is evaluated.Copyright