Aiko Yakeno

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...

Modification of quasi-streamwise vortical structure in a drag-reduced turbulent channel flow with spanwise wall oscillation

The conditional averaging technique is applied to quasi-streamwise vortices in order to clarify the impact of their structural modification on the resultant drag reduction effect in a turbulent channel flow subjected to spanwise harmonic wall oscillation. The quantitative contributions of quadrant Reynolds shear stresses induced by the quasi-streamwise vortices are calculated on the basis of the Fukagata-Iwamoto-Kasagi identity [K. Fukagata, K. Iwamoto, and N. Kasagi, “Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows,” Phys. Fluids 14, L73 (2002)]. It is found that the Q2 event characterized by upwelling of low-speed fluid away from the wall governs the skin friction drag reduction at relatively small oscillation periods, whereas the Q4 event characterized by downwelling of high speed fluid toward the wall slightly contributes to drag reduction at small oscillation periods, and then to drag increase significantly with increasing the oscillation period. Detailed inves...

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.

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

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.

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.

Wall-Turbulence Structure with Pressure Gradient Around 2D Hump

Direct numerical simulation around a two-dimensional hump shape is conducted at the Reynolds number \(Re_{h} = 16,000\), based on the hump height. We investigate wall-turbulence structures around the hump in order to predict and control them to suppress separation. At this Reynolds number, specific striped wall-turbulence structure appears at the leading-edge near the wall surface. Its spanwise length-scale is close to that of the streak in a fully-developed turbulent channel flow. That is \(\lambda _{y} = 0.08\) scaled with the hump height, which corresponds to \(\lambda _{y}^{+} = 150\) in the local viscous unit. We identify two more different spanwise-correlated scales, \(\lambda _{y} = 0.40\) and 0.13 around the hump. Spanwise length-scale of \(\lambda _{y} = 0.40\) is around \(\lambda _{y}^{+} = 600\). On the other hand, the other scale \(\lambda _{y} = 0.13\) is not dependent on the local viscous scale.

Investigation of Maximum Velocity Induced by Body-Force Fields for Simpler Modeling of Plasma Actuators

The relation between the parameters of the body-force field generated by a plasma actuator and the maximum induced velocity in quiescent air is investigated by expressing the body-force distribution as the Gaussian function of the spatial coordinates. The aim of this study is to identify the dominant parameters for modeling of the body-force distribution. For that purpose, the parametric study using numerical simulations and dimensional analysis are conducted to derive the nondimensional key parameters. It is found that the nondimensional maximum induced velocity is determined by the Reynolds number calculated by three parameters: the total induced momentum per unit time, the height of the center of gravity of the body-force distribution, and the standard deviation from the center of gravity. In addition, the relation for the Gaussian body-force distribution turns out to be applicable to a conventional model, i.e, the Suzen model, even though the shapes of the distribution differ. Thus, we conclude that the three body-force parameters above are the key parameters for the maximum velocity induced by a plasma actuator.

LES of Separated-flow Controlled by DBD Plasma Actuator around NACA 0015 over Reynolds Number Range of 10^4-10^6

We have conducted high-fidelity large-eddy simulations on the separated flow around an airfoil with control by the DBD plasma actuator over a wide Reynolds number range. The Reynolds numbers based on a chord length were set to 63,000, 260,000 and 1,600,000. For the no control cases, the flow separates near the leading edge in laminar state at Reynolds numbers of 63,000 and 260,000, and massive turbulent separation occurs at Reynolds number of 1,600,000. The separation control with the burst actuation can achieve the flow reattachment through the promotion of the turbulent transition for the Reynolds numbers of 63,000 and 260,000, resulting in the improvement in both the lift and drag. On the other hand, the lift coefficient can be mainly increased over 45 % through the large-scale vortex paring induced by the burst plasma actuation for the Reynolds number of 1,600,000. The effects of the burst frequency on the separation control are evaluated based on the improvement of the aerodynamic performance. In this evaluation, the effective burst frequency non-dimensionalized by a chord length and freestream velocity (F = fc/u∞) comes to change with the Reynolds number. While relatively high burst frequencies (F ≈ 5) show the good improvement in the lift-drag ratio at Reynolds number of 63,000, the lower burst frequency (F ≈ 1) shows the highest improvement at Reynolds number of 1,600,000. On the other hand, when the non-dimensional burst frequency based on the momentum thickness and edge velocity of the separation shear-layer (Fθs) is considered, the high liftdrag ratio can be recognized at Fθs ≈ 10−2 for all the Reynolds number conditions. ∗Postdoctoral Fellow, Department of Space Flight Engineering and AIAA Member. †Engineer,Engineering Solution Division. ‡Postdoctoral Fellow, Department of Space Flight Engineering and AIAA Member. §Graduate Student, Department of Aeronautics and Astronautics and AIAA Member. ¶Postdoctoral Fellow, Department of Space Flight Engineering and AIAA Member. ∥Assistant Professor, Department of Space Flight Engineering and AIAA Member. ∗∗Professor, Department of Space Flight Engineering and AIAA Fellow.