We conduct large-eddy simulations of separated airfoil flows with control by a dielectric-barrier-discharge plasma actuator over a wide range of Reynolds numbers. The Reynolds numbers based on the chord length (Re) are set at Re = 5.0 × 10³, 1.0 × 10⁴, 6.3 × 10⁴, 2.6 × 10⁵, and 1.6 × 10⁶. These Reynolds numbers cover most of the conditions used in the previous studies on separation control by a plasma actuator. The burst frequency nondimensionalized by the chord length and freestream velocity (F⁺) is used as the computational parameter, and the effective burst actuation and control mechanisms at each Reynolds number condition are investigated. With regard to cases without the control, the flows separate near the leading edge in the laminar state at the Reynolds number range of 10³–10⁵, and a substantial turbulent separation occurs at the Reynolds number of 1.6 × 10⁶. Separation control with a high burst frequency [F⁺ ≃ O(10)] can cause early flow reattachment through the promotion of turbulent transition of a separation shear-layer for Re = 6.3 × 10⁴ and 2.6 × 10⁵. Flow reattachment is mainly caused by momentum entrainment into the boundary layer by fine-scale turbulent vortices. On the other hand, the large-scale spanwise vortices play an important role at F⁺ = 1 for Re = 1.0 × 10⁴ and 1.6 × 10⁶. In these cases, the dynamics of the spanwise vortices show similar behavior and the pairing of these vortices significantly contributes to the separation control by increasing the momentum entrainment. The optimum value of F⁺ changes with a Reynolds number. In contrast, when a nondimensional burst frequency based on the characteristics of the separation shear-layer ($F_{{\theta }_s}$ ) is considered, a high lift-to-drag ratio is found at $F_{{\theta }_s}\simeq O\left(10^{-2}\right)$ for all Reynolds numbers. This demonstrates that one of the effective burst frequencies is closely related to the scale of the separation shear-layer, especially for the spanwise vortex shed from the separation shear-layer.

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.

The flow control over the blades of a small horizontal-axis wind turbine (HAWT) model using a dielectric barrier discharge plasma actuator (DBD-PA) was studied based on large-eddy simulations. The numerical simulations were performed with a high-resolution computational method, and the effects of the DBD-PA on the flow fields around the blades were modeled as a spatial body force distribution. The DBD-PA was installed at the leading edge of the blades, and its impacts on the flow fields and axial torque generation were discussed. The increase in the ratios of the computed, cycle-averaged axial torque reasonably agreed with that of the available experimental data. In addition, the computed results presented a maximum of 19% increase in the cycle-averaged axial torque generation by modulating the operating parameters of the DBD-PA because of the suppression of the leading edge separation when the blade’s effective angles of attack were relatively high. Thus, the suppression of the leading edge separation by flow control can lead to a delay in the breakdown of the tip vortex as a secondary effect.

With the developments of computer technologies, three-dimensional direct numerical simulations (DNS) of turbulent combustion have been realized with a detailed or reduced kinetic mechanism. The 3D DNS gives detailed information about turbulent flames, while there are few experimental techniques which have high accuracy enough to compare with DNS. In this paper, after showing summary of recent DNS of turbulent premixed flames, newly-developed laser diagnostics are presented. Simultaneous CH-OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) are used to investigate the local flame structure of the turbulent premixed flames. From CH-OH PLIF and PIV measurements, flame fronts are identified, and the curvature of the flame front and the tangential strain rate at the flame front are evaluated. The experimental results are compared with 3D DNS of hydrogen-air and methane-air turbulent premixed flames. The flame displacement speeds in turbulent premixed flames have been measured directly by the CH double-pulsed PLIF. Since the time interval of the successive CH PLIF can be selected arbitrarily, both of the large scale dynamics and local displacement of the flame front can be obtained. As an application of laser diagnostics for development of high-efficient and low-emission combustors, reconstruction of 3D flame structure is shown by using multiple-plane OH PLIF.

To achieve a usable range measurement feature using a Vehicle-Mounted monocular camera for ADAS (Advantage Drive Assist System), a pitch angle estimation with high accuracy is required. We propose a method for estimating pitch angle with a non-occurrence of cumulative error, and the initial pitch angle can be estimated simultaneously with only a Vehicle-Mounted monocular camera. We use the Harris-corner algorithm and the pyramid Lucas-Kanade method to detect the optical flow of feature points between adjacent frames from the monocular camera. With the result of the optical flow detection using Structure from Motion method to estimate the camera ego-motion parameters, including the rotation matrix and the translation vector, and to optimize the estimated ego-motion parameters using the Gauss-Newton method. In addition, we propose a method of estimating pitch angle relative to the road surface from the translation vector. The pitch angle and the pitch angle rate decomposed from the rotation matrix of the adjacent frames are composed using an average transfer method, to achieve the high accuracy pitch estimation. Further, the effectiveness of the proposed method is confirmed by experiment.

Large-eddy simulations of the flows over an NACA0015 airfoil were conducted to investigate a flow control authority of a dielectric barrier discharge plasma actuator at pre-stall angles of attack. The Reynolds number was set to 63,000, and angles of attack were set to 4, 6, 8, and 10 degrees. The plasma actuator was installed at 5% chord length from the airfoil’s leading edge. Good flow control authority was confirmed in terms of lift-to-drag ratio increase and drag reduction. These improvements mainly result from the reduction of the pressure drug, which is due to the change in pressure distribution accompanying the movement and shrink of the laminar separation bubble on the airfoil surface. Additionally, although flow control using a burst drive with a nondimensional burst frequency of six improves the lift-to-drag ratio at all angles of attack, the phenomena leading to the improvement differ between near-stall angles (10 and 12 degrees) and the other lower angles. At near-stall angles, the turbulent transition is rapidly promoted by PA, and the flow is reattached. Whereas, at the lower angles, the transport of two-dimensional vortex structures, which maintain their structures up to downstream and suppress the turbulent transition, makes the flow reattachment.