Plasma actuators (PA) can be utilized as fluid control devices without moving parts, but further improvement in drive efficiency is necessary. Herein, string-type PAs with up to 12 insulated conductive wires were evaluated to replace sheet-type PAs having a single encapsulated electrode. The thrust–power ratio of string-type PAs with eight or more wires is nine times that of a single-wire PA. This is due to the substantial increase in the width of the encapsulated electrode and the discrete arrangement of conductors in the streamwise direction. To determine the factors influencing the performance of PAs with discrete encapsulated electrodes, sheet-type PAs with and without discretized encapsulated electrodes and with the same configuration as string-type PAs were characterized. The measurement results revealed that no significant difference exists in the plasma extension length (L_DBD) between sheet-type PAs without and with discretization under the same applied voltage, but 25% and 45% decreases in the thrust and power consumption, respectively, were observed compared to those of string-type PAs. The discretization of the encapsulated electrodes in the sheet-type plasma actuator increased the thrust–power ratio by 30%. Efficient non-mechanical fluid control using dielectric barrier discharge is therefore possible with string-type PAs with discrete electrodes.

An experimental investigation of active flow control on a three-dimensional (3D) curved surface bluff body was conducted by using a string-type plasma actuator. The 3D bluff body model tested in this study was composed of a quarter sphere and a half cylinder, and the Reynolds number based on the diameter of half cylinder was set at 1.3 × 10⁴. The modulation drive was adopted for flow control, and the control effects of variations in dimensionless burst frequency (f_m⁺) normalized by the width of the model and freestream velocity were studied. Velocity distributions analyzed by particle image velocimetry showed that the recirculation region behind the model shrank due to the flow control. The static pressure distributions on the back surface of the model tended to decrease under any f_m⁺ set in this study, especially in the ranges of 0.40 ≤ f_m⁺ ≤ 0.64. The drag coefficient reached its maximum value under the similar ranges of f_m⁺. Although the aerodynamic wake sharpening was observed due to the flow control, the entrainment of separated flow into the back surface of the model was enhanced. This scenario of wake manipulation was considered to be responsible for increasing drag acting on the model.

Numerical simulations of the interaction between the vortex ring and the vortex tube was performed, with the aim of control of tumbulent mixing layer. The vortex tube is the simple model of the rolling-up vortices in the mixing layer. The simulation was made by the three-dimensional vortex blob method. The ratio of circulation of vortex tube to vortex ring Γ_R/Γ_F was varied in the range of 0.5-2.0. If Γ_R/Γ_F=0.5 the vortex was stretched and involved into the vortex tube. If Γ_R/Γ_F=1.0 the cut and reconnection occurred between a part of the vortex ring and a part of the vortex tube. But if Γ_R/Γ_F exceeds about 1.5 the vortex ring was found to pass through the vortex tube. The initial position of the vortex ring has significant effects on the interaction. A portion in the vortex ring whose circulation is the same sign as the vortex tube, combined with the vortex tube to generate the large vortex. On the other hand, another portion in the vortex ring of the opposite sign generate a new vortex pair.

The effects of cross flow and fluctuating cross flow on the aerodynamic performance of multirotor drone propellers were investigated by using wind tunnel testing and numerical simulations. Three hovering propellers were examined and were designed based on Adkins and Liebeck theory with modifications. Thrust coefficient C_T and power coefficient C_P of each propeller were experimentally measured and Figure of Merit FM was evaluated. From the cross flow experiments and numerical simulations, C_T, C_P and FM calculated were in reasonable agreement with the experimental results at high cross flow velocity. The results implicated that a stall might occur at near the root of the propeller as well as demonstrated that the starting point of the propeller design played a role in aerodynamic performance under the cross flow conditions. Moreover, from the fluctuating cross flow experiments, C_T, C_P and FM fluctuated in time and the propellers are more susceptible to cross flow and fluctuating cross flow at lower rotational speed in terms of the amount of change of aerodynamic forces.

Coherent structures in a turbulent plane mixing layer are comparatively studied by wavelet transform and proper orthogonal decomposition (POD) analysis, using the database of the mixing layer obtained by Bonnet et al. (1994). The POD analysis yields more coherent vorticity distributions of the large-scale structures in the mixing layer, suggesting that the first mode represents the most dominant structure. The wavelet transform indicates the low-frequency modulation of vorticity of the large-scale structure. The POD analysis fails to show the large-scale structure in the central part of the mixing layer due to the effects of the random components, whereas the wavelet transform can extract the coherent structure if the vortex-passage period is chosen as the scale of the wavelet.