# Special Issue \"Active Flow Control Technologies for Energy and Propulsive Systems\"
Boundary layer modification is reached through a propagating train of vortices, which are created by an oscillating diaphragm within a cavity inside the body. In reference [14], synthetic jet actuators and plasma DBD actuators have been compared in terms of active separation control performance on a highly loaded subsonic compressor stator cascade. The plasma actuator is slightly more efficient for the reduction of flow separation in the region just downstream of the blade actuators. However, at the same mechanical power delivered by the actuator to the fluid, the synthetic jet actuator SJAs are more advantageous than the continuous jet actuator CJAs and slightly outperform plasma actuator application from the pressure loss reduction and pressure rise viewpoints. The fluidic jets have low power requirements, whereas the power consumption would be prohibitive for the PA configuration that shows low fluid mechanic efficiency. Another area of interest, other than flow around streamlined airfoils or turbine blade, is flow control around bluff bodies such as large pylons, e.g., onshore wind turbines, tidal turbine mounts or the risers used for offshore platforms, deep-water wind turbines or bridge sections. Synthetic jet actuators could modify the wake produced by a structure and hence reduce the magnitude and/or frequency that a structure oscillates at, with a beneficial effect on reducing the risk of failure. In reference [15], a numerical study was performed to investigate the effect of varying synthetic jet actuation frequency on drag and lift coefficients, pressure coefficient and the turbulence intensity in the wake of a circular cylinder in cross-flow. A significant modification of the drag coefficient for a circular cylinder in cross-flow at Reynolds equal to 3900 is achieved by varying the actuation frequency of the synthetic jet. The passive flow control has also been of great interest in many studies. Vortex generator, distributed roughness, streamlining, and uniform blowing and suction are among various devices that are employing for the passive flow control technique. Vortex generators (VGs) are increasingly used, especially in the field of wind turbines, as flow control devices to improve rotor blade aerodynamic performance. Nevertheless, VGs may produce excess residual drag in some applications. The so-called sub-boundary layer VGs can lead to a significant flow-separation control with lower drag than the conventional VGs. In reference [16], an investigation has been performed about how well the simulations can reproduce the physics of the flow of the primary vortex generated by rectangular sub-boundary layer VGs mounted on a flat plate with a negligible pressure gradient with an angle of attack of the vane to the oncoming flow of 18\({}^{\circ}\). The computational results show good agreement with the experimental data provided by the Advanced Aerodynamic Tools of Large Rotors (AVATAR) European project for the development and validation of aerodynamic models. Finally, the results indicate that the highest VG seems to be more suitable for separation control applications. Finally, control of the flow could be also useful for heat transfer enhancement in heat exchangers. Thermal augmentation techniques can be divided into active and passive. The passive techniques with the tube inserts are most frequently used because they do not require the direct application of external power. Twisted tape is the most commonly used solution. The thermal augmentation of twisted tape is given by its ability to induce swirl flow and increase the turbulence intensity close to the tube wall, which can promote mixed flow from the near-wall and central regions. In reference [17], the effects of ball turbulators (BTs) on the heat transfer and fluid friction characteristics in a circular tube are investigated through numerical simulation. The performance evaluation criterion (PEC) data underline that the use of a smaller ball diameter ratio and a smaller spacer length are preferred.