1. makes a case for electrohydrodynamic effec ts as the reason behind the some of the sho ck wave alteration,101-103the general consensus is that most of the effects seen are a product of the heating from the plasma. Computationalstudiesalsoindicatethisresult.104
2. Concerning drag reduction, an initial study showed that the drag coefficient for a sphere in the presence of a WIG was significantly reduced for subsonic flow.111The same experiment for supersonic speeds showed that the drag coefficient was higher using a WIG than with typical airflow. Other plasma sources constructed for drag reduction have proven to be more effective since then. Ganiev et al. reported a reduction in the drag coefficient of about 50% from a subsonic speed to Mach 4 using a plasma jet placed at the tip of a somewhat blunt body.112Plasma jets appear to be inefficient for streamlined shapes.98At the time of Ref. 112, many other publications also described drag reduction with plasma jets and other forms of focused energy addition. A thorough list of these early publications can be found in Ref. 113. However, the large drag reduction by the plasma jet injection appears to be more directly related to the counterflow jet instead of the thermal effects of the plasma.114Aswasdiscussed,theuse of plasma jets was eventually deemed unrealistic for MHD flight applications in the 1960's. Although many of these current systems have been met with enthusiasm, scaling the power requirements to flight vehicles or missiles may pose insurmountable problems with current technology. Although new publications continue to emerge with different plasma sources and test geometries, very little of it is predominantly different from what was carried out at the beginning of this decade. In order to overcome the skepticism resulting from problems including but not limited to power consumption, scaling, and hypersonic interaction at true flight conditions, the science of plasma control for aerodynamics must transition into realistic systems. If a forward facing plasma jet is viable, then it certainly should be able to be demonstrated on the front of a missile. Perhaps an inlet system can be constructed and ground tested with surface actuators that create or manipulate shock waves to minimize inlet spillage. It is understandable that some of the models of full-scale hypersonic systems have not been constructed due to the cost, but plasma control definitelyneeds tobebetter proven experimentallyas partofmore flight-readysystems.
3. Considering the physics involved, a dielectric barrier discharge is similar to a glow discharge. Where a glow discharge has an air gap, a DBD contains a gap of dielectric material between the anode and cathode. Typical materials like glass, polymers, and ceramics have a much higher resistivity than air, allowing for the electrodes to be placed closer to one another. Closer placement increases the electric field around the electrodes and ultimately raises the Coulomb force in Eq. (10) without the occurrence of electrical breakdown. The dielectric barrier is self-limiting as it prevents charge accumulation over the barrier material to prevent arcing. DBDs have been recognized since the mid-19th century, with the their first application being the production of ozone.115Since that time, research has continued to grow and now applications includ e surface trea tment, reduction of pollutants, lasers, and plasma display panels. Systems using glow discharges often use low pressure, but the discharges were stabilized across the barrier atatmosphericpressurebeginning inthe1980's.116
4. Dielectric barrier discharges constructed for aerodynamic flow control applications appeared in the literature near the end of the 1990's.117,118In the decade since those reports, research into aerodynamic flow control with DBDs has rapidly increased both experimentally and computationally. A number of reviews have been written,47,119-121which probably indicates a variety of opinionson their applicability. At low speeds, DBD actuators have a significant effect on boundary layer flow. Figure 23 shows a notable image of flow reattachment made possible by an array of DBD actuators produced by Roth et al.122This actuator system works at atmospheric pressure, and has been named the One Atmosphere Uniform Glow Discharge Plasma (OAUGDPTM). The ionization is created with a high voltage Figure23. Smoke visualization shows flow reattachment on a NACA 0015 airfoil at a 12° angle of attack by an array of EFC actuators. The freestream flow speed is 2.6 m/s(fromRef. 122).
5. conducted using higher free stream speeds. Opaits et al. investigated DBD control of a NACA 0015 airfoil with free stream speeds of 20 to 75 m/s at atmospheric pressure.125The stall angle was raised with the DBD actuators with u∞= 75 m/s, and a change in pressure distribution was also recorded. Similarly, Roupassov et al. measured changes in the pressure distribution for a NACA 0015 airfoil at speeds up to 110 m/s.126In this case, the electrodes were placed parallel to the flow, and it appears that the pressure distribution incurs a greater change with the DBD actuator when the airfoil is c lose it its sta ll angle. One attempt was made recently to mount a DBD actuator on the leading edge of the wing of a Jantar Standard SZD-48-3 sailplane.127It appears that the DBD system was able to affect the separation and lift characteristics of thewing surface,but thedata werenotparticularly reliable andrefined tests areneeded.