As aerospace vehicles approach hypersonic speeds (Mach 5 and above), the aerodynamic and thermal challenges they encounter become significantly more complex than those in subsonic or even supersonic regimes. Among the most critical issues are intense shock waves, boundary layer separation, and severe thermal loads. Traditional mechanical flow control methods—such as flaps, jets, or synthetic jets—struggle to keep pace with the fast, high-enthalpy flows encountered at hypersonic velocities. In recent years, the use of plasmas for flow control has emerged as a promising and actively researched alternative.
Plasma technologies, when properly implemented, enable targeted manipulation of specific flow properties such as local pressure and temperature gradients, boundary layer characteristics, and even shock position and strength. These capabilities suggest broad utility for plasma-based flow control not only in improving vehicle stability and maneuverability, but also in reducing heat loads—one of the most formidable design constraints in hypersonic systems.
Two Regimes in Hypersonic Plasma Flow Control
At Mach numbers below about Mach 12, the gas temperature in shock and boundary layers is below 3000 K or so. Although such temperature is very high, the “natural” thermal ionization of air is still quite low. To utilize plasma flow control in this regime, the plasma must be created artificially, e.g. by applying a strong electric field as done in laboratory electric discharges. But since the temperature in shock and boundary layers increases with Mach number approximately quadratically and the ionization fraction (i.e. the percentage of molecules releasing their electrons) exponentially increases with temperature, the thermal ionization becomes high above Mach 12, and there is no need to create the plasma artificially.
Applications in Shock-Wave/Boundary-Layer Interactions (SWBLI)
One of the most damaging phenomena in hypersonic flows is the shock-wave/boundary-layer interaction (SWBLI), which often leads to boundary layer separation, unsteady pressure loads, and enhanced convective heat transfer. Several experimental studies and numerical simulations have demonstrated that strategically placed nonequilibrium plasma actuators can mitigate these effects.
For example, using a localized magnetic field near the surface, placing a pair of electrodes flush with the surface, and applying a proper voltage to those electrodes, an electric discharge running along the surface while being accelerated by the Lorentz force can be created. Such magnetically accelerated surface discharges have been shown to affect flow separation and/or to shift turbulent pulsations to higher frequencies thus reducing dynamic loads on the structure.
This dynamic control of boundary layers could bring great benefits in both supersonic and hypersonic flight.
Magnetic Heat Shield and Aerodynamic Control
Application of a static magnetic field at the vehicle nose or leading edge can increase the shock stand-off distance and reduce heat flux to the surface, thus reducing the stringent requirements for thermal protection system (TPS). This magnetohydrodynamic (MHD) concept, known as magnetic heat shielding, could help with aerobraking during reentry and could also extend the lifespan of reusable hypersonic vehicles.
Recent research also showed that creating independently controlled MHD “patches” around the circumference of a reentry vehicle would enable pitch, yaw, and roll control and could also be used to enhance lift, which would go well beyond what is possible with conventional techniques relying on physical control surfaces.
Virtual Shapes
Localized heating using steady or pulsed plasma energy deposition can create artificial obstacles to the flow, and these virtual shapes or virtual surfaces can dynamically change aerodynamics, e.g. by increasing the lift-to-drag ratio.
Airbreathing hypersonic propulsion could also benefit from plasma-created virtual shapes. For example, creating a plasma-heated region just upstream of the scramjet inlet, called a “Virtual Cowl”, would control the inlet by non-mechanical means, which would be particularly important in off-design conditions.