Plasma Activated Boundary Layer Control for Hypersonic Transition Delay: A Theoretical and Numerical Investigation

Abstract

Laminar-to-turbulent transition in hypersonic boundary layers results in a significant increase in surface heat flux and skin friction, posing a critical challenge for the design of hypersonic vehicles. This paper theoretically investigates the potential of Plasma-Activated Kinetics (PAK) as a novel active flow control technique for hypersonic transition delay. We propose that energy deposition from a non-thermal, nanosecond-pulsed dielectric barrier discharge (ns-DBD) plasma actuator can strategically alter the boundary layer\'s stability landscape. Through a multi-physics framework, we analyse three primary mechanisms: (1) the creation of a localised, steady thermal bump that modifies the base floe profile, (2) the generation of stabilising species gradients (vibrational non-equilibrium) that regulate the Mack mode stability, and (3) the direct damping of the second-mode instability waves through periodic, anti-phase thermal perturbations. Linear stability theory (LST) and parabolized stability equation (PSE) analyses are employed to quantify the increase in transition Reynolds number. Theoretical results imply that PAK, by selectively targeting the most unstable modes (in particular the second mode), can attain an enormous downstream shift in transition location (>30% increase in transition Reynolds number) with minimal energy input compared to standard thermal safety systems. This work establishes a foundational theoretical basis for PAK as a promising technology for enhancing hypersonic vehicle performance and thermal control.

Introduction

Boundary-layer transition control is a major challenge in hypersonic flight because turbulent flow dramatically increases heat transfer, threatening thermal protection systems and vehicle performance. Passive control methods have limited effectiveness under varying flight conditions, motivating the use of adaptive, active flow control. Recent advances in plasma physics—specifically nanosecond-pulsed dielectric barrier discharge (ns-DBD) actuators—offer a promising solution by depositing localized thermal energy into the boundary layer without mechanical components.

This paper presents a theoretical investigation of Plasma-Activated Kinetics (PAK) as a mechanism to delay laminar-to-turbulent transition in hypersonic boundary layers. The study examines three complementary stabilization mechanisms: (1) base-flow modification through localized steady heating, (2) stabilization of second-mode (Mack mode) instabilities via vibrational nonequilibrium and plasma-generated species gradients, and (3) active wave cancellation using high-frequency, phase-controlled thermal forcing.

Localized plasma-induced heating modifies near-wall density and viscosity, producing a less inflectional velocity profile that stabilizes first-mode instabilities and increases the critical Reynolds number for transition. At higher Mach numbers, plasma-generated vibrational nonequilibrium alters the acoustic properties of the boundary layer, detuning the resonance mechanism responsible for second-mode amplification and significantly reducing its growth rate. In its most advanced form, PAK enables active wave cancellation, where phase-shifted plasma pulses destructively interfere with instability waves, analogous to active noise control.

A hierarchical theoretical methodology combining compressible CFD, multi-temperature modeling, linear and parabolized stability analyses, and N-factor transition prediction is used to evaluate these mechanisms. Results indicate that steady plasma heating can delay first-mode transition by approximately 15%, while vibrational nonequilibrium effects can reduce second-mode growth rates by 30–50%, substantially extending the laminar flow region. Active wave cancellation is shown to be theoretically viable but technologically challenging due to stringent bandwidth, phase accuracy, and spatial coupling requirements.

Overall, the study concludes that plasma-based transition control is most effective when applied as a hybrid strategy: steady or low-frequency plasma actuation conditions the base flow and suppresses first-mode instabilities, creating favorable conditions for targeted, high-frequency control of second-mode instabilities. This integrated PAK approach offers a promising theoretical pathway toward adaptive hypersonic boundary-layer control.

Conclusion

This theoretical investigation demonstrates that Plasma-Activated Kinetics (PAK) holds significant promise for delaying transition in hypersonic flight. By strategically depositing thermal energy using ns-DBD plasma actuators, we can stabilize the boundary layer through several complementary paths: modifying the base flow, introducing stabilizing species and vibrational gradients, and employing direct anti-phase wave cancellation. Our linear stability analyses suggest that the transition Reynolds number can be increased by more than 30%, which could represent a major breakthrough in managing the intense heat loads faced by hypersonic vehicles. The most practical approach for immediate use involves steady or low-frequency pulsed heating to suppress first-mode instabilities, alongside vibrational non-equilibrium to dampen second-mode growth. While the logic behind closed-loop active wave cancellation is sound, its successful implementation remains a longer-term goal that depends on future advances in high-bandwidth actuator design and robust sensing. Moving forward, research must focus on high-fidelity direct numerical simulations (DNS) to validate these theoretical models, as well as experimental work to precisely characterize the thermal and chemical perturbations induced by ns-DBDs in high-enthalpy environments.

References

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Copyright

Copyright © 2026 Annmariya Shimmy, Krishna Priya K. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.