Absolute Instabilities in Heated Jets

When entering a planet’s atmosphere, spacecraft induce a strong compression shock and must be protected from the resulting extreme heat flux by a thermal protection system made of either reusable or ablative materials. To characterise these materials, the harsh flow conditions of atmospheric entry are reproduced in plasma wind tunnels, where a jet of gas heated up to ionisation is directed at material samples for prolonged testing. Unfortunately, heated jets exhibit complex dynamic behaviours, resulting in oscillations that increase the uncertainties in the experiments.At sufficient Reynolds numbers, the dynamic behaviour of heated jets shifts from an amplifier to a self-sustained oscillator type via a Hopf bifurcation, if the centreline-to-ambient density ratio falls below a given threshold. This change is known in the literature to be related to the onset of absolute instabilities in the flow. However, this type of instability is usually studied for a simplified description of the gas, which is not suitable for the case of a plasma wind tunnel.This doctoral work investigates the nature of the instabilities responsible for the oscillations observed in a plasma jet, similar to the one in the VKI Plasmatron facility. The analysis is carried out by comparing results from different numerical methods, including linear stability analyses (both local and global) and direct numerical simulations. The thesis first describes the effect of high-temperature gas models on the stability of synthetic jets found in the literature, before analysing the case of Plasmatron.The analysis of synthetic jets with real-gas effects shows that the onset of the first dissociation reactions in the flow has a strong influence on the prevailing type of instability. Furthermore, if a sufficiently long region of absolute instability is present in the jet, the flow bifurcates to a periodic limit cycle, and steady state solutions become inadequate to describe the flow and its dynamic behaviour. In this case, a stability analysis of the time-averaged state can accurately reproduce the results of direct numerical simulations. In the case of Plasmatron, a large region of absolute instability is revealed in the plasma jet, suggesting that the observed oscillations are caused (in part) by a global non-linear mode and that the flow has entered a limit cycle. Trends of the absolute instability frequency with respect to the driving parameters of Plasmatron are in agreement with experimental observations.The present work confirms that global stability features of heated jet flows are very sensitive to subtle changes of the undisturbed or time-averaged state, which results from technological constraints in the case of Plasmatron. Furthermore, this thesis has shown the relevance of including high-temperature gas effects in the stability analysis of high-enthalpy jets.Doctorat en Sciences de l'ingénieur et technologie