Viviana Lago

Optical spectroscopy investigation of N2–CH4 plasma jets simulating Titan atmospheric entry conditions

Planetary probes penetrating at supersonic speed into high atmosphere require the development of composite materials for thermal protection of the surface exposed to a high enthalpy flux. Rarefied arc plasmas can be used to simulate the atmospheric re-entry plasmas. The aim of this paper is to describe the latest results of optical emission measurements of the CH (A–X) system, the CN violet (B–X) bands and the NH (A–X) electronic transitions in a N2–CH4 plasma source (Titans atmosphere) and in a gas mixture of Ar–CH4. In order to deduce the plasma parameters, such as rotational and vibrational temperatures, of these molecular species in the plasma environment, numerical simulation codes have been implemented. In this context, rotational temperatures near 7000u2009K for CN and 3500u2009K to 2800u2009K for the hydrides NH and CH, respectively, are observed. The vibrational temperature of the CH molecule is around 3800u2009K while those of the CN and NH molecules are 9500u2009K and 7900u2009K, respectively.

Electron and vibrational temperatures in hypersonic CO2–N2 plasma jets

At Laboratoire dA?rothermique low pressure stationary arc jets are used to simulate the plasma flow around a space probe during a hypersonic entry into a planetary atmosphere. This paper focuses on the experimental investigation of the species present in the flow produced at the exit of the supersonic nozzle in CO2?N2 plasma mixtures similar to the Mars atmosphere. The thermal nonequilibrium of the internal modes of some radicals is examined together with the electron density and temperature.

Fluidic thrust vectoring of an axisymmetrical nozzle: an analytical model

An engineering-type analysis is proposed and used to investigate the performances of thrust vectoring by fluidic injection in the divergent of a supersonic axisymmetrical convergent-divergent nozzle. This method includes several approaches which consist mainly of a fluidic obstacle height evaluation and the prediction of the separation line that results upstream of the fluidic obstacle. The construction of the separation line is also based on some separation correlations proposed in the literature. The nozzle thrust deviation is then calculated by taking into account the injectant fluid momentum rate contribution and the integration of the pressure acting on the nozzle inner wall. The sensitivity of the model versus some separation criteria is discussed. The results of the analytical model are compared with the experiments conducted recently by the authors. The comparison shows a very good agreement for some of the separation criteria over the whole range of injected to main flow-rate ratios.

Sliding discharge optical emission characteristics

In this work, several optical studies in an atmospheric pressure sliding plasma sheet have been performed. This discharge is generated using two electrodes flush mounted on an insulating flat plate (upper electrodes), and a third electrode flush placed on the opposite side of the plate facing the upper inter electrode gap (lower electrode). A DC negative voltage is applied to one of the two upper electrodes and to the lower electrode, while the other upper electrode is biased with an AC voltage. In this configuration a sliding discharge is produced on the flat plate within the upper electrodes gap. The sliding discharge optical emission of the spectral bands corresponding to the 0-0 transition of the second positive system of N2 (lambda = 337.1 nm) and the first negative system of N2 + (lambda = 391.4 nm) have been measured. Also the light spatial distribution in the plasma sheet has been studied using a CCD camera coupled to interferential filters corresponding to the wavelengths investigated. The reduced electric field in the plasma sheet has been derived from the measurement of the intensity ratio of the nitrogen lines. This study has been realized varying the amplitude of the DC voltage and the amplitude and frequency of the AC voltage. The reduced electric field strength is found to be almost constant for all the experimental conditions, with a value of 500 plusmn 100 Td (1 Td = 1.10-17 V cm2).

Influence of a plasma actuator on aerodynamic forces over a flat plate interacting with a rarefied Mach 2 flow

Purpose n n n n nThe purpose of this paper is to describe experimental and numerical investigations focussed on the shock wave modification induced by a dc glow discharge. The model is a flat plate in a rarefied Mach 2 air flow, equipped with a plasma actuator composed of two electrodes. The natural flow without actuation exhibits a shock wave with a hyperbolic shape. When the discharge is on, the shock wave shape remains hyperbolic but the shock wave is pushed forward, leading to an increase in the shock wave angle. In order to discriminate thermal effects from purely plasma ones, the plasma actuator is then replaced by an heating element. n n n n nDesign/methodology/approach n n n n nThe experimental study is carried out with the super/hypersonic wind tunnel MARHy located at the ICARE Laboratory in Orleans. The experimental configuration with the heating element is simulated with a code using the 2D full compressible Navier-Stokes equations adapted for the rarefied conditions. n n n n nFindings n n n n nFor heating element temperatures equal to the flat plate wall surface ones with the discharge on, experimental and numerical investigations showed that the shock wave angle was lower with the heating element, only 50 percent of the values got with the plasma actuator, meaning that purely plasma effects must also be considered to fully explain the flow modifications observed. The results obtained with the numerical simulations are then used to calculate the aerodynamic forces, i.e. the drag and the lift. These numerical results are then extrapolated to the plasma actuator case and it was found that the drag coefficient rises up to 13 percent when the plasma actuator is used, compared to only 5 percent with the heating element. n n n n nOriginality/value n n n n nThis paper matters in the topic of atmospheric entries where flow control, heat management and aerodynamic forces are of huge importance.

Experimental investigation of the properties of a glow discharge used as plasma actuator applied to rarefied supersonic flow control around a flat plate

This paper describes experimental investigations focused on the glow discharge created by a plasma actuator and used to shock wave modification over a flat plate in a Mach 2 air flow. The model is equipped with a plasma actuator composed of two electrodes. A weakly ionized plasma is created above the plate by generating a glow discharge with a negative dc potential applied to the upstream electrode. ICCD images of the discharge without and with the Mach 2 flow show the influence of the flow field on the discharge morphology. In addition, ICCD images of the modified flow revealed that when the discharge is ignited, the shock wave angle increased with the applied voltage. Thermal measurements of the flat plate surface carried out with an IR camera showed that the spatial temperature distribution is not uniform along the plate and its maximum, near the leading edge, increases with the applied voltage. Previous results showed that surface heating is responsible for roughly 50% of the shock wave angle increase, meaning that purely plasma effects must also be considered to fully explain the flow modifications observed. The focus of this paper is the study of the properties of the glow discharge to better understand the interaction between the supersonic flow and the purely plasma effects which are responsible of flow field modifications, in particular ionization degree and thermal disequilibrium upstream the model.

Efficiency of plasma actuator ionization in shock wave modification in a rarefied supersonic flow over a flat plate

This paper describes experimental and numerical investigations focused on the shock wave modification, induced by a dc glow discharge, of a Mach 2 flow under rarefied regime. The model under investigation is a flat plate equipped with a plasma actuator composed of two electrodes. The glow discharge is generated by applying a negative potential to the upstream electrode, enabling the creation of a weakly ionized plasma. The natural flow (i.e. without the plasma) exhibits a thick laminar boundary layer and a shock wave with a hyperbolic shape. Images of the flow obtained with an ICCD camera revealed that the plasma discharge induces an increase in the shock wave angle. Thermal effects (volumetric, and at the surface) and plasma effects (ionization, and thermal non-equilibrium) are the most relevant processes explaining the observed modifications. The effect induced by the heating of the flat plate surface is studied experimentally by replacing the upstream electrode by a heating element, and numerically by modifying the thermal boundary condition of the model surface. The results show that for a similar temperature distribution over the plate surface, modifications induced by the heating element are lower than those produced by the plasma. This difference shows that other effects than purely thermal effects are involved with the plasma actuator. Measurements of the electron density with a Langmuir probe highlight the fact that the ionization degree plays an important role into the modification of the flow. The gas properties, especially the isentropic exponent, are indeed modified by the plasma above the actuator and upstream the flat plate. This leads to a local modification of the flow conditions, inducing an increase in the shock wave angle.

Experimental and Numerical Study of Thrust-Vectoring Effects by Transverse Gas Injection into a Propulsive Axisymmetric C-D Nozzle

Transverse gas injection into a divergent section of a convergent-divergent rocket nozzle have been investigated for thrust vectoring effects as the segment part of the CNES Perseus program. Cold flow experiments on conical and truncated ideal nozzles have been conducted in the hypersonic windtunnel EDITH. These experiments are compared and complemented by the numerical investigation results of the compressible FANS numerical solver CPS C. Number of parameters and performance criteria have been examined and reported. It is found that pertinent vector force is possible to be achieved via such fluidic thrust vectoring system.

Shock modification induced by a DC discharge: numerical and experimental study

The present work is devoted to the study of the influence of an electrical DC discharge applied over a flat plate subjected to a supersonic and rarefied air flow. The main topic of this paper concerns the 2D Navier-Stokes numerical simulation of the flow developed around the model and submitted to the effects induced by the presence of glow plasma created above the flat plate. Different effects like, surface plate temperature, gas volume temperature and plate geometry, were considered in the numerical simulation and compared with the experimental results obtained in a wind tunnel producing a laminar Mach 2 air flow at ambient pressure equivalent to the 80 km altitude atmospheric pressure.

Effect of surface heating on shock wave modification by a plasma actuator in a rarefied supersonic flow over a flat plate

This paper describes experimental and numerical investigations focused on the shock wave modification induced by a plasma actuator. The studied model is a flat plate placed in a rarefied Mach 2 air flow. The plasma actuator is composed of two metallic electrodes set on the upper surface of the model. A negative dc potential is applied to the upstream electrode in order to generate an abnormal glow discharge and, thus, to create a weakly ionized plasma around the model. ICCD images of the flow allow the shock wave to be detected and its angle to be estimated. When the discharge is ignited, the shock wave angle increases with the discharge current. In addition, an IR camera is used to measure the increase in the surface temperature. The longitudinal distributions of the surface temperature are used as boundary conditions in the numerical simulations of the surface heating induced by the plasma actuator. This type of thermal effect is reproduced experimentally with a heating element designed to this purpose. For the same surface heating, experimental results show that the shock wave angle is higher in the case of the plasma actuator: surface heating is responsible for roughly 50% of the shock wave angle modification. The numerical simulations are used to studied the aerodynamic forces modifications in the case of the surface heating. The results are extrapolated to the plasma actuator in order to estimate the variations in the drag and lift coefficients.