Simon Demange

Absolute instability in Plasma jet

During an atmospheric entry, spacecrafts are protected from the extreme temperature developing behind the bow shock by a Thermal Protection System made of ablative materials (TPS). To design the TPS, materials are tested in the VKI Plasmatron Inductively Coupled Plasma wind tunnel, however, relevant phenomena such as ablation, catalysis and transition, are strongly coupled with the quality of the plasma jet flow. Unfortunately, perturbations due to jet hydrodynamic instabilities have been observed experimentally and perturb the experiments. Instabilities found in jets can display two distinct natures: either convective if the flow acts as an amplifier with initial perturbations, either absolute if it acts as an oscillator. However, distinguishing these two types experimentally is not straightforward. Numerically, both convective and absolute types can be accurately estimated by the Linear Stability Theory. Retrieving the frequencies and amplification rates of disturbances growing in the jet. Convective instabilities in the Plasmatron have been previously studied at VKI, without matching all experimental results. Literature studies on absolute instabilities do not cover the operative conditions reached in the facility, leaving the nature of the instabilities observed not completely understood. This research project will investigate the absolute instabilities which may develop in Plasmatron, as a new approach to reduce the uncertainties on the ow field quantities. Modules will be developed and added to the VESTA toolkit, to determine the nature of instabilities developing for all operative conditions. The stability computations will use velocity and temperature proles for a mixture of gases in Local Thermodynamic Equilibrium, fitted to simulations reproducing the experimental conditions. The results will be validated against experimental observations, and will serve to complete absolute instability models and design more efficient TPS.

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Andrea Fagnani

Comprehensive characterization of the aerothermomechanical response of space debris to atmospheric entry plasmas

Space debris, that is, all man-made objects in Earth's orbit which are non-functional, represent the trace of sixty years of space activities. Self-sustained growth, promoted by collisions among fragments, and impact with operating satellites can hinder future access to space. To achieve a sustainable space environment, the Design for Demise (D4D) strategy proposes to conceive a space object for the safe end-of-life disposal through a destructive atmospheric re-entry. Aerothermal heating and aerodynamic forces should be exploited to break up and completely destroy any spacecraft component. Experience, however, demonstrates that several parts, made up of titanium, steel and silicate materials, can survive re-entry almost intact, thus representing risk on ground.

To safely implement the D4D strategy, accurate computational models are required for the prediction of the spacecraft thermal degradation. In turn, these models need empirical data for validation. The overarching objective of this research is to advance the predictive capabilities of the aerothermal demise of space debris by performing multi-scale experiments and modelling to characterize the thermal decomposition of metallic and silicate components.

The project is articulated in three main parts. First, we will provide a comprehensive theoretical framework for the description of the thermal degradation of metallic and silicate materials in high-enthalpy reacting flows, also addressing the extension of the current ground-to-flight scaling methodology. Next, starting from the computational tools developed at VKI and NASA Ames Research Center, we will improve specific features in order to deal with melting-ablation problems, with the aim of applying them to the ground test environment to correctly design the experimental conditions. Finally, the core of the work will focus on the multi-scale experimental characterization, with plasma wind tunnel tests and micro-scale post-test analysis, leveraging core competences and facilities of the von Karman Institute for Fluid Dynamics and the Electrochemical and Surface Engineering research group (SURF) at the VUB.

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Vincent Fitzgerald Giangaspero

Modeling strategy for blackout analysis of re-entry phases in space exploration missions

During the re-entry phase of a spacecraft into a planetary atmosphere, the high temperatures generated around the vehicle result in the formation of a plasma flow. Plasma interacts with electromagnetic waves disrupting all communication, navigation and telemetry signals, leading to the well-known radio blackout problem. For re-entry vehicles, radio blackout typically lasts several minutes, depending on the angle of re-entry and the particular flight trajectory of the spacecraft. Communication or radio blackout is an important issue for hypersonic vehicles and it is extremely important to develop strategies for propagating telemetry during hypersonic flight. With future space missions aimed towards Mars, including NASA’s Mars 2020 and ESA’s Exo-Mars 2020, there is a strong motivation to improve the understanding of the physics regarding the communication blackout.

The goal of this research is to develop and validate more effective numerical tools to properly predict radio blackout, both for Earth and Mars re-entry missions, by the development of a multi physics fully-integrated code. Furthermore, one of the most promising mitigation methods for blackout, which is based on electromagnetic manipulation of plasma, will be tested for Mars atmosphere, a novelty for this field. With a limited amount of blackout work performed, this research will create a greater understanding and will contribute filling the unsolved gap in blackout modelling and mitigation methods.

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Maria Teresa Scelzo

Characterisation of Cryogenic Slush for Future Launcher Engines

VKI Supervisors: Laura Peveroni, Prof. Jean-Marie Buchlin
ULB Supervisor: Prof. Gerard Degrez

Liquid hydrogen is widely used as fuel in rocket engines due to the high specific impulse provided. However, several problems are related to this choice, such us low density, temperature stratification and short holding time. Moreover, strong sloshing can occur due to the liquid state. A promising way to reduce these problems is to use slush, that is a solid-liquid mixture with 15% higher density and 18% higher specific enthalpy. The employment of this form of propellant results in promising new concept rocket engines. However, the presence of solid particle in the liquid suspension changes the flow properties: the rheological behaviour becomes non-Newtonian for high particle concentration and, due to the heat transfer, the solid fraction may partially melt changing slush features. Hence, the standard correlations for the liquid flow in tubes are no longer valid. An observed phenomenon of drag reduction appears to be interesting for improving system efficiency, but it has never been completely understood. Therefore, the optimization of the slush transport through circular pipelines and narrowing, requires the measurement of the slush physical features, along with their modelling.

The goal of this research project is to provide a complete understanding of the slush behaviour at different flow conditions such as velocity, mass fraction concentration and particle distribution. Due to the high risks and extremely high costs involved in using hydrogen, the research will be carried out for slush nitrogen. An intense experimental campaign will furnish a complete database concerning pressure drops, heat transfer and phase change phenomena. This database will be used to design improved engineering correlations as well as revised models for Eulerian-Eulerian numerical approaches available in existing softwares, to be used in the design of slush transport and storage systems.

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