About 100 tonnes of small meteoroids enter the terrestrial atmosphere every day. As a meteoroid interacts with the atmosphere, its surface temperature can reach several thousands of Kelvins, causing its ablation. Among the various strategies for the observation of meteors, radio-based techniques have proved to be a simple yet valuable tool for collecting large amounts of data. Ground-radio stations can detect free electron densities in the meteoroid trail, which are initially produced by the hyperthermal collisions of the ablated species with the incoming freestream. Therefore, the ability to predict the ionisation intensity and the rate of dissipation of the plasma trail becomes essential for the correct interpretation of the radio signal. However, the current models are drastically simplified; they use zero-dimensional approaches and disregard nonequilibrium phenomena, such as rarefied gas effects. This work aims to provide a detailed description of the degradation process of a meteoroid and the physico-chemical dynamics driving the ablated vapour around the body and in the extended plasma trail. Each improvement in the modelling is expected to impact not only our understanding of the physical problem but also the estimates on mass fluxes and the statistical outcome of the collected observational data. Chapter 3 describes the nonequilibrium collisional processes around millimetre-sized meteoroids, at certain selected conditions along the entry path. These conditions are chosen in agreement with the trajectory and flow regime analysis performed in Chapter 2. Particular attention is given to the production of free electrons, on which the intensity of the radio signal depends. We carry out the investigation in the framework of the Direct Simulation Monte Carlo (DSMC) method, where we include specific models to tackle both gas-surface interactions and gas-phase encounters. With respect to classical meteor theory, the introduced evaporation boundary condition can take into account condensation fluxes and the backscattering of molecules at the wall. Moreover, a database of elastic and reactive cross sections is developed for the ablated vapour. This set of data may be useful to scientists as a reference database for future theoretical and numerical studies. Collisional processes and shielding effects prove to be meaningful for events below 100 km or intense ablation rates, and they could be even more significant for bigger meteoroids. Ionisation comes mainly from the hyperthermal encounters between air and vapour, rather than from thermalised metal-metal collisions. Chapter 4 deals with the experimental characterisation of the material response of some meteorite samples. These experiments allow testing the evaporation model from Chapter 3 and yield insight into the ablation mechanisms. We present the results from two campaigns, carried out in the VKI Plasmatron wind tunnel and the NASA Ames laser ablation facility, so as to span a range of evaporation conditions. Both tests confirm strong degassing of light species and oxidation, and they indicate that accurate ablation modelling requires describing the mechanical removal of the molten layer into the hypersonic flow. Geochemical characterisation of the recovered material demonstrates the successful use of these type of facilities to reproduce fusion crusts similar to those collected on the ground. Finally, measurements of the surface temperatures, coupled with thermal response modelling of the material, help in refining the understanding of the surface energy balance. Despite the simplicity of the numerical model, simulations and pyrometry data are found to agree reasonably well in laser experiments, while they needed a tuned contribution of the oxidation heat flux in the Plasmatron tests. In Chapter 5, we study the processes which lead to the extinction of the plasma trail. For this purpose, we employ the simulations obtained in Chapter 3 as initial conditions to carry out detailed chemical and multicomponent diffusion calculations of the extended trail (up to several kilometres) using a Lagrangian approach. In this approach, a fluid element reactor marches along assigned streamlines, calculating multicomponent mass diffusion in the radial direction and detailed chemical reactions. Chemical reactions have negligible effects in the neutralisation process of underdense trails, which are dominated by mass diffusion. Also, a constant diffusion coefficient, as used in standard models, is sufficient to reproduce the numerical profiles. The influence of chemistry and differential diffusion could be enhanced in overdense meteors. Therefore this aspect deserves future investigation. Finally, we link the dissipation of the electrons to the reflected radio echo, so as to estimate the resulting signal in the framework of the classical underdense meteor theory. On the whole, the procedure presented represents a standalone methodology, which can provide meteor physical parameters at given trajectory conditions, without the need to rely on standard lumped models. In light of the experimental results of Chapter 4, in Chapter 6, we present a numerical procedure that allows studying melting in the presence of a rarefied gas phase. In these flows, the condensed phase can be treated as a continuous medium, whereas a kinetic treatment is needed for the gas. We propose a computational approach in which both phases are simulated by particle schemes: the DSMC method for the vapour and the Smoothed Particle Hydrodynamics (SPH) method for the solid and the liquid. This approach is computationally more intensive than the one of Chapter 3, as it proposes to describe the dynamics of the three phases comprehensively. In this case, a two-way coupling between phases is pursued, at the expense of other possibly relevant phenomena such as evaporation or chemical nonequilibrium of the gas, which are not taken into account. We show the details of the thermal and dynamic coupling methodology, along with some verification test cases in simplified configurations. Finally, as a proof-of-concept, we consider the melting of a solid cylinder immersed in a rarefied hypersonic stream. The dynamics of the molten layer under the influence of the external flow is analysed.
Ogni giorno circa 100 tonnellate di piccoli meteoroidi entrano nell'atmosfera terrestre. A causa delle loro piccole dimensioni e delle enormi velocità, i meteoroidi bruciano ad altitudini elevate, dove sono responsabili degli strati ricchi di metalli nella mesosfera e nella termosfera inferiore che svolgono un ruolo essenziale nei processi chimici atmosferici. Tra le varie strategie per l'osservazione delle stelle cadenti, le tecniche radio si sono rivelate uno strumento semplice ma prezioso per raccogliere grandi quantità di dati, 24 ore su 24 indipendentemente dalle condizioni di visibilità. Le stazioni radio di terra possono rilevare il plasma nella scia luminosa, che è prodotto dalle collisioni della roccia vaporizzata con il flusso d'aria ipersonico. Pertanto, la capacità di prevedere l'intensità di ionizzazione e la velocità di dissipazione della scia di plasma diventa essenziale per la corretta interpretazione del segnale radio. Tuttavia, gli approcci attuali sono drasticamente semplificati e ignorano gli effetti di collisione nel gas rarefatto. Questo lavoro mira a fornire una descrizione dettagliata del processo di degradazione del meteoroide e di quei fenomeni fisico-chimici che guidano la dinamica del vapore ablato intorno al corpo e nella sua scia. In primo luogo, abbiamo studiato il comportamento del gas simulando le molecole direttamente alla scala cinetica. In una seconda fase, abbiamo sviluppato una procedura per esaminare la neutralizzazione della coda della meteora. Inoltre, abbiamo dedicato particolare attenzione allo studio delle interazioni gas-superficie. Questa indagine è stata supportata da esperimenti a terra in una galleria del vento al plasma. Infine, abbiamo sviluppato e verificato una metodologia computazionale, che tiene conto dell'accoppiamento termico e dinamico del vapore con la fase condensata. Siamo fiduciosi che questa tesi abbia messo insieme le tessere fondamentali di uno sforzo di modellazione più ampio ed ambizioso che aiuterà gli astronomi a ridurre le incertezze nell'interpretazione degli echi radio, aprendo la strada all'adozione di approci computazionali sofisticati nel campo della scienza delle meteore.
Development of an ablation model for rarefied flows with application to meteoroid entry
BARISELLI, FEDERICO
2020/2021
Abstract
About 100 tonnes of small meteoroids enter the terrestrial atmosphere every day. As a meteoroid interacts with the atmosphere, its surface temperature can reach several thousands of Kelvins, causing its ablation. Among the various strategies for the observation of meteors, radio-based techniques have proved to be a simple yet valuable tool for collecting large amounts of data. Ground-radio stations can detect free electron densities in the meteoroid trail, which are initially produced by the hyperthermal collisions of the ablated species with the incoming freestream. Therefore, the ability to predict the ionisation intensity and the rate of dissipation of the plasma trail becomes essential for the correct interpretation of the radio signal. However, the current models are drastically simplified; they use zero-dimensional approaches and disregard nonequilibrium phenomena, such as rarefied gas effects. This work aims to provide a detailed description of the degradation process of a meteoroid and the physico-chemical dynamics driving the ablated vapour around the body and in the extended plasma trail. Each improvement in the modelling is expected to impact not only our understanding of the physical problem but also the estimates on mass fluxes and the statistical outcome of the collected observational data. Chapter 3 describes the nonequilibrium collisional processes around millimetre-sized meteoroids, at certain selected conditions along the entry path. These conditions are chosen in agreement with the trajectory and flow regime analysis performed in Chapter 2. Particular attention is given to the production of free electrons, on which the intensity of the radio signal depends. We carry out the investigation in the framework of the Direct Simulation Monte Carlo (DSMC) method, where we include specific models to tackle both gas-surface interactions and gas-phase encounters. With respect to classical meteor theory, the introduced evaporation boundary condition can take into account condensation fluxes and the backscattering of molecules at the wall. Moreover, a database of elastic and reactive cross sections is developed for the ablated vapour. This set of data may be useful to scientists as a reference database for future theoretical and numerical studies. Collisional processes and shielding effects prove to be meaningful for events below 100 km or intense ablation rates, and they could be even more significant for bigger meteoroids. Ionisation comes mainly from the hyperthermal encounters between air and vapour, rather than from thermalised metal-metal collisions. Chapter 4 deals with the experimental characterisation of the material response of some meteorite samples. These experiments allow testing the evaporation model from Chapter 3 and yield insight into the ablation mechanisms. We present the results from two campaigns, carried out in the VKI Plasmatron wind tunnel and the NASA Ames laser ablation facility, so as to span a range of evaporation conditions. Both tests confirm strong degassing of light species and oxidation, and they indicate that accurate ablation modelling requires describing the mechanical removal of the molten layer into the hypersonic flow. Geochemical characterisation of the recovered material demonstrates the successful use of these type of facilities to reproduce fusion crusts similar to those collected on the ground. Finally, measurements of the surface temperatures, coupled with thermal response modelling of the material, help in refining the understanding of the surface energy balance. Despite the simplicity of the numerical model, simulations and pyrometry data are found to agree reasonably well in laser experiments, while they needed a tuned contribution of the oxidation heat flux in the Plasmatron tests. In Chapter 5, we study the processes which lead to the extinction of the plasma trail. For this purpose, we employ the simulations obtained in Chapter 3 as initial conditions to carry out detailed chemical and multicomponent diffusion calculations of the extended trail (up to several kilometres) using a Lagrangian approach. In this approach, a fluid element reactor marches along assigned streamlines, calculating multicomponent mass diffusion in the radial direction and detailed chemical reactions. Chemical reactions have negligible effects in the neutralisation process of underdense trails, which are dominated by mass diffusion. Also, a constant diffusion coefficient, as used in standard models, is sufficient to reproduce the numerical profiles. The influence of chemistry and differential diffusion could be enhanced in overdense meteors. Therefore this aspect deserves future investigation. Finally, we link the dissipation of the electrons to the reflected radio echo, so as to estimate the resulting signal in the framework of the classical underdense meteor theory. On the whole, the procedure presented represents a standalone methodology, which can provide meteor physical parameters at given trajectory conditions, without the need to rely on standard lumped models. In light of the experimental results of Chapter 4, in Chapter 6, we present a numerical procedure that allows studying melting in the presence of a rarefied gas phase. In these flows, the condensed phase can be treated as a continuous medium, whereas a kinetic treatment is needed for the gas. We propose a computational approach in which both phases are simulated by particle schemes: the DSMC method for the vapour and the Smoothed Particle Hydrodynamics (SPH) method for the solid and the liquid. This approach is computationally more intensive than the one of Chapter 3, as it proposes to describe the dynamics of the three phases comprehensively. In this case, a two-way coupling between phases is pursued, at the expense of other possibly relevant phenomena such as evaporation or chemical nonequilibrium of the gas, which are not taken into account. We show the details of the thermal and dynamic coupling methodology, along with some verification test cases in simplified configurations. Finally, as a proof-of-concept, we consider the melting of a solid cylinder immersed in a rarefied hypersonic stream. The dynamics of the molten layer under the influence of the external flow is analysed.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/177659