Browsing by Author "Su, C. C."

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3D Direct Simulation Monte Carlo Modelling of the Inner Gas Coma of Comet 67P/Churyumov-Gerasimenko: A Parameter Study
(2016-03)
Liao, Y.
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Su, C. C.
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Marschall, R.
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Wu, J. S.
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Rubin, M.
Direct Simulation Monte Carlo (DSMC) is a powerful numerical method to study rarefied gas flows such as cometary comae and has been used by several authors over the past decade to study cometary outflow. However, the investigation of the parameter space in simulations can be time consuming since 3D DSMC is computationally highly intensive. For the target of ESA\'s Rosetta mission, comet 67P/Churyumov-Gerasimenko, we have identified to what extent modification of several parameters influence the 3D flow and gas temperature fields and have attempted to establish the reliability of inferences about the initial conditions from in situ and remote sensing measurements. A large number of DSMC runs have been completed with varying input parameters. In this work, we present the simulation results and conclude on the sensitivity of solutions to certain inputs. It is found that among cases of water outgassing, the surface production rate distribution is the most influential variable to the flow field.
A comparison of multiple Rosetta data sets and 3D model calculations of 67P/Churyumov-Gerasimenko coma around equinox (May 2015)
(2019-08-01)
Marschall, R.
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Rezac, L.
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Kappel, D.
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Su, C. C.
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Gerig, S. -B.
We have used the latest available shape model for gas and dust simulations of the inner coma of comet 67P/Churyumov-Gerasimenko for the period around May 2015 (equinox). We compare results from a purely insolation-driven model with a complementary set of observations made by ROSINA, VIRTIS, MIRO, and OSIRIS within the same period. The observations include - for the first time - inverted MIRO measurements of gas density, temperature and bulk velocity to constrain the model. The comparisons show that, as in November 2014 (Marschall et al., 2016), insolation-driven activity does not provide an adequate fit to the data. Both VIRTIS and MIRO observations indicate that emissions from the Hatmehit and Imhotep regions of the nucleus are strongly depleted in total gas, H2O, and dust emissions in this case. The MIRO inversion provides a challenging constraint to the models as a consequence of the terminator orbit and nucleus pointing of the spacecraft. Nonetheless a consistent picture with a dominance of outgassing from the Hapi region, even at equinox, is clearly evident. An inhomogeneous model consistent with models proposed for the November 2014 time-frame was constructed and provides a better fit to the data. As far as we are aware this is the first time comae data from four Rosetta instruments have been used to constrain within one self-contained model the emission distribution at the nucleus surface and study the dynamics of the gas and dust outflow.
Cliffs versus plains: Can ROSINA/COPS and OSIRIS data of comet 67P/Churyumov-Gerasimenko in autumn 2014 constrain inhomogeneous outgassing?
(2017-09-01)
Marschall, R.
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Mottola, S.
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Su, C. C.
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Liao, Y.
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Rubin, M.
Context. This paper describes the modelling of gas and dust data acquired in the period August to October 2014 from the European Space Agency's Rosetta spacecraft when it was in close proximity to the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. With our 3D gas and dust comae models this work attempts to test the hypothesis that cliff activity on comet 67P/Churyumov-Gerasimenko can solely account for the local gas density data observed by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the dust brightnesses seen by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) in the considered time span. Methods. The model uses a previously developed shape model of the nucleus. From this, the water sublimation rates and gas temperatures at the surface are computed. The gas expansion is modelled with a 3D Direct Simulation Monte Carlo algorithm. A dust drag algorithm is then used to compute dust volume number densities in the coma, which are then converted to brightnesses using Mie theory and a line-of-sight integration. Furthermore we have studied the impact of topographic re-radiation on the models. Results. We show that gas activity from only cliff areas produces a fit to the ROSINA/COPS data that is as statistically good as a purely insolation-driven model. In contrast, pure cliff activity does not reproduce the dust brightness observed by OSIRIS and can thus be ruled out. On the other hand, gas activity from the Hapi region in addition to cliff activity produces a statistically better fit to the ROSINA/COPS data than purely insolation-driven outgassing and also fits the OSIRIS observations rather well. We found that topographic re-radiation does not contribute significantly to the sublimation behaviour of H2O but plays an important role in how the gas flux interacts with the irregular shape of the nucleus. Conclusions. We demonstrate that fits to the observations are non-unique. We can conclude however that gas and dust activity from cliffs and the Hapi region are consistent with the ROSINA/COPS and OSIRIS data sets for the considered time span and are thus a plausible solution. Models with activity from low gravitational slopes alone provide a statistically inferior solution.
Implementation of a transient adaptive sub-cell module for the parallel-DSMC code using unstructured grids
(2010-08-01)
Su, C. C.
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Tseng, K. C.
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Cave, H. M.
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Wu, J. S.
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Lian, Y. Y.
A method of transient adaptive sub-cells (TAS) suitable for unstructured grids that is modified from the existing one for the structured grids of DSMC is Introduced. The TAS algorithm is implemented within the framework of a parallelized DSMC code (PDSC). Benchmarking tests are conducted for steady driven cavity flow, steady hypersonic flow over a two-dimensional cylinder, steady hypersonic flow over a cylinder/flare and the unsteady vortex shedding behind a two-dimensional cylinder The use of TAS enables a reduction in the computational expense of the simulation since larger sampling cells and less simulation particles can be employed. Furthermore, the collision quality of the simulation is maintained or improved and the preservation of property gradients and vorticity at the scale of the sub-cells enables correct unsteady vortex shedding frequencies to be predicted The use of TAS in a parallel-DSMC code allows simulations of unsteady processes at a level to be carried out efficiently, accurately and with acceptable computational time (C) 2010 Elsevier Ltd All rights reserved
Modelling observations of the inner gas and dust coma of comet 67P/Churyumov-Gerasimenko using ROSINA/COPS and OSIRIS data: First results
(2016-05-01)
Marschall, R.
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Su, C. C.
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Liao, Y.
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Thomas, N.
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Altwegg, K.
Context. This paper describes the initial modelling of gas and dust data acquired in August and September 2014 from the European Space Agency's Rosetta spacecraft when it was in close proximity to the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. This work is an attempt to provide a self-consistent model of the innermost gas and dust coma of the comet, as constrained by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) data set for the gas and by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) data set for the dust. Methods. The model uses a previously developed shape model for the nucleus, and from this the water sublimation rate and gas temperatures at the surface are computed with a simple thermal model. The gas expansion is modelled with a 3D parallel implementation of a Direct Simulation Monte Carlo algorithm. A dust drag algorithm is then used to produce dust densities in the coma, which are then converted to brightnesses using Mie theory and a line-of-sight integration. Results. We show that a purely insolation-driven model for surface outgassing does not produce a reasonable fit to ROSINA/COPS data. A stronger source in the "neck" region of the nucleus (region Hapi) is needed to match the observed modulation of the gas density in detail. This agrees with OSIRIS data, which shows that the dust emission from the "neck" was dominant in the August-September 2014 time frame. The current model matches this observation reasonably if a power index of 2-3 for the dust size distribution is used. A better match to the OSIRIS data is seen by using a single large particle size for the coma. Conclusions. We have shown possible solutions to the gas and dust distributions in the inner coma, which are consistent with ROSINA and OSIRIS data.
On deviations from free-radial outflow in the inner coma of comet 67P/Churyumov-Gerasimenko
(2018-09-01)
Gerig, S. -B.
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Marschall, R.
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Thomas, N.
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Bertini, I.
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Bodewits, D.
The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) onboard the European Space Agency's Rosetta spacecraft acquired images of comet 67P/Churyumov-Gerasimenko (67P) and its surrounding dust coma starting from May 2014 until September 2016. In this paper we present methods and results from analysis of OSIRIS images regarding the dust outflow in the innermost coma of 67P. The aim is to determine the global dust outflow behaviour and place constraints on physical processes affecting particles in the inner coma. We study the coma region right above the nucleus surface, spanning from the nucleus centre out to a distance of about 50 km comet centric distance (approximately 25 average comet radii). We primarily adopt an approach used by Thomas and Keller (1990) to study the dust outflow. We present the effects on azimuthally-averaged values of the dust reflectance of non-radial flow and non-point-source geometry, acceleration of dust particles, sublimation of icy dust particles after ejection from the surface, dust particle fragmentation, optical depth effects and the influence of gravitationally bound particles. All of these physical processes could modify the observed distribution of light scattered by the dust coma. In the image analysis, profiles of azimuthally averaged dust brightness as a function of impact parameter b (azimuthal average, "(A) over bar -curve") were fitted with a simple function that best fits the shape of our profile curves (f (b; u, v, w, z) = u/b(v) + wb + z). The analytical fit parameters (u, v, w, z), which hold the key information about the dust outflow behaviour, were saved in a comprehensive database. Through statistical analysis of these information, we show that the spatial distribution of dust follows free-radial outflow behaviour (i.e. force-free radial outflow with constant velocity) beyond distances larger than similar to 11.9 km from the comet centre, which corresponds to a relative distance of about 6 average comet radii from the comet centre. Hence, we conclude that beyond this distance, and on average, fragmentation and gravitationally bound particles are negligible processes in determining the optically scattered light distribution in the innermost coma. Closer to the nucleus we observe dust outflow behaviour that deviates from free-radial outflow. A comparison of our result profiles with numerical models using a Direct Simulation Monte Carlo (DSMC) approach with dust particle distributions calculated using a test particle approach has been used to demonstrate the influence of a complex shape and particle acceleration on the azimuthal average profiles. We demonstrate that, while other effects such as fragmentation or sublimation of dust particles cannot be ruled out, acceleration of the dust particles and effects arising from the shape of the irregular nucleus (non-point source geometry) are sufficient to explain the observed dust outflow behaviour from image data analysis. As a by-product of this work, we have calculated "Af rho" values for the 1/r regime. We found a peak in the coma activity in terms of Af rho (normalised to a phase angle of 90) of similar to 210 cm 20 days after perihelion. Furthermore, based on simplified models of particle motion within bound orbits, it is shown that limits on the total cross-sectional area of bound particles might be derived through further analysis. An example is given. (C) 2018 The Authors. Published by Elsevier Inc.
Redistribution of particles across the nucleus of comet 67P/Churyumov-Gerasimenko
(2015-11-01)
Thomas, N.
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Davidsson, B.
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El-Maarry, M. R.
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Fornasier, S.
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Giacomini, L.
Context. We present an investigation of the surface properties of areas on the nucleus of comet 67P/Churyumov-Gerasimenko. Aims. We aim to show that transport of material from one part of the cometary nucleus to another is a significant mechanism that influences the appearance of the nucleus and the surface thermal properties. Methods. We used data from the OSIRIS imaging system onboard the Rosetta spacecraft to identify surface features on the nucleus that can be produced by various transport mechanisms. We used simple calculations based on previous works to establish the plausibility of dust transport from one part of the nucleus to another. Results. We show by observation and modeling that "airfall" as a consequence of non-escaping large particles emitted from the neck region of the nucleus is a plausible explanation for the smooth thin deposits in the northern hemisphere of the nucleus. The consequences are also discussed. We also present observations of aeolian ripples and ventifacts. We show by numerical modeling that a type of saltation is plausible even under the rarified gas densities seen at the surface of the nucleus. However, interparticle cohesive forces present difficulties for this model, and an alternative mechanism for the initiation of reptation and creep may result from the airfall mechanism. The requirements on gas density and other parameters of this alternative make it a more attractive explanation for the observations. The uncertainties and implications are discussed.
Simulations of subsonic vortex-shedding flow past a 2D vertical plate in the near-continuum regime by the parallelized DSMC code
(2012-08-01)
Tseng, K. C.
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Kuo, T. C.
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Lin, S. C.
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Su, C. C.
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Wu, J. S.
A general-purpose Parallel Direct Simulation Monte Carlo Code, named PDSC, is used to simulate near-continuum subsonic flow past a 2D vertical plate for studying the vortex-shedding phenomena. An unsteady time-averaging sampling method and a post-processing procedure called DREAM (DSMC Rapid Ensemble Averaging Method) have also been implemented, reducing the overall computational expense and improving the sampling quality of time-dependent flow problems in the rarefied flow regime. Parametric studies, including the temporal variable time step (TVTS) factor, the number of particles per cell, the domain size, and the Reynolds number, have been conducted, obtaining the Strouhal number and various aerodynamic coefficients of the flow. Results are compared to experimental data in the continuum regime available in the literature, demonstrating the capacity of PDSC and DREAM to simulate near-continuum vortex-shedding problems within acceptable computational time. (C) 2012 Elsevier B.V. All rights reserved.
The spatial distribution of water in the inner coma of Comet 9P/Tempel 1: Comparison between models and observations
(2014-07-01)
Finklenburg, S.
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Thomas, N.
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Su, C. C.
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Wu, J. -S.
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機械工程學系
The near nucleus coma of Comet 9P/Tempel 1 has been simulated with the 3D Direct Simulation Monte Carlo (DSMC) code PDSC++ (Su, C.-C. [2013]. Parallel Direct Simulation Monte Carlo (DSMC) Methods for Modeling Rarefied Gas Dynamics. PhD Thesis, National Chiao Tung University, Taiwan) and the derived column densities have been compared to observations of the water vapour distribution found by using infrared imaging spectrometer on the Deep Impact spacecraft (Feaga, L.M., A\'Hearn, M.F., Sunshine, J.M., Groussin, O., Farnham, T.L. [2007]. Icarus 191(2), 134-145. http://dx.doi.org/10.1016/j.icarus.2007.04.038). Modelled total production rates are also compared to various observations made at the time of the Deep Impact encounter. Three different models were tested. For all models, the shape model constructed from the Deep Impact observations by Thomas et al. (Thomas, P.C., Veverka, J., Belton, M.J.S., Hidy, A., A\'Hearn, M.F., Farnham, T.L., et al. [2007]. Icarus, 187(1), 4-15. http://dx.doi.org/10.1016/j.icarus.2006.12.013) was used. Outgassing depending only on the cosine of the solar insolation angle on each shape model facet is shown to provide an unsatisfactory model. Models constructed on the basis of active areas suggested by Kossacki and Szutowicz (Kossacki, K, Szutowicz, S. [2008]. Icarus, 195(2), 705-724. http://dx.doi.org/ 10.1016/j.icarus.2007.12.014) are shown to be superior. The Kossacki and Szutowicz model, however, also shows deficits which we have sought to improve upon. For the best model we investigate the properties of the outflow. (C) 2014 Elsevier Inc. All rights reserved.
Two-level virtual mesh refinement algorithm in a parallelized DSMC Code using unstructured grids
(2011-09-01)
Su, C. C.
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Tseng, K. C.
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Wu, J. S.
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Cave, H. M.
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Jermy, M. C.
In this paper, a virtual mesh refinement (VMR) algorithm for unstructured grids is developed and verified in the parallelized direct simulation Monte Carlo code (PDSC). This algorithm is a two-level virtual mesh refinement, in which the background mesh is refined based on an initial DSMC simulation. The refined cells are arranged in a way similar to the structured grid, which makes the particle tracing on them very efficient, unlike unstructured grids. These virtual cells are used for particle collision and sampling. Only those refined cells which contain the centroid of background cells are used for outputting macroscopic data. Two hypersonic flows with purely quadrilateral and mixed triangular-quadrilateral mesh were used to verify this new mesh-refining algorithm. Results show that the case using VMR can faithfully reproduce the benchmark case with a much reduced computational time. (C) 2011 Elsevier Ltd. All rights reserved.
Water vapor deposition from the inner gas coma onto the nucleus of Comet 67P/Churyumov-Gerasimenko
(2018-08-01)
Liao, Y.
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Marschall, R.
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Su, C. C.
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Wu, J. S.
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Lai, I. L.
Rosetta has detected water ice existing on the surface of Comet 67P/Churyurnov-Gerasimenko in various types of features. One of particular interest is the frost-like layer observed at the edge of receding shadows during the whole mission, interpreted as the recondensation of a thin layer of water ice. Two possible mechanisms, (1) subsurface ice sublimation and (2) gas coma deposition, have been proposed for producing this recondensation process and diurnal cycles of water ice. Previous studies have demonstrated both mechanisms based on simplified models. More precise and modem models are yet insufficient when addressing the gas-coma-deposition mechanism. We aim to study the recondensation from the inner water gas coma of the 67P/Churyumov-Gerasimenko with more physical constraints including the OSIRIS images, nucleus shape model, and insolation conditions. We compute, for the First time, the backflux distributions from the coma with various boundary conditions. Numerical simulations of this gas-coma-deposition process show that the equivalent water ice deposition can be up to several microns in an hour of accumulation time close to the perihelion passage, which is comparable with the simulation results of the other subsurface-ice sublimation mechanism.