Simulations of Re-Entry Vehicles by Using DSMC with Chemical-Reaction Module

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76448/1/AIAA-2006-8084-520.pd

Limitations in the determination of surface emission distributions on comets through modelling of observational data -- A case study based on Rosetta observations

The European Space Agency's (ESA) Rosetta mission has returned a vast data set of measurements of the inner gas coma of comet 67P/Churyumov-Gerasimenko. These measurements have been used by different groups to determine the distribution of the gas sources at the nucleus surface. The solutions that have been found differ from each other substantially and illustrate the degeneracy of this issue. It is the aim of this work to explore the limitations that current gas models have in linking the coma measurements to the surface. In particular, we discuss the sensitivity of Rosetta's ROSINA/COPS, VIRTIS, and MIRO instruments to differentiate between vastly different spatial distributions of the gas emission from the surface. We have applied a state of the art 3D DSMC gas dynamics code to simulate the inner gas coma of different models that vary in the fraction of the surface that contains ice and in different sizes of active patches. These different distributions result in jet interactions that differ in their dynamical behaviour. We have found that ROSINA/COPS measurements by themselves cannot detect the differences in our models. While ROSINA/COPS measurements are important to constrain the regional inhomogeneities of the gas emission, they can by themselves not determine the surface emission distribution of the gas sources to a spatial accuracy of better than a few hundred metres (400 m). Any solutions fitting the ROSINA/COPS measurements is hence fundamentally degenerate, be it through a forward or inverse model. Only other instruments with complementary measurements can potentially lift this degeneracy as we show here for VIRTIS and MIRO. Finally, as a by-product, we have explored the effect of our activity distributions on lateral flow at the surface that may be responsible for some of the observed aeolian features.Comment: Icarus (in press

On understanding multi-instrument Rosetta data of the innermost dust and gas coma of comet 67P/Churyumov-Gerasimenko - results, strengths, and limitations of models

Numerical models are powerful tools for understanding the connection between the emitted gas and dust from the surface of comets and the subsequent expansion into space where remote sensing instruments can perform measurements. We will present such a predictive model which can provide synthetic measurements for multiple instruments on board ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko (hereafter 67P). We will demonstrate why a multi instrument approach is essential and how models can be used to constrain the gas and dust source distribution on the surface

The structure of sphere wakes at intermediate Reynolds numbers in still and turbulent environments.

The structure of sphere wakes was studied for sphere Reynolds numbers in the range of 30-4000 in still environments and for sphere Reynolds numbers in the range of 130-1600 in turbulent environments. Measurements included dye traces illuminated by a laser light sheet for flow visualization and laser velocimetry for streamwise and crosstream velocities. Classical similarity results for turbulent and laminar wakes were used to interpret and correlate the measurements. There are three wake regions identified in still environments: a fast-decay wake region that was observed only when vortex shedding was present (Re280), followed in succession by turbulent and laminar wake regions. Mean velocities within the turbulent and laminar wake regions scaled according to classical similarity theories, with transition between these regions at conditions where their estimates of mean streamwise velocities along the axis were the same: this occurred at local wake Reynolds numbers, Re\sb{\rm w}10, which is defined in terms of centerline velocity and wake width. Within the turbulent wake region, turbulence intensities along the axis were roughly 85% for Re\sb{\rm w}70; however, as the onset of the laminar wake region was approached, turbulence intensities along the axis became proportional to Re\sb{\rm w}\sp{\rm 7/4}, which is consistent with scaling proposed some years ago by Batchelor and Townsend, and Philips, for the final decay period of axisymmetric turbulent wakes. The structure of sphere wakes were measured in the turbulent environments with streamwise integral scales much larger than the sphere diameter and Kolmogorov length scales less than the sphere diameter. At these conditions, some phenomena observed for sphere wakes at comparable Reynolds numbers in nonturbulent environments were suppressed; instead, while the wakes were turbulent, their mean streamwise velocities scaled like self-preserving laminar wakes but with enhanced viscosities due to turbulence. Effective turbulent viscosities were relatively independent of position and the ratios of integral length scales and Kolmogorov length scales to sphere diameters, however, they progressively increased with sphere Reynolds numbers. Low and high Reynolds number regimes were observed, separated by a transition regime involving sphere Reynolds numbers in the range of 300-600 which was associated with the onset of vortex shedding in the near wake region. Wake mixing properties within the laminar-like turbulent wake region were affected by both the sphere Reynolds number and the ambient turbulence intensity in the low Reynolds number regime; in contrast, wake mixing properties mainly varied with the sphere Reynolds number in the high Reynolds number regime where wake turbulence was dominated by wake generated turbulence. Observed Strouhal numbers associated with eddy shedding were generally lower than the values observed earlier in turbulence-free environments.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104222/1/9501066.pdfDescription of 9501066.pdf : Restricted to UM users only

A Simple Approach for Regenerating Electrolyzed Hydrogen Production Using Non-De-Ionized Water Sources

This research focuses on using natural renewable water resources, filters, and performance recovery systems to reduce the cost of generating pure hydrogen for Proton Exchange Membrane Fuel Cells (PEMFCs). This study uses de-ionized (DI) water, tap water, and river water from upstream as the water source. Water from these sources passes through 1 μm PP filters, activated carbon, and reverse osmosis for filtering. The filtered water then undergoes hydrogen production experiments for a duration of 6000 min. Performance recovery experiments follow directly after hydrogen production experiments. The hydrogen production experiments show the following: DI water yielded a hydrogen production rate of 27.13 mL/min; unfiltered tap water produced 15.41 mL/min; unfiltered upstream river water resulted in 10.03 mL/min; filtered tap water yielded 19.24 mL/min; and filtered upstream river water generated 18.54 mL/min. Performance recovery experiments conducted by passing DI water into PEMFCs for 15 min show that the hydrogen generation rate of tap water increased to 25.73 mL/min, and the rate of hydrogen generation of upstream river water increased to 22.58 mL/min. In terms of cost-effectiveness, under the same volume of hydrogen production (approximately 600 kg/year), using only DI water costs 1.8-times more than the cost of using filtered tap water in experiments

Development of a High-Power-Factor Power Supply for an Atmospheric-Pressure Plasma Jet

This paper presents the design and implementation of a miniaturized high-voltage power supply with power factor correction (PFC) for atmospheric-pressure plasma jet (APPJ) applications. The sinusoidal output frequency and voltage of the power supply can be controlled independently from 16 to 24 kHz and from 1 to 10 kVpeak, respectively. A helium APPJ load is used to assess the performance of the developed power supply. It is shown that the developed high-voltage power supply operates effectively, and the designed PFC converter improves the input current distortion of the power supply. Not only the power factor of the power supply is increased from 0.41 to 0.95, but it also provides a low-ripple DC voltage, which reduces the high-voltage ripple of the output from 730 to 50 Vp-p. In this paper, the proposed design integrates the PFC converter into the high-voltage power supply so that the developed power supply has better electrical characteristics and the overall power supply can be significantly miniaturized