Effects of pressure, oxygen concentration, and forced convection on flame spread rate of Plexiglas, Nylon and Teflon

Experiments were conducted in which the burning of cylindrical materials in a flowing oxidant stream was studied. Plexiglas, Nylon, and Teflon fuel specimens were oriented such that the flames spread along the surface in a direction opposed to flowing gas. Correlations of flame spread rate were obtained that were power law relations in terms of pressure, oxygen concentration, and gas velocity

Intensity measurements with a curved crystal gamma ray spectrometer

Having made several sets of intensity measurements we are now in a position to comment on the performance of the curved crystal spectrometers as instruments for measuring relative intensities of X -rays and soft X-rays. It must be admitted that the difficulties of making such measurements accurately have proved greater than was anticipated.In Chapter 3 it was seen that sensitivity calibration was in practice a different procedure, very liable to error on account of the "step by step" nature of the method adopted. This method was adopted because of the absence of suitable sources to provide a more reliable method. The comparison, in Chapter 5, of the RaD -ray with the RaE L X -rays showed that the curve obtained was, in fact, unreliable for this energy difference.However, the difficulties are not merely practical ones. Though the calibration curve may be unreliable for large energy differences it does show that the sensitivity of the spectrometers in their present form is rapidly varying in a non -uniform way over the energy region in which they are most useful. Thus good sensitivity -energy calibration is inherently difficult, quite apart from the practical difficulty of finding suitable calibrating lines.The problem, then, is to make the sensitivity more uniform over the range for which the present spectrometers are useful.We should like, in the first place, to see if this can be done without making any drastic alteration in the way in which they are used (i.e. with an extended source and photographic recording).The sensitivity has to be increased at both the low and high energy ends of the curve. The steep fall -off at low energies is to a certain extent due to decreasing sensitivity of the emulsion but the main cause is rapidly increasing absorption in the path between source and detector. This could be reduced in two ways. Firstly, by placing the whole apparatus in vacuum to eliminate ahsorption in air and, secondly, by using a thinner crystal. The reduction in crystal thickness must not be carried too far, however, since reflections from the whole crystal thickness contribute to the image. While reduction in intensity due to this may be less than the gain due to smaller absorption for low energies, the reverse is likely to be true for higher energies where absorption is unimportant. The crystals at present employed are 0.25 mm. thick, in which distance a line of energy 10 Rev is reduced by a factor of 7 by absorption, whereas one of 100 Rev is reduced by a factor of 1.01. If the crystal thickness is halved the absorption factors become 2.6 and 1.005. This means that for the 10 Rev line the nett gain in intensity would be 7/2.6 x 1/2 = 1.35 whereas the 100 Key would be reduced by half. One must, therefore, ensure that the thickness is not reduced by so much that the sensitivity gained at low energies is lost two - or three -fold at high energies. From the rough figures quoted it can be seen that the thickness should not be reduced by a factor of more than 2 if the instrument is to be used over the whole range of 0 - 100 Kev or so. Of course if higher energies are not required then a further reduction could be tolerated.Assuming the intensity of the image to be proportional to de⁻μᵈ where d = crystal thickness, and μ = total linear absorption coefficient of the crystal for a given energy, then the optimum thickness for that energy is 1/μ. The thickness must be chosen to give the most uniform response over the range required.The fall -off in sensitivity at high energies is due to decreasing reflecting power of the crystal planes and decreasing sensitivity of the emulsion. The reflecting power of the crystal cannot be increased except by using a different crystal but not much improvement is possible here. In any case if it were increased it would be increased for all energies and would not, therefore, produce more uniform sensitivity.The total reflection from the crystal could of course be increased by using a thicker crystal but here all the remarks about reducing thickness apply in reverse. This would however be profitable if the spectrometer were to be used in the range 50 - 100 Kev, say. Notice, however, that the optimum thickness for 100 Kev is about 1.8 cm. and, even supposing a good enough crystal could be obtained of such a thickness, it would be very difficult to bend it with the required precision.The sensitivity of the plate could be increased by using a thicker emulsion but for high energies the gain in sensitivity is approximately proportional to the increase in thickness, and as G5 emulsions several hundred microns thick are very difficult to process, not much can conveniently be gained in this way. In addition, background stain is increased so that contrast between a line and the background is not much improved. A more promising possibility is to increase the absorption of the emulsion by soaking it in a solution of a lead compound before exposing - or by using specially manufactured emulsion containing lead.To sum up, therefore, we can say that if a spectrometer is to be used for intensity measurements over its entire useful range (up to 120 Kev) then a little improvement can be expected by evacuating the apparatus and impregnating the emulsion with lead. If, however, we do not wish to cover the whole range at once, but could divide it into a few smaller ranges (even two, from 0 - 25 Key, and from 25 Kev upwards) and could use a different spectrometer for each energy band, then considerable improvement might be expected by a judicious choice of crystal thickness. Fo instance, the optimum thickness for 100 Kev mentioned above would give about 3C times the intensity of the crystal at present used.For energies above 120 Key the instruments would have to be used like Dumond's (1947) - with a line source on the focussing circle and a counter with suitable collimation on the other side of the crystal. The performance of such a spectrometer as regards precision intensity measurements is beyond the scope of the present research but it is quite evident that here again the sensitivity must vary rapidly - though presumably continuously decreasing for energies above 100 Kev or so

A Search for N2+ in Spectra of Comet C/2002C1 (Ikeya-Zhang)

We report low- and high-resolution spectra of comet C/2002 C1 (Ikeya-Zhang) from McDonald Observatory. The comet had a well-developed ion tail including CO+, CO2+, CH+, and H2O+. We used our high-resolution spectra to search for N2+. None was detected and we placed upper limits on N2+/CO+ of 5.4 times 10^{-4}. N2+ was detected in the low-resolution spectra but we show that this emission was probably telluric in origin (if cometary, we derive N2+/CO+ = 5.5 times 10^{-3}, still very low). We discuss the implications for the conditions in the early solar nebula of the non-detection of N2+. These depend on whether the H2O ice was deposited in the amorphous or crystalline form. If H2O was deposited in its crystalline form, the detection of CO+ but not N2+ has implications for H2O/H2 in the early solar nebula.Comment: Accepted for publication in ApJ (Letters) - 10 Sept 200

Photometry, spectrophotometry and polarimetry of comet P/Encke during fall of 1979

Broadband S-20, B and V magnitudes of P/Encke were obtained with the digital area photometer, using an Image Dissector Scanner (IDS) detector on the 2.7 m telescope at McDonald Observatory during August 1979. The notation V(S-20) is used for S-20 magnitudes transformed to V magnitudes. The variation in the V(S20) magnitudes (26, 5 minute integrations) on the best photometric night (21 August) was small and random indicating either a lack of rotational albedo variations or, more likely, a masking of the nucleus by the outburst activity. A spectrum covering the region from 3630 to 4900 A at a resolution of 5 A was obtained on 27 August with the IDS spectrograph. The spectrum was featureless, showing no emission at the CN or CO+ wavelengths

The CN Isotopic Ratios In Comets

Our aim is to determine the isotopic ratios (12)C/(13)C and (14)N/(15)N in a variety of comets and link these measurements to the formation and evolution of the solar system. The (12)C/(13)C and (14)N/(15)N isotopic ratios are measured for the CN radical by means of high-resolution optical spectra of the R branch of the B-X (0, 0) violet band. 23 comets from different dynamical classes have been observed, sometimes at various heliocentric and nucleocentric distances, in order to estimate possible variations of the isotopic ratios in parent molecules. The (12)C/(13)C and (14)N/(15)N isotopic ratios in CN are remarkably constant (average values of, respectively, 91.0 +/- 3.6 and 147.8 +/- 5.7) within our measurement errors, for all comets whatever their origin or heliocentric distance. While the carbon isotopic ratio does agree with the terrestrial value (89), the nitrogen ratio is a factor of two lower than the terrestrial value (272), indicating a fractionation in the early solar system, or in the protosolar nebula, common to all the comets of our sample. This points towards a common origin of the comets independently of their birthplaces, and a relationship between HCN and CN.NSFAstronom

Finite element models of wire rope for vibration analysis

The usefulness of wire rope in shock and vibration isolation is briefly reviewed and its modeling for the purpose of vibration analysis is addressed. A model of a nominally straight segment of wire rope is described in which the rope structure is represented by a maiden, or central, strand of wire with one (or more) strand(s) wrapped around it in a helix (helices). The individual strands are modeled using finite elements and MSC NASTRAN. Small linear segments of each wire are modeled mathematically by dividing them lengthwise into triangular prisms representing each prism by a solid NASTRAN element. To model pretensioning and allow for extraction of internal force information from the NASTRAN model, the wound strands are connected to the maiden strand and each other using spring (scalar elastic) elements. Mode shapes for a length of wire rope with one and fixed to a moving base and the other attached to a point mass, are presented. The use of the NASTRAN derived mode shapes to approximate internal normal forces in equations of motion for vibration analyses is considered

The McDonald Observatory Planet Search: New Long-Period Giant Planets, and Two Interacting Jupiters in the HD 155358 System

We present high-precision radial velocity (RV) observations of four solar-type (F7-G5) stars - HD 79498, HD 155358, HD 197037, and HD 220773 - taken as part of the McDonald Observatory Planet Search Program. For each of these stars, we see evidence of Keplerian motion caused by the presence of one or more gas giant planets in long-period orbits. We derive orbital parameters for each system, and note the properties (composition, activity, etc.) of the host stars. While we have previously announced the two-gas-giant HD 155358 system, we now report a shorter period for planet c. This new period is consistent with the planets being trapped in mutual 2:1 mean-motion resonance. We therefore perform an in-depth stability analysis, placing additional constraints on the orbital parameters of the planets. These results demonstrate the excellent long-term RV stability of the spectrometers on both the Harlan J. Smith 2.7 m telescope and the Hobby-Eberly telescope.Comment: 38 pages, 10 figures, 6 tables. Accepted for publication in Ap

Failure mechanisms of graphene under tension

Recent experiments established pure graphene as the strongest material known to mankind, further invigorating the question of how graphene fails. Using density functional theory, we reveal the mechanisms of mechanical failure of pure graphene under a generic state of tension. One failure mechanism is a novel soft-mode phonon instability of the $K_1$-mode, whereby the graphene sheet undergoes a phase transition and is driven towards isolated benzene rings resulting in a reduction of strength. The other is the usual elastic instability corresponding to a maximum in the stress-strain curve. Our results indicate that finite wave vector soft modes can be the key factor in limiting the strength of monolayer materials

Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes

From millimeter and optical observations of the Jupiter-family comet 17P/Holmes performed soon after its huge outburst of October 24, 2007, we derive 14 N/15N = 139 +/- 26 in HCN, and 14N/15N = 165 +/- 40 in CN, establishing that HCN has the same non-terrestrial isotopic composition as CN. The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp) after a reanalysis of previously published measurements. These results are compatible with HCN being the prime parent of CN in cometary atmospheres. The 15N excess relative to the Earth atmospheric value indicates that N-bearing volatiles in the solar nebula underwent important N isotopic fractionation at some stage of Solar System formation. HCN molecules never isotopically equilibrated with the main nitrogen reservoir in the solar nebula before being incorporated in Oort-cloud and Kuiper-belt comets. The 12C/13C ratios in HCN and CN are measured to be consistent with the terrestrial value.Comment: Accepted for publication in the Astrophysical Journal (Letters) 4 page

Quantifying prosthetic and intact limb use in upper limb amputees via egocentric video: an unsupervised, at-home study

Analysis of the manipulation strategies employed by upper-limb prosthetic device users can provide valuable insights into the shortcomings of current prosthetic technology or therapeutic interventions. Typically, this problem has been approached with survey or lab-based studies, whose prehensile-grasp-focused results do not necessarily give accurate representations of daily activity. In this work, we capture prosthesis-user behavior in the unstructured and familiar environments of the participants own homes. Compact head-mounted video cameras recorded ego-centric views of the hands during self-selected household chores. Over 60 hours of video was recorded from 8 persons with unilateral amputation or limb difference (6 transradial, 1 transhumeral, 1 shoulder). Of this, almost 16 hours of video data was analyzed by human experts using the 22-category ‘TULIP’ custom manipulation taxonomy, producing the type and duration of over 27,000 prehensile and non-prehensile manipulation tags on both upper limbs, permitting a level of objective analysis not previously possible with this population. Our analysis included unique observations on non-prehensile manipulations occurrence, determining that 79% of transradial body-powered device manipulations were non-prehensile, compared to 60% for transradial myoelectric devices. Conversely, only 16-19% of intact limb activity was non-prehensile. Additionally, multi-grasp terminal devices did not lead to increased activity compared to 1DOF devices