Rotational Instabilities and Centrifugal Hangup

One interesting class of gravitational radiation sources includes rapidly rotating astrophysical objects that encounter dynamical instabilities. We have carried out a set of simulations of rotationally induced instabilities in differentially rotating polytropes. An $n$=1.5 polytrope with the Maclaurin rotation law will encounter the $m$=2 bar instability at $T/|W| \gtrsim 0.27$. Our results indicate that the remnant of this instability is a persistent bar-like structure that emits a long-lived gravitational radiation signal. Furthermore, dynamical instability is shown to occur in $n$=3.33 polytropes with the $j$-constant rotation law at $T/|W| \gtrsim 0.14$. In this case, the dominant mode of instability is $m$=1. Such instability may allow a centrifugally-hung core to begin collapsing to neutron star densities on a dynamical timescale. If it occurs in a supermassive star, it may produce gravitational radiation detectable by LISA.Comment: 13 pages (includes 11 figures) and 1 separate jpeg figure; to appear in Astrophysical Sources of Gravitational Radiation, AIP conference proceedings, edited by Joan M. Centrell

Assessment of the operating characteristics of the SSME LOX turbopump pump-end bearing

A bearing/shaft model of the SSME LOX turbopump was developed using the SHABERTH bearing/shaft math modeling computer code. A previously developed bearing/shaft thermal model of the SSME LOX turbopump turbine and bearing was used in conjunction with SHABERTH to evaluate the thermomechanical operating characteristics of the LOX turbopump end bearings. Results show that for the two unmounted diametrical clearances evaluated (4.0 mils and 6.3 mils), the inboard pump end bearing supports about 81% of the isolator load for the small clearance and 77% of the isolator load for the larger clearance. Bearing clearance changes due to thermal effects were 40% for the 4.0 mil diametrical clearance case and 19% for the 6.3 mil clearance case evaluated. The thermal analysis included evaluation of bearing temperatures for a subcooled case and a saturated case. Results indicate that no drastic temperature change occurred between the two cases. Since the rolling element and race surfaces of the subcooled case were at temperatures sufficiently high enough to be vapor blanketed, exceeding saturation temperature at the bearing inlet did not increase surface temperatures greatly

Advanced rocket engine cryogenic turbopump bearing thermal model

A lumped node thermal model was developed representing the Space Shuttle Main Engine (SSME) liquid oxygen (LOX) turbopump turbine end bearings operating in a cryogenically cooled bearing tester. Bearing elements, shaft, carrier, housing, cryogen flow characteristics, friction heat, and fluid viscous energy are included in the model. Heat transfer characteristics for the regimes of forced convection boiling are modeled for liquid oxygen (LOX) and liquid nitrogen (LN2). Large temperature differences between the cryogenic fluid and baring contact surfaces require detailed nodal representation in these areas. Internal loads and friction heat are affected by temperature dependent operating clearances requiring iterations between bearing thermal and mechanical models. Analyses indicate a thermal-mechanical coupling resulting in reduced operating clearances, increased loading and heating which can contribute to premature bearing failure. Contact surfaces operate at temperatures above local saturation resulting in vapor rather than liquid in the contacts, precluding possible liquid film lubrication. Elevated temperatures can reduce lubrication, increase friction, and reduce surface hardness supporting a surface failure mode rather than subsurface fatigue

Ground vibration during the bentonite tunnelling process

The research was carried out during the bentonite shield tunnel drive for the Acton Grange trunk outfall sewer at Warrington, Cheshire. This tunnel is driven through cohesionless Drift deposits beneath a built-up urban environment, with a cover of less than 6m. The environmental effects of the ground vibration caused by the excavation process are investigated with particular regard to ground settlement by compaction. The geology of the area and the technical and commercial factors which led to the choice of the bentonite tunnelling system are described. Previous work on compaction by vibration is critically reviewed and methods to assess a soil's potential for compaction are given. The vibration instrumentation is described and relevant wave propagation theory is developed with emphasis on body waves from underground sources. Vibration data were recorded from transducers located in boreholes, on the pavement surface, on the tunnelling machine and on the concrete tunnel lining. These records were processed to characterise the vibrations in terms of peak particle velocities, frequency spectra and spatial attenuation.The maximum measured ground vibration (expressed in terms of resultant peak particle velocity) was 3.90 mm/s. The vibration was characterised by random high velocity particle motions resulting from impacts between the machine's disc cutters and glacial boulders in the tunnel face. Surface and subsurface settlement measurements were made along the tunnel line and structural damage to property above the tunnel was observed. Laboratory tests and other field data showed that the ground in this area was likely to settle at levels of vibration lower than those measured from the tunnelling machine. The vibration caused by the excavation process caused ground compaction which contributed to ground settlement and the ensuing damage to the overlying structures. The vibration was not likely to have damaged these properties directly but did cause considerable nuisance to the residents

Gravitational Radiation From and Instabilities in Compact Stars and Compact Binary Systems.

We have examined three types of compact astrophysical systems that are possible sources of detectable gravitational wave radiation (GWR): nonaxisymmetric pulsars; rapidly rotating compact stars undergoing the bar-mode instability; and coalescing compact binaries. Our analysis of nonaxisymmetric pulsars, based on the assumption that any equatorial asymmetries present in these objects were rotationally induced, indicates that nearby millisecond pulsars are generally better candidates for the detection of GWR than the Crab pulsar, which has been the object of an ongoing search for GWR (Tsubono 1991). Our finite difference hydrodynamics (FDH) simulation of an object encountering the rotationally induced bar-mode instability results in an ellipsoidal final configuration which, although gradually becoming more axisymmetric, persists for several orbits, continuously emitting GWR. We also have examined the stability and coalescence of equal mass binaries with polytropic, white dwarf (WD), and neutron star (NS) equations of state (EOS). In order for our explicit FDH code to be able to follow the coalescence of a binary system, it must proceed on a dynamical timescale. Hence, we began our investigation by performing FDH tests of the dynamical stability of individual models constructed along equilibrium sequences of binaries with the same total mass M\sb{T} and EOS but decreasing separation, in order to determine if any models on these sequences were unstable to merger on a dynamical timescale. Our simulations indicate that no points of instability exist on the WD EOS sequences with M\sb{T} =.500 M\sb{\odot} and 2.03 M\sb\odot or on the polytropic EOS sequences with polytropic indices n = 1.5 and 1.0. However, binary models on the n = 0.5 polytropic sequence and on two realistic NS EOS sequences were dynamically unstable to merger. Again using our FDH code, we followed the evolution of the binary with the minimum total energy and angular momentum on the n = 0.5 sequence through coalescence. At the end of the simulation, the ellipsoidal central object is encircled by spiral arms, ejected from the system during the merger, that have wrapped around on themselves and is continuing to emit low amplitude GWR

Optical carrier wave shocking: detection and dispersion

Carrier wave shocking is studied using the Pseudo-Spectral Spatial Domain (PSSD) technique. We describe the shock detection diagnostics necessary for this numerical study, and verify them against theoretical shocking predictions for the dispersionless case. These predictions show Carrier Envelope Phase (CEP) and pulse bandwidth sensitivity in the single-cycle regime. The flexible dispersion management offered by PSSD enables us to independently control the linear and nonlinear dispersion. Customized dispersion profiles allow us to analyze the development of both carrier self-steepening and shocks. The results exhibit a marked asymmetry between normal and anomalous dispersion, both in the limits of the shocking regime and in the (near) shocked pulse waveforms. Combining these insights, we offer some suggestions on how carrier shocking (or at least extreme self-steepening) might be realised experimentally.Comment: 9 page

Bearing tester data compilation, analysis and reporting and bearing math modeling, volume 1

Thermal and mechanical models of high speed angular contact ball bearings operating in LOX and LN2 were developed and verified with limited test data in an effort to further understand the parameters that determine or effect the SSME turbopump bearing operational characteristics and service life. The SHABERTH bearing analysis program which was adapted to evaluate shaft bearing systems in cryogenics is not capable of accommodating varying thermal properties and two phase flow. A bearing model with this capability was developed using the SINDA thermal analyzer. Iteration between the SHABERTH and the SINDA models enable the establishment of preliminary bounds for stable operation in LN2. These limits were established in terms of fluid flow, fluid inlet temperature, and axial load for a shaft speed of 30,000 RPM

3-Dimensional Core-Collapse

In this paper, we present the results of 3-dimensional collapse simulations of rotating stars for a range of stellar progenitors. We find that for the fastest spinning stars, rotation does indeed modify the convection above the proto-neutron star, but it is not fast enough to cause core fragmentation. Similarly, although strong magnetic fields can be produced once the proto-neutron star cools and contracts, the proto-neutron star is not spinning fast enough to generate strong magnetic fields quickly after collapse and, for our simulations, magnetic fields will not dominate the supernova explosion mechanism. Even so, the resulting pulsars for our fastest rotating models may emit enough energy to dominate the total explosion energy of the supernova. However, more recent stellar models predict rotation rates that are much too slow to affect the explosion, but these models are not sophisticated enough to determine whether the most recent, or past, stellar rotation rates are most likely. Thus, we must rely upon observational constraints to determine the true rotation rates of stellar cores just before collapse. We conclude with a discussion of the possible constraints on stellar rotation which we can derive from core-collapse supernovae.Comment: 34 pages (5 of 17 figures missing), For full paper, goto http://qso.lanl.gov/~clf/papers/rot.ps.gz accepted by Ap