Excitation Mechanisms of the Nitrogen First‐Positive and First‐Negative Radiation at High Temperature

The kinetic mechanisms responsible for the excitation of the first-positive and first-negative emission of nitrogen have been investigated in a re-examination of previously reported shock-tube measurements of the nonequilibrium radiation for these systems. The rate coefficients of the collisional quenching reactions, N_2(A^(3)Σ^(+)_u)(^(k^(N)_(-2))⇒) N_2(X^(1)Σ^(+)_g) + N(^(4)S) and N^(+)_2(B^(2)Σ^(+)_u) + N_2(X^(1)Σ^(+)_g)(^(k^(N)_(q))⇒) N^(+)_2(X^(2)Σ^(+)_g) or N^(+)_2(A(^(2)II_u)+N_2(X^(1)Σ^(+)_g) were found to be given by the empirical expressions, k_(2^(N))=5.1x10^(-3)T^(-2.23) cm^3 sec^(-1) and k_(q^(N_2))=1.9x10^(-2)T^(-2.33)cm^3 sec^(-1), respectively, over the approximate temperature range 6000-14 000°K

Solar Energy Resource Potential in Alaska

Solar energy applications are receiving attention in Alaska as in much of the rest of the country. Solar energy possibilities for Alaska include domestic water heating, hot-water or hot-air collection for space heating, and the use of passive solar heating in residential or commercial buildings. As a first analysis, this study concentrated on applying solar energy to domestic hot-water heating needs (not space heating) in Alaska, and an analysis of solar hot-water heating economics was performed using the F-CHART solar energy simulation computer program. Results indicate that solar energy cannot compete economically with oil-heated domestic hot water at any of the five study locations in Alaska, but that it may be economical in comparison with electrically heated hot water if solar collector systems can be purchased and installed for $20 to$25 per square foot.This work was made possible by a grant from the Solar Planning Office, West, 3333 Quebec, Denver, Colorado. It was performed as the Alaskan response to a western regional solar energy planning grant from the U. S. Department of Energy. The authors wish to acknowledge the support and cooperation of the Alaska State Department of Commerce, Division of Energy and Power Development, through whose efforts the grant was made available, especially Clarissa Quinlan, Grant Peterson, and Don Markle

Hydrodynamic Simulations of the Bardeen-Petterson Effect

We present SPH simulations of accretion discs in orbit about rotating compact objects such as black holes and neutron stars, and study the structure of warped discs produced by the Bardeen-Petterson effect. We calculate the transition radius out to which the disc specific angular momentum vector is aligned with that of the black hole. We focus on the parameter regime where the warp dynamics are controlled by bending wave propagation, but also consider models in which warps are subject to diffusion rather than wave transport, and are able to consider the fully nonlinear regime. Because of hydrodynamic or pressure effects, for the parameter range investigated, the transition radius is always found to be much smaller than that obtained by Bardeen & Petterson (1975). For discs with midplane Mach numbers of about 10, the transition occurs between 10 - 16 gravitational radii, whereas for a Mach number of about 30 it occurs at around 30 gravitational radii. A thicker disc with a Mach number of 5 is found to produce no discernible warped structure. The rate of black hole - disc alignment is found to be consistent with the ideas of Ress (1978), with the alignment torque behaving as if it arises from the accreted material transferring its misaligned component of angular momentum at the larger transition radius of Bardeen & Petterson (1975). The inclusion of Einstein precession in the calculations modified both the warped disc structure and, consistent with linear analysis, produced an increased alignment rate by up to a factor of 4 because of the effect that a non Keplerian potential has on the propagation of warps.Comment: 18 pages, 14 figures. Accepted for publication in M.N.R.A.S. A version with posctcript figures included can be obtained from http://www.maths.qmw.ac.uk/~rp

Deformation and fracture of thin sheet aluminum-lithium alloys: The effect of cryogenic temperatures

The objective is to characterize the fracture behavior and to define the fracture mechanisms for new Al-Li-Cu alloys, with emphasis on the role of indium additions and cryogenic temperatures. Three alloys were investigated in rolled product form: 2090 baseline and 2090 + indium produced by Reynolds Metals, and commercial AA 2090-T81 produced by Alcoa. The experimental 2090 + In alloy exhibited increases in hardness and ultimate strength, but no change in tensile yield strength, compared to the baseline 2090 composition in the unstretched T6 condition. The reason for this behavior is not understood. Based on hardness and preliminary Kahn Tear fracture experiments, a nominally peak-aged condition was employed for detailed fracture studies. Crack initiation and growth fracture toughness were examined as a function of stress state and microstructure using J(delta a) methods applied to precracked compact tension specimens in the LT orientation. To date, J(delta a) experiments have been limited to 23 C. Alcoa 2090-T81 exhibited the highest toughness regardless of stress state. Fracture was accompanied by extensive delamination associated with high angle grain boundaries normal to the fatigue precrack surface and progressed microscopically by a transgranular shear mechanism. In contrast the two peak-aged Reynolds alloys had lower toughness and fracture was intersubgranular without substantial delamination. The influences of cryogenic temperature, microstructure, boundary precipitate structure, and deformation mode in governing the competing fracture mechanisms will be determined in future experiments. Results contribute to the development of predictive micromechanical models for fracture modes in Al-Li alloys, and to fracture resistant materials

A geometric and structural approach to the analysis and design of biological circuit dynamics: a theory tailored for synthetic biology

Much of the progress in developing our ability to successfully design genetic circuits with predictable dynamics has followed the strategy of molding biological systems to fit into conceptual frameworks used in other disciplines, most notably the engineering sciences. Because biological systems have fundamental differences from systems in these other disciplines, this approach is challenging and the insights obtained from such analyses are often not framed in a biologically-intuitive way. Here, we present a new theoretical framework for analyzing the dynamics of genetic circuits that is tailored towards the unique properties associated with biological systems and experiments. Our framework approximates a complex circuit as a set of simpler circuits, which the system can transition between by saturating its various internal components. These approximations are connected to the intrinsic structure of the system, so this representation allows the analysis of dynamics which emerge solely from the system's structure. Using our framework, we analyze the presence of structural bistability in a leaky autoactivation motif and the presence of structural oscillations in the Repressilator

An Exact, Three-Dimensional, Time-Dependent Wave Solution in Local Keplerian Flow

We present an exact three-dimensional wave solution to the shearing sheet equations of motion. The existence of this solution argues against transient amplification as a route to turbulence in unmagnetized disks. Moreover, because the solution covers an extensive dynamical range in wavenumber space, it is an excellent test of the dissipative properties of numerical codes.Comment: 22 pages, 4 figures. To appear Apj Dec 1 200