Periodic fluid transients in rectangular ducts with transverse magnetic fields,II

Summary This paper treats periodic fluid transients in liquid metals contained in constant-area rectangular ducts with uniform, transverse, applied magnetic fields and with insulating walls parallel to the magnetic field and perfectly conducting walls perpendicular to it. The magnetic Reynolds and Mach numbers are assumed to be small, and the Reynolds number is assumed to be large. First, asymptotic solutions are obtained for small values of the interaction parameter and then approximate solutions are obtained for arbitrary values of the interaction parameter using a Galerkin method with modified Fourier series.

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Résumé Cet article traite des ondes de compression périodiques dans un métal liquide contenu dans une conduite rectangulaire à section rectangulaire constante, à laquelle un champ magnétique transverse et uniforme est appliqué et dont deux parois sont isolantes et parallèles au champ magnétique, les deux autres étant parfaitement conductrices. On admet que le nombre magnétique de Reynolds et le nombre de Mach sont petits et que le nombre de Reynolds est grand. Quand le nombre d'intéraction est petit, les solutions seront asymptotiques et quand le nombre d'intéraction est arbitrarire, on trouvera les solutions approximatives par la méthode de Galerkin avec les series modifiées de Fourier.

     

Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask

We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.     

Ultrahigh-resolution plastic graded-index fused image plates

Fiber-optic imaging systems such as the medical endoscope, the boroscope, the fused-image faceplate, and the image conduit are now made from glass step-index (SI) fibers, and the image resolution of the SI fiber-optic imaging systems is limited to ~5 microm. Ultrahigh-resolution fiber-optic fused-image plates with fiber diameter sizes of 5 and 2.8 microm were fabricated with plastic graded-index (GRIN) fibers. The measured image resolutions of the 5-microm SI and GRIN-based guides were comparable, and the resolution of the plastic GRIN image guides improved as the fiber diameter was reduced from 5 to 2.8 microm.     

Certain yawed magnetogasdynamic flows

Certain steady yawed magnetogasdynamic flows, in which the magnetic field is everywhere parallel to the velocity field, are related to certain reduced three-dimensional compressible gas flows having zero magnetic field. Under a restriction, the reduced flows are linked, by certain reciprocal relations, to a four parameter class of plane gas flows. In the instance of constant entropy an approximation method is suggested for obtaining magnetogasdynamic flows from the corresponding plane, irrotational gasdynamic flows and examples are given.

Nomenclature

magnetogasdynamic flow variables H magnetic intensity - q fluid velocity - fluid density - p pressure - s entropy - Q _t, H _t component of q, H in the x–y plane - w , h component of q, H perpendicular to the x–y plane reduced gasdynamic flow factor of proportionality - q* fluid velocity - * fluid density - p* pressure - Q _t ^* =u*î+v*, w* components of q* - l arbitrary constant - A _v Alfvén speed - Q _t, , p fluid velocity, density, pressure of the reciprocal gas dynamic flow - L, n, k, arbitrary constants - , velocity potential, stream function - angle made by Q _t, Q _t ^* , and V with the x-axis - adiabatic gas constant - a ²=(–1)/2 constant - M Mach number - W constant value of w* - E approximate constant value of g(p) - * modified potential function - modified velocity coordinate - +i - complex potential of the irrotational flow - B arbitrary constant - V incompressible flow velocity - V modified fluid velocity - X _p, Y _p points on the profile     

A model for adiabatic supersonic turbulent boundary layers

An asymptotic analysis of the equations describing supersonic turbulent flow over an adiabatic wall is carried out for high Reynolds numbers, Re, and mainstream Mach numbers, M _e=O(1). A general expression for the adiabatic-wall temperature is derived. The asymptotic theory constrains the types of turbulence models that are suitable to represent the effects of viscous dissipation. A simple algebraic turbulence model is proposed and comparisons with measured total enthalpy profile data show good agreement, capturing the overshoot observed in total enthalpy near the boundarylayer edge.This work was supported by NASA Langley Research Center under Grant NAG-1-832 and the Air Force Office of Scientific Research under Grants AFOSR-91-0069 and F49620-93-0130; Dr. Ruban was supported by a grant from United Technologies Corporation.     

The concentration field in a turbulent channel flow with polymer injection at the wall

Instantaneous concentration profiles have been measured in turbulent water channel flows at 5 axial locations immediately downstream of a line, wall injection of a dyed 700 ppm polymer solution and for comparison, dyed water. Concentration was deduced from a line of fluoresced radiation that was stimulated by a laser beam directed through the dyed injectant and normal to the channel wall. Both statistical and time-resolved results show how the turbulent mixing is modified and damped when the injectant is a polymer solution. A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988     

The hydrochloride and hydrobromide salt forms of (S)‐amphetamine

Despite the high profile of amphetamine, there have been relatively few structural studies of its salt forms. The lack of any halide salt forms is surprising as the typical synthetic route for amphetamine initially produces the chloride salt. (S)‐Amphetamine hydrochloride [systematic name: (2S)‐1‐phenylpropan‐2‐aminium chloride], C₉H₁₄N⁺·Cl⁻, has a Z′ = 6 structure with six independent cation–anion pairs. That these are indeed crystallographically independent is supported by different packing orientations of the cations and by the observation of a wide range of cation conformations generated by rotation about the phenyl–CH₂ bond. The supramolecular contacts about the anions also differ, such that both a wide variation in the geometry of the three N—H...Cl hydrogen bonds formed by each chloride anion and differences in C—H...Cl contacts are apparent. (S)‐Amphetamine hydrobromide [systematic name: (2S)‐1‐phenylpropan‐2‐aminium bromide], C₉H₁₄N⁺·Br⁻, is broadly similar to the hydrochloride in terms of cation conformation, the existence of three N—H...X hydrogen‐bond contacts per anion and the overall two‐dimensional hydrogen‐bonded sheet motif. However, only the chloride structure features organic bilayers and Z′ > 1.     

Biocompatible Infinite‐Coordination‐Polymer Nanoparticle–Nucleic‐Acid Conjugates for Antisense Gene Regulation

Herein, we report the synthesis of DNA‐functionalized infinite‐coordination‐polymer (ICP) nanoparticles as biocompatible gene‐regulation agents. ICP nanoparticles were synthesized from ferric nitrate and a ditopic 3‐hydroxy‐4‐pyridinone (HOPO) ligand bearing a pendant azide. Addition of Fe^III to a solution of the ligand produced nanoparticles, which were colloidally unstable in the presence of salts. Conjugation of DNA to the Fe^III–HOPO ICP particles by copper‐free click chemistry afforded colloidally stable nucleic‐acid nanoconstructs. The DNA–ICP particles, when cross‐linked through sequence‐specific hybridization, exhibited narrow, highly cooperative melting transitions consistent with dense DNA surface loading. The ability of the DNA–ICP particles to enter cells and alter protein expression was also evaluated. Our results indicate that these novel particles carry nucleic acids into mammalian cells without the need for transfection agents and are capable of efficient gene knockdown.     

Scale-free distribution of avian influenza outbreaks

Using global case data for the period from 25 November 2003 to 10 March 2007, we construct a network of plausible transmission pathways for the spread of avian influenza among domestic and wild birds. The network structure we obtain is complex and exhibits scale-free (although not necessarily small-world) properties. Communities within this network are connected with a distribution of links with infinite variance. Hence, the disease transmission model does not exhibit a threshold and so the infection will continue to propagate even with very low transmissibility. Consequentially, eradication with methods applicable to locally homogeneous populations is not possible. Any control measure needs to focus explicitly on the hubs within this network structure.