On the acoustic boundary condition in the presence of flow

The boundary condition on the acoustic perturbation velocity at an impermeable surface in a flow is considered for the cases in which the surface generates a sound field by vibration or is acoustically deformed by an incident sound field. It is shown that in general the condition is not equivalent to the requirement of continuity of acoustic particle displacement in the direction normal to the unperturbed surface.     

Additional information from the measurements by Hushfar,Rogers and Stair of the infrared chemiluminescence from the reaction N+O2 → NO+O

Hushfar et al. measured the chemiluminescent first-overtone radiation at 2.7 μm from the reaction N+O₂ → NO+O, which is followed by the fast quenching reaction N+O → N₂+O. From the rate of overtone photons per O₂ molecule and the spectrum approximated by a Boltzmann distribution, they computed ?₂, the number of overtone photons per NO-forming reaction in the low-density limit when no quenching occurs. We show ?₂ is essentially independent of the spectrum, infer the limits on the fundamental-band photon efficiency, ?₁, and obtain the initial population distribution of NO(2 ? χ ? 6).     

Investigation of a coaxial boosted-output hollow-cathode lamp for atomic absorption

A version of the boosted-output, hollow-cathode lamp has been studied as a possible light source for atomic absorption. Our version of this lamp utilizes a secondary electrical discharge to enhance the emission. passing axially through the center of an open cylindrical cathode. Eventually the increase in intensity is accompanied by a decrease in the absorbance of test solutions due to line-broadening in the lamp. Using the criterion of equivalent absorbance, lamps of this design provided ten to twenty times the intensity of commercial hollow-cathode lamps for the elements studied: aluminum, molybdenum, titanium vanadium and copper.     

Gravitational contribution to the Casimir energy in Kaluza-Klein theories

The Casimir energy of the gravitational field in Kaluza-Klein theories is investigated. The mathematical techniques needed to compute the contribution of a single graviton loop to the quantum effective potential on a background manifold of (Minkowski space) ? (N-sphere) are developed. In these computations the cosmological constant plays a dynamical role, acting like a mass for the graviton. The numerical work for the case N = 1 is done explicitly, and a solution to the one-loop corrected equations of motion is found, although it is not stable. The possibility of an imaginary part to the effective potential for N > 1 is noted, and its existence is attributed to tachyonic terms in the mode sum.     

Yielding of glassy polymers in the second quadrant of principal stress space

Second quadrant crazing and shear yielding studies were performed on glassy poly(methyl methacrylate) by means of combined torsion-tension loading. The results are in quantitative agreement with the shear and normal stress yielding criteria proposed by Sternstein and Ongchin. It is shown that four distinct regions of material response exist in the second quadrant and, depending on the stress state, 1) no crazing and no shear yielding, 2) crazing alone, 3) shear yielding alone, or 4) crazing and shear yielding can occur. An analysis of stress field induced brittle-ductile transitions is presented which is in agreement with other studies of high-pressure yielding.     

Compton scattering from deuterium and the polarizabilities of the neutron

A new program of Compton scattering on deuterium is under way at the tagged-photon facility at MAX-Lab in Lund, Sweden. We will measure differential cross sections between 60° and 150° over the photon energy range 60–115?MeV in 5?MeV steps, with the ultimate goal of obtaining new precision information on the electric and magnetic polarizabilities of the neutron.     

Characterization of high intensity focused ultrasound transducers using acoustic streaming

A new approach for characterizing high intensity focused ultrasound (HIFU) transducers is presented. The technique is based upon the acoustic streaming field generated by absorption of the HIFU beam in a liquid medium. The streaming field is quantified using digital particle image velocimetry, and a numerical algorithm is employed to compute the acoustic intensity field giving rise to the observed streaming field. The method as presented here is applicable to moderate intensity regimes, above the intensities which may be damaging to conventional hydrophones, but below the levels where nonlinear propagation effects are appreciable. Intensity fields and acoustic powers predicted using the streaming method were found to agree within 10% with measurements obtained using hydrophones and radiation force balances. Besides acoustic intensity fields, the streaming technique may be used to determine other important HIFU parameters, such as beam tilt angle or absorption of the propagation medium.     

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