Application of the opto-acoustic effect to the operation of optically pumped far-infrared gas lasers

A method is reported which allows the operation of optically pumped farinfrared (FIR) gas lasers in a simple way: A spectrophone is used to control and to stabilize the pump laser frequency in order to achieve most efficient optical pumping of the FIR laser gas.     

Polarization of neutrons from the reaction H(d, n)He

The angular dependence of the polarization of neutrons from the reaction ³H(d, n)⁴He was measured at deuteron energies of 3.35, 4.35 and 5.35 MeV for reaction angles up to 165° lab. Polarization values were derived from the left-right asymmetry in n- scattering.     

CAN: an example of nonclassical acoustic nonlinearity in solids

A new class of nonlinear acoustic phenomena has been observed for acoustic wave interaction with simulated and realistic nonbonded contact interfaces (cracked defects) in solids. "Nonclassical" effects are due to substantially asymmetric stiffness characteristics of the interface for normal stress that results in specific contact acoustic nonlinearity (CAN). The asymmetry in the contact restoring forces causes the stiffness parametric modulation and instability of oscillations, which results in acoustic wave fractional subharmonic generation. The CAN subharmonics and higher harmonics reveal threshold dynamic behaviour, evident hysteresis, and instability effects.     

Air-coupled ultrasound inspection of various materials

Conventional ultrasound inspection, a standard non-destructive testing method, uses a coupling medium (e.g. water) because of impedance mismatch. This liquid contact is a drawback because it prevents inspection of many materials. There is a need, then, for air-coupled ultrasound testing, which is now feasible because of low impedance focused narrow band transducers and sensitive electronics, both of which improve the signal-to-noise ratio. We present results obtained on fibre-reinforced plastics, water sensitive materials (e.g. reinforced ceramics), and "shape adaptive" structures to reveal delaminations, impacts, and growth of internal defects. Actuators embedded in "adaptive" structures are used as transmitters while the receiver records the signals. Thus it is possible to image defect areas and non-linear behaviour of potential defects.     

Numerical analysis of secondary and tertiary states of fluid flow and their stability properties

In fluid systems exhibiting a gradual transition to a turbulent state in dependence on external parameters the increasing complexity of motion is usually caused by sequences of bifurcations in the solution space. Through the consideration of the configuration of highest symmetry compatible with the basic physical problem most bifurcations can be identified by their symmetry breaking properties. Because it can incorporate all available symmetries in the selection of expansion functions, the Galerkin method turns out to be especially useful for the numerical analysis of highly symmetric fluid flow and their instabilities. By allowing for the broken symmetries, the nonlinear evolution of the flow can be followed numerically through several bifurcations. A complete stability analysis with respect to arbitrary infinitesimal disturbances is possible on the basis of Floquet's theory. This method has been applied in cases of Taylor-Couette flow, Rayleigh-Bénard convection, and shear flows with cubic profiles.     

A review of time-domain impedance modelling and applications

Time-domain impedance boundary conditions are reviewed and similarities between the models are shown. The extended Helmholtz resonator model is used with a time-domain impedance boundary condition for impedance eduction and simulation of the noise radiation from a lined aeroengine nozzle. Model parameters obtained by impedance eduction from flow duct measurements and then used to predict the noise radiation for realistic aeroengine nozzle flow and geometry. The prediction of sound radiation from a coaxial jet nozzle at approach conditions with the educed model parameters is performed in 2D and 3D. Various features of the numerical method that enable it to be used for realistic applications are presented. This includes the suppression of flow instabilities in the linear inviscid solution and analysis of the solution to prove the global conservation of acoustic energy in each individual result. It is found that the current approach is too time consuming for optimisation, even for a 2D simulation using parallel processing. Thus, finally the potential of porting a CAA method to the graphics processing unit (GPU) is shown in one example. The GPU based computation is about 100 times faster. The results are encouraging, even though the GPU variant needs further validation.     

He4-He4 interaction potential from low energy total cross section measurements

Total relative and absolute cross sections for He⁴-He⁴ have been measured between 110 and 1100 m/s velocity of the primary beam at scattering chamber temperatures of 5.0 and 77 K. Our measurements are sensitive to the low energy repulsive and the attractive part of the potential. The resulting potential is able to describe scattering measurements of other authors; it is also in very good agreement with ab initio calculations of Bertoncini and Wahl.     

Synthesis and Structural Characterization of Divalent Transition Metal Alkynylamidinate Complexes

A series of new alkynylamidinate complexes of selected first and second row transition metals has been synthesized and fully characterized. Treatment of MCl₂ precursors (M=Mn, Fe, Co) with 2 equiv. of the lithium alkynylamidinates Li[c-C₃H₅−C≡C−C(NR′)₂] ⋅ THF (R′=ⁱPr (2), Cy (cyclohexyl) ( 2 )) afforded a series of binuclear complexes of the type M₂[c-C₃H₅−C≡C−C(NR)₂-κN:κN′]₂[c-C₃H₅−C≡C−C(NR)₂-κ²N,N′]₂ ( 3 : M=Mn, R=Cy; 4 a : M=Fe, R=ⁱPr; 4 b : M=Fe, R=Cy; 5 : M=Co, R=ⁱPr) with no significant metal-metal bonding. In marked contrast, a similar reaction of CrCl₂ with 2 equiv. of 1 afforded the homoleptic dinuclear chromium(II) complex Cr₂[c-C₃H₅−C≡C−C(NⁱPr)₂-κN:κN′]₄ ( 6 ) which supposedly comprises a Cr−Cr quadruple bond. Complex 6 could also be prepared in a more rational way and in better yield (61 %) by using dichromium(II) tetraacetate, Cr₂(OAc)₄, as starting material. Related reactions employing dimolybdenum(II) tetraacetate, Mo₂(OAc)₄, and 2 or 3 equiv. of 1 afforded the mixed-ligand paddle wheel-type complexes trans-Mo₂(OAc-κO:κO′)₂([c-C₃H₅−C≡C−C(NⁱPr)₂-κN:κN′]₂ ( 7 ) and Mo₂(OAc-κO:κO′)([c-C₃H₅−C≡C−C(NⁱPr)₂-κN:κN′]₃ ( 8 ). All title compounds were structurally characterized through single-crystal X-ray diffraction and spectroscopic techniques (NMR, IR, Raman).