Effects of small changes in initial conditions on mixing layer three-dimensionality

Conclusions An experimental study which shows the effects of relatively small changes in the initial conditions on the development of the three-dimensional structure of a plane mixing layer originating from laminar boundary layers has been completed. It was shown that while the exact shapes and positions of the streamwise vortex structures are not the same for the two initial conditions, their overall distribution, reorganization and decay are very similar. The present results imply that while some of the specific details of the streamwise vortex structure may be facility dependent, a relatively strong structure, which produces significant three-dimensionality, should form in all mixing layers originating from laminar boundary layers. After some initial readjustments, the structure will appear in the form of counter-rotating pairs of streamwise vortices which, in the mean, grow with the mixing layer and decay in strength.The present results also serve as a warning that small changes in initial conditions may significantly affect the Reynolds stress distributions in the near-field. The most likely mechanism for this is through the effects of very small changes in initial boundary layer properties on the details (strength and location) of the spanwise vortex roll-up. Despite these relatively large differences in the near-field, both mixing layers attain comparable turbulence structure and growth rates in the far-field. In addition, the behavior of the streamwise vortex structure does not appear to be affected by the differences in the near-field Reynolds stress distributions.     

Pressure-sensitive paint in aerodynamic testing

Pressure-sensitive paint (PSP) is a relatively new aerodynamic measurement tool with the unique capability of providing a field measurement of pressure over a test surface. An introductory review of this technology is presented, which is confined to the application of the PSP method to aircraft development wind tunnel testing. This is at present the primary application area and thus the focus of research on the use of the method, and is the authors' own area of research. Described are PSP fundamentals, the various elements comprising PSP technology, and current limitations and considerations in applying this technology. Experimental results are presented to illustrate the present capability of the method. The few publications currently available on this subject in the open literature are also referenced.     

Cold chemistry with electronically excited Ca+ Coulomb crystals

Rate constants for chemical reactions of laser-cooled Ca(+) ions and neutral polar molecules (CH(3)F, CH(2)F(2), or CH(3)Cl) have been measured at low collision energies (/k(B)=5-243 K). Low kinetic energy ensembles of (40)Ca(+) ions are prepared through Doppler laser cooling to form "Coulomb crystals" in which the ions form a latticelike arrangement in the trapping potential. The trapped ions react with translationally cold beams of polar molecules produced by a quadrupole guide velocity selector or with room-temperature gas admitted into the vacuum chamber. Imaging of the Ca(+) ion fluorescence allows the progress of the reaction to be monitored. Product ions are sympathetically cooled into the crystal structure and are unambiguously identified through resonance-excitation mass spectrometry using just two trapped ions. Variations of the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of (40)Ca(+) involved in the laser-cooling cycle, and these are modeled by solving the optical Bloch equations for the eight-level system. Systematic variation of the steady-state populations over a series of reaction experiments allows the extraction of bimolecular rate constants for reactions of the ground state ((2)S(1/2)) and the combined excited states ((2)D(3/2) and (2)P(1/2)) of (40)Ca(+). These results are analyzed in the context of capture theories and ab initio electronic structure calculations of the reaction profiles. In each case, suppression of the ground state rate constant is explained by the presence of a submerged or real barrier on the ground state potential surface. Rate constants for the excited states are generally found to be in line with capture theories.     

Structural and functional diversity of glutaredoxins in yeast

Glutaredoxins are defined as thiol disulfide oxidoreductases that reduce disulfide bonds employing reduced glutathione as electron donor. They constitute a complex family of proteins with a diversity of enzymatic and functional properties. Thus, dithiol glutaredoxins are able to reduce disulfide bonds and deglutathionylate mixed disulfides between glutathione and cysteine protein residues. They could act regulating the redox state of sulfhydryl residues of specific proteins, while thioredoxins (another family of thiol disulfide oxidoreductases which employ NADPH as electron donor) would be the general sulfhydryl reductants. Some dithiol glutaredoxins such as human Grx2 form dimers bridged by one iron-sulfur cluster, which acts as a sensor of oxidative stress, therefore regulating the activity of the glutaredoxin. The ability to interact with iron-sulfur clusters as ligands is also characteristic of monothiol glutaredoxins with a CGFS-type active site. These do not display thiol oxidoreductase activity, but have roles in iron homeostasis. The three members of this subfamily in Saccharomyces cerevisiae participate in the synthesis of the iron-sulfur clusters in mitochondria (Grx5), or in signalling the iron status inside the cell for regulation of iron uptake and intracellular iron relocalization (Grx3 and Grx4). Such a role in iron metabolism seems to be evolutionary conserved. Fungal cells also contain membrane-associated glutaredoxins structurally and enzymatically similar to dithiol glutaredoxins, which may act as redox regulators at the early stages of the protein secretory machinery.     

Rapid passage signals from a vibrationally excited target molecule: a pump and probe experiment with continuous wave quantum cascade lasers

Two 5 μm continuous wave quantum cascade lasers are used to perform a counterpropagating pump and probe experiment on a low pressure sample of nitric oxide. The strong pump field excites a fundamental rovibrational transition and the weaker probe field is tuned to the corresponding rotationally resolved hot band transition. When both light fields are in resonance, rapid passage is observed in the hot band absorption lineshape arising from a minimally damped and velocity-selected sample of molecules in the v=1 state. The measured rapid passage signals are well described by a two-level model based on the optical Bloch equations.     

Fundamentals of Plasma Polymerization

The plasma polymerization of ethylene is used as an example through which to discuss the elementary steps involved in forming a polymer in an electric discharge. The relationship of the experimentally controlled variables to the rate of formation of first generation active species is discussed. These species are related, in turn, to the overall rate of polymerization through a simple model. Two asymptotic conditions are discussed which correspond to minimal and total conversion of monomer to polymer. The dependence of polymer deposition rate on monomer flow rate predicted by the model is found to correspond very closely to that observed experimentally. The predicted effect of gas pressure on polymer deposition rate also agrees with that found experimentally.     

Analytic regularization in Soft-Collinear Effective Theory

In high-energy processes which are sensitive to small transverse momenta, individual contributions from collinear and soft momentum regions are not separately well-defined in dimensional regularization. A simple possibility to solve this problem is to introduce additional analytic regulators. We point out that in massless theories the unregularized singularities only appear in real-emission diagrams and that the additional regulators can be introduced in such a way that gauge invariance and the factorized eikonal structure of soft and collinear emissions is maintained. This simplifies factorization proofs and implies, at least in the massless case, that the structure of Soft-Collinear Effective Theory remains completely unchanged by the presence of the additional regulators. Our formalism also provides a simple operator definition of transverse parton distribution functions.