DNS and Modelling of Passive Scalar Transport in Turbulent Channel Flow with a Focus on Scalar Dissipation Rate Modelling

A direct numerical simulation of turbulent channel flow with an imposed mean scalar gradient is analyzed with a focus on passive scalar flux modelling and in particular the treatment of the passive scalar dissipation equation. The Prandtl number is 0.71 and the Reynolds number based on the wall friction velocity and the channel half width is 265. Budgets are presented for the passive scalar variance and its dissipation rate, as well as for the individual scalar flux components. These form a basis for a discussion of modelling issues related to explicit algebraic scalar flux modelling. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dose mapping for documentation of radiation sterilization

The radiation sterilization standards EN 552 and ISO 11137 require that dose mapping in real or simulated product be carried in connection with the process qualification. This paper reviews the recommendations given in the standards and discusses the difficulties and limitations of practical dose mapping. The paper further gives recommendations for effective dose mapping including traceable dosimetry, documented procedures for placement of dosimeters, and evaluation of measurement uncertainties.     

Novel low‐loss waveguide delay lines using Vernier ring resonators for on‐chip multi‐λ microwave photonic signal processors

In this paper, novel photonic delay lines (DLs) using Vernier/non‐identical ring resonators (VRRs) are proposed and demonstrated, which are capable of simultaneous generation of multiple different delays at different wavelengths (frequencies). The simple device architectures and full reconfigurability allow the DLs to be integrated with other functional building blocks in photonic integrated circuits to realize on‐chip, complex multi‐λ microwave photonic signal processors with reduced system complexity. To prove the concept, DLs using VRRs in cascaded and coupled configurations have been fabricated in TriPleX^TM waveguide technology, which enables a very low delay‐induced loss of approximately 0.18 dB/100 ps. The fabricated DLs demonstrated simultaneous generation of four incremental delays, where a maximum incremental step of 550 ps and a corresponding top delay of 1650 ps were measured for a bandwidth up to 1 GHz. To our knowledge, this is the first report on VRRs for delay generation functionalities.     

Crystalline Anions Based on Classical N-Heterocyclic Carbenes

Herein, the first stable anions K[SIPr^Bp] ( 4 a-K ) and K[IPr^Bp] ( 4 b-K ) (SIPr^Bp=BpC{N(Dipp)CH₂}₂, IPr^Bp=BpC{N(Dipp)CH}₂; Bp=4-PhC₆H₄; Dipp=2,6-iPr₂C₆H₃) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a-K and 4 b-K are prepared by KC₈ reduction of the neutral radicals [SIPr^Bp] ( 3 a ) and [IPr^Bp] ( 3 b ), respectively. The radicals 3 a and 3 b as well as [Me-IPr^Bp] 3 c (Me-IPr^Bp=BpC{N(Dipp)CMe}₂) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPr^Bp)Br ( 2 a ), (IPr^Bp)Br ( 2 b ), and (Me−IPr^Bp)Br ( 2 c ) with KC₈. The cyclic voltammograms of 2 a–2 c exhibit two one-electron reversible redox processes in −0.5 to −2.5 V region that correspond to the radicals 3 a–3 c and the anions ( 4 a–4 c )⁻. Computational calculations suggest a closed-shell singlet ground state for ( 4 a–4 c )⁻ with the singlet-triplet energy gap of 17–24 kcal mol⁻¹.     

Boron-Centered Lewis Superacid through Redox-Active Ligands: Application in C−F and S−F Bond Activation

A series of redox-responsive ferrocenyl-substituted boranes and boronic esters were synthesized. Oxidation of the ferrocenyl ligand to the ferrocenium resulted in a drastic increase in the Lewis acidity beyond the strength of SbF₅, which was investigated experimentally and computationally. The resulting highly Lewis acidic boron compounds were used for catalytic C−F and S−F bond activation.     

A Water- and Base-Stable Iminopyridine-Based Cage That Can Bind Larger Organic Anions

Catalysis inside molecular cages and capsules has attracted an increasing amount of attention over the last decade. While many examples of the catalysis of reactions with cationic intermediates and transition states are known, those with anionic counterparts are scarce. One limiting factor is access to suitably sized cationic iminopyridine-based cages that are stable towards water and anionic/nucleophilic guest molecules. This study aimed at identifying such suitable cages. In this full paper, we describe the journey that finally led to the synthesis of a novel iminopyridine-based tetrahedron that can bind larger organic anions with binding constants of up to 850 M⁻¹ in MeCN−d₃/H₂O=9 : 1. Importantly, it also displays stability in basic aqueous acetonitrile. Surprisingly, investigations towards catalysis of reactions with anionic transition states did not indicate rate accelerations in the presence of the cage.