Injection and mixing in a scramjet combustor: DES and RANS studies

The low momentum flux ratio jet in the HyShot II scramjet combustor is studied by DES (Detached Eddy Simulation) and RANS (Reynolds-Averaged Navier–Stokes) methods. The flow structure near the injector, shock pattern in the symmetry plane as well as the instantaneous coherent structures are presented and explained. Further insight into the flow physics is obtained by visualizing instantaneous coherent structures. The formation of Ω-shaped vortices, which was previously observed in experiments but never well-studied numerically, is discussed in detail. A new schematic of flow physics is proposed to enhance the understanding of the low momentum flux ratio jet. Compared to the DES result, the RANS method is unable to capture the dynamics of turbulent structures. The DES method provides much detailed information about mixing patterns and a more reliable mixing efficiency than the RANS result. The RANS method over-predicts the eddy-viscosity during turbulence modeling and suppresses unsteady turbulent fluctuations by time averaging, which results in a 25% over-estimation of the mixing efficiency.     

Phenomenological modeling of viscous electrostrictive polymers

A common usage for electroactive polymers (EAPs) is in different types of actuators, where advantage is taken of the deformation of the polymer due to an electric field. It turns out that time-dependent effects are present in these applications. One of these effects is the viscoelastic behavior of the polymer material. In view of the modeling and simulation of applications for EAP within a continuum mechanics setting, a phenomenological framework for an electro-viscoelastic material model is elaborated in this work. The different specific models are fitted to experimental data available in the literature. While the experimental data used for inherent electrostriction is restricted to small strains, a large strain setting is used for the model in order to account for possible applications where the polymers undergo large deformations, such as in pre-strained actuators.     

Ultrafast excited state dynamics of a bithiophene-isoindigo copolymer obtained by direct arylation polycondensation and its application in indium tin oxide-free solar cells

A low band gap copolymer, P2TI (E_g = 1.6.eV), with bithiophene as donor and isoindigo as acceptor units is designed and synthesized by the direct arylation polycondensation (DAP) method. Absorbance of the polymer spans from 300 to 780 nm. It exhibits an absorption coefficient (ε) of 96 L/g cm in solution at its maxima. A HOMO level of −5.42 eV and LUMO level of −3.72 eV is measured by square wave voltammetry. ITO-free solar cells fabricated using a P2TI :PCBM71 bulk heterojunction shows a moderate efficiency of 1.02% with a high open circuit voltage of 0.81 V. An intramolecular charge transfer state is found in the relaxation of P2TI in solution, which is generated with a time constant of 2 ps as measured by femtosecond-transient absorption spectroscopy. Charge carriers were generated in <250 fs in P2TI: PCBM71 films. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1475–1483     

Controlled nitroxide‐mediated radical polymerization of methyl and phenyl vinyl ketone

The article describes unprecedented nitroxide‐mediated radical polymerization of methyl and phenyl vinyl ketone (MVK and PVK) using a sterically highly hindered alkoxyamine as initiator/regulator. It is shown that controlled polymerization of PVK is far more difficult to achieve than controlled MVK polymerization. Whereas for MVK high conversion resulting in polyvinyl ketone with low polydispersity index is readily obtained, the PVK polymerization provides good results only in the presence of free nitroxide and styrene as additives. Vinyl ketone polymerizations are analyzed by ESI mass spectrometry. These MS studies provide insights into the problems associated with the controlled nitroxide‐mediated polymerization of PVK. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012