Trailing edge noise reduction of wind turbine blades by active flow control

In the current study, we investigate a route to reduction of the turbulent boundary layer–trailing edge interaction noise. The trailing edge noise is generated by surface pressure fluctuations beneath a turbulent boundary and scattered at the trailing edge of wind turbine blades. Trailing edge noise is considered to be the dominant noise source of modern wind turbines. Therefore, efforts are constantly made to attenuate the noise. Today, noise emission can be reduced by proper airfoil design or passive devices, such as trailing edge serrations. A further improved candidate technology for trailing edge noise attenuation is active flow control in the form of wall‐normal suction. With active flow control, the boundary layer features responsible for trailing edge noise generation can be manipulated, and correspondingly the trailing edge noise can be reduced. Detailed experimental investigations were performed at the Universities of Tel‐Aviv and Stuttgart. The tests showed that steady wall‐normal suction has a positive effect on the trailing edge noise by reducing the boundary layer thickness, and with it the integral length scales of the eddies within the boundary layer. Copyright © 2014 John Wiley & Sons, Ltd.     

Alkene oxidation catalyzed by transition metal substituted Keggin-type heteropolyanions

Alkene oxidations with various oxidants (tert-butyl hydroperoxide, iodosylbenzene and molecular oxygen in the presence of isobutyraldehyde (IBA)) catalyzed by transition metal monosubstituted heteropolyanions, PW₁₁MO ₃₉ ^n– (PW₁₁M; M=Co^II, Mn^II, Cu^II, Ti^IV, Ru^IV, V^V and Nb^V), have been studied. Orders of catalytic activity of PW₁₁M are different for the oxidants studied. Radical chain mechanisms are proposed fort-BuOOH and O₂/IBA. Preliminary coordination of the oxidant to PW₁₁M is not a necessary step of its homolytic activation. Epoxidation with PhIO requires its coordination to the catalyst and most likely includes the formation of active metal-oxo species.     

The mechanism of the failure of the dispersion-strengthened Cu–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanosystem

The method of “in situ tensile testing in SEM” is suitable for investigations of fracture mechanisms because it enables to observe and document deformation processes directly, thank to which the initiation and development of plastic deformation and fracture can be reliably described. The deformation and fracture mechanisms of Cu–Al₂O₃ nanomaterials with 5 vol.% of Al₂O₃ phase has been analyzed using technique of the “in situ tensile testing in SEM.” It has been shown that the deformation process causes break-up of large Al₂O₃ particles and decohesion of smaller ones. The final fracture path is influenced also by boundaries of nanograins, through which the principal crack propagates towards the sample exterior surface. Based on the experimental observations, a model of damage and/or fracture mechanisms has been proposed.     

Positron Annihilation Study of Zirconia Nanopowders and Nanoceramics Stabilized by Magnesia and Ceria

Positron lifetime (LT) and coincidence Doppler broadening (CDB) measurements on nanopowders and ceramics of ceria‐ and magnesia‐stabilized zirconia (CeSZ and MgSZ, respectively) are presented. The nanopowders were prepared by the coprecipitation technique. Effects of nanopowder calcination and sintering at various temperatures were investigated. In the nanopowders, the two kinds of open‐volume defects associated with grain boundaries (GBs) could be identified via positron trapping: (i) vacancy‐like misfit defects situated along GBs and (ii) larger defects at the intersections of at least three GBs (triple points). CDB measurements on CeSZ compacted nanopowders indicated a segregation of Ce ions along GBs. A few percent fractions of positrons were found to form positronium localized in pores of ≈1.8 nm diameter in compacted nanopowders. Sintering of nanopowders at 1500°C appeared to be sufficient for disappearance of pores and triple point defects. In sintered ceramics, contrary to compacted nanopowders, positrons were trapped in zirconium vacancies in grain interiors.     

Microstructure changes of polyurethane by inclusion of chemically modified carbon nanotubes at low filler contents

The surface of multi-walled carbon nanotubes (MWCNTs) was modified to introduce acidic groups in either covalent or van der Waals interaction bonding environments to establish cross-linking sites with a host polymer. Nanocomposites based on a polyurethane matrix (PU) containing chemically functionalised multi-walled carbon nanotubes (MWCNTs) have been shown to alter its mechanical performance depending on the nature of the surface functional groups on MWCNTs, which correlates to the type of bonding interaction of the surface group and also the dispersibility of MWCNTs and their influence on the domain structure of polyurethane. The stress at break for nanocomposites containing 0.25 wt% of acid-oxidised MWCNTs (MWCNT-ox), bearing covalently attached carboxylic, lactone and phenolic groups, was twice that of the native PU and Young’s Modulus for the nanocomposites increased by four times. Whereas when hemin, which contains carboxylic functionality, was immobilised to the surface of pure MWCNTs, the improvement in Young’s Modulus was only around twice that of pure PU. Differences in the disaggregation of MWCNTs into PU were observed between the samples as well as variation of the native domain structure of PU. The results also infer that the purification of MWCNTs from acid-oxidative lattice fragments (fulvic acids) is vital prior to conducting surface chemistry and polymerisation in order to ensure maximum mechanical performance enhancement in their reinforcement of the host polymer.     

Elastic scattering and light transport in three-dimensional collagen gel constructs: a mathematical model and computer simulation approach

A mathematical modeling approach for elastic scattering and light propagation is presented, which can be used to obtain the scattering coefficient, the index of refraction, and the distribution of the collagen fibrils in a gel. Collagen fibrils can be realistically represented by small cylindrical particles. The analysis of the scattering of light by such particles provides the scattering coefficient. Light transport in multilayered tissues has been modeled and the collagen fibrils scattering coefficient has been considered as main input parameters. Assuming that a gel is composed of fibrils with the same diameter, it is possible to obtain all the input parameters of the model and, therefore, a simulated spectrum. This can be repeated for several diameters. Considering a gel composed of fibrils with different diameters, it is possible to obtain a best-fitting simulated spectrum as a weighted sum (least-square-error based) of the spectra corresponding to several fibril diameters, and, therefore, obtain an estimate of the percentages of fibrils of each diameter in the gel. Moreover, the scattering coefficient and refractive index, which are also provided by the model, are relevant parameters as they relate to tissue properties in their own right.     

Changes of liquid-crystalline polymer structure with temperature

Summary Thermal changes in the structural state of liquid crystalline polymers with mesogenic side groups and the kinetics of mesophase formation were studied by means of smallangle X-ray scattering.Part 3: V.Tsulcruk,O.Lokhonya,V.Shilov,V.Kuzmina,Yu.Lipatov Makromol.Chem,,Rapid Commun.4,595(1983)