Supercritical and subcritical dynamic flow-induced instabilities of a small-scale wind turbine blade placed in uniform flow

There is a growing interest in extracting more power per turbine by increasing the rotor size in offshore wind turbines. As a result, the turbine blades will become longer and therefore more flexible, and a flexible blade is susceptible to flow-induced instabilities. In order to design and build stable large wind turbine blades, the onset of possible flow-induced instabilities should be considered in the design process. Currently, there is a lack of experimental work on flow-induced instabilities of wind turbine blades. In the present study, a series of experiments were conducted and flow-induced instabilities were observed in wind turbine blades. A small-scale flexible blade based on the NREL 5 MW reference wind turbine blade was built using three-dimensional printing technique. The blade was placed in the test section of a wind tunnel and was subjected to uniform oncoming flow, representing the case of a parked wind turbine blade. The blade׳s tip displacement was measured using a non-contacting displacement measurement device as the oncoming wind speed was increased. At a critical wind speed, the blade became unstable and experienced limit cycle oscillations. The amplitude of these oscillations increased with increasing wind speed. Both supercritical and subcritical dynamic instabilities were observed. The instabilities were observed at different angles of attack and for blades both with and without a geometric twist. It was found that the blade twist had a significant influence on the observed instability: a blade without a twist experienced a strong subcritical instability.     

Towards more rational techniques for the isolation of valuable essential oils from plants

Attention is drawn to the use of new and clean alternative methods for the isolation of essential oils from plants. A critical overview is presented of conventional methods (based on either organic solvent extraction or distillation) and new alternatives (including microwave-assisted extraction (MAE) as well as supercritical CO₂ (SC-CO₂) extraction and subcritical water extraction). The advantages and disadvantages of each technique are reported and special emphasis is given to the use of continuous subcritical water extraction which emerges as clearly advantageous over conventional techniques (by avoiding the use of organic solvents and considerably shortening the extraction time, as well as increasing the efficiency) and recent techniques, such as MAE (by increasing the efficiency) and SC-CO₂ extraction (by avoiding the co-extraction of cuticular waxes and lipids and the need for a sample drying stage prior to extraction).     

Retention mechanisms in subcritical water reversed-phase chromatography

Differences in the properties of subcritical water and conventional water/acetonitrile and water/methanol mobile phases for reversed phase separations are explored. Using van’t Hoff plots enthalpies and entropies of transfer are compared among the mobile phases while linear solvation energy relationships are used to quantify contributions to retention based on a solute's polarizability, dipolarity, hydrogen bond donating ability, hydrogen bond accepting ability, and molecular size. Results suggest the presence of acetonitrile or methanol in the mobile phase may decrease dispersive interactions of the solute with the stationary phase compared to subcritical water, thereby lowering enthalpic contributions to retention. Enthalpic contributions are found to drive the retention of a methylene group in all systems studied.     

A numerical method for subcritical and supercritical open channel flow calculation

A marching finite volume method is presented for the calculation of two-dimensional, subcritical and supercritical, steady open channel flow including the usually neglected terms of slope and bottom friction. The channel flow will be assumed to be homogeneous, incompressible, two-dimensional and viscous with wind and Coriolis forces neglected. A hydrostatic pressure distribution is assumed throughout the flow field. The numerical technique used is a combination of the finite element and finite difference methods. A transformation is introduced through which quadrilaterals in the physical domain are mapped into squares in the computational domain. The governing system of PDEs is thus transformed into an equivalent system applied over a square grid network. Comparisons with other numerical solutions as well as with measurements for various open channel configurations show that the proposed approach is a comparatively accurate, reliable and fast technique.     

Invariant solutions in a channel flow using a minimal restricted nonlinear model

Simulations using a Restricted Nonlinear (RNL) system, where mean flow distortion resulting from Reynolds stress feedback regenerates rolls, is applied in a channel flow under subcritical conditions. This quasi-linear restriction of the dynamics is used to study invariant solutions located in the bulk of the flow found recently by Rawat et al. (2016) [14]. It is shown that the RNL system truncated to a single streamwise mode for the perturbation supports invariant solutions that are found to bifurcate from a relative periodic orbit into a travelling wave solution when the spanwise size is increasing. In particular, the travelling wave solution exhibits a spanwise localized structure that remains unchanged for large values of the spanwise extent as the invariant solution lying on the lower branch found by Rawat et al. (2016) [14]. In addition, travelling wave solutions provided by this minimal RNL system are self-similar with respect to the Reynolds number based on the centreline velocity, and the half-channel height varying from 2000 to 5000.     

Sharp conditions of global existence and scattering for a focusing energy-critical Schrödinger equation

We consider a focusing energy-critical Schrödinger equation with subcritical perturbations and address question related to the sharp criterion of global existence and scattering. By analyzing the variational characteristics of this equation, we established two types of invariant flows. Then approximating this equation by the energy-critical nonlinear Schrödinger equations with the same initial data and combining the properties of the invariant flows, we obtain the sharp conditions of global existence for this equation. Moreover, when the solution is globally defined, we prove the scattering.     

Chemical recycling of networked polystyrene derivatives using subcritical water in the presence of an aminoalcohol

Subcritical water is a benign and effective media for polymer degradation. On subcritical water treatment in the presence of an aminoalcohol, unsaturated polyesters crosslinked with styrene were decrosslinked, and a linear polystyrene derivative bearing hydroxy-terminated side-chains was recovered. After modification of the hydroxy groups with maleic anhydride, the polystyrene derivative was re-crosslinked with styrene to form a networked structure again. The resulting solid was degradable by subcritical water treatment in the presence of the aminoalcohol to give another polystyrene derivative bearing hydroxy groups. These processes could be repeated successfully, demonstrating the applicability as a novel recycling system of thermosetting resins. The polystyrene derivative was also re-crosslinked again on heating with an alternative copolymer of styrene and maleic anhydride due to the formation of linkage between the hydroxy groups and carboxylic anhydride moieties.     

Industrial application of green chromatography--I. Separation and analysis of niacinamide in skincare creams using pure water as the mobile phase

In this work, chromatographic separation of niacin and niacinamide using pure water as the sole component in the mobile phase has been investigated. The separation and analysis of niacinamide have been optimized using three columns at different temperatures and various flow rates. Our results clearly demonstrate that separation and analysis of niacinamide from skincare products can be achieved using pure water as the eluent at 60 °C on a Waters XTerra MS C18 column, a Waters XBridge C18 column, or at 80 °C on a Hamilton PRP-1 column. The separation efficiency, quantification quality, and analysis time of this new method are at least comparable with those of the traditional HPLC methods. Compared with traditional HPLC, the major advantage of this newly developed green chromatography technique is the elimination of organic solvents required in the HPLC mobile phase. In addition, the pure water chromatography separations described in this work can be directly applied in industrial plant settings without further modification of the existing HPLC equipment.     

Extracting and trapping biogenic volatile organic compounds stored in plant species

Biogenic volatile organic compounds (BVOCs), released by practically all plants, have important atmospheric and ecological consequences. Because BVOC-emission measurements are especially tedious, complex and extremely variable between species, two approaches have been used in scientific studies to try to estimate BVOC-emission types and rates from plant species. The first, which has known little success, involves grouping species according to plant-taxonomy criteria (typically, genus and family). The second involves studying the correlation between BVOC content and emission (i.e. how leaf content could be used to estimate emissions). The latter strategy has provided controversial results, partly because BVOCs are amazingly chemically diverse, and, as a result, techniques used to study plant BVOC content, which we review, cannot be equally adequate for all analytes.In order to choose an adequate technique, two patterns must be distinguished. Specifically stored compounds - mainly monoterpenes and sesquiterpenes that dominate the essential oil obtained from a plant - are permanently and massively present in specific storage structures (e.g., secretory cavities, trichomes) of the order of μg/g-mg/g and usually allow emissions to occur during stress periods when terpenes are weakly synthesized. These BVOCs can be studied directly through traditional extraction techniques (e.g., hydrodistillation) and novel techniques (e.g., application of microwaves and ultrasound), and indirectly by trapping techniques involving the collection, within adsorbent material, of BVOCs present in the headspace of a plant.Non-specifically stored compounds (e.g., isoprene, 2-methyl-3-buten-2-ol, and, in species without storage structures, monoterpenes and sesquiterpenes) can only be temporarily accumulated in leaf aqueous and lipid phases in small concentrations of the order of ng/g. As a result, studying their concentration in leaves requires the use of trapping techniques, more sensitive to trace amounts. Unlike for specifically stored BVOCs, knowledge of the concentration of non-specifically stored BVOCs cannot provide any information regarding the emission potential of a species but, instead, provides crucial information to understand why BVOC emissions may be uncoupled from the physiological processes that drive their synthesis.We describe both extracting and trapping techniques and discuss them in terms of the technical choices that may cause losses of thermolabile constituents, chemical transformations, different volatile recoveries and suitability to represent plant content of BVOCs faithfully. The second part of this review addresses technical shortcomings and biological and environmental factors that may alter the correlations between BVOC content and emission from plants.