Development of an aeroelastic stability boundary for a rotor in autorotation

<p>For the present study, a mathematical model AMRA was created to simulate the aeroelastic behaviour of a rotor during autorotation. Our model: Aeroelastic Model of a Rotor in Autorotation (AMRA) captures transverse bending and teeter, torsional twist and lag-wise motion of the rotor blade and hence it is used to investigate couplings between blade flapping, torsion and rotor speed. Lagrange’s method was used for the modelling of blade flapping and chord-wise bending. Torsional twist of the rotor blade was modelled with the aid of finite element method (FEM), and blade transverse bending could also be modelled in FEM. The model can switch between using a full FEM model for bending and torsion, or a FEM model for torsion and simple blade teeter, depending on the complexity that the user requires.</p> <p>The AMRA model was verified against experimental data obtained during a CAA sponsored flight test programme of the G-UNIV autogyro. Published results of modal analysis of helicopter rotor blades and other data published in open literature were used to validate the FEM model of the rotor blade. The first torsional natural frequency of the ’McCutcheon’ rotor blades was measured with the aid of high-speed camera and used for validation of the FEM model of blade torsional twist. As a further verification of the modelling method, Aérospatiale Puma helicopter rotor blade data were compared on a Southwell plot showing comparison between experimental results and AMRA estimation.</p> <p>The aeromechanical behaviour of the rotor during both axial flight and forward flight in autorotation was investigated. A significant part of the research was focused on investigation of the effect of different values of torsional and flexural stiffness, and the relative positions of blade shear centre/elastic axis and centre of mass of the blade on stability during the autorotation.</p> <p>The results obtained with the aid of the model demonstrate the interesting, and unique, characteristics of the autorotative regime - with instabilities possible in bending and torsion, but also in rotorspeed. Coupled rotor speed/flap/twist oscillations (flutter and divergence) occur if the torsional stiffness of the blade is lower than a critical value, or if the blade centre of mass is significantly aft of the blade twisting axis, as is the case in helicopter pitch-flap flutter. The instability shown here, however, is specific to the autogyro, or autorotating rotor, as it is coupled with rotorspeed, and so differs from both helicopter rotor flutter and fixed-wing flutter. The coupling with rotorspeed allows a combined flutter and divergence instability, where the rotor begins to flutter in rotorspeed, teeter angle and torsional twist and, once the rotorspeed had dropped below a critical value, then moves into divergence in flap and rotorspeed. It was found that the aeroelastic behaviour of a rotor in autorotation is significantly affected by the strong coupling of blade bending stiffness and teeter angle with rotorspeed, and the strong coupling between blade aeroelastic twist and rotor torque.</p&gt

Low-dimensional Characterization and Control of Bluff-body Wakes. Report 0006

This report is a summary of research conducted for the EPSRC on contract GR/L/59030— “Low-dimensional characterization and control of bluff-body wakes”. The motivation for the work was to investigate active control schemes for stabilizing the low-Reynolds number bluff-body wake, which is an archetypal unstable flow exhibiting self-sustained flow oscillations as a result of global flow instability. Control of bluff-body wake oscillations is of use in drag reduction, noise suppression and prevention of flow induced structural oscillations. Moreover, suppression of closely related unstable flows, such as growth of a dynamic stall vortex on a pitching helicopter blade, may be possible using a similar strategy. To this end, a numerical model of an unstable bluff-body flow was developed and validated by comparison with published literature. Various control strategies involving low-dimensional models of the flow and combinations of distributed sensors and actuators in the near and far wake were investigated. The control results of this study are unique, in that successful control of the flow has been demonstrated further away from criticality than by any other scheme. These results provide a base for continued research in this area and in other related flows (for instance control of helicopter dynamic stall) and has contributed to a publication [1] and several others in preparation

Observations of the Vortex Ring State

This paper considers the vortex ring state, a flow condition usually associated with the descent of a rotor into its own wake. The phenomenon is investigated through experiments on simple rotor systems, and a comparison is then made with observations of a flow generated by a specially designed open core, annular jet that generates a mean flow velocity profile similar to the mean flow in a rotor wake in hover. In an experimentally simulated descent, the jet flow generates a flow state that shares many features of the rotor vortex ring state

The effect of a barnacle-shaped excrescence on the hydrodynamic performance of a tidal turbine blade section

Efficient tidal turbine designs rely upon the hydrodynamic performance of the turbine blade sections. A significant consideration for the likely power generation capacity of a tidal turbine is the effect of biofouling on the blade performance. A turbine blade surface is susceptible to large scale macrofouling, mainly from encrusters, such as barnacles and molluscs, colonising the developing surface. This paper considers the case of when a barnacle attaches to the upper (suction) surface of the blade section. Results of experiments to investigate the unsteady flow characteristics of the blade section are presented, and the modification of the hydrodynamic performance coefficients due to the presence of a barnacle is evaluated. The barnacle has no significant effect upon the lift in steady flow and unsteady flow, but there is a very large increase in the drag. Dependent upon the degree of barnacle encrustation, the effect on a turbine blade drag may be significant and lead to a degradation of a turbine predicted performance

Genetic drivers of kidney defects in the digeorge syndrome

BACKGROUND The DiGeorge syndrome, the most common of the microdeletion syndromes, affects multiple organs, including the heart, the nervous system, and the kidney. It is caused by deletions on chromosome 22q11.2; the genetic driver of the kidney defects is unknown. METHODS We conducted a genomewide search for structural variants in two cohorts: 2080 patients with congenital kidney and urinary tract anomalies and 22,094 controls. We performed exome and targeted resequencing in samples obtained from 586 additional patients with congenital kidney anomalies. We also carried out functional studies using zebrafish and mice. RESULTS We identified heterozygous deletions of 22q11.2 in 1.1% of the patients with congenital kidney anomalies and in 0.01% of population controls (odds ratio, 81.5; P = 4.5×1014). We localized the main drivers of renal disease in the DiGeorge syndrome to a 370-kb region containing nine genes. In zebrafish embryos, an induced loss of function in snap29, aifm3, and crkl resulted in renal defects; the loss of crkl alone was sufficient to induce defects. Five of 586 patients with congenital urinary anomalies had newly identified, heterozygous protein-Altering variants, including a premature termination codon, in CRKL. The inactivation of Crkl in the mouse model induced developmental defects similar to those observed in patients with congenital urinary anomalies. CONCLUSIONS We identified a recurrent 370-kb deletion at the 22q11.2 locus as a driver of kidney defects in the DiGeorge syndrome and in sporadic congenital kidney and urinary tract anomalies. Of the nine genes at this locus, SNAP29, AIFM3, and CRKL appear to be critical to the phenotype, with haploinsufficiency of CRKL emerging as the main genetic driver

Trailing edge flap flow control for dynamic stall

Results of a series of dynamic stall tests in a wind tunnel of an aerofoil fitted with a pitching, trailing edge flap are presented. Phase angle, amplitude, motion profile and duration of the flap wereinvestigated to assess the potential of the flap of mitigating the adverse effects of dynamic stall. The tests were a continuation of the investigations by Feszty et al. (2004) using a computational fluid dynamics method, and the results broadly confirm their conclusions. The results presented in this paper also confirm the observations from experimental work Gerontakos and Lee (2006) and Lee and Gerrontakos (2007) at lower Reynolds number in dynamic stall by strong suction being generated over the aerofoil lower surface, and it is the modification to the lower surface shape by the flap that creates this effect. The dynamic stall vortex acts to enhance the lower surface suction, and careful flap phasing and flap motion profile shaping can make the control more effective

The architectural evolution of self-immolative polymers

© 2020 Elsevier Ltd Self-immolative polymers (SIPs) depolymerize to small molecules through a cascade of reactions following cleavage of the polymer backbone or a specific terminal or focal point moiety by a stimulus. They have been developed using the principles of self-immolative spacers and low ceiling temperature (Tc) polymers. Key developments over the past couple of decades have been the polymerization of spacers to enable long reaction cascades, and the introduction of stimuli-responsive end-caps which have allowed depolymerization to be triggered in a controlled manner in response to a wide array of stimuli. This review will focus on the architectural evolution of SIPs over the past two decades from oligomers to dendrimers, linear polymers, cyclic polymers, graft copolymers, networks, and hyperbranched systems. We will discuss how the architecture influences the triggering and propagation of the reaction cascade and highlight how different architectures can provide advantages and disadvantages in terms of their synthesis and properties

The flow field around a rotor in axial descent

Measurements of the flow field around a model rotor descending axially into its own vortex wake have been performed using particle image velocimetry (PIV). At low descent rates, the expected cylindrical down-flow structure below the rotor is observed. At slightly higher descent rate, the flow enters the so-called vortex ring state (VRS) where the vorticity from the rotor accumulates into a toroidal structure near the rotor tips, and a large recirculation zone forms above the rotor disk. In the VRS, the flow below the rotor shows a significant upwards component, with a small up-flow zone penetrating right up to the rotor disk. Measurements show there to be a range of descent rates just before the onset of the VRS over which the flow may be interpreted to be in an incipient VRS condition. In this range, analyses of individual PIV measurements indicate that the flow near the rotor intermittently switches between the down-flow topology found at lower descent rates and the flow topology found in the fully developed VRS. The frequency of excursions of the flow into the VRS topology increases as the descent rate of the rotor is increased until, at high enough descent rate, the flow remains locked within its toroidal state

Polyglyoxylamides: Tuning Structure and Properties of Self-Immolative Polymers

© 2018 American Chemical Society. Self-immolative polymers (SIPs) are a class of stimuli-responsive materials that undergo controlled end-to-end depolymerization in response to stimuli. Their unique degradation and amplification properties have made them of interest for a diverse array of applications including sensors, vehicles for controlled release, and transient objects. Thus far, a limited number of SIP backbones exist, each with its own advantages and limitations. We report here the preparation and study of polyglyoxylamides (PGAms) as a new class of SIPs. PGAms were synthesized by simple postpolymerization modifications of poly(ethyl glyoxylate) (PEtG). While retaining the important stimuli-responsive depolymerization properties of polyglyoxylates, PGAms exhibited much different thermal properties, and some were even water-soluble. Furthermore, a depolymerizable PGAm analogue of poly(ethylene glycol) was prepared, demonstrating the capability to synthesize more complex PGAm graft copolymers. Overall, PGAms are a new class of SIPs with unique combinations of physical, thermal, and degradative properties that provide avenues for novel applications