Theoretical analysis of perching and hovering maneuvers

Unsteady aerodynamic phenomena are encountered in a large number of modern aerospace and non-aerospace applications. Leading edge vortices (LEVs) are of particular interest because of their large impact on the forces and performance. In rotorcraft applications, they cause large vibrations and torsional loads (dynamic stall), affecting the performance adversely. In insect flight however, they contribute positively by enabling high-lift flight. Identifying the conditions that result in LEV formation and modeling their effects on the flow is an important ongoing challenge. Perching (airfoil decelerates to rest) and hovering (zero freestream velocity) maneuvers are of special interest. In earlier work by the authors, a Leading Edge Suction Parameter (LESP) was developed to predict LEV formation for airfoils undergoing arbitrary variation in pitch and plunge at a constant freestream velocity. In this research, the LESP criterion is extended to situations where the freestream velocity is varying or zero. A point-vortex model based on this criterion is developed and results from the model are compared against those from a computational fluid dynamics (CFD) method. Abstractions of perching and hovering maneuvers are used to validate the low-order model's performance in highly unsteady vortex-dominated flows, where the time-varying freestream/translational velocity is small in magnitude compared to the other contributions to the velocity experienced by the leading edge region of the airfoil. Time instants of LEV formation, flow topologies and force coefficient histories for the various motion kinematics from the low-order model and CFD are obtained and compared. The LESP criterion is seen to be successful in predicting the start of LEV formation and the point-vortex method is effective in modeling the flow development and forces on the airfoil. Typical run-times for the low-order method are between 30-40 seconds, making it a potentially convenient tool for control/design applications

Comment on “Effects of focused ion beam milling on the nanomechanical behavior of a molybdenum-alloy single crystal” Appl. Phys. Lett. 91, 111915 (2007)

While this article provides insight into differences in mechanics between Ga+-irradiated and “pure” surfaces of molybdenum, there are several statements that are either inaccurate or poorly stated. It is clear that when a surface is directly irradiated by orthogonal ion beam (0.07–0.21 mW), a focused ion beam (FIB) damage layer will likely form and affect the strength. However, this finding does not provide adequate foundation for raising the question of FIB-induced hardening in nanopillars, given the vast differences between these experiments and procedure used in pillar fabrication. These issues would cause considerable confusion and result in disservice to mechanical testing community if not clarified

Comparative Analysis of Experimental and Numerical Investigation on Thermophysicalproperties in Hydro Carbon Mixtures using Jouyban-Acree Model at Various Temperatures

The thermophysical properties of liquid mixtures provide additional information regarding molecular interactions. A perusal of the literature revealed that the predictions of thermophysical properties of liquid mixtures are scarce. With an aim, the thermophysical properties of viscosity, excess molar volume VE and viscosity deviations Δη of liquid mixtures are predicted by using various nonlinear models. In this reseach Jouyban-Acree viscosity models have been used for predicting viscosity of Acetophenone with P-xylene and 1, 4 Dioxane with Benzene at different mole fractions measured at various temperatures in the atmospheric pressure condition. From experimentation excess volumes, VE, and deviations in viscosities, Δη, of mixtures at infinite dilutions have been obtained. The measured systems show positive VE and negative Δη with increasing temperatures. From the positive excess molar volume, when aromatics, which exist in a highly associated form in the pure state, are mixed with polar solvents (ketones), the monomerization occurs and new specific interactions appear in the solution.The negative viscosity deviation depends on the size and shape of the molecules and molecular interactions. These measured data tailored to the Jouyban-Acree nonlinear models to derive the binary coefficients Jouyban-Acree model is more adequate for the thermo physical and the standard deviation was found to be < 2.06 %. The molecular interactions existing between the components and comparison of liquid mixtures were also discussed

Calcification in a marginal sea - influence of seawater [Ca2+] and carbonate chemistry on bivalve shell formation

In estuarine coastal systems such as the Baltic Sea, mussels suffer from low salinity which limits their distribution. Anthropogenic climate change is expected to cause further desalination which will lead to local extinctions of mussels in the low saline areas. It is commonly accepted that mussel distribution is limited by osmotic stress. However, along the salinity gradient environmental conditions for biomineralization are successively becoming more adverse as a result of reduced [Ca2+] and dissolved inorganic carbon (CT) availability. In larvae, calcification is an essential process starting during early development with formation of the prodissoconch I (PD I) shell which is completed under optimal conditions within 2 days. Experimental manipulations of seawater [Ca2+] start to impair PD I formation in Mytilus larvae at concentrations below 3 mM, which corresponds to conditions present in the Baltic at salinities below 8 g kg-1. In addition, lowering dissolved inorganic carbon to critical concentrations (< 1 mM) similarly affected PD I size which was well correlated with calculated ΩAragonite and [Ca2+][HCO3-]/[H+] in all treatments. Comparing results for larvae from the western Baltic with a population from the central Baltic revealed significantly higher tolerance of PD I formation to lowered [Ca2+] and [Ca2+][HCO3-]/[H+] in the low saline adapted population. This may result from genetic adaptation to the more adverse environmental conditions prevailing in the low saline areas of the Baltic. The combined effects of lowered [Ca2+] and adverse carbonate chemistry represent major limiting factors for bivalve calcification and can thereby contribute to distribution limits of mussels in the Baltic Sea

Twinning and the mechanical behavior of magnesium alloys at very high strain rates

The dynamic mechanical behavior of magnesium and its alloys is a subject of interest primarily because of its high specific strength. This makes it attractive for structural components and vehicles. The hexagonal close packed crystal structure of magnesium makes it highly anisotropic in terms of its mechanical behavior. Extension twinning is a dominant deformation mechanism in these materials. This is often reflected in a characteristic sigmoidal profile of the stress–strain curve when crystals are compressed along directions perpendicular to the crystallographic c-axis. Past experiments have been limited to strain rates of 103 s–1 . This study focuses on microstructural twinning effects on the mechanical behavior of AZ31 magnesium alloy at higher strain rates. We perform very high-strain rate experiments on AZ31 magnesium alloy, using a miniature Kolsky compression bar apparatus coupled with a high speed camera for whole field imaging. This experiment is capable of achieving strain rates on the order of 105 s–1 . Experiments at these strain rates have shown substantial plastic deformation without failure when compared with the lower rates of loading. This is evidence of deformation mechanisms that tend to delay failure in the material. We also observe a change in the hardening rates between these experiments and experiments done at 103 s–1. Examination of the microstructure of deformed samples gives us information about the relative activation and growth of deformation mechanisms that cause plastic deformation at these rates

Okonkwo’s Fall: Multiple Perspectives

This article aims at exploring the causes of the fall of Okonkwo, the protagonist of the Nigerian Novel. Things Fall Apart by the renowned novelist, Chinua Achebe. Though the novel mainly deals with the fall of Igbo Culture where Okonkwo has played the sheet anchor role in the novel, Things Fall Apart at the hands of British establishment in Nigeria, the other vital reasons that make him vulnerable will also be discussed at length in this article. It is from the study of the novel, it is established that the Igbo society that refuses to change itself could be one of the reasons for the fall. However, it is very clear that the changes should take place spontaneously and not by force which the Igbo society has been the victim and the representative, Okonkwo its scapegoat. The writers of the article make sure that the reading of this article will be an eye opener in terms of Nigerian consciousness as revealed in the novel, Things Fall Apart

Novel framework for optimized digital forensic for mitigating complex image attacks

Digital Image Forensic is significantly becoming popular owing to the increasing usage of the images as a media of information propagation. However, owing to the presence of various image editing tools and softwares, there is also an increasing threats over image content security. Reviewing the existing approaches of identify the traces or artifacts states that there is a large scope of optimization to be implmentation to further enhance teh processing. Therfore, this paper presents a novel framework that performs cost effective optmization of digital forensic tehnqiue with an idea of accurately localizing teh area of tampering as well as offers a capability to mitigate the attacks of various form. The study outcome shows that propsoed system offers better outcome in contrast to existing system to a significant scale to prove that minor novelty in design attribute could induce better improvement with respect to accuracy as well as resilience toward all potential image threats

Dynamic mechanical response and microstructural effects in commercially hot-pressed boron carbide under different loading rates and stress states

Boron carbide, with its high hardness and superior compressive strength, has become an attractive engineering material. Certain applications, notably ballistic protection, have drawn attention to its dynamic behavior. Like other brittle materials, the mechanical behavior of boron carbide is strongly related to the intrinsic microstructural flaws, e.g., non-metallic inclusions that form separate phases (graphite, BN, AlN) or voids. The critically sized and/or critically spaced flaws play important roles in dynamic failure processes as potential nucleation sites, and their relative importance depends on a combination of several mechanical properties. This study investigated the failure of a commercially hot-pressed boron carbide subjected to dynamic uniaxial and biaxial compression. The process was studied by loading small-size cubic specimens in a compression Kolsky Bar at strain-rates of ~103 s‑1. A high-speed camera was used to capture the failure process. SEM/EDS analysis was used to determine the chemical composition of the material matrix and flaws, and to identify critical actors in the failure process from the postmortem fragments. To study the strain-rate sensitivity, the mechanical behavior was also evaluated in the quasi-static regime (~10‑3 s–1) by using a servo-hydraulic testing machine in compression. The strain rate sensitivity for this material is typical of other advanced ceramics; the strength under dynamic compression was higher compared with the quasi-static strength. SEM/EDS examination showed that boron carbide has a microstructure with a complex superposition of inclusions of different composition and properties. It appears that the larger graphite-based inclusions are key factors contributing to the dynamic failure of this material

DNA methylation of ESR-1 and N-33 in colorectal mucosa of patients with Ulcerative Colitis (UC)

Introduction: Epigenetic marking such as DNA methylation influence gene transcription and chromosomal stability and may also be affected by environmental exposures. Few studies exist on alteration in DNA methylation profiles (genomic and gene specific methylation) in patients with Ulcerative Colitis (UC) and none assessing its relationship with lifestyle exposures. Aims & Methods: To assess genomic methylation and promoter methylation of the ESR-1 (oestrogen receptor - 1) and N-33 (tumour suppressor candidate-3) genes in the macroscopically normal mucosa of UC patients as well as to investigate effects of anthropometric and lifestyle exposures on DNA methylation. Sixty eight subjects were recruited (24 UC and 44 age and sex matched controls). Colorectal mucosal biopsies were obtained and DNA was extracted. Genomic DNA methylation was quantified using the tritium-labelled cytosine extension assay (3[H] dCTP) whilst gene specific methylation was quantified using the COBRA method. Results: The methylation level of both ESR-1 and N-33 genes were significantly higher in UC subjects compared with controls (7.9% vs 5.9%; p = 0.015 and 66% vs 9.3%; p < 0.001 respectively). There was no detectable difference in global DNA methylation between patients with UC and age and sex matched controls. No associations between indices of DNA methylation and anthropometric measures or smoking patterns were detected. Conclusions: For the first time, we have shown increased methylation in the promoter regions of the putative tumour suppressor gene N-33 in macroscopically normal mucosa of patients with UC. In addition, we have confirmed that methylation of ESR-1 promoter is higher in UC patients compared with age and sex matched controls. These findings suggests that, inactivation through methylation of the putative tumour suppressor genes N-33 and ESR-1, may not be associated with colorectal carcinogenesis in UC

An Accretion Model for Anomalous X-Ray Pulsars

We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star towards a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time, and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.Comment: 15 pages and 3 Postscript figure