13,028 research outputs found
Drag cancellation by added-mass pumping
A submerged body subject to a sudden shape-change experiences large forces
due to the variation of added-mass energy. While this phenomenon has been
studied for single actuation events, application to sustained propulsion
requires studying \textit{periodic} shape-change. We do so in this work by
investigating a spring-mass oscillator submerged in quiescent fluid subject to
periodic changes in its volume. We develop an analytical model to investigate
the relationship between added-mass variation and viscous damping and
demonstrate its range of application with fully coupled fluid-solid
Navier-Stokes simulations at large Stokes number. Our results demonstrate that
the recovery of added-mass kinetic energy can be used to completely cancel the
viscous damping of the fluid, driving the onset of sustained oscillations with
amplitudes as large as four times the average body radius . A quasi-linear
relationship is found to link the terminal amplitude of the oscillations ,
to the extent of size change , with peaking at values from 4 to 4.75
depending on the details of the shape-change kinematics. In addition, it is
found that pumping in the frequency range of
is required for
sustained oscillations. The results of this analysis shed light on the role of
added-mass recovery in the context of shape-changing bodies and
biologically-inspired underwater vehicles.Comment: 10 pages, 6 Figures, under review in JFM Rapid
Optimal Trajectory Tracking for an Autonomous UAV
The aim of the present project is the design of optimal flight trajectories for an automomous aerial vehicle which is expected to reach the desired locations in the operational environment expressed in terms of planned waypoints. The navigation must be performed with the vehicle's best effort, i.e. with the lowest cost. Hence, we want to minimize the input energy, a function of the inputs for the mathematical model which describes the dynamics of the vehicle. The trajectory must satisfy all the constraints and pass through all the planned waypoints. Assuming the vehicle as a point mass model, the best solution has been investigated through a genetic algorithm search procedure. The optimisation problem has been solved by modifying a micro-genetic algorithm software which was initially developed by D.L. Carroll. Between all the possible trajectories we select the more "realistic" connections among the waypoints. First of all, we have left out the trajectories with discontinuity in the derivatives as these are not feasible by the real aircraft. The polynomial spline is a suitable candidate to solve our problem. The algorithm splits the trajectory in sub-trajectories which join a sequence of three waypoints. Starting from the first three waypoints, the following sub-trajectories are superimposed keeping the first waypoint coincident with the last of the previous sub-trajectory. The sequence of polynomials is initialized assuming that jumps in the direction of flight are avoided pointing the heading angle in the presumed direction of flight. The optimal trajectory is a trade-off amongst three factors: the required energy cost, the minimum distance from the required waypoint and the feasibility of the trajectory. Results obtained with this optimization procedure are presente
Pediatric non alcoholic fatty liver disease: old and new concepts on development, progression, metabolic insight and potential treatment targets
Nonalcoholic fatty liver disease (NAFLD) is the leading cause of chronic liver disease in children. NAFLD has emerged to be extremely prevalent, and predicted by obesity and male gender. It is defined by hepatic fat infiltration >5% hepatocytes, in the absence of other causes of liver pathology. It includes a spectrum of disease ranging from intrahepatic fat accumulation (steatosis) to various degrees of necrotic inflammation and fibrosis (non-alcoholic steatohepatatis [NASH]). NAFLD is associated, in children as in adults, with severe metabolic impairments, determining an increased risk of developing the metabolic syndrome. It can evolve to cirrhosis and hepatocellular carcinoma, with the consequent need for liver transplantation. Both genetic and environmental factors seem to be involved in the development and progression of the disease, but its physiopathology is not yet entirely clear. In view of this mounting epidemic phenomenon involving the youth, the study of NAFLD should be a priority for all health care systems. This review provides an overview of current and new clinical-histological concepts of pediatric NAFLD, going through possible implications into patho-physiolocical and therapeutic perspectives
Capture of Leptophilic Dark Matter in Neutron Stars
Dark matter particles will be captured in neutron stars if they undergo
scattering interactions with nucleons or leptons. These collisions transfer the
dark matter kinetic energy to the star, resulting in appreciable heating that
is potentially observable by forthcoming infrared telescopes. While previous
work considered scattering only on nucleons, neutron stars contain small
abundances of other particle species, including electrons and muons. We perform
a detailed analysis of the neutron star kinetic heating constraints on
leptophilic dark matter. We also estimate the size of loop induced couplings to
quarks, arising from the exchange of photons and Z bosons. Despite having
relatively small lepton abundances, we find that an observation of an old,
cold, neutron star would provide very strong limits on dark matter interactions
with leptons, with the greatest reach arising from scattering off muons. The
projected sensitivity is orders of magnitude more powerful than current dark
matter-electron scattering bounds from terrestrial direct detection
experiments.Comment: 26 pages, 8 figures, 3 tables, 2 appendices. Discussion extended,
references added, matches JCAP published versio
Existence of periodic orbits near heteroclinic connections
We consider a potential with two different global minima
and, under a symmetry assumption, we use a variational approach to
show that the Hamiltonian system \begin{equation} \ddot{u}=W_u(u), \hskip 2cm
(1) \end{equation} has a family of -periodic solutions which, along a
sequence , converges locally to a heteroclinic solution
that connects to . We then focus on the elliptic system
\begin{equation} \Delta u=W_u(u),\;\; u:R^2\rightarrow R^m, \hskip 2cm (2)
\end{equation} that we interpret as an infinite dimensional analogous of (1),
where plays the role of time and is replaced by the action functional
We assume that
has two different global minimizers in the set of maps that connect to . We work in a symmetric
context and prove, via a minimization procedure, that (2) has a family of
solutions , which is -periodic in , converges to
as and, along a sequence
, converges locally to a heteroclinic solution that
connects to .Comment: 36 pages, 4 figure
Wrinkling in engineering fabrics: a comparison between two different comprehensive modelling approaches
We consider two ‘comprehensive’ modelling approaches for engineering fabrics. We distinguish the two approaches using the terms ‘semi-discrete’ and ‘continuum’, reflecting their natures. We demonstrate a fitting procedure, used to identify the constitutive parameters of the continuum model from predictions of the semi-discrete model, the parameters of which are in turn fitted to experimental data. We, then, check the effectiveness of the continuum model by verifying the correspondence between semi-discrete and continuum model predictions using test cases not previously used in the identification process. Predictions of both modelling approaches are compared against full-field experimental kinematic data, obtained using stereoscopic digital image correlation techniques, and also with measured force data. Being a reduced order model and being implemented in an implicit rather than an explicit finite-element code, the continuum model requires significantly less computational power than the semi-discrete model and could therefore be used to more efficiently explore the mechanical response of engineering fabrics
To what extent does photoperiod affect cattle reproduction? Clinical perspectives of melatonin administration
The seasonality of reproduction in most mammals is dictated by photoperiod, temperature and nutrition. Melatonin, mainly synthesized in the pineal gland, is generally accepted as the active mediator of photoperiod responses including reproduction. While non-pregnant heifers and cows show continuous sexual activity and are therefore not seasonal breeders, it has been suggested that photo-periodicity may influence the appearance of puberty in heifers and the onset of parturition. Further, the light/dark ratio may influence endocrine patterns of gestation and a shorter light period correlates with the incidence of twin pregnancies. This review considers specific aspects of the effects of photoperiod and melatonin on reproduction in dairy cattle and discusses the clinical applications of melatonin
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