Selfdual variational principles are introduced in order to construct solutions for Hamiltonian and other dynamical systems which satisfy a variety of linear and nonlinear boundary conditions including many of the standard ones. These principles lead to new variational proofs of the existence of parabolic flows with prescribed initial conditions, as well as periodic, anti-periodic and skew-perio...

Time-periodic shear flows can give rise to Parametric Instability (PI), as in the case of the Mathieu equation (Stoker, 1950; Nayfeh and Mook, 1995). This mechanism results from a resonance between the oscillatory basic state and waves that are superimposed on it. Farrell and Ioannou (1996a, b) explain that PI occurs because the snapshots of the velocity profile are subject to transient growth....

A numerical method is presented for the computation of externally forced Stokes flows bounded by the plane z=0 and satisfying periodic boundary conditions in the x and y directions. The motivation for this work is the simulation of flows generated by cilia, which are hair-like structures attached to the surface of cells that generate flows through coordinated beating. Large collections of cilia...

We present exact calculations of flow polynomials F (G, q) for lattice strips of various fixed widths Ly and arbitrarily great lengths Lx, with several different boundary conditions. Square, honeycomb, and triangular lattice strips are considered. We introduce the notion of flows per face fl in the infinite-length limit. We study the zeros of F (G, q) in the complex q plane and determine exactl...

Initially homogeneous uspensions of bubbles in basaltic magmas and lava flows may become unstable and form rising waves or layers of bubbles. We derive a set of model equations for the two-phase (bubble-liquid) system and present the results of a linear stability analysis and numerical simulations. Periodic vesicle layers are preserved in Columbia l•iver flows with a spacing of •1 m; the predic...

The IEEE 802.15.4 protocol offers great potential in several application fields, such as industrial Wireless Sensor Networks (WSNs). An industrial automation cell (field level) is characterized mostly by periodic traffic flows. Using the IEEE 802.15.4 protocol, it is necessary to manage Guaranteed Time Slots (GTSs) allocation and at the same time ensure adequate performance both to other period...

The paper considers the so-called dynamic phasor model as a basis for harmonic analysis of a class of switching systems. The dynamic phasor model is a powerful tool for exploring cyclic properties of dynamic systems. It is shown that there is a connection between the dynamic phasor model and the harmonic transfer function of a linear time periodic system and this connection is used to extend th...

A 63-yr-old man was noted to breathe with a periodic pattern of ventilation during sleep, both before and after operation for coronary artery grafting, and 6 weeks later after aortic aneurysm repair. Periodic ventilation was associated with cyclic oxygen desaturations and increases in heart rate and arterial pressure. Administration of oxygen was found to abolish consistently the periodic patte...

This paper investigates the localization within the Xenopus egg of the factors responsible for the periodic activities such as the cyclic rounding-up and flattening, related to the cleavage cycle. Denuded eggs were bisected along the boundary line between the animal and the vegetal hemispheres immediately after being rotated through 90 degrees off the vertical axis (Early Bisection). The result...