Encapsulated formulation of the Selective Frequency Damping method

We present an alternative "encapsulated" formulation of the Selective Frequency Damping method for finding unstable equilibria of dynamical systems, which is particularly useful when analysing the stability of fluid flows. The formulation makes use of splitting methods, which means that it can be wrapped around an existing time-stepping code as a "black box". The method is first applied to a scalar problem in order to analyse its stability and highlight the roles of the control coefficient $\chi$ and the filter width $\Delta$ in the convergence (or not) towards the steady-state. Then the steady-state of the incompressible flow past a two-dimensional cylinder at $Re=100$, obtained with a code which implements the spectral/hp element method, is presented

Optical frequency combs from high-order sideband generation

We report on the generation of frequency combs from the recently-discovered phenomenon of high-order sideband generation (HSG). A near-band gap continuous-wave (cw) laser with frequency $f_\text{NIR}$ was transmitted through an epitaxial layer containing GaAs/AlGaAs quantum wells that were driven by quasi-cw in-plane electric fields $F_\text{THz}$ between 4 and 50 kV/cm oscillating at frequencies $f_\text{THz}$ between 240 and 640 GHz. Frequency combs with teeth at $f_\text{sideband}=f_\text{NIR}+nf_\text{THz}$ ($n$ even) were produced, with maximum reported $n>120$, corresponding to a maximum comb span $>80$ THz. Comb spectra with the identical product $f_\text{THz}\times F_\text{THz}$ were found to have similar spans and shapes in most cases, as expected from the picture of HSG as a scattering-limited electron-hole recollision phenomenon. The HSG combs were used to measure the frequency and linewidth of our THz source as a demonstration of potential applications

Wind Dispersal in Californian Desert Plants: Experimental Studies and Conceptual Considerations

Because of the important role wind is alleged to have in dispersal of fruits and seeds in desert plants, diaspores were collected for experimental study of 14 species from two Sonoran Desert localities of Imperial Co., California. Field observations on natural dispersal of these species also were made. Although all 14 species were not judged to be primarily adapted to anemochory, tests on air transport capability were made using a calibrated and modified seed-blowing machine. Diaspores were tested individually and results for 20 trials averaged for each experimental condition. In one series of tests, lofting ability was determined. In other tests, a Plexiglas wind tunnel was used; the bottom of the tunnel was lined with Plexiglas, wood, and sand for three types of trials respectively. Mean surface area and mean mass were determined for the seeds and fruits ofthe 14 species. These figures and the ratio between surface area and mass were compared to results from the lofting and horizontal movement tests. A high presentation surface area/mass ratio was positively correlated with ease of horizontal movement (tumbling chiefly) and ease of lofting. Excellence at transport in air appears variously countered in the anemochorous species by diaspore characteristics which seem to insure lodging in crevices or depressions. More than one kind of transport in air (e.g., tumbling, floating, skidding) can be identified and particular species may exhibit these in varying proportions. Seeds with high mass have nil wind-dispersal ability and thereby may be adapted to reaching (and staying in) depressions, washes, etc. High static friction is another mechanism which maximizes lodging ability. As Asteraceae show, anemochorous ability does not always run counter to lodging ability: capability of a diaspore to attach to hairs or skin of animals runs parallel to ability to lodge in soil crevices. In the study areas and in other desert localities, ants may play some role in movement of many kinds of seeds and fruits over short distances, but destruction by ants often is excessive