Transition to turbulence in duct flow

The transition of the flow in a duct of square cross-section is studied. Like in the similar case of the pipe flow, the motion is linearly stable for all Reynolds numbers; this flow is thus a good candidate to investigate the 'bypass' path to turbulence. Initially the so-called 'linear optimal perturbation problem' is formulated and solved, yielding optimal disturbances in the form of longitudinal vortices. Such optimals, however, fail to elicit a significant response from the system in the nonlinear regime. Thus, streamwise-inhomogeneous, sub-optimal disturbances are focussed upon; nonlinear quadratic interactions are immediately evoked by such initial perturbations and an unstable streamwise-homogeneous large amplitude mode rapidly emerges. The subsequent evolution of the flow, at a value of the Reynolds number at the edge between fully developed turbulence and relaminarization, shows the alternance of patterns with two pairs of large scale vortices near opposing parallel walls. Such edge states bear a resemblance to optimal disturbance

Welfare budget lessons from Pre-Industrial England: why the ‘big society’ idea may not work

Cutting welfare spending is unlikely to lead to an increase in private voluntary work and charitable giving, explain Nina Boberg-Fazlic and Paul Sharp. Using historical data from late eighteenth and early nineteenth century England, they illustrate how parts of the country that saw increased levels of spending under the Poor Laws also enjoyed higher levels of charitable income

Progress report on the Heavy Ions in Space (HIIS) experiment

One of the objectives of the Heavy Ions In Space (HIIS) experiment is to investigate heavy ions which appear at Long Duration Exposure Facility (LDEF) below the geomagnetic cutoff for fully-ionized galactic cosmic rays. Possible sources of such 'below-cutoff' particles are partially-ionized solar energetic particles, the anomalous component of cosmic rays, and magnetospherically-trapped particles. In recent years, there have also been reports of below-cutoff ions which do not appear to be from any known source. Although most of these observations are based on only a handful of ions, they have led to speculation about 'partially-ionized galactic cosmic rays' and 'near-by cosmic ray sources'. The collecting power of HIIS is order of magnitude larger than that of the instruments which reported these results, so HIIS should be able to confirm these observations and perhaps discover the source of these particles. Preliminary results on below-cutoff heavy-ions are reported. Observations to possible known sources of such ions are compared. A second objective of the HIIS experiment is to measure the elemental composition of ultraheavy galactic cosmic rays, beginning in the tin-barium region of the periodic table. A report on the status of this analysis is presented

Anomalous scaling behavior in Takens-Bogdanov bifurcations

A general algorithm is presented for estimating the nonlinear instability threshold, $\sigma_c$, for subcritical transitions in systems where the linearized dynamics is significantly non-normal (i.e. subcritical bifurcations of {\em Takens-Bogdanov} type). The $N$-dimensional degenerate node is presented as an example. The predictions are then compared to numerical studies with excellent agreement.Comment: 6 page

Localised edge states nucleate turbulence in extended plane Couette cells

We study the turbulence transition of plane Couette flow in large domains where localised perturbations are observed to generate growing turbulent spots. Extending previous studies on the boundary between laminar and turbulent dynamics we determine invariant structures intermediate between laminar and turbulent flow. In wide but short domains we find states that are localised in spanwise direction, and in wide and long domains the states are also localised in downstream direction. These localised states act as critical nuclei for the transition to turbulence in spatially extended domains.Comment: 15 pages, 5 figure

Skadegörarna utmanar skogen

Invasiva skadegörare är främmande arter som som hamnat i en ny miljö och därigenom ofta orsakar stor skada. Skadegörare i skog består till största delen av olika insekter och svamp-patogener som orsakar sjukdomar. Introduktionen av invasiva skadegörare till Europas skogar har mer än fördubblats under de senaste 100 åren. De invasiva skadegörarna sprids effektivt via handeln med växter och växtmaterial, men även i förpackningsmaterial, som lastpallar. Svamppatogener introduceras oftast genom import av levande plantor medan skadliga insekter som barkborrar och nematoder oftast transporteras med trä och träprodukter. Den potentiella kostnaden orsakade av invasiva skadegörare är enorm. Att hantera almsjukan, en patogen som påverkat ett trädslag med liten utbredning i Sverige, har uppskattats kosta 20-54 miljoner kronor per år. Inkluderas värdeförluster ökar kostnaden med upp till 37 gånger! Skulle de dominerande trädslagen gran eller tall hotas av en invasiv art på liknande sätt som almen skulle kostnaderna snabbt kunna blir mycket stora. Dessutom går stora värden i kulturlandskapet och i biologisk mångfald förlorade

Information extraction and transmission techniques for spaceborne synthetic aperture radar images

Information extraction and transmission techniques for synthetic aperture radar (SAR) imagery were investigated. Four interrelated problems were addressed. An optimal tonal SAR image classification algorithm was developed and evaluated. A data compression technique was developed for SAR imagery which is simple and provides a 5:1 compression with acceptable image quality. An optimal textural edge detector was developed. Several SAR image enhancement algorithms have been proposed. The effectiveness of each algorithm was compared quantitatively

On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high Reynolds number flow over an Ahmed body

We investigate a hierarchy of eddy-viscosity terms in POD Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to Large Eddy Simulation data for a flow around the vehicle-like bluff body call Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (1994). Robustness of the model solution with respect to initial conditions, eddy viscosity values and model order is only achieved for state-dependent eddy viscosities as proposed by Noack, Morzynski & Tadmor (2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier-Stokes based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.Comment: Submitted to the Journal of Fluid Mechanic