A new nonlinear travelling-wave solution for a flow through an isothermal square duct is discovered. The solution is found by a continuation approach in parameter space, starting from a case where the fluid is heated internally. The Reynolds number for which the travelling wave emerges is much lower than that of the solutions discovered recently by an analysis based on the self-sustaining process (Wedin et al., Phys. Rev. E, vol. 79, 2009, p. 065305; Uhlmann et al., Advances in Turbulence XII, 2009, pp. 585–588). Furthermore, the new travelling-wave solution is shown to be unstable from the onset.

A total of fifteen adult specimens of the so-called ‘lessepsian sprinter’ Fistularia commersonii were captured by bottom trawlers and gill-netters operating along the Latium coasts, Italy, in October–November 2007. This non-native species is established in the eastern Mediterranean but it has been sporadically reported beyond the Strait of Sicily. Never before has the species been recorded in such large numbers from a single locality in the western Mediterranean. These records may suggest an incoming establishment of a self-sustained population of the species also in the western Mediterranean and they document for the first time the presence of a lessepsian fish in Latium waters. Biometry and meristics of the available specimens of F. commersonii and data on their diet and reproductive condition are given.

A male specimen of Leucoraja circularis was captured in January 2005 by a bottom trawler operating south-east off the Anzio coast (central Tyrrhenian Sea). Biometric and meristic characters of the specimen are presented. This capture represents the first documented record of the species in the central Tyrrhenian Sea.

The first confirmed records of two blonde rays (Raja brachyura) in the Ligurian Sea for 140 years are described. Two specimens were caught off Cogoleto (western Ligurian Sea) and they were deposited in the Civic Museum of Natural History ‘Giacomo Doria’ of Genoa. Morphometric and meristic features have been reported.

The first case of albinism in the kitefin shark, Dalatias licha, is reported here through the study of one specimen caught by professional bottom trawl in the Gulf of Genoa, Ligurian Sea. The albino surface was quantified and morphometric and meristic characteristics were compared with those of normal specimens of the same sex and equivalent length, caught within the same area. The shark body presented a partial reduction in pigmentation and no morphometric differences between the albino specimen and the normal individuals were observed.

Two possible initial paths of transition to turbulence in simple shear flows are examined. The first is the – by now classical – transient (or algebraic) growth scenario which may have an important role in the by-pass transition of those flows for which traditional eigen-analysis predicts asymptotic stability. Transient growth is optimally excited by certain initial disturbances now known as "optimal perturbations"; they can be found through a classical variational analysis initiated by Farrell (1988). The second path starts with the exponential amplification, in nominally subcritical conditions, of modal disturbances developing over a base flow mildly distorted with respect to its idealized counterpart. The base flow distortion of given norm that excites the largest growth of the instability wave is called the “minimal defect", and its study was initiated by Bottaro et al. (2003). Both paths provide feasible initial conditions for the transition process and it is likely that in most practical situations algebraic and exponential growth mechanisms are concurrently at play in provoking transition to turbulence in shear flows.

Control theory is used to determine optimal disturbances in pipe flow and the forcing, in the form of blowing and suction at the wall, capable of attenuating them. An approach is adopted, based on a parabolic approximation of the linear Navier–Stokes equations, which is appropriate when dealing with asymptotically elongated (in the streamwise direction) flow structures. A cost functional is introduced and maximized in the optimal-perturbation problem or minimized, for a given inflow perturbation, in the optimal-control problem. The extrema of the cost functional are reached by means of an iterative technique, based on the numerical solution of the equations for the state and the adjoint state, coupled via transfer and optimality conditions. Central to the control is the determination of the Green's function expressing the receptivity of the flow to wall forcing. A considerable reduction in output disturbance energy, as compared to the uncontrolled case, is obtained for control laws operating both over a long section of the pipe or over shorter strips. Finally, a robust control is sought, by simultaneously computing the worst inflow condition and the corresponding best control at the wall.

We explore the spatial growth of disturbances developing on top of a base flow given by the Hagen–Poiseuille profile, which has been modified by a small axisymmetric and axially invariant distortion. Such deviations from the ideal parabolic profile may, for instance, occur in experiments as a result of experimental uncertainties. The optimal distortion (i.e. the distortion with a prescribed norm that induces the maximum growth rate) is computed by a variational technique. Unstable modes are found to exist for very small values of the norm of the deviation at low Reynolds numbers, and the instability is governed by an inviscid mechanism. The growth of these modes and the ensuing transition to turbulence is then studied by means of direct numerical simulations. Two possible paths of transition are found, one based on the exponential amplification of axisymmetric disturbances and the subsequent formation of $\uLambda$-vortices and the other based on the growth and breakdown of streamwise streaks.