Fluid dynamics in the functional foregut of xylem-sap feeding insects: A comparative study of two Xylella fastidiosa vectors.

Xylem sap sucking insects are adapted to ingest fluids under tension. Although much has been learned about such feeding strategy, this adaptation still poses several unresolved questions, including how these insects ingest against strong xylem sap tension. Xylem sap-feeding insects are vectors of the plant pathogenic xylem-limited bacterium Xylella fastidiosa. This bacterium colonizes the cuticular lining of the foregut of vectors in a persistent manner. We used micro-computed tomography and scanning electron microscopy to investigate the foregut morphometry of two X. fastidiosa vector species: Philaenus spumarius and Graphocephala atropunctata (Hemiptera: Aphrophoridae and Cicadellidae, respectively). On the basis of morphometric data, we built a hydrodynamic model of the foregut of these two insect species, focusing on the precibarium, a region previously shown to be colonized by X. fastidiosa and correlated with pathogen acquisition from and inoculation to plants. Our data show that space in the P. spumarius functional foregut could potentially harbor twice as many cells as similar space in G. atropunctata, although the opposite trend has been observed with biological samples. Average flow velocity of ingested fluid depended on the percentage of the cibarium volume exploited for suction: if the entire volume were used, velocities were in the range of meters per second. In contrast, velocities on the order of those found in the literature (about 10 cm/s) were attained if only 5% of the cibarium volume were exploited. Simulated bacterial colonization of the foregut was analyzed in relation to hydrodynamics and pressure needed for insects to ingest. Our model is designed to represent the diameter reduction of the food canal in both insect species when infected with X. fastidiosa. Results indicated that full bacterial colonization significantly increased the mean sap-sucking flow velocity. In particular, the colonization increased the maximum section-averaged velocity in the G. atropunctata more than two times and the net pressure needed to mantain the flow in the precibarium when colonized is relevant (about 0.151 MPa) if compared to a standard xylem sap tension (1 MPa). Bacterial colonization also influenced the sucking process of the G. atropunctata, by hindering the formation of a recirculation zone (or eddy), that characterizd the flow in the distal part of the precibarium when bacteria were absent. On the other hand, considering the pressure the insect must generate to feed, X. fastidiosa colonization probably influences fitness of the G. atropunctata more than that of P. spumarius.

Monitoring for Coastal Resilience: Preliminary Data from Five Italian Sandy Beaches.

Video-monitoring can be exploited as a valuable tool to acquire continuous, high-quality information on the evolution of beach morphology at a low cost and, on such basis, perform beach resilience analyses. This manuscript presents preliminary results of an ongoing, long-term monitoring programme of five sandy Italian beaches along the Adriatic and Tyrrhenian sea. The project aims at analyzing nearshore morphologic variabilities on a time period of several years, to link them to resilience indicators. The observations indicate that most of the beach width variations can be linked to discrete variations of sandbar systems, and most of all to an offshore migration and decay of the outermost bars. Further, the largest net shoreline displacements across the observation period are experienced by beaches with a clear NOM (Net Offshore Migration)-type evolution of the seabed.

Extra Strain Rates in an unsteady spilling breaking wave.

We analyze the extra strain rates that characterize a curved, thin mixing layer induced at an unsteady spilling breaker. We focus on the flow curvature, which induces some extra rates of strain that should be accounted for in algebraic-type turbulence closures. The comparison between the analytical formulation proposed by Brocchini and co-workers for a single-phase turbulent thin layer of fluid and the data, obtained from a Particle Image Velocimetry (PIV) dedicated experimental program, reveals that the order of magnitude of the extra rates of strain induced by the streamline curvature, is comparable with that of the simple shear. This differs from what observed for the geometric curvature terms and from what occurs at hydraulic jumps, typically used to model steady breakers.

Summertime conditions of a muddy estuarine environment: the EsCoSed project contribution.

As part of the Estuarine Cohesive Sediments (EsCoSed) project, a field experiment was performed in a highly engineered environment, acting as a natural laboratory, to study the physico-chemical properties of estuarine sediments and the associated hydro-morphodynamics during different seasons. The present contribution focuses on the results obtained from the summertime monitoring of the most downstream part of the Misa River (Senigallia, Italy). The measured hydrodynamics suggested a strong interaction between river current, wave forcing and tidal motion; flow velocities, affected by wind waves traveling upstream, changed significantly along the water column in both direction and magnitude. Surficial salinities in the estuary were low in the upper reaches of the estuary and exceeded 10 psu before the river mouth. Montmorillonite dominated the clay mineral assemblage, suggesting that large, low density flocs with high settling velocities (>1 mm s(-1)) may dominate the suspended aggregate materials.

A reasoned overview on Boussinesq-type models: the interplay between physics, mathematics and numerics.

This paper, which is largely the fruit of an invited talk on the topic at the latest International Conference on Coastal Engineering, describes the state of the art of modelling by means of Boussinesq-type models (BTMs). Motivations for using BTMs as well as their fundamentals are illustrated, with special attention to the interplay between the physics to be described, the chosen model equations and the numerics in use. The perspective of the analysis is that of a physicist/engineer rather than of an applied mathematician. The chronological progress of the currently available BTMs from the pioneering models of the late 1960s is given. The main applications of BTMs are illustrated, with reference to specific models and methods. The evolution in time of the numerical methods used to solve BTMs (e.g. finite differences, finite elements, finite volumes) is described, with specific focus on finite volumes. Finally, an overview of the most important BTMs currently available is presented, as well as some indications on improvements required and fields of applications that call for attention.