Alessandro Stocchino

The horizontal mixing induced by large-scale vortical features (macrovortices) in a straight compound channel flow are analysed with specific emphasis on possible similarities with those charac- teristic of a shear layer induced by the meeting of three shallow streams of different speed (shear layer model). An experimental investigation based on PTV and PIV analysis of free surface ve- locities forms

The linear and weakly nonlinear stability of a uniform flow in an infinitely wide open channel with erodible bottom is studied. Under suitable conditions the flow is found to be unstable, leading to the formation of dunes and antidunes. At a linear level, the corresponding regions of existence are presented and compared with experimental data. A weakly nonlinear analysis is

ABSTRACT The incidence of Abdominal Aortic Aneurysms (AAA) varies between 3% and 6% of the elder population, especially men over sixty years of age. Moreover, familiarity, smoking and peripherical atherosclerosis are known to be important risk factors. In the United States AAA ruptures cause about 9000 deaths every year. About 33000 elective surgical treatments per year are performer with a mortality of 1400–2800 patients. The percentage of success of surgical treatment remains strictly related to the diagnosis of the stage of the AAA. Mortality rate for elective surgical intervention (asymptomatic aneurysms) is 5%. On the contrary mortality rate for emergency surgical intervention is about 70% [1]. Selection of candidates to surgery is still based on measurement of aortic diameter, with a threshold size of 55 mm. The rate of AAA rupture increases from 21% to 46% for diameters between 50 and 70 mm. The main diagnostic tool currently adopted is the CT that allows for obtaining precise information regarding the dimensions and the morphology of the aneurysm and possible proximal extension (juxarenal, infrarenal, thorac-abdominal) and distal (iliac and hypogastric) of the aneurysm, intramural thrombus. Since rupture occurs when the aneurysm wall fails to withstand the forces acting on it, in vivo data on AAA wall pathology could identify patients at risk of rupture. End-stage aneurysm disease and especially aneurysm rupture are characterized by extensive inflammation of the arterial wall. Although the stimulus for this enhanced infiltration is not known, recent insights into the pathophysiology of aneurysm formation, growth, and rupture indicate a close relationship between increased mechanical stress and the activation of infiltrated lymphocytes and macrophages [2]. This increased inflammatory activity results in the progressive breakdown of the aortic wall, aneurysm dilatation and, ultimately, rupture. A further complication is represented by the mechanical actions exerted on the arterial wall by the internal blood flow. The blood flow is strongly influenced by the presence of an AAA, which modifies significantly the geometry of the aorta. Shear stresses and pressure are important factors to understand the formation and evolution of an AAA. Another important aspect is related to the formation of the thrombus that is thought to be relevant for the wall degeneration, see [3] for a comprehensive review of these topics.

... to assess the role of flow unsteadiness (and thus of coupling or decoupling bed and ... The kinematic and dynamic boundary conditions to be associated with (2) then readat the ... the leading order, the Exner's equation (11) does not produce any additional information, since under ...

The linear and weakly nonlinear stability of a uniform flow in an infinitely wide open channel with erodible bottom is studied. Under suitable conditions the flow is found to be unstable, leading to the formation of dunes and antidunes. At a linear level, the corresponding regions of existence are presented and compared with experimental data. A weakly nonlinear analysis is

ABSTRACT A linear stability analysis for dune and ripple formation is presented that implements a rotational two-dimensional flow model valid in the smooth as well as in the transitional and rough flow regimes. Sediment is assumed to be transported as bedload, disregarding the role of suspension. Therefore, the main mechanism driving instability, for both ripples and dunes, is the phase lag between bed shear stress and bed elevation. Ripples are shown to be confined to relatively low values of the Shields parameter and of the particle Reynolds number. For higher values of the Shields parameter and of the particle Reynolds number (and thus of the Froude number and of the roughness Reynolds number), ripples are replaced by dunes. The present analysis ultimately allows for a successful unification of the theories of dune and ripple formation and for a clarification of the debated role of ripples on the formation of dunes. A good agreement between predicted and observed wavelengths for both ripples and dunes is found.

ABSTRACT The linear stability of a uniform flow in an infinitely wide erodible channel is investigated with respect to disturbances of the bed that are periodic in both the transverse and the longitudinal directions. A rotational flow and sediment transport model, originally developed to study the formation of two-dimensional dunes and antidunes, is straightforwardly extended to cover variations in the lateral direction. Sediment is assumed to be transported as bed load, disregarding the role of suspension. Following a standard linearization procedure, a dispersion relationship is obtained that expresses the growth rate and the celerity of the sand wave as a function of the streamwise and spanwise wavenumbers and of the relevant flow and sediment parameters. Regions of instabilities in the space of the parameters are found, which can be associated with bed forms of different kinds, spanning from dunes and antidunes to alternate bars. Therefore, the present theory allows for a unified view of the formation of two- and three-dimensional bed forms in rivers in terms of the relevant flow and sediment parameters.

ABSTRACT Experimental measurements of the collision and rebound of a spherical particle with a solid wall in both newtonian and non-newtonian flow are provided. Experiments are carried out in a cubic Plexiglas tank where spheres of different materials, with a di- ameter of 5 and 10 mm, are free to settle in a still fluid. The latter consists of water, in the first set of observations, and a solution of Carboxylmethylcellulose (CMC) in wa- ter, in the second set. The rheology of the solution has been investigated and found to behave as an Hershel-Bulkley fluid. Different viscosities have been obtained varying CMC concentrations, providing a transparent media for Particle Image Velocimetry acquisition. The position and the velocity of the particle are estimated measuring in time the position of its centroid. Images have been acquired by IDT Cross correlation camera with a frame rate of 30Hz synchronized with a Nd:Yag laser by New Wave Re- search. Simultaneous measurements of both particle velocity and flow field allow for the identification of flow structures triggered by the sedimentation of the sphere and its impact on the horizontal wall. Comparison between the intensity of the ejections of the two different fluids observed whenever the particle approaches the wall can shed some light on particle wall collision mechanism in non-newtonian flow. Implications can be sought in the mechanism of collision of coarse particles in a muddy matrix, a process known to play an important role in debris flow dynamics.

The action of wind blowing over a closed basin ultimately results in a steady shear-induced circulation pattern and in a leeward rising of the free surface—and a corresponding windward lowering—known as wind set-up. If the horizontal dimensions of the basin are large with respect to the average flow depth, the occurrence of local quasi-equilibrium conditions can be expected, i.e. the flow can be assumed to be locally driven only by the wind stress and by the opposing free surface gradient due to set-up. This wind-induced flow configuration shows a strong similarity with turbulent Couette–Poiseuille flow, the one dimensional flow between parallel plates generated by the simultaneous action of a constant pressure gradient and of the shear induced by the relative motion of the plates. A two-equation turbulence closure is then employed to perform a numerical study of turbulent Couette–Poiseuille flows for different values of the ratio of the shear stresses at the two walls. The resulting eddy viscosity vertical distributions are analyzed in order to devise analytical profiles of eddy viscosity that account for the effect of wind. The results of this study, beside allowing for a physical insight on the turbulence process of this class of flows, will allow for a more accurate description of the wind effect to be included in the formulation of quasi-3D and 3D models of lagoon hydrodynamics.