Suppression of von K\'arm\'an vortex streets past porous rectangular cylinders

Although the stability properties of the wake past impervious bluff bodies have been widely examined in the literature, similar analyses regarding the flow around and through porous ones are still lacking. In this work, the effect of the porosity and permeability on the wake patterns of porous rectangular cylinders is numerically investigated at low to moderate Reynolds numbers in the framework of direct numerical simulation combined with local and global stability analyses. A modified Darcy-Brinkman formulation is employed here so as to describe the flow behavior inside the porous media, where also the convective terms are retained to correctly account for the inertial effects at high values of permeability. Different aspect ratios of the cylinder are considered, varying the thickness-to-height ratios, t/d, from 0.01 (flat plate) to 1.0 (square cylinder). The results show that the permeability of the bodies has a strong effect in modifying the characteristics of the wakes and of the associated flow instabilities, while the porosity weakly affects the resulting flow patterns. In particular, the fluid flows through the porous bodies and, thus, as the permeability is progressively increased, the recirculation regions, initially attached to the rear part of the bodies, at first detach from the body and, eventually, disappear even in the near wakes. Global stability analyses lead to the identification of critical values of the permeability above which any linear instability is prevented. Moreover, a different scaling of the non-dimensional permeability allows to identify a general threshold for all the configurations here studied that ensures the suppression of vortex shedding, at least in the considered parameter space.Comment: 31 pages and 17 figure

Documenting and modeling the accretion of surface and subsoil organic carbon in agricultural Inceptisols reclaimed from Mediterranean sea marshes in Sardinia

High input agriculture in productive Inceptisols that were reclaimed from sea marshes offers an opportunity to study the increase of soil organic carbon (SOC) in soils with originally low SOC. We documented the current SOC content and its distribution with depth for several soil profiles

Prescription appropriateness of anti-diabetes drugs in elderly patients hospitalized in a clinical setting: evidence from the REPOSI Register

Diabetes is an increasing global health burden with the highest prevalence (24.0%) observed in elderly people. Older diabetic adults have a greater risk of hospitalization and several geriatric syndromes than older nondiabetic adults. For these conditions, special care is required in prescribing therapies including anti- diabetes drugs. Aim of this study was to evaluate the appropriateness and the adherence to safety recommendations in the prescriptions of glucose-lowering drugs in hospitalized elderly patients with diabetes. Data for this cross-sectional study were obtained from the REgistro POliterapie-Società Italiana Medicina Interna (REPOSI) that collected clinical information on patients aged ≥ 65 years acutely admitted to Italian internal medicine and geriatric non-intensive care units (ICU) from 2010 up to 2019. Prescription appropriateness was assessed according to the 2019 AGS Beers Criteria and anti-diabetes drug data sheets.Among 5349 patients, 1624 (30.3%) had diagnosis of type 2 diabetes. At admission, 37.7% of diabetic patients received treatment with metformin, 37.3% insulin therapy, 16.4% sulfonylureas, and 11.4% glinides. Surprisingly, only 3.1% of diabetic patients were treated with new classes of anti- diabetes drugs. According to prescription criteria, at admission 15.4% of patients treated with metformin and 2.6% with sulfonylureas received inappropriately these treatments. At discharge, the inappropriateness of metformin therapy decreased (10.2%, P < 0.0001). According to Beers criteria, the inappropriate prescriptions of sulfonylureas raised to 29% both at admission and at discharge. This study shows a poor adherence to current guidelines on diabetes management in hospitalized elderly people with a high prevalence of inappropriate use of sulfonylureas according to the Beers criteria

Behavior of wake flows past porous bluff bodies

In the present thesis, the wake flows at low Reynolds numbers past porous bluff bodies are investigated, in the context of flow control. The mathematical model for the flow inside the porous medium is based on the Volume Averaged Navier Stokes Equations. The resulting formulation takes into account the penetration of the viscous and inertial terms in the porous medium. At the interface between the surface and the pure fluid the continuity of the velocities and stresses is considered. The mathematical model for the flow through a porous medium has been then validated against Direct Numerical Simulations described in the literature, finding an excellent agreement; the presence of the inertial term in the equations is fundamental to well capture the flow behavior near the interface. The stability analysis is carried out within the framework of the global linear stability theory, of the sensitivity analysis and of the local spatio-temporal analysis. The introduced analytic tools have been applied to the case of the porous rectangular cylinders, varying the thickness-to-height ratio, up to the two limit cases of the flat plate and the square cylinder. In all the configurations it is noted that the recirculation bubble detaches from the body and moves downstream, as the permeability is increased. At the same time the recirculation bubble becomes smaller and disappears, resulting in a stabilization of the flow and in an inhibition of the vortex shedding. This behavior has been observed for all values of the Reynolds number. The detachment of the recirculation bubble implies the displacement of the global instability core, that has been verified both with the sensitivity analysis and some Direct Numerical Simulations. Moreover, it is found that, for proper combinations of the flow parameters, the flow is globally unstable but without a recirculation region in the wake. The local spatio-temporal analysis is shown to provide an explanation of this behavior, also demonstrating that the marginal stability curve in the space of the parameters can be related to a specific region in the wake, in which the velocity on the symmetry axis is less than the 5% of the free-stream velocity. The results for different values of the thickness of the rectangular bodies indicate that the behavior is the same introducing a parameter corrected with the thickness-to-height ratio of the rectangular cylinder. The steady and axisymmetric flow, for a disk and a sphere, has also been carried out using the global stability and sensitivity analysis. The results show an analogous behavior to that of the rectangular cylinders, while the flow around a sphere can exhibit a penetrating recirculation bubble, besides the already illustrated phenomena

Dripping down the rivulet

This paper is associated with a video winner of a 2018 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion

Instability of a thin viscous film flowing under an inclined substrate: the emergence and stability of rivulets

We study the pattern formation of a thin film flowing under an inclined planar substrate. The phenomenon is studied in the context of the Rayleigh-Taylor instability using the lubrication equation. Inspired by experimental observations, we numerically study the thin film response to a streamwise-invariant sinusoidal initial condition. The numerical response shows the emergence of predominant streamwise-aligned structures, modulated along the direction perpendicular to the flow, called rivulets. Oscillations of the thickness profile along the streamwise direction do not grow significantly when the inclination is very large or the liquid layer very thin. However, for small inclinations or thick films, streamwise perturbations grow on rivulets. A secondary stability analysis of one-dimensional and steady rivulets reveals a strong stabilization mechanism for large inclinations or very thin films. The theoretical results are compared with experimental measurements of the streamwise oscillations of the rivulet profile, showing a good agreement. The emergence of rivulets is investigated by studying the impulse response. Both the experimental observation and the numerical simulation show a marked anisotropy favouring streamwise-aligned structures. A weakly nonlinear model is proposed to rationalize the levelling of all but streamwise-aligned structures

Instability of a thin viscous film flowing under an inclined substrate: steady patterns

The flow of a thin film coating the underside of an inclined substrate is studied. We measure experimentally spatial growth rates and compare them to the linear stability analysis of a flat film modelled by the lubrication equation. When forced by a stationary localized perturbation, a front develops that we predict with the group velocity of the unstable wave packet. We compare our experimental measurements with numerical solutions of the nonlinear lubrication equation with complete curvature. Streamwise structures dominate and saturate after some distance. We recover their profile with a one-dimensional lubrication equation suitably modified to ensure an invariant profile along the streamwise direction and compare them with the solution of a purely two-dimensional pendent drop, showing overall a very good agreement. Finally, those different profiles agree also with a two-dimensional simulation of the Stokes equations

To drip or not to drip ::pattern formation of a thin film flowing under an inclined plane

We discuss the pattern formation of a thin film flowing under an inclined plane, with theoretical, experimental and numerical analyses, in the context of the Rayleigh-Taylor instability and in the absence of inertia

On the stability of wake flows past porous bluff bodies

Inspired by nature and motivated by engineering applications, the study of the flow past and through porous media has received a growing interest in our community over the years. In this work, we present results from a numerical study aimed to investigate the effect of the porosity and permeability on the transition scenario of the wakes of porous bluff bodies at low-moderate Reynolds numbers. This work is developed in the framework of direct numerical simulation and linear stability analysis. First, the two-dimensional flow past porous rectangular cylinders is investigated, considering thickness-to-height ratios ranging from 0.01 (flat plate) to 1.0 (square cylinder). Then, the cases of the flow past a porous sphere and a disk are studied. The results show that the permeability of the bodies has a strong effect in modifying the characteristics of the flow instabilities, while the porosity weakly affects the flow patterns. In particular, since the fluid can flow through the porous obstacles, the recirculation regions detach from the body first and then disappear in the near wakes when the permeability is increased. Lastly, for all the configurations here presented, critical values of the permeability are furthermore identified above which any flow instabilities are prevented