Drag Reduction in Turbulent Wall-Bounded Flows of Realistic Polymer Solutions.

Suspensions of DNA macromolecules (0.8 wppm, 60 kbp), modeled as finitely extensible nonlinear elastic dumbbells coupled to the Newtonian fluid, show drag reduction up to 27% at friction Reynolds number 180, saturating at the previously unachieved Weissenberg number ≃10^{4}. At a large Weissenberg number, the drag reduction is entirely induced by the fully stretched polymers, as confirmed by the extensional viscosity field. The polymer extension is strongly non-Gaussian, in contrast to the assumptions of classical viscoelastic models.

The Fluid-Dynamics of Endo Vascular Aneurysm Sealing (EVAS) System failure.

PURPOSE: The main objective of this work is to investigate hemodynamics phenomena occurring in EVAS (Endo Vascular Aneurysm Sealing), to understand if and how they could lead to type 1a endoleaks and following re-intervention. To this aim, methods based on computational fluid mechanics are implemented as a tool for checking the behavior of a specific EVAS configuration, starting from the post-operative conditions. Pressure and velocity fields are detailed and compared, for two configurations of the Nellix, one as attained after correct implantation and the other in pathological conditions, as a consequence of migration or dislocation of endobags. METHODS: The computational fluid dynamics (CFD) approach is used to simulate the behavior of blood within a segment of the aorta, before and after the abdominal bifurcation. The adopted procedure allows reconstructing the detailed vascular geometry from high-resolution computerized tomography (CT scan) and generating the mesh on which the equations of fluid mechanics are discretized and solved, in order to derive pressure and velocity field during heartbeats. RESULTS: The main results are obtained in terms of local velocity fields and wall pressures. Within the endobags, velocities are usually quite regular during the whole cardiac cycle for the post-implanted condition, whereas they are more irregular for the migrated case. The largest differences among the two cases are observed in the shape and location of the recirculation region in the rear part of the aorta and the region between the endobags, with the formation of a gap due to the migration of one or both of the two. In this gap, the pressure fields are highly different among the two conditions, showing pressure peaks and pressure gradients at least four times larger for the migrated case in comparison to the post-implanted condition. CONCLUSIONS: In this paper, the migration of one or both endobags is supposed to be related to the existing differential pressures acting in the gap formed between the two, which could go on pushing the two branches one away from the other, thus causing aneurysm re-activation and endoleaks. Regions of flow recirculation and low-pressure drops are revealed only in case of endobag migration and in presence of an aneurysm. These regions are supposed to lead to possible plaque formation and atherosclerosis.

Exact regularized point particle (ERPP) method for particle-laden wall-bounded flows in the two-way coupling regime.

The Exact Regularized Point Particle (ERPP) method is extended to treat the interphase momentum coupling between particles and fluid in the presence of walls by accounting for the vorticity generation due to the particles close to solid boundaries. The ERPP method overcomes the limitations of other methods by allowing the simulation of an extensive parameter space (Stokes number, mass loading, particle-to-fluid density ratio and Reynolds number) and of particle spatial distributions that are uneven (few particles per computational cell). The enhanced ERPP method is explained in detail and validated by considering the global impulse balance. In conditions when particles are located close to the wall, a common scenario in wall-bounded turbulent flows, the main contribution to the total impulse arises from the particle-induced vorticity at the solid boundary. The method is applied to direct numerical simulations of particle-laden turbulent pipe flow in the two-way coupling regime to address the turbulence modulation. The effects of the mass loading, the Stokes number and the particle-to-fluid density ratio are investigated. The drag is either unaltered or increased by the particles with respect to the uncoupled case. No drag reduction is found in the parameter space considered. The momentum stress budget, which includes an extra stress contribution by the particles, provides the rationale behind the drag behaviour. The extra stress produces a momentum flux towards the wall that strongly modifies the viscous stress, the culprit of drag at solid boundaries.

Pressure control in interfacial systems: Atomistic simulations of vapor nucleation.

A large number of phenomena of scientific and technological interest involve multiple phases and occur at constant pressure of one of the two phases, e.g., the liquid phase in vapor nucleation. It is therefore of great interest to be able to reproduce such conditions in atomistic simulations. Here we study how popular barostats, originally devised for homogeneous systems, behave when applied straightforwardly to heterogeneous systems. We focus on vapor nucleation from a super-heated Lennard-Jones liquid, studied via hybrid restrained Monte Carlo simulations. The results show a departure from the trends predicted for the case of constant liquid pressure, i.e., from the conditions of classical nucleation theory. Artifacts deriving from standard (global) barostats are shown to depend on the size of the simulation box. In particular, for Lennard-Jones liquid systems of 7000 and 13 500 atoms, at conditions typically found in the literature, we have estimated an error of 10-15 kBT on the free-energy barrier, corresponding to an error of 104-106 s-1σ-3 on the nucleation rate. A mechanical (local) barostat is proposed which heals the artifacts for the considered case of vapor nucleation.

Activated Wetting of Nanostructured Surfaces: Reaction Coordinates, Finite Size Effects, and Simulation Pitfalls.

A liquid in contact with a textured surface can be found in two states, Wenzel and Cassie. In the Wenzel state the liquid completely wets the corrugations while in the Cassie state the liquid is suspended over the corrugations with air or vapor trapped below. The superhydrophobic properties of the Cassie state are exploited for self-cleaning, drag reduction, drug delivery, etc., while in the Wenzel state most of these properties are lost; it is therefore of great fundamental and technological interest to investigate the kinetics and mechanism of the Cassie-Wenzel transition. Computationally, the Cassie-Wenzel transition is often investigated using enhanced sampling ("rare events") techniques based on the use of collective variables (CVs). The choice of the CVs is a crucial task because it affects the free-energy profile, the estimation of the free-energy barriers, and the evaluation of the mechanism of the process. Here we investigate possible simulation artifacts introduced by common CVs adopted for the study of the Cassie-Wenzel transition: the average particle density in the corrugation of a textured surface and the coarse-grained density field at various levels of coarse graining. We also investigate possible additional artifacts associated with finite size effects. We focus on a pillared surface, a system often used in technological applications. We show that the use of a highly coarse-grained density (a single CV) of the fluid in the interpillar region brings to severe artifacts: errors of hundreds of kBT in the difference of free energy between the Cassie and Wenzel states, of tens of kBT in the estimate of the free-energy barriers, and erroneous wetting mechanisms. A proper description of the wetting mechanism and its energetics apparently requires a fine discretization of the density field. Concerning the finite-size effects, we have found that the typical systems employed in simulations of the Cassie-Wenzel transition, containing a single pillar within periodic boundary conditions, prevent the complete break of translational symmetry of the liquid-vapor meniscus during the process. Capturing this break of symmetry is crucial for describing the transition state along the wetting process and the early stage of the opposite process, the Wenzel-Cassie transition.

Intrusion and extrusion of a liquid on nanostructured surfaces.

Superhydrophobicity is connected to the presence of gas pockets within surface asperities. Upon increasing the pressure this 'suspended' state may collapse, causing the complete wetting of the rough surface. In order to quantitatively characterize this process on nanostructured surfaces, we perform rare-event atomistic simulations at different pressures and for several texture geometries. Such an approach allows us to identify for each pressure the stable and metastable states and the free energy barriers separating them. Results show that, by starting from the superhydrophobic state and increasing the pressure, the suspended state abruptly collapses at a critical intrusion pressure. If the pressure is subsequently decreased, the system remains trapped in the metastable state corresponding to the wet surface. The liquid can be extruded from the nanostructures only at very negative pressures, by reaching the critical extrusion pressure (spinodal for the confined liquid). The intrusion and extrusion curves form a hysteresis cycle determined by the large free energy barriers separating the suspended and wet states. These barriers, which grow very quickly for pressures departing from the intrusion/extrusion pressure, are shown to strongly depend on the texture geometry.

Shock wave formation in the collapse of a vapor nanobubble.

In this Letter, the dynamics of a collapsing vapor bubble is addressed by means of a diffuse-interface formulation. The model cleanly captures, through a unified approach, all the critical features of the process, such as phase change, transition to supercritical conditions, thermal conduction, compressibility effects, and shock wave formation and propagation. Rather unexpectedly for pure vapor bubbles, the numerical experiments show that the process consists in the oscillation of the bubble associated with the emission of shock waves in the liquid, and with the periodic disappearance and reappearance of the liquid-vapor interface due to transition to super- or subcritical conditions. The results identify the mechanism of shock wave formation as strongly related to the transition of the vapor to the supercritical state, with a progressive steepening of a focused compression wave evolving into a shock which is eventually reflected as an outward propagating wave in the liquid.

Tilting angle and water slippage over hydrophobic coatings.

Hydrophobic coatings, such as octadecyltrichlorosilane or n-alkyl monolayers, enhance the slippage of liquids on solid walls. For a given alkyl chain length, the main structural parameter for homogeneous coatings is the tilt angle between coating molecules and the surface normal. In this paper, ab initio calculations are used to calculate the equilibrium configuration of coating molecules, showing that the tilt angle easily changes from 0° to 30° depending on the specific head group binding the solid substrate. These values are used to set up classical molecular dynamics of water slippage over the coatings using different water models (Transferable Intermolecular Potential 3 point (TIP3P), Transferable Intermolecular Potential 4 point (TIP4P) and TIP4P/2005). The slippage is found to be robust with respect to the coating tilting, while a slight dependence on the water model is observed.

Molecular dynamics simulation of ratchet motion in an asymmetric nanochannel.

The persistence of ratchet effects, i.e., nonzero mass flux under a zero-mean time-dependent drive, when many-body interactions are present, is studied via molecular dynamics (MD) simulations of a simple liquid flowing in an asymmetric nanopore. The results show that (i) ratchet effects persist under many-body density correlations induced by the forcing; (ii) two distinct linear responses (flux proportional to the drive amplitude) appear under strong loads. One regime has the same conductivity of linear response theory up to a forcing of about 10 kT, while the second displays a smaller conductivity, the difference in responses is due to geometric effects alone. (iii) Langevin simulations based on a naive mapping of the many-body equilibrium bulk diffusivity, D, onto the damping rate, gamma are also found to yield two distinct linear responses. However, in both regimes, the flux is significantly smaller than the one of MD simulations.

Scaling properties in the production range of shear dominated flows.

In large Reynolds number turbulence, isotropy is recovered as the scale is reduced and homogeneous-isotropic scalings are eventually observed. This picture is violated in many cases, e.g., wall bounded flows, where, due to the shear, different scaling laws emerge. This effect has been ascribed to the contamination of the inertial range by the larger anisotropic scales. The issue is addressed here by analyzing both numerical and experimental data for a homogeneous shear flow. In fact, under strong shear, the alteration of the scaling exponents is not induced by the contamination from the anisotropic sectors. Actually, the exponents are universal properties of the isotropic component of the structure functions of shear dominated flows. The implications are discussed in the context of turbulence near solid walls, where improved closure models would be advisable.

Double scaling and intermittency in shear dominated flows.

The nature of intermittency in shear dominated flows changes with respect to homogeneous and isotropic conditions since the process of energy transfer is affected by the turbulent kinetic energy production associated with the Reynolds stresses. For these flows, a new form of refined similarity law is able to describe the increased level of intermittency. Ideally a length scale associated with the mean shear separates the two ranges, i.e., the classical Kolmogorov-like inertial range, below, and the shear dominated range, above. In the present paper we give evidence of the coexistence of the two regimes and we support the conjecture that the statistical properties of the dissipation field are practically insensible to the mean shear. This allows for a theoretical prediction of the scaling exponents of structure functions in the shear dominated range based on the known intermittency corrections for isotropic flows. The prediction is found to closely match the available numerical and experimental data. The analysis shows that the larger anisotropic scales of shear turbulence display universality, and determines the modality by which the dissipation field fixes the properties of turbulent fluctuations in the shear dominated range.