Alignment of Nonspherical Active Particles in Chaotic Flows.

We study the orientation statistics of spheroidal, axisymmetric microswimmers, with shapes ranging from disks to rods, swimming in chaotic, moderately turbulent flows. Numerical simulations show that rodlike active particles preferentially align with the flow velocity. To explain the underlying mechanism, we solve a statistical model via the perturbation theory. We show that such an alignment is caused by correlations of fluid velocity and its gradients along particle paths combined with fore-aft symmetry breaking due to both swimming and particle nonsphericity. Remarkably, the discovered alignment is found to be a robust kinematical effect, independent of the underlying flow evolution. We discuss its possible relevance for aquatic ecology.

Validation of a questionnaire about knowledge and perception of biological risk among biomedical students of Sapienza University of Rome.

BACKGROUND ADN AIM: Healthcare workers and Biomedical students are continuously exposed to biological risk in their clinical practice. The objective of this study was to evaluate the validity and reliability of an Italian questionnaire on the knowledge and perception of biological risk in Biomedical students at the beginning of their professional training. MATERIALS AND METHODS: An electronic questionnaire was administered to students attending the second semester of the first year of Biomedical Courses at Sapienza University of Rome. The questionnaire consists of 40 questions divided into five sections collecting sociodemographic data, health status and level of knowledge and perception of biological risk. The statistical analysis was performed with Statistical Package for Social Sciences (SPSS) version 25. RESULTS: A total of 309 individuals answered the online questionnaire, with a response rate of 83.5%. The analysis of internal consistency was performed by two dichotomous variables that measured the knowledge level on hygiene behaviour and gloves use. The analysis showed a standardized Cronbach's alpha equal to 0.765, corresponding to a good reliability. A better reliability was found out among physiotherapy and medical students, with a Cronbach's alpha equal to 0.944 and 0.881, respectively. Regarding vaccines, 97.7% of the sample was given a Hepatitis B vaccination and 98.7% of students consider vaccinations essential for healthcare workers. CONCLUSIONS: Results of Cronbach's alpha showed a good reliability of the questionnaire. First-year Biomedical students may be exposed to occupational biological risk mainly because of their inexperience. A training educational path should be implemented in order to acquire competences, knowledge, attitudes and practical skills, correct behaviors and a personal and professional responsibility.

Point-particle method to compute diffusion-limited cellular uptake.

We present an efficient point-particle approach to simulate reaction-diffusion processes of spherical absorbing particles in the diffusion-limited regime, as simple models of cellular uptake. The exact solution for a single absorber is used to calibrate the method, linking the numerical parameters to the physical particle radius and uptake rate. We study the configurations of multiple absorbers of increasing complexity to examine the performance of the method by comparing our simulations with available exact analytical or numerical results. We demonstrate the potential of the method to resolve the complex diffusive interactions, here quantified by the Sherwood number, measuring the uptake rate in terms of that of isolated absorbers. We implement the method in a pseudospectral solver that can be generalized to include fluid motion and fluid-particle interactions. As a test case of the presence of a flow, we consider the uptake rate by a particle in a linear shear flow. Overall, our method represents a powerful and flexible computational tool that can be employed to investigate many complex situations in biology, chemistry, and related sciences.

Scale-dependent colocalization in a population of gyrotactic swimmers.

We study the small scale clustering of gyrotactic swimmers transported by a turbulent flow, when the intrinsic variability of the swimming parameters within the population is considered. By means of extensive numerical simulations, we find that the variety of the population introduces a characteristic scale R^{*} in its spatial distribution. At scales smaller than R^{*} the swimmers are homogeneously distributed, while at larger scales an inhomogeneous distribution is observed with a fractal dimension close to what observed for a monodisperse population characterized by mean parameters. The scale R^{*} depends on the dispersion of the population and it is found to scale linearly with the standard deviation both for a bimodal and for a Gaussian distribution. Our numerical results, which extend recent findings for a monodisperse population, indicate that in principle it is possible to observe small scale, fractal clustering in a laboratory experiment with gyrotactic cells.

Irreversibility of the two-dimensional enstrophy cascade.

We study the time irreversibility of the direct cascade in two-dimensional turbulence by looking at the time derivative of the square vorticity along Lagrangian trajectories, a quantity called metenstrophy. By means of extensive direct numerical simulations we measure the time irreversibility from the asymmetry of the probability density function of the metenstrophy and we find that it increases with the Reynolds number of the cascade, similarly to what is found in three-dimensional turbulence. A detailed analysis of the different contributions to the enstrophy budget reveals a remarkable difference with respect to what is observed for the energy cascade, in particular the role of the statistics of the forcing to determine the degree of irreversibility.

Anomalous diffusion in confined turbulent convection.

Turbulent convection in quasi-one-dimensional geometry is studied by means of high-resolution direct numerical simulations within the framework of Rayleigh-Taylor turbulence. Geometrical confinement has dramatic effects on the dynamics of the turbulent flow, inducing a transition from superdiffusive to subdiffusive evolution of the mixing layer and arresting the growth of kinetic energy. A nonlinear diffusion model is shown to reproduce accurately the above phenomenology. The model is used to predict, without free parameters, the spatiotemporal evolution of the heat flux profile and the dependence of the Nusselt number on the Rayleigh number.

Control of particle clustering in turbulence by polymer additives.

We study the clustering properties of inertial particles in a turbulent viscoelastic fluid. The investigation is carried out by means of direct numerical simulations of turbulence in the Oldroyd-B model. The effects of polymers on the small-scale properties of homogeneous turbulence are considered in relation with their consequences on clustering of particles, both lighter and heavier than the carrying fluid. We show that, depending on particle and flow parameters, polymers can either increase or decrease clustering.

Shell model for quasi-two-dimensional turbulence.

We discuss the possibility to introduce geometrical constraints in shell models of turbulence in order to mimic the turbulent dynamics that takes place in fluid layers with large aspect ratio. By using a scale-dependent set of coupling parameters, we are able to resolve both scales larger and smaller than a geometrical dimension of the flow. The proposed model is able to resolve with high accuracy the split energy cascade phenomenon recently observed in such flows, and allows us to investigate in detail the scaling properties of turbulent convection confined in narrow convective cells.

Nonlinear diffusion model for Rayleigh-Taylor mixing.

The complex evolution of turbulent mixing in Rayleigh-Taylor convection is studied in terms of eddy diffusivity models for the mean temperature profile. It is found that a nonlinear model, derived within the general framework of Prandtl mixing theory, reproduces accurately the evolution of turbulent profiles obtained from numerical simulations. Our model allows us to give very precise predictions for the turbulent heat flux and for the Nusselt number in the ultimate state regime of thermal convection.

Shear turbulence on a sparse spectral grid.

We simulate turbulence in a plane Couette geometry by a spectral method intermediate between full resolution and the complete elimination of small modes common in large eddy simulations. The wave number grid is sparse in spanwise and downstream direction, with a total number of modes proportional to Re(3/4) lnRe. At a Reynolds number of 2000 we could suppress more than 80% of the modes and still obtain a fairly accurate resolution of the boundary layer structures and friction factors. For a wide range of resolutions, the mean velocities are described by logarithmic profiles, with von Karman constant near a value of 0.4.

Lagrangian statistics and temporal intermittency in a shell model of turbulence.

We study the statistics of single-particle Lagrangian velocity in a shell model of turbulence. We show that the small-scale velocity fluctuations are intermittent, with scaling exponents connected to the Eulerian structure function scaling exponents. The observed reduced scaling range is interpreted as a manifestation of the intermediate dissipative range, as it disappears in a Gaussian model of turbulence.