Enrico Fonda

Liquid nitrogen in fluid dynamics: visualization and velocimetry using frozen particles.

High-Reynolds-number flows are common both in nature and industrial applications, but are difficult to attain in laboratory settings using standard test fluids such as air and water. To extend the Reynolds number range, water and air have been replaced at times by low-viscosity fluids such as pressurized air, sulfur hexafluoride, and cryogenic nitrogen gas, as well as liquid and gaseous helium. With a few exceptions, liquid nitrogen has been neglected despite the fact that it has a kinematic viscosity of about a fifth of that of water at room temperature. We explore the use of liquid nitrogen here. In particular, we study the use of frozen particles for flow visualization and velocimetry in liquid nitrogen. We create particles in situ by injecting a gaseous mixture of room-temperature nitrogen and an additional seeding gas into the flow. We present a systematic study of potential seeding gases to determine which create particles with the best fidelity and optical properties. The technique has proven capable of producing sub-micrometer sized tracers that allow particle tracking and particle image velocimetry. We review possible high-Reynolds-number experiments using this technique, and discuss the merits and challenges of using liquid nitrogen as a test fluid.

Lattice Boltzmann Inverse Kinetic Approach for Classical Incompressible Fluids

Despite the abundant literature on the subject appeared in the last few years, the lattice Boltzmann method (LBM) is probably the one for which a complete understanding is not yet available. As an example, an unsolved theoretical issue is related to the construction of a discrete kinetic theory which yields exactly the fluid equations, i.e., is non‐asymptotic (here denoted as LB inverse kinetic theory). The purpose of this paper aims at investigating discrete inverse kinetic theories (IKT) for incompressible fluids. We intend to show that the discrete IKT can be defined in such a way to satisfy, in particular, the requirement of completeness, i.e., all fluid fields are expressed as moments of the kinetic distribution function and all hydrodynamic equations can be identified with suitable moment equations of an appropriate inverse kinetic equation IKE.

The Computational Complexity of Traditional Lattice‐Boltzmann Methods for Incompressible Fluids

It is well‐known that customary direct solution methods (based on the discretization of the fluid fields) for the fluid equations of incompressible fluids may be affected by a high computational complexity. This is due primarily to the numerical solution of the Poisson equation for the fluid pressure and occurs when the scale‐length of turbulent fluctuations becomes comparable to the discretization scale which characterizes the numerical solution method. An alternative, which can reduce significantly the complexity caused by the numerical solution of the fluid equations for incompressible fluids, may be achieved by so‐called particle simulation methods. In such a case the dynamics of fluids is approximated in terms of a set of test particles which advance in time in terms of suitable evolution equations defined in such a way to satisfy identically the Poisson equation. Particle simulation methods rely typically on appropriate kinetic models for the fluid equations which permit the evaluation of the fluid ...