Mathematical modelling of a laminar suspension flow in the flat inclined channel

The laminar flow of a suspension consisting of viscous incompressible fluid with solid spherical particles of the same size in a flat inclined channel is investigated in this work. The mathematical model is formulated in a one-fluid approximation in a three-dimensional statement and solved in the OpenFOAM software package. The results of mathematical modeling are compared with experimental data. The study of the dynamics of the distribution of solid spherical particles in the flow and sedimentation along the length of the channel depending on the size of particles and the angle of inclination of the channel relative to the horizon is carried out.

Fluid Models of Parallel Service Systems Under FCFS

In parallel service systems, customers of various types are served simultaneously by several servers that have some dierent and some overlapping skills or capacities. Examples include customer routing in call centers, scheduling of hospital operating rooms, and driver assignment in ride-hailing services. First come first served (FCFS) is a natural and widespread resource allocation policy, yet its performance both during the transient stage and in the long run is often difficult to analyze. In particular, matching rates, which indicate what fraction from all services are of a specific server customer type, are difficult to evaluate under FCFS. In Fluid Models of Parallel Service Systems under FCFS, Yuval Nov, Gideon Weiss, and Hanqin Zhang formulate a stochastic queueing model for such systems and study its fluid approximation. The paper demonstrates the usefulness and the limitations of exploring fluid performance in evaluating system behavior in terms of stability, responsiveness, and utilization.

Numerical simulation of the thermal regime of an underground spent fuel storage facility (built-in structure variant)

The results of a numerical simulation of the thermal regime of an underground facility for long-term storage of spent nuclear fuel in a built-in reinforced concrete structure are presented. Two computer models were constructed in a three-dimensional formulation in the COMSOL programme. The first model is based on the incompressible fluid approximation, while the second model is based on the "incompressible ideal gas" approximation. The mathematical basis of models: the continuity equation, Navier - Stokes equations averaged by Reynolds, the standard (k - ε) turbulence model, and the general heat transfer equation. Consideration of mixed convection conditions is implemented in the "incompressible ideal gas" approximation, where the air density is a function of temperature only. The most thermally stressful arrangement of spent fuel placement is investigated: U-Zr - defective - U-Be. The air rate is varied in the range from 21 to 0.656 m3/s. Numerical experiments were performed for up to 5 years of fuel storage. The principal difference between the non-stationary structure of the velocity fields predicted in the "incompressible ideal gas" model and the "frozen" picture of the aerodynamic parameters in the incompressible fluid model is emphasized. It is shown that the requirements for exceeding the temperature limit values are met when the object operates under conservative ventilation conditions (rate 0.656 m3/s) with a minimum of costs for organizing ventilation. The dynamics of heat flows directed into the rock mass through the base and from the surface of the built-in structure of the U-Zr fuel compartment to the air environment are analyzed. The predominance of the heat flow from the surface of the structure and the different times when the curves of the heat flow dynamics reach their maximum values are noted. The heat flow to the array reaches its maximum significantly faster than to the air.

Fluid Approximation–based Analysis for Mode-switching Population Dynamics

Fluid approximation results provide powerful methods for scalable analysis of models of population dynamics with large numbers of discrete states and have seen wide-ranging applications in modelling biological and computer-based systems and model checking. However, the applicability of these methods relies on assumptions that are not easily met in a number of modelling scenarios. This article focuses on one particular class of scenarios in which rapid information propagation in the system is considered. In particular, we study the case where changes in population dynamics are induced by information about the environment being communicated between components of the population via broadcast communication. We see how existing hybrid fluid limit results, resulting in piecewise deterministic Markov processes, can be adapted to such models. Finally, we propose heuristic constructions for extracting the mean behaviour from the resulting approximations without the need to simulate individual trajectories.

A numerical approach to the non-uniqueness problem of cosmic ray two-fluid equations at shocks

ABSTRACT Cosmic rays (CRs) are frequently modelled as an additional fluid in hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of astrophysical flows. The standard CR two-fluid model is described in terms of three conservation laws (expressing conservation of mass, momentum, and total energy) and one additional equation (for the CR pressure) that cannot be cast in a satisfactory conservative form. The presence of non-conservative terms with spatial derivatives in the model equations prevents a unique weak solution behind a shock. We investigate a number of methods for the numerical solution of the two-fluid equations and find that, in the presence of shock waves, the results generally depend on the numerical details (spatial reconstruction, time stepping, the CFL number, and the adopted discretization). All methods converge to a unique result if the energy partition between the thermal and non-thermal fluids at the shock is prescribed using a subgrid prescription. This highlights the non-uniqueness problem of the two-fluid equations at shocks. From our numerical investigations, we report a robust method for which the solutions are insensitive to the numerical details even in absence of a subgrid prescription, although we recommend a subgrid closure at shocks using results from kinetic theory. The subgrid closure is crucial for a reliable post-shock solution and also its impact on large-scale flows because the shock microphysics that determines CR acceleration is not accurately captured in a fluid approximation. Critical test problems, limitations of fluid modelling, and future directions are discussed.

Cosmological perturbations for two cold fluids in ΛCDM

The cosmic large-scale structure of our Universe is comprised of baryons and cold dark matter (CDM). Yet it is customary to treat these two components as a combined single-matter fluid with vanishing pressure, which is justified only for sufficiently large scales and late times. Here we go beyond the single-fluid approximation and develop the perturbation theory for two gravitationally coupled fluids while still assuming vanishing pressure. We mostly focus on perturbative expansions in powers of D (or D+), the linear structure growth of matter in a ΛCDM Universe with cosmological constant Λ. We derive in particular (1) explicit recursion relations for the two fluid densities, (2) complementary all-order results in the Lagrangian-coordinates approach, as well as (3) the associated component wavefunctions in a semi-classical approach to cosmic large-scale structure. In our companion paper we apply these new theoretical results to generate novel higher-order initial conditions for cosmological hydrodynamical simulations.