Nonreflecting boundary conditions are essential elements in the computation of many compressible flows. Such simulations are very sensitive to the treatment of acoustic waves at boundaries. Nonreflecting conditions allow acoustic waves to propagate through boundaries with zero or small levels of reflection into the domain. However, perfectly nonreflecting conditions must be avoided because they can lead to ill-posed problems for the mean flow. Various methods have been proposed to construct boundary conditions that can be sufficiently nonreflecting for the acoustic field while still making the mean flow problem well-posed. A widely used technique for nonreflecting outlets is analyzed (

This paper presents an analytical/numerical study of the effects of the mean flow on thermoacoustic instabilities. Simple quasi-1D configurations such as a 1D premixed flame in a duct connected to a nozzle are considered in order to investigate to what extent the frequency of oscillation and growth rate are modified when the Mach number is not zero. It is demonstrated that the zero Mach number assumption for the mean flow can lead to significant errors, especially when the mean flow is not isentropic, a condition which is always met in combustion applications. The analysis confirms that terms involving the mean velocity may contribute to the disturbance energy equation as much as the flame forcing ('Rayleigh') term. Besides, the net effect of the non zero Mach number terms on the stability of the modes strongly depends on both the boundary conditions and the flame response. For moderate Mach number values of order 0.05, the errors made by assuming that the mean flow is at rest are large enough to change the stability of the frequencies of interest in an academic combustor.

In Coulter counters, cells counting and volumetry are achieved by monitoring their electrical print when they flow through a sensing zone. However, the volume measurement may be impaired by the cell dynamics, which may be difficult to control. In this paper, numerical simulations of the dynamics and electrical signature of red blood cells in a Coulter counter are presented, accounting for the deformability of the cells. In particular, a specific numerical pipeline is developed to overcome the challenge of the multi‐scale nature of the problem. It consists in segmenting the whole computation of the cell dynamics and electrical response in a series of dedicated computations, with a saving of one order of magnitude in computational time. This numerical pipeline is used with rigid spheres and deformable red blood cells in an industrial Coulter counter geometry, and compared with experimental measurements. The simulations not only reproduce electrical signatures typical of those measured experimentally, but also allow an analysis of the electrical signature in terms of the heterogeneity of the electrical field and dynamics of the particles in the measurement zone. This study provides a methodology for computing the sizing of rigid or deformable particles by Coulter counters, opening the way to a better understanding of cells signatures in such devices.

A methodology for the computation of azimuthal combustion instabilities which can occur in annular combustors is proposed in this work. A thermoacoustic numerical tool using the n−τ model for the coupling of acoustic and combustion is required to solve the Helmholtz equation in reactive media. The methodology is based on the Independence Sector Assumption in Annular Combustor (ISAAC) which states that the heat release fluctuations in a given sector are driven only by the fluctuating mass flow rates due to the velocity perturbations through its own swirler. This assumption is first discussed with respect to a Large Eddy Simulation of an annular combustor. The methodology is then applied to an academic annular test case which exhibits amplified or damped, standing or rotating azimuthal eigenmodes depending on parameters n and τ .