In this work, a constant-pressure model capable to simulate the overlap of chambers in rotary internal combustion engines is proposed. It refers as a chamber overlap when two adjacent chambers are in communication through the same port, which could occur in some rotary internal combustion engines. The proposed model is thermodynamic (or zero-dimensional (0D)) in nature and is designed for application in engine simulators that combine one-dimensional (1D) gasdynamic models with thermodynamic ones. Since the equations of the proposed model depend on the flow direction and on the flow regime, a robust and reliable solution strategy is developed. The model is assessed using a two-dimensional (2D) problem and is applied in the simulation of a rotary internal combustion engine. Results for this last problem are compared with other common approaches used in the simulation of rotary engines, showing the importance of effects such as the interaction between overlapping chambers and the dynamics of the flow.

Abdominal aortic aneurysm (AAA) is a critical condition with high rupture risk, and the maximum diameter alone is insufficient for prediction. Pulse wave velocity (PWV), a surrogate of aortic stiffness, can be estimated using 4D-Flow magnetic resonance imaging (MRI), but requires validation under dilated conditions. This in vitro study examined the relationship between PWV and stiffness by comparing healthy and aneurysmal compliant aortic models. Two latex phantoms were fabricated to represent normal and AAA geometries. A circulatory MRI-compatible system simulated physiological inlet flow, with flow rates measured across perpendicular planes using 4D- and 2D-Flow MRI. PWV was derived from transit times of the systolic upstroke and interplane distances. Complementary 1D numerical simulations and laboratory flowmeter measurements were performed. Although the wall elasticity and thickness were identical, PWV in the healthy model ranged from 6.2 to 7.7 m/s and in the aneurysmal model it ranged from 14.2 to 15 m/s. This increase was confirmed by temporal overlap of thoracic flow curves and reduced slope in the transit time–distance regression. Results were consistent across simulations, 2D-Flow, and flowmeter data. Findings highlight that indirect 4D-Flow assessment of thoracic stiffness in the presence of AAA must account for wave reflections introduced by dilation, which significantly alter PWV estimation.

In the boundary element method (BEM), the Galerkin weighting technique allows to obtain numerical solutions of a boundary integral equation (BIE), giving the Galerkin boundary element method (GBEM). In three-dimensional (3D) spatial domains, the nested double surface integration of GBEM leads to a significantly larger computational time for assembling the linear system than with the standard collocation method. In practice, the computational time is roughly an order of magnitude larger, thus limiting the use of GBEM in 3D engineering problems. The standard approach for reducing the computational time of the linear system assembling is to skip integrations whenever possible. In this work, a modified assembling algorithm for the element matrices in GBEM is proposed for solving integral kernels that depend on the exterior unit normal. This algorithm is based on kernels symmetries at the element level and not on the flow nor in the mesh. It is applied to a BIE that models external creeping flows around 3D closed bodies using second-order kernels, and it is implemented using OpenMP. For these BIEs, the modified algorithm is on average 32% faster than the original one.

Modeling turbulent fluxes in RANS (Reynolds-Averaged Navier-Stokes) equations is a nontrivial task. Unlike molecular diffusion, turbulent scales are tipically not well separated from those over which mean properties vary, and therefore the very common analogy between them is a poor one. When the interest lies in simulating the initial stage of a process (e.g. the zone close to a nozzle discharging material) some modifications to the diffusion coefficients must be done in order to represent the process adequately. Moreover, when the turbulent dispersion of a puff of material or a burning kernel needs to be predicted, it is necessary to consider that not all of the turbulent eddies are effective in diffusing the material. Eddies much larger than the puff primarily convect it in a random way, and this must also be included in the diffusion process if the puff growth is to be computed accurately. In this work, lagrangian simulations are performed in a simplified turbulence configuration. Then, dispersion statistics are used to obtain an expression for the turbulent diffusivity appropriate for simulating the growth of a puff and related phenomena, such as the kernel produced after a spark discharge.