Evaluation of an improved mixing plane interface for OpenFOAM

A mixing plane interface provides a circumferentially averaging rotor-statorcoupling interface, which is extremely useful in practical turbomachinery simulations. Itallows fundamentally transient problems to be studied in steady-state, using simplified meshcomponents having periodic properties, and with the help of a multiple reference frames(MRF) approach. An improved version of the mixing plane interface for the community-drivenversion of OpenFOAM is presented. This new version of the mixing plane introduces a perfield,user-selectable mixing option for the flow fields at the interface, including the possibilityto use a mass-flow averaging algorithm for the velocity field. We show that the quality of themass-flow transfer can be improved by a proper selection of the mixing options at theinterface. This paper focuses on the evaluation of the improved mixing plane interface forvarious steady-state simulations of incompressible flows, applied to a simple 2D validation testcase, and to more complex 3D turbomachinery cases

Eulerian Multi-Fluid Model for Polydisperse Flows

This work restricts the term multiphase only to disperse flows, where one of the phases is present in the form of particles, droplets or bubbles, which are suspended within the continuous phase. The dispersed elements can vary in size. The proposed method uses the classes method in the Euler-Euler framework to handle the flow's polydisperse nature. With this approach, every droplet/bubble/particle class is treated like a different phase in the calculation, i.e. every size class has its continuity and momentum equation. However, the pressure is shared among all phases. The derived model is tested for various polydisperse flows, which display the developed model's capability to predict such complex dynamic behaviour. These test cases include complex bubbly flows and dense spray (where droplet sizes vary significantly)

Implementation of integral viscoelastic constitutive models in OpenFOAM® computational library

This work reports the implementation and verification of a new so lver in OpenFOAM® open source computational library, able to cope with integral viscoelastic models based on the integral upper-convected Maxwell model. The code is verified through the comparison of its predictions with analytical solutions and numerical results obtained with the differential upper-convected Maxwell modelCAPES, FCT projects PEsT-C/CTM/LA0025/2013, PTDC/MAT/121185/2010 and FEDE

Numerical Capture and Validation of a Massively Separated Bluff-Body Wake

A flow over a bluff-body is numerically investigated and validated using a Detached-Eddy Simulation (DES) technique at Re=21,400. An incompressible solver that is nominally second-order accurate employing an implicit constant backward time-stepping scheme with blended upwind-central differencing spatial discretization is used to study the massively separated wake that is generated. Measurements are taken up to 6 downstream characteristic lengths, evaluating the wake time-averaged first- and second-moment statistics alongside near-wall boundary layer quantities and surface-force integrals. Results advocate the use of DES methods, which are found to be significantly more accurate for capturing wake statistics, compared to two different Reynolds-Averaged (RANS) models calibrated with an identical grid. Although comparative accuracy can be obtained with the RANS techniques for the boundary layer and surface-forces, these techniques are unsuitable for modeling wake statistics as they are inherently dissipative, evident through early velocity recovery when evaluated against experimental data

On the formation of string cavitation inside fuel injectors

The formation of vortex or ‘string’ cavitation has been visualised in the flow upstream of the injection hole inlet of an automotive-sized optical diesel fuel injector nozzle operating at pressures up to 2,000 bar. Three different nozzle geometries and three-dimensional flow simulations have been employed to describe how, for two adjacent nozzle holes, their relative positions influenced the formation and hole-to-hole interaction of the observed string cavitation vortices. Each hole was shown to contain two counter-rotating vortices: the first extending upstream on axis with the nozzle hole into the nozzle sac volume and the second forming a single ‘bridging’ string linked to the adjacent hole. Steady-state and transient fuel injection conditions were shown to produce significantly different nozzle-flow characteristics with regard to the formation and interaction of these vortices in the geometries tested, with good agreement between the experimental and simulation results being achieved. The study further confirms that the visualised vortices do not cavitate themselves but act as carriers of gas-phase components within the injector flow

Barrier dysfunction or drainage reduction: differentiating causes of CSF protein increase

BACKGROUND Cerebrospinal fluid (CSF) protein analysis is an important element in the diagnostic chain for various central nervous system (CNS) pathologies. Among multiple existing approaches to interpreting measured protein levels, the Reiber diagram is particularly robust with respect to physiologic inter-individual variability, as it uses multiple subject-specific anchoring values. Beyond reliable identification of abnormal protein levels, the Reiber diagram has the potential to elucidate their pathophysiologic origin. In particular, both reduction of CSF drainage from the cranio-spinal space as well as blood-CNS barrier dysfunction have been suggested ρas possible causes of increased concentration of blood-derived proteins. However, there is disagreement on which of the two is the true cause. METHODS We designed two computational models to investigate the mechanisms governing protein distribution in the spinal CSF. With a one-dimensional model, we evaluated the distribution of albumin and immunoglobulin G (IgG), accounting for protein transport rates across blood-CNS barriers, CSF dynamics (including both dispersion induced by CSF pulsations and advection by mean CSF flow) and CSF drainage. Dispersion coefficients were determined a priori by computing the axisymmetric three-dimensional CSF dynamics and solute transport in a representative segment of the spinal canal. RESULTS Our models reproduce the empirically determined hyperbolic relation between albumin and IgG quotients. They indicate that variation in CSF drainage would yield a linear rather than the expected hyperbolic profile. In contrast, modelled barrier dysfunction reproduces the experimentally observed relation. CONCLUSIONS High levels of albumin identified in the Reiber diagram are more likely to originate from a barrier dysfunction than from a reduction in CSF drainage. Our in silico experiments further support the hypothesis of decreasing spinal CSF drainage in rostro-caudal direction and emphasize the physiological importance of pulsation-driven dispersion for the transport of large molecules in the CSF