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A direct numerical simulation of a three-dimensional diffuser at Reynolds number Re = 10,000 (based on inlet bulk velocity) has been performed using a low-dissipation finite element code. The geometry chosen for this work is the Stanford diffuser, introduced by Cherry et al. (Int. J. Heat Fluid Flow 29:803–811, 2008). Results have been exhaustively compared with the published data with a quite good agreement. Additionally, further turbulent statistics have been provided such as the Reynolds stresses or the turbulent kinetic energy. A proper orthogonal decomposition and a dynamic mode decomposition analyses of the main flow variables have been performed to identify the main characteristics of the large-scale motions. A combined, self-induced movement of the large-scales has been found to originate in the top-right expansion corner with two clear features. A low-frequency diagonal cross-stream travelling wave first reported by Malm et al. (J. Fluid Mech. 699:320–351, 2012), has been clearly identified in the spatial modes of the stream-wise velocity components and the pressure, associated with the narrow band frequency of St¿[0.083,0.01] . This movement is caused by the geometrical expansion of the diffuser in the cross-stream direction. A second low-frequency trait has been identified associated with the persisting secondary flows and acting as a back and forth global accelerating-decelerating motion located on the straight area of the diffuser, with associated frequencies of St<0.005 . The smallest frequency observed in this work has been St=0.0013 . This low-frequency observed in the Stanford diffuser points out the need for longer simulations in order to obtain further turbulent statistics., The research leading to this work has been partially funded by the European Project NextSim which has received funding from the European High-Performance Computing Joint Undertaking (JU) under grant agreement No 956104 and co-founded by the Spanish Agencia Estatal de Investigacion (AEI) under grant agreement PCI2021-121962. Benet Eiximeno also acknowledges the financial support by the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (Refs: PID2020-116937RB-C21 and PID2020-116937RB-C22). Oriol Lehmkuhl has been partially supported by a Ramon y Cajal postdoctoral contract (Ref: RYC2018-025949-I). He also acknowledges the support of the European Project HiFi-TURB which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 814837. We also acknowledge the Barcelona Supercomputing Center for awarding us access to the MareNostrum IV machine based in Barcelona, Spain. The authors acknowledge the support of Departament de Recerca i Universitats de la Generalitat de Catalunya to the Research Group Large-scale Computational Fluid Dynamics (Code: 2021 SGR 00902)., Peer Reviewed, Postprint (published version)

A DNS of a three dimensional diffuser has been performed with the in-house solver Alya. The resulting dataset presents a good agreement with existing numerical and experimental data, even without smoothing of the sharp corners. Discrepancies in the duct area have been found to be due to uncertainties, however, in the diffuser region differences are more noticeably between experimental and numerical data, where they all exhibit an oscillating behavior. The POD analysis of this flow has quantified a slow mixing mechanism inside the diffuser with a period of 4 flow-through times. This is due to the slow motions in the diffuser that are only represented 3 times over the 21 flow-through times in which statistics are gathered. This is considered to be the main reason of discrepancy between all datasets. While DNS of turbulent flow at are feasible nowadays, temporal dynamics of the Standford diffuser present a challenge as the integration time needed becomes larger. The current dataset is, nevertheless, valid for further analysis and activities and will be made available online., This work was funded as part of the European Project HiFi-TURB which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 814837., Peer Reviewed, Postprint (author's final draft)

A finite-volume discretization over unstructured meshes is the most used formulation to solve Navier-Stokes equations by many general purpose CFD packages codes such as OpenFOAM or ANSYS-Fluent., This work is supported by the Ministerio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00), and the FusionCAT project (001-P-001722) cofinanced by the European Union Regional Development Fund within the framework of the ERDF Operational Program of Catalonia 2014-2020 with a grant of 50% of total cost eligible. D. Santos acknowledges a FI AGAUR-Generalitat de Catalunya fellowship (2022FI B 00173)., Postprint (author's final draft)

Buoyancy-driven flows have always been an important subject of scientific studies with numerous applications in environment and technology., F.X.T., X.A-F., D.S. and A.O. are supported by theMinisterio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00). F.X.T. and A.O. are supported by the FusionCAT project (001-P-001722) co-financed by the European Union Regional Development Fund within the framework of the ERDF Operational Program of Catalonia 2014-2020 with a grant of 50% of total cost eligible. D.S. is supported by the FI predoctoral grant (2020FI B 00839) of the Catalan Agency for Management of University and Research Grants (AGAUR). Calculations were carried out on MareNostrum 4 supercomputer at BSC. The authors thankfully acknowledge these institutions., Postprint (author's final draft)

The final publication is available at Springer via http://dx.doi.org/10.1007/s40571-019-00264-x, In this work, a new model for the analysis of incompressible fluid flows with massive instabilities at different scales is presented. It relies on resolving all the instabilities at all scales without any additional model, i.e., following the direct numerical simulation style. Nevertheless, the computation is carried out at two levels or scales, termed the coarse and the fine. The fine-scale simulation is performed on representative volume elements providing the homogenized stress tensor as a function of several dimensionless numbers characterizing the flow. Consequently, the effect of the fine-scale instabilities is transferred to the coarse level as a homogenized stress tensor, a procedure inspired by standard multi-scale methods used in solids. The present proposal introduces a new way for the treatment of the flow at the fine scale, simulating not only the coarse scale but also the fine scale with all the necessary detail, but without incurring in the excessive computational cost of the classical DNS. Another interesting aspect of the present proposal is the use of a Lagrangian formulation for convecting the eddies simulated on the fine mesh through the coarse domain. Several examples showing the potentiality of this methodology for the simulation of homogeneous flows are presented., The authors express their most sincere gratitude to Xavier Oliver, Alfredo Huespe, and Pablo Sanchez for many fruitful discussions and valuable advises regarding the multi-scale methods. We sincerely thank Eugenio Oñate for continuous support and multiple suggestions for alleviating the problems faced in the development of the method.We also thank Pablo Becker for his trials which allowed us to obtain first unstable solutions on an RVE. Axel Larreteguy wishes to acknowledge the support from UADE and Banco Santander RIO through Grant BSR181. Juan Gimenez and Norberto Nigro wish to acknowledge CONICET, Universidad Nacional del Litoral (CAI+D 2016 PJ 50020150100018LI), and Agencia Nacional de Promoción Científica y Tecnológica (PICT 2016-2908)., Peer Reviewed, Postprint (author's final draft)

The dynamics of binary droplets collision is of huge importance in different fields, from multiphase reactors (Dudukovic et al in Chem Eng Sci 54:1975–1995, 1999 [11]), raindrop formation, ink-jet printing, spray combustion, emulsion stability, turbine blade cooling, spray coating (Ashgriz and Poo in J Fluid Mech 221:183–204, 1990 [5]), and to drug delivery. Due to the complexity of the nature of droplets collision, this topic is one of the most challenging areas in the field of fluid dynamics., We acknowledge Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (ENE2017-88697-R). A. Amani acknowledges the financial support of (AGAUR) of Generalitat de Catalunya research scholarship (2016 FI_B 01059). N. Balcázar acknowledges the Programa Torres Quevedo MINECO (PTQ-14-07186)., Postprint (published version)

Management and follow-up of chronic aortic dissections continues to be a clinical challenge due to progressive aortic dilatation. To predict dilatation, guidelines suggest follow-up of the aortic diameter. However, dilatation is triggered by haemodynamic parameters (pressure and wall shear stresses (WSS)), and geometry of false (FL) and true lumen (TL). We aimed at a better understanding of TL and FL haemodynamics by performing in-silico (CFD) and in-vitro studies on an idealized dissected aorta and compared this to a typical patient. We observed an increase in diastolic pressure and wall stress in the FL and the presence of diastolic retrograde flow. The inflow jet increased WSS at the proximal FL while a large variability in WSS was induced distally, all being risk factors for wall weakening. In-silico, in-vitro and in-vivo findings were very similar and complementary, showing that their combination can help in a more integrated and extensive assessment of aortic dissections, improving understanding of the haemodynamic conditions and related clinical evolution., Peer Reviewed, Postprint (published version)