# R. BorrellBarcelona Supercomputing Center · Department of Computer Applications in Science and Engineering

R. Borrell

Senior Researcher

## About

79

Publications

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## Publications

Publications (79)

CFD users of supercomputers usually resort to rule-of-thumb methods to select the number of subdomains (partitions) when relying on MPI-based parallelization. One common approach is to set a minimum number of elements or cells per subdomain, under which the parallel efficiency of the code is “known” to fall below a subjective level, say 80%. The si...

CFD users of supercomputers usually resort to rule-of-thumb methods to select the number of subdomains (partitions) when relying on MPI-based parallelization. One common approach is to set a minimum number of elements or cells per subdomain, under which the parallel efficiency of the code is "known" to fall below a subjective level, say 80%. The si...

Synthetic (zero net mass flux) jets are an active flow control technique to manipulate the flow field in wall-bounded and free-shear flows. The present paper focuses on the role of the periodic actuation mechanisms on the boundary layer of a SD7003 airfoil at \(Re=U_{\infty } C/\nu =6\times 10^4\). Here, Reynolds number is defined in terms of the f...

Modern supercomputers allow the simulation of complex phenomena with increased accuracy. Eventually, this requires finer geometric discretizations with larger numbers of mesh elements. In this context, and extrapolating to the Exascale paradigm, meshing operations such as generation, adaptation or partition, become a critical bottleneck within the...

Multiphysics problems involve the couplings of different sets of partial differential equations. Partitioned methods consider the individual solutions of each set, which upon iterating, converge to the monolithic solution. The main drawback of partitioned methods is the additional iterative loop, which can be done a la Jacobi (parallel) or a la Gau...

High fidelity Computational Fluid Dynamics simulations are generally associated with large computing requirements, which are progressively acute with each new generation of supercomputers. However, significant research efforts are required to unlock the computing power of leading-edge systems, currently referred to as pre-Exascale systems, based on...

One of the main challenges of civil aviation is the construction of more efficient airplanes in terms of fuel consumption and noise emissions. Research and development on the aerodynamics of the full airplane are one of the priorities established by the Advisory Council for Aeronautics Research in Europe. In this context, high fidelity simulations...

Synthetic (zero net mass flux) jets are an active flow control technique to manipulate the flow field in wall-bounded and free-shear flows. The fluid necessary to actuate on the boundary layer is intermittently injected through an orifice and is driven by the motion of a diaphragm located on a sealed cavity below the surface [1].

This work studies the implementation of wall modeling for large eddy simulation in a finite element context. It provides a detailed description of how the approach used by the finite volume and finite differences communities is adapted to the finite element context. The new implementation is as simple and easy to implement as the classical finite e...

The Sparse Matrix Vector Product (SpMV) is one of the main operations of iterative solvers, and, in a parallel context, it is also the siege of point-to-point communications between the neighboring MPI processes. The parallel SpMV is built in such a way that it gives, up to round off errors, the same result as its sequential counterpart. In this re...

The variety of computing architectures competing in the exascale race makes the portability of codes of major importance. In this work, the HPC² code is presented as a fully-portable, algebra-based framework suitable for heterogeneous computing. In its application to CFD, the algorithm of the time-integration phase relies on a reduced set of only t...

Larger supercomputers allow the simulation of more complex phenomena with increased accuracy. Eventually this requires finer and thus also larger geometric discretizations. In this context, and extrapolating to the Exascale paradigm, meshing operations such as generation, deformation, adaptation/regeneration or partition/load balance, become a crit...

It is well known that the wake topology in the flow past a circular cylinder remains almost unchanged up to Reynolds number \(\sim 10^5\) Williamson (Annu Rev Fluid Mech 28(1), 477–539 (1996)) [1]. Then, at \(Re\sim 2\times 10^5\) major changes take place entailing flow separation, turbulence transition in the detached shear layers, reattachment of...

The fluid-structure interaction (FSI) problem has received great attention in the last few years, mainly because it is present in many physical systems, industrial applications, and almost every biological system. In the parallel computational field, outstanding advances have been achieved for the individual components of the problem, allowing, for...

This paper investigates the effects of surface roughness on the flow past a circular cylinder at subcritical to transcritical Reynolds numbers. Large eddy simulations of the flow for sand grain roughness of size k/D = 0.02 are performed (D is the cylinder diameter). Results show that surface roughness triggers the transition to turbulence in the bo...

Nowadays, high performance computing (HPC) systems experience a disruptive moment with a variety of novel architectures and frameworks, without any clarity of which one is going to prevail. In this context, the portability of codes across different architectures is of major importance. This paper presents a portable implementation model based on an...

Since 2011, the European project Mont-Blanc has been focused on enabling ARM-based technology for HPC, developing both hardware platforms and system software. The latest Mont-Blanc prototypes use system-on-chip (SoC) devices that combine a CPU and a GPU sharing a common main memory. Specific developments of parallel computing software and well-suit...

The variety of HPC architectures competing in the exascale race makes the portability of codes of major importance. In this work, the HPC² (HPC Hybrid Portable Code) code is presented as a fully portable CFD code for modeling of incompressible turbulent flows, based on an algebraic operational approach. As a result, at the time-integration phase th...

This paper focuses on the effects of surface roughness in the flow past a circular cylinder at different Reynolds numbers. Large eddy simulations of the flow, from subcritical to transcritical Reynolds numbers and at relatively high equivalent sand grain roughness of k s /D = 0.02 are performed. In order to determine the effects of the surface roug...

This paper presents some recent efforts carried out on the expansion of the scalability of TermoFluids multi-physics Computational Fluid Dynamics (CFD) code, aiming to achieve petascale capacity for a single simulation. We describe different aspects that we have improved in our code in order to efficiently run it on 131,072 CPU-cores. This work has...

In the present work large eddy simulations of the flow past a rough cylinder are performed at a Reynolds number of Re = 4.2 × 10⁵ and an equivalent sand-grain surface roughness height ks = 0.02D. In order to determine the effects of the surface roughness on the boundary layer transition and as a consequence on the wake topology, results are compare...

Large-eddy simulations (LES) of the flow past a circular cylinder are used to investigate the flow topology
and the vortex shedding process at Reynolds numbers $Re=2.5 \times10^5- 8.5\times10^5$. This range encompasses both the critical and super-critical regimes. As the flow enters the critical regime, major changes
occur which affect the flow con...

In this paper, we report the development of a parallel mesh multiplication code to subdivide a base mesh (containing tetrahedral, pyramids, prisms or hexahedral) into a finer mesh without user intervention, taking care of the quality of refined meshes generated by the algorithm. First, a coarse mesh is generated with an unstructured mesh generator...

CFD optimizations are characterized by long and highly variable objective function evaluation times. The aim of this study is to improve the parallel performance using a new load balancing master-worker parallelization strategy. First the new method is introduced and compared with the standard parallelization strategy. Finally two illustrative exam...

In this paper a parallel adaptive mesh refinement (AMR) strategy for large eddy simulations (LES) of turbulent flows is presented. The underlying discretization of the Navier–Stokes equations is based on a finite-volume symmetry-preserving formulation, with the aim of preserving the symmetry properties of the continuous differential operators and e...

The flow past a circular cylinder is associated with different types of instabilities which involve the wake, the separated shear layers and the boundary layer.

The flow around airfoils in full stall is a problem of great interest in aerodynamics and specifically for the design of turbo-machines (turbines, propellers, wind turbines, etc.).

It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence
transition, reattachment of the flow, and further turbulent separation of the boundary layer. The transition to turbulence in the boundary layer causes the delaying of the separation point and an important reduction of the drag f...

The flow past a circular cylinder at critical and supercritical Reynolds number combines flow sepa-ration, turbulence transition, reattachment of the flow and further turbulent separation of the boundary layer. The transition to turbulence causes the delaying of the separation point and, an important reduction of the drag force on the cylinder surf...

In this paper advanced turbulence simulations at Reynolds numbers in the range of 1.4 × 10^5 -8.5 × 10^5 will be carried out by means of large-eddy simulations. Nu-merical simulations using unstructured grids up to 90 million of control volumes have been performed on Marenostrum Supercomputer. One of the major outcomes of this work is to shed some...

This work aims at investigating, by means of a direct numerical simulation, the flow over a clean spherical bubble fixed on the axis of a turbulent pipe flow. The simulation is performed by means of a parallel unstructured symmetry-preserving formu-lation on a mesh of 5.4M cells. The main features of the turbulent flow are described by analyzing th...

The preconditioned conjugate gradient (PCG) is one of the most prominent iterative methods for the solution of sparse linear systems with symmetric and positive definite matrix that arise, for example, in the modeling of incompressible flows. The method relies on a set of basic linear algebra operations which determine the overall performance. Ther...

The Volume-of-Fluid (VOF) is one of the most widely used methods for interface tracking in the simulation of multi-fluid flows. The interface between different fluids is generated from the volume fraction scalar fields, which account for the ratio of volume of each fluid in each control volume. Then, an advection equation is solved to obtain the ne...

It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow and further turbulent separation of the boundary layer. In the critical regime, the transition to turbulence in the boundary layer causes the delaying of the separation point and, an important...

This paper describes a hybrid MPI-CUDA parallelization strategy for the direct numerical simulation of incompressible flows using unstructured meshes. Our in-house MPI-based unstructured CFD code has been extended in order to increase its performance by means of GPU co-processors. Therefore, the main goal of this work is to take advantage of the cu...

In this paper a parallel adaptive mesh refinement for LES simulation of turbulent flows is presented. The AMR scheme applies a cell- based refinement technique to get enough grid-resolution to solve the small scales structures, adapting the mesh according to physics- based refinement criteria. A flexible tree data structure is used to keeping track...

The Boltzmann transport equation is solved in the context of radiative heat transfer, for an isotropically scattering medium with reflecting boundaries. Under these circumstances, the different ordinates of the angular flux are mutually coupled. We explore here the use of a parallel sweep-based block diagonal pre-conditioner as a complement of the...

This work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures
formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 air...

Direct numerical simulations of the flow over a sphere have been performed. The computations have been carried out in the sub-critical regime at Re = 3700 and Re = 10,000 (based on the free-stream velocity and the sphere diameter). A parallel unstructured symmetry-preserving formulation has been used for simulating the flow. Computations have been...

In this paper a radial basis function (RBF) interpolation method is implemented to be applied in computational fluid dynamics (CFDs) problems with dynamic meshes. The method has been tested with two challenging examples of dynamic meshing, the deformation of a pitching airfoil and a three-dimensional movement of a sphere, both discretized over visc...

The Boltzmann Transport Equation is solved on unstructured meshes using the Discrete Ordinates Method. The flux for each ordinate is swept across the computational grid, within a source iteration loop that accounts for the coupling between the different ordinates. In this paper, a spatial domain decomposition strategy is used to divide the work amo...

Traditionally, turbulence modelling of industrial flows in complex geometries have been solved using RANS models and unstructured meshes based solvers. The lack of precision of RANS models in these situations and the increase of computational power, together with the emergence of new high-efficiency sparse parallel algorithms, have made possible th...

The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = UrefD/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the presen...

External car aerodynamics study has great importance in overall car efficiency and ride stability, being a key element in successful automotive design. The flow over car geometries shows three dimensional and unsteady turbulent characteristics. Additionally, vortex shedding, flow reattachment and recirculation bubbles are also found around the bluf...

Natural convection heat transfer in cavities has been studied extensively in the literature due to its relevance to many engineering areas such as low temperature solar collectors, design of buildings, cooling of nuclear reactors, etc.

Flow around cylindrical structures is of relevance for many practical applications. Knowledge of flow-related unsteady loading of such structures is crucial for hydro - and aerodynamic control and design. In order to obtain a deeper knowledge of this kind of flow, a DNS have been performed at ReD = 3900 (ReD = UrefD/ν). The instantaneous velocity s...

In the published article Rodriguez et al. (2011) one of the author’s names was misspelled and should have read Ricard Borrell.(Received October 10 2011)(Reviewed October 10 2011)(Accepted October 10 2011)

This work is devoted to the development of efficient parallel algorithms for the direct numerical simulation (DNS) of incompressible flows on modern supercomputers. In doing so, a Poisson equation needs to be solved at each time-step to project the velocity field onto a divergence-free space. Due to the non-local nature of its solution, this ellipt...

The direct numerical simulation of the flow over a sphere is performed. The computations are carried out in the sub-critical regime at Re = 3700 (based on the free-stream velocity and the sphere diameter). A parallel unstructured symmetry-preserving formulation is used for simulating the flow. At this Reynolds number, flow separates laminarly near...

In the context of time-accurate numerical simulation of incompressible flows, a Poisson equation needs to be solved at least once per time-step to project the velocity field onto a divergence-free space. Due to the non-local nature of its solution, this elliptic system is one of the most time consuming and difficult to parallelise parts of the code...

The Ahmed body car is a semi-rectangular vehicle with a rounded front part and a slant back. Flow over this generic body reproduce
the basic fluid-dynamics features of real cars with a typical fastback geometry and its simplified topology allows easy comparisons
between experimental and numerical works.

Turbulent natural convection of a fluid inside an enclosure heated from below (Rayleigh-Bénard convection), has been object
of many theoretical and experimental investigations (Grötzbach, 1983; Niemela et al., 2000). Over the past decades numerical simulations have become a powerful tool for providing extensive data in turbulence structures
and flo...

The numerical simulation of interfacial and free surface flows is a vast and interesting topic in the areas of engineering and fundamental physics, such as the study of liquid-gas interfaces, formation of droplets, bubbles and sprays, combustion problems with liquid and gas reagents, study of wave motion and others. One of the most powerful and rob...

The Navier–Stokes and continuity equations can be written as
$$\rho \frac{{\partial u}}{{\partial t}} + C(u)u + Du + Gp = 0$$
$$Mu\text{ } = \text{ }0$$
where u ∈ Rm
and p ∈ Rq
are the velocities vectors and pressure, respectively. The matrices C (u), D ∈ Rm ×m
are the convective and diffusive operators, respectively. Note the u-dependence of the c...

In this paper a parallel direct Poisson solver for DNS simulation of turbulent flows statistically homogeneous in one spatial direction is presented. It is based on a Fourier diagonalization and a Schur decomposition on the spanwise and streamwise directions respectively. Numerical experiments carried out in order to test the robustness and efficie...

Traditionally turbulence modeling of industrial flows in complex geometries have been solved using RANS models and unstructured meshes based solvers. The lack of precision of RANS models in these situations and the increase of computer power, together with the emergence of new high-efficiency sparse parallel algorithms, make possible the use of mor...