Ivan Bermejo-Moreno

Solving the compressible navier-stokes equations on up to 1.97 million cores and 4.1 trillion grid points

We present weak and strong scaling studies as well as performance analyses of the Hybrid code, a finite-difference solver of the compressible Navier-Stokes equations on structured grids used for the direct numerical simulation of isotropic turbulence and its interaction with shock waves. Parallelization is achieved through MPI, emphasizing the use of nonblocking communication with concurrent computation. The simulations, scaling and performance studies were done on the Sequoia, Vulcan and Vesta Blue Gene/Q systems, the first two accounting for a total of 1,966,080 cores when used in combination. The maximum number of grid points simulated was 4.12 trillion, with a memory usage of approximately 1.6 PB. We discuss the use of hyperthreading, which significantly improves the parallel performance of the code on this architecture.

Geometry of enstrophy and dissipation, grid resolution effects and proximity issues in turbulence

We perform a multi-scale non-local geometrical analysis of the structures extracted from the enstrophy and kinetic energy dissipation-rate, instantaneous fields of a numerical database of incompressible homogeneous isotropic turbulence decaying in time obtained by DNS in a periodic box. Three different resolutions are considered: 256^3, 512^3 and 1024^3 grid points, with k_(max)η(overbar) approximately 1, 2 and 4, respectively, the same initial conditions and Re_λ ≈ 77. This allows a comparison of the geometry of the structures obtained for different resolutions. For the highest resolution, structures of enstrophy and dissipation evolve in a continuous distribution from blob-like and moderately stretched tube-like shapes at the large scales to highly stretched sheet-like structures at the small scales. The intermediate scales show a predominance of tube-like structures for both fields, much more pronounced for the enstrophy field. The dissipation field shows a tendency towards structures with lower curvedness than those of the enstrophy, for intermediate and small scales. The 256^3 grid resolution case (k_(max)η(overbar) ≈ 1) was unable to detect the predominance of highly stretched sheet-like structures at the smaller scales in both fields. The same non-local methodology for the study of the geometry of structures, but without the multi-scale decomposition, is applied to two scalar fields used by existing local criteria for the eduction of tube- and sheet-like structures in turbulence, Q and [A_ij]_+, respectively, obtained from invariants of the velocity-gradient tensor and alike in the 1024^3 case. This adds the non-local geometrical characterization and classification to those local criteria, assessing their validity in educing particular geometries. Finally, we introduce a new methodology for the study of proximity issues among structures of different fields, based on geometrical considerations and non-local analysis, by taking into account the spatial extent of the structures. We apply it to the four fields previously studied. Tube-like structures of Q are predominantly surrounded by sheet-like structures of [A_ij]_+, which appear at closer distances. For the enstrophy, tube-like structures at an intermediate scale are primarily surrounded by sheets of smaller scales of the enstrophy and structures of dissipation at the same and smaller scales. A secondary contribution results from tubes of enstrophy at smaller scales appearing at farther distances. Different configurations of composite structures are presented.

Lagrangian evolution of the invariants of the velocity gradient tensor in a turbulent boundary layer

Lagrangian mean evolution of the invariants of the velocity gradient tensor in different regions of a turbulent boundary layer is investigated using data from a direct numerical simulation of a zero pressure gradient turbulent boundary layer. Conditional mean trajectories (CMTs) are calculated for the evolution of invariants based on their mean rate of change, conditioned on their location in the (RA, QA) plane, which determines the focal or non-focal nature of flow at that point. CMTs are calculated over a larger range of gradients than previously reported boundary layer measurements and show a distinct difference in topological evolution depending on the resolution and the range of invariants considered. In the present case, CMTs for strong gradients in all regions of the boundary layer pass around a focus at the origin and asymptote towards the right-hand side of a saddle point located near the right-hand side of the line dividing unstable focal and unstable nodal structures, consistent with viscous di...

Wall-Modeled Large-Eddy Simulations of a Supersonic Turbulent Flow in a Square Duct

Wall-modeled large-eddy simulation (WMLES) is used to investigate the corner vortices generated by a supersonic, turbulent flow through a square duct. Confidence in the solver is established through several channel flow validation cases which cover a range of Reynolds numbers and flow regimes. These studies show the ability of the wall-model to approximate wall-resolved LES at reduced computational cost. The WMLES is then applied to an incompressible, turbulent duct flow. Comparisons with the DNS calculations of Huser & Biringen are used to analyze the effects that the wall-model has on the secondary structures created by this fully-confined flow. Particular attention is paid to how variations in the wallmodel parameters influence the mean and turbulence quantities of interest. Finally, the “shock-free” wind tunnel tests conducted by Davis & Gessner are the subject of WMLES investigations. The inlet conditions for this supersonic, square duct flow are a Mach number of 3.91 and a unit Reynolds number of 2.0 x 10 7 1/m. Simulation results for the fully turbulent, downstream portion of the duct are compared with experimental measurements at two streamwise locations. The WMLES is shown to sustain secondary flow structures in this highly turbulent, compressible environment.