Massively parallel direct numerical simulations of forced compressible turbulence: a hybrid MPI/OpenMP approach

A highly scalable simulation code for turbulent flows which solves the fully compressible Navier-Stokes equations is presented. The code, which supports one, two and three dimensional domain decompositions is shown to scale well on up to 262,144 cores. Introducing multiple levels of parallelism based on distributed message passing and shared-memory paradigms results in a reduction of up to 33% of communication time at large core counts. The code has been used to generate a large database of homogeneous isotropic turbulence in a stationary state created by forcing the largest scales in the flow. The scaling of spectra of velocity and density fluctuations are presented. While the former follow classical theories strictly valid for incompressible flows, the latter presents a more complicated behavior. Fluctuations in velocity gradients and derived quantities exhibit extreme though rare fluctuations, a phenomenon known as intermittency. The simulations presented provide data to disentangle Reynolds and Mach number effects.