Until now, jobs running on HPC clusters were tied to the node where their execution started. We have removed that limitation by integrating a user level Checkpoint/Restart library into a Resource Manager, fully transparent to both the user and running application. This opens the door to a whole new set of tools and scheduling possibilities based on the fact that jobs can be migrated, checkpointed and restarted on a different place or in a different moment, while providing fault-tolerance for every job running on the cluster. This is of utmost importance in the future generation of exascale HPC clusters, where an increasing degree and complexities of efficient scheduling make it challenging to obtain the required degree of parallelism demanded by the applications.
Time-accurate three-dimensional Reynolds-averaged Navier -Stokes simulations have been performed to explore the flow unsteadiness of the subscale J-2S thrust optimized contour rocket nozzle in over-expansion at prescribed air supply conditions in the chamber. The results show the development of periodic low-frequency, high-amplitude, self-sustained oscillations at the simulated nozzle pressure ratios (NPRs). The cap-shock pattern is responsible for the two-coaxial jet topology that appears. The complex three-dimensional evolution of the flowfield is characterized by a periodical jet flapping, axial oscillations, and azimuthal fluctuations with or without swirl, depending on the NPR. According to the simulations, jet flapping quasi-restricted to a preferred plane may occur during the firings, being supported by the reported thrust and side-load measurements. The computed dominant frequencies corresponding to the longitudinal and transverse oscillation modes show an excellent agreement with the experimental counterpart. The analysis of the forces spectra and flowfields has identified that synchronization between side-load (transverse) and thrust (longitudinal) oscillations develops in consonance with the observations. Besides, their strong coupling is favoured by the response of the shocks to the pulsating pressure near the large recirculation bubble that arises behind the cap-shock. A discussion on the self-oscillating, high-coherent flow phenomenon is addressed. The present three-dimensional study has captured for the first time the phenomenon of cyclic transition between reattached flow separation mode and non-reattached mode, where the alternating bursting and formation of a separation bubble is accompanied by the large amplitude axial displacement of the shock pattern. Furthermore, the role of the separation bubble dynamics on triggering high side-load peaks at fixed NPRs is numerically confirmed.
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