Contributed Talk - Splinter Computation

Friday, 25 September 2020, 14:54   (virtual room B)

Exploring small- and large-scale dynamo growth with graphics processing units

Miikka Väisälä, Johannes Pekkilä, Maarit Käpylä, Matthias Rheinhardt, Hsien Shang, R. Krasnopolsky
ASIAA, Aalto University, MPS, Nordita

We have developed a code, Astaroth, which is able to compute high-order stencils such as finite difference operations effectively on graphics processing units (GPUs). Astaroth functions as a general stencil computing application programming interface (API), where performed computations, can be instructed via a Domain Specific Language (DSL). Astaroth was originally built in mind for performing magnetohydrodynamics (MHD) computations using high-order finite difference methods. High-order stencils with multiple coupled fields, however, have been difficult to efficiently compute on GPUs due to memory access latencies. However, Astaroth is able to minimize these latencies by utilizing the principles of vertex pipelining familiar from computer graphics. On a single GPU node we achieve more than 30 times speedup compared to a CPU node. As a first science case with Astaroth, we have performed direct numerical simulation using resistive isothermal MHD and a forcing function with periodic domain. We used both helical and non-helical forcing to produce both large- and small-scale dynamos, performing runs with multiple resolutions and dense Reynolds number spacing. We explored the dynamo growth in particular. Large- and small-scale dynamos follow their own growth rates, until at high Reynolds numbers they become comparable, with small scale dynamo seemingly driving the growth. Small-scale dynamo growth rate follows a logarithmic profile. We also found that signatures of the small-scale dynamo are present at the large-scale dynamo growth, even with mid-range Reynolds numbers. We estimated mean-field coefficients based on the Second Order Correlation Approximation (SOCA). The computed estimates of the mean-field term were able to produce comparable growth rates for the large-scale dynamo, but they did not reproduce the high Reynolds number growth rates, which is likely explainable by the presence of small-scale dynamo. Alpha quenching was observed towards the high Reynolds numbers.