Fully resolved simulation of wall-bounded bubbly flows: can we do it faster?

Dr. Pengyu Shi (FWDC) & Dr. Nina Elkina (FWCI)

2022, November 22nd

Wall-bounded bubbly flows are widely encountered in a variety of technical and natural systems. Fully resolved simulation of such flows is still challenging, as the length scale characterizing the wall-bubble interaction becomes very small when the bubble goes close to the wall. In the first part of this hybrid talk, we show simulation results for a highly simplified configuration where a single bubble rises close to a rigid wall. These simulations are carried out using JADIM and Basilisk, both are popular in the multiphase flow community. It turns out that, to get results in the fully developed regime, the wall-time required may exceeds a year, even with the optimized MPI configuration. Close inspection indicates that it is the implicit solvers for the NS equations, in particular the need to resolve the Poisson equation, that deteriorates the MPI Parallel Efficiency. As a matter of fact logically simple explicit methods are much easier to attain almost linear scaling on modern multi-core architectures as well as to exploit high nominal performance of a single core. Indeed iterative schemes resulting from implicit discretization may show a good convergence paid by a large number of logical switches. Therefore explicit or semi-implicit methods for the NS equations are advantageous for parallel codes.

However not much can be done for the elliptic Poisson equation which is often used to ensure the incompressibility condition. In the second part of our talk we will present first results on the status of the explicit matrix-free method applied to the NS equations without a need to solve the Poisson equation at all.