An efficient phase-field method for turbulent multiphase flows

TL;DR

This paper introduces a new efficient phase-field method for simulating large-scale three-dimensional turbulent multiphase flows, significantly reducing computational costs while maintaining accuracy.

Contribution

It presents a novel discretization scheme for the biharmonic term and a multi-resolution algorithm, enabling high-performance simulations on massive meshes with reduced computational expense.

Findings

Achieved efficient large-scale simulations on meshes up to 8 billion nodes

Validated method through drop deformation and bubble breakup cases

Successfully simulated drop coalescence in turbulent convection

Abstract

With the aim of efficiently simulating three-dimensional multiphase turbulent flows with a phase-field method, we propose a new discretization scheme for the biharmonic term (the 4th-order derivative term) of the Cahn-Hilliard equation. This novel scheme can significantly reduce the computational cost while retaining the same accuracy as the original procedure. Our phase-field method is built on top of a direct numerical simulation solver, named AFiD (www.afid.eu) and open-sourced by our research group. It relies on a pencil distributed parallel strategy and a FFT-based Poisson solver. To deal with large density ratios between the two phases, a pressure split method [1] has been applied to the Poisson solver. To further reduce computational costs, we implement a multiple-resolution algorithm which decouples the discretizations for the Navier-Stokes equations and the scalar equation: while a stretched wall-resolving grid is used for the Navier-Stokes equations, for the Cahn-Hilliard equation we use a fine uniform mesh. The present method shows excellent computational performance for large-scale computation: on meshes up to 8 billion nodes and 3072 CPU cores, a multiphase flow needs only slightly less than 1.5 times the CPU time of the single-phase flow solver on the same grid. The present method is validated by comparing the results to previous studies for the cases of drop deformation in shear flow, including the convergence test with mesh refinement, and breakup of a rising buoyant bubble with density ratio up to 1000. Finally, we simulate the breakup of a big drop and the coalescence of O(10^3) drops in turbulent Rayleigh-B\'enard convection at a Rayleigh number of $10^8$, observing good agreement with theoretical results.