Exploring the Applications of Filtered Linear Forcing Technique on Two-Way Coupled Particle-Laden HIT Flows

Turbulence forcing is a common technique in HIT simulation which adds an artificial force to the Navier-Stokes (NS) equations to counteract energy decay due to viscosity. For particle-laden flows, forcing is necessary as particle response times may be longer than the characteristic lifetime of the turbulent eddies. It is important that the presence of particles does not affect the forcing or the nature of the resulting turbulence. This work investigates the suitability of the FLF method for this task and compares it to a common method of turbulence forcing which we call ordinary linear forcing (OLF). FLF improves upon OLF by applying a low-pass filter, and allows for a separation of scales between forced scales and scales resolved by the simulation [Palmore and Desjardins, Phys. Rev. Fluids, 2018]. Direct Numerical Simulation (DNS) is used to solve the gas phase NS equations and is coupled with Lagrangian Particle Tracking (LPT) to solve particle motion in a triply periodic cubic domain. The domain has N=256³ grid cells and is set up for the gas phase to represent standard atmospheric conditions. Simulations with particles of varying Stokes Numbers (0.1 < St < 100) and volume fractions (5e-5 < VF < 5e-2) are demonstrated in HIT flow with Re_λ ≈ 70.  The results verify that FLF shows a separation of scales and the particles only introduce energy into the flow at higher frequency wavenumbers in the two-way coupled regime. Apart from the energy spectrum, the Q criterion is observed to analyze particle-vorticity interactions.