Drag Source Term Assembly Timing in Two-Way Coupled FDS

[19ap]

Hello everyone,

we are currently analysing two-way coupled Eulerian–Lagrangian particle settling simulations in FDS and would like to clarify one specific detail regarding the assembly of the gas-phase momentum source term due to particle drag.

From the Technical Reference Guide (9.1 Particle Transport in the Gas Phase), it is clear that in two-way coupling the drag force acting on each Lagrangian particle is added to the gas momentum equation as a volume-specific source term (sum of drag forces per cell divided by cell volume). However, the documentation does not explicitly describe at which point in the time-integration sequence the particle positions used for this accumulation are evaluated.

More specifically, within a single global time step:

1. The gas phase is advanced.
2. Lagrangian particles are advanced.
3. Drag forces are accumulated into the gas momentum source arrays.

Our question concerns step (3):
When assembling the momentum source term in a given cell, are the drag contributions summed from particles that resided in that cell at the beginning of the time step, or from particles whose updated positions fall inside that cell after particle advection within the same time step?
In other words, is the drag source term spatially evaluated using particle locations at time level n or at time level n+1?

Our understanding is that typical Eulerian–Lagrangian implementations assemble drag contributions based on the particle’s updated position after advection within the time step, rather than on its position at the beginning of the step. However, we would appreciate clarification on this point.

Thank you in advance for your help.

[drjfloyd]

The source file main.f90 contains the order of evaluation within FDS. The source file part.f90 is where all the particle routines are.

In main:

! Perform droplet mass/energy transfer, then move all particles, then transfer momentum to the gas.
! Solid particle mass/energy transfer is handled by WALL_BC.

COMPUTE_WALL_BC_2A: DO NM=LOWER_MESH_INDEX,UPPER_MESH_INDEX
CALL PARTICLE_MASS_ENERGY_TRANSFER(T,DT,NM)
CALL MOVE_PARTICLES(T,DT,NM)
IF (PARTICLE_DRAG) CALL PARTICLE_MOMENTUM_TRANSFER(DT,NM)
ENDDO COMPUTE_WALL_BC_2A