[jenc24]

The Brownian force (https://cpp.openfoam.org/v11/Brownia...8C_source.html) is said to be calculated as in Ahmadi et. al (https://doi.org/10.1080/02786829208959550), as
.

The force is then added to the list of forces acting on a particle (as an explicit contribution) as , where is the mass of the particle. The sum of forces gives the net force, which is then integrated to obtain the new particle position.

The article states, that the Brownian motion force should be varying through the timestep, as shown here: https://bugs.openfoam.org/file_downl...=1616&type=bug,
whereas in case of current implementation, it is clearly constant for each timestep.

Moreover, tracking the time evolution of the RMS displacement of particles, given by

gives a wrong result; time evolution is obtained, instead of as shown in theory (Ahmadi). This phenomena most likely stems from the fact that Brownian force is calculated and added to the sum of all forces just like any other force (drag, lift, ...), which is most likely incorrect.

A thought on this matter from anyone will be greatly appreciated.