The virtual massless particle tracing assumes that the flow fields around the actuators are 2D, and the particle velocity equals the flow velocity at the particle position. The flow fields used for particle tracing are obtained from the PIV experiments. To get the flow velocity at any location within the observation region, cubic interpolation is used to interpolate the 2D velocity-gridded **data** obtained from the PIV experiment. The center of mass of the particles is
calculated by X COM = ∑ i=1 M X i _ M .
Here, M is the total number of the particles and X i is the coordinate of the ith particle.