The video was taken with a high-speed video camera at 200 frames per second. Below is a picture of the sphere in the gravel bed with particles in the flow illuminated by a laser sheet. The instantaneous downstream and vertical force on the particle were measured by a force transducer located under a false wall of the flume. Velocity vectors were measured by a hybrid PIV-PTV algorithm. Ejections in the flow gives rise to stagnation of flow at the front of the particle and high lift but very low drag.
Sweeps produce flow under the particle which gives negative lift but high drag.
When the particle is lower in the bed, flow under the grain is more difficult, and ejections produce small lift and little drag. Whereas, sweeps produce large drag and large fluctuations in lift.