Low Reynolds-number moment on asymmetric bodie

The moment induced on asymmetric particles by a low Reynolds-number flow is examined. A flat disk and a half sphere are suspended in low-velocity airflow and the moment exerted on the particles is measured as a function of Reynolds number and approach angle. Object Reynolds-numbers range between 20 and 200, and the measured moment is in the nNm range. A moment coefficient (a moment normalized with the drag force and object radius) is calculated from the moment measurements and found to generally decrease with increasing Reynolds number. Stable approach angle positions are identified for the objects in the flow, and it is shown that the objects tend to rotate towards the position of maximum drag (local extremum) from other approach angles. The present measurements are consistent with previous qualitative observations of free-falling particles.