The far wake trajectory of the tumbling tip vortex effect of a reduced-scale, 1 m diame-
ter, four-bladed rotor during hover is studied using vortex methods combined with a center
of mass analysis approach. Measurements of all three components of the velocity field are
acquired using a stereo PIV system synchronized to capture up to 800 wake age of the
vortex with 10 offsets during hover conditions. The nominal operating condition of the
rotor is at a rotational speed of 1520RPM, corresponding to Rec = 248,000 with a chord
length of 58.5mm. The rotor was operated with a pitch of 7.2± 0.5 and a CT / of 0.029.
The far wake vortex tumbling phenomenon is captured and described. It is shown that
tip vortices from two blades tumble through approximately 180 of rotation before they
coalesce. It is seen that the parent vortices are stronger than the daughter vortex, but due
to vortex stretching the daughter vortex has a smaller radius with greater swirl strength.
An accurate characterization and prediction of the trajectory of the far wake vortex tum-
bling can enhance the ability to predict and alleviate the resuspension of particles during
brownout as well as provide a database for far wake validation of CFD codes.