Tip Vortex Formation and Evolution to the Near Wake of a Rotor in ...

core radius ( non-dim. by chord )0.150.10.0500 60 120 180 240 300 360 420 480 540vortex age ( degrees )Figure 10 - Evolution of the core radius on the ABSVI.5 Periodic Fluctuations in Core Axial VelocityThe evolution of the peak axial velocity is shown inFigure 13. The data from the front of the rotor has beenremoved for clarity. On the ABS, the peak velocitystarts at a little over 20% of the tip-speed and rapidlyincreases to nearly 60% of the tip-speed. This behavioris then followed by a gradual decay back down to 20%of the tip-speed by a vortex age of 160°. The rear of therotor shows peak decaying from 22% of the tip-speed to8% 108° of vortex age. On the RBS, the peak axialvelocity reaches nearly 80% of the tip-speed. A sharpdrop follows this peak to 23% of the tip-speed by 5° ofvortex age. After that point, it exhibits harmonicoscillations. Inspection of the other velocities shows thesame oscillations. The period of this oscillation is nearly10° of rotor azimuth. This corresponds to a Strouhalnumber, based on blade tip thickness, of 0.16.0.00RBSABS-0.05-0.10z/R-0.15-0.20-0.25-0.30Rear view-0.35-0.40-1 -0.5 0 0.5 1y/RFigure 11 – Wake boundaries on the ABS and RBS0.04rear0-0.04z/R-0.08-0.12-0.16frontView from the RBS-1 -0.5 0 0.5 1x/RFigure 12 – Wake boundaries on the front and rearVa /(! R)0.80.60.40.2Strouhal No. = 0.16ABSRBSRear00 48 96 144vortex age ( degrees )Figure 13 - Evolution of the peak axial velocity (frontstation removed for clarity)Another interesting feature of Figure 13 is the axialvelocity on the RBS reaching nearly 80% of the tipspeed.The magnitude of this velocity alludes to thepeak axial velocity reaching tip-speed during formationperhaps due to the no-slip boundary condition. Seedparticle dynamics may explain why velocities of thismagnitude are not observed at the other stations. On theRBS the blade and freestream directions are the same.Therefore the velocity difference between seed particlesin the freestream and the local fluid velocity around theblade is much smaller than at the other stations. As aresult, the seed particles can more quickly adjust to thelocal fluid velocity. Measurements were made to extendthe data on the evolution of the axial velocity out tovortex age of 540°. By a vortex age of 160° the numberof data points inside the vortex core had diminished tothe point that no conclusion could be made on the peakaxial velocity.Copyright © American Helicopter Society. Presented at the 57 th AHS Forum, June 2001