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

square tip and is heavily tip-loaded due to the constantchord,rectangular planform. The Reynolds number ofthe tests is high enough to involve transition on theblades, and both the freestream turbulence intensity(roughly 0.15%), the blade chord, collective pitch of 10degrees, and the surface roughness of the blade withrough grit over the leading edge up to quarter-chord,and the wooden surface aft of that, all contribute to thedevelopment of turbulent boundary layers. The seedparticles used are not monodisperse in size distribution,and some of them are expected to be in the regimewhere the spinout error must be considered. Except forthe tip Mach number, these are fairly realistic conditionsfor interpretation and application of the results, as willbe seen.IV. Experimental SetupThese experiments were conducted in Georgia Tech’sJohn Harper Wind Tunnel. The facility is a closedcircuit tunnel with a 2.13m x 2.74m test section. Airspeed is continuously variable up to 60 m/s. Turbulencelevels are 0.2%. The rotor shaft extends through the testsection ceiling to the centerline of the tunnel, and istilted forward by 6° to simulate forward flight. Attachedto this shaft is a two-bladed, teetering, untwisted,untapered, NACA 0015 rotor. The radius and bladechord are 9.457m and 85.7mm respectively. Testconditions for these experiments were a rotor RPM of1050 and an advance ratio of 0.10. An optical sensor isused to generate a TTL 1/rev pulse used for phaseaveraging measurements.Figure 1 - Forward flight experimental setupSingle velocity component measurements were madeusing a laser velocimeter (LV) on the advancing bladeside (ABS). The multi-line output from a 5W argon-ionlaser is used to power the laser Doppler velocimeter.The green line (514.5nm) is separated and then split intotwo beams, frequency shifted and coupled in to a fiberoptic probe in a TSI Colorburst module. The probe ismounted in the test section to a three-axis lineartraverse. The probe and traverse are well outside of therotor wake to minimize their effects on the rotor flowfield. All of these measurements were made inbackscatter mode. The measurement volume is ellipsoidwith a diameter of 90.5µm and a length of 1.31mm. Themeasurement grid consisted of a course grid with5.08mm steps and a finer grid centered on the blade-tipwith 2.54mm steps. The origin for these measurementsis located at the blade-tip trailing edge when ψ = 90°.One hundred thousand points were collected at eachmeasurement location. The data were phase averagedand then sorted by rotor azimuth into bins 0.5° wide.Figure 2 - ABS measurement gridMineral oil is used for the LV seeding. The seeder islocated downstream, so particles are drawn through 4turning vanes, the fan, honeycomb and screens beforeentering the test section. Particle diameter rangesbetween 1 and 5 µm. based on Multiple-Ratio Single-Particle sizing performed in the 1980s.Since previous work [19-20] has shown that theflowfield is periodic to within 1° of rotor azimuth, thethree-dimensional velocity field can be reconstructedfrom single velocity component measurements. Theability to reconstruct the flow field is vital to thesemeasurements, as the rotor interferes withmeasurements for certain azimuths at certain locations.To alleviate these problems, each velocity componentwas measured from two different probe orientations.The two resulting datasets had missing data for differentazimuths. The datasets were then combined to form onecomplete dataset.Copyright © American Helicopter Society. Presented at the 57 th AHS Forum, June 2001