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V. RAGHAV JOURNAL OF THE AMERICAN HELICOPTER SOCIETY Fig. 3. High Advance Ratio Facility: configured for radial flow. The forward flight rotor results presented here started with extensive PIV studies of the radial velocity field over the retreating blade of a two- bladed teetering rotor in forward flight (Ref. 20). PIV was primarily used to study the radial flow over the dynamically stalled blade in forward flight using the setup depicted in Fig. 3(a). Radial flow measurements were obtained at four stations using the apertures shown in Fig. 3(b). To demonstrate the validity of the radial flow results, static stall tests were conducted on the blade at an angle of attack of 13.5◦ and a freestream velocity of 15 ft/s (4.6 m/s). Velocity measurements in chordwise axial planes were made at 12 locations between r/R = 0.514 and 0.971 with spatial resolution increasing toward the tip, as shown in Figs. 4(a) and 4(b). At each position, a calibration board was used to quantify spatial coordinates. Corrections for the tilt of the laser-illuminated measuring window with respect to the camera image plane were incorporated into the DaVis software. PIV considerations The equipment used with the LaVision setup included a New Wave Research Solo Nd:YAG laser, an ImagerIntense charge-coupled device (CCD) camera, and the DaVis imaging software. The laser was double pulsed and frequency doubled to produce 532 nm visible light. The resulting beam diameter was about 3.5 mm with a pulse width of 5 ns. The ImagerIntense camera had a 1376 (h)×1040 (v) pixel viewing area. The scan rate of the CCD sensor was 16 MHz. The focal length of the lens system was 50 mm. The aperture of the camera was set at f/4. One hundred image pairs were captured at each measurement location. Since the mean flow direction was normal to the image plane, it was important to choose a pulse separation (t) that was short enough to ensure that the out-of-plane movement did not exceed 1/3 of the Fig. 4. High Advance Ratio Facility: configured for chordwise flow. sheet thickness as discussed in Ref. 31. A delay of 100 μs was cho- sen to produce sufficient frame-to-frame particle matching. Accurate PIV measurements also depended on the interrogation window size. The signal-to-noise ratio begins to degrade if particle displacements exceed one quarter of the interrogation window (Ref. 31). Optimally, 7–10 parti- cles in each interrogation window are necessary (Ref. 32). Using results by Yang et al. (Ref. 18), a maximum axial velocity of 8 m/s was esti- mated while the radial velocity was limited to 3 m/s. A 32 × 32 pixel window would be sufficiently large so that particles traveling less than 10.6 m/s would not travel more than eight pixels or one quarter of the total interrogation window. Velocities were calculated from the spatial cross-correlation of the images. The average particle displacement in each interrogation window was revealed by the location of the highest correlation peak. With the pixel size of the ImagerIntense camera averaging 6.5 μm, the particle size ranged from 1.06 to 1.97 pixels. The optimal particle diameter for digital cross-correlation is approximately 2 pixels. An interrogation window overlap of 75% and a second interrogation pass with a reduced window size increased the data yield and the signal- to-noise ratio of the correlation peak. Therefore, the first pass utilized an interrogation window of 64 × 64 pixels, whereas a 32 × 32 pixel win- dow was used on the second pass. Postprocessing of the vector images consisted of an applied vector range and a median filter. As erroneous vectors appeared at the edges of the camera-viewing window, these pro- cesses greatly reduced this noise. Further details on the experimental methodology are presented in DiOttavio et al. (Ref. 20). 022005-4PDF Image | Radial Flow Rotating Blade Retreating Blade Stall
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