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V. RAGHAV JOURNAL OF THE AMERICAN HELICOPTER SOCIETY Fig. 10. Average strength of structures with respect to the upper and lower shear layers. Fig. 11. Radial variation of the ensemble-averaged separation line, with error bars showing standard deviation. Quantitative estimates. The average vorticity entrained in the discrete structures at various radial stations is depicted in Fig. 9. The key rev- elation is that the average strength of the structures and their variance increases on moving outboard on the blade. This fact along with the notion that the discrete structures are “driven” by the radial jet shear layer (discussed in preceding section) can now be used to reconcile the behavior of the radial velocity. In other words, a higher amount of vor- ticity generated from the surface is entrained in the discrete structures on moving outboard. Subsequently, these discrete structures that were observed to “peel” or “jump” off the blade (see Fig. 7) caused the evident decrease in peak magnitude of the radial velocity on moving outboard on the blade (Fig. 6). A quantitative comparison of the average strength of the structures to the average vorticity in the shear layers bounding the radial jet is illus- trated in Fig. 10. Two shear layers were considered for this analysis. First, the lower shear layer (LSL) is the boundary layer between the solid blade surface and the peak of the radial jet profile. It is produced by the shear between the radially accelerated flow and the no-slip condition at the sur- face and is the stronger of the two shear layers. Second, the upper shear layer (USL) is the layer of fluid above the point of peak radial velocity. It is produced by the jet velocity decay into the largely stagnant zone above it. The LSL was observed to be almost an order of magnitude (factors of 7 or 8 have been observed) stronger than the discrete structures at inboard locations. It then weakens, even as the structures themselves get stronger, so that it is only three to four times as strong as the structures at the most outboard locations evaluated. Thus at these outboard locations, the discrete structures carry away 25%–30% of the jet. However, the strength of the individual vortical structures is much less than 2% of the vorticity estimated from the radial velocity profile for the USL. Hence it can be inferred that this jet decay occurs mostly through the breakoff of the discrete structures. Chordwise flow The notable feature in the chordwise plane is the location and behav- ior of the separation line. At each radial station examined, the separation point was determined by plotting the normal velocity 2 mm from the blade surface and determining the point where an abrupt increase in its magnitude was observed. This is a practical way of determining separa- tion from velocity data where resolving the boundary layer itself is im- practical. Figure 11 shows the radial variation of the ensemble-averaged separation line with the error bars, showing the standard deviation of the location of the separation point. At inboard locations (r/R < 0.6), stall occurred very close to the blade leading edge as seen in Fig. 12, which indicates the severity of dynamic stall. This is a classic feature also observed in other studies (Ref. 4). On the outboard sections of the blade, the separation point moved aft toward the trailing edge. This is attributed to the influence of the downwash induced by the tip vortex. Fig. 12. Instantaneous vorticity observed via PIV at different r/R. (a) Span station, r/R = 0.571, (b) span station, r/R = 0.743. 022005-8PDF Image | Radial Flow Rotating Blade Retreating Blade Stall
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