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AN EXPLORATION OF RADIAL FLOW ON A ROTATING BLADE IN RETREATING BLADE STALL 2013 (a) Comparison of tip speed ratio and reduced (b) Comparison of variation of angle of attack frequency at = 270 ( = 30 for wind turbine) across the span at different azimuths ( = 30 ) for wind turbine Fig. 2. Comparison of dynamic similarity parameters. stall experiments (Ref. 24). For convenience of comparison to the rotor setup, the azimuth angle ψ has been defined differently from the regular convention. ψ = 0 starts at the right side of the turbine looking upwind (see Fig. 1). The equations used to derive the dynamic similarity parameters are summarized in the Appendix. Figure 2 illustrates the similarities in the dynamic parameters of tip speed ratio, reduced frequency, and angle of attack. The rotor setup is in the same general operating regime as the wind turbine at yaw angles of 20◦−30◦. Clearly, the reduced frequencies and tip speed ratios (as per the definition) are in the same regime. The range of angle of attack that the blades experience is comparable between setups. The rotating blades in each case are subjected to the same conditions; hence, the behavior and characteristics of the fluid flow over the blades should be similar. Present scope and objectives In this paper, the phenomenon of radial flow development over a rotor blade in the dynamic stall regime of forward flight is examined. The paper seeks to answer the question on whether the radial flow is of first-order significance to the prediction of dynamic stall. Experimental Setup and Methodology The experiment was conducted in the high advance ratio experimental setup constructed in the test section of the closed circuit John J. Harper 7 ft × 9 ft (2.13 m × 2.74 m) wind tunnel in the School of Aerospace Engineering at The Georgia Institute of Technology. The motor was positioned below the rotor with manually adjustable cyclic and collec- tive pitch. To simplify the operations, a teetering rotor hub design was preferred. Studies by Bousman (Refs. 27, 28) correlating flight data with predictions show that the event of primary interest in load prediction is the large excursion in blade pitching moment that accompanies the separation and convection of the dynamic stall vortex. One school of thought suggests that the timing of dynamic stall on full-scale helicopter rotor blades depends on shock–vortex interaction phenomena in the highly accelerated flow over the leading upper surface of the blade (Refs. 29, 30). The shock terminating this region may trigger or interact with the separation of the dynamic stall vortex. Thus it may be rightly extrapolated that experimental studies where the critical Mach Table 1. High Advance Ratio Facility rotor specifications Description Rotor blade airfoil Blade mass total Blade span Blade chord Disc radius Solidity Precone Maximum collective Maximum cyclic Maximum TPP tilt Blade aspect ratio Motor Height Value NACA0012 1.747 0.622 0.178 0.889 0.0895 1.6 10 6.5 16 3.49 3.73 1.575 Unit kg m m m deg deg deg deg kW m 022005-3 number is not exceeded are irrelevant to predicting the stall line. But it should be noted that the flow field downstream of the stall line is incompressible, and studies in this region can be conducted ignoring compressibility. Furthermore, the large aspect ratio that is typical of helicopter and wind turbine blades is not essential to study the physics of the radial flow. This is because the stalled regions develop well inboard, eliminating aspect ratio effects. Thus a moderate aspect ratio, low tip Mach number test case is adequate for the purposes of studying the radial flow on a rotating blade in dynamic stall. These reasons form the basis for the design of the experimental setup used in this work. Given the objectives and motivation of studying radial flow behind the stall line, a rotor rpm of 200 and a tunnel speed of 20 ft/s (6.1 m/s) was chosen. The operating advance ratio is 0.33, which is sufficient to induce dynamic stall as demonstrated in Ref. 20 by flow visualization. This condition results in a chordwise relative velocity of about 10.5 ft/s (3.21 m/s) at the rotor midradius at the 270◦ azimuth of the 35-inches (0.889-m) radius rotor. A collective of 10◦ and a cyclic of −5◦ were used to create a 15◦ pitch on the retreating blade at 270◦. However, due to the flapping of the blade, the effective angle of attack observed on the retreating blade is 16.8◦. These angles agree with the angles corresponding to the onset of dynamic stall for a NACA0012 airfoil as reported by McAlister et al. in Ref. 3. Further details of the facility and test are provided in Table 1.PDF Image | Radial Flow Rotating Blade Retreating Blade Stall
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