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Abstract Unsteady airfoil experiments were conducted in a high-pressure wind tunnel at chord Reynolds numbers of Re c = 3.0 × 10 6 . A moderately thick NACA0021 airfoil was pitched from rest beyond the static stall angle in six individual ramp tests with increasing and decreasing angles of attack. The variant types of motion of the pitching maneuvers were characterized by constant angular velocity, angular acceleration and angular jerk, respectively. The ramp-up experiments revealed a substantial and time-dependent excess of the aerodynamic forces from static values in all three test cases and exhibited a distinct time delay as a consequence of the variant motion types. Similarly, the ramp-down experiments were largely impacted by the progression of the pitching motion, resulting in pronounced differences in the temporal development of lift and drag. Results are shown as time series of integrated forces and surface pressure distributions.
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Dynamic stall at high Reynolds numbers induced by ramp-type pitching motions
The transient pressure field around a moderately thick airfoil is studied as it undergoes ramp-type pitching motions at high Reynolds numbers and low Mach numbers. A unique set of laboratory experiments were performed in a high-pressure wind tunnel to investigate dynamic stall at chord Reynolds numbers in the range of $$0.5\times 10^6\leq Re _c\leq 5.5\times 10^6$$ in the absence of compressibility effects. In addition to variations of mean angle and amplitude, pitching manoeuvres at reduced frequencies in the range of $$0.01\leq k\leq 0.40$$ were studied by means of surface-pressure measurements. Independently of the parameter variations, all test cases exhibit a nearly identical stall behaviour characterized by a gradual trailing-edge stall, in which the dynamic stall vortex forms approximately at mid-chord. The location of the pitching window with respect to the Reynolds-number-dependent static stall angle is found to define the temporal development of the stall process. The time until stall onset is characterized by a power law, where a small excess of the static stall angle results in a drastically prolonged stall delay. The reduced frequency exhibits a decrease in impact on the stall development in the case of angle-limited pitching manoeuvres. Beyond a critical reduced frequency, both load magnitudes and vortex evolution become reduced frequency independent and instead depend on the geometry of the motion and the convective time scale, respectively. Overall, the characteristics of vortex evolution induced by dynamic stall show remarkable similarities to the framework of optimal vortex formation reported in Gharib et al. ( J. Fluid Mech. , vol. 360, 1998, pp. 121–140). The data from this study are publicly available at https://doi.org/10.34770/b3vq-sw14 .
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- Award ID(s):
- 1652583
- PAR ID:
- 10338821
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 938
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2022.70
