Large-eddy simulations (LESs) of low-Reynolds-number flow (Re=50,000) over a NACA0018 airfoil are performed to investigate flow control at the stall angle of attack (15 deg) by low-amplitude surface waves (actuations) of different types (backward/forward traveling and standing waves) on the airfoil’s suction side. It is found that the backward (toward downstream) traveling waves, inspired from aquatic swimmers, are more effective than forward traveling and standing wave actuations. The results of simulations show that a backward traveling wave with a reduced frequency f∗=4 (f∗=fL/U, where f is frequency; L, chord length; and U, free flow velocity), a nondimensional wavelength λ∗=0.2 (λ∗=λ/L, where λ is dimensional wavelength), and a nondimensional amplitude a∗=0.002 (a∗=a/L, where a is dimensional amplitude) can suppress stall. In contrast, the flow over the airfoil with either standing or forward traveling wave actuations separates from the leading edge similar to the baseline. Consequently, the backward traveling wave creates the highest lift-to-drag ratio. For traveling waves at a higher amplitude (a∗=0.008), however, the shear layer becomes unstable from the actuation point and creates periodic coherent structures. Therefore, the lift coefficient decreases compared with the low-amplitude case.
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Mechanisms of Morphing Wall Flow Control by Traveling Waves over an Airfoil
The main two mechanisms of morphing wall flow control are direct injection of momentum in the streamwise direction and indirect momentum transfer via triggering instabilities. Traveling waves have been shown to perform better than standing waves, probably because they can use both mechanisms. However, the relative importance of the two mechanisms is not known. To differentiate between the mechanisms, a range of parameters (frequency, amplitude, and starting location) at stall (15 deg angle of attack) and poststall (20 deg angle of attack) is tested using wall-resolved large-eddy simulations with a sharp-interface curvilinear immersed boundary method at a low Reynolds number of [Formula: see text] over a NACA0018 airfoil. The results of the simulations demonstrate that the flow is reattached within a range of nondimensional frequencies, actuation amplitudes, and starting locations of oscillation at the stall and poststall angles of attack. Significant lift enhancement and drag reduction are also observed within these ranges. The nondimensional frequency range at which the flow is reattached is found to be similar to the dominant nondimensional frequencies of leading-edge vortex shedding of the unactuated airfoil. These indicate that the indirect transfer of momentum is the dominant mechanism because direct injection of momentum increases with the increase of amplitude and frequency; that is, separation should reduce as they increase. Nevertheless, direct injection of momentum improves the performance relative to pure excitations of standing waves when instabilities are triggered.
more » « less- Award ID(s):
- 1905355
- NSF-PAR ID:
- 10483394
- Publisher / Repository:
- AIAA
- Date Published:
- Journal Name:
- AIAA Journal
- Volume:
- 61
- Issue:
- 4
- ISSN:
- 0001-1452
- Page Range / eLocation ID:
- 1687 to 1707
- 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/10.2514/1.J062449
