Abstract
The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈2×10⁵–4×10⁵. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation.
Highlights
There are many examples in which an airfoil operates close to stall, either intentionally to maximize lift, or inadvertently, such as in the case of a wind turbine blade experiencing a sudden gust
Validation of measurement procedure By way of validation of the measurement procedure and of the wind tunnel, initial measurements were made of the noise due to a NACA0012 airfoil of 0.2 m chord at high angle of attack and compared against the spectra predicted by the BPM model [1], which provides estimates for the Experimental noise eff=12°
In this paper the sound generated by an airfoil has been investigated experimentally over a wide range of angles of attack, including prestall and poststall conditions
Summary
There are many examples in which an airfoil operates close to stall, either intentionally to maximize lift, or inadvertently, such as in the case of a wind turbine blade experiencing a sudden gust. Airfoils operating close to stall generally suffer a degradation in aerodynamic performance At these conditions, airfoils are well known to be noisy, characterized by large increases in low-frequency noise [1]. Increasing the airfoil angle of attack produces an increase in the adverse pressure gradient over the airfoil suction side, which induces a separation of the boundary layer from the surface. As long as this separation is incipient, the separated layer is still able to reattach to the surface in a relatively short distance. Its main focus is to investigate the cause of these pressure fluctuations, which has received comparatively little attention in the literature
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