In order to progress in the area of aeroacoustics, experimental measurements are necessary. Not only are they required for engineering applications in acoustics and noise engineering, but also they are necessary for developing models of acoustic phenomenon around us. One measurement of particular importance is acoustic impedance. Acoustic Impedance is the measure of opposition of acoustical flow due to the acoustic pressure. It indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a given frequency and can be a characteristic of the medium.The aim of the present paper is to give a synthetic overview of the literature on impedance measurements and to discuss the advantage and disadvantage of each measurement technique. In this work, we investigate the three main categories of impedance measurement techniques, namely reverberation chamber techniques, impedance tube techniques, and far-field techniques. Theoretical principles for each technique are provided along with a discussion on historical development and recent advancements for each technique.
The interaction of a rotating conical flow with a solid surface generates a centrifugal instability. This occurs in the flow over the wings of certain types of aircraft at high angles of attack. Efforts at our laboratory have detected such structures using near-surface flow diagnostics, and shown that they can be effectively alleviated using passive flow control near the surface. Their alleviation removes the narrowband spectral peak at the nominal location of vertical fins on these aircraft. This paper explores the substitution of active flow control techniques that remain conformal to the surface and are only powered during high angle of attack operation. The occurrence of the phenomenon and its 15-dB alleviation with geometric fences are shown on a rounded-edge 42-degree swept, cropped delta wing at 25 degrees angle of attack. The feasibility and power requirements for the plasma actuator are estimated in this paper. The generation of counter-rotating vortices using a double dielectric barrier discharge actuator is demonstrated.
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