Planar nearfield acoustical holography in high-speed, subsonic flow
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The objective is to develop a NAH procedure that includes the effects of the high-speed, subsonic flow of a fluid medium. Recently, the speed of a transportation has significantly increased, e.g., to close to the speed of sound. As a result, the NAH data measured with a microphone array attached to an aircraft or train include significant airflow effects. Here, the convective wave equation along with the convective Euler's equation is used to derive the proposed NAH procedure. A mapping function between static and moving fluid medium cases is also derived from the convective wave equation. Then, a new wave number filter defined by mapping the static wave number filter is proposed. For the purpose of validating the proposed NAH procedure, a monopole simulation at Mach = -0.6 is conducted. The reconstructed acoustic fields obtained by applying the proposed NAH procedure to the simulation data match well with the exact fields. Through an experiment with two loudspeakers performed in a wind tunnel at the airflow speed of Mach = -0.12, it is also shown that the proposed NAH procedure can be used to successfully reconstruct the sound fields radiated from the two loudspeakers.
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