Abstract
A numerical method is presented for calculating the optimal lining impedance for the inlet duct of a let engine. The calculation is based on the condition that the total radiated power from the duct shall be a minimum. Several situations are considered: first, the case of an axially symmetric duct in a stationary acoustic medium; second, that of a body of arbitrary geometry in a stationary medium; and finally, the case of an axially symmetric duct in the presence of spinning modes and axial air flow. In each case, the method may yield the optimal lining impedances for a given discrete frequency, flow velocity, and duct geometry. To demonstrate the application of the method, a number of cases are investigated. First, a cylindrical duct closed at one end by a vibrating piston is used to make comparisons with predictions obtained analytically by Rice. Then a more complex problem of a bell-mouth compressor is investigated. A discussion of the results obtained is presented together with a critique of the potential difficulties of the method as applied to complex duet configurations. [This work was carried out under contract with NASA, Washington.]
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